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Widening the Window of Spin-Crossover Temperatures in Bis(formazanate)iron(II) Complexes via Steric and Noncovalent Interactions
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  • Francesca Milocco
    Francesca Milocco
    Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands
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  • Folkert de Vries
    Folkert de Vries
    Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands
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  • Harmke S. Siebe
    Harmke S. Siebe
    Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands
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  • Silène Engbers
    Silène Engbers
    Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands
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  • Serhiy Demeshko
    Serhiy Demeshko
    Institut für Anorganische Chemie, Universität Göttingen, Tammannstraße 4, 37077 Göttingen, Germany
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  • Franc Meyer
    Franc Meyer
    Institut für Anorganische Chemie, Universität Göttingen, Tammannstraße 4, 37077 Göttingen, Germany
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    Orcidhttp://orcid.org/0000-0002-8613-7862
  • Edwin Otten*
    Edwin Otten
    Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands
    *Email [email protected]
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    Orcidhttp://orcid.org/0000-0002-5905-5108
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Inorganic Chemistry

Cite this: Inorg. Chem. 2021, 60, 3, 2045–2055
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https://doi.org/10.1021/acs.inorgchem.0c03593
Published January 19, 2021

Copyright © 2021 American Chemical Society. This publication is licensed under CC-BY-NC-ND.

Abstract

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Bis(formazanate)iron(II) complexes undergo a thermally induced S = 0 to S = 2 spin transition in solution. Here we present a study of how steric effects and π-stacking interactions between the triarylformazanate ligands affect the spin-crossover behavior, in addition to electronic substituent effects. Moreover, the effect of increasing the denticity of the formazanate ligands is explored by including additional OMe donors in the ligand (7). In total, six new compounds (2–7) have been synthesized and characterized, both in solution and in the solid state, via spectroscopic, magnetic, and structural analyses. The series spans a broad range of spin-crossover temperatures (T1/2) for the LS ⇌ HS equilibrium in solution, with the exception of compound 6 which remains high-spin (S = 2) down to 210 K. In the solid state, 6 was shown to exist in two distinct forms: a tetrahedral high-spin complex (6a, S = 2) and a rare square-planar structure with an intermediate-spin state (6b, S = 1). SQUID measurements, 57Fe Mössbauer spectroscopy, and differential scanning calorimetry indicate that in the solid state the square-planar form 6b undergoes an incomplete spin-change-coupled isomerization to tetrahedral 6a. The complex that contains additional OMe donors (7) results in a six-coordinate (NNO)2Fe coordination geometry, which shifts the spin-crossover to significantly higher temperatures (T1/2 = 444 K). The available experimental and computational data for 7 suggest that the Fe···OMe interaction is retained upon spin-crossover. Despite the difference in coordination environment, the weak OMe donors do not significantly alter the electronic structure or ligand-field splitting, and the occurrence of spin-crossover (similar to the compounds lacking the OMe groups) originates from a large degree of metal–ligand π-covalency.

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Synopsis

A series of Fe(II) complexes with formazanate ligands are reported, and ligand substituent effects on structure and spin-crossover properties are examined. These ligand modifications allow isolation of compounds with tetrahedral geometries in both low- and high-spin ground states as well as an intermediate-spin square-planar complex. Steric properties, π-stacking interactions, and additional donor substituents lead to a wide range of spin-crossover temperatures (T1/2) in this class of compounds.

Introduction

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The geometry of transition metal complexes is dependent on the electronic structure, (1) and it is often the case that the geometry preferred on steric grounds is overridden in favor of a different one by electronic effects. (2) In four-coordinate complexes two extreme geometries can be observed: the sterically favored tetrahedral and the electronically stabilized square-planar structure. While complexes with a d8 configuration have been thoroughly investigated, the balance between steric and electronic effects on the geometry of compounds with a lower d-electron count is not well established. In the case of first-row transition metals such as Fe(II), the electronic stabilization is typically small, and therefore these compounds tend to adopt a tetrahedral configuration. (1a,2,3) Therefore, to observe square-planar Fe(II) complexes, specific requirements are usually needed that result in intermediate-spin (S = 1) compounds: (1b) (i) macrocyclic ligands that enforce a planar geometry around the metal center (4) or (ii) strong field ligands, e.g., phosphines, that provide a greater ligand field stabilization energy compared to nitrogen and oxygen donors (in the case of mono- and bidentate ligands), often in combination with ortho-substituted aryl coligands. (1b,5) Exceptions to this where Fe(II) square-planar structures were observed have been sporadically reported. (6) Furthermore, while isomerization between tetrahedral and square-planar geometries is a well-established phenomenon for cobalt(II), (7) nickel(II), (7a) and copper(II), (7a,8) it is rare for iron(II). (6e)
In the simplistic terms of crystal field theory, the spin state of a complex in a certain geometry is determined by the orbital splitting (Δ) and the pairing energy (PE). (1a) When the values of these two parameters are comparable, various electronic configurations, differing in the spin state, may be accessible. This opens the possibility of switching between different spin states by using external stimuli (e.g., temperature, pressure, or light), leading to the phenomenon of spin-crossover (SCO). (9) While the major representatives in the category of spin-crossover compounds are six-coordinate Fe(II) complexes with nitrogen donor ligands, (9c,10) pioneering work on four-coordinate Fe(II) compounds has been conducted by the groups of Chirik, (11) Smith, (12) Peters, (13) and ours. (14) To illustrate the relationship between geometry and spin state in Fe(II) complexes, a comparison of the expected splitting of the d-orbital manifold in common coordination geometries is provided in Figure 1A–C. The role of ligand design in tuning the SCO properties, such as the spin-crossover temperature (T1/2), is well recognized. (15) However, predicting the effect of changes in steric/electronic properties of the ligand and spin-crossover energetics remains very challenging due to the small energy differences involved.

Figure 1

Figure 1. Common ligand field splitting diagrams for octahedral (A), square-planar (B), and tetrahedral (C) geometries and unusual ligand field splitting for the pseudo-tetrahedral geometries found in bis(formazanate)iron(II) complexes (D). (14)

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Following our report of a four-coordinate Fe(II) spin-crossover complex with formazanate ligands, (14a) we recently established that spin-crossover is a general feature of this class of compounds. (14b) The stability of the low-spin (S = 0) state for these compounds is ascribed to an unusual splitting pattern of the d-orbitals in this geometry. Specifically, the formazanate ligands, which are good π-acceptor ligands, are engaged in π-backdonation with the metal, and this allows the formation of a highly covalent metal–ligand bond, stabilizing one of the d-orbitals (the antibonding dyz orbital that belongs to the t2 set in a conventional tetrahedral complex), which gives rise to an “inverted” ligand field with an approximate “two-over-three” splitting pattern (Figure 1D). We demonstrated that it is possible to tune the SCO properties of bis(formazanate)iron(II) complexes by substituent effects that are purely electronic in nature. (14b) In the present work, we extend these studies to include steric effects as well as π-stacking interactions between the triarylformazanate ligands. Included in this analysis are nonsymmetric ligands that have two different N–Ar substituents. In addition, we describe the effect of additional OMe donor groups in the ligand. The aim of this work is to obtain comprehensive insight into how the spin-crossover properties of this class of compounds may be modulated via modification of the ligand.

Results and Discussion

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The bis(formazanate) iron complexes 2–7 were synthesized following a procedure previously reported by us (14b) starting from the iron precursor Fe[N(SiMe3)2]2 as depicted in Scheme 1. Complex 1 has already been extensively studied in our previous work, (14a) and it is therefore included in the discussion as reference compound. Besides compounds 1 and 2, all the others feature nonsymmetric ligands that have two different N–Ar substituents. The effect of an electron-withdrawing perfluorinated ring (Ar = C6F5) is studied either in the C–Ar3 position (2 and 4), as the N–Ar1 group (5), or in both positions (6). At the same time the influence of the electron-donating, sterically demanding mesityl group (Ar5 = Mes) is investigated in the N–Ar position either alone (3) or in combination with the perfluorinated ring (4, 5, and 6). Furthermore, the ortho-anisyl group (Ar1 = o-An) is introduced in the N–Ar position in compound 7, increasing the coordination ability of the formazanate to a tridentate monoanionic ligand.

Scheme 1

Scheme 1. Synthesis of Compounds 1–7
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Solid-State Characterization

While attempts to obtain single crystals suitable for X-ray diffraction were not successful for 2 and 5, the other compounds could be obtained in crystalline form. Single-crystal X-ray diffraction studies for complexes 3, 4, 6, and 7 allowed determination of their molecular structure, and pertinent metrical parameters are collected in Table 1. Overall, the structure of compound 3 is very similar to 1: it has relatively short Fe–N distances averaging to 1.831 Å and a flattened tetrahedral geometry around the Fe center (angle between the ligand coordination planes of 64.06(9)°), features that are indicative of a low-spin Fe(II) center. (14a) The steric pressure exerted by the N-Mes groups is evinced by the N(Mes)–Fe–N(Mes) angle of 109.19(6)°, which is noticeably larger than the N(Ph)–Fe–N(Ph) angle (100.68(6)°). The N-mesityl rings in 3 are engaged in off-center π-stacking interactions (Figure 2) both within the same molecule (interplanar angle of 2.77°; distance between Mes centroids and the least-squares plane of the other Mes ring of 3.200/3.229 Å) and between neighboring molecules (centroid-to-plane distance of 3.604/3.702 Å, Figure 2). Complex 4 shows similar intramolecular interactions between N-Mes groups (interplanar angle of 2.22°), but in this case the π-stacking does not extend to adjacent molecules (Figure S2). In contrast to 3, the Fe–N bonds are long (1.9610(12)–1.9946(11) Å), and the angle between the formazanate coordination planes is increased to 83.21(7)°, indicating that 4 is high-spin in the solid.

Figure 2

Figure 2. Crystal structure of compound 3 showing the π-stacking interactions between the mesityl rings. The Fe center, ligand backbone, and the mesityl rings are shown as 50% probability ellipsoids and the remaining atoms as wireframe; hydrogen atoms are removed for clarity.

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Table 1. Selected Bond Lengths (Å) and Angles (deg) in Compounds 1, 3, 4, 6, and 7 at 100 K (Unless Stated otherwise)
 1a346ab6b7
Fe(1)–N(1)1.8278(15)1.8192(14)1.9946(11)2.030(2)1.9259(9)1.877(2)
Fe(1)–N(4)1.8207(15)1.8351(13)1.9610(12)1.9851(19)1.9461(9)1.883(2)
Fe(1)–N(5)1.8330(16)1.8242(13)1.9864(12)2.035(2) 1.874(2)
Fe(1)–N(8)1.8174(16)1.8449(13)1.9616(12)1.9966(19) 1.895(2)
Fe(1)–O(1)     2.1128(18)
Fe(1)–O(2)     2.1029(19)
∠(NFeN)/(NFeN)c60.97(10)64.06(9)83.21(7)89.31(12)0.00(0)81.67(14)
Fe out-of-planed0.0010.0180.1160.5820.7000.220
0.0460.1190.1160.580 0.224
a

Data taken from ref (14a).

b

Structure measured at 200 K.

c

Dihedral angle between the coordination planes defined by the N–Fe–N atoms.

d

Displacement of the Fe atom out of the plane defined by the four N atoms of each ligand backbone.

Compound 6, in which the ligands are highly asymmetric from an electronic point of view (N–C6F5 and N-Mes), was obtained in two distinctly different forms (6a/b) depending on the crystallization conditions. A batch of crystals was obtained from hot hexane (6a) and analyzed by single-crystal X-ray diffraction. It shows a distorted tetrahedral environment around Fe, with off-center π-stacking between the N-Mes rings (interplanar angle of 2.87°) (Figure 3). The Fe–N distances of 6a (1.9851(19)–2.030(2) Å) and the angle between the formazanate coordination planes (89.31(12)°) indicate a high-spin Fe center. Surprisingly, crystals obtained by diffusion of hexane into a THF solution of 6 show that under these conditions it crystallizes as a square-planar complex (6b, Figure 3).

Figure 3

Figure 3. Molecular structure of compounds 6a and 6b showing 50% probability ellipsoids; hydrogen atoms omitted for clarity. The inset for each shows the Fe(NNCNN)2 core of the structure with the N–Fe–N planes and the dihedral angle.

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The square-planar geometry has the N–Ar groups in an anti-relationship, which allows off-center parallel intramolecular π-stacking interactions between the electron-rich Mes and the electron-deficient C6F5 groups (interplanar angle = 10.14°; distance = 3.259 Å, Figure 4). In addition, off-center parallel stacking between the C–C6F5 rings of neighboring molecules (centroid-to-plane distance of 3.222 Å) and a weaker intermolecular interaction between the N–Ar groups (interplanar angle of 10.14° and centroid-to-plane distance of 3.459 Å) are observed. (16) While the FeN4 fragment is planar (enforced by the crystallographic symmetry), the FeNNCNN six-membered chelate rings are puckered with the Fe center displaced out of the ligand plane. The Fe–N bonds in square-planar 6b (1.9259(9)–1.9461(9) Å) are shorter than those found in high-spin FeN4 complexes, such as tetrahedral 6a and in distorted-planar iron bis(amidinate) complexes reported by Hessen et al. (2.0528–2.0697 Å). (6a) The similarity of the metrical parameters in 6b to those in intermediate-spin Fe(II) porphyrins (e.g., 1.972(4) Å in Fe(TPP) (4a)) suggests that 6b also has an S = 1 ground state. To the best of our knowledge, this is the first example of an intermediate-spin Fe(II) complex with bidentate nitrogen donor ligands which adopts a square-planar geometry in the solid state. Although solution studies (vide infra) indicate that 6 is high-spin in toluene, the accessibility of a square-planar polymorph for 6 suggests that controlling the strength of π-stacking interactions is a viable approach to change the geometric preference and thus spin state in this class of compounds.

Figure 4

Figure 4. Crystal structure of compound 6b illustrating the π-stacking interactions between the aromatic rings, showing 50% probability ellipsoids. Parts of the molecule are shown as wireframe, and hydrogen atoms are removed for clarity.

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57Fe Mössbauer spectroscopy was employed to elucidate the electronic structure of 6. A quadrupole doublet with isomer shift δ = 0.75 mm/s and quadrupole splitting |ΔEq| = 1.21 mm/s was observed for a batch of crystals for the tetrahedral complex 6a (Figure 5 B). In contrast, crystals of square-planar 6b have a lower isomer shift (δ = 0.54 mm/s) and a higher quadrupole splitting (|ΔEq| = 2.73 mm/s) (Figure 5A). The Mössbauer spectra of both batches differ significantly from low-spin (S = 0) bis(formazanate)iron compounds, which have isomer shifts (δ) around 0 mm/s and an |ΔEq| of ca. 2 mm/s. (14) The isomer shift of 6a is indicative of a high-spin state (S = 2) (17) and indeed is comparable to that in the high-spin bis(formazanate)iron complex Fe(PhNNCPhNNPh) (δ = 0.60 mm/s). (14b) On the other hand, the isomer shift for 6b is in agreement with an intermediate spin state (S = 1), similar to the one reported for Fe(TPP) (δ = 0.50 mm/s). (4a) A crude powder of a pristine sample of 6 (i.e., not purified by crystallization) shows a Mössbauer spectrum identical with that of 6b and remains unchanged between 7 and 300 K (Figure 5C and Figure S4a–c). The magnetic susceptibility measurement of the powder sample of 6 recorded on a SQUID magnetometer gave χMT ≈ 1.1 cm3 mol–1 K, supporting the assignment of an intermediate-spin state (Figure 6). The magnetic susceptibility in the solid state stays constant up to 390 K, and then it suddenly increases, approaching a value of 2.5 cm3 mol–1 K at 400 K, which is lower than the expected value for a high-spin state S = 2 but could be an indication of an incomplete spin transition. To further probe this, the sample used for the SQUID measurement was subsequently analyzed by Mössbauer spectroscopy (Figure 5D). After 6b was heated to 400 K, the major species (82%) has a quadrupole doublet with δ = 0.74 mm/s and |ΔEq| = 1.17 mm/s, which are in good agreement with the values obtained for 6a. Thus, this indicates that square-planar, intermediate-spin 6b switches at least partially to tetrahedral, high-spin 6a in the solid state. Differential scanning calorimetry analysis of a fresh powder sample of 6 shows an endothermic transition at 412 K with an onset temperature around 397 K (followed by subsequent decomposition) and corroborates a spin transition in the solid state at high temperature.

Figure 5

Figure 5. 57Fe Mössbauer spectra at 80 K in the solid state of 6b (A), 6a (B), and a powder sample of 6 before (C) and after (D) heating to 400 K for SQUID measurements. The red line in the spectrum of heated 6 represents the main species with 82% area, and the gray subspectra are unknown impurities.

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Figure 6

Figure 6. Magnetic susceptibility data for a powder sample of 6 in the solid state (heating to 400 K and subsequent cooling). The solid black line shows the best fit curve for S = 1 with the parameters g = 2.10 and D = 11.2 cm–1 (100% IS). The dashed red line shows the spin-only value for an S = 2 system.

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Lastly, compound 7 containing formazanate ligands with an additional OMe donor moiety was characterized. Single-crystal X-ray diffraction allowed determination of the molecular structure as shown in Figure 7. It shows a distorted octahedral geometry where both the formazanate moieties act as tridentate ligands. The Fe–N bond lengths, which average 1.882 Å, are shorter than those reported for an octahedral monoformazanate iron(II) cationic complex (Fe–N average of 1.974 Å), (18) which reflects the relatively poor donor ability of the OMe groups. Nevertheless, the Fe–O bonds in 7 are relatively short (2.1128(18) and 2.1029(19) Å) and in agreement with it having a low-spin ground state. These metrical parameters stand in marked contrast to those reported by Hannedouche for an iron complex with related β-diketiminate ligands, which interact with only one o-OMe group (Fe–O distance = 2.465 Å) to form a five-coordinate complex that has a high-spin ground state based on the metrical data. (19)
Table 2. 57Fe Mössbauer Parameters (δ = Isomer Shift in mm s–1; |ΔEq| = Quadrupole Splitting in mm s–1) for Compounds 1, 3, and 6a
 136a6b6b
δ0.030.050.750.540.55
|ΔEq|2.051.991.212.732.72
a

Measured in the solid state at 80 K.

b

Powder sample of the crude product before crystallization.

Figure 7

Figure 7. Molecular structures of 7 showing 50% probability ellipsoids. One of the N–Ph rings is shown as wireframe, and hydrogen atoms are omitted for clarity.

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Variable-Temperature NMR and UV/Vis Spectroscopy in Solution

We subsequently studied the spin-crossover behavior in solution by monitoring the spectral changes as a function of temperature. The NMR chemical shifts for all compounds are found to be temperature dependent but at low temperature do not follow the Curie behavior that is expected for a paramagnet: instead, the NMR resonances of all compounds except 6 converge into the diamagnetic range of the spectrum, suggestive of population of the S = 0 state. With 1 as reference, the changes induced by the different ligand substituents are discussed below. The enthalpy and entropy differences (ΔH/ΔS) that describe the LS ⇌ HS equilibrium as well as the spin-crossover temperature (T1/2) for the series of compounds are collected in Table 3, and a plot of the high-spin fraction as a function of temperature is shown in Figure 8.

Figure 8

Figure 8. Temperature dependence of the high-spin fraction (γHS) of compounds 1–5 and 7 in toluene-d8, including error bars for T1/2 (γHS = 0.5). The liquid range for toluene is indicated with the color gradient at the temperature axis.

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Table 3. Thermodynamic Parameters for the Equilibrium between S = 0 and S = 2 Spin States in Toluene-d8 Solution for Compounds 1–7
 1a2b34b567b
ΔH (kJ mol–1)22.2 ± 0.38.5 ± 0.426.3 ± 0.112.6 ± 1.019.0 ± 0.4–37.5 ± 1.6
ΔS (J mol–1 K–1)64 ± 145 ± 478 ± 167 ± 570 ± 1–85 ± 5
T1/2c (K)345 ± 7192 ± 18340 ± 2188 ± 21271 ± 8–444 ± 34
a

Data reproduced from ref (14a).

b

Estimated from fitting a limited temperature range.

c

The uncertainty in T1/2 is obtained by using error propagation from ΔH and ΔS.

For compound 2, which has a symmetrical ligand with a highly electron-withdrawing C–Ar3 group, the variable-temperature 1H and 19F NMR spectra in toluene-d8 are indicative of an equilibrium between the high- and low-spin states. Although the former is predominant even at 207 K (the lowest temperature that could be reached inside the NMR probe), and fitting the temperature dependence of the chemical shifts thus is somewhat less accurate, it is clear from the data that the thermodynamic values that describe the spin equilibrium are much decreased in 2 (ΔH = 8.5 ± 0.4 kJ mol–1, ΔS = 45 ± 4 J mol–1 K–1) compared to 1. This can be attributed to the decrease in σ-donor strength of the ligands, which results in a smaller ligand-field splitting and destabilization of the low-spin state.
The introduction of an electron-rich, sterically demanding mesityl ring as an N–Ar group in compound 3 resulted in larger differences between both spin states, with ΔH = 26.3 ± 0.1 kJ mol–1 and ΔS = 78 ± 1 J mol–1 K–1 from fitting the NMR data. The increase in these values stands in contrast to the expected effect of electron-donating substituents at that position, since the N–Ar groups predominantly influence metal–ligand π-bonding. (14b) However, it is clear from the crystallographic data of 3 (vide supra) that the N–Mes rings are engaged in noncovalent interactions (stacking), and we conclude that these attractive forces act to stabilize the more compact low-spin state.
The two effects discussed above were subsequently combined in compound 4. While the crystallographic data indicate that 4 is high spin in the solid state, the solution data clearly indicate that the S = 0 state is populated at low temperature. The combination of two opposing effects on the relative stability of the low-spin state results in thermodynamic parameters for the spin-state equilibrium in 4 (ΔH = 12.6 ± 1.0 kJ mol–1; ΔS = 67 ± 5 J mol–1 K–1) that are intermediate between those of compounds 2 and 3.
Subsequently, we evaluated the influence of a highly electron-withdrawing N–C6F5 substituent that is present in compounds 5 and 6. Changing the N–Ph group in 3 to N–C6F5 in 5 results in a noticeable decrease in ΔH and ΔS to values of 19.0 ± 0.4 kJ mol–1 and 70 ± 1 J mol–1 K–1, respectively. In the absence of structural data for 5, we refrain from a detailed interpretation of these values. It is noted, however, that this result runs counter to the expectation that an electron-withdrawing N–Ar group leads to increased ΔH/ΔS due to stronger metal–ligand π-bonding.
For compound 6, which has an additional C6F5 substituent at the C–Ar3 position, the solution characterization data are indicative of a high-spin ground state also at low temperature, as shown by a magnetic moment of 4.9–5.1 μB across the temperature range studied (217–348 K). The spectral changes in the VT 1H NMR studies provide evidence for a spin equilibrium at the lowest temperatures (a departure from Curie behavior), which may indicate population of either the intermediate-spin (S = 1) state that is observed by crystallography or a low-spin (S = 0) complex similar to that present for the other compounds. However, the population of a different spin state is too small at these temperatures to allow an unambiguous interpretation of the changes that occur.
Finally, the spin-crossover properties of compound 7 were evaluated in solution. At room temperature, the 1H NMR spectrum of 7 shows resonances in the diamagnetic range, and the number of signals is indicative of C2v symmetry. While most peaks are sharp, those corresponding to the o-CH (N–Ph) and the OMe groups appear broadened, suggesting that also 7 may show a temperature-dependent equilibrium between a LS (S = 0) diamagnetic state and a HS (S = 2) paramagnetic state. Indeed, when the temperature was increased, the resonances of 7 broaden substantially and shift away from their diamagnetic values (Figure 9).

Figure 9

Figure 9. 1H NMR spectra of 7 recorded between 247 and 397 K (toluene-d8, 500 MHz).

