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Venturing Past Uranium: Synthesis of a Np(IV) Polyoxomolybdate–Alkoxide Sandwich Complex
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  • Leyla R. Valerio
    Leyla R. Valerio
    Department of Chemistry, University of Rochester, Rochester, New York 14627, United States
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  • Dominic Shiels
    Dominic Shiels
    Department of Chemistry, University of Rochester, Rochester, New York 14627, United States
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  • Lauren M. Lopez
    Lauren M. Lopez
    H. C. Brown Laboratory, James Tarpo Jr. and Margaret Tarpo, Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States
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  • Andrew W. Mitchell
    Andrew W. Mitchell
    H. C. Brown Laboratory, James Tarpo Jr. and Margaret Tarpo, Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States
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  • Matthias Zeller
    Matthias Zeller
    H. C. Brown Laboratory, James Tarpo Jr. and Margaret Tarpo, Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States
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    Orcidhttps://orcid.org/0000-0002-3305-852X
  • Suzanne C. Bart*
    Suzanne C. Bart
    H. C. Brown Laboratory, James Tarpo Jr. and Margaret Tarpo, Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States
    *Email: [email protected]
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    Orcidhttps://orcid.org/0000-0002-8918-9051
  • Ellen M. Matson*
    Ellen M. Matson
    Department of Chemistry, University of Rochester, Rochester, New York 14627, United States
    *Email: [email protected]
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    Orcidhttps://orcid.org/0000-0003-3753-8288
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Inorganic Chemistry

Cite this: Inorg. Chem. 2024, 63, 48, 22639–22649
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https://doi.org/10.1021/acs.inorgchem.4c04428
Published November 20, 2024

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Abstract

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The synthesis of a Np(IV) polyoxomolybdate–alkoxide sandwich complex, (TBA)2[Np{Mo5O13(OMe)4NO}2] (TBA = tetrabutylammonium), is reported. This compound represents a rare example of a neptunium polyoxometalate cluster isolated outside of water, allowing for characterization of its electrochemical properties in nonaqueous solvents. Complexation of An(IV) cations fine-tunes the redox properties of the cluster, with the observed four reversible reductive events varying slightly both in potential and peak separation depending on the actinide present. The new Np(IV) complex also shows an irreversible event assigned to oxidation of Np(IV) to Np(V). New methodology for facile 17O enrichment of (TBA)2[Mo5O13(OMe)4NO][Na(MeOH)] is presented, which provides a simple pathway to 17O enriched analogues of the sandwich complexes discussed (Zr(IV), Hf(IV), Th(IV), U(IV), U(V), Np(IV)). 17O NMR spectroscopy subsequently provides insights into both the nature of metal–oxygen bonding, as well as the influence of unpaired f-electrons on the local environment of the oxygen nuclei.

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Synopsis

Reaction of a lacunary polyoxomolybdate−alkoxide with neptunium(IV) results in the formation of a rare neptunium−polyoxometalate complex isolated outside of water.

