ACS Publications. Most Trusted. Most Cited. Most Read
My Activity
CONTENT TYPES
RETURN TO ISSUEPREVFeatured ArticleNEXT

Electronic Structure of a Diiron Complex: A Multitechnique Experimental Study of [(dppf)Fe(CO) 3]+/0

  • Mario Winkler
    Mario Winkler
    Institute of Physical Chemistry, University of Stuttgart, Pfaffenwaldring 55, Stuttgart 70569, Germany
  • Marc Schnierle
    Marc Schnierle
    Institute of Inorganic Chemistry, University of Stuttgart, Pfaffenwaldring 55, Stuttgart 70569, Germany
  • Felix Ehrlich
    Felix Ehrlich
    Institute of Physical Chemistry, University of Stuttgart, Pfaffenwaldring 55, Stuttgart 70569, Germany
  • Kim-Isabelle Mehnert
    Kim-Isabelle Mehnert
    Institute of Physical Chemistry, University of Stuttgart, Pfaffenwaldring 55, Stuttgart 70569, Germany
  • David Hunger
    David Hunger
    Institute of Physical Chemistry, University of Stuttgart, Pfaffenwaldring 55, Stuttgart 70569, Germany
    More by David Hunger
  • Alena M. Sheveleva
    Alena M. Sheveleva
    Department of Chemistry and Photon Science Institute, University of Manchester, Oxford Road, Manchester M13 9PL, U.K.
  • Lukas Burkhardt
    Lukas Burkhardt
    Department of Chemistry and Center for Sustainable Systems Design, Paderborn University, Warburger Strasse 100, Paderborn 33098, Germany
  • Matthias Bauer
    Matthias Bauer
    Department of Chemistry and Center for Sustainable Systems Design, Paderborn University, Warburger Strasse 100, Paderborn 33098, Germany
  • Floriana Tuna
    Floriana Tuna
    Department of Chemistry and Photon Science Institute, University of Manchester, Oxford Road, Manchester M13 9PL, U.K.
  • Mark R. Ringenberg*
    Mark R. Ringenberg
    Institute of Inorganic Chemistry, University of Stuttgart, Pfaffenwaldring 55, Stuttgart 70569, Germany
    *Email: [email protected] (M.R.R.).
  • , and 
  • Joris van Slageren*
    Joris van Slageren
    Institute of Physical Chemistry, University of Stuttgart, Pfaffenwaldring 55, Stuttgart 70569, Germany
    *Email: [email protected] (J.v.S.).
Cite this: Inorg. Chem. 2021, 60, 5, 2856–2865
Publication Date (Web):February 11, 2021
https://doi.org/10.1021/acs.inorgchem.0c03259
Copyright © 2021 American Chemical Society
  • Open Access
  • Editors Choice

Article Views

4552

Altmetric

-

Citations

LEARN ABOUT THESE METRICS
PDF (2 MB)
Supporting Info (1)»

Abstract

Here we explore the electronic structure of the diiron complex [(dppf)Fe(CO)3]0/+ [10/+; dppf = 1,1′-bis(diphenylphosphino)ferrocene] in two oxidation states by an advanced multitechnique experimental approach. A combination of magnetic circular dichroism, X-ray absorption and emission, high-frequency electron paramagnetic resonance (EPR), and Mössbauer spectroscopies is used to establish that oxidation of 10 occurs on the carbonyl iron ion, resulting in a low-spin iron(I) ion. It is shown that an unequivocal result is obtained by combining several methods. Compound 1+ displays slow spin dynamics, which is used here to study its geometric structure by means of pulsed EPR methods. Surprisingly, these data show an association of the tetrakis[3,5-bis(trifluoromethylphenyl)]borate counterion with 1+.

This publication is licensed under

CC-BY-NC-ND 4.0.
  • cc licence
  • by licence
  • nc licence
  • nd licence

Synopsis

A multitechnique experimental investigation establishes the oxidation site for an iron-based hydrogen-evolving catalyst that is also a single-molecule magnet and a potential molecular quantum bit. Furthermore, measurements reveal a close geometric association of the weakly coordinating counterion with the complex, underlining the need to consider the role of counterions in chemical reactivity.

Introduction

ARTICLE SECTIONS
Jump To

In essentially all fields of catalysis, there is the dichotomy between stable, but poorly defined heterogeneous catalysts (1) and more fragile, but well-characterized homogeneous catalysts. (2) The latter are more amenable to detailed mechanistic studies, and nature also uses a homogeneous approach to catalysis, e.g., in the formation of H2 by the different classes of hydrogenase enzymes. (3) Interestingly, most manmade homogeneous electrocatalysts for hydrogen-evolving reactions (HERs) employ cobalt, whereas nature uses iron and nickel for these purposes. (2) The design principles for manmade catalysts include (i) the presence of free coordination sites and a suitable electronic structure to allow the generation of hydride species and (ii) a ligand coordination sphere that stabilizes the metal species responsible for proton reduction, e.g., due to metal–ligand interaction (ligand noninnocence). (2)
Recently, some of us have reported the synthesis and investigation of complex [(dppf)Fe(CO)3]0/+ [10/+; dppf = 1,1′-bis(diphenylphosphino)ferrocene; Scheme 1], which features the redox-active and highly adaptable ferrocenyl-based ligand platform. Notably, the terminal hydride species [(dppf)FeH(CO)3]BF4 (Scheme 1) proved accessible. The same work revealed noninnocent behavior of the dppf ligand in the cationic complex 1+ in the form of electron delocalization in Fe···Fe interaction. (4) However, the size of the system precluded performing electronic structure calculations above the density functional theory (DFT) level: A meaningful multireference calculation, e.g., based on complete-active-space self-consistent-field methods, would require an active space consisting of the 3d orbitals of the two iron atoms (10 in total), at least some π or π* orbitals of the Cp ligands, and the π* orbitals of the carbonyl ligands, which is not feasible with the current computational possibilities. This made us ask whether the noninnocent behavior of 1+ could be better explored using advanced spectroscopic techniques in order to provide a deeper understanding of this phenomenon. Therefore, we embarked on a detailed study of the electronic and geometric structures of 10/+ via a multitechnique approach, and the results are discussed here. As a collateral result, we have found that 1+ displays field-induced single-ion-magnet behavior and quantum coherence at low temperatures.

Scheme 1

Scheme 1. Structures of 10/+ and [(dppf)FeH(CO)3]+ Showing the Two Isomers of 1+ with Respective SP and TBP Carbonyl Iron Coordination Geometries

Results and Discussion

ARTICLE SECTIONS
Jump To

Electrocatalytic HER Performance of 1

The zerovalent tricarbonyl iron component of 1 can be protonated using strong acids (HBF4) to afford the metal hydride [FeH(dppf)(CO)3]+ (1H+), resulting in the formal two-electron oxidation of the tricarbonyl metal center. The 1H NMR spectrum of 1H+ contains an upfield triplet at −6.6 ppm, consistent with a terminal hydride coupled to two equivalent phosphines. The molecular structure was determined crystallographically and reported. (4) The terminal hydride 1H+ exhibits an irreversible anodic wave at +0.5 V versus Fc0/+ (Fc0/+ = [FeCp2]0/+), which is consistent with the oxidation of the dppf ligand, as well as an irreversible cathodic wave at −1.5 V versus Fc0/+ (Figure 1A). Complex 1 was screened for electrocatalytic reduction of H+ by cyclic voltammetry (CV) in acetonitrile (CH3CN). To this end, 1 equiv of p-toluenesulfonic acid (HOTs; pKa = 8.45 in CH3CN) (5) was added to the solution, and immediately after that, a CV was run. The voltammograms under acidic conditions displayed a new reduction wave at −1.5 V (vs Fc0/+) that is attributed to catalytic proton reduction. Concurrently, the anodic peak currents of the oxidation waves decreased. Further equivalents of HOTs were added until the catalytic peak current of the new reduction wave no longer increased. The catalytic peak current (icat,p) was compared to the peak current of the initial oxidation wave (ia,init). The plot of icat,p/ia,init as a function of the number of HOTs equivalents saturates at an icat,p/ia,init ratio of 1.6 (Figure 1B). In the concentration-independent region, the hydrogen production rate k can be obtained from the current ratio, using the equation , with v the CV scan speed. (6) Following this procedure, we arrive at a hydrogen production rate of k = 0.5 s–1. These results indicate that the diiron system 1 can indeed be a HER catalyst, albeit at very slow rates. In comparison, the rate of [FeFe] hydrogenase HER would be 700 s–1, (7) and rates as high as 33000 s–1 have been reported for molecular systems. (8) In order to rationally design improved iron-based HER catalysts, a deeper understanding of the electronic and geometric structures of 1 and its relationship to its HER activity would be helpful.

Figure 1

Figure 1. (A) Cyclic voltammograms (scan rate 100 mV s–1), recorded on a solution of 1 in CH3CN with the repeated addition of HOTs equivalents. (B) Catalytic/initial anodic current ratio as a function of the number of added HOTs equivalents.