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The variable-temperature NMR data can be modeled with the equilibrium parameters ΔH = 37.5 ± 1.6 kJ mol–1 and ΔS = 85 ± 5 J mol–1 K–1. The increase in ΔH compared to the other compounds discussed above indicates that there is a substantial additional enthalpic penalty upon changing the spin state from singlet to quintet. A key question surrounding the spin-crossover in 7 is whether or not the Fe···OMe interaction is retained in solution; that is, does it involve a change in the coordination sphere around the Fe center, or does the ligand maintain the same coordination mode in both spin states? Several lines of experimental and computational evidence point toward retention of the tridentate NNO coordination mode of the ligand in both spin states, resulting in an octahedral geometry for 7 throughout. First, although 7 is predominantly low-spin at room temperature, its OMe resonance is somewhat broadened. This is likely because it is in close proximity to the paramagnetic center and is thus noticeably affected, also when the population of high-spin 7 is still very low. In addition, the spin-state equilibrium in 7 is characterized by a value of ΔS (85 ± 5 J mol–1 K–1) that is only marginally larger than that of the others; loss of the Fe···OMe interaction in the high-spin state is expected to lead to a much larger entropy change. Finally, we performed density functional theory calculations on 7 in both spin states, with and without the Fe···OMe interaction (7calc and 7′calc, respectively; see the Supporting Information for details). The results of geometry optimizations with a def2-TZVP basis set (20) using either pure (BP86) (21) or hybrid functionals (TPSSh, (22) B3LYP (23)) all indicate that structures with the Fe···OMe interaction are favored over those in which the OMe group points away from the metal center. The optimized geometries for 7calc in the low-spin state have short Fe–O bonds of 2.14–2.21 Å, which are elongated to 2.43–2.51 Å in the S = 2 minima; the shortest bonds are found for the TPSSh geometries and the longest for BP86. Although, as expected, there are large differences between these functionals for the computed energy differences between the different spin states, (24) it is important to note that the calculations indicate that coordination of the OMe groups is stabilizing in both spin states, regardless of the functional used (ΔGcalc > 23.7 kJ mol–1). Analysis of the frontier molecular orbitals of (low-spin) 7calc shows that the additional interaction with the weak OMe donor groups does not lead to a substantial change in ligand-field strength in comparison to a structure in which the OMe groups are rotated away from the metal center (7′calc; see Figures S54 and S55 for a comparison of the canonical DFT orbitals). In fact, the HOMO–LUMO energy gap at the BP86/def2-TZVP level is somewhat smaller in the structure with the Fe···OMe interaction (LS-7calc: 9227 cm–1) compared to without (LS-7′calc: 10466 cm–1). (25) Analysis of the intrinsic bonding orbitals (26) at the BP86/def2-TZVP geometry shows that while the change from a four- to six-coordinate environment does result in somewhat different localized Fe orbitals, in both cases there clearly is a substantial degree of metal–ligand π-covalency (Figure 10). A similar analysis using the B3LYP and TPSSh functionals at the corresponding minima for 7calc shows that the intrinsic bonding orbitals are qualitatively similar for TPSSh. On the other hand, the B3LYP results show less covalent Fe–N bonds, which is reflected by a lower N-contribution to the relevant IBOs and a smaller Wiberg bond index for the Fe–N bonds (Table S6).

Figure 10

Figure 10. Representation of intrinsic bonding orbitals at the BP86/def2-TZVP minima, both with (7calc; (A)) and without Fe–O interaction (7′calc; (B)).

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To corroborate the NMR data, we subsequently performed variable-temperature UV–vis spectroscopic measurements on all compounds. Dilute solutions in toluene (ca. 10–5 M) were analyzed at temperatures down to 183 K. The fact that we could access lower temperatures in the UV–vis spectrometer was particularly helpful in the analysis of compounds 2 and 4, for which spin-crossover has a relatively low T1/2. The thermodynamic parameters obtained from the fitting of the UV–vis data are congruent with those found from the NMR analysis (see Table S4).
Although the UV–vis spectra of the compounds in this series often are equilibrium mixtures that contain both spin states, the data at the extremes of the temperature range represent predominantly low- or high-spin (at low or high temperature, respectively), and these were taken to extract the absorption maxima of the other spin state by scaled subtraction (see the Supporting Information for details). The only exception is compound 6, the UV–vis spectrum of which does not change appreciably with temperature, and 6 is predominantly found in the high-spin state (Figure 11A). The LS spectra for 2–5 show two intense bands in the visible range with absorption maxima between 375–445 and 515–575 nm which are assigned to ligand-based π–π* transitions. (14) In the HS state, the two bands are bathochromically shifted (around 390–510 and 580–630 nm, respectively), and the lowest energy band shows a significantly lower intensity (Figure S24).

Figure 11

Figure 11. UV/vis spectra in toluene for (A) compound 6 recorded between 183 and 293 K and (B) compound 7 recorded between 293 and 383 K.

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The UV–vis spectrum of compound 7 is distinct from the others as it shows three absorption maxima in the LS state (λ = 459, 608, and 828 nm; see Figure 11B). The lowest-energy transition in 7 is much broader and occurs at significantly lower energy than in the other compounds. Thus, the presence of the additional OMe donor groups in 7 results in an additional low-lying excited state that is a distinguishing feature of this compound.

Conclusions

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In this work, we have extended the series of bis(formazanate)iron complexes to systems featuring nonsymmetric ligands with two different N–Ar substituents. We have demonstrated that the spin-crossover behavior of this class of compounds may be modulated via modification of the ligand using different strategies: electronic effects, steric effects, π-stacking interactions, and ligand denticity. The formazanate ligands reported in this work allowed crystallographic characterization of structures with different coordination geometries and spin states: pseudo-tetrahedral low-spin (3), tetrahedral high-spin (4, 6a), square-planar intermediate-spin (6b), and octahedral low-spin (7). Moreover, 6b is shown to thermally switch in the solid state to 6a, undergoing an incomplete spin-change-coupled square-planar–tetrahedral isomerization, which is rare for iron(II) compounds. The combination of sterics, π-stacking interactions, and electronic effects provides a plethora of tools that can be used to substantially affect spin-crossover behavior in this class of compounds. Overall, we were able to tune the system to obtain solution spin-crossover properties that range from very low T1/2 (∼190 K in 2 and 4) to well above room temperature (444 K in 7). Computational data suggest that the spin-crossover in the six-coordinate bis(formazanate)iron(II) complex (7) is of similar nature to that previously described for the four-coordinate derivatives (14) and originates from a large degree of covalency in the Fe–N bonds due to metal → ligand π-back-donation. Given the relevance of understanding and tuning spin-state-dependent reactivity, we anticipate that this study provides useful insight into ways to fine-tune the spin-state energetics in Fe(II) complexes.

Experimental Section

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General Considerations

All manipulations were performed under nitrogen or argon by using standard glovebox, Schlenk, and vacuum-line techniques. THF (Aldrich, anhydrous, 99.8%) was dried by percolation over columns of Al2O3 (Fluka); toluene, hexane, and pentane (Aldrich, anhydrous, 99.8%) were passed over columns of Al2O3 (Fluka), BASF R3-11-supported Cu oxygen scavenger, and molecular sieves (Aldrich, 4 Å). THF-d8 (Euriso-top) and Tol-d8 (Aldrich) were vacuum transferred from Na/K alloy and stored under nitrogen.
The compounds 2H, (27)3H, (28)4H, (29)5H, (28)6H, (30) and Fe[N(SiMe3)2]2 (31) were synthesized according to the literature procedures. Ligand 7H was prepared according to a slightly adapted version of a literature method (28,32) (see the Supporting Information for a detailed description). Sodium carbonate (Merck), tetrabutylammonium bromide (Sigma-Aldrich, 99%), o-anisidine (Sigma-Aldrich, >99%), hydrochloric acid (Boom B.V., 37–38%), and sodium nitrite (Sigma-Aldrich, 99%) were used as received.
NMR spectra were recorded on a Varian Mercury 400, Inova 500, or Bruker 600 MHz spectrometer. The 1H and 13C NMR spectra were referenced internally by using the residual solvent resonances and reported in ppm relative to TMS (0 ppm). The assignments of NMR resonances were aided by COSY, HMQC, HSQC, and HMBC experiments using standard pulse sequences.
Elemental analyses were performed by the analytical laboratory of the Institute of Inorganic Chemistry at the University of Göttingen using an Elementar Vario EL III instrument.

Synthesis of Fe[PhNNC(C6F5)NNPh]2 (2)

A dark-orange solution of L2H (516.5 mg, 1.32 mmol) in THF (20 mL) was added to a green solution of Fe[N(SiMe3)2]2 (247.9 mg, 0.66 mmol) in THF (10 mL). The reaction mixture was stirred overnight at room temperature, leading to a red-brick-colored solution. The volatiles were removed under a vacuum, and the product was extracted in THF. Slow diffusion of hexane into the THF solution at −30 °C resulted in 303.8 mg of dark-brown powder (0.36 mmol, 55%). 1H NMR (500 MHz, THF-d8, 25 °C): δ 24.61 (4H, Ph m-CH), −9.31 (2H, Ph p-CH), −13.40 (br, 4H, Ph o-CH) ppm. 19F NMR (470 MHz, THF-d8, −65 °C): δ – 111.79 (2F, C6F5, o-CF), −130.81 (1F, C6F5, p-CF), −157.34 ppm (2F, C6F5, m-CF). HMQC-NMR (125 MHz, THF-d8, +25 °C): δ 107.2 (Ph m-CH), 3.98 ppm (Ph p-CH). The signal of Ph o-CH in the HMQC spectrum was not visible due to line broadening.

Synthesis of Fe[PhNNC(p-Tol)NNMes]2 (3)

Fe[N(SiMe3)2]2 (0.71 g 1.88 mmol) was dissolved in THF (15 mL), and a solution of L3H (1.31 g, 3.66 mmol) in THF (15 mL) was added. The reaction mixture was stirred for 2 days at room temperature, leading to a dark-red solution. The solution was filtered, and the volatiles were removed under a vacuum. Recrystallization by slow diffusion of hexane into a THF solution gave 0.62 g of brown powder (0.83 mmol, 44% yield). 1H NMR (600 MHz, THF-d8, 25 °C): δ 12.48 (2H, Ph m-CH), 10.94 (2H, p-Tol m-CH), 10.56 (1H, Mes m-CHA), 9.76 (3H, p-Tol p-CH3), 8.66 (6H, Mes o-CH3), 8.45 (1H, Mes m-CHB), 3.96 (2H, p-Tol o-CH), 0.42 (1H, Ph p-CH), −0.70 (3H, Mes p-CH3), −1.44 ppm (2H, Ph o-CH). 1H NMR (500 MHz, THF-d8, −55 °C): δ 8.06 (2H, p-Tol o-CH), 7.37 (2H, p-Tol m-CH), 7.25 (2H, Ph m-CH), 7.11 (1H, Mes m-CHA), 7.05 (1H, Ph p-CH), 6.65 (1H, Mes m-CHB), 6.47 (2H, Ph o-CH), 2.58 (6H, p-Tol p-CH3 and Mes o-CH3A), 1.69 (3H, Mes o-CH3B), 0.41 ppm (3H, Mes p-CH3). 13C NMR (125 MHz, THF-d8, −55 °C): δ 144.5 (Ph ipso-C), 142.3 (Mes ipso-C), 141.4 (NCN), 137.8 (ipso-C), 135.6 (ipso-C), 134.2 (ipso-C), 133.9 (Mes m-C), 133.7 (Ph p-C), 133.3 (p-tol o-C), 133.2 (Ph o-C), 132.9 (p-tol m-C), 131.9 (Ph m-C), 24.7 (Mes o-CH3), 24.2 (p-tol p-CH3), 22.8 (Mes p-CH3), 22.0 ppm (Mes o-CH3). Anal. Calcd for C46H46N8Fe: C, 72.06; H, 6.05; N, 14,61. Found: C, 72.54; H 5.82; N, 14.12.

Synthesis of Fe[PhNNC(C6F5)NNMes]2 (4)

A dark-orange solution of L4H (478.3 mg, 1.11 mmol) in toluene (40 mL) was added to a green solution of Fe[N(SiMe3)2]2 (190.5 mg, 0.506 mmol) in toluene (10 mL). The reaction mixture was stirred for 2 days at room temperature, leading to a brown solution. The volatiles were removed under a vacuum, and the product was extracted into toluene. Slow diffusion of hexane into the toluene solution resulted in 189.6 mg of dark-brown crystals (0.206 mmol, 41%). 1H NMR (500 MHz, toluene-d8, 25 °C): δ 35.51 (3H), 30.98 (3H), 28.44 (2H Ph m-CH), 21.38 (1H), 12.61 (1H), −8.71 (3H), −19.26 (1H, Ph p-CH), −23.52 (2H Ph o-CH) ppm. 19F NMR (470 MHz, toluene-d8, 25 °C): δ −103.93 (2F, C6F5, o-CF), −125.22 (1F, C6F5, p-CF), −147.38 ppm (2F, C6F5, m-CF).

Synthesis of Fe[C6F5NNC(p-Tol)NNMes]2 (5)

A red-brick-colored solution of L5H (1.41 g, 3.16 mmol) in toluene (40 mL) was added to a green solution of Fe[N(SiMe3)2]2 (0.60 g, 1.58 mmol) in toluene (10 mL). The reaction mixture was stirred overnight at room temperature, leading to a brown solution. The solution was filtered, and the volatiles were removed under a vacuum; the obtained dark solid was quickly washed with cold hexane, giving 1.28 g (1.35 mmol, 85%) of crude product. Any attempt to recrystallize the product was unsuccessful. 1H NMR (400 MHz, C6D6, 25 °C): δ 24.23 (3H), 17.22 (2H), 15.52 (3H), 14.15 (2H), 12.84 (6H, Mes o-CH3), −6.20 ppm (2H). 19F NMR (375 MHz, C6D6, 25 °C): δ −91.63 (1F, p-CF), −163.25 ppm (2F, m-CF). The signal of C6F5o-CF was not visible due to line broadening.

Synthesis of Fe[C6F5NNC(C6F5)NNMes]2 (6)

A dark-orange solution of 6H (1.352 g, 2.587 mmol) in THF (40 mL) was added to a green solution of Fe[N(SiMe3)2]2 (0.489 g 1.299 mmol) in THF (20 mL). The reaction mixture was stirred for 5 h, leading to a brown solution. The volatiles were removed under a vacuum, and a brown solid was collected in 70% yield (1.009 g, 0.919 mmol). The solid was recrystallized from refluxing hexane which afforded crystals of 6a suitable for X-ray diffraction. Alternatively, diffusion of hexane into a THF solution afforded single crystals of 6b. 1H NMR (500 MHz, toluene-d8, 25 °C): δ 20.94 (3H, Mes p-CH3), 14.77 (2H, Mes m-CH), 11.22 ppm (6H, Mes o-CH3). 19F NMR (375 MHz, toluene-d8, 25 °C): δ −93.78 (1F, p-CF), −106.37 (2F, o-CF), −130.67 (1F, p-CF), −154.58 (2F, m-CF), −160.23 ppm (2F, m-CF). The signal of o-CF of N–C6F5 was not visible due to line broadening. Note: NMR spectra are identical for 6a and 6b. Anal. Calcd for C44H22N8F20Fe: C, 48.11; H, 2.02; N, 10,20. Found: C, 48.25; H 1.85; N, 10.04.

Synthesis of Fe[PhNNC(p-Tol)NN(o-An)]2·0.5(THF) (7)

A fuchsia THF (10 mL) solution of L7H (96.4 mg, 0.28 mmol) was added to a green solution of Fe[N(SiMe3)2]2 (52.7 mg, 0.14 mmol) in 5 mL of THF. The reaction mixture was stirred for 3 days at room temperature, leading to a brown solution that was filtered through a 0.2 μm syringe filter, and slow diffusion of hexane into the THF solution afforded 7 as dark needles in 70% yield (76.6 mg, 0.098 mmol). 1H NMR (400 MHz, C6D6, 25 °C): δ = 8.43 (d, J = 7.6 Hz, 1H, o-An δCH), 8.36 (d, J = 7.9 Hz, 2H, p-tolyl o-CH), 7.37 (d, J = 7.7 Hz, 2H, p-tolyl m-CH), 7.08 (t, J = 7.2 Hz, 1H, o-An γCH), 6.84 (d, J = 6.7 Hz, 2H, Ph m-CH), 6.75 (t, J = 7.5 Hz, 1H, o-An βCH), 6.64 (t, J = 7.3 Hz, 1H, Ph p-CH), 6.41 (m, 1H, Ph o-CH), 6.30 (d, J = 7.9 Hz, 1H, o-An αCH), 3.58 (m, 1H, THF),a 2.78 (s, 3H, o-An OCH3), 2.41 (s, 3H, p-tolyl CH3), 1.42 ppm (m, 1H, THF).a13C NMR (151 MHz, C6D6, 25 °C): δ = 169.7 (Ph ipso-C), 152.8 (NCN), 151.6 (o-An ipso-COCH3), 150.4 (o-An ipso-C), 137.0 (p-tolyl ipso-CCH3), 136.4 (p-tolyl ipso-C), 128.8 (p-tolyl m-CH), 127.4 (p-tolyl o-CH), 127.3 (Ph m-CH), 126.6 (Ph p-CH) 124.6 (Ph o-CH), 124.4 (o-An βCH), 123.2 (o-An γCH), 118.6 (o-An δCH), 112.0 (o-An αCH), 67.8 (THF),a 56.5 (o-An OCH3), 25.8 (THF),a 21.0 ppm (p-tolyl CH3). Anal. Calcd for C44H42N8O2.5Fe: C 67.87, H 5.44, N 14.39. Found: C 68.02, H 5.43, N 14.02.

X-ray Crystallography

Single crystals of compounds 3, 4, 6a, 6b, and 7 (directly obtained from the mother liquor) were mounted on top of a cryoloop and transferred into the cold nitrogen stream (100 K; 200 K for 6a) of a Bruker-AXS D8 Venture diffractometer. Data collection and reduction was done by using the Bruker software suite APEX3. (33) The final unit cell was obtained from the xyz centroids of 9772 (3), 9845 (4), 9892 (6a), 9813 (6b), and 9876 (7) reflections after integration. A multiscan absorption correction was applied for compounds 3, 4, 6a, and 6b based on the intensities of symmetry-related reflections measured at different angular settings (SADABS). For compound 7, a numerical absorption correction was applied after indexing of the crystal faces in APEX3. The structures were solved by direct methods using SHELXS, (34) and refinement of the structure was performed by using SHLELXL. (35) From the refinement of 4 it was clear that the hexane solvent molecule was disordered. A two-site disorder model was used to describe this. The site-occupancy factor for the major disorder component refined to 0.72. Several of the atoms in the disordered solvent molecule gave nonpositive definite displacement parameters when refined freely, and ultimately DFIX and ISOR instructions were applied. The structure of 6a was measured at 200 K because the data at 100 K indicated an (incomplete) phase transition (not further investigated). For all structures, the hydrogen atoms were generated by geometrical considerations, constrained to idealized geometries and allowed to ride on their carrier atoms with an isotropic displacement parameter related to the equivalent displacement parameter of their carrier atoms. Crystal data and details on data collection and refinement are presented in Table 4.
Table 4. Crystallographic Data for Compounds 3, 4, 6a, 6b, and 7
 346a6b7
chem formulaC46H46N8FeC50H46F10FeN8C44H22F20FeN8C44H22F20FeN8C46H46FeN8O3
Mr766.761004.801098.541098.54814.76
cryst systtriclinictriclinicmonoclinictriclinicmonoclinic
color, habitred, blockred, blockbrown, blockred, blockgreen, needle
size (mm)0.23 × 0.18 × 0.110.42 × 0.26 × 0.090.30 × 0.17 × 0.150.34 × 0.20 × 0.090.49 × 0.06 × 0.02
space groupP-1P-1P21/cP-1P21/c
a (Å)8.5063(5)12.3467(8)12.7118(6)7.2786(6)12.0477(5)
b (Å)11.3217(7)12.4167(8)20.5795(10)12.5743(10)24.0684(9)
c (Å)21.5733(13)15.4105(10)17.4336(7)12.6798(9)14.6588(5)
α (deg)93.547(2)86.354(2)90118.595(2)90
β (deg)94.372(2)88.205(2)95.991(2)99.168(3)109.140(2)
γ (deg)109.254(2)83.349(2)9094.529(3)90
V (Å3)1947.3(2)2341.2(3)4535.8(4)989.67(13)4015.6(3)
Z22414
ρcalc, g cm–31.3081.4251.6091.8431.348
radiation [Å]Mo Kα 0.71073Mo Kα 0.71073Mo Kα 0.71073Mo Kα 0.71073Cu Kα 1.54178
μ(Mo Kα), mm–10.4320.4070.4580.525 
μ(Cu Kα), mm–1    3.433
F(000)808103621925481712
temp (K)100(2)100(2)200(2)100(2)100(2)
θ range (deg)2.76–27.163.04–27.922.88–26.382.88–27.943.68–65.14
data collected (h, k, l)–10:10; –13:14; –27:27–16:16; –16:16; –20:20–15:15; –25:25; –21:21–9:9; –16:16; –16:16–14:14; –28:28; –17:16
no. of rflns collected6024070899549184768532490
no. of indpndt collected853811211908947556743
observed reflns Fo ≥ 2.0σ(Fo)72389251634645575373
R(F) (%)3.463.364.322.554.61
wR(F2) (%)8.038.059.932.5512.01
GooF1.0451.0401.0307.071.037
weighting a, b0.0267, 1.52600.0311, 1.26770.0324, 3.46910.0372, 0.61250.0552, 2.9707
params refined504687664334527
min, max resid dens–0.427, 0.295–0.280, 0.375–0.260, 0.264–0.451, 0.379–0.346, 0.689

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c03593.

  • Full experimental and characterization data, computational details (PDF)

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CCDC 2036148–2036152 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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Author Information

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  • Corresponding Author
  • Authors
    • Francesca Milocco - Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands
    • Folkert de Vries - Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands
    • Harmke S. Siebe - Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands
    • Silène Engbers - Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands
    • Serhiy Demeshko - Institut für Anorganische Chemie, Universität Göttingen, Tammannstraße 4, 37077 Göttingen, Germany
    • Franc Meyer - Institut für Anorganische Chemie, Universität Göttingen, Tammannstraße 4, 37077 Göttingen, Germany;  Orcidhttp://orcid.org/0000-0002-8613-7862
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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Financial support from The Netherlands Organization for Scientific Research (NWO) is gratefully acknowledged (VIDI grant to E.O.). We thank the Center for Information Technology of the University of Groningen for their support and for providing access to the Peregrine high performance computing cluster as well as Prof. Wesley Browne and Dr. Johannes Klein (University of Groningen) for access to VT-UV/vis spectroscopy. S.D. and F.M. acknowledge support from the Universität Göttingen.

Additional Note

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a Crystals of 7 contain one THF per iron complex, but drying results in loss of part of the THF solvate molecules.

References

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This article references 35 other publications.