Introduction

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There has been a renewed interest in studying the fundamental chemistry of actinides due to their importance in fields such as spent nuclear fuel processing and environmental remediation. (1−4) The majority of studies have focused on uranium and thorium, mainly due to their prevalence as carbon neutral fuel sources and the ease of handling their weakly radioactive 238U and 232Th isotopes. In comparison, less progress has been achieved for transuranium elements, which are present in minor amounts in spent nuclear fuels. This class of elements has reduced availability compared to U/Th, and requires radiological facilities to safely handle these higher specific-activity α-emitters. The chemistry of neptunium (237Np), (5) however, offers an opportunity to study transuranium elements with relatively few obstacles compared to its later actinide counterparts. This element has gained popularity in recent years because of its exciting solution-phase redox chemistry that can span oxidation states of +2 to +7, stabilized using a variety of ligand frameworks, similar to the earlier actinides.
Recently, polyoxometalates (POMs), which are anionic molecular metal oxide clusters typically made from tungsten(VI), molybdenum(VI), or vanadium(V), have emerged as suitable frameworks for the formation of molecular actinide complexes. (6−13) The stability and ease by which these clusters crystallize has facilitated detailed structural characterization, and for comparisons to be made with transition metal- or lanthanide-containing derivatives. (14−17) Moreover, the high molecular weight (∼1,000 to 20 000 g/mol) of POMs allows for stoichiometric reactions to be carried out with exceptionally small quantities of actinide starting materials, a property which is especially important when looking to study the highly radioactive and rare transuranium elements. (18,19)
Structurally characterized transuranium POM complexes that leverage polyoxotungstates clusters as “ligands” have been established. Early efforts in this space utilized trilacunary polyoxometalate anions to complex high-valent neptunyl and plutonyl cations. (20,21) For example, 2:2 or 2:3 cluster complexes incorporating [UO2]2+, [NpO2]2+, [NpO2]+, and [PuO2]2+ into a polyoxotungstate framework have been reported, forming complexes of the type [Na2(AnO2)2(α-EW9O34)2]n– (An = U, n = 12; An = Np, n = 14) and K11[K3(PuO2)3(GeW9O34)2]·12H2O. (22−24) More recently, lacunary POM complexes incorporating transuranium cations with lower charges have been accessed, though many have only been identified in solution-phase studies due to the radioactivity of the heavier transuranium elements. (25−27) Moisy and co-workers detail structural characterization of a series of An(IV) sandwich complexes using the heteropolyanion [P2W17O61]10− to form An(P2W17O61)216− (An = Th, U, Np, and Pu). (18) Structural and spectroscopic characterization has also been obtained for a mixed-ligand polyoxometalate complex of Np(IV), which leverages crystallization from a mixture of [Np(W5O18)2]8– and [Np(BW11O39)2]14– to form K10.5H0.5[Np(BW11O39)(W5O18)]·15H2O. (6) Seminal work from Deblonde and co-workers utilizes heteropolyoxotungstate clusters to complex microgram quantities of Am(III) and Cm(III) cations, with structural and spectroscopic characterization obtained for the complexes. (19)
Despite the relative prevalence of literature detailing polyoxotungstate actinide complexes, far fewer reports utilize molybdenum analogues, attributed to the lower stabilities of lacunary polyoxomolybdates. (28) Interestingly, however, a subset of actinide literature details the role of molybdenum in spent nuclear fuel reprocessing and how the chemical behavior of neptunyl(V) is affected by the molybdate ion. (29,30) Given understanding the interactions between molybdate ions and neptunium is relevant to energy and the environment, we proposed utilizing a polyoxomolybdate cluster as a ligand for neptunium would provide fundamental insights into the bonding, redox reactivity, and electronic communication between the cluster scaffold and Np ion.
Our research team has previously described the synthesis and characterization of a series of polyoxomolybdate–alkoxide sandwich complexes of the general formula, (TBA)2[M{Mo5O13(OMe)4NO}2] (TBA = tetrabutylammonium), incorporating M(IV) cations (M = Zr(IV), Hf(IV), Th(IV), U(IV)), herein referred to as 2-Zr(Mo5)2, 3-Hf(Mo5)2, 4-Th(Mo5)2, and 5-U(Mo5)2. (31) Complexation of An(IV) ions enhanced the redox properties of the cluster, with the complexes reversibly accepting up to four electrons. It was hypothesized that the large size of the actinide cations was important, as increasing the separation between the {Mo5} subunits appears to stabilize the reduced complexes by minimizing intramolecular repulsion. We were intrigued to continue with these studies by examining the effect that a smaller Np(IV) ion would have on the overall electronic structure and redox properties of this family. Herein, we extend the series of M(IV) containing sandwich complexes, generating [Np{Mo5O13(OMe)4NO}2]2– 7-Np(Mo5)2 (Figure 1). Characterization by single crystal X-ray diffraction (SCXRD) reveals that the bond metrics of the neptunium derivative lie between the transition metal and larger actinide derivatives. 17O enrichment and analysis by 17O NMR spectroscopy of the M(IV) series provides insight into the nature of the M–O bonding and the influence of unpaired f-electrons on the oxygen nuclear environments. Examination of the electrochemical properties of 7-Np(Mo5)2 reveal subtle differences in the polyoxoalkoxide-based redox processes when compared to the other sandwich complexes, exemplifying the ability of actinides to influence ligand based redox events. Cyclic voltammetry reveals an irreversible oxidation of Np(IV) to Np(V) in (TBA)2[Np{Mo5O13(OMe)4NO}2], occurring at 1.54 V, with the presence of an irreversible cluster based oxidation occurring at a similar potential. These are proposed to contribute to the instability of the Np(V) derivative compared to the previously reported U(V) compound. (31)