Electronic Structure of 10/+

In the parent compound 10, the ferrocene iron is formally in the 2+ oxidation state, while the carbonyl iron is formally zerovalent. Previous DFT calculations and spectroelectrochemical observations indicated that oxidation occurs on the tricarbonyl iron rather than on the ferrocene iron. Furthermore, two species were observed in solution electron paramagnetic resonance (EPR) at X band, assigned to a species whose carbonyl iron is in a square-pyramidal (SP) coordination geometry and a species where this iron atom is in a trigonal-bipyramidal (TBP) geometry. In the latter species, computational indications for significant metal–metal interactions were found, but these provided only a cursory understanding of this interaction. (4) However, the electronic structure of especially 1+ is perhaps not entirely accurately described by previously used monodeterminantal methods such as DFT. Because the size of the system prohibits more advanced electronic structure calculations, we used the time-proven strategy of the multitechnique experimental approach to investigate physical phenomena, combined with comparisons to data recorded on similar compounds. Here we intended to answer questions including the site at which the first oxidation of 10 takes place, as well as clarification of the two structural isomers of 1+ that were previously found, and last a better description of the Fe–Fe interaction in 1+.
First information on the electronic structure of a compound can be garnered from electronic absorption spectra. Here we recorded magnetic circular dichroism (MCD) spectra, i.e., the relative absorption difference between left and right circularly polarized light in an applied magnetic field. (9) Because the MCD effect is a signed quantity, the effective resolution in MCD is often higher than that in plain electronic absorption spectroscopy. (10) The MCD intensity can be divided into three mechanisms (A, B, and C terms), where the last is active only in paramagnetic systems but usually leads to much stronger signals. (9) The MCD spectrum of diamagnetic 10 (Figures 2 and S1 and S2) is generally rather weak and displays one feature with positive (at 26300 cm–1) and negative (at 27800 cm–1) parts, which we attribute to a small high-spin iron(III) impurity. In contrast, the MCD spectrum of 1+(BArF4) (BArF4 = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate; the anion will be omitted in the text from here on) shows two rather strong bands of opposite signs in the near-IR (NIR; 9900 and 12700 cm–1) as well as a number of weaker features in the visible (Table S1). Bands at similar NIR energies were previously observed in UV/vis/NIR spectroelectrochemistry. The former was assigned on the basis of time-dependent DFT calculations to a metal-to-metal charge-transfer transition, where the electron density is transferred from the ferrocene iron to the carbonyl iron. (4) Because charge transfer proceeds from the more highly oxidized metal ion to the less highly oxidized metal ion, this transition can be seen as a reverse intervalence charge transfer. This assignment is corroborated with the relatively high MCD intensity compared to the absorption spectrum, in line with the expectation that metal-centered transitions have stronger MCD intensities than ligand-based ones. To further verify the DFT result that oxidation occurs on the carbonyl iron rather than the ferrocene iron, we have recorded the MCD spectrum of [Fc](BArF4) for comparison (Figure 2), which displays a very intense, highly structured band at ca. 15500 cm–1, with clear C-term-like temperature and field dependence. This transition has been previously observed in MCD (11) and high-resolution matrix spectroscopy experiments (12,13) and was assigned to a transition that involves charge transfer from the Cp π orbitals to the iron ion. The structure is due to vibronic coupling to the totally symmetric metal–ligand vibration. (12) Clearly, the Fc+ and 1+ spectra are qualitatively completely different, suggesting that oxidation of 10 indeed takes place on the carbonyl iron. MCD measurements on low-spin iron(I) complexes [FeCl(PP)] (PP is a diphosphino ligand) have been reported. (14) In these, bands with positive MCD intensities in the NIR (7500–8000 cm–1) were observed and attributed to d-d transitions. However, although these transition energies are similar to those observed for 1+, the reported origin is not the same, making a direct comparison difficult.

Figure 2

Figure 2. MCD spectra recorded on powder mulls of 10, 1+, and [Fc](BArF4) in Fluorolube at T = 1.5 K. Applied fields were B = 5 T (1+/0) and 100 mT ([Fc]+).

A modern element-specific method that is exquisitely suitable for determination of the local electronic structures of atoms is high-energy-resolution, fluorescence-detected X-ray absorption near-edge structure (HERFD-XANES). (15,16) In preedge transitions, the primary photoelectron ends up in a bound and previously unoccupied state, i.e., a LUMO of the system. A complementary technique is valence-to-core X-ray emission spectroscopy (VtC-XES), where an electron is excited into the continuum and the subsequent X-ray emission because of the transfer of an electron from an occupied valence orbital (a HOMO) to the 1s core hole resulting from the photoionization in K-edge spectroscopy. Figure S3 displays the VtC-XES spectra recorded at the Fe K-edge on powder samples of 10 and 1+. Both spectra display several higher-energy, intense bands (the Kβ2,5 region) and lower-energy, less intense bands (the Kβ″ region). Ferrocenes and low-valent iron carbonyls have been extensively studied in the past via VtC-XES, which allows a proper literature analysis of the observed spectral features and deviations. (17−19) The main difference between 10 and 1+ appears to be the absence of the high-energy shoulder at 7111 eV for the latter. Typically, high-energy features above 7110 eV are due to transitions from 3d orbitals, while features below 7110 eV are ascribed to transitions from ligand orbitals with considerable Fe 4p orbital contributions. (17) It has been reported that the VtC-XES spectra of ferrocene (Fc) and ferrocenium (Fc+) lack high-energy 3d-localized features because of the absence of 3d–4p hybridization in 5-fold symmetry. (20,21) In contrast, TBP iron carbonyls and phosphines exhibit significant hybridization of 4p with both 3dxy and 3dx2y2 orbitals. For this reason, the significant intensity decrease of the high-energy feature around 7110 eV and especially the absence of the shoulder around 7111 eV upon oxidation of 10 to 1+ is attributed to changes on the iron carbonyl site. The most likely responsible change is a reduced 3d occupancy at the carbonyl iron site that decreases the number of possible 3d-to-1s transitions. All features below 7110 eV are a superposition of features due to transitions from orbitals localized on the CO or Cp moieties. (17,19)
Core-to-core (CtC-)XES can be used to investigate the metal-ion spin states and covalency in the system (22,23) but only in a series of related complexes. Previous studies suggested that the mainline moves down in energy with increasing spin. Figure S4 shows CtC-XES spectra recorded using the Kβ transitions of 10 and 1+. The mainline of 10 at around 7057.8 eV is much narrower than that of 1+ at around 7058.3 eV. Additionally, both the intensity of the Kβ′ satellite at 7045 eV and the satellite–mainline splitting increase, going from 10 to 1+, which is in line with a higher total spin of the latter system. The difference of roughly 2.4 eV between the mainline energy in 10 and the shoulder in 1+ is much larger than the value reported for the mainline energy difference in pure Fc versus [Fc]+, which is a first indication that the high-energy mainline shoulder is related to a transition from an orbital located on the iron carbonyl site. (20) Hence, we conclude that the oxidation most likely affects mainly the carbonyl iron center and that the high-energy shoulder reflects the oxidation-associated changes at that center.
The HERFD-XANES spectrum of 10 (Figure 3 and S5) displays an intense 4p feature at ca. 7120 eV, previously observed for dppf0. (21) Two further preedge features were observed at around 7113.3 and at 7115.0 eV, respectively. A feature at an energy similar to that of the former for Fe2(CO)9, (24) and at an energy similar to that of the latter for dppf0, (21) was reported, and the 10 XANES spectrum is thus essentially a superposition of ferrocene-like and iron(0) carbonyl-like XANES spectra. Fe2(CO)9 and dppf0 reference spectra were measured in higher concentrations, leading to self-absorption effects that strongly increase the prepeak and preedge feature intensities. No peak was observed at 7111.6 eV, where a peak for dppf+ was reported, (21) confirming the oxidation state of the ferrocene iron in 10 as 2+.

Figure 3

Figure 3. HERFD-XANES spectra of powder samples of 10 and 1+, as well as of the reference compounds [Fc](PF6), (21) dppf, (21) Fe2(CO)9, (24) and [FeBr(dpbz)2]. (25)

The (partial) separation of the mainline peaks of the iron atoms for 1+ allows one to record site-selective HERFD-XANES spectra, monitoring the emission on either the high- or low-energy side of the CtC-XES mainline, while scanning the excitation energy through the Fe K-edge. The HERFD-XANES spectrum of 1+ (Figures 3 and S6) recorded by monitoring the CtC intensity at the high-energy shoulder (7060.2 eV) has a single prepeak feature at 7115 eV, and no strong 4p feature is observable. The spectrum differs significantly from that of Fe2(CO)9. In contrast, the spectrum monitored at the low-energy slope (7056.7 eV) does contain a strong 4p feature at ca. 7120 eV and a double prepeak feature. Double prepeak features were observed for ferrocenium derivatives (20,21) but also for the low-spin iron(I) compound [FeBr(dpbz)2]. (25) A 4p peak cannot be clearly observed for the iron(I) species but also cannot be ruled out. Thus, on the basis of the prepeak features, scenarios in which the oxidation of 10 to 1+ occurs on the ferrocene or carbonyl iron are both possible. In the former, the low-energy side of the CtC-XES mainline would correspond to a low-spin iron(III) ion in a ferrocenium moiety and the high-energy side to the neutral carbonyl iron. In the latter scenario, the high-energy side of XES corresponds to a low-spin iron(I) carbonyl ion and the low-energy side to a diamagnetic iron(II) of a ferrocene moiety. In correspondence with VtC-XES and nonresonant CtC-XES, the latter scenario is most likely and in agreement with the MCD results, as well as IR spectroelectrochemistry. In the latter, the oxidation site was assigned to the carbonyl iron on the basis of the large blue shift of the carbonyl stretching frequency upon oxidation, consistent with a loss of electron density on the carbonyl iron and a concurrent decrease of metal–CO back-bonding. (4) Nevertheless, we have further investigated the issue.
EPR spectra are very sensitive to small changes in the electronic structure, which are reflected in the g-tensor values, according to the spin Hamiltonian = μBŜ·g·B0. Measuring at higher frequencies and concomitantly higher fields increases the effective g-value resolution and allows a more accurate determination of the g-tensor values. Figure 4 displays high-frequency EPR (HFEPR) spectra recorded on a pressed powder pellet of 1+ at 10 K and frequencies between 90 and 375 GHz, plotted as a function of the effective g value. The spectra display three features that are stationary on the g-value axis, demonstrating their origin in g-value anisotropy. The better resolution at higher frequencies underlines the value of higher frequencies in EPR. There are no further features across the entire available field range of 0–15 T (Figure S7), precluding the presence of high-spin species (S > 1/2) with zero-field-splitting gaps of less than ca. 25 cm–1 (the sum of effective energies corresponding to the highest field strength and microwave frequency). Simulating the spectra, using the spin Hamiltonian given above, and assuming S = 1/2 yield g values of gxx = 1.991, gyy = 2.006, and gzz = 2.046 (giso = 2.014), indicating weak rhombic g-value anisotropy for 1+. These values are rather close to the values found for other low-spin iron(I) complexes (Table S2). In contrast, EPR spectra of low-spin iron(III) ferrocenium derivatives are, without exception, axial and highly anisotropic (Table S2) with gxx = gyy ≈1.7 and gzz ≈ 3.6. Thus, the EPR results strongly favor the carbonyl iron as the oxidation site. The HFEPR spectra do not show any splitting due to superhyperfine interactions with the two phosphorus atoms of dppf. This is because the sample was measured as a solid rather than in solution, and thus broadening of the EPR lines is expected as a result of intermolecular dipolar interactions. Similar observations were made at X-band frequencies (ca. 9.5 GHz). Here poorly hyperfine-resolved spectra were obtained in the solid state as well (Figure S8) because of dipolar broadening. In contrast, measurements on frozen solutions produced much better resolved spectra, including clear hyperfine splitting (Figure S9). These frozen solution spectra are fully in agreement with previously published fluid solution spectra in terms of the g value and 31P hyperfine coupling constant. (4)