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    Molecular stereochemistry of two intermediate-spin complexes. Iron(II) phthalocyanine and manganese(II) phthalocyanine
    Kirner, John F.; Dow, W.; Scheidt, W. Robert
    Inorganic Chemistry (1976), 15 (7), 1685-90CODEN: INOCAJ; ISSN:0020-1669.
    The mol. stereochem. of Fe(II) phthalocyanine (Pc) and Mn(II) phthalocyanine was detd. by x-ray diffraction methods. The phthalocyanine ligand constrains the metal ion to effectively square-planar coordination and to an intermediate spin state. The FeII-N bond distance of 1.926(1) and the MnII-N bond length of 1.938(3) Å are wholly consistent with the assignment of an intermediate-spin ground state. Both complexes crystallize as the β polymorph. Crystal data are: for FePc, space group P21/a, a 19.392(5), b 4.786(2), c 14.604(4) Å, β 120.85(1)°, d.(exptl.) 1.61, d.(calcd.) 1.623, and Z = 2, required mol. symmetry ‾1; for MnPc, space group P21/a, a 19.400(4), b 4.761(2), c 14.613(3) Å, β 120.74(1)°, d.(exptl.) 1.61, d.(calcd.) 1.625, and Z = 2, required mol. symmetry ‾1. Final discrepancy indices are: FePc, R1 0.045, R2 0.057; MnPc, R1 0.066, R2 0.066.
    (c) Strauss, S. H.; Silver, M. E.; Ibers, J. A. Iron(II) octaethylchlorine: structure and ligand affinity comparison with its porphyrin and isobacteriochlorin homologs. J. Am. Chem. Soc. 1983, 105, 4108– 4109,  DOI: 10.1021/ja00350a069
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    4c
    Iron(II) octaethylchlorine: structure and ligand affinity comparison with its porphyrin and isobacteriochlorin homologs
    Strauss, Steven H.; Silver, Michael E.; Ibers, James A.
    Journal of the American Chemical Society (1983), 105 (12), 4108-9CODEN: JACSAT; ISSN:0002-7863.
    trans-7,8-Dihydro-2,3,7,8,12,13,17,18-octaethylporphyrinatoiron(II), Fe(OEC), is orthorhombic, space group Pbcn, with a 21.880(9), b 15.795(6), and c 8.554(4) Å at -150°; Z = 4. The structure was refined anisotropically by least-squares to a final R(F2) = 0.105. The Fe and 4 N atoms are rigorously planar, while the rest of the chlorin macrocycle is significantly S4 ruffled. The affinity of Fe(OEC) and its octaethylporphyrin and octaethylisobacteriochlorin homologs for the weak σ-donor ligands THF and EtSH is strongly macrocycle dependent. This dependence may have a structural basis whereby reduced macrocycles more easily distort, adjusting their core size to accommodate the changing requirements of the metal upon complexation of a weak ligand.
  5. 5
    Chatt, J.; Shaw, B. L. Alkyls and aryls of transition metals. Part IV. Cobalt(II) and iron(II) derivatives. J. Chem. Soc. 1961, 285,  DOI: 10.1039/jr9610000285
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    Alkyls and aryls of transition metals. IV. Cobalt(II) and iron(II) derivatives
    Chatt, J.; Shaw, B. L.
    Journal of the Chemical Society (1961), (), 285-90CODEN: JCSOA9; ISSN:0368-1769.
    cf. CA 54, 20936d. The prepn. and properties of stable organometallic complexes of the types trans-[MR2-(PR2')2] (M = Co, Fe; R, R' = org. radicals) were described. The groups R were ortho-substituted aryl groups, where the substituents were somewhat bulky. These were the first planar complexes of Co(II) and Fe(II) that had only monodentate ligands. Their configurations were established by their magnetic and elec. dipole moments. Possible reasons for the unusual configurations and stabilities of these organometallic derivs. were discussed.
  6. 6
    (a) Nijhuis, C. A.; Jellema, E.; Sciarone, T. J. J.; Meetsma, A.; Budzelaar, P. H. M.; Hessen, B. First-Row Transition Metal Bis(amidinate) Complexes; Planar Four-Coordination of FeII Enforced by Sterically Demanding Aryl Substituents Eur. Eur. J. Inorg. Chem. 2005, 2005, 2089– 2099,  DOI: 10.1002/ejic.200500094
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    (b) Wurzenberger, X.; Piotrowski, H.; Klufers, P. A stable molecular entity derived from rare iron(II) minerals: the square-planar high-spin d6 Fe(II)O4 chromophore Angew. Angew. Chem., Int. Ed. 2011, 50, 4974– 4978,  DOI: 10.1002/anie.201006898
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    6b
    A Stable Molecular Entity Derived from Rare Iron(II) Minerals: The Square-Planar High-Spin-d6 FeIIO4 Chromophore
    Wurzenberger, Xaver; Piotrowski, Holger; Kluefers, Peter
    Angewandte Chemie, International Edition (2011), 50 (21), 4974-4978, S4974/1-S4974/4CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A strongly alk. (LiOH or NaOH) aq. soln. of furanose mimic anhydroerythritol (meso-oxolane-3,4-diol, H2L) and FeCl2 gave a reddish-blue soln., from which, depending on the conditions, orange-red crystals of the tetrahydrates Li2[FeL2]·4H2O (1) or Na2[FeL2]·4H2O (2), or violet crystals of the nonahydrate Na2[FeL2]·9H2O (3) were grown. The bidentate oxolanediolato ligand stabilizes the rare high-spin ferrous SP-4 coordination in 1 as revealed by x-ray crystallog. and SQUID magnetometer measurements. The structure of 2 is also SP-4 as revealed by x-ray crystallog. Compd. 3 is slightly distorted toward tetrahedron. Three related mononuclear high-spin [FeIIX4]2- (X = Cl, F, OH) species and their high-spin [MnIIX4]2- analogs were analyzed in a DFT approach using the unrestricted-B3LYP/tzvp level of theory. Enhanced Jahn-Teller flattening can stabilize square planar high-spin d6 centers despite their "wrong" spin state by a marked sepn. of the neg. charge on the ligand atoms and the stereochem. active d electrons. This result reveals square-planar high-spin centers to be building blocks of reasonable stability.
    (c) Cantalupo, S. A.; Fiedler, S. R.; Shores, M. P.; Rheingold, A. L.; Doerrer, L. H. High-spin square-planar Co(II) and Fe(II) complexes and reasons for their electronic structure. Angew. Chem., Int. Ed. 2012, 51, 1000– 1005,  DOI: 10.1002/anie.201106091
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    6c
    High-Spin Square-Planar CoII and FeII Complexes and Reasons for Their Electronic Structure
    Cantalupo, Stefanie A.; Fiedler, Stephanie R.; Shores, Matthew P.; Rheingold, Arnold L.; Doerrer, Linda H.
    Angewandte Chemie, International Edition (2012), 51 (4), 1000-1005, S1000/1-S1000/18CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The high-spin, square planar complexes [K(DME)2]2[M(ddfp)2] (M = CoII, FeII, ddfp = perfluoropinacolate) were prepd. and characterized by x-ray crystallog., temp.-dependent magnetic susceptibility, cyclic voltammetry, and other methods. The tetrahedral analog [K(DME)2]2[Zn(ddfp)2] was also prepd. and characterized crystallog. The combination of high-spin electron configuration and square-planar geometry is made possible by ligand constraints that generate five non-degenerate d-orbitals with ligand-based π-donation, a relatively weak ligand-field splitting, and no intervening ligand-based π-acceptor mol. orbitals.
    (d) Pinkert, D.; Demeshko, S.; Schax, F.; Braun, B.; Meyer, F.; Limberg, C. A Dinuclear Molecular Iron(II) Silicate with Two High-Spin Square-Planar FeO4 Units. Angew. Chem., Int. Ed. 2013, 52, 5155– 5158,  DOI: 10.1002/anie.201209650
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    6d
    A Dinuclear Molecular Iron(II) Silicate with Two High-Spin Square-Planar FeO4 Units
    Pinkert, Denise; Demeshko, Serhiy; Schax, Fabian; Braun, Beatrice; Meyer, Franc; Limberg, Christian
    Angewandte Chemie, International Edition (2013), 52 (19), 5155-5158CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Dinuclear iron complex [L'2Fe2][Na(OEt2)2]2 (1) (H3L' = ligand with elimination of "Me2SiO" from 3-(1,1-dimethylethyl)-3-[(hydroxydimethylsilyl)oxy]-1,1,5,5-tetramethyl-1,5-trisiloxanediol) was synthesized and studied by x-ray diffraction anal., temp.-dependent magnetic susceptibility and Mossbauer spectroscopy.
    (e) Liu, Y.; Luo, L.; Xiao, J.; Wang, L.; Song, Y.; Qu, J.; Luo, Y.; Deng, L. Four-Coordinate Iron(II) Diaryl Compounds with Monodentate N-Heterocyclic Carbene Ligation: Synthesis, Characterization, and Their Tetrahedral-Square Planar Isomerization in Solution. Inorg. Chem. 2015, 54, 4752– 4760,  DOI: 10.1021/acs.inorgchem.5b00138
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    6e
    Four-Coordinate Iron(II) Diaryl Compounds with Monodentate N-Heterocyclic Carbene Ligation: Synthesis, Characterization, and Their Tetrahedral-Square Planar Isomerization in Solution
    Liu, Yuesheng; Luo, Lun; Xiao, Jie; Wang, Lei; Song, You; Qu, Jingping; Luo, Yi; Deng, Liang
    Inorganic Chemistry (2015), 54 (10), 4752-4760CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    The salt elimination reactions of (IPr2Me2)2FeCl2 (IPr2Me2 = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) with the corresponding aryl Grignard reagents afford [(IPr2Me2)2FeAr2] (Ar = Ph, 3; C6H4-p-Me, 4; C6H4-p-tBu, 5; C6H3-3,5-(CF3)2, 6) in good yields. X-ray crystallog. studies revealed the presence of both tetrahedral and trans square planar isomers for 3 and 6 and the tetrahedral structures for 4 and 5. Magnetic susceptibility and 57Fe Mossbauer spectrum measurements on the solid samples indicated the high-spin (S = 2) and intermediate-spin (S = 1) nature of the tetrahedral and square planar structures, resp. Soln. property studies, including soln. magnetic susceptibility measurement, variable-temp. 1H and 19F NMR, and absorption spectroscopy, on 3-6, as well as an 57Fe Mossbauer spectrum study on a frozen THF soln. of tetrahedral [(IPr2Me2)257FePh2] suggest the coexistence of tetrahedral and trans square planar structures in soln. phase. D. functional theory calcns. on (IPr2Me2)2FePh2 disclosed that the tetrahedral and trans square planar isomers are close in energy and that the geometry isomerization can occur by spin-change-coupled geometric transformation on four-coordinate iron(II) center.
  7. 7
    (a) Holm, R. H.; Chakravorty, A.; Theriot, L. J. The Synthesis, Structures, and Solution Equilibria of Bis(pyrrole-2-aldimino)metal(II) Complexes. Inorg. Chem. 1966, 5, 625– 635,  DOI: 10.1021/ic50038a028
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    7a
    The synthesis, structures, and solution equilibria of bis-(pyrrole-2-aldimino)metal(II) complexes
    Holm, R. H.; Chakravorty, A.; Theriot, L. J.
    Inorganic Chemistry (1966), 5 (4), 625-35CODEN: INOCAJ; ISSN:0020-1669.
    Synthesis of an extensive series of bis(pyrrole-2-aldimino)metal(II) complexes with M(II) = Co, Ni, Pd, Cu, and Zn and various alkyl groups (R) appended to the azomethine N was effected by a nonaq. chelation reaction in tetrahydrofuran. Preliminary single crystal x-ray results for complexes with R = tert-Bu reveal that Co, Ni, and Zn complexes are isomorphous, but appreciable differences in the cell consts. of the Ni complex indicate that it is not truly isostructural with the tetrahedral Co and Zn complexes. The Cu complex exists in 2 cryst. modifications, neither of which is isomorphous with the Co-Ni-Zn series. Spectral and magnetic studies in soln. show that the tert-Bu Co complex is tetrahedral whereas the corresponding Cu complex is distorted from planarity to an unknown extent. Cu complexes with less bulky R groups are planar. The tert-Bu Ni complex is pseudo-tetrahedral; complexes with sec-alkyl groups such as iso-Pr are involved in a configurational equil. between planar and pseudo-tetrahedral forms. The paramagnetic Ni complexes show large isotropic proton hyperfine contact shifts. Spin d. calcns. for the coordinated ligand system are used as the basis of proton resonance assignments. It is concluded that in the pseudo-tetrahedral form spin imbalance exists in the highest filled ligand π-mol. orbital, and that, in addn., there is an underlying spin imbalance in the highest filled σ-mol. orbital, the result of which is observable in the proton resonance spectra. Thermodynamic parameters characterizing the structural change were obtained for the Ni complexes from the temp. dependence of the proton contact shifts. A quant. comparison of the stabilization of tetrahedral Ni(II) by pyrrole-2-aldimine, salicylaldimine, and β-keto amine ligand systems is presented.
    (b) Wolny, J. A.; Rudolf, M. F.; Ciunik, Z.; Gatner, K.; Wołowiec, S. Cobalt(II) triazene 1-oxide bis(chelates). A case of planar (low spin)–tetrahedral (high spin) isomerism. J. Chem. Soc., Dalton Trans. 1993, 1611– 1622,  DOI: 10.1039/DT9930001611
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    7b
    Cobalt(II) triazene 1-oxide bis(chelates). A case of planar (low spin)-tetrahedral (high spin) isomerism
    Wolny, Juliusz A.; Rudolf, Mikolaj F.; Ciunik, Zbigniew; Gatner, Kazimierz; Wolowiec, Stanislaw
    Journal of the Chemical Society, Dalton Transactions: Inorganic Chemistry (1972-1999) (1993), (10), 1611-22CODEN: JCDTBI; ISSN:0300-9246.
    High- and low-spin [CoL2] (HL = 3-phenyl-1-triazene 1-oxide derivs.) were prepd. and isolated. The low-spin complexes possess square-planar structure and the high-spin complexes are tetrahedral. The mol. structure of high-spin [CoL2] [HL = 3-(4-methylphenyl)-1-methyl-1-triazene 1-oxide] was detd.; triclinic, space group P‾1, a 7.970(5), b 10.174(5), c 11.676(5) Å, α 87.18(4), β 74.31(4), γ 74.06(4)°, Z = 2, R = 0.041. For some complexes the isolation of both planar (low-spin) and tetrahedral (high-spin) isomers or of their conglomerates is possible depending on the synthesis conditions. The crystal structure of square-planar [Ni(ON(Me)NNC6H4Me-4)2], which is isomorphous with the low-spin isomer of [Co(ON(Me)NNC6H4Me-4)2], was detd.: triclinic, space group P‾1, a 7.495(2), b 7.694(5), c 8.612(3) Å, α 64.64(5), β 87.84(2), γ 78.64(3)°, Z = 1, R = 0.0271. The complexes exhibit a planar-tetrahedral equil. in noncoordinating solvents, the ΔHΘ and ΔSΘ values of which, detd. from soln. magnetic susceptibility measurements, at 1-15 kJ mol-1 and 5-30 J K-1 mol-1, resp. The electrochem. properties of the complexes are given.
    (c) Ingleson, M. J.; Pink, M.; Fan, H.; Caulton, K. G. Exploring the reactivity of four-coordinate PNPCoX with access to three-coordinate spin triplet PNPCo. Inorg. Chem. 2007, 46, 10321– 10334,  DOI: 10.1021/ic701171p
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    7c
    Exploring the reactivity of four-coordinate PNPCoX with access to three-coordinate spin triplet PNPCo
    Ingleson, Michael J.; Pink, Maren; Fan, Hongjun; Caulton, Kenneth G.
    Inorganic Chemistry (2007), 46 (24), 10321-10334CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    (PNP)CoX, where PNP is (tBu2PCH2SiMe2)2N- and X is Cl, I, N3, OAr, OSO2CF3, and N(H)Ar, are reported. Some of these show magnetic susceptibility, color, and 1H NMR evidence of being in equil. between a blue, tetrahedral S = 3/2 state and a red, planar S = 1/2 state; the equil. populations are influenced by subtle solvent effects (e.g., benzene and cyclohexane are different), as well as by temp. Attempted oxidn. to Co(III) with O2 occurs instead at phosphorus, giving [P(O)NP(O)]CoX species. The single O-atom transfer reagent PhI:O likewise oxidizes P. Even I2 oxidizes P to give the pendant phosphonium species (tBu2P(I)CH2SiMe2NSiMe2CH2PtBu2)CoI2 with a tetrahedral S = 3/2 cobalt; the solid-state structure shows intermol. PI···ICo interactions. Attempted alkyl metathesis of PNPCoX inevitably results in redn., forming PNPCo, which is a spin triplet with planar T-shaped coordination geometry with no agostic interaction. Triplet PNPCo binds N2(weakly) and CO (whose low CO stretching frequency indicates strong PNP → Co donor power), but not ethene or MeCCMe.
  8. 8
    Gaazo, J. Plasticity of the coordination sphere of copper(II) complexes, its manifestation and causes. Coord. Chem. Rev. 1976, 19, 253– 297,  DOI: 10.1016/S0010-8545(00)80317-3
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    (a) Cambi, L.; Szegö, L. Über die magnetische Susceptibilität der komplexen Verbindungen. Ber. Dtsch. Chem. Ges. B 1931, 64, 2591– 2598,  DOI: 10.1002/cber.19310641002
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    (b) Kahn, O.; Martinez, C. J. Spin-Transition Polymers: From Molecular Materials Toward Memory Devices. Science 1998, 279, 44– 48,  DOI: 10.1126/science.279.5347.44
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    Spin-transition polymers: from molecular materials toward memory devices
    Kahn, O.; Martinez, C. Jay
    Science (Washington, D. C.) (1998), 279 (5347), 44-48CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    A review with 53 refs. Some 3dn (4 ≤ n ≤ 7) transition metal compds. exhibit a cooperative transition between a low-spin (LS) and a high-spin (HS) state. This transition is abrupt and occurs with a thermal hysteresis, which confers a memory effect on the system. The intersite interactions and thus the cooperativity are magnified in polymeric compds. such as [Fe(Rtrz)3]A2·nH2O in which the Fe2+ ions are triply bridged by 4-R-substituted-1,2,4-triazole mols. Moreover, in these compds., the spin transition is accompanied by a well-pronounced change of color between violet in the LS state and white in the HS state. The transition temps. of these materials can be fine tuned, using an approach based on the concept of a mol. alloy. In particular, it is possible to design a compd. for which room temp. falls in the middle of the thermal hysteresis loop. These materials have many potential applications, for example, as temp. sensors, as active elements of various types of displays, and in information storage and retrieval.
    (c) Gütlich, P.; Garcia, Y.; Goodwin, H. A. Spin crossover phenomena in Fe(ii) complexes. Chem. Soc. Rev. 2000, 29, 419– 427,  DOI: 10.1039/b003504l
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    Spin crossover phenomena in Fe(II) complexes
    Gutlich, Philipp; Garcia, Yann; Goodwin, Harold A.
    Chemical Society Reviews (2000), 29 (6), 419-427CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
    A review with 58 refs. The behavior of spin crossover compds. is among the most striking and fascinating shown by relatively simple mol. species. This review aims to draw attention to the various ways in which spin crossover phenomena are manifested in Fe(II) complexes, to offer some rationalization for these, and to highlight their possible applications. Typical examples have been selected along with more recent ones to give an overall view of the scope and development of the area. The article is structured to provide the basic material for those who wish to enter the field of spin crossover.
    (d) Sato, O.; Tao, J.; Zhang, Y. Z. Control of magnetic properties through external stimuli. Angew. Chem., Int. Ed. 2007, 46, 2152– 2187,  DOI: 10.1002/anie.200602205
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    9d
    Control of magnetic properties through external stimuli
    Sato, Osamu; Tao, Jun; Zhang, Yuan-Zhu
    Angewandte Chemie, International Edition (2007), 46 (13), 2152-2187CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A review. The magnetic properties of many magnetic materials can be controlled by external stimuli. The principal focus here is on the thermal, photochem., electrochem., and chem. of phase transitions that involve changes in magnetization. Mol. compds. described herein range from metal complexes through pure org. compds. to composite materials. Most of the review is devoted to the properties. valence-tautomeric compds., mol. magnets, and spin-crossover complexes, which could find future applications in memory devices or optical switches.
    (e) Bousseksou, A.; Molnar, G.; Salmon, L.; Nicolazzi, W. Molecular spin crossover phenomenon: recent achievements and prospects. Chem. Soc. Rev. 2011, 40, 3313– 3335,  DOI: 10.1039/c1cs15042a
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    9e
    Molecular spin crossover phenomenon: Recent achievements and prospects
    Bousseksou, Azzedine; Molnar, Gabor; Salmon, Lionel; Nicolazzi, William
    Chemical Society Reviews (2011), 40 (6), 3313-3335CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
    A review. Recently we assisted a strong renewed interest in the fascinating field of mol. spin crossover complexes by (1) the emergence of nanosized spin crossover materials through direct synthesis of coordination nanoparticles and nanopatterned thin films as well as by (2) the use of novel sophisticated high spatial and temporal resoln. exptl. techniques and theor. approaches for the study of spatiotemporal phenomena in cooperative spin crossover systems. Besides generating new fundamental knowledge on size-redn. effects and the dynamics of the spin crossover phenomenon, this research aims also at the development of practical applications such as sensor, display, information storage and nanophotonic devices. In this crit. review, we discuss recent work in the field of mol.-based spin crossover materials with a special focus on these emerging issues, including chem. synthesis, phys. properties and theor. aspects as well (223 refs.).
  10. 10
    Halcrow, M. A. The spin-states and spin-transitions of mononuclear iron(II) complexes of nitrogen-donor ligands. Polyhedron 2007, 26, 3523– 3576,  DOI: 10.1016/j.poly.2007.03.033
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    10
    The spin-states and spin-transitions of mononuclear iron(II) complexes of nitrogen-donor ligands
    Halcrow, Malcolm A.
    Polyhedron (2007), 26 (14), 3523-3576CODEN: PLYHDE; ISSN:0277-5387. (Elsevier B.V.)
    A review of mononuclear iron(II) complexes with heterocyclic N-donor ligation is presented. A brief introduction to spin-crossover chem. and low-temp. spin-trapping is provided, since many of these compds. undergo thermal spin-transitions upon cooling or heating. These are highlighted, and the structural changes underlying spin-crossover are discussed where this is known. Materials showing spin-trapping behavior following thermal quenching or irradn. at very low temps. are also described.
  11. 11
    Bowman, A. C.; Milsmann, C.; Bill, E.; Turner, Z. R.; Lobkovsky, E.; DeBeer, S.; Wieghardt, K.; Chirik, P. J. Synthesis and Electronic Structure Determination of N-Alkyl-Substituted Bis(imino)pyridine Iron Imides Exhibiting Spin Crossover Behavior. J. Am. Chem. Soc. 2011, 133, 17353– 17369,  DOI: 10.1021/ja205736m
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    11
    Synthesis and Electronic Structure Determination of N-Alkyl-Substituted Bis(imino)pyridine Iron Imides Exhibiting Spin Crossover Behavior
    Bowman, Amanda C.; Milsmann, Carsten; Bill, Eckhard; Turner, Zoe R.; Lobkovsky, Emil; DeBeer, Serena; Wieghardt, Karl; Chirik, Paul J.
    Journal of the American Chemical Society (2011), 133 (43), 17353-17369CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Three new N-alkyl substituted bis(imino)pyridine iron imide complexes, (iPrPDI)FeNR (iPrPDI = 2,6-(2,6-iPr2-C6H3-N:CMe)2C5H3N; R = 1-adamantyl (1Ad), cyclooctyl (CyOct), and 2-adamantyl (2Ad)) were synthesized by addn. of the appropriate alkyl azide to the iron bis(dinitrogen) complex, (iPrPDI)Fe(N2)2. SQUID magnetic measurements on the isomeric iron imides, (iPrPDI)FeN1Ad and (iPrPDI)FeN2Ad, established spin crossover behavior with the latter example having a more complete spin transition in the exptl. accessible temp. range. X-ray diffraction on all three alkyl-substituted bis(imino)pyridine iron imides established essentially planar compds. with relatively short Fe-Nimide bond lengths and two-electron redn. of the redox-active bis(imino)pyridine chelate. Zero- and applied-field Mossbauer spectroscopic measurements indicate diamagnetic ground states at cryogenic temps. and established low isomer shifts consistent with highly covalent mols. For (iPrPDI)FeN2Ad, Mossbauer spectroscopy also supports spin crossover behavior and allowed extn. of thermodn. parameters for the S = 0 to S = 1 transition. X-ray absorption spectroscopy and computational studies were also performed to explore the electronic structure of the bis(imino)pyridine alkyl-substituted imides. An electronic structure description with a low spin ferric center (S = 1/2) antiferromagnetically coupled to an imidyl radical (Simide = 1/2) and a closed-shell, dianionic bis(imino)pyridine chelate (SPDI = 0) is favored for the S = 0 state. An iron-centered spin transition to an intermediate spin ferric ion (SFe = 3/2) accounts for the S = 1 state obsd. at higher temps. Other possibilities based on the computational and exptl. data are also evaluated and compared to the electronic structure of the bis(imino)pyridine iron N-aryl imide counterparts.
  12. 12
    (a) Scepaniak, J. J.; Harris, T. D.; Vogel, C. S.; Sutter, J.; Meyer, K.; Smith, J. M. Spin Crossover in a Four-Coordinate Iron(II) Complex. J. Am. Chem. Soc. 2011, 133, 3824– 3827,  DOI: 10.1021/ja2003473
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    12a
    Spin Crossover in a Four-Coordinate Iron(II) Complex
    Scepaniak, Jeremiah J.; Harris, T. David; Vogel, Carola S.; Sutter, Jorg; Meyer, Karsten; Smith, Jeremy M.
    Journal of the American Chemical Society (2011), 133 (11), 3824-3827CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The four-coordinate iron(II) phosphoraniminato complex PhB(MesIm)3Fe-N=PPh3 undergoes an S = 0 to S = 2 spin transition with TC = 81 K, as detd. by variable-temp. magnetic measurements and Mossbauer spectroscopy. Variable-temp. single-crystal X-ray diffraction revealed that the S = 0 to S = 2 transition is assocd. with an increase in the Fe-C and Fe-N bond distances and a decrease in the N-P bond distance. These structural changes have been interpreted in terms of electronic structure theory.
    (b) Mathoniere, C.; Lin, H. J.; Siretanu, D.; Clerac, R.; Smith, J. M. Photoinduced single-molecule magnet properties in a four-coordinate iron(II) spin crossover complex. J. Am. Chem. Soc. 2013, 135, 19083– 19086,  DOI: 10.1021/ja410643s
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    12b
    Photoinduced Single-Molecule Magnet Properties in a Four-Coordinate Iron(II) Spin Crossover Complex
    Mathoniere, Corine; Lin, Hsiu-Jung; Siretanu, Diana; Clerac, Rodolphe; Smith, Jeremy M.
    Journal of the American Chemical Society (2013), 135 (51), 19083-19086CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The four-coordinate Fe-(II) complex, PhB(MesIm)3FeNPPh3 (1) was previously reported to undergo a thermal spin-crossover (SCO) between high-spin (HS, S = 2) and low-spin (LS, S = 0) states. This complex is photoactive <20 K, undergoing a photoinduced LS to HS spin state change, as detd. by optical reflectivity and photomagnetic measurements. With continuous white light irradn., 1 displays slow relaxation of the magnetization, i.e. single-mol. magnet (SMM) properties, at temps. <5 K. This complex provides a structural template for the design of new photoinduced mononuclear SMMs based on the SCO phenomenon.
    (c) Lin, H. J.; Siretanu, D.; Dickie, D. A.; Subedi, D.; Scepaniak, J. J.; Mitcov, D.; Clerac, R.; Smith, J. M. Steric and electronic control of the spin state in three-fold symmetric, four-coordinate iron(II) complexes. J. Am. Chem. Soc. 2014, 136, 13326– 13332,  DOI: 10.1021/ja506425a
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    12c
    Steric and Electronic Control of the Spin State in Three-Fold Symmetric, Four-Coordinate Iron(II) Complexes
    Lin, Hsiu-Jung; Siretanu, Diana; Dickie, Diane A.; Subedi, Deepak; Scepaniak, Jeremiah J.; Mitcov, Dmitri; Clerac, Rodolphe; Smith, Jeremy M.
    Journal of the American Chemical Society (2014), 136 (38), 13326-13332CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The 3-fold sym., four-coordinate Fe(II) phosphoraminimato complexes PhB(MesIm)3Fe-N=PRR'R'' (PRR'R'' = PMePh2, PMe2Ph, PMe3, and PPr3) undergo a thermally induced S = 0 to S = 2 spin-crossover in fluid soln. Smaller phosphoraminimato ligands stabilize the low-spin state, and an excellent correlation is obsd. between the characteristic temp. of the spin-crossover (T1/2) and the Tolman cone angle (θ). Complexes with para-substituted triarylphosphoraminimato ligands (p-XC6H4)3P=N- (X = H, Me and OMe) also undergo spin-crossover in soln. These isosteric phosphoraminimato ligands reveal that the low-spin state is stabilized by more strongly donating ligands. This control over the spin state provides important insights for modulating the magnetic properties of four-coordinate Fe(II) complexes.
  13. 13
    Creutz, S. E.; Peters, J. C. Spin-State Tuning at Pseudo-tetrahedral d6 Ions: Spin Crossover in [BP3]FeII–X Complexes. Inorg. Chem. 2016, 55, 3894– 3906,  DOI: 10.1021/acs.inorgchem.6b00066
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    13
    Spin-State Tuning at Pseudo-tetrahedral d6 Ions: Spin Crossover in [BP3]FeII-X Complexes
    Creutz, Sidney E.; Peters, Jonas C.
    Inorganic Chemistry (2016), 55 (8), 3894-3906CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    Low-coordinate transition-metal complexes that undergo spin crossover remain rare. We report here a series of four-coordinate, pseudo-tetrahedral P3FeII-X complexes supported by tris(phosphine)borate P3 ([PhBPR3]-) and phosphiniminato X-type ligands (-N=PR'3) that, in combination, tune the spin-crossover behavior of the system. Most of the reported iron complexes undergo spin crossover at temps. near or above room temp. in soln. and in the solid state. The change in spin state coincides with a significant change in the degree of π-bonding between Fe and the bound N atom of the phosphiniminato ligand. Spin crossover is accompanied by striking changes in the UV-visible (UV-vis) absorption spectra, which allows for quant. modeling of the thermodn. parameters of the spin equil. These spin equil. have also been studied by numerous techniques including paramagnetic NMR (NMR), IR, and M.ovrddot.ossbauer spectroscopies; X-ray crystallog.; and solid-state superconducting quantum interference device (SQUID) magnetometry. These studies allow qual. correlations to be made between the steric and electronic properties of the ligand substituents and the enthalpy and entropy changes assocd. with the spin equil.
  14. 14
    (a) Travieso-Puente, R.; Broekman, J. O. P.; Chang, M.-C.; Demeshko, S.; Meyer, F.; Otten, E. Spin-Crossover in a Pseudo-tetrahedral Bis(formazanate) Iron Complex. J. Am. Chem. Soc. 2016, 138, 5503– 5506,  DOI: 10.1021/jacs.6b01552
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    14a
    Spin-Crossover in a Pseudo-tetrahedral Bis(formazanate) Iron Complex
    Travieso-Puente, Raquel; Broekman, J. O. P.; Chang, Mu-Chieh; Demeshko, Serhiy; Meyer, Franc; Otten, Edwin
    Journal of the American Chemical Society (2016), 138 (17), 5503-5506CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Spin-crossover in a pseudo-tetrahedral bis(formazanate) iron(II) complex (1) is described. Structural, magnetic, and spectroscopic analyses indicate that this compd. undergoes thermal switching between an S = 0 and an S = 2 state, which is very rare in four-coordinate complexes. The transition to the high-spin state is accompanied by an increase in Fe-N bond lengths and a concomitant contraction of intraligand N-N bonds. The latter suggests that stabilization of the low-spin state is due to the π-acceptor properties of the ligand. One-electron redn. of 1 leads to the formation of the corresponding anion, which contains a low-spin (S = 1/2) Fe(I) center. The findings are rationalized by electronic structure calcns. using d. functional theory.
    (b) Milocco, F.; de Vries, F.; Bartels, I. M. A.; Havenith, R. W. A.; Cirera, J.; Demeshko, S.; Meyer, F.; Otten, E. Electronic Control of Spin-Crossover Properties in Four-Coordinate Bis(formazanate) Iron(II) Complexes. J. Am. Chem. Soc. 2020, 142, 20170– 20181,  DOI: 10.1021/jacs.0c10010
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    14b
    Electronic Control of Spin-Crossover Properties in Four-Coordinate Bis(formazanate) Iron(II) Complexes
    Milocco, Francesca; de Vries, Folkert; Bartels, Imke M. A.; Havenith, Remco W. A.; Cirera, Jordi; Demeshko, Serhiy; Meyer, Franc; Otten, Edwin
    Journal of the American Chemical Society (2020), 142 (47), 20170-20181CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The transition between spin states in d-block metal complexes has important ramifications for their structure and reactivity, with applications ranging from information storage materials to understanding catalytic activity of metalloenzymes. Tuning the ligand field (ΔO) by steric and/or electronic effects has provided spin-crossover compds. for several transition metals in the periodic table, but this has mostly been limited to coordinatively satd. metal centers in octahedral ligand environments. Spin-crossover complexes with low coordination nos. are much rarer. Here we report a series of four-coordinate, (pseudo)tetrahedral Fe(II) complexes with formazanate ligands and demonstrate how electronic substituent effects can be used to modulate the thermally induced transition between S = 0 and S = 2 spin states in soln. All six compds. undergo spin-crossover in soln. with T1/2 above room temp. (300-368 K). While structural anal. by X-ray crystallog. shows that the majority of these compds. are low-spin in the solid state (and remain unchanged upon heating), we find that packing effects can override this preference and give rise to either rigorously high-spin (6) or gradual spin-crossover behavior (5) also in the solid state. D. functional theory calcns. are used to delineate the empirical trends in soln. spin-crossover thermodn. In all cases, the stabilization of the low-spin state is due to the π-acceptor properties of the formazanate ligand, resulting in an "inverted" ligand field, with an approx. "two-over-three" splitting of the d-orbitals and a high degree of metal-ligand covalency due to metal → ligand π-backdonation. The computational data indicate that the electronic nature of the para-substituent has a different influence depending on whether it is present at the C-Ar or N-Ar rings, which is ascribed to the opposing effect on metal-ligand σ- and π-bonding.
  15. 15
    (a) Feltham, H. L. C.; Barltrop, A. S.; Brooker, S. Spin crossover in iron(II) complexes of 3,4,5-tri-substituted-1,2,4-triazole (Rdpt), 3,5-di-substituted-1,2,4-triazolate (dpt – ), and related ligands. Coord. Chem. Rev. 2017, 344, 26– 53,  DOI: 10.1016/j.ccr.2016.10.006
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    15a
    Spin crossover in iron(II) complexes of 3,4,5-tri-substituted-1,2,4-triazole (Rdpt), 3,5-di-substituted-1,2,4-triazolate (dpt-), and related ligands
    Feltham, Humphrey L. C.; Barltrop, Alexis S.; Brooker, Sally
    Coordination Chemistry Reviews (2017), 344 (), 26-53CODEN: CCHRAM; ISSN:0010-8545. (Elsevier B.V.)