Figure 1

Figure 1. General schematic for the synthesis of sandwich complexes from 1-NaMo5, extended to Np(IV) in this work. (31)

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Experimental Section

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General Considerations

All air- and moisture-sensitive manipulations with neptunium were performed using standard Schlenk techniques or in an MBraun negative pressure UHP argon atmosphere drybox. All air- and moisture-sensitive manipulations with thorium or uranium were carried out using a standard high-vacuum line, Schlenk techniques, or an MBraun inert atmosphere drybox containing an atmosphere of purified dinitrogen. The MBraun glovebox was equipped with a cold well designed for freezing samples in liquid nitrogen as well as a −35 °C freezer for cooling samples and crystallizations. Solvents for sensitive manipulations were dried and deoxygenated using literature procedures with a Seca solvent purification system or a glass contour solvent purification system (Pure Process Technology, LLC) and stored over activated 4 Å molecular sieves (Fisher Scientific) prior to use. Deuterated solvents were purchased from Cambridge Isotope Laboratories, dried with molecular sieves, and degassed by three freeze–pump–thaw cycles. 40% 17O enriched H2O was purchased from CortecNet and used as received. (TBA)4[Mo8O26], (32) (TBA)2[Mo5O13(OMe)4NO][Na(MeOH)] (1-NaMo5), (33) and NpCl4(DME)2 (34) were synthesized according to literature procedures.

Safety Considerations

Caution! 237Np represents a health risk due to its α and γ emission and its decay to the short-lived 233Pa isotope (t1/2 = 27.0 days), which is a strong β and γ emitter. All studies with Np were conducted in a laboratory equipped for radioactive materials. All studies were modeled on depleted uranium prior to working with 237Np. Depleted uranium (primary isotope 238U) is a weak α-emitter (4.197 MeV) with a half-life of 4.47 × 109 years, and 232Th is a weak α-emitter (4.082 MeV) with a half-life of 1.41 × 1010 years; manipulations and reactions should be carried out in monitored fume hoods or in an inert atmosphere drybox in a radiation laboratory equipped with α and β counting equipment.
The scarcity of neptunium in combination with the relatively high specific radioactivity of 237Np requires syntheses to be performed on small scales (<15 mg Np). Fortunately, the high molecular weight of polyoxometalates allows for stoichiometric reactions to be performed with exceptionally small quantities of actinide starting materials, making extension of this chemistry to neptunium favorable. To ensure this, reactions using optimized quantities of depleted uranium as a model for neptunium were undertaken and proved to be successful.

Synthesis of (TBA)2[Np{Mo5O13(OMe)4NO}2] (7-Np(Mo5)2)

NpCl4(DME)2 (10 mg, 0.018 mmol, 1 equiv; DME = dimethoxyethane) was weighed into a 5 mL vial and sealed with a septum cap before being taken out of the glovebox. In a separate 20 mL vial, (TBA)2[Mo5O13(OMe)4NO][Na(MeOH)] (49 mg, 0.035 mmol, 2 equiv) was dissolved in methanol (MeOH, 3 mL) in air and added dropwise to the solid NpCl4(DME)2 with a syringe. A green-yellow precipitate formed immediately upon addition. The suspension was gently shaken for 5 min before being opened to air and filtered over Celite in a glass pipette plugged with a microfiber glass filter. A fine green powder was collected on the Celite bed and washed with MeOH (5 mL). The product was extracted with dichloromethane (DCM) until washings ran clear (5 mL). The solvent was removed under a N2 flow and the product was subsequently dried under reduced pressure overnight before being brought into the glovebox. Yield: 38 mg, 0.016 mmol, 90%. Yellow-green single crystals were obtained by vapor diffusion of Et2O into a saturated solution of the product in acetonitrile (MeCN) at room temperature. 1H NMR (400 MHz, CDCl3): δ 4.73 (s, 24H), 3.10 (s, 16H), 1.61 (s, 16H), 1.29 (s, 16H), 1.02 (m, 24H). 17O NMR (54.2 MHz, CDCl3): δ 114.3 (μ5-O), 493.9 (Mo–O–Mo), 846.4 (Mo═O), 964.9 (Mo–O–Np). λmax (DCM) = 763 nm (ε = 158 M–1 cm–1), 900 nm (ε = 92 M–1 cm–1), 990 nm (ε = 56 M–1 cm–1), 1,240 nm (ε = 24 M–1 cm–1), and 1,390 nm (ε = 25 M–1 cm–1). Broad and intense absorption also observed from 400 to 700 nm.