Figure 4

Figure 4. High-frequency EPR spectra recorded on a pressed powder pellet of 1+ at T = 10 K and different frequencies as indicated (black lines). Red lines are simulations using gxx = 1.991, gyy = 2.006, and gzz = 2.046 (giso = 2.014).

We have further investigated compounds 10/+ by means of 57Fe Mössbauer spectroscopy, comparing them with various model compounds. The room temperature Mössbauer spectrum of 10 displays two doublets (Figure 5 and Table S3) that can be fitted with an isomer shift of δ = −0.13 mm s–1 and ΔEQ = 2.16 mm s–1 for the first doublet and δ = +0.42 mm s–1 and a quadrupole splitting of ΔEQ = 2.29 mm s–1 for the second. The values for the former doublet are rather close to those obtained for the iron carbonyl compound [Fe(PPh3)2(CO)3] (δ = −0.17 mm s–1 and ΔEQ = 2.63 mm s–1), while the latter are close to those obtained for dppf itself (δ = +0.43 mm s–1 and ΔEQ = 2.29 mm s). Accordingly, the former doublet is assigned to the carbonyl iron and the latter to the ferrocene iron. The isomer shift difference agrees with a higher electron density on the iron atom coordinated to electron-releasing Cp ligands compared to that of the iron atom coordinated to π-accepting CO ligands.

Figure 5

Figure 5. Room temperature Mössbauer spectra of powder samples of 10 and 1+, as well as of the reference compounds dppf, Fc, [Fc](BArF4), and [(PPh3)2Fe(CO)3].

The room temperature Mössbauer spectrum of 1+ is rather more complicated (Figure 5) and features at least three doublets. The outermost lines could be reproduced by using isomer-shift and quadrupole-splitting values (Table S3) that are close to those found for the dppf iron atom in 10, suggesting that this iron atom is less affected by the oxidation process. In contrast, the room temperature Mössbauer spectrum of ferrocenium consists of a single line at δ = +0.42 mm s–1 without measurable quadrupole splitting. The innermost part of the Mössbauer spectrum of 1+ was reproduced by using a sum of two doublets with isomer shifts of −0.04 and +0.33 mm s–1 and quadrupole splittings of 0.50 and 0.80 mm s–1, respectively. These doublets are assigned to the oxidized carbonyl iron ion, which exists in two isomers at room temperature with TBP and SP geometries, respectively. (4) Mössbauer data for low-spin iron(I) appear to be quite rare, with the exception of various dinitrosyl iron complexes. (26,27) These complexes possess isomer shifts in quite a wide range, which can be understood in view of the noninnocence of the NO ligands. The most relevant low-spin iron(I) complex for which Mössbauer data have been reported is [Fe(nacnac)(CO)3], where nacnac = N,N′-bis(2,4-diisopropylphenyl)-1,3-diketiminate, which has δ = +0.12 mm s–1 and ΔEQ = 0.77 mm s–1. (28) The coordination geometry around iron is SP for this compound, and the Mössbauer parameters are comparable to, but still significantly different from, those of the SP isomer of 1+, reflecting the different ligand properties of nacnac and dppf.
Previous solution EPR investigations revealed that the dynamic equilibrium between the two isomers of 1+ shifts to the TBP isomer with decreasing temperature. Hence, we investigated the temperature dependence of the Mössbauer spectra down to 3 K (Figure 6). Indeed, upon a decrease in the temperature below 50 K, the Mössbauer spectra show only features due to one isomer. This allows us to determine the relative amount of each isomer as a function of the temperature (Figure S10) and to unambiguously assign the two sets of Mössbauer parameters to each of the TBP and SP species (Table S3).

Figure 6

Figure 6. Mössbauer spectra recorded on a 60 mg powder sample of 1+ at different temperatures as indicated. Solid lines are Lorentzian line deconvolutions.

Spin Dynamics and Geometric Structure of 1+

Interestingly, at the lowest temperature of 3 K, the Mössbauer spectrum of 1+ broadens significantly, which can be indicative of slow paramagnetic relaxation on the Mössbauer time scale of 10–7 s. (29) Alternatively, these slight broadenings may be experimental artifacts due to the active pumping of the sample space, required to achieve temperatures below the helium boiling point. With the help of Blume–Tjon theory, (30) the Mössbauer line shape can be related to paramagnetic relaxation times in the sample. For very long relaxation times, the Mössbauer spectrum splits into a sextet by magnetic hyperfine interaction between the electron-spin magnetic moment and nuclear spin, as is, for example, observed for ferrocenium (Figure S11). (31,32) Using the functionality implemented in the MossWin software and assuming that additional broadening relative to the line width at 7 K is only due to relaxation effects, the 3 K spectrum was fitted, yielding relaxation times of 100 ± 50 ns for the carbonyl iron and 0.6 ± 0.1 ns for the dppf iron (Figure S12 and accompanying text). The finite relaxation time obtained for the dppf iron atom may indicate a small but finite spin density on this iron. These measurements underline the power of Mössbauer spectroscopy to investigate spin–lattice relaxation (SLR) dynamics on an atomic level at relaxation rates that are not accessible with other techniques. Indeed, alternating-current (ac) susceptibility measurements of a pressed powder pellet of 1+ in zero external field revealed no signs of slow paramagnetic relaxation, which would be evidenced by significant out-of-phase ac susceptibility signals (Figure S13). However, such out-of-phase signals are clearly observed at 1.8 K upon the application of moderate external fields up to 2 T (Figure S13). The substantial slowing down of magnetization dynamics due to the application of an external magnetic field is attributed to the suppression of efficient magnetization relaxation by (hyperfine-induced) quantum tunneling of the magnetization in zero field. Metal complexes that display slow magnetization dynamics in applied fields are often called field-induced single-ion magnets, although there is no physical barrier to relaxation of the magnetic moment involved here (although 1+ possesses two metal ions, its magnetism is essentially due to the paramagnetic carbonyl iron). From fits of the out-of-phase component of the ac susceptibility (χ″) versus frequency, the relaxation times at each field were obtained (Figure S14). The plot of the relaxation time τ as a function of the field was fitted with a Brons–Van Vleck-like equation τ–1 = cB4 + d(1 + eB2)/(1 + fB2). (33) Here the first term parametrizes the direct process of SLR, and the second the influence of internal fields on this process. In the latter term, the e parameter describes the influence of the field on the relaxation of interacting spins, and the f parameter describes the suppression of internal SLR mechanisms by the external field. The fit resulted in the following parameters: c = 0.2(2) T–4 s–1, d = 56(9) s–1, e = 0.1(1) T–2, and f = 30(1) T–2. These parameters all appear to be on the lower end of the range of published data (Table S4), (33−36) suggesting limited influence of the external field on the SLR. However, published data all deal with vanadium(IV) complexes, and trends are not easily observed even within that class of compounds. The temperature dependence of the relaxation behavior was studied in an external field of 1.25 T (Figure S15 and Table S5), where for the lowest temperature again two relaxation processes are clearly observed.
In order to shed further light on the relaxation processes at 1.25 T, we turned to pulsed Q-band EPR. The observation of spin echoes proves that 1+ possesses measurable quantum coherence times, which is one of the Di Vincenzo criteria for quantum bits. The electron-spin–echo (ESE)-detected spectra, recorded on a 1 mM dichloromethane/toluene (1:1) solution at 10 K (Figure S16), display a rich structure that could be fitted by using a rhombic g and hyperfine tensors [ = μBŜ·g·B0 + Ŝ·A·Î; gxx = 1.9850, gyy = 2.0035, and gzz = 2.0420; hyperfine coupling values were eventually derived from electron–nuclear double resonance (ENDOR) measurements], where the latter arises because of coupling with two 31P atoms. The hyperfine splitting is only resolved for the central g-tensor component, in contrast to what was observed in the X-band spectra (Figures S9 and S17). This is due to g strain, leading to broader lines at higher EPR frequencies and concomitantly higher magnetic fields. In fact, pulsed X-band spectra display hyperfine structure for both components of the pseudoaxial g tensor (Figure S17). We investigated the spin dynamics processes of SLR (Figure S18) and spin–spin relaxation (SSR; Figure S19) in more detail by means of inversion recovery and Hahn-echo pulse sequences. The temperature dependences of the T1 (SLR) and TM (SSR or phase memory) time constants as a function of the temperature are shown in Figure S20 and Table S6. The SLR time depends strongly on the temperature. Interestingly, the relaxation times from the ac superconducting quantum interference device (SQUID) measurements are about an order of magnitude shorter. This is likely due to the different nature of the two samples: the SQUID measurements are carried out on pellets of the pure powder, whereas the EPR measurements involved dilute frozen solutions. In the former, the intermolecular distances are much shorter, and the ensuing stronger intermolecular dipolar spin–spin interactions may enhance SLR. Furthermore, the phonon spectrum of the solid versus frozen solution matrixes may be different, which also impacts the SLR. The phase memory time decreases only slowly with the temperature from 5.54 μs at 15 K to 1.23 μs at 70 K. The individual Hahn-echo decay curves display an oscillatory behavior superimposed on the decay, which we attribute to electron-spin–echo envelope modulation (ESEEM; Figure S21), which is due to hyperfine coupling. (37) The modulation frequency of ca. 2.75 MHz can be understood as the difference between the hyperfine constant and nuclear Larmor frequency (ωmod = |msA – ωI|) with A the hyperfine constant (found to be 44 MHz in the ESE) and ωI the nuclear Larmor frequency. From this equation, a nuclear Larmor frequency of 19 ± 3 MHz is found, which is the same as the theoretical Larmor frequency of 31P (21.51 MHz) within experimental error, allowing us to conclude that it is that nucleus that is coupled to the electron spin.
Hyperfine coupling constants can be investigated in more detail by more advanced pulsed EPR techniques, such as hyperfine-sublevel-correlation (HYSCORE; Figure S22) and ENDOR spectroscopies. The favorable spin dynamics times of 1+ allowed us to embark on such studies. Figure 7 shows the (+,+) quadrant of a frozen solution pulsed HYSCORE spectrum of 1+ at 10 K. HYSCORE is a 2D ESEEM experiment in which each nuclear spin is correlated between α- and β-electron-spin manifolds. (37) The (+,+) quadrant of the spectrum obtained after Fourier transformation of the recorded HYSCORE data typically contains features due to weakly coupled nuclei, i.e., those for which the Larmor frequency ωI/2π is larger than half the hyperfine coupling A. The distance of the peak from the diagonal is a measure of the hyperfine coupling strength. Peaks in the (−,+) quadrant of the spectrum arise from strongly coupled nuclei, where the distance to the antidiagonal relates to the Larmor frequencies of the nuclei involved. The exact shapes of the peaks contain a wealth of information about hyperfine anisotropies and quadrupole splittings for high-spin nuclei. The HYSCORE spectrum in Figure 7, recorded at an applied magnetic field of B0 = 343.9 mT, essentially corresponds to gzz, i.e., molecules are excited, whose g-tensor z axes are aligned with the field (Figure S17). Two spectral features are observed in the (+,+) quadrant, namely, two peaks split off from the diagonal (the peak at 3.8 MHz on the diagonal is spurious) and a strong feature at 14.6 MHz. The former is attributed to weakly coupled 13C nuclei with A = 1.8 MHz and A = 3.3 MHz. From these hyperfine coupling constants, we can derive the distance between the electron and carbon nuclear spins. To this end, we assume the point-dipole approximation to hold and write the hyperfine tensor A as
(1)
Here Aiso is the isotropic hyperfine coupling constant, 1 the unit matrix, T the dipolar coupling constant (which contains the distance information), and ρ the rhombic contribution. In the expression for T, ge and gN are the electron and nuclear Landé factors and μB and μN the Bohr and nuclear magnetons, respectively. R is the distance between the electron and nuclear spins. It turned out that we cannot assume collinearity of the g and A tensors, and we converted the A tensor to the molecular coordinate frame given by the g-tensor axis by means of a Euler angle rotation. In this way, we derive T = 0.5 MHz, corresponding to a distance of 5.41 Å (Table 1), and therefore we assign this signal to distant carbon atoms, including those on the Cp rings, in the counterion, and in the solvent. The second feature is due to the 1H nuclei whose hyperfine coupling is too weak to be resolved (the order of magnitude is 1 MHz; Table 1). The 31P nuclei that might be expected to turn up in the (−,+) quadrant of the HYSCORE spectrum (not shown in Figure 7) fall outside of the measurement window because of the fact that their hyperfine coupling is far too strong. Similar results are obtained for other field settings (Figures S23 and S24).