    A review. A general introduction to spin crossover involving iron(II) is provided. Then the 26 examples of 3,4,5-tri-substituted-1,2,4-triazole (mostly Rdpt and Rppt) ligands and the 11 examples of 3,5-di-substituted-1,2,4-triazole (including Hdpt) ligands that are of interest to this review are introduced. The 26 examples of 3,4,5-tri-substituted-1,2,4-triazole ligands fall into main two categories: (a) the 13 examples of sym. 4-substituted 3,5-di(2-pyridyl)triazole (Rdpt) ligands and (b) the 13 ligands closely related to the Rdpt ligands but featuring Ph (known as Rppt ligands), pyrazine or pyrimidine rings in place of one of more of the pyridine rings. In contrast, of the 11 examples of 3,5-di-substituted-1,2,4-triazole (HL, including Hdpt) ligands, only 2 are sym. (3,5-di(2-pyridyl)triazole and 3,5-di(2-pyrazyl)triazole ligands); the other 9 are non-sym. as two different rings are attached to the 3 and 5 positions of the triazole ring. In all but one example, in iron complexes of the 3,5-di-substituted-1,2,4-triazole ligands (HL, including Hdpt) the ligand deprotonates and coordinates as an anionic 3,5-di-substituted-1,2,4-triazolate (L-) ligand. This field was reviewed in 2008, and considered the 17 such complexes published up until 2007. Hence the present review provides a comprehensive update on the substantial progress that has been made in this field since then. Specifically it covers the 92 new iron(II) complexes of these 37 ligands, published since then and before 1 Dec. 2015. Of the 92 of new iron(II) complexes reported, 72 have been structurally characterized, and almost half of them, 40, are SCO-active. A wide range of nuclearities, from mono- to di- to tri- to penta- and poly-nuclear, as well as a range of binding modes, are obsd., so this review is organised into sections accordingly. In each section, after briefly summarizing the findings of the 2008 review, the new developments are detailed. Finally, the findings of this rich avenue of investigation into such ligands are summarized, indicating a bright future ahead.
    (b) Constable, E. C.; Baum, G.; Bill, E.; Dyson, R.; van Eldik, R.; Fenske, D.; Kaderli, S.; Morris, D.; Neubrand, A.; Neuburger, M.; Smith, D. R.; Wieghardt, K.; Zehnder, M.; Zuberbühler, A. D. Control of Iron(II) Spin States in 2,2′:6′,2″-Terpyridine Complexes through Ligand Substitution. Chem. - Eur. J. 1999, 5, 498– 508,  DOI: 10.1002/(SICI)1521-3765(19990201)5:2<498::AID-CHEM498>3.0.CO;2-V
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    15b
    Control of iron(II) spin states in 2,2':6',2''-terpyridine complexes through ligand substitution
    Constable, Edwin C.; Baum, Gerhard; Bill, Eckhard; Dyson, Raylene; Van Eldik, Rudi; Fenske, Dieter; Kaderli, Susan; Morris, Darrell; Neubrand, Anton; Neuburger, Markus; Smith, Diane R.; Wieghardt, Karl; Zehnder, Margareta; Zuberbuhler, Andreas D.
    Chemistry - A European Journal (1999), 5 (2), 498-508CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH)
    6- And 6,6''-aryl-substituted 2,2':6',2''-terpyridine ligands and their Fe(II) complexes were prepd. The introduction of Ph substituents at both the 6- and 6'' - positions leads exclusively to the formation of orange high-spin Fe(II) complexes while the presence of a single 6-Ph substituent results in spin-crossover systems. [Fe(2)2]X2 (2 = 4,6-diphenyl-2,2':6',2''-terpyridine; X = ClO4 or PF6) were studied in detail, and the solid-state x-ray structures of both the low- and high-spin forms are reported. Mossbauer spectroscopic and magnetic susceptibility measurements are reported, and the temp. and pressure dependence of the high-spin/low-spin transition were studied. An x-ray structural study of [Fe(2)2](ClO4)2 is also reported; this complex is highly distorted with two very long Fe···N contacts of over 2.4 Å and is best regarded as a four-coordinate Fe complex.
  16. 16
    (a) Bacchi, S.; Benaglia, M.; Cozzi, F.; Demartin, F.; Filippini, G.; Gavezzotti, A. X-ray Diffraction and Theoretical Studies for the Quantitative Assessment of Intermolecular Arene–Perfluoroarene Stacking Interactions. Chem. - Eur. J. 2006, 12, 3538– 3546,  DOI: 10.1002/chem.200501248
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    17a
    X-ray diffraction and theoretical studies for the quantitative assessment of intermolecular arene-perfluoroarene stacking interactions
    Bacchi, Sergio; Benaglia, Maurizio; Cozzi, Franco; Demartin, Francesco; Filippini, Giuseppe; Gavezzotti, Angelo
    Chemistry - A European Journal (2006), 12 (13), 3538-3546CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The arene-perfluoroarene stacking interaction was studied by exptl. and theor. methods. Compds. with different possibilities for formation of this recognition motif in the solid state were synthesized, and their crystal structures detd. by single-crystal x-ray diffraction. The crystal packing of these compds., as well as the packing of related compds. retrieved from crystallog. databases, were analyzed with quant. crystal potentials: total lattice energies and the cohesive energies of closest mol. pairs in the crystals were calcd. The arene-perfluoroarene recognition motif emerges as a dominant interaction in the nonhydrogen-bonding compds. studied here, to the point that asym. dimers formed over the stacking motif carry over to asym. units made of two mols. in the crystal both for pure compds. and for mol. complexes; however, inter-ring distances and angles range from 3.70 to 4.85 Å and from 5 to 21°, resp. Pixel energy partitioning reveals that whenever arom. rings stack, the largest cohesive energy contribution comes from dispersion, which roughly amts. to 20 kJ mol-1 per Ph ring, while the coulombic term is minor but significant enough to make a difference between the arene-arene or perfluoroarene-perfluoroarene interactions on the one hand, and arene-perfluoroarene interactions on the other, whereby the latter are favored by ∼10 kJ mol-1 per Ph ring. No evidence of special interaction which can be attributed to H···F confrontation was recognizable.
    (b) Salonen, L. M.; Ellermann, M.; Diederich, F. Aromatic rings in chemical and biological recognition: energetics and structures. Angew. Chem., Int. Ed. 2011, 50, 4808– 4842,  DOI: 10.1002/anie.201007560
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    17b
    Aromatic rings in chemical and biological recognition: energetics and structures
    Salonen, Laura M.; Ellermann, Manuel; Diederich, Francois
    Angewandte Chemie, International Edition (2011), 50 (21), 4808-4842CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A review. This review describes a multidimensional treatment of mol. recognition phenomena involving arom. rings in chem. and biol. systems. It summarizes new results reported since the appearance of an earlier review in 2003 in host-guest chem., biol. affinity assays and biostructural anal., data base mining in the Cambridge Structural Database (CSD) and the Protein Data Bank (PDB), and advanced computational studies. Topics addressed are arene-arene, perfluoroarene-arene, S···arom., cation-π, and anion-π interactions, as well as hydrogen bonding to π systems. The generated knowledge benefits, in particular, structure-based hit-to-lead development and lead optimization both in the pharmaceutical and in the crop protection industry. It equally facilitates the development of new advanced materials and supramol. systems, and should inspire further utilization of interactions with arom. rings to control the stereochem. outcome of synthetic transformations.
    (c) Wheeler, S. E. Local Nature of Substituent Effects in Stacking Interactions. J. Am. Chem. Soc. 2011, 133, 10262– 10274,  DOI: 10.1021/ja202932e
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    17c
    Local Nature of Substituent Effects in Stacking Interactions
    Wheeler, Steven E.
    Journal of the American Chemical Society (2011), 133 (26), 10262-10274CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Popular explanations of substituent effects in π-stacking interactions hinge upon substituent-induced changes in the aryl π-system. This entrenched view has been used to explain substituent effects in countless stacking interactions over the past 2 decades. However, for a broad range of stacked dimers, it is shown that substituent effects are better described as arising from local, direct interactions of the substituent with the proximal vertex of the other ring. Consequently, substituent effects in stacking interactions are additive, regardless of whether the substituents are on the same or opposite rings. Substituent effects are also insensitive to the introduction of heteroatoms on distant parts of either stacked ring. This local, direct interaction viewpoint provides clear, unambiguous explanations of substituent effects for myriad stacking interactions that are in accord with robust computational data, including DFT-D and new benchmark CCSD(T) results. Many of these computational results cannot be readily explained using traditional π-polarization-based models. Analyses of stacking interactions based solely on the sign of the electrostatic potential above the face of an arom. ring or the mol. quadrupole moment face a similar fate. The local, direct interaction model provides a simple means of analyzing substituent effects in complex arom. systems and also offers simple explanations of the crystal packing of fluorinated benzenes and the recently published dependence of the stability of protein-RNA complexes on the regiochem. of fluorinated base analogs [J. Am. Chem. Soc.2011, 133, 3687-3689].
  17. 17
    Gütlich, P. Fifty Years of Mössbauer Spectroscopy in Solid State Research - Remarkable Achievements, Future Perspectives. Z. Anorg. Allg. Chem. 2012, 638, 15– 43,  DOI: 10.1002/zaac.201100416
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    18
    Fifty Years of Moessbauer Spectroscopy in Solid State Research - Remarkable Achievements, Future Perspectives
    Guetlich, Philipp
    Zeitschrift fuer Anorganische und Allgemeine Chemie (2012), 638 (1), 15-43CODEN: ZAACAB; ISSN:0044-2313. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Moessbauer spectroscopy was founded more than fifty years ago based on an outstanding discovery by the young German physicist Rudolf Ludwig Moessbauer while working on his Ph.D. thesis. He discovered the recoilless nuclear resonance fluorescence of gamma radiation and was awarded the Nobel Prize in Physics in 1961 as one of the youngest recipients of this most prestigious award. His discovery led to the development of a new technique for measurements of hyperfine interactions between nuclear moments and electromagnetic fields. This method, with highest sharpness of tuning of 10-13, yields information on valence state, symmetry, magnetic behavior, phase transition, lattice dynamics and other solid state properties.
  18. 18
    Milocco, F.; Demeshko, S.; Meyer, F.; Otten, E. Ferrate(II) complexes with redox-active formazanate ligands. Dalton Trans. 2018, 47, 8817– 8823,  DOI: 10.1039/C8DT01597J
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    Ferrate(II) complexes with redox-active formazanate ligands
    Milocco, Francesca; Demeshko, Serhiy; Meyer, Franc; Otten, Edwin
    Dalton Transactions (2018), 47 (26), 8817-8823CODEN: DTARAF; ISSN:1477-9226. (Royal Society of Chemistry)
    The synthesis of mono(formazanate) Fe complexes is described. In the presence of Bu4N halides, salt metathesis reactions afford the ferrate(II) complexes [Bu4N][LFeX2] (L = PhNNC(p-tol)NNPh; X = Cl, Br) in good yield, and the products were characterized. The high-spin ferrate(II) complexes show cyclic voltammograms that are consistent with reversible, ligand-based 1-electron redn. The halides in these ferrate(II) compds. are labile, and are displaced by 4-methoxyphenyl isocyanide (4 equiv) as evidenced by formation of the low-spin, cationic octahedral complex [LFe(CNC6H4(p-OMe))4][Br]. Thus, a straightforward route to mono(formazanate) Fe(II) complexes was established.
  19. 19
    Lepori, C.; Guillot, R.; Hannedouche, J. C1-symmetric β-Diketiminatoiron(II) Complexes for Hydroamination of Primary Alkenylamines. Adv. Synth. Catal. 2019, 361, 714– 719,  DOI: 10.1002/adsc.201801464
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    20
    C1-symmetric β-Diketiminatoiron(II) Complexes for Hydroamination of Primary Alkenylamines
    Lepori, Clement; Guillot, Regis; Hannedouche, Jerome
    Advanced Synthesis & Catalysis (2019), 361 (4), 714-719CODEN: ASCAF7; ISSN:1615-4150. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The synthesis and solid-state characterization of an array of well-defined low-coordinate C1-sym. β-diketiminatoiron(II) alkyl complexes featuring steric and electronic variations on one of the N-aryl substituents of the β-diketiminate ligand scaffold are reported. All complexes display unique catalytic abilities of promoting the selective exo-cyclohydroamination of unprotected 2,2-diphenylpent-4-en-1-amine under mild reactions conditions. The incorporation of a potentially coordinative ortho-methoxy substituent on one of the N-aryl rings of the β-diketiminate skeleton, in conjunction with a more crowded 2,6-diisopropylphenyl group on the other, affords a far more active catalyst C1-sym. β-diketiminatoiron(II) alkyl complex than the authors' previously reported C2-sym. β-diketiminatoiron(II) alkyl complex (Ar1 = Ar2 = 2,4,6-(Me)3C6H2)/cyclopentylamine system. Comparative studies let the authors postulate that this superior activity of C1-sym. β-diketiminatoiron(II) alkyl complex , when compared with other complexes , is likely arising from steric effects and/or the coordinating ability of the ortho-methoxy substituent. The scope and limitations of this novel C1-sym. β-diketiminatoiron(II) alkyl complex are also presented.
  20. 20
    Weigend, F.; Ahlrichs, R. Balanced basis sets of split valence, triple zeta valence and quadruple zeta valence quality for H to Rn: Design and assessment of accuracy. Phys. Chem. Chem. Phys. 2005, 7, 3297– 3305,  DOI: 10.1039/b508541a
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    Balanced basis sets of split valence, triple zeta valence and quadruple zeta valence quality for H to Rn: Design and assessment of accuracy
    Weigend, Florian; Ahlrichs, Reinhart
    Physical Chemistry Chemical Physics (2005), 7 (18), 3297-3305CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    Gaussian basis sets of quadruple zeta valence quality for Rb-Rn are presented, as well as bases of split valence and triple zeta valence quality for H-Rn. The latter were obtained by (partly) modifying bases developed previously. A large set of more than 300 mols. representing (nearly) all elements-except lanthanides-in their common oxidn. states was used to assess the quality of the bases all across the periodic table. Quantities investigated were atomization energies, dipole moments and structure parameters for Hartree-Fock, d. functional theory and correlated methods, for which we had chosen Moller-Plesset perturbation theory as an example. Finally recommendations are given which type of basis set is used best for a certain level of theory and a desired quality of results.
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    Becke, A. D. Density-functional exchange-energy approximation with correct asymptotic behavior. Phys. Rev. A 1988, 38, 3098– 3100,  DOI: 10.1103/PhysRevA.38.3098
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    Density-functional exchange-energy approximation with correct asymptotic behavior
    Becke, A. D.
    Physical Review A: Atomic, Molecular, and Optical Physics (1988), 38 (6), 3098-100CODEN: PLRAAN; ISSN:0556-2791.
    Current gradient-cor. d.-functional approxns. for the exchange energies of at. and mol. systems fail to reproduce the correct 1/r asymptotic behavior of the exchange-energy d. A gradient-cor. exchange-energy functional is given with the proper asymptotic limit. This functional, contg. only one parameter, fits the exact Hartree-Fock exchange energies of a wide variety of at. systems with remarkable accuracy, surpassing the performance of previous functionals contg. two parameters or more.
  22. 22
    (a) Staroverov, V. N.; Scuseria, G. E.; Tao, J.; Perdew, J. P. Comparative assessment of a new nonempirical density functional: Molecules and hydrogen-bonded complexes. J. Chem. Phys. 2003, 119, 12129– 12137,  DOI: 10.1063/1.1626543
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    Comparative assessment of a new nonempirical density functional: Molecules and hydrogen-bonded complexes
    Staroverov, Viktor N.; Scuseria, Gustavo E.; Tao, Jianmin; Perdew, John P.
    Journal of Chemical Physics (2003), 119 (23), 12129-12137CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
    A comprehensive study is undertaken to assess the nonempirical meta-generalized gradient approxn. (MGGA) of Tao, Perdew, Staroverov, and Scuseria (TPSS) against 14 common exchange-correlation energy functionals. Principal results are presented in the form of statistical summaries of deviations from expt. for the G3/99 test set (223 enthalpies of formation, 86 ionization potentials, 58 electron affinities, 8 proton affinities) and three addnl. test sets involving 96 bond lengths, 82 harmonic vibrational frequencies, and 10 hydrogen-bonded complexes, all computed using the 6-311++G(3df,3pd) basis. The TPSS functional matches, or exceeds in accuracy all prior nonempirical constructions and, unlike semiempirical functionals, consistently provides a high-quality description of diverse systems and properties. The computational cost of self-consistent MGGA is comparable to that of ordinary GGA, and exact exchange (unavailable in some codes) is not required. A one-parameter global hybrid version of the TPSS functional is introduced and shown to give further improvement for most properties.
    (b) Tao, J.; Perdew, J. P.; Staroverov, V. N.; Scuseria, G. E. Climbing the Density Functional Ladder: Nonempirical Meta--Generalized Gradient Approximation Designed for Molecules and Solids. Phys. Rev. Lett. 2003, 91, 146401,  DOI: 10.1103/PhysRevLett.91.146401
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    23b
    Climbing the Density Functional Ladder: Nonempirical Meta-Generalized Gradient Approximation Designed for Molecules and Solids
    Tao, Jianmin; Perdew, John P.; Staroverov, Viktor N.; Scuseria, Gustavo E.
    Physical Review Letters (2003), 91 (14), 146401/1-146401/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
    The electron d., its gradient, and the Kohn-Sham orbital kinetic energy d. are the local ingredients of a meta-generalized gradient approxn. (meta-GGA). We construct a meta-GGA d. functional for the exchange-correlation energy that satisfies exact constraints without empirical parameters. The exchange and correlation terms respect two paradigms: one- or two-electron densities and slowly varying densities, and so describe both mols. and solids with high accuracy, as shown by extensive numerical tests. This functional completes the third rung of "Jacob's ladder" of approxns., above the local spin d. and GGA rungs.
  23. 23
    Becke, A. D. Density-functional thermochemistry. III. The role of exact exchange. J. Chem. Phys. 1993, 98, 5648– 5652,  DOI: 10.1063/1.464913
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    Density-functional thermochemistry. III. The role of exact exchange
    Becke, Axel D.
    Journal of Chemical Physics (1993), 98 (7), 5648-52CODEN: JCPSA6; ISSN:0021-9606.
    Despite the remarkable thermochem. accuracy of Kohn-Sham d.-functional theories with gradient corrections for exchange-correlation, the author believes that further improvements are unlikely unless exact-exchange information is considered. Arguments to support this view are presented, and a semiempirical exchange-correlation functional (contg. local-spin-d., gradient, and exact-exchange terms) is tested for 56 atomization energies, 42 ionization potentials, 8 proton affinities, and 10 total at. energies of first- and second-row systems. This functional performs better than previous functionals with gradient corrections only, and fits expt. atomization energies with an impressively small av. abs. deviation of 2.4 kcal/mol.
  24. 24
    (a) Harvey, J. N. In Principles and Applications of Density Functional Theory in Inorganic Chemistry I; Springer: Berlin, 2004; pp 151– 184.
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    Spinning around in Transition-Metal Chemistry
    Swart, Marcel; Gruden, Maja
    Accounts of Chemical Research (2016), 49 (12), 2690-2697CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
    A review. The great diversity and richness of transition metal chem., such as the features of an open d-shell, opened a way to numerous areas of scientific research and technol. applications. Depending on the nature of the metal and its environment, there are often several energetically accessible spin states, and the progress in accurate theor. treatment of this complicated phenomenon is presented in this Account. The spin state energetics of a transition metal complex can be predicted theor. on the basis of d. functional theory (DFT) or wave function based methodol., where DFT has advantages since it can be applied routinely to medium-to-large-sized mols. and spin-state consistent d. functionals are now available. Addnl. factors such as the effect of the basis set, thermochem. contributions, solvation, relativity, and dispersion, have been investigated by many researchers, but challenges in unambiguous assignment of spin states still remain. The first DFT studies showed intrinsic spin-state preferences of hybrid functionals for high spin and early generalized gradient approxn. functionals for low spin. Progress in the development of d. functional approxns. (DFAs) then led to a class of specially designed DFAs, such as OPBE, SSB-D, and S12g, and brought a very intriguing and fascinating observation that the spin states of transition metals and the SN2 barriers of org. mols. are somehow intimately linked. Among the many noteworthy results that emerged from the search for the appropriate description of the complicated spin state preferences in transition metals, we mainly focused on the examn. of the connection between the spin state and the structures or coordination modes of the transition metal complexes. Changes in spin states normally lead only to changes in the metal-ligand bond lengths, but to the best of our knowledge, the dapsox ligand showed the first example of a transition-metal complex where a change in spin state leads also to changes in the coordination, switching between pentagonal-bipyramidal and capped-octahedron. Moreover, we have summarized the results of the thorough study that cor. the exptl. assignment of the nature of the recently synthesized Sc3+ adduct of [FeIV(O)(TMC)]2+ (TMC = 1,4,8,11-tetramethylcyclam) and firmly established that the Sc3+-capped iron-oxygen complex corresponds to high-spin FeIII. Last, but not least, we have provided deeper insight and rationalization of the observation that unlike in metalloenzymes, where the FeIV-oxo is usually obsd. with high spin, biomimetic FeIV-oxo complexes typically have a intermediate spin state. Energy decompn. analyses on the trigonal-bypiramidal (TBP) and octahedral model systems with ammonia ligands have revealed that the interaction energy of the prepd. metal ion in the intermediate spin state is much smaller for the TBP structure. This sheds light on the origin of the intermediate spin state of the biomimetic TBP FeIV-oxo complexes.
  25. 25
    These orbital splitting energies are relatively small; spin-crossover complexes commonly have significantly larger values for Δ. See for example:Hauser, A. Ligand Field Theoretical Considerations. In Spin Crossover in Transition Metal Compounds I; Gütlich, P., Goodwin, H. A., Eds.; Springer Berlin Heidelberg: Berlin, Heidelberg, 2004, p 49.
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    Intrinsic Atomic Orbitals: An Unbiased Bridge between Quantum Theory and Chemical Concepts
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    Modern quantum chem. can make quant. predictions on an immense array of chem. systems. However, the interpretation of those predictions is often complicated by the complex wave function expansions used. Here we show that an exceptionally simple algebraic construction allows for defining at. core and valence orbitals, polarized by the mol. environment, which can exactly represent SCF wave functions. This construction provides an unbiased and direct connection between quantum chem. and empirical chem. concepts, and can be used, for example, to calc. the nature of bonding in mols., in chem. terms, from first principles. In particular, we find consistency with electronegativities (χ), C 1s core-level shifts, resonance substituent parameters (σR), Lewis structures, and oxidn. states of transition-metal complexes.
    (b) Knizia, G.; Klein, J. E. M. N. Electron Flow in Reaction Mechanisms—Revealed from First Principles. Angew. Chem., Int. Ed. 2015, 54, 5518– 5522,  DOI: 10.1002/anie.201410637
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    The "curly arrow" of Robinson and Ingold is the primary tool for describing and rationalizing reaction mechanisms. Despite this approach's ubiquity and stellar success, its phys. basis has never been clarified and a direct connection to quantum chem. has never been found. Here we report that the bond rearrangements expressed by curly arrows can be directly obsd. in ab initio computations, as transformations of intrinsic bond orbitals (IBOs) along the reaction coordinate. Our results clarify that curly arrows are rooted in phys. reality-a notion which has been challenged before-and show how quantum chem. can directly establish reaction mechanisms in intuitive terms and unprecedented detail.
  27. 27
    Kamphuis, A. J.; Milocco, F.; Koiter, L.; Pescarmona, P. P.; Otten, E. Highly Selective Single-Component Formazanate Ferrate(II) Catalysts for the Conversion of CO2 into Cyclic Carbonates. ChemSusChem 2019, 12, 3635– 3641,  DOI: 10.1002/cssc.201900740
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    Highly Selective Single-Component Formazanate Ferrate(II) Catalysts for the Conversion of CO2 into Cyclic Carbonates
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    ChemSusChem (2019), 12 (15), 3635-3641CODEN: CHEMIZ; ISSN:1864-5631. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The development of new families of active and selective single-component catalysts based on earth-abundant metal is of interest from a sustainable chem. perspective. In this context, anionic mono(formazanate) Fe(II) complexes bearing labile halide ligands, which possess both Lewis acidic and nucleophilic functionalities, were developed as novel single-component homogeneous catalysts for the reaction of CO2 with epoxides to produce cyclic carbonates. The influence of the halide ligand and the electronic properties of the formazanate ligand backbone on the catalytic activity are studied by employing the Fe(II) complexes with and without an addnl. nucleophile. Very high selectivity is achieved towards the formation of the cyclic carbonate products from various terminal and internal epoxides without the need of a cocatalyst. Crystallog. data are given.
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    Chang, M.-C.; Roewen, P.; Travieso-Puente, R.; Lutz, M.; Otten, E. Formazanate Ligands as Structurally Versatile, Redox-Active Analogues of β-Diketiminates in Zinc Chemistry. Inorg. Chem. 2015, 54, 379– 388,  DOI: 10.1021/ic5025873
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    Formazanate Ligands as Structurally Versatile, Redox-Active Analogues of β-Diketiminates in Zinc Chemistry
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    Inorganic Chemistry (2015), 54 (1), 379-388CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    A range of tetrahedral bis(formazanate)zinc complexes with different steric and electronic properties of the formazanate ligands were synthesized. The solid-state structures for several of these were detd. by x-ray crystallog., which showed that complexes with sym., unhindered ligands prefer coordination to the Zn center via the terminal N atoms of the NNCNN ligand backbone. Steric or electronic modifications can override this preference and give rise to solid-state structures in which the formazanate ligand forms a 5-membered chelate by binding to the metal center via an internal N atom. In soln., these compds. show dynamic equil. that involve both 5- and 6-membered chelates. All compds. are intensely colored, and the effect of the ligand substitution pattern on the UV-visible absorption spectra was evaluated. Their cyclic voltammetry is reported, which shows that all compds. may be electrochem. reduced to radical anionic (L2Zn-) and dianionic (L2Zn2-) forms. While unhindered NAr substituents lie in the plane of the ligand backbone (Ar = Ph), the introduction of sterically demanding substituents (Ar = Mes) favors a perpendicular orientation in which the NMes group is no longer in conjugation with the backbone, resulting in hypsochromic shifts in the absorption spectra. The redox potentials in L2Zn compds. may be altered in a straightforward manner over a relatively wide range (∼700 mV) via the introduction of electron-donating or -withdrawing substituents on the formazanate framework.
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    Chang, M. C.; Otten, E. Synthesis and ligand-based reduction chemistry of boron difluoride complexes with redox-active formazanate ligands. Chem. Commun. 2014, 50, 7431– 7433,  DOI: 10.1039/C4CC03244F
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    Synthesis and ligand-based reduction chemistry of boron difluoride complexes with redox-active formazanate ligands
    Chang, M.-C.; Otten, E.
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    Mono(formazanate) boron difluoride complexes (LBF2), which show remarkably facile and reversible ligand-based redox-chem., were synthesized by transmetalation of bis(formazanate) zinc complexes with boron trifluoride. The one-electron redn. product [LBF2]-[Cp2Co]+ and a key intermediate for the transmetalation reaction, the six-coordinate zinc complex (L(BF3))2Zn were isolated and fully characterized.
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    (a) Chang, M. C.; Chantzis, A.; Jacquemin, D.; Otten, E. Boron difluorides with formazanate ligands: redox-switchable fluorescent dyes with large stokes shifts. Dalton Trans. 2016, 45, 9477– 9484,  DOI: 10.1039/C6DT01226D
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    The synthesis of a series of (formazanate)boron difluorides and their 1-electron redn. products is described. The neutral compds. are fluorescent with large Stokes shifts. DFT calcns. suggest that a large structural reorganization accompanies photoexictation and accounts for the large Stokes shift. Redn. of the neutral boron difluorides occurs at the ligand and generates the corresponding radical anions. These complexes are non-fluorescent, allowing switching of the emission by changing the ligand oxidn. state.
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    The absence of residual solvent in metal precursors can be of key importance for the successful prepn. of metal complexes or materials. Herein, the authors describe methods for the quantitation of residual coordinated THF that binds to Fe[N(SiMe3)2]2, a commonly used iron synthon, when prepd. according to common literature procedures. A simple method for quantitation of the amt. of residual coordinated THF using 1H NMR spectroscopy is highlighted. Finally, a detailed synthetic procedure is described for the synthesis of THF-free Fe[N(SiMe3)2]2.
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    Chemical Communications (Cambridge, United Kingdom) (2007), (2), 126-128CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
    Formazans, R'NHN:CRN:NR' (R ≠ R' = p-tolyl, Ph) react with boron triacetate to produce boratatetrazines I, which can be reduced by cobaltocene to yield borataverdazyl radical anions, e.g., II-the first boron contg. verdazyl radicals.
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    A short history of SHELX
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    An account is given of the development of the SHELX system of computer programs from SHELX-76 to the present day. In addn. to identifying useful innovations that have come into general use through their implementation in SHELX, a crit. anal. is presented of the less-successful features, missed opportunities and desirable improvements for future releases of the software. An attempt is made to understand how a program originally designed for photog. intensity data, punched cards and computers over 10000 times slower than an av. modern personal computer has managed to survive for so long. SHELXL is the most widely used program for small-mol. refinement and SHELXS and SHELXD are often employed for structure soln. despite the availability of objectively superior programs. SHELXL also finds a niche for the refinement of macromols. against high-resoln. or twinned data; SHELXPRO acts as an interface for macromol. applications. SHELXC, SHELXD and SHELXE are proving useful for the exptl. phasing of macromols., esp. because they are fast and robust and so are often employed in pipelines for high-throughput phasing. This paper could serve as a general literature citation when one or more of the open-source SHELX programs (and the Bruker AXS version SHELXTL) are employed in the course of a crystal-structure detn.
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    Crystal structure refinement with SHELXL
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    Acta Crystallographica, Section C: Structural Chemistry (2015), 71 (1), 3-8CODEN: ACSCGG; ISSN:2053-2296. (International Union of Crystallography)
    The improvements in the crystal structure refinement program SHELXL have been closely coupled with the development and increasing importance of the CIF (Crystallog. Information Framework) format for validating and archiving crystal structures. An important simplification is that now only one file in CIF format (for convenience, referred to simply as 'a CIF') contg. embedded reflection data and SHELXL instructions is needed for a complete structure archive; the program SHREDCIF can be used to ext. the and files required for further refinement with SHELXL. Recent developments in SHELXL facilitate refinement against neutron diffraction data, the treatment of H atoms, the detn. of abs. structure, the input of partial structure factors and the refinement of twinned and disordered structures. SHELXL is available free to academics for the Windows, Linux and Mac OS X operating systems, and is particularly suitable for multiple-core processors.