17O Enrichment of (TBA)2[Mo5O13(OMe)4NO][Na(MeOH)(H2O)]

(TBA)2[Mo5O13(OMe)4NO][Na(MeOH)] (100 mg, 0.07 mmol, 1 equiv) was dissolved in anhydrous MeOH (5 mL) forming a purple solution. 40% 17O enriched H2O (6.5 μL, 0.36 mmol, 5 equiv) was added and the solution was stirred at 50 °C for 3 h. The solution was allowed to cool temperature and then the volatiles were removed under vacuum. 17O NMR spectroscopy of the crude material showed successful 17O enrichment but also revealed the presence of a significant amount of residual water. Therefore, the material was purified by recrystallization. Vapor diffusion of Et2O into a saturated solution of the crude material dissolved in anhydrous MeOH led to the formation of large purple crystals. The mother liquor was decanted, washed with Et2O (can lead to loss of crystallinity), and dried under vacuum overnight (73 mg, 72% yield). 1H NMR (500 MHz, CD3OD): δ 1.03 (t, J = 7.3 Hz, 24 H), 1.44 (h, J = 7.4 Hz, 16 H), 1.68 (m, 16 H), 3.26 (m, 16 H), 4.65 (−OMe), 4.87 (H2O). 17O NMR (67.8 MHz, CD3OD): δ −38.8 (Na–OH2), −15.6 (H2O), 18.3 (μ5-O), 417.2 (Mo–O–Mo), 800.7 (Mo–O–Na), 837.4 (Mo═O). Anal. Calcd for C37H90N3O19Mo5Na (mol wt 1399.868 g mol–1): C, 31.75%; H 6.48%; N, 3.00%. Found: C, 32.03%; H 6.25%; N, 3.37%.
It was noted that the material also readily loses 17O enrichment upon dissolution in wet MeOH, presumably due to O atom exchange with the H2O present in the solvent. Therefore, to avoid loss of enrichment, the reaction, purification, and subsequent reactions should all be performed in anhydrous solvents where possible.
17O enriched analogues of (TBA)2[M{Mo5O13(OMe)4NO}2] (M = Zr(IV), Hf(IV), Th(IV), U(IV), Np(IV)) were prepared according to methods described previously (and below), using the 17O enriched material prepared here. (31) Characterization data were consistent with what was previously described, (31) while 17O NMR spectra are shown in Figure 6 and Figures S10–S15. A modified method was used to prepare the U(V) containing analogue and is described below.

Direct Synthesis of (TBA)[U(V){Mo5O13(OMe)4NO}2] (6-U(Mo5)2)

In a 20 mL scintillation vial, (TBA)2[Mo5O13(OMe)4NO][Na(MeOH)] (100 mg, 0.07 mmol, 2 equiv) was dissolved in MeCN (5 mL). The purple solution was added to solid UCl4 (14 mg, 0.036 mmol, 1 equiv) in a separate vial with stirring. This led to an immediate formation of a dark brown solution. The mixture was stirred for 10 min before adding to a separate vial containing solid [NO][PF6] (63 mg, 0.36 mmol, 10 equiv). A brown suspension formed immediately which was stirred for 3 min before passing through a bed of Celite. The solid was washed with a small amount of MeCN (2 × 1 mL) and then extracted with DCM until the washing ran clear (approximately 10 mL). The volatiles were removed under vacuum to leave a dark brown solid (41 mg, 57% yield). 17O NMR (67.8 MHz, CD2Cl2) δ 76.2 (μ5-O), 569.0 (Mo–O–Mo), 940.4 (Mo–O–U), 960.0 (Mo═O). Additional characterization was in line with what was previously reported. (31)