Figure 7

Figure 7. X-band HYSCORE spectrum (blue) recorded on a 2 mM frozen solution of 1+ in dichloromethane/toluene (1:1) at 10 K at an applied field of B0 = 343.9 mT, using a short delay time of 136 ns after the first pulse in the four-pulse HYSCORE sequence. The simulation is shown in red (see the text for details).

Table 1. Hyperfine Coupling Constants and Derived Electron–Nuclear Spin Distances, All Obtained from the Fit of HYSCORE and ENDOR Data
nucleusaiso/MHzT/MHzρdistance/Å
13C2.30.505.41
31P(1,2)39.53.250.7232.90
1H(1)01.403.84
1H(2)00.9504.37
1H(3)00.4005.83
1H(4)00.2506.82
19F(1)1.401.1504.10
19F(2)1.200.9504.37
19F(3)1.331.0804.19
A complementary method to assess hyperfine couplings is pulsed ENDOR spectroscopy. Figure 8 displays the richly structured pulsed Q-band Davies ENDOR spectrum recorded on a frozen solution of 1+, using an applied field of 1200.6 mT, which corresponds to the middle g value (gyy). The spectrum displays a structured feature at ca. 42 MHz, which we attribute to the upper component of the two 31P ENDOR lines situated at A/2 ± νL, where νL is the phosphorus Larmor frequency. The peaks at ca. 51 MHz belong to weakly coupled 1H spins. The small feature at 17 MHz is an artifact caused by the presence of the second harmonic of the radio frequency caused by the power amplifier. (38) Finally, the small but distinct and significant feature at 48 MHz is assigned to weakly coupled 19F nuclei, in view of its frequency. This is interesting because these nuclei must come from the BArF4 counterion, which is thus shown to be in close association with the cationic complex. No peaks due to 13C (expected around 12.9 MHz) were observed. Similar results are obtained at other fields and at X-band frequencies (Figures S25 and S26). In view of the high data quality, we embarked on a full analysis, aiming for a single set of hyperfine parameters that would fit all six ENDOR spectra. During our extensive analysis and simulation, it turned out that the limited excitation bandwidth of the microwave pulses and the noncollinearity between the g and A tensors play important roles. Finally, we managed to arrive at a single minimal set of parameters that reproduced all six ENDOR spectra rather well (Figures S25 and S26). From the hyperfine coupling parameters, we extracted the relevant distances with the help of eq 1 (Table 1). For 31P, we find a single distance of 2.90 Å, which is significantly longer than the experimental (CO)3Fe–P distance in 10 of 2.29 Å. (39) No crystal structure for 1+ has been reported for 1+, but DFT calculations for the different isomers gave Fe–P distances of 2.26–2.33 Å. (4) This suggests either that the spin density is not fully localized or that the obtained distance is not correct because of nondipolar contributions to the hyperfine interaction. At least four distinct 1H nuclei are required to reproduce the related part of the ENDOR spectra. Finally, for the 19F nuclei, three distinct hyperfine couplings were required, which are assigned to the three fluorine atoms of a single −CF3 group from the counterion. The bulky counterion in close proximity of the carbonyl iron moiety in 1+ is unusual because such ion pairs are thought to be essentially unobservable with such weakly coordinating anions.

Figure 8

Figure 8. Pulsed Q-band ENDOR spectrum recorded on a 4 mM frozen solution of 1+ in dichloromethane/toluene (1:1) at 10 K and an applied field of B0 = 1200.6 mT.

Conclusion

ARTICLE SECTIONS
Jump To

We have studied the electronic structure of the dinuclear iron complex [(dppf)Fe(CO)3] (1), with the aim to gain further insight into the electron structure of both iron sites. We show that a multitechnique experimental approach is required to establish the oxidation locus in this complex. The work has revealed that the cationic complex 1+ is present in two isomers in the solid state at room temperature, and temperature-dependent Mössbauer measurements gave the relative abundance of the two isomers down to 5 K. Complex 1 is also shown to be multifunctional because investigation of its spin dynamics revealed that it displays field-induced single-molecule-magnet behavior and is also a potential molecular qubit. The favorable spin dynamics properties allowed us to investigate aspects of the geometrical structure, revealing the surprising close association of the bulky counterion with the complex. This provides insight into the high electrophilicity of 1+, which may underline its relative stability compared to other iron(I) species, e.g., [Fe(CO)3(PR3)2]+, where ion pairing cannot be completely ignored and underlines the necessity to consider the influence of counterions on reactivity even if they are weakly coordinating.

Experimental Section

ARTICLE SECTIONS
Jump To

Compound 10 was synthesized from dppf and Fe2(CO)9 as previously described. (4) The one-electron-oxidized product 1+ was obtained by chemical oxidation with [Fc](BArF4). The purity and identity were checked by IR spectroscopy. MCD spectra were recorded on a homemade spectrometer constructed around an Aviv 42CD spectrometer and an Oxford Instruments 10T Spectromag optical cryomagnet. (40,41) X-ray absorption and emission experiments were carried out at the ID26 beamline of European Synchrotron Radiation Facility (ESRF), as recently described. (17) Mössbauer spectra were obtained on a homemade spectrometer based on a RCPTM MS-96 Mössbauer spectrometer equipped with a Ritverc Co57 in a Rh-matrix source, a YAP:Ce scintillating crystal detector, and a Janis SVT-400 helium-bath cryostat. Spectra were calibrated against α-iron at room temperature and fitted using the MossWinn 4.01 program. X-Band EPR spectra were recorded on a Bruker EMX spectrometer. Magnetometric measurements were carried out on a Quantum Design MPMS3 SQUID magnetometer. For high-frequency EPR (42) and for basic pulsed Q-band EPR, (43) home-built spectrometers were used. X-Band HYSCORE and ENDOR measurements were performed on a Bruker Elexsys 580 spectrometer at the National EPR Facility at The University of Manchester. All EPR spectra were simulated using the Easyspin toolbox for Matlab. (44)

Supporting Information

ARTICLE SECTIONS
Jump To

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c03259.