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  • Abstract

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    Figure 1

    Figure 1. Common ligand field splitting diagrams for octahedral (A), square-planar (B), and tetrahedral (C) geometries and unusual ligand field splitting for the pseudo-tetrahedral geometries found in bis(formazanate)iron(II) complexes (D). (14)

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    Scheme 1

    Scheme 1. Synthesis of Compounds 1–7
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    Figure 2

    Figure 2. Crystal structure of compound 3 showing the π-stacking interactions between the mesityl rings. The Fe center, ligand backbone, and the mesityl rings are shown as 50% probability ellipsoids and the remaining atoms as wireframe; hydrogen atoms are removed for clarity.

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    Figure 3

    Figure 3. Molecular structure of compounds 6a and 6b showing 50% probability ellipsoids; hydrogen atoms omitted for clarity. The inset for each shows the Fe(NNCNN)2 core of the structure with the N–Fe–N planes and the dihedral angle.

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    Figure 4

    Figure 4. Crystal structure of compound 6b illustrating the π-stacking interactions between the aromatic rings, showing 50% probability ellipsoids. Parts of the molecule are shown as wireframe, and hydrogen atoms are removed for clarity.

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    Figure 5

    Figure 5. 57Fe Mössbauer spectra at 80 K in the solid state of 6b (A), 6a (B), and a powder sample of 6 before (C) and after (D) heating to 400 K for SQUID measurements. The red line in the spectrum of heated 6 represents the main species with 82% area, and the gray subspectra are unknown impurities.

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    Figure 6

    Figure 6. Magnetic susceptibility data for a powder sample of 6 in the solid state (heating to 400 K and subsequent cooling). The solid black line shows the best fit curve for S = 1 with the parameters g = 2.10 and D = 11.2 cm–1 (100% IS). The dashed red line shows the spin-only value for an S = 2 system.

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    Figure 7

    Figure 7. Molecular structures of 7 showing 50% probability ellipsoids. One of the N–Ph rings is shown as wireframe, and hydrogen atoms are omitted for clarity.

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    Figure 8

    Figure 8. Temperature dependence of the high-spin fraction (γHS) of compounds 1–5 and 7 in toluene-d8, including error bars for T1/2 (γHS = 0.5). The liquid range for toluene is indicated with the color gradient at the temperature axis.

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    Figure 9

    Figure 9. 1H NMR spectra of 7 recorded between 247 and 397 K (toluene-d8, 500 MHz).

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    Figure 10

    Figure 10. Representation of intrinsic bonding orbitals at the BP86/def2-TZVP minima, both with (7calc; (A)) and without Fe–O interaction (7′calc; (B)).

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    Figure 11

    Figure 11. UV/vis spectra in toluene for (A) compound 6 recorded between 183 and 293 K and (B) compound 7 recorded between 293 and 383 K.

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  • References

    This article references 35 other publications.