Physical Measurements

1H NMR (NMR = nuclear magnetic resonance) spectra for neptunium compounds were recorded at room temperature on a Bruker AV-III-HD-400 spectrometer operating at 400.13 MHz. All chemical shifts are reported relative to 1H residual chemical shifts of chloroform-d (7.24 ppm). 1H NMR spectra for all other compounds (Th, U) were recorded at room temperature on a 400 MHz Bruker AVANCE spectrometer or a 500 MHz Bruker AVANCE spectrometer locked on the signal of deuterated solvents. All chemical shifts are reported relative to the chosen deuterated solvent as a standard. 17O NMR were collected at room temperature on a Bruker AV-III-HD-400 spectrometer (at 54.2 MHz) or a 500 MHz Bruker AVANCE spectrometer (at 67.8 MHz), with the spectrometer locked on the signal of the deuterated solvents and all chemical shifts given relative to an external standard of D2O. Cyclic voltammetry (CV) for the neptunium complex was performed using a three-electrode setup inside a negative-pressure argon glovebox (MBraun UniLab, USA) using a CH Instrument 620E potentiostat. The concentration of the cluster and the supporting electrolyte (TBAPF6) were kept at 1 and 100 mM, respectively, throughout all measurements. CVs were recorded using a 3 mm diameter glassy carbon working electrode (CH Instruments, USA), a Pt wire auxiliary electrode (CH Instruments, USA), and a silver wire quasi-reference electrode. Ferrocene was used as an internal standard after completion of the measurements, and potentials were referenced versus the Fc+/0 couple. Electronic absorption measurements were recorded inside a negative pressure drybox at room temperature in anhydrous MeCN in sealed 1 cm quartz cuvettes using a JASCO V-770 UV–vis–NIR spectrophotometer equipped with a fiber optic stage and sample holder for 7-Np(Mo5)2.

X-ray Crystallography

Single crystals suitable for X-ray diffraction were coated with poly(isobutylene) oil in the glovebox and quickly transferred to the goniometer head of a Bruker Quest diffractometer with a fixed chi angle, a sealed tube fine focus X-ray tube, single crystal curved graphite incident beam monochromator, a Photon II area detector and an Oxford Cryosystems low temperature device. Examination and data collection were performed with Mo Kα radiation (λ = 0.71073 Å) at 150 K.

Results and Discussion

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To expand upon prior work with actinide(IV) cations, a Np(IV) adduct of the lacunary polyoxomolybdate–alkoxide cluster (TBA)2[Mo5O13(OMe)4NO][Na(MeOH)] (1-NaMo5) was synthesized. The addition of a purple solution of 1-NaMo5 in MeOH to half an equivalent of pink NpCl4(DME)2 results in the immediate precipitation of a fine green solid, which is isolated by filtration and extraction with DCM to afford (TBA)2[Np{Mo5O13(OMe)4NO}2] (7-Np(Mo5)2) (see Experimental Section for additional details). Analysis of the solid via 1H NMR spectroscopy reveals a set of four resonances from +3 to +1 ppm, assigned to the associated TBA cations. An additional resonance at 4.73 ppm is assigned to the bridging alkoxide functionalities of the [Mo5O13(OMe)4NO]3– fragment (Figure 2).

Figure 2

Figure 2. 1H NMR spectra of 4-Th(Mo5)2, 5-U(Mo5)2, 6-U(Mo5)2, and 7-Np(Mo5)2 ordered by increasing 5f electron count. Spectra recorded in CDCl3 at room temperature.