  • Additional MCD data with discussion on 10, VtC- and CtC-XES spectra for 1+/0, magnified portions of the HERFS-XANES spectra of 1+/0 and reference compounds, HFEPR spectra for 1+ plotted as a function of the field, table of the HFEPR fit data with literature values of reference compounds, X-band EPR spectra recorded on powders and frozen solutions of 1+, table of the Mössbauer fit data and literature values of reference compounds, temperature dependence of the fractions of the two isomers of 1+ as derived from VT Mössbauer data, Mössbauer spectrum of [Fc]+(BArF4), Blume–Tjon fit of the 3 K Mössbauer spectrum of 1+ and discussion, ac SQUID measurement data at different fields, frequencies, and temperatures, fits of the ac relaxation data, table of ac relaxation fit data with literature values of reference compounds, echo-detected EPR spectra for 1+ at X and Q bands, inversion recovery and Hahn-echo decay curves, table and figure with spin relaxation times at different temperatures, ESEEM curve and its Fourier transform, HYSCORE pulse sequence, X-band HYSCORE spectra, X- and Q-band ENDOR spectra, and Easyspin simulation file (PDF)

Terms & Conditions

Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.

Author Information

ARTICLE SECTIONS
Jump To

  • Corresponding Authors
  • Authors
    • Mario Winkler - Institute of Physical Chemistry, University of Stuttgart, Pfaffenwaldring 55, Stuttgart 70569, Germany
    • Marc Schnierle - Institute of Inorganic Chemistry, University of Stuttgart, Pfaffenwaldring 55, Stuttgart 70569, Germany
    • Felix Ehrlich - Institute of Physical Chemistry, University of Stuttgart, Pfaffenwaldring 55, Stuttgart 70569, Germany
    • Kim-Isabelle Mehnert - Institute of Physical Chemistry, University of Stuttgart, Pfaffenwaldring 55, Stuttgart 70569, Germany
    • David Hunger - Institute of Physical Chemistry, University of Stuttgart, Pfaffenwaldring 55, Stuttgart 70569, Germany
    • Alena M. Sheveleva - Department of Chemistry and Photon Science Institute, University of Manchester, Oxford Road, Manchester M13 9PL, U.K.
    • Lukas Burkhardt - Department of Chemistry and Center for Sustainable Systems Design, Paderborn University, Warburger Strasse 100, Paderborn 33098, GermanyOrcidhttp://orcid.org/0000-0003-0747-9811
    • Matthias Bauer - Department of Chemistry and Center for Sustainable Systems Design, Paderborn University, Warburger Strasse 100, Paderborn 33098, GermanyOrcidhttp://orcid.org/0000-0002-9294-6076
    • Floriana Tuna - Department of Chemistry and Photon Science Institute, University of Manchester, Oxford Road, Manchester M13 9PL, U.K.
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

ARTICLE SECTIONS
Jump To

This work was financially supported by the Deutsche Forschungsgemeinschaft (CRC1333 Projects C2, S1), Leverhulme Trust UK (RF-2018-545/4), EPSRC U.K. National Electron Paramagnetic Resonance Facility (NS/A000055/1), The Royal Society (IEC/R2/170250 and NIF/R1/180441), and German BMBF through Projects Syn-XAS (05K18PPA) and FocusPP64 (05K19PP1). The provision of beamtime at beamline ID26 of the ESRF is gratefully acknowledged.

References

ARTICLE SECTIONS
Jump To

This article references 44 other publications.