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      Open-Shell Organometallics as a Bridge between Werner-Type and Low-Valent Organometallic Complexes. The Effect of the Spin State on the Stability, Reactivity, and Structure
      Poli, Rinaldo
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      (b) Hawrelak, E. J.; Bernskoetter, W. H.; Lobkovsky, E.; Yee, G. T.; Bill, E.; Chirik, P. J. Square planar vs tetrahedral geometry in four coordinate iron(II) complexes. Inorg. Chem. 2005, 44, 3103– 3111,  DOI: 10.1021/ic048202+
      1b
      Square Planar vs Tetrahedral Geometry in Four Coordinate Iron(II) Complexes
      Hawrelak, Eric J.; Bernskoetter, Wesley H.; Lobkovsky, Emil; Yee, Gordon T.; Bill, Eckhard; Chirik, Paul J.
      Inorganic Chemistry (2005), 44 (9), 3103-3111CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      The geometric preferences of a family of four coordinate, iron(II) d6 complexes of the general form L2FeX2 have been systematically evaluated. Treatment of Fe2(Mes)4 (Mes = 2,4,6-Me3C6H2) with monodentate phosphine and phosphite ligands furnished square planar trans-P2Fe(Mes)2 derivs. Identification of the geometry has been accomplished by a combination of soln. and solid-state magnetometry and, in two cases (P = PMe3, PEt2Ph), x-ray diffraction. In contrast, both tetrahedral and square planar coordination has been obsd. upon complexation of chelating phosphine ligands. A combination of crystallog. and magnetic susceptibility data for (depe)Fe(Mes)2 (depe = 1,2-bis(diethylphosphino)ethane) established a tetrahedral mol. geometry whereas SQUID magnetometry and Moessbauer spectroscopy on samples of (dppe)Fe(Mes)2 (dppe = 1,2-bis(diphenylphosphino)ethane) indicated a planar mol. When dissolved in chlorinated solvents, the latter compd. promotes chlorine atom abstraction, forming tetrahedral (dppe)Fe(Mes)Cl and (dppe)FeCl2. Ligand substitution reactions have been studied for both structural types and are rapid on the NMR time scale at ambient temp.
    2. 2
      Hartwig, J. F. Organotransition Metal Chemistry: From Bonding to Catalysis; University Science Books: Sausalito, CA, 2010.
      There is no corresponding record for this reference.
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      Cirera, J.; Ruiz, E.; Alvarez, S. Stereochemistry and spin state in four-coordinate transition metal compounds. Inorg. Chem. 2008, 47, 2871– 2889,  DOI: 10.1021/ic702276k
      3
      Stereochemistry and Spin State in Four-Coordinate Transition Metal Compounds
      Cirera, Jordi; Ruiz, Eliseo; Alvarez, Santiago
      Inorganic Chemistry (2008), 47 (7), 2871-2889CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      A systematic DFT computational study of the stereochem. assocd. with each spin state for first transition series four-coordinate dn (n = 0-10) homoleptic metal complexes is presented. The stereochem. of [MMe4]x- complexes in the 21 spin configurations analyzed can be predicted from the d orbital occupation in the ideal tetrahedral geometry, grouped in three families with tetrahedral, square planar, or intermediate structures that can be described in some cases as sawhorses. The effect of the following factors on the spin state and stereochem. preferences has also been studied: (a) substitution of the σ-donor Me ligands by π-donor chlorides, (b) a high (+ 4) oxidn. state of the metal, and (c) substitution of the metal atom by a second transition series one. Through those factors, low-spin tetrahedral structures can be achieved, as summarized by a magic cube.
    4. 4
      (a) Collman, J. P.; Hoard, J. L.; Kim, N.; Lang, G.; Reed, C. A. Synthesis, stereochemistry, and structure-related properties of alpha, beta, gamma, delta-tetraphenylporphinatoiron(II). J. Am. Chem. Soc. 1975, 97, 2676– 2681,  DOI: 10.1021/ja00843a015
      4a
      Synthesis, stereochemistry, and structure-related properties of α,β,γ,δ-tetraphenylporphinatoiron(II)
      Collman, James P.; Hoard, J. L.; Kim, Nancy; Lang, George; Reed, Christopher A.
      Journal of the American Chemical Society (1975), 97 (10), 2676-81CODEN: JACSAT; ISSN:0002-7863.
      Addnl. data considered in abstracting and indexing are available from a source cited in the original document. Redn. of chloro-α,β,γ,δ-tetraphenylporphinatoiron(III) by bis(acetylacetonato)chromium(II) under anaerobic conditions yielded α,β,γ,δ-tetraphenylporphinatoiron(II), Fe(TPP), which crystd. from benzene-EtOH soln. in the tetragonal system. Pertinent crystal data are: space group, I‾42d; α 15.080(9) and c 14.043(9) Å; d. (exptl.) = 1.38 and d. (calcd.) = 1.39 for Z = 4; required mol. symmetry S4. Intensities of 1164 reflections having (sin θ)/λ < 0.71 Å-1, recorded with Zr-filtered Mo Kα radiation on a Picker FACS-I diffractometer, were used in anisotropic full-matrix least-squares refinement of the 112 structural parameters. The length of the FeII-N bonds in the intermediate-spin (S = 1) Fe(TPP) mol. is 1.972(4) Å, as compared with 2.086(4) Å in high-spin (S = 2) 2-methylimidazole-α,β,γ,δ-tetraphenylporphinatoiron(II), with 2.004(3) Å in low-spin (S = 0) bis(piperidine)-α,β,γ,δ-tetraphenylporphinatoiron(II, and with Cu-N = 1.981(7) Å in Cu(TPP). The foregoing assignment of spin states to the 3 Fe(II) porphyrins is fully confirmed by the Moessbauer spectra recorded for them in zero and in large applied magnetic fields at 4.2-300°K. Recrystd. Fe(TPP) conforming to the theor. compn. has an effective magnetic moment of ∼4.4 μB at room temp. Detn. of the anisotropic magnetic susceptibilities as a function of temp. for the tetragonal crystal, when considered in conjunction with the Moessbauer data, should lead to a more detailed specification of the electronic configuration of the intermediate-spin iron(II) atom in Fe(TPP) than that given in this paper.
      (b) Kirner, J. F.; Dow, W.; Scheidt, W. R. Molecular stereochemistry of two intermediate-spin complexes. Iron(II) phthalocyanine and manganese(II) phthalocyanine. Inorg. Chem. 1976, 15, 1685– 1690,  DOI: 10.1021/ic50161a042
      4b
      Molecular stereochemistry of two intermediate-spin complexes. Iron(II) phthalocyanine and manganese(II) phthalocyanine
      Kirner, John F.; Dow, W.; Scheidt, W. Robert
      Inorganic Chemistry (1976), 15 (7), 1685-90CODEN: INOCAJ; ISSN:0020-1669.
      The mol. stereochem. of Fe(II) phthalocyanine (Pc) and Mn(II) phthalocyanine was detd. by x-ray diffraction methods. The phthalocyanine ligand constrains the metal ion to effectively square-planar coordination and to an intermediate spin state. The FeII-N bond distance of 1.926(1) and the MnII-N bond length of 1.938(3) Å are wholly consistent with the assignment of an intermediate-spin ground state. Both complexes crystallize as the β polymorph. Crystal data are: for FePc, space group P21/a, a 19.392(5), b 4.786(2), c 14.604(4) Å, β 120.85(1)°, d.(exptl.) 1.61, d.(calcd.) 1.623, and Z = 2, required mol. symmetry ‾1; for MnPc, space group P21/a, a 19.400(4), b 4.761(2), c 14.613(3) Å, β 120.74(1)°, d.(exptl.) 1.61, d.(calcd.) 1.625, and Z = 2, required mol. symmetry ‾1. Final discrepancy indices are: FePc, R1 0.045, R2 0.057; MnPc, R1 0.066, R2 0.066.
      (c) Strauss, S. H.; Silver, M. E.; Ibers, J. A. Iron(II) octaethylchlorine: structure and ligand affinity comparison with its porphyrin and isobacteriochlorin homologs. J. Am. Chem. Soc. 1983, 105, 4108– 4109,  DOI: 10.1021/ja00350a069
      4c
      Iron(II) octaethylchlorine: structure and ligand affinity comparison with its porphyrin and isobacteriochlorin homologs
      Strauss, Steven H.; Silver, Michael E.; Ibers, James A.
      Journal of the American Chemical Society (1983), 105 (12), 4108-9CODEN: JACSAT; ISSN:0002-7863.
      trans-7,8-Dihydro-2,3,7,8,12,13,17,18-octaethylporphyrinatoiron(II), Fe(OEC), is orthorhombic, space group Pbcn, with a 21.880(9), b 15.795(6), and c 8.554(4) Å at -150°; Z = 4. The structure was refined anisotropically by least-squares to a final R(F2) = 0.105. The Fe and 4 N atoms are rigorously planar, while the rest of the chlorin macrocycle is significantly S4 ruffled. The affinity of Fe(OEC) and its octaethylporphyrin and octaethylisobacteriochlorin homologs for the weak σ-donor ligands THF and EtSH is strongly macrocycle dependent. This dependence may have a structural basis whereby reduced macrocycles more easily distort, adjusting their core size to accommodate the changing requirements of the metal upon complexation of a weak ligand.
    5. 5
      Chatt, J.; Shaw, B. L. Alkyls and aryls of transition metals. Part IV. Cobalt(II) and iron(II) derivatives. J. Chem. Soc. 1961, 285,  DOI: 10.1039/jr9610000285
      5
      Alkyls and aryls of transition metals. IV. Cobalt(II) and iron(II) derivatives
      Chatt, J.; Shaw, B. L.
      Journal of the Chemical Society (1961), (), 285-90CODEN: JCSOA9; ISSN:0368-1769.
      cf. CA 54, 20936d. The prepn. and properties of stable organometallic complexes of the types trans-[MR2-(PR2')2] (M = Co, Fe; R, R' = org. radicals) were described. The groups R were ortho-substituted aryl groups, where the substituents were somewhat bulky. These were the first planar complexes of Co(II) and Fe(II) that had only monodentate ligands. Their configurations were established by their magnetic and elec. dipole moments. Possible reasons for the unusual configurations and stabilities of these organometallic derivs. were discussed.
    6. 6
      (a) Nijhuis, C. A.; Jellema, E.; Sciarone, T. J. J.; Meetsma, A.; Budzelaar, P. H. M.; Hessen, B. First-Row Transition Metal Bis(amidinate) Complexes; Planar Four-Coordination of FeII Enforced by Sterically Demanding Aryl Substituents Eur. Eur. J. Inorg. Chem. 2005, 2005, 2089– 2099,  DOI: 10.1002/ejic.200500094
      There is no corresponding record for this reference.
      (b) Wurzenberger, X.; Piotrowski, H.; Klufers, P. A stable molecular entity derived from rare iron(II) minerals: the square-planar high-spin d6 Fe(II)O4 chromophore Angew. Angew. Chem., Int. Ed. 2011, 50, 4974– 4978,  DOI: 10.1002/anie.201006898
      6b
      A Stable Molecular Entity Derived from Rare Iron(II) Minerals: The Square-Planar High-Spin-d6 FeIIO4 Chromophore
      Wurzenberger, Xaver; Piotrowski, Holger; Kluefers, Peter
      Angewandte Chemie, International Edition (2011), 50 (21), 4974-4978, S4974/1-S4974/4CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A strongly alk. (LiOH or NaOH) aq. soln. of furanose mimic anhydroerythritol (meso-oxolane-3,4-diol, H2L) and FeCl2 gave a reddish-blue soln., from which, depending on the conditions, orange-red crystals of the tetrahydrates Li2[FeL2]·4H2O (1) or Na2[FeL2]·4H2O (2), or violet crystals of the nonahydrate Na2[FeL2]·9H2O (3) were grown. The bidentate oxolanediolato ligand stabilizes the rare high-spin ferrous SP-4 coordination in 1 as revealed by x-ray crystallog. and SQUID magnetometer measurements. The structure of 2 is also SP-4 as revealed by x-ray crystallog. Compd. 3 is slightly distorted toward tetrahedron. Three related mononuclear high-spin [FeIIX4]2- (X = Cl, F, OH) species and their high-spin [MnIIX4]2- analogs were analyzed in a DFT approach using the unrestricted-B3LYP/tzvp level of theory. Enhanced Jahn-Teller flattening can stabilize square planar high-spin d6 centers despite their "wrong" spin state by a marked sepn. of the neg. charge on the ligand atoms and the stereochem. active d electrons. This result reveals square-planar high-spin centers to be building blocks of reasonable stability.
      (c) Cantalupo, S. A.; Fiedler, S. R.; Shores, M. P.; Rheingold, A. L.; Doerrer, L. H. High-spin square-planar Co(II) and Fe(II) complexes and reasons for their electronic structure. Angew. Chem., Int. Ed. 2012, 51, 1000– 1005,  DOI: 10.1002/anie.201106091
      6c
      High-Spin Square-Planar CoII and FeII Complexes and Reasons for Their Electronic Structure
      Cantalupo, Stefanie A.; Fiedler, Stephanie R.; Shores, Matthew P.; Rheingold, Arnold L.; Doerrer, Linda H.
      Angewandte Chemie, International Edition (2012), 51 (4), 1000-1005, S1000/1-S1000/18CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The high-spin, square planar complexes [K(DME)2]2[M(ddfp)2] (M = CoII, FeII, ddfp = perfluoropinacolate) were prepd. and characterized by x-ray crystallog., temp.-dependent magnetic susceptibility, cyclic voltammetry, and other methods. The tetrahedral analog [K(DME)2]2[Zn(ddfp)2] was also prepd. and characterized crystallog. The combination of high-spin electron configuration and square-planar geometry is made possible by ligand constraints that generate five non-degenerate d-orbitals with ligand-based π-donation, a relatively weak ligand-field splitting, and no intervening ligand-based π-acceptor mol. orbitals.
      (d) Pinkert, D.; Demeshko, S.; Schax, F.; Braun, B.; Meyer, F.; Limberg, C. A Dinuclear Molecular Iron(II) Silicate with Two High-Spin Square-Planar FeO4 Units. Angew. Chem., Int. Ed. 2013, 52, 5155– 5158,  DOI: 10.1002/anie.201209650
      6d
      A Dinuclear Molecular Iron(II) Silicate with Two High-Spin Square-Planar FeO4 Units
      Pinkert, Denise; Demeshko, Serhiy; Schax, Fabian; Braun, Beatrice; Meyer, Franc; Limberg, Christian
      Angewandte Chemie, International Edition (2013), 52 (19), 5155-5158CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Dinuclear iron complex [L'2Fe2][Na(OEt2)2]2 (1) (H3L' = ligand with elimination of "Me2SiO" from 3-(1,1-dimethylethyl)-3-[(hydroxydimethylsilyl)oxy]-1,1,5,5-tetramethyl-1,5-trisiloxanediol) was synthesized and studied by x-ray diffraction anal., temp.-dependent magnetic susceptibility and Mossbauer spectroscopy.
      (e) Liu, Y.; Luo, L.; Xiao, J.; Wang, L.; Song, Y.; Qu, J.; Luo, Y.; Deng, L. Four-Coordinate Iron(II) Diaryl Compounds with Monodentate N-Heterocyclic Carbene Ligation: Synthesis, Characterization, and Their Tetrahedral-Square Planar Isomerization in Solution. Inorg. Chem. 2015, 54, 4752– 4760,  DOI: 10.1021/acs.inorgchem.5b00138
      6e
      Four-Coordinate Iron(II) Diaryl Compounds with Monodentate N-Heterocyclic Carbene Ligation: Synthesis, Characterization, and Their Tetrahedral-Square Planar Isomerization in Solution
      Liu, Yuesheng; Luo, Lun; Xiao, Jie; Wang, Lei; Song, You; Qu, Jingping; Luo, Yi; Deng, Liang
      Inorganic Chemistry (2015), 54 (10), 4752-4760CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      The salt elimination reactions of (IPr2Me2)2FeCl2 (IPr2Me2 = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) with the corresponding aryl Grignard reagents afford [(IPr2Me2)2FeAr2] (Ar = Ph, 3; C6H4-p-Me, 4; C6H4-p-tBu, 5; C6H3-3,5-(CF3)2, 6) in good yields. X-ray crystallog. studies revealed the presence of both tetrahedral and trans square planar isomers for 3 and 6 and the tetrahedral structures for 4 and 5. Magnetic susceptibility and 57Fe Mossbauer spectrum measurements on the solid samples indicated the high-spin (S = 2) and intermediate-spin (S = 1) nature of the tetrahedral and square planar structures, resp. Soln. property studies, including soln. magnetic susceptibility measurement, variable-temp. 1H and 19F NMR, and absorption spectroscopy, on 3-6, as well as an 57Fe Mossbauer spectrum study on a frozen THF soln. of tetrahedral [(IPr2Me2)257FePh2] suggest the coexistence of tetrahedral and trans square planar structures in soln. phase. D. functional theory calcns. on (IPr2Me2)2FePh2 disclosed that the tetrahedral and trans square planar isomers are close in energy and that the geometry isomerization can occur by spin-change-coupled geometric transformation on four-coordinate iron(II) center.
    7. 7
      (a) Holm, R. H.; Chakravorty, A.; Theriot, L. J. The Synthesis, Structures, and Solution Equilibria of Bis(pyrrole-2-aldimino)metal(II) Complexes. Inorg. Chem. 1966, 5, 625– 635,  DOI: 10.1021/ic50038a028
      7a
      The synthesis, structures, and solution equilibria of bis-(pyrrole-2-aldimino)metal(II) complexes
      Holm, R. H.; Chakravorty, A.; Theriot, L. J.
      Inorganic Chemistry (1966), 5 (4), 625-35CODEN: INOCAJ; ISSN:0020-1669.
      Synthesis of an extensive series of bis(pyrrole-2-aldimino)metal(II) complexes with M(II) = Co, Ni, Pd, Cu, and Zn and various alkyl groups (R) appended to the azomethine N was effected by a nonaq. chelation reaction in tetrahydrofuran. Preliminary single crystal x-ray results for complexes with R = tert-Bu reveal that Co, Ni, and Zn complexes are isomorphous, but appreciable differences in the cell consts. of the Ni complex indicate that it is not truly isostructural with the tetrahedral Co and Zn complexes. The Cu complex exists in 2 cryst. modifications, neither of which is isomorphous with the Co-Ni-Zn series. Spectral and magnetic studies in soln. show that the tert-Bu Co complex is tetrahedral whereas the corresponding Cu complex is distorted from planarity to an unknown extent. Cu complexes with less bulky R groups are planar. The tert-Bu Ni complex is pseudo-tetrahedral; complexes with sec-alkyl groups such as iso-Pr are involved in a configurational equil. between planar and pseudo-tetrahedral forms. The paramagnetic Ni complexes show large isotropic proton hyperfine contact shifts. Spin d. calcns. for the coordinated ligand system are used as the basis of proton resonance assignments. It is concluded that in the pseudo-tetrahedral form spin imbalance exists in the highest filled ligand π-mol. orbital, and that, in addn., there is an underlying spin imbalance in the highest filled σ-mol. orbital, the result of which is observable in the proton resonance spectra. Thermodynamic parameters characterizing the structural change were obtained for the Ni complexes from the temp. dependence of the proton contact shifts. A quant. comparison of the stabilization of tetrahedral Ni(II) by pyrrole-2-aldimine, salicylaldimine, and β-keto amine ligand systems is presented.
      (b) Wolny, J. A.; Rudolf, M. F.; Ciunik, Z.; Gatner, K.; Wołowiec, S. Cobalt(II) triazene 1-oxide bis(chelates). A case of planar (low spin)–tetrahedral (high spin) isomerism. J. Chem. Soc., Dalton Trans. 1993, 1611– 1622,  DOI: 10.1039/DT9930001611
      7b
      Cobalt(II) triazene 1-oxide bis(chelates). A case of planar (low spin)-tetrahedral (high spin) isomerism
      Wolny, Juliusz A.; Rudolf, Mikolaj F.; Ciunik, Zbigniew; Gatner, Kazimierz; Wolowiec, Stanislaw
      Journal of the Chemical Society, Dalton Transactions: Inorganic Chemistry (1972-1999) (1993), (10), 1611-22CODEN: JCDTBI; ISSN:0300-9246.
      High- and low-spin [CoL2] (HL = 3-phenyl-1-triazene 1-oxide derivs.) were prepd. and isolated. The low-spin complexes possess square-planar structure and the high-spin complexes are tetrahedral. The mol. structure of high-spin [CoL2] [HL = 3-(4-methylphenyl)-1-methyl-1-triazene 1-oxide] was detd.; triclinic, space group P‾1, a 7.970(5), b 10.174(5), c 11.676(5) Å, α 87.18(4), β 74.31(4), γ 74.06(4)°, Z = 2, R = 0.041. For some complexes the isolation of both planar (low-spin) and tetrahedral (high-spin) isomers or of their conglomerates is possible depending on the synthesis conditions. The crystal structure of square-planar [Ni(ON(Me)NNC6H4Me-4)2], which is isomorphous with the low-spin isomer of [Co(ON(Me)NNC6H4Me-4)2], was detd.: triclinic, space group P‾1, a 7.495(2), b 7.694(5), c 8.612(3) Å, α 64.64(5), β 87.84(2), γ 78.64(3)°, Z = 1, R = 0.0271. The complexes exhibit a planar-tetrahedral equil. in noncoordinating solvents, the ΔHΘ and ΔSΘ values of which, detd. from soln. magnetic susceptibility measurements, at 1-15 kJ mol-1 and 5-30 J K-1 mol-1, resp. The electrochem. properties of the complexes are given.
      (c) Ingleson, M. J.; Pink, M.; Fan, H.; Caulton, K. G. Exploring the reactivity of four-coordinate PNPCoX with access to three-coordinate spin triplet PNPCo. Inorg. Chem. 2007, 46, 10321– 10334,  DOI: 10.1021/ic701171p
      7c
      Exploring the reactivity of four-coordinate PNPCoX with access to three-coordinate spin triplet PNPCo
      Ingleson, Michael J.; Pink, Maren; Fan, Hongjun; Caulton, Kenneth G.
      Inorganic Chemistry (2007), 46 (24), 10321-10334CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      (PNP)CoX, where PNP is (tBu2PCH2SiMe2)2N- and X is Cl, I, N3, OAr, OSO2CF3, and N(H)Ar, are reported. Some of these show magnetic susceptibility, color, and 1H NMR evidence of being in equil. between a blue, tetrahedral S = 3/2 state and a red, planar S = 1/2 state; the equil. populations are influenced by subtle solvent effects (e.g., benzene and cyclohexane are different), as well as by temp. Attempted oxidn. to Co(III) with O2 occurs instead at phosphorus, giving [P(O)NP(O)]CoX species. The single O-atom transfer reagent PhI:O likewise oxidizes P. Even I2 oxidizes P to give the pendant phosphonium species (tBu2P(I)CH2SiMe2NSiMe2CH2PtBu2)CoI2 with a tetrahedral S = 3/2 cobalt; the solid-state structure shows intermol. PI···ICo interactions. Attempted alkyl metathesis of PNPCoX inevitably results in redn., forming PNPCo, which is a spin triplet with planar T-shaped coordination geometry with no agostic interaction. Triplet PNPCo binds N2(weakly) and CO (whose low CO stretching frequency indicates strong PNP → Co donor power), but not ethene or MeCCMe.
    8. 8
      Gaazo, J. Plasticity of the coordination sphere of copper(II) complexes, its manifestation and causes. Coord. Chem. Rev. 1976, 19, 253– 297,  DOI: 10.1016/S0010-8545(00)80317-3
      There is no corresponding record for this reference.
    9. 9
      (a) Cambi, L.; Szegö, L. Über die magnetische Susceptibilität der komplexen Verbindungen. Ber. Dtsch. Chem. Ges. B 1931, 64, 2591– 2598,  DOI: 10.1002/cber.19310641002
      There is no corresponding record for this reference.
      (b) Kahn, O.; Martinez, C. J. Spin-Transition Polymers: From Molecular Materials Toward Memory Devices. Science 1998, 279, 44– 48,  DOI: 10.1126/science.279.5347.44
      9b
      Spin-transition polymers: from molecular materials toward memory devices
      Kahn, O.; Martinez, C. Jay
      Science (Washington, D. C.) (1998), 279 (5347), 44-48CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      A review with 53 refs. Some 3dn (4 ≤ n ≤ 7) transition metal compds. exhibit a cooperative transition between a low-spin (LS) and a high-spin (HS) state. This transition is abrupt and occurs with a thermal hysteresis, which confers a memory effect on the system. The intersite interactions and thus the cooperativity are magnified in polymeric compds. such as [Fe(Rtrz)3]A2·nH2O in which the Fe2+ ions are triply bridged by 4-R-substituted-1,2,4-triazole mols. Moreover, in these compds., the spin transition is accompanied by a well-pronounced change of color between violet in the LS state and white in the HS state. The transition temps. of these materials can be fine tuned, using an approach based on the concept of a mol. alloy. In particular, it is possible to design a compd. for which room temp. falls in the middle of the thermal hysteresis loop. These materials have many potential applications, for example, as temp. sensors, as active elements of various types of displays, and in information storage and retrieval.
      (c) Gütlich, P.; Garcia, Y.; Goodwin, H. A. Spin crossover phenomena in Fe(ii) complexes. Chem. Soc. Rev. 2000, 29, 419– 427,  DOI: 10.1039/b003504l
      9c
      Spin crossover phenomena in Fe(II) complexes
      Gutlich, Philipp; Garcia, Yann; Goodwin, Harold A.
      Chemical Society Reviews (2000), 29 (6), 419-427CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
      A review with 58 refs. The behavior of spin crossover compds. is among the most striking and fascinating shown by relatively simple mol. species. This review aims to draw attention to the various ways in which spin crossover phenomena are manifested in Fe(II) complexes, to offer some rationalization for these, and to highlight their possible applications. Typical examples have been selected along with more recent ones to give an overall view of the scope and development of the area. The article is structured to provide the basic material for those who wish to enter the field of spin crossover.
      (d) Sato, O.; Tao, J.; Zhang, Y. Z. Control of magnetic properties through external stimuli. Angew. Chem., Int. Ed. 2007, 46, 2152– 2187,  DOI: 10.1002/anie.200602205
      9d
      Control of magnetic properties through external stimuli
      Sato, Osamu; Tao, Jun; Zhang, Yuan-Zhu
      Angewandte Chemie, International Edition (2007), 46 (13), 2152-2187CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A review. The magnetic properties of many magnetic materials can be controlled by external stimuli. The principal focus here is on the thermal, photochem., electrochem., and chem. of phase transitions that involve changes in magnetization. Mol. compds. described herein range from metal complexes through pure org. compds. to composite materials. Most of the review is devoted to the properties. valence-tautomeric compds., mol. magnets, and spin-crossover complexes, which could find future applications in memory devices or optical switches.
      (e) Bousseksou, A.; Molnar, G.; Salmon, L.; Nicolazzi, W. Molecular spin crossover phenomenon: recent achievements and prospects. Chem. Soc. Rev. 2011, 40, 3313– 3335,  DOI: 10.1039/c1cs15042a
      9e
      Molecular spin crossover phenomenon: Recent achievements and prospects
      Bousseksou, Azzedine; Molnar, Gabor; Salmon, Lionel; Nicolazzi, William
      Chemical Society Reviews (2011), 40 (6), 3313-3335CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