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To our surprise, the 1H NMR spectrum obtained from the reaction with Np(IV) is nearly identical to that acquired for the Th(IV) (5f0) analogue despite the former having three unpaired electrons. The spectrum is also drastically different from the U(IV) (5f2) sandwich complex, where the paramagnetism of the actinide center results in TBA resonances that appear from −2.5 to −5 ppm as well as a resonance assigned to the methoxide groups of the cluster at 9.61 ppm. In our original report, 1H DOSY NMR spectroscopy revealed that for 5-U(Mo5)2 the anionic cluster unit and TBA cations were tight ion pairs in solutions with high dielectric constants (strongest in CDCl3), leading to the large magnetic interactions that were observed. (31) It was initially postulated that a neptunium derivative would accordingly display similar or more intense magnetic interactions compared to 5-U(Mo5)2, as a consequence of the 5f3 electron configuration of the neptunium ion. Rather, the 1H NMR spectrum of 7-Np(Mo5)2 resembles its diamagnetic congeners, which serves as an initial indication that the protons within the complex are magnetically decoupled from the paramagnetic actinide center. It is important to note that the 1H NMR spectrum of 7-Np(Mo5)2 is also similar to that of the U(V) (5f1) analogue, suggesting that there are likely multiple factors that dictate the extent of observed paramagnetic shifting within the complexes (e.g., electronic configuration, cation size, distance of the protons from the center). Evidently, the actinide center within these systems has a significant impact on the local environment of the protons within these complexes, and 5-U(Mo5)2 likely has unique magnetic properties that are significantly different from the other isolated derivatives.
To further characterize the product, crystals of 7-Np(Mo5)2 suitable for single crystal X-ray diffraction (SCXRD) were grown by vapor diffusion of diethyl ether into a saturated solution of the product in acetonitrile at room temperature. Refinement of the data confirmed the structural composition of 7-Np(Mo5)2 as the eight-coordinate species (TBA)2[Np{Mo5O13(OMe)4NO}2] (Figure 3, Table 1). The solid-state structure features a central neptunium ion in a square antiprismatic geometry, sandwiched between two cluster units. The average bond distances between the molybdenum and oxygen atoms within the [Mo5O13(OMe)4NO]3– unit are consistent with those observed in the transition metal(IV), Th(IV), and U(IV) congeners, highlighting that incorporation of either transition metal or actinide ions does not cause significant perturbations in the solid-state molecular structure of the cluster units. The average M–O bond lengths of approximately 2.35 Å for 7-Np(Mo5)2 are shorter than those of 4-Th(Mo5)2 and 5-U(Mo5)2 (2.41 and 2.36 Å, respectively), but longer than 2-Zr(Mo5)2 and 3-Hf(Mo5)2 (2.20 and 2.19 Å, respectively). This can be rationalized on the basis of the effective ionic radius of the Np(IV) cation (0.98 Å), (35) which is larger than the transition metal derivatives, but smaller than the previously isolated actinide clusters (Hf < Zr ≪ Np < U < Th). This is likely the result of increasing nuclear charge moving from left to right across the f-block, which simultaneously causes a contraction and lowering of energy of the 5f orbitals and a decrease in ionic radius of the actinide elements across the series. (36,37) It is also reasonable to suggest that the Np–O bonds formed upon complexation with the POM assembly likely contain a small amount of covalent character. Though An(IV) cations typically form electrostatic interactions with POMs due to their higher charge, the lowering of energy of the 5f orbitals for Np ions is hypothesized to increase orbital overlap with the cluster unit, resulting in stronger bonds between the actinide and cluster in 7-Np(Mo5)2 compared to 4-Th(Mo5)2 and 5-U(Mo5)2, which is evidenced experimentally by the shortening of the M–O bond lengths in 7-Np(Mo5)2 compared to 4-Th(Mo5)2 and 5-U(Mo5)2. (36,37) Another consequence of this phenomena in the solid-state structure of 7-Np(Mo5)2 is the distance between the central μ5-oxo in each cluster unit. The distance is 6.812(5) Å for 7-Np(Mo5)2, sitting between the M(IV) (M = Zr, Hf) and An(IV) congeners, which exhibit distances of 6.940(2) Å and 6.843(6) Å for 4-Th(Mo5)2 and 5-U(Mo5)2 and 6.652(3) Å and 6.643(3) Å for 2-Zr(Mo5)2 and 3-Hf(Mo5)2.

Figure 3

Figure 3. Single crystal X-ray diffraction structure of 7-Np(Mo5)2 with probability ellipsoids set at 50%. The tetrabutylammonium cations and some disorder has been masked for clarity.