  1. 1
    Zeng, M.; Li, Y. Recent advances in heterogeneous electrocatalysts for the hydrogen evolution reaction. J. Mater. Chem. A 2015, 3 (29), 1494214962,  DOI: 10.1039/C5TA02974K
  2. 2
    Thoi, V. S.; Sun, Y.; Long, J. R.; Chang, C. J. Complexes of earth-abundant metals for catalytic electrochemical hydrogen generation under aqueous conditions. Chem. Soc. Rev. 2013, 42 (6), 23882400,  DOI: 10.1039/C2CS35272A
  3. 3
    Schilter, D.; Camara, J. M.; Huynh, M. T.; Hammes-Schiffer, S.; Rauchfuss, T. B. Hydrogenase Enzymes and Their Synthetic Models: The Role of Metal Hydrides. Chem. Rev. 2016, 116 (15), 86938749,  DOI: 10.1021/acs.chemrev.6b00180
  4. 4
    Ringenberg, M. R.; Wittkamp, F.; Apfel, U.-P.; Kaim, W. Redox Induced Configurational Isomerization of Bisphosphine–Tricarbonyliron(I) Complexes and the Difference a Ferrocene Makes. Inorg. Chem. 2017, 56 (13), 75017511,  DOI: 10.1021/acs.inorgchem.7b00957
  5. 5
    Eckert, F.; Leito, I.; Kaljurand, I.; Kütt, A.; Klamt, A.; Diedenhofen, M. Prediction of acidity in acetonitrile solution with COSMO-RS. J. Comput. Chem. 2009, 30 (5), 799810,  DOI: 10.1002/jcc.21103
  6. 6
    Pool, D. H.; DuBois, D. L. [Ni(PPh2NAr2)2(NCMe)][BF4]2 as an electrocatalyst for H2 production: PPh2NAr2 = 1,5-(di(4-(thiophene-3-yl)phenyl)-3,7-diphenyl-1,5-diaza-3,7-diphosphacyclooctane). J. Organomet. Chem. 2009, 694 (17), 28582865,  DOI: 10.1016/j.jorganchem.2009.04.010
  7. 7
    Wilson, A. D.; Newell, R. H.; McNevin, M. J.; Muckerman, J. T.; Rakowski DuBois, M.; DuBois, D. L. Hydrogen Oxidation and Production Using Nickel-Based Molecular Catalysts with Positioned Proton Relays. J. Am. Chem. Soc. 2006, 128 (1), 358366,  DOI: 10.1021/ja056442y
  8. 8
    Helm, M. L.; Stewart, M. P.; Bullock, R. M.; DuBois, M. R.; DuBois, D. L. A Synthetic Nickel Electrocatalyst with a Turnover Frequency Above 100,000 s–1 for H2 Production. Science 2011, 333 (6044), 863866,  DOI: 10.1126/science.1205864
  9. 9
    Mason, W. R. A Practical Guide to Magnetic Circular Dichroism; John Wiley & Sons: Hoboken, NJ, 2007.
  10. 10
    McMaster, J.; Carducci, M. D.; Yang, Y.-S.; Solomon, E. I.; Enemark, J. H. Electronic Spectral Studies of Molybdenyl Complexes. 2. MCD Spectroscopy of [MoOS4]- Centers. Inorg. Chem. 2001, 40 (4), 687702,  DOI: 10.1021/ic0005846
  11. 11
    Rowe, M. D.; McCaffery, A. J. Electronic structure of ferricenium ion from absorption, MCD, and ESR studies. J. Chem. Phys. 1973, 59 (7), 37863794,  DOI: 10.1063/1.1680551
  12. 12
    Fulara, J.; Filipkowski, K.; Maier, J. P. Electronic Transition of Ferrocenium: Neon Matrix and CASPT2 Studies. J. Phys. Chem. C 2017, 121 (20), 1069410697,  DOI: 10.1021/acs.jpcc.6b10391
  13. 13
    Ishimura, K.; Hada, M.; Nakatsuji, H. Ionized and excited states of ferrocene: Symmetry adapted cluster–configuration–interaction study. J. Chem. Phys. 2002, 117 (14), 65336537,  DOI: 10.1063/1.1504709
  14. 14
    Kneebone, J. L.; Fleischauer, V. E.; Daifuku, S. L.; Shaps, A. A.; Bailey, J. M.; Iannuzzi, T. E.; Neidig, M. L. Electronic Structure and Bonding in Iron(II) and Iron(I) Complexes Bearing Bisphosphine Ligands of Relevance to Iron-Catalyzed C–C Cross-Coupling. Inorg. Chem. 2016, 55 (1), 272282,  DOI: 10.1021/acs.inorgchem.5b02263
  15. 15
    Bauer, M. HERFD-XAS and valence-to-core-XES: new tools to push the limits in research with hard X-rays?. Phys. Chem. Chem. Phys. 2014, 16 (27), 1382713837,  DOI: 10.1039/C4CP00904E
  16. 16
    Hämäläinen, K.; Siddons, D. P.; Hastings, J. B.; Berman, L. E. Elimination of the inner-shell lifetime broadening in x-ray-absorption spectroscopy. Phys. Rev. Lett. 1991, 67 (20), 28502853,  DOI: 10.1103/PhysRevLett.67.2850
  17. 17
    Burkhardt, L.; Vukadinovic, Y.; Nowakowski, M.; Kalinko, A.; Rudolph, J.; Carlsson, P.-A.; Jacob, C. R.; Bauer, M. Electronic Structure of the Hieber Anion [Fe(CO)3(NO)]– Revisited by X-ray Emission and Absorption Spectroscopy. Inorg. Chem. 2020, 59 (6), 35513561,  DOI: 10.1021/acs.inorgchem.9b02092
  18. 18
    Burkhardt, L.; Holzwarth, M.; Plietker, B.; Bauer, M. Detection and Characterization of Hydride Ligands in Iron Complexes by High-Resolution Hard X-ray Spectroscopy and Implications for Catalytic Processes. Inorg. Chem. 2017, 56 (21), 1330013310,  DOI: 10.1021/acs.inorgchem.7b02063
  19. 19
    Burkhardt, L.; Mueller, C.; Groß, O. A.; Sun, Y.; Sitzmann, H.; Bauer, M. The Bonding Situation in the Dinuclear Tetra-Hydrido Complex [{5CpFe}2(μ-H)4] Revisited by Hard X-Ray Spectroscopy. Inorg. Chem. 2019, 58 (10), 66096618,  DOI: 10.1021/acs.inorgchem.8b03032
  20. 20
    Lancaster, K. M.; Finkelstein, K. D.; DeBeer, S. Kβ. X-ray Emission Spectroscopy Offers Unique Chemical Bonding Insights: Revisiting the Electronic Structure of Ferrocene. Inorg. Chem. 2011, 50 (14), 67676774,  DOI: 10.1021/ic200822b
  21. 21
    Atkins, A. J.; Bauer, M.; Jacob, C. R. The chemical sensitivity of X-ray spectroscopy: high energy resolution XANES versus X-ray emission spectroscopy of substituted ferrocenes. Phys. Chem. Chem. Phys. 2013, 15 (21), 80958105,  DOI: 10.1039/c3cp50999k
  22. 22
    Glatzel, P.; Bergmann, U. High resolution 1s core hole X-ray spectroscopy in 3d transition metal complexes—electronic and structural information. Coord. Chem. Rev. 2005, 249 (1), 6595,  DOI: 10.1016/j.ccr.2004.04.011
  23. 23
    Pollock, C. J.; Delgado-Jaime, M. U.; Atanasov, M.; Neese, F.; DeBeer, S. Kβ Mainline X-ray Emission Spectroscopy as an Experimental Probe of Metal–Ligand Covalency. J. Am. Chem. Soc. 2014, 136 (26), 94539463,  DOI: 10.1021/ja504182n
  24. 24
    Atkins, A. J.; Bauer, M.; Jacob, C. R. High-resolution X-ray absorption spectroscopy of iron carbonyl complexes. Phys. Chem. Chem. Phys. 2015, 17 (21), 1393713948,  DOI: 10.1039/C5CP01045D
  25. 25
    Messinis, A. M.; Luckham, S. L. J.; Wells, P. P.; Gianolio, D.; Gibson, E. K.; O’Brien, H. M.; Sparkes, H. A.; Davis, S. A.; Callison, J.; Elorriaga, D.; Hernandez-Fajardo, O.; Bedford, R. B. The highly surprising behaviour of diphosphine ligands in iron-catalysed Negishi cross-coupling. Nat. Catal. 2019, 2 (2), 123133,  DOI: 10.1038/s41929-018-0197-z
  26. 26
    Hess, J. L.; Hsieh, C.-H.; Brothers, S. M.; Hall, M. B.; Darensbourg, M. Y. Self-Assembly of Dinitrosyl Iron Units into Imidazolate-Edge-Bridged Molecular Squares: Characterization Including Mössbauer Spectroscopy. J. Am. Chem. Soc. 2011, 133 (50), 2042620434,  DOI: 10.1021/ja208384d
  27. 27
    Schiewer, C. E.; Müller, C. S.; Dechert, S.; Bergner, M.; Wolny, J. A.; Schünemann, V.; Meyer, F. Effect of Oxidation and Protonation States on [2Fe–2S] Cluster Nitrosylation Giving {Fe(NO)2}9 Dinitrosyl Iron Complexes (DNICs). Inorg. Chem. 2019, 58 (1), 769784,  DOI: 10.1021/acs.inorgchem.8b02927
  28. 28
    MacLeod, K. C.; Vinyard, D. J.; Holland, P. L. A Multi-iron System Capable of Rapid N2 Formation and N2 Cleavage. J. Am. Chem. Soc. 2014, 136 (29), 1022610229,  DOI: 10.1021/ja505193z
  29. 29
    Gütlich, P.; Bill, E.; Trautwein, A. X. Mössbauer Spectroscopy and Transition Metal Chemistry; Springer: Berlin, 2011.
  30. 30
    Blume, M.; Tjon, J. A. Mössbauer Blume Tjon. Phys. Rev. 1968, 165, 446,  DOI: 10.1103/PhysRev.165.446
  31. 31
    Reiners, M.; Baabe, D.; Schweyen, P.; Freytag, M.; Jones, P. G.; Walter, M. D. Teaching Ferrocenium How to Relax: A Systematic Study on Spin–Lattice Relaxation Processes in tert-Butyl-Substituted Ferrocenium Derivatives. Eur. J. Inorg. Chem. 2017, 2017 (2), 388400,  DOI: 10.1002/ejic.201600873
  32. 32
    Ding, M.; Hickey, A. K.; Pink, M.; Telser, J.; Tierney, D. L.; Amoza, M.; Rouzières, M.; Ozumerzifon, T. J.; Hoffert, W. A.; Shores, M. P.; Ruiz, E.; Clérac, R.; Smith, J. M. Magnetization Slow Dynamics in Ferrocenium Complexes. Chem. - Eur. J. 2019, 25 (45), 1062510632,  DOI: 10.1002/chem.201900799
  33. 33
    Tesi, L.; Lucaccini, E.; Cimatti, I.; Perfetti, M.; Mannini, M.; Atzori, M.; Morra, E.; Chiesa, M.; Caneschi, A.; Sorace, L.; Sessoli, R. Quantum coherence in a processable vanadyl complex: new tools for the search of molecular spin qubits. Chem. Sci. 2016, 7, 2074,  DOI: 10.1039/C5SC04295J
  34. 34
    Atzori, M.; Benci, S.; Morra, E.; Tesi, L.; Chiesa, M.; Torre, R.; Sorace, L.; Sessoli, R. Structural Effects on the Spin Dynamics of Potential Molecular Qubits. Inorg. Chem. 2018, 57 (2), 731740,  DOI: 10.1021/acs.inorgchem.7b02616
  35. 35
    Atzori, M.; Tesi, L.; Benci, S.; Lunghi, A.; Righini, R.; Taschin, A.; Torre, R.; Sorace, L.; Sessoli, R. Spin Dynamics and Low Energy Vibrations: Insights from Vanadyl-Based Potential Molecular Qubits. J. Am. Chem. Soc. 2017, 139 (12), 43384341,  DOI: 10.1021/jacs.7b01266
  36. 36
    Atzori, M.; Morra, E.; Tesi, L.; Albino, A.; Chiesa, M.; Sorace, L.; Sessoli, R. Quantum Coherence Times Enhancement in Vanadium(IV)-based Potential Molecular Qubits: the Key Role of the Vanadyl Moiety. J. Am. Chem. Soc. 2016, 138 (35), 1123411244,  DOI: 10.1021/jacs.6b05574
  37. 37
    Schweiger, A.; Jeschke, G. Principles of Pulse Electron Paramagnetic Resonance; Oxford University Press: Oxford, U.K., 2001.
  38. 38
    Jeschke, G. ESR Spectroscopy in Membrane Biophysics; Springer: Boston, MA, 2007; pp 1747.
  39. 39
    Kim, T.-J.; Kwon, K.-H.; Kwon, S.-C.; Baeg, J.-O.; Shim, S.-C.; Lee, D.-H. Iron complexes of 1,1′-bis(diphenylphosphino)ferrocene (BPPF) as efficient catalysts in the synthesis of carbamates. X-ray crystal structure of (BPPF)Fe(CO)3. J. Organomet. Chem. 1990, 389 (2), 205217,  DOI: 10.1016/0022-328X(90)85412-R
  40. 40
    Rechkemmer, Y.; Fischer, J. E.; Marx, R.; Dörfel, M.; Neugebauer, P.; Horvath, S.; Gysler, M.; Brock-Nannestad, T.; Frey, W.; Reid, M. F.; van Slageren, J. Comprehensive Spectroscopic Determination of the Crystal Field Splitting in an Erbium Single-Ion Magnet. J. Am. Chem. Soc. 2015, 137, 1311413120,  DOI: 10.1021/jacs.5b08344
  41. 41
    Rechkemmer, Y. Spectroscopic Investigations of the Magnetic Anisotropy of Lanthanide and Cobalt based Molecular Nanomagnets. Ph.D. Thesis, University of Stuttgart, Stuttgart, Germany, 2016.
  42. 42
    Neugebauer, P.; Bloos, D.; Marx, R.; Lutz, P.; Kern, M.; Aguila, D.; Vaverka, J.; Laguta, O.; Dietrich, C.; Clérac, R.; van Slageren, J. Ultra-broadband EPR spectroscopy in field and frequency domains. Phys. Chem. Chem. Phys. 2018, 20 (22), 1552815534,  DOI: 10.1039/C7CP07443C
  43. 43
    Tkach, I.; Baldansuren, A.; Kalabukhova, E.; Lukin, S.; Sitnikov, A.; Tsvir, A.; Ischenko, M.; Rosentzweig, Y.; Roduner, E. A Homebuilt ESE Spectrometer on the Basis of a High-Power Q-Band Microwave Bridge. Appl. Magn. Reson. 2008, 35 (1), 95112,  DOI: 10.1007/s00723-008-0141-5
  44. 44
    Stoll, S.; Schweiger, A. EasySpin, a comprehensive software package for spectral simulation and analysis in EPR. J. Magn. Reson. 2006, 178 (1), 4255,  DOI: 10.1016/j.jmr.2005.08.013

Cited By

ARTICLE SECTIONS
Jump To

This article is cited by 1 publications.