      A review. Recently we assisted a strong renewed interest in the fascinating field of mol. spin crossover complexes by (1) the emergence of nanosized spin crossover materials through direct synthesis of coordination nanoparticles and nanopatterned thin films as well as by (2) the use of novel sophisticated high spatial and temporal resoln. exptl. techniques and theor. approaches for the study of spatiotemporal phenomena in cooperative spin crossover systems. Besides generating new fundamental knowledge on size-redn. effects and the dynamics of the spin crossover phenomenon, this research aims also at the development of practical applications such as sensor, display, information storage and nanophotonic devices. In this crit. review, we discuss recent work in the field of mol.-based spin crossover materials with a special focus on these emerging issues, including chem. synthesis, phys. properties and theor. aspects as well (223 refs.).
    10. 10
      Halcrow, M. A. The spin-states and spin-transitions of mononuclear iron(II) complexes of nitrogen-donor ligands. Polyhedron 2007, 26, 3523– 3576,  DOI: 10.1016/j.poly.2007.03.033
      10
      The spin-states and spin-transitions of mononuclear iron(II) complexes of nitrogen-donor ligands
      Halcrow, Malcolm A.
      Polyhedron (2007), 26 (14), 3523-3576CODEN: PLYHDE; ISSN:0277-5387. (Elsevier B.V.)
      A review of mononuclear iron(II) complexes with heterocyclic N-donor ligation is presented. A brief introduction to spin-crossover chem. and low-temp. spin-trapping is provided, since many of these compds. undergo thermal spin-transitions upon cooling or heating. These are highlighted, and the structural changes underlying spin-crossover are discussed where this is known. Materials showing spin-trapping behavior following thermal quenching or irradn. at very low temps. are also described.
    11. 11
      Bowman, A. C.; Milsmann, C.; Bill, E.; Turner, Z. R.; Lobkovsky, E.; DeBeer, S.; Wieghardt, K.; Chirik, P. J. Synthesis and Electronic Structure Determination of N-Alkyl-Substituted Bis(imino)pyridine Iron Imides Exhibiting Spin Crossover Behavior. J. Am. Chem. Soc. 2011, 133, 17353– 17369,  DOI: 10.1021/ja205736m
      11
      Synthesis and Electronic Structure Determination of N-Alkyl-Substituted Bis(imino)pyridine Iron Imides Exhibiting Spin Crossover Behavior
      Bowman, Amanda C.; Milsmann, Carsten; Bill, Eckhard; Turner, Zoe R.; Lobkovsky, Emil; DeBeer, Serena; Wieghardt, Karl; Chirik, Paul J.
      Journal of the American Chemical Society (2011), 133 (43), 17353-17369CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Three new N-alkyl substituted bis(imino)pyridine iron imide complexes, (iPrPDI)FeNR (iPrPDI = 2,6-(2,6-iPr2-C6H3-N:CMe)2C5H3N; R = 1-adamantyl (1Ad), cyclooctyl (CyOct), and 2-adamantyl (2Ad)) were synthesized by addn. of the appropriate alkyl azide to the iron bis(dinitrogen) complex, (iPrPDI)Fe(N2)2. SQUID magnetic measurements on the isomeric iron imides, (iPrPDI)FeN1Ad and (iPrPDI)FeN2Ad, established spin crossover behavior with the latter example having a more complete spin transition in the exptl. accessible temp. range. X-ray diffraction on all three alkyl-substituted bis(imino)pyridine iron imides established essentially planar compds. with relatively short Fe-Nimide bond lengths and two-electron redn. of the redox-active bis(imino)pyridine chelate. Zero- and applied-field Mossbauer spectroscopic measurements indicate diamagnetic ground states at cryogenic temps. and established low isomer shifts consistent with highly covalent mols. For (iPrPDI)FeN2Ad, Mossbauer spectroscopy also supports spin crossover behavior and allowed extn. of thermodn. parameters for the S = 0 to S = 1 transition. X-ray absorption spectroscopy and computational studies were also performed to explore the electronic structure of the bis(imino)pyridine alkyl-substituted imides. An electronic structure description with a low spin ferric center (S = 1/2) antiferromagnetically coupled to an imidyl radical (Simide = 1/2) and a closed-shell, dianionic bis(imino)pyridine chelate (SPDI = 0) is favored for the S = 0 state. An iron-centered spin transition to an intermediate spin ferric ion (SFe = 3/2) accounts for the S = 1 state obsd. at higher temps. Other possibilities based on the computational and exptl. data are also evaluated and compared to the electronic structure of the bis(imino)pyridine iron N-aryl imide counterparts.
    12. 12
      (a) Scepaniak, J. J.; Harris, T. D.; Vogel, C. S.; Sutter, J.; Meyer, K.; Smith, J. M. Spin Crossover in a Four-Coordinate Iron(II) Complex. J. Am. Chem. Soc. 2011, 133, 3824– 3827,  DOI: 10.1021/ja2003473
      12a
      Spin Crossover in a Four-Coordinate Iron(II) Complex
      Scepaniak, Jeremiah J.; Harris, T. David; Vogel, Carola S.; Sutter, Jorg; Meyer, Karsten; Smith, Jeremy M.
      Journal of the American Chemical Society (2011), 133 (11), 3824-3827CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      The four-coordinate iron(II) phosphoraniminato complex PhB(MesIm)3Fe-N=PPh3 undergoes an S = 0 to S = 2 spin transition with TC = 81 K, as detd. by variable-temp. magnetic measurements and Mossbauer spectroscopy. Variable-temp. single-crystal X-ray diffraction revealed that the S = 0 to S = 2 transition is assocd. with an increase in the Fe-C and Fe-N bond distances and a decrease in the N-P bond distance. These structural changes have been interpreted in terms of electronic structure theory.
      (b) Mathoniere, C.; Lin, H. J.; Siretanu, D.; Clerac, R.; Smith, J. M. Photoinduced single-molecule magnet properties in a four-coordinate iron(II) spin crossover complex. J. Am. Chem. Soc. 2013, 135, 19083– 19086,  DOI: 10.1021/ja410643s
      12b
      Photoinduced Single-Molecule Magnet Properties in a Four-Coordinate Iron(II) Spin Crossover Complex
      Mathoniere, Corine; Lin, Hsiu-Jung; Siretanu, Diana; Clerac, Rodolphe; Smith, Jeremy M.
      Journal of the American Chemical Society (2013), 135 (51), 19083-19086CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      The four-coordinate Fe-(II) complex, PhB(MesIm)3FeNPPh3 (1) was previously reported to undergo a thermal spin-crossover (SCO) between high-spin (HS, S = 2) and low-spin (LS, S = 0) states. This complex is photoactive <20 K, undergoing a photoinduced LS to HS spin state change, as detd. by optical reflectivity and photomagnetic measurements. With continuous white light irradn., 1 displays slow relaxation of the magnetization, i.e. single-mol. magnet (SMM) properties, at temps. <5 K. This complex provides a structural template for the design of new photoinduced mononuclear SMMs based on the SCO phenomenon.
      (c) Lin, H. J.; Siretanu, D.; Dickie, D. A.; Subedi, D.; Scepaniak, J. J.; Mitcov, D.; Clerac, R.; Smith, J. M. Steric and electronic control of the spin state in three-fold symmetric, four-coordinate iron(II) complexes. J. Am. Chem. Soc. 2014, 136, 13326– 13332,  DOI: 10.1021/ja506425a
      12c
      Steric and Electronic Control of the Spin State in Three-Fold Symmetric, Four-Coordinate Iron(II) Complexes
      Lin, Hsiu-Jung; Siretanu, Diana; Dickie, Diane A.; Subedi, Deepak; Scepaniak, Jeremiah J.; Mitcov, Dmitri; Clerac, Rodolphe; Smith, Jeremy M.
      Journal of the American Chemical Society (2014), 136 (38), 13326-13332CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      The 3-fold sym., four-coordinate Fe(II) phosphoraminimato complexes PhB(MesIm)3Fe-N=PRR'R'' (PRR'R'' = PMePh2, PMe2Ph, PMe3, and PPr3) undergo a thermally induced S = 0 to S = 2 spin-crossover in fluid soln. Smaller phosphoraminimato ligands stabilize the low-spin state, and an excellent correlation is obsd. between the characteristic temp. of the spin-crossover (T1/2) and the Tolman cone angle (θ). Complexes with para-substituted triarylphosphoraminimato ligands (p-XC6H4)3P=N- (X = H, Me and OMe) also undergo spin-crossover in soln. These isosteric phosphoraminimato ligands reveal that the low-spin state is stabilized by more strongly donating ligands. This control over the spin state provides important insights for modulating the magnetic properties of four-coordinate Fe(II) complexes.
    13. 13
      Creutz, S. E.; Peters, J. C. Spin-State Tuning at Pseudo-tetrahedral d6 Ions: Spin Crossover in [BP3]FeII–X Complexes. Inorg. Chem. 2016, 55, 3894– 3906,  DOI: 10.1021/acs.inorgchem.6b00066
      13
      Spin-State Tuning at Pseudo-tetrahedral d6 Ions: Spin Crossover in [BP3]FeII-X Complexes
      Creutz, Sidney E.; Peters, Jonas C.
      Inorganic Chemistry (2016), 55 (8), 3894-3906CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      Low-coordinate transition-metal complexes that undergo spin crossover remain rare. We report here a series of four-coordinate, pseudo-tetrahedral P3FeII-X complexes supported by tris(phosphine)borate P3 ([PhBPR3]-) and phosphiniminato X-type ligands (-N=PR'3) that, in combination, tune the spin-crossover behavior of the system. Most of the reported iron complexes undergo spin crossover at temps. near or above room temp. in soln. and in the solid state. The change in spin state coincides with a significant change in the degree of π-bonding between Fe and the bound N atom of the phosphiniminato ligand. Spin crossover is accompanied by striking changes in the UV-visible (UV-vis) absorption spectra, which allows for quant. modeling of the thermodn. parameters of the spin equil. These spin equil. have also been studied by numerous techniques including paramagnetic NMR (NMR), IR, and M.ovrddot.ossbauer spectroscopies; X-ray crystallog.; and solid-state superconducting quantum interference device (SQUID) magnetometry. These studies allow qual. correlations to be made between the steric and electronic properties of the ligand substituents and the enthalpy and entropy changes assocd. with the spin equil.
    14. 14
      (a) Travieso-Puente, R.; Broekman, J. O. P.; Chang, M.-C.; Demeshko, S.; Meyer, F.; Otten, E. Spin-Crossover in a Pseudo-tetrahedral Bis(formazanate) Iron Complex. J. Am. Chem. Soc. 2016, 138, 5503– 5506,  DOI: 10.1021/jacs.6b01552
      14a
      Spin-Crossover in a Pseudo-tetrahedral Bis(formazanate) Iron Complex
      Travieso-Puente, Raquel; Broekman, J. O. P.; Chang, Mu-Chieh; Demeshko, Serhiy; Meyer, Franc; Otten, Edwin
      Journal of the American Chemical Society (2016), 138 (17), 5503-5506CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Spin-crossover in a pseudo-tetrahedral bis(formazanate) iron(II) complex (1) is described. Structural, magnetic, and spectroscopic analyses indicate that this compd. undergoes thermal switching between an S = 0 and an S = 2 state, which is very rare in four-coordinate complexes. The transition to the high-spin state is accompanied by an increase in Fe-N bond lengths and a concomitant contraction of intraligand N-N bonds. The latter suggests that stabilization of the low-spin state is due to the π-acceptor properties of the ligand. One-electron redn. of 1 leads to the formation of the corresponding anion, which contains a low-spin (S = 1/2) Fe(I) center. The findings are rationalized by electronic structure calcns. using d. functional theory.
      (b) Milocco, F.; de Vries, F.; Bartels, I. M. A.; Havenith, R. W. A.; Cirera, J.; Demeshko, S.; Meyer, F.; Otten, E. Electronic Control of Spin-Crossover Properties in Four-Coordinate Bis(formazanate) Iron(II) Complexes. J. Am. Chem. Soc. 2020, 142, 20170– 20181,  DOI: 10.1021/jacs.0c10010
      14b
      Electronic Control of Spin-Crossover Properties in Four-Coordinate Bis(formazanate) Iron(II) Complexes
      Milocco, Francesca; de Vries, Folkert; Bartels, Imke M. A.; Havenith, Remco W. A.; Cirera, Jordi; Demeshko, Serhiy; Meyer, Franc; Otten, Edwin
      Journal of the American Chemical Society (2020), 142 (47), 20170-20181CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      The transition between spin states in d-block metal complexes has important ramifications for their structure and reactivity, with applications ranging from information storage materials to understanding catalytic activity of metalloenzymes. Tuning the ligand field (ΔO) by steric and/or electronic effects has provided spin-crossover compds. for several transition metals in the periodic table, but this has mostly been limited to coordinatively satd. metal centers in octahedral ligand environments. Spin-crossover complexes with low coordination nos. are much rarer. Here we report a series of four-coordinate, (pseudo)tetrahedral Fe(II) complexes with formazanate ligands and demonstrate how electronic substituent effects can be used to modulate the thermally induced transition between S = 0 and S = 2 spin states in soln. All six compds. undergo spin-crossover in soln. with T1/2 above room temp. (300-368 K). While structural anal. by X-ray crystallog. shows that the majority of these compds. are low-spin in the solid state (and remain unchanged upon heating), we find that packing effects can override this preference and give rise to either rigorously high-spin (6) or gradual spin-crossover behavior (5) also in the solid state. D. functional theory calcns. are used to delineate the empirical trends in soln. spin-crossover thermodn. In all cases, the stabilization of the low-spin state is due to the π-acceptor properties of the formazanate ligand, resulting in an "inverted" ligand field, with an approx. "two-over-three" splitting of the d-orbitals and a high degree of metal-ligand covalency due to metal → ligand π-backdonation. The computational data indicate that the electronic nature of the para-substituent has a different influence depending on whether it is present at the C-Ar or N-Ar rings, which is ascribed to the opposing effect on metal-ligand σ- and π-bonding.
    15. 15
      (a) Feltham, H. L. C.; Barltrop, A. S.; Brooker, S. Spin crossover in iron(II) complexes of 3,4,5-tri-substituted-1,2,4-triazole (Rdpt), 3,5-di-substituted-1,2,4-triazolate (dpt – ), and related ligands. Coord. Chem. Rev. 2017, 344, 26– 53,  DOI: 10.1016/j.ccr.2016.10.006
      15a
      Spin crossover in iron(II) complexes of 3,4,5-tri-substituted-1,2,4-triazole (Rdpt), 3,5-di-substituted-1,2,4-triazolate (dpt-), and related ligands
      Feltham, Humphrey L. C.; Barltrop, Alexis S.; Brooker, Sally
      Coordination Chemistry Reviews (2017), 344 (), 26-53CODEN: CCHRAM; ISSN:0010-8545. (Elsevier B.V.)
      A review. A general introduction to spin crossover involving iron(II) is provided. Then the 26 examples of 3,4,5-tri-substituted-1,2,4-triazole (mostly Rdpt and Rppt) ligands and the 11 examples of 3,5-di-substituted-1,2,4-triazole (including Hdpt) ligands that are of interest to this review are introduced. The 26 examples of 3,4,5-tri-substituted-1,2,4-triazole ligands fall into main two categories: (a) the 13 examples of sym. 4-substituted 3,5-di(2-pyridyl)triazole (Rdpt) ligands and (b) the 13 ligands closely related to the Rdpt ligands but featuring Ph (known as Rppt ligands), pyrazine or pyrimidine rings in place of one of more of the pyridine rings. In contrast, of the 11 examples of 3,5-di-substituted-1,2,4-triazole (HL, including Hdpt) ligands, only 2 are sym. (3,5-di(2-pyridyl)triazole and 3,5-di(2-pyrazyl)triazole ligands); the other 9 are non-sym. as two different rings are attached to the 3 and 5 positions of the triazole ring. In all but one example, in iron complexes of the 3,5-di-substituted-1,2,4-triazole ligands (HL, including Hdpt) the ligand deprotonates and coordinates as an anionic 3,5-di-substituted-1,2,4-triazolate (L-) ligand. This field was reviewed in 2008, and considered the 17 such complexes published up until 2007. Hence the present review provides a comprehensive update on the substantial progress that has been made in this field since then. Specifically it covers the 92 new iron(II) complexes of these 37 ligands, published since then and before 1 Dec. 2015. Of the 92 of new iron(II) complexes reported, 72 have been structurally characterized, and almost half of them, 40, are SCO-active. A wide range of nuclearities, from mono- to di- to tri- to penta- and poly-nuclear, as well as a range of binding modes, are obsd., so this review is organised into sections accordingly. In each section, after briefly summarizing the findings of the 2008 review, the new developments are detailed. Finally, the findings of this rich avenue of investigation into such ligands are summarized, indicating a bright future ahead.
      (b) Constable, E. C.; Baum, G.; Bill, E.; Dyson, R.; van Eldik, R.; Fenske, D.; Kaderli, S.; Morris, D.; Neubrand, A.; Neuburger, M.; Smith, D. R.; Wieghardt, K.; Zehnder, M.; Zuberbühler, A. D. Control of Iron(II) Spin States in 2,2′:6′,2″-Terpyridine Complexes through Ligand Substitution. Chem. - Eur. J. 1999, 5, 498– 508,  DOI: 10.1002/(SICI)1521-3765(19990201)5:2<498::AID-CHEM498>3.0.CO;2-V
      15b
      Control of iron(II) spin states in 2,2':6',2''-terpyridine complexes through ligand substitution
      Constable, Edwin C.; Baum, Gerhard; Bill, Eckhard; Dyson, Raylene; Van Eldik, Rudi; Fenske, Dieter; Kaderli, Susan; Morris, Darrell; Neubrand, Anton; Neuburger, Markus; Smith, Diane R.; Wieghardt, Karl; Zehnder, Margareta; Zuberbuhler, Andreas D.
      Chemistry - A European Journal (1999), 5 (2), 498-508CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH)
      6- And 6,6''-aryl-substituted 2,2':6',2''-terpyridine ligands and their Fe(II) complexes were prepd. The introduction of Ph substituents at both the 6- and 6'' - positions leads exclusively to the formation of orange high-spin Fe(II) complexes while the presence of a single 6-Ph substituent results in spin-crossover systems. [Fe(2)2]X2 (2 = 4,6-diphenyl-2,2':6',2''-terpyridine; X = ClO4 or PF6) were studied in detail, and the solid-state x-ray structures of both the low- and high-spin forms are reported. Mossbauer spectroscopic and magnetic susceptibility measurements are reported, and the temp. and pressure dependence of the high-spin/low-spin transition were studied. An x-ray structural study of [Fe(2)2](ClO4)2 is also reported; this complex is highly distorted with two very long Fe···N contacts of over 2.4 Å and is best regarded as a four-coordinate Fe complex.
    16. 16
      (a) Bacchi, S.; Benaglia, M.; Cozzi, F.; Demartin, F.; Filippini, G.; Gavezzotti, A. X-ray Diffraction and Theoretical Studies for the Quantitative Assessment of Intermolecular Arene–Perfluoroarene Stacking Interactions. Chem. - Eur. J. 2006, 12, 3538– 3546,  DOI: 10.1002/chem.200501248
      17a
      X-ray diffraction and theoretical studies for the quantitative assessment of intermolecular arene-perfluoroarene stacking interactions
      Bacchi, Sergio; Benaglia, Maurizio; Cozzi, Franco; Demartin, Francesco; Filippini, Giuseppe; Gavezzotti, Angelo
      Chemistry - A European Journal (2006), 12 (13), 3538-3546CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The arene-perfluoroarene stacking interaction was studied by exptl. and theor. methods. Compds. with different possibilities for formation of this recognition motif in the solid state were synthesized, and their crystal structures detd. by single-crystal x-ray diffraction. The crystal packing of these compds., as well as the packing of related compds. retrieved from crystallog. databases, were analyzed with quant. crystal potentials: total lattice energies and the cohesive energies of closest mol. pairs in the crystals were calcd. The arene-perfluoroarene recognition motif emerges as a dominant interaction in the nonhydrogen-bonding compds. studied here, to the point that asym. dimers formed over the stacking motif carry over to asym. units made of two mols. in the crystal both for pure compds. and for mol. complexes; however, inter-ring distances and angles range from 3.70 to 4.85 Å and from 5 to 21°, resp. Pixel energy partitioning reveals that whenever arom. rings stack, the largest cohesive energy contribution comes from dispersion, which roughly amts. to 20 kJ mol-1 per Ph ring, while the coulombic term is minor but significant enough to make a difference between the arene-arene or perfluoroarene-perfluoroarene interactions on the one hand, and arene-perfluoroarene interactions on the other, whereby the latter are favored by ∼10 kJ mol-1 per Ph ring. No evidence of special interaction which can be attributed to H···F confrontation was recognizable.
      (b) Salonen, L. M.; Ellermann, M.; Diederich, F. Aromatic rings in chemical and biological recognition: energetics and structures. Angew. Chem., Int. Ed. 2011, 50, 4808– 4842,  DOI: 10.1002/anie.201007560
      17b
      Aromatic rings in chemical and biological recognition: energetics and structures
      Salonen, Laura M.; Ellermann, Manuel; Diederich, Francois
      Angewandte Chemie, International Edition (2011), 50 (21), 4808-4842CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A review. This review describes a multidimensional treatment of mol. recognition phenomena involving arom. rings in chem. and biol. systems. It summarizes new results reported since the appearance of an earlier review in 2003 in host-guest chem., biol. affinity assays and biostructural anal., data base mining in the Cambridge Structural Database (CSD) and the Protein Data Bank (PDB), and advanced computational studies. Topics addressed are arene-arene, perfluoroarene-arene, S···arom., cation-π, and anion-π interactions, as well as hydrogen bonding to π systems. The generated knowledge benefits, in particular, structure-based hit-to-lead development and lead optimization both in the pharmaceutical and in the crop protection industry. It equally facilitates the development of new advanced materials and supramol. systems, and should inspire further utilization of interactions with arom. rings to control the stereochem. outcome of synthetic transformations.
      (c) Wheeler, S. E. Local Nature of Substituent Effects in Stacking Interactions. J. Am. Chem. Soc. 2011, 133, 10262– 10274,  DOI: 10.1021/ja202932e
      17c
      Local Nature of Substituent Effects in Stacking Interactions
      Wheeler, Steven E.
      Journal of the American Chemical Society (2011), 133 (26), 10262-10274CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Popular explanations of substituent effects in π-stacking interactions hinge upon substituent-induced changes in the aryl π-system. This entrenched view has been used to explain substituent effects in countless stacking interactions over the past 2 decades. However, for a broad range of stacked dimers, it is shown that substituent effects are better described as arising from local, direct interactions of the substituent with the proximal vertex of the other ring. Consequently, substituent effects in stacking interactions are additive, regardless of whether the substituents are on the same or opposite rings. Substituent effects are also insensitive to the introduction of heteroatoms on distant parts of either stacked ring. This local, direct interaction viewpoint provides clear, unambiguous explanations of substituent effects for myriad stacking interactions that are in accord with robust computational data, including DFT-D and new benchmark CCSD(T) results. Many of these computational results cannot be readily explained using traditional π-polarization-based models. Analyses of stacking interactions based solely on the sign of the electrostatic potential above the face of an arom. ring or the mol. quadrupole moment face a similar fate. The local, direct interaction model provides a simple means of analyzing substituent effects in complex arom. systems and also offers simple explanations of the crystal packing of fluorinated benzenes and the recently published dependence of the stability of protein-RNA complexes on the regiochem. of fluorinated base analogs [J. Am. Chem. Soc.2011, 133, 3687-3689].
    17. 17
      Gütlich, P. Fifty Years of Mössbauer Spectroscopy in Solid State Research - Remarkable Achievements, Future Perspectives. Z. Anorg. Allg. Chem. 2012, 638, 15– 43,  DOI: 10.1002/zaac.201100416
      18
      Fifty Years of Moessbauer Spectroscopy in Solid State Research - Remarkable Achievements, Future Perspectives
      Guetlich, Philipp
      Zeitschrift fuer Anorganische und Allgemeine Chemie (2012), 638 (1), 15-43CODEN: ZAACAB; ISSN:0044-2313. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Moessbauer spectroscopy was founded more than fifty years ago based on an outstanding discovery by the young German physicist Rudolf Ludwig Moessbauer while working on his Ph.D. thesis. He discovered the recoilless nuclear resonance fluorescence of gamma radiation and was awarded the Nobel Prize in Physics in 1961 as one of the youngest recipients of this most prestigious award. His discovery led to the development of a new technique for measurements of hyperfine interactions between nuclear moments and electromagnetic fields. This method, with highest sharpness of tuning of 10-13, yields information on valence state, symmetry, magnetic behavior, phase transition, lattice dynamics and other solid state properties.
    18. 18
      Milocco, F.; Demeshko, S.; Meyer, F.; Otten, E. Ferrate(II) complexes with redox-active formazanate ligands. Dalton Trans. 2018, 47, 8817– 8823,  DOI: 10.1039/C8DT01597J
      19
      Ferrate(II) complexes with redox-active formazanate ligands
      Milocco, Francesca; Demeshko, Serhiy; Meyer, Franc; Otten, Edwin
      Dalton Transactions (2018), 47 (26), 8817-8823CODEN: DTARAF; ISSN:1477-9226. (Royal Society of Chemistry)