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Table 1. Pertinent Bond Distances (Å) for (TBA)2[Np{Mo5O13(OMe)4NO}2] (7-Np(Mo5)2)a
Complex2-Zr(Mo5)23-Hf(Mo5)24-Th(Mo5)25-U(Mo5)27-Np(Mo5)2
M–O2.2012.1912.4102.3582.349
μ5-O−μ5-O6.6526.6436.9406.8436.812
O–O2.7512.7513.1053.0122.987
Ionic Radius (42)0.840.831.051.000.98
a

Distances for 2-Zr(Mo5)2, 3-Hf(Mo5)2, 4-Th(Mo5)2, and 5-U(Mo5)2 are included for comparison. (31)

Structurally characterized Np–POM sandwich complexes are rare and most have been synthesized in water. (6,18) The majority leverage Lindqvist or Keggin anions that feature tungsten as the framework metal. For example, the structurally characterized polyoxotungstoneptunates [Np(W5O18)2]8– and [Np(SiW11O39)2]12– feature average Np–O bond distances of 2.34 and 2.35 Å, respectively, which are either similar or identical to 7-Np(Mo5)2. (6) Moreover, Moisy and co-workers have reported the isolation of a series of An(IV) polyoxometalate clusters with the general formula [An(P2W17O61)2]16– (An = Th(IV), U(IV), Np(IV), Pu(IV), Am(IV)). (18) They observe similar bond metrics and trends (i.e., shortening of An–O contacts) due to differences in the ionic radius of the central metal cation sandwiched between the lacunary clusters.
The pronounced difference in color between 5-U(Mo5)2 (brown) and 7-Np(Mo5)2 (light green) prompted our interest in characterizing 7-Np(Mo5)2 via electronic absorption spectroscopy (Figure 4) to compare to the other An(IV) derivatives. The obtained UV–vis–NIR spectrum displays broad and relatively intense absorptions across the visible region, similar to that observed for the U(IV) derivative. Polyoxometalates are unique because the cluster can be treated as a singular unit that operates with a set of delocalized orbitals involving multiple metals. To this point, reports of metal-to-POM charge transfer have become increasingly common. (38−40) In our original report, the broad absorptions in the visible region of the brown 5-U(Mo5)2 cluster were tentatively assigned to uranium–molybdenum charge transfer bands given similar mechanisms of absorption have been reported for U–polyoxotungstate complexes exhibiting charge transfer character between U(IV) and W(VI). (8,17,41) Taking into account the differences in color between the uranium and neptunium compounds, it is hypothesized that the broad and intense absorption observed from 400 to 700 nm in 7-Np(Mo5)2 is a result of metal-to-ligand charge transfer (MLCT) from the neptunium metal center to the atoms within the cluster unit (Np(5f) → Mo(4d)), with additional minor contributions from ligand-to-metal charge transfer (LMCT) (O(2p)→ Np(5f)). The band at 763 nm (ε = 158 M–1 cm–1) is assigned to a Np(IV) f–f transition, and has been previously observed in reports of tetravalent neptunium coordination complexes. (42−45) Analysis of the near-infrared region of the electronic absorbance spectrum reveals weak f–f transitions, consistent with the assignment of a 4+ oxidation state (5f3 electron configuration) of neptunium. Notably, 7-Np(Mo5)2 has absorption bands at approximately 900 nm (ε = 92 M–1 cm–1), 990 nm (ε = 56 M–1 cm–1), 1,240 nm (ε = 24 M–1 cm–1), and 1,390 nm (ε = 25 M–1 cm–1), which are similar to other reports of tetravalent neptunium complexes and match quite well with the vis–NIR absorption spectrum of [Np(W5O18)2]8– recorded in aqueous solution. (6,42,43,46) The scarcity of literature detailing the electronic transitions of Np–polyoxomolybdate complexes in organic solvent highlights the importance of developing this technique to probe the electronic structure of transuranium–POM complexes.

Figure 4

Figure 4. Electronic absorption spectrum of 7-Np(Mo5)2 collected at room temperature in acetonitrile. The inset shows the near-infrared region of the spectrum to highlight f–f transitions of the Np(IV) center.