  1. Yixian Pan, Marc Schnierle, Daniel Auweiler, Mark R. Ringenberg. Electrochemical Reduction Mechanism of [(η5-C5H5)Fe(dppf)(CO)]+ (dppf = 1,1′-Bis(diphenylphosphino)ferrocene). Organometallics 2021, 40 (6) , 760-765. https://doi.org/10.1021/acs.organomet.1c00010
  • Abstract

    Scheme 1

    Scheme 1. Structures of 10/+ and [(dppf)FeH(CO)3]+ Showing the Two Isomers of 1+ with Respective SP and TBP Carbonyl Iron Coordination Geometries

    Figure 1

    Figure 1. (A) Cyclic voltammograms (scan rate 100 mV s–1), recorded on a solution of 1 in CH3CN with the repeated addition of HOTs equivalents. (B) Catalytic/initial anodic current ratio as a function of the number of added HOTs equivalents.

    Figure 2

    Figure 2. MCD spectra recorded on powder mulls of 10, 1+, and [Fc](BArF4) in Fluorolube at T = 1.5 K. Applied fields were B = 5 T (1+/0) and 100 mT ([Fc]+).

    Figure 3

    Figure 3. HERFD-XANES spectra of powder samples of 10 and 1+, as well as of the reference compounds [Fc](PF6), (21) dppf, (21) Fe2(CO)9, (24) and [FeBr(dpbz)2]. (25)

    Figure 4

    Figure 4. High-frequency EPR spectra recorded on a pressed powder pellet of 1+ at T = 10 K and different frequencies as indicated (black lines). Red lines are simulations using gxx = 1.991, gyy = 2.006, and gzz = 2.046 (giso = 2.014).

    Figure 5

    Figure 5. Room temperature Mössbauer spectra of powder samples of 10 and 1+, as well as of the reference compounds dppf, Fc, [Fc](BArF4), and [(PPh3)2Fe(CO)3].

    Figure 6

    Figure 6. Mössbauer spectra recorded on a 60 mg powder sample of 1+ at different temperatures as indicated. Solid lines are Lorentzian line deconvolutions.

    Figure 7

    Figure 7. X-band HYSCORE spectrum (blue) recorded on a 2 mM frozen solution of 1+ in dichloromethane/toluene (1:1) at 10 K at an applied field of B0 = 343.9 mT, using a short delay time of 136 ns after the first pulse in the four-pulse HYSCORE sequence. The simulation is shown in red (see the text for details).

    Figure 8

    Figure 8. Pulsed Q-band ENDOR spectrum recorded on a 4 mM frozen solution of 1+ in dichloromethane/toluene (1:1) at 10 K and an applied field of B0 = 1200.6 mT.

  • References

    ARTICLE SECTIONS
    Jump To

    This article references 44 other publications.