      The synthesis of mono(formazanate) Fe complexes is described. In the presence of Bu4N halides, salt metathesis reactions afford the ferrate(II) complexes [Bu4N][LFeX2] (L = PhNNC(p-tol)NNPh; X = Cl, Br) in good yield, and the products were characterized. The high-spin ferrate(II) complexes show cyclic voltammograms that are consistent with reversible, ligand-based 1-electron redn. The halides in these ferrate(II) compds. are labile, and are displaced by 4-methoxyphenyl isocyanide (4 equiv) as evidenced by formation of the low-spin, cationic octahedral complex [LFe(CNC6H4(p-OMe))4][Br]. Thus, a straightforward route to mono(formazanate) Fe(II) complexes was established.
    19. 19
      Lepori, C.; Guillot, R.; Hannedouche, J. C1-symmetric β-Diketiminatoiron(II) Complexes for Hydroamination of Primary Alkenylamines. Adv. Synth. Catal. 2019, 361, 714– 719,  DOI: 10.1002/adsc.201801464
      20
      C1-symmetric β-Diketiminatoiron(II) Complexes for Hydroamination of Primary Alkenylamines
      Lepori, Clement; Guillot, Regis; Hannedouche, Jerome
      Advanced Synthesis & Catalysis (2019), 361 (4), 714-719CODEN: ASCAF7; ISSN:1615-4150. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The synthesis and solid-state characterization of an array of well-defined low-coordinate C1-sym. β-diketiminatoiron(II) alkyl complexes featuring steric and electronic variations on one of the N-aryl substituents of the β-diketiminate ligand scaffold are reported. All complexes display unique catalytic abilities of promoting the selective exo-cyclohydroamination of unprotected 2,2-diphenylpent-4-en-1-amine under mild reactions conditions. The incorporation of a potentially coordinative ortho-methoxy substituent on one of the N-aryl rings of the β-diketiminate skeleton, in conjunction with a more crowded 2,6-diisopropylphenyl group on the other, affords a far more active catalyst C1-sym. β-diketiminatoiron(II) alkyl complex than the authors' previously reported C2-sym. β-diketiminatoiron(II) alkyl complex (Ar1 = Ar2 = 2,4,6-(Me)3C6H2)/cyclopentylamine system. Comparative studies let the authors postulate that this superior activity of C1-sym. β-diketiminatoiron(II) alkyl complex , when compared with other complexes , is likely arising from steric effects and/or the coordinating ability of the ortho-methoxy substituent. The scope and limitations of this novel C1-sym. β-diketiminatoiron(II) alkyl complex are also presented.
    20. 20
      Weigend, F.; Ahlrichs, R. Balanced basis sets of split valence, triple zeta valence and quadruple zeta valence quality for H to Rn: Design and assessment of accuracy. Phys. Chem. Chem. Phys. 2005, 7, 3297– 3305,  DOI: 10.1039/b508541a
      21
      Balanced basis sets of split valence, triple zeta valence and quadruple zeta valence quality for H to Rn: Design and assessment of accuracy
      Weigend, Florian; Ahlrichs, Reinhart
      Physical Chemistry Chemical Physics (2005), 7 (18), 3297-3305CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      Gaussian basis sets of quadruple zeta valence quality for Rb-Rn are presented, as well as bases of split valence and triple zeta valence quality for H-Rn. The latter were obtained by (partly) modifying bases developed previously. A large set of more than 300 mols. representing (nearly) all elements-except lanthanides-in their common oxidn. states was used to assess the quality of the bases all across the periodic table. Quantities investigated were atomization energies, dipole moments and structure parameters for Hartree-Fock, d. functional theory and correlated methods, for which we had chosen Moller-Plesset perturbation theory as an example. Finally recommendations are given which type of basis set is used best for a certain level of theory and a desired quality of results.
    21. 21
      Becke, A. D. Density-functional exchange-energy approximation with correct asymptotic behavior. Phys. Rev. A 1988, 38, 3098– 3100,  DOI: 10.1103/PhysRevA.38.3098
      22
      Density-functional exchange-energy approximation with correct asymptotic behavior
      Becke, A. D.
      Physical Review A: Atomic, Molecular, and Optical Physics (1988), 38 (6), 3098-100CODEN: PLRAAN; ISSN:0556-2791.
      Current gradient-cor. d.-functional approxns. for the exchange energies of at. and mol. systems fail to reproduce the correct 1/r asymptotic behavior of the exchange-energy d. A gradient-cor. exchange-energy functional is given with the proper asymptotic limit. This functional, contg. only one parameter, fits the exact Hartree-Fock exchange energies of a wide variety of at. systems with remarkable accuracy, surpassing the performance of previous functionals contg. two parameters or more.
    22. 22
      (a) Staroverov, V. N.; Scuseria, G. E.; Tao, J.; Perdew, J. P. Comparative assessment of a new nonempirical density functional: Molecules and hydrogen-bonded complexes. J. Chem. Phys. 2003, 119, 12129– 12137,  DOI: 10.1063/1.1626543
      23a
      Comparative assessment of a new nonempirical density functional: Molecules and hydrogen-bonded complexes
      Staroverov, Viktor N.; Scuseria, Gustavo E.; Tao, Jianmin; Perdew, John P.
      Journal of Chemical Physics (2003), 119 (23), 12129-12137CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
      A comprehensive study is undertaken to assess the nonempirical meta-generalized gradient approxn. (MGGA) of Tao, Perdew, Staroverov, and Scuseria (TPSS) against 14 common exchange-correlation energy functionals. Principal results are presented in the form of statistical summaries of deviations from expt. for the G3/99 test set (223 enthalpies of formation, 86 ionization potentials, 58 electron affinities, 8 proton affinities) and three addnl. test sets involving 96 bond lengths, 82 harmonic vibrational frequencies, and 10 hydrogen-bonded complexes, all computed using the 6-311++G(3df,3pd) basis. The TPSS functional matches, or exceeds in accuracy all prior nonempirical constructions and, unlike semiempirical functionals, consistently provides a high-quality description of diverse systems and properties. The computational cost of self-consistent MGGA is comparable to that of ordinary GGA, and exact exchange (unavailable in some codes) is not required. A one-parameter global hybrid version of the TPSS functional is introduced and shown to give further improvement for most properties.
      (b) Tao, J.; Perdew, J. P.; Staroverov, V. N.; Scuseria, G. E. Climbing the Density Functional Ladder: Nonempirical Meta--Generalized Gradient Approximation Designed for Molecules and Solids. Phys. Rev. Lett. 2003, 91, 146401,  DOI: 10.1103/PhysRevLett.91.146401
      23b
      Climbing the Density Functional Ladder: Nonempirical Meta-Generalized Gradient Approximation Designed for Molecules and Solids
      Tao, Jianmin; Perdew, John P.; Staroverov, Viktor N.; Scuseria, Gustavo E.
      Physical Review Letters (2003), 91 (14), 146401/1-146401/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
      The electron d., its gradient, and the Kohn-Sham orbital kinetic energy d. are the local ingredients of a meta-generalized gradient approxn. (meta-GGA). We construct a meta-GGA d. functional for the exchange-correlation energy that satisfies exact constraints without empirical parameters. The exchange and correlation terms respect two paradigms: one- or two-electron densities and slowly varying densities, and so describe both mols. and solids with high accuracy, as shown by extensive numerical tests. This functional completes the third rung of "Jacob's ladder" of approxns., above the local spin d. and GGA rungs.
    23. 23
      Becke, A. D. Density-functional thermochemistry. III. The role of exact exchange. J. Chem. Phys. 1993, 98, 5648– 5652,  DOI: 10.1063/1.464913
      24
      Density-functional thermochemistry. III. The role of exact exchange
      Becke, Axel D.
      Journal of Chemical Physics (1993), 98 (7), 5648-52CODEN: JCPSA6; ISSN:0021-9606.
      Despite the remarkable thermochem. accuracy of Kohn-Sham d.-functional theories with gradient corrections for exchange-correlation, the author believes that further improvements are unlikely unless exact-exchange information is considered. Arguments to support this view are presented, and a semiempirical exchange-correlation functional (contg. local-spin-d., gradient, and exact-exchange terms) is tested for 56 atomization energies, 42 ionization potentials, 8 proton affinities, and 10 total at. energies of first- and second-row systems. This functional performs better than previous functionals with gradient corrections only, and fits expt. atomization energies with an impressively small av. abs. deviation of 2.4 kcal/mol.
    24. 24
      (a) Harvey, J. N. In Principles and Applications of Density Functional Theory in Inorganic Chemistry I; Springer: Berlin, 2004; pp 151– 184.
      There is no corresponding record for this reference.
      (b) Swart, M.; Gruden, M. Spinning around in Transition-Metal Chemistry. Acc. Chem. Res. 2016, 49, 2690– 2697,  DOI: 10.1021/acs.accounts.6b00271
      25b
      Spinning around in Transition-Metal Chemistry
      Swart, Marcel; Gruden, Maja
      Accounts of Chemical Research (2016), 49 (12), 2690-2697CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
      A review. The great diversity and richness of transition metal chem., such as the features of an open d-shell, opened a way to numerous areas of scientific research and technol. applications. Depending on the nature of the metal and its environment, there are often several energetically accessible spin states, and the progress in accurate theor. treatment of this complicated phenomenon is presented in this Account. The spin state energetics of a transition metal complex can be predicted theor. on the basis of d. functional theory (DFT) or wave function based methodol., where DFT has advantages since it can be applied routinely to medium-to-large-sized mols. and spin-state consistent d. functionals are now available. Addnl. factors such as the effect of the basis set, thermochem. contributions, solvation, relativity, and dispersion, have been investigated by many researchers, but challenges in unambiguous assignment of spin states still remain. The first DFT studies showed intrinsic spin-state preferences of hybrid functionals for high spin and early generalized gradient approxn. functionals for low spin. Progress in the development of d. functional approxns. (DFAs) then led to a class of specially designed DFAs, such as OPBE, SSB-D, and S12g, and brought a very intriguing and fascinating observation that the spin states of transition metals and the SN2 barriers of org. mols. are somehow intimately linked. Among the many noteworthy results that emerged from the search for the appropriate description of the complicated spin state preferences in transition metals, we mainly focused on the examn. of the connection between the spin state and the structures or coordination modes of the transition metal complexes. Changes in spin states normally lead only to changes in the metal-ligand bond lengths, but to the best of our knowledge, the dapsox ligand showed the first example of a transition-metal complex where a change in spin state leads also to changes in the coordination, switching between pentagonal-bipyramidal and capped-octahedron. Moreover, we have summarized the results of the thorough study that cor. the exptl. assignment of the nature of the recently synthesized Sc3+ adduct of [FeIV(O)(TMC)]2+ (TMC = 1,4,8,11-tetramethylcyclam) and firmly established that the Sc3+-capped iron-oxygen complex corresponds to high-spin FeIII. Last, but not least, we have provided deeper insight and rationalization of the observation that unlike in metalloenzymes, where the FeIV-oxo is usually obsd. with high spin, biomimetic FeIV-oxo complexes typically have a intermediate spin state. Energy decompn. analyses on the trigonal-bypiramidal (TBP) and octahedral model systems with ammonia ligands have revealed that the interaction energy of the prepd. metal ion in the intermediate spin state is much smaller for the TBP structure. This sheds light on the origin of the intermediate spin state of the biomimetic TBP FeIV-oxo complexes.
    25. 25
      These orbital splitting energies are relatively small; spin-crossover complexes commonly have significantly larger values for Δ. See for example:Hauser, A. Ligand Field Theoretical Considerations. In Spin Crossover in Transition Metal Compounds I; Gütlich, P., Goodwin, H. A., Eds.; Springer Berlin Heidelberg: Berlin, Heidelberg, 2004, p 49.
      There is no corresponding record for this reference.
    26. 26
      (a) Knizia, G. Intrinsic Atomic Orbitals: An Unbiased Bridge between Quantum Theory and Chemical Concepts. J. Chem. Theory Comput. 2013, 9, 4834– 4843,  DOI: 10.1021/ct400687b
      27a
      Intrinsic Atomic Orbitals: An Unbiased Bridge between Quantum Theory and Chemical Concepts
      Knizia, Gerald
      Journal of Chemical Theory and Computation (2013), 9 (11), 4834-4843CODEN: JCTCCE; ISSN:1549-9618. (American Chemical Society)
      Modern quantum chem. can make quant. predictions on an immense array of chem. systems. However, the interpretation of those predictions is often complicated by the complex wave function expansions used. Here we show that an exceptionally simple algebraic construction allows for defining at. core and valence orbitals, polarized by the mol. environment, which can exactly represent SCF wave functions. This construction provides an unbiased and direct connection between quantum chem. and empirical chem. concepts, and can be used, for example, to calc. the nature of bonding in mols., in chem. terms, from first principles. In particular, we find consistency with electronegativities (χ), C 1s core-level shifts, resonance substituent parameters (σR), Lewis structures, and oxidn. states of transition-metal complexes.
      (b) Knizia, G.; Klein, J. E. M. N. Electron Flow in Reaction Mechanisms—Revealed from First Principles. Angew. Chem., Int. Ed. 2015, 54, 5518– 5522,  DOI: 10.1002/anie.201410637
      27b
      Electron Flow in Reaction Mechanisms-Revealed from First Principles
      Knizia, Gerald; Klein, Johannes E. M. N.
      Angewandte Chemie, International Edition (2015), 54 (18), 5518-5522CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The "curly arrow" of Robinson and Ingold is the primary tool for describing and rationalizing reaction mechanisms. Despite this approach's ubiquity and stellar success, its phys. basis has never been clarified and a direct connection to quantum chem. has never been found. Here we report that the bond rearrangements expressed by curly arrows can be directly obsd. in ab initio computations, as transformations of intrinsic bond orbitals (IBOs) along the reaction coordinate. Our results clarify that curly arrows are rooted in phys. reality-a notion which has been challenged before-and show how quantum chem. can directly establish reaction mechanisms in intuitive terms and unprecedented detail.
    27. 27
      Kamphuis, A. J.; Milocco, F.; Koiter, L.; Pescarmona, P. P.; Otten, E. Highly Selective Single-Component Formazanate Ferrate(II) Catalysts for the Conversion of CO2 into Cyclic Carbonates. ChemSusChem 2019, 12, 3635– 3641,  DOI: 10.1002/cssc.201900740
      28
      Highly Selective Single-Component Formazanate Ferrate(II) Catalysts for the Conversion of CO2 into Cyclic Carbonates
      Kamphuis, Aeilke J.; Milocco, Francesca; Koiter, Luuk; Pescarmona, Paolo P.; Otten, Edwin
      ChemSusChem (2019), 12 (15), 3635-3641CODEN: CHEMIZ; ISSN:1864-5631. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The development of new families of active and selective single-component catalysts based on earth-abundant metal is of interest from a sustainable chem. perspective. In this context, anionic mono(formazanate) Fe(II) complexes bearing labile halide ligands, which possess both Lewis acidic and nucleophilic functionalities, were developed as novel single-component homogeneous catalysts for the reaction of CO2 with epoxides to produce cyclic carbonates. The influence of the halide ligand and the electronic properties of the formazanate ligand backbone on the catalytic activity are studied by employing the Fe(II) complexes with and without an addnl. nucleophile. Very high selectivity is achieved towards the formation of the cyclic carbonate products from various terminal and internal epoxides without the need of a cocatalyst. Crystallog. data are given.
    28. 28
      Chang, M.-C.; Roewen, P.; Travieso-Puente, R.; Lutz, M.; Otten, E. Formazanate Ligands as Structurally Versatile, Redox-Active Analogues of β-Diketiminates in Zinc Chemistry. Inorg. Chem. 2015, 54, 379– 388,  DOI: 10.1021/ic5025873
      29
      Formazanate Ligands as Structurally Versatile, Redox-Active Analogues of β-Diketiminates in Zinc Chemistry
      Chang, Mu-Chieh; Roewen, Peter; Travieso-Puente, Raquel; Lutz, Martin; Otten, Edwin
      Inorganic Chemistry (2015), 54 (1), 379-388CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      A range of tetrahedral bis(formazanate)zinc complexes with different steric and electronic properties of the formazanate ligands were synthesized. The solid-state structures for several of these were detd. by x-ray crystallog., which showed that complexes with sym., unhindered ligands prefer coordination to the Zn center via the terminal N atoms of the NNCNN ligand backbone. Steric or electronic modifications can override this preference and give rise to solid-state structures in which the formazanate ligand forms a 5-membered chelate by binding to the metal center via an internal N atom. In soln., these compds. show dynamic equil. that involve both 5- and 6-membered chelates. All compds. are intensely colored, and the effect of the ligand substitution pattern on the UV-visible absorption spectra was evaluated. Their cyclic voltammetry is reported, which shows that all compds. may be electrochem. reduced to radical anionic (L2Zn-) and dianionic (L2Zn2-) forms. While unhindered NAr substituents lie in the plane of the ligand backbone (Ar = Ph), the introduction of sterically demanding substituents (Ar = Mes) favors a perpendicular orientation in which the NMes group is no longer in conjugation with the backbone, resulting in hypsochromic shifts in the absorption spectra. The redox potentials in L2Zn compds. may be altered in a straightforward manner over a relatively wide range (∼700 mV) via the introduction of electron-donating or -withdrawing substituents on the formazanate framework.
    29. 29
      Chang, M. C.; Otten, E. Synthesis and ligand-based reduction chemistry of boron difluoride complexes with redox-active formazanate ligands. Chem. Commun. 2014, 50, 7431– 7433,  DOI: 10.1039/C4CC03244F
      30
      Synthesis and ligand-based reduction chemistry of boron difluoride complexes with redox-active formazanate ligands
      Chang, M.-C.; Otten, E.
      Chemical Communications (Cambridge, United Kingdom) (2014), 50 (56), 7431-7433CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
      Mono(formazanate) boron difluoride complexes (LBF2), which show remarkably facile and reversible ligand-based redox-chem., were synthesized by transmetalation of bis(formazanate) zinc complexes with boron trifluoride. The one-electron redn. product [LBF2]-[Cp2Co]+ and a key intermediate for the transmetalation reaction, the six-coordinate zinc complex (L(BF3))2Zn were isolated and fully characterized.
    30. 30
      (a) Chang, M. C.; Chantzis, A.; Jacquemin, D.; Otten, E. Boron difluorides with formazanate ligands: redox-switchable fluorescent dyes with large stokes shifts. Dalton Trans. 2016, 45, 9477– 9484,  DOI: 10.1039/C6DT01226D
      31a
      Boron difluorides with formazanate ligands: redox-switchable fluorescent dyes with large stokes shifts
      Chang, M.-C.; Chantzis, A.; Jacquemin, D.; Otten, E.
      Dalton Transactions (2016), 45 (23), 9477-9484CODEN: DTARAF; ISSN:1477-9226. (Royal Society of Chemistry)
      The synthesis of a series of (formazanate)boron difluorides and their 1-electron redn. products is described. The neutral compds. are fluorescent with large Stokes shifts. DFT calcns. suggest that a large structural reorganization accompanies photoexictation and accounts for the large Stokes shift. Redn. of the neutral boron difluorides occurs at the ligand and generates the corresponding radical anions. These complexes are non-fluorescent, allowing switching of the emission by changing the ligand oxidn. state.
      (b) Chang, M.-C. Formazanate as redox-active, structurally versatile ligand platform: Zinc and boron chemistry. PhD thesis, University of Groningen, 2016.
      There is no corresponding record for this reference.
    31. 31
      Broere, D. L.; Coric, I.; Brosnahan, A.; Holland, P. L. Quantitation of the THF Content in Fe[N(SiMe3)2]2.xTHF. Inorg. Chem. 2017, 56, 3140– 3143,  DOI: 10.1021/acs.inorgchem.7b00056
      32
      Quantitation of the THF Content in Fe[N(SiMe3)2]2·xTHF
      Broere, Daniel L. J.; Coric, Ilija; Brosnahan, Anna; Holland, Patrick L.
      Inorganic Chemistry (2017), 56 (6), 3140-3143CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      The absence of residual solvent in metal precursors can be of key importance for the successful prepn. of metal complexes or materials. Herein, the authors describe methods for the quantitation of residual coordinated THF that binds to Fe[N(SiMe3)2]2, a commonly used iron synthon, when prepd. according to common literature procedures. A simple method for quantitation of the amt. of residual coordinated THF using 1H NMR spectroscopy is highlighted. Finally, a detailed synthetic procedure is described for the synthesis of THF-free Fe[N(SiMe3)2]2.
    32. 32
      Gilroy, J. B.; Ferguson, M. J.; McDonald, R.; Patrick, B. O.; Hicks, R. G. Formazans as β-diketiminate analogues. Structural characterization of boratatetrazines and their reduction to borataverdazyl radical anions. Chem. Commun. 2007, 126– 128,  DOI: 10.1039/B609365E
      33
      Formazans as β-diketiminate analogues. Structural characterization of boratatetrazines and their reduction to borataverdazyl radical anions
      Gilroy, Joe B.; Ferguson, Michael J.; McDonald, Robert; Patrick, Brian O.; Hicks, Robin G.
      Chemical Communications (Cambridge, United Kingdom) (2007), (2), 126-128CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
      Formazans, R'NHN:CRN:NR' (R ≠ R' = p-tolyl, Ph) react with boron triacetate to produce boratatetrazines I, which can be reduced by cobaltocene to yield borataverdazyl radical anions, e.g., II-the first boron contg. verdazyl radicals.
    33. 33
      Bruker. APEX3, SAINT and SADABS; Bruker AXS Inc.: Madison, WI, 2016.
      There is no corresponding record for this reference.
    34. 34
      Sheldrick, G. A short history of SHELX. Acta Crystallogr., Sect. A 2008, 64, 112– 122,  DOI: 10.1107/S0108767307043930
      35
      A short history of SHELX
      Sheldrick, George M.
      Acta Crystallographica, Section A: Foundations of Crystallography (2008), 64 (1), 112-122CODEN: ACACEQ; ISSN:0108-7673. (International Union of Crystallography)
      An account is given of the development of the SHELX system of computer programs from SHELX-76 to the present day. In addn. to identifying useful innovations that have come into general use through their implementation in SHELX, a crit. anal. is presented of the less-successful features, missed opportunities and desirable improvements for future releases of the software. An attempt is made to understand how a program originally designed for photog. intensity data, punched cards and computers over 10000 times slower than an av. modern personal computer has managed to survive for so long. SHELXL is the most widely used program for small-mol. refinement and SHELXS and SHELXD are often employed for structure soln. despite the availability of objectively superior programs. SHELXL also finds a niche for the refinement of macromols. against high-resoln. or twinned data; SHELXPRO acts as an interface for macromol. applications. SHELXC, SHELXD and SHELXE are proving useful for the exptl. phasing of macromols., esp. because they are fast and robust and so are often employed in pipelines for high-throughput phasing. This paper could serve as a general literature citation when one or more of the open-source SHELX programs (and the Bruker AXS version SHELXTL) are employed in the course of a crystal-structure detn.
    35. 35
      Sheldrick, G. Crystal structure refinement with SHELXL. Acta Crystallogr., Sect. C 2015, 71, 3– 8,  DOI: 10.1107/S2053229614024218
      36
      Crystal structure refinement with SHELXL
      Sheldrick, George M.
      Acta Crystallographica, Section C: Structural Chemistry (2015), 71 (1), 3-8CODEN: ACSCGG; ISSN:2053-2296. (International Union of Crystallography)
      The improvements in the crystal structure refinement program SHELXL have been closely coupled with the development and increasing importance of the CIF (Crystallog. Information Framework) format for validating and archiving crystal structures. An important simplification is that now only one file in CIF format (for convenience, referred to simply as 'a CIF') contg. embedded reflection data and SHELXL instructions is needed for a complete structure archive; the program SHREDCIF can be used to ext. the and files required for further refinement with SHELXL. Recent developments in SHELXL facilitate refinement against neutron diffraction data, the treatment of H atoms, the detn. of abs. structure, the input of partial structure factors and the refinement of twinned and disordered structures. SHELXL is available free to academics for the Windows, Linux and Mac OS X operating systems, and is particularly suitable for multiple-core processors.

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