    1. 1
      Zeng, M.; Li, Y. Recent advances in heterogeneous electrocatalysts for the hydrogen evolution reaction. J. Mater. Chem. A 2015, 3 (29), 1494214962,  DOI: 10.1039/C5TA02974K
    2. 2
      Thoi, V. S.; Sun, Y.; Long, J. R.; Chang, C. J. Complexes of earth-abundant metals for catalytic electrochemical hydrogen generation under aqueous conditions. Chem. Soc. Rev. 2013, 42 (6), 23882400,  DOI: 10.1039/C2CS35272A
    3. 3
      Schilter, D.; Camara, J. M.; Huynh, M. T.; Hammes-Schiffer, S.; Rauchfuss, T. B. Hydrogenase Enzymes and Their Synthetic Models: The Role of Metal Hydrides. Chem. Rev. 2016, 116 (15), 86938749,  DOI: 10.1021/acs.chemrev.6b00180
    4. 4
      Ringenberg, M. R.; Wittkamp, F.; Apfel, U.-P.; Kaim, W. Redox Induced Configurational Isomerization of Bisphosphine–Tricarbonyliron(I) Complexes and the Difference a Ferrocene Makes. Inorg. Chem. 2017, 56 (13), 75017511,  DOI: 10.1021/acs.inorgchem.7b00957
    5. 5
      Eckert, F.; Leito, I.; Kaljurand, I.; Kütt, A.; Klamt, A.; Diedenhofen, M. Prediction of acidity in acetonitrile solution with COSMO-RS. J. Comput. Chem. 2009, 30 (5), 799810,  DOI: 10.1002/jcc.21103
    6. 6
      Pool, D. H.; DuBois, D. L. [Ni(PPh2NAr2)2(NCMe)][BF4]2 as an electrocatalyst for H2 production: PPh2NAr2 = 1,5-(di(4-(thiophene-3-yl)phenyl)-3,7-diphenyl-1,5-diaza-3,7-diphosphacyclooctane). J. Organomet. Chem. 2009, 694 (17), 28582865,  DOI: 10.1016/j.jorganchem.2009.04.010
    7. 7
      Wilson, A. D.; Newell, R. H.; McNevin, M. J.; Muckerman, J. T.; Rakowski DuBois, M.; DuBois, D. L. Hydrogen Oxidation and Production Using Nickel-Based Molecular Catalysts with Positioned Proton Relays. J. Am. Chem. Soc. 2006, 128 (1), 358366,  DOI: 10.1021/ja056442y
    8. 8
      Helm, M. L.; Stewart, M. P.; Bullock, R. M.; DuBois, M. R.; DuBois, D. L. A Synthetic Nickel Electrocatalyst with a Turnover Frequency Above 100,000 s–1 for H2 Production. Science 2011, 333 (6044), 863866,  DOI: 10.1126/science.1205864
    9. 9
      Mason, W. R. A Practical Guide to Magnetic Circular Dichroism; John Wiley & Sons: Hoboken, NJ, 2007.
    10. 10
      McMaster, J.; Carducci, M. D.; Yang, Y.-S.; Solomon, E. I.; Enemark, J. H. Electronic Spectral Studies of Molybdenyl Complexes. 2. MCD Spectroscopy of [MoOS4]- Centers. Inorg. Chem. 2001, 40 (4), 687702,  DOI: 10.1021/ic0005846
    11. 11
      Rowe, M. D.; McCaffery, A. J. Electronic structure of ferricenium ion from absorption, MCD, and ESR studies. J. Chem. Phys. 1973, 59 (7), 37863794,  DOI: 10.1063/1.1680551
    12. 12
      Fulara, J.; Filipkowski, K.; Maier, J. P. Electronic Transition of Ferrocenium: Neon Matrix and CASPT2 Studies. J. Phys. Chem. C 2017, 121 (20), 1069410697,  DOI: 10.1021/acs.jpcc.6b10391
    13. 13
      Ishimura, K.; Hada, M.; Nakatsuji, H. Ionized and excited states of ferrocene: Symmetry adapted cluster–configuration–interaction study. J. Chem. Phys. 2002, 117 (14), 65336537,  DOI: 10.1063/1.1504709
    14. 14
      Kneebone, J. L.; Fleischauer, V. E.; Daifuku, S. L.; Shaps, A. A.; Bailey, J. M.; Iannuzzi, T. E.; Neidig, M. L. Electronic Structure and Bonding in Iron(II) and Iron(I) Complexes Bearing Bisphosphine Ligands of Relevance to Iron-Catalyzed C–C Cross-Coupling. Inorg. Chem. 2016, 55 (1), 272282,  DOI: 10.1021/acs.inorgchem.5b02263
    15. 15
      Bauer, M. HERFD-XAS and valence-to-core-XES: new tools to push the limits in research with hard X-rays?. Phys. Chem. Chem. Phys. 2014, 16 (27), 1382713837,  DOI: 10.1039/C4CP00904E
    16. 16
      Hämäläinen, K.; Siddons, D. P.; Hastings, J. B.; Berman, L. E. Elimination of the inner-shell lifetime broadening in x-ray-absorption spectroscopy. Phys. Rev. Lett. 1991, 67 (20), 28502853,  DOI: 10.1103/PhysRevLett.67.2850
    17. 17
      Burkhardt, L.; Vukadinovic, Y.; Nowakowski, M.; Kalinko, A.; Rudolph, J.; Carlsson, P.-A.; Jacob, C. R.; Bauer, M. Electronic Structure of the Hieber Anion [Fe(CO)3(NO)]– Revisited by X-ray Emission and Absorption Spectroscopy. Inorg. Chem. 2020, 59 (6), 35513561,  DOI: 10.1021/acs.inorgchem.9b02092
    18. 18
      Burkhardt, L.; Holzwarth, M.; Plietker, B.; Bauer, M. Detection and Characterization of Hydride Ligands in Iron Complexes by High-Resolution Hard X-ray Spectroscopy and Implications for Catalytic Processes. Inorg. Chem. 2017, 56 (21), 1330013310,  DOI: 10.1021/acs.inorgchem.7b02063
    19. 19
      Burkhardt, L.; Mueller, C.; Groß, O. A.; Sun, Y.; Sitzmann, H.; Bauer, M. The Bonding Situation in the Dinuclear Tetra-Hydrido Complex [{5CpFe}2(μ-H)4] Revisited by Hard X-Ray Spectroscopy. Inorg. Chem. 2019, 58 (10), 66096618,  DOI: 10.1021/acs.inorgchem.8b03032
    20. 20
      Lancaster, K. M.; Finkelstein, K. D.; DeBeer, S. Kβ. X-ray Emission Spectroscopy Offers Unique Chemical Bonding Insights: Revisiting the Electronic Structure of Ferrocene. Inorg. Chem. 2011, 50 (14), 67676774,  DOI: 10.1021/ic200822b
    21. 21
      Atkins, A. J.; Bauer, M.; Jacob, C. R. The chemical sensitivity of X-ray spectroscopy: high energy resolution XANES versus X-ray emission spectroscopy of substituted ferrocenes. Phys. Chem. Chem. Phys. 2013, 15 (21), 80958105,  DOI: 10.1039/c3cp50999k
    22. 22
      Glatzel, P.; Bergmann, U. High resolution 1s core hole X-ray spectroscopy in 3d transition metal complexes—electronic and structural information. Coord. Chem. Rev. 2005, 249 (1), 6595,  DOI: 10.1016/j.ccr.2004.04.011
    23. 23
      Pollock, C. J.; Delgado-Jaime, M. U.; Atanasov, M.; Neese, F.; DeBeer, S. Kβ Mainline X-ray Emission Spectroscopy as an Experimental Probe of Metal–Ligand Covalency. J. Am. Chem. Soc. 2014, 136 (26), 94539463,  DOI: 10.1021/ja504182n
    24. 24
      Atkins, A. J.; Bauer, M.; Jacob, C. R. High-resolution X-ray absorption spectroscopy of iron carbonyl complexes. Phys. Chem. Chem. Phys. 2015, 17 (21), 1393713948,  DOI: 10.1039/C5CP01045D
    25. 25
      Messinis, A. M.; Luckham, S. L. J.; Wells, P. P.; Gianolio, D.; Gibson, E. K.; O’Brien, H. M.; Sparkes, H. A.; Davis, S. A.; Callison, J.; Elorriaga, D.; Hernandez-Fajardo, O.; Bedford, R. B. The highly surprising behaviour of diphosphine ligands in iron-catalysed Negishi cross-coupling. Nat. Catal. 2019, 2 (2), 123133,  DOI: 10.1038/s41929-018-0197-z
    26. 26
      Hess, J. L.; Hsieh, C.-H.; Brothers, S. M.; Hall, M. B.; Darensbourg, M. Y. Self-Assembly of Dinitrosyl Iron Units into Imidazolate-Edge-Bridged Molecular Squares: Characterization Including Mössbauer Spectroscopy. J. Am. Chem. Soc. 2011, 133 (50), 2042620434,  DOI: 10.1021/ja208384d
    27. 27
      Schiewer, C. E.; Müller, C. S.; Dechert, S.; Bergner, M.; Wolny, J. A.; Schünemann, V.; Meyer, F. Effect of Oxidation and Protonation States on [2Fe–2S] Cluster Nitrosylation Giving {Fe(NO)2}9 Dinitrosyl Iron Complexes (DNICs). Inorg. Chem. 2019, 58 (1), 769784,  DOI: 10.1021/acs.inorgchem.8b02927
    28. 28
      MacLeod, K. C.; Vinyard, D. J.; Holland, P. L. A Multi-iron System Capable of Rapid N2 Formation and N2 Cleavage. J. Am. Chem. Soc. 2014, 136 (29), 1022610229,  DOI: 10.1021/ja505193z
    29. 29
      Gütlich, P.; Bill, E.; Trautwein, A. X. Mössbauer Spectroscopy and Transition Metal Chemistry; Springer: Berlin, 2011.
    30. 30
      Blume, M.; Tjon, J. A. Mössbauer Blume Tjon. Phys. Rev. 1968, 165, 446,  DOI: 10.1103/PhysRev.165.446
    31. 31
      Reiners, M.; Baabe, D.; Schweyen, P.; Freytag, M.; Jones, P. G.; Walter, M. D. Teaching Ferrocenium How to Relax: A Systematic Study on Spin–Lattice Relaxation Processes in tert-Butyl-Substituted Ferrocenium Derivatives. Eur. J. Inorg. Chem. 2017, 2017 (2), 388400,  DOI: 10.1002/ejic.201600873
    32. 32
      Ding, M.; Hickey, A. K.; Pink, M.; Telser, J.; Tierney, D. L.; Amoza, M.; Rouzières, M.; Ozumerzifon, T. J.; Hoffert, W. A.; Shores, M. P.; Ruiz, E.; Clérac, R.; Smith, J. M. Magnetization Slow Dynamics in Ferrocenium Complexes. Chem. - Eur. J. 2019, 25 (45), 1062510632,  DOI: 10.1002/chem.201900799
    33. 33
      Tesi, L.; Lucaccini, E.; Cimatti, I.; Perfetti, M.; Mannini, M.; Atzori, M.; Morra, E.; Chiesa, M.; Caneschi, A.; Sorace, L.; Sessoli, R. Quantum coherence in a processable vanadyl complex: new tools for the search of molecular spin qubits. Chem. Sci. 2016, 7, 2074,  DOI: 10.1039/C5SC04295J
    34. 34
      Atzori, M.; Benci, S.; Morra, E.; Tesi, L.; Chiesa, M.; Torre, R.; Sorace, L.; Sessoli, R. Structural Effects on the Spin Dynamics of Potential Molecular Qubits. Inorg. Chem. 2018, 57 (2), 731740,  DOI: 10.1021/acs.inorgchem.7b02616
    35. 35
      Atzori, M.; Tesi, L.; Benci, S.; Lunghi, A.; Righini, R.; Taschin, A.; Torre, R.; Sorace, L.; Sessoli, R. Spin Dynamics and Low Energy Vibrations: Insights from Vanadyl-Based Potential Molecular Qubits. J. Am. Chem. Soc. 2017, 139 (12), 43384341,  DOI: 10.1021/jacs.7b01266
    36. 36
      Atzori, M.; Morra, E.; Tesi, L.; Albino, A.; Chiesa, M.; Sorace, L.; Sessoli, R. Quantum Coherence Times Enhancement in Vanadium(IV)-based Potential Molecular Qubits: the Key Role of the Vanadyl Moiety. J. Am. Chem. Soc. 2016, 138 (35), 1123411244,  DOI: 10.1021/jacs.6b05574
    37. 37
      Schweiger, A.; Jeschke, G. Principles of Pulse Electron Paramagnetic Resonance; Oxford University Press: Oxford, U.K., 2001.
    38. 38
      Jeschke, G. ESR Spectroscopy in Membrane Biophysics; Springer: Boston, MA, 2007; pp 1747.
    39. 39
      Kim, T.-J.; Kwon, K.-H.; Kwon, S.-C.; Baeg, J.-O.; Shim, S.-C.; Lee, D.-H. Iron complexes of 1,1′-bis(diphenylphosphino)ferrocene (BPPF) as efficient catalysts in the synthesis of carbamates. X-ray crystal structure of (BPPF)Fe(CO)3. J. Organomet. Chem. 1990, 389 (2), 205217,  DOI: 10.1016/0022-328X(90)85412-R
    40. 40
      Rechkemmer, Y.; Fischer, J. E.; Marx, R.; Dörfel, M.; Neugebauer, P.; Horvath, S.; Gysler, M.; Brock-Nannestad, T.; Frey, W.; Reid, M. F.; van Slageren, J. Comprehensive Spectroscopic Determination of the Crystal Field Splitting in an Erbium Single-Ion Magnet. J. Am. Chem. Soc. 2015, 137, 1311413120,  DOI: 10.1021/jacs.5b08344
    41. 41
      Rechkemmer, Y. Spectroscopic Investigations of the Magnetic Anisotropy of Lanthanide and Cobalt based Molecular Nanomagnets. Ph.D. Thesis, University of Stuttgart, Stuttgart, Germany, 2016.
    42. 42
      Neugebauer, P.; Bloos, D.; Marx, R.; Lutz, P.; Kern, M.; Aguila, D.; Vaverka, J.; Laguta, O.; Dietrich, C.; Clérac, R.; van Slageren, J. Ultra-broadband EPR spectroscopy in field and frequency domains. Phys. Chem. Chem. Phys. 2018, 20 (22), 1552815534,  DOI: 10.1039/C7CP07443C
    43. 43
      Tkach, I.; Baldansuren, A.; Kalabukhova, E.; Lukin, S.; Sitnikov, A.; Tsvir, A.; Ischenko, M.; Rosentzweig, Y.; Roduner, E. A Homebuilt ESE Spectrometer on the Basis of a High-Power Q-Band Microwave Bridge. Appl. Magn. Reson. 2008, 35 (1), 95112,  DOI: 10.1007/s00723-008-0141-5
    44. 44
      Stoll, S.; Schweiger, A. EasySpin, a comprehensive software package for spectral simulation and analysis in EPR. J. Magn. Reson. 2006, 178 (1), 4255,  DOI: 10.1016/j.jmr.2005.08.013
  • Supporting Information

    Supporting Information

    ARTICLE SECTIONS
    Jump To

    The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c03259.

    • Additional MCD data with discussion on 10, VtC- and CtC-XES spectra for 1+/0, magnified portions of the HERFS-XANES spectra of 1+/0 and reference compounds, HFEPR spectra for 1+ plotted as a function of the field, table of the HFEPR fit data with literature values of reference compounds, X-band EPR spectra recorded on powders and frozen solutions of 1+, table of the Mössbauer fit data and literature values of reference compounds, temperature dependence of the fractions of the two isomers of 1+ as derived from VT Mössbauer data, Mössbauer spectrum of [Fc]+(BArF4), Blume–Tjon fit of the 3 K Mössbauer spectrum of 1+ and discussion, ac SQUID measurement data at different fields, frequencies, and temperatures, fits of the ac relaxation data, table of ac relaxation fit data with literature values of reference compounds, echo-detected EPR spectra for 1+ at X and Q bands, inversion recovery and Hahn-echo decay curves, table and figure with spin relaxation times at different temperatures, ESEEM curve and its Fourier transform, HYSCORE pulse sequence, X-band HYSCORE spectra, X- and Q-band ENDOR spectra, and Easyspin simulation file (PDF)


    Terms & Conditions

    Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.

Pair your accounts.

Export articles to Mendeley

Get article recommendations from ACS based on references in your Mendeley library.

Pair your accounts.

Export articles to Mendeley

Get article recommendations from ACS based on references in your Mendeley library.

You’ve supercharged your research process with ACS and Mendeley!

STEP 1:
Click to create an ACS ID

Please note: If you switch to a different device, you may be asked to login again with only your ACS ID.

Please note: If you switch to a different device, you may be asked to login again with only your ACS ID.

Please note: If you switch to a different device, you may be asked to login again with only your ACS ID.

MENDELEY PAIRING EXPIRED
Your Mendeley pairing has expired. Please reconnect