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In our previous work, cyclic voltammetry (CV) experiments showed the actinide complexes 4-Th(Mo5)2 and 5-U(Mo5)2 could reversibly accept up to four electrons (with the uranium derivative also showing a reversible one electron oxidation). (31) The +4 charge of the central metal was found to be a defining factor for the observation of reversible redox events, with analogous sandwich complexes containing both Ba(II) and Bi(III) showing more complex and often irreversible redox chemistry. Furthermore, the increased size of the actinide centers was postulated to be an important factor in the stabilization of the more highly reduced states by minimizing the electrostatic repulsion between the {Mo5} units, with 2-Zr(Mo5)2 and 3-Hf(Mo5)2 showing only two reversible reduction events.
Like uranium, neptunium is redox active with access to a wide range of oxidation states between +2 and +7 (with +5 the most common in aqueous media). In acidic solutions, the Np(IV)/Np(III) couple is anodically shifted by approximately 0.7 V compared to the corresponding U(IV)/U(III) couple. (5,47) This trend was preserved when the actinide cations were bound to polyoxometalates, with the respective An(IV)/An(III) couples of [Np(P2W17O61)2]n− and [U(P2W17O61)2]n− reported at −0.95 and −1.73 V. (48) There are only a limited number of studies where Np(IV)/Np(III) redox couples are reported in nonaqueous media; similar trends of anodic shifting of the Np(IV)/Np(III) redox couple versus the corresponding U(IV)/U(III) redox couple were observed. (47,49,50) Indeed, most recently, this was extended by La Pierre and co-workers, who reported cyclic voltammetry measurements on a series of tetravalent imidophosphorane complexes, including uranium and neptunium derivatives. Although the An(IV)/An(III) redox couple was not observed for either the Np or U derivative, reversible An(V)/An(IV) couples were observed at −0.70 V for Np and −1.57 V for U (both vs Fc0/+). (51) This exemplifies how the Np(V)/Np(IV) redox couple is also anodically shifted compared to uranium, demonstrating that the trend is not specific to the An(IV)/An(III) redox couple but appears to be a generic feature of neptunium redox chemistry.
Given that the previously reported cyclic voltammogram of 5-U(Mo5)2 showed the presence of a reversible U(V)/U(IV), and no U(IV)/U(III) couple, we were intrigued to determine how the redox properties of the analogous neptunium complex would compare. The cyclic voltammogram of 7-Np(Mo5)2, obtained in MeCN with TBAPF6 as the supporting electrolyte, is shown in Figure 5. The corresponding CVs of 4-Th(Mo5)2 and 5-U(Mo5)2 are included for comparison (with dashed traces showing the more oxidizing regions). It is clear that 7-Np(Mo5)2 possesses similar reduction properties to the other actinide containing sandwich complexes. The first two 1e– reduction events of 7-Np(Mo5)2 (E1/2 = −0.87, −1.43 V) appear very similar to those of 4-Th(Mo5)2 (E1/2 = −0.93, −1.37 V) and 5-U(Mo5)2 (E1/2 = −0.80, −1.32 V) and are assigned to addition of a single electron to each {Mo5} unit (i.e., Mo(VI) → Mo(V) reduction). The second two events, assigned to addition of a second electron to each of the {Mo5} units, are less well resolved for 7-Np(Mo5)2, with larger differences between the anodic and cathodic peak potentials observed. This is indicative of a change in rate of electron transfer, likely switching from a diffusion limited process to electron transfer rate limited process. When combined with the fact that the fourth reduction event of 7-Np(Mo5)2 was observed at −2.27 V (ca. 0.2–0.3 V lower than for 4-Th(Mo5)2 and 5-U(Mo5)2), these results indicate that the more highly reduced states of 7-Np(Mo5)2 are harder to access, and may in turn be marginally less stable. This can be rationalized by the smaller size of the Np(IV) ion as compared to its early actinide counterparts; the average An–O bond length is lower for 7-Np(Mo5)2 than for either 4-Th(Mo5)2 or 5-U(Mo5)2, leading to closer {Mo5} units and increased intramolecular charge repulsion. This would expectedly then place the redox characteristics of 7-Np(Mo5)2 in an intermediate position between 4-Th(Mo5)2/5-U(Mo5)2 and 2-Zr(Mo5)2/3-Hf(Mo5)2 (which have shorter Zr–O and Hf–O bond lengths and can only be reversibly reduced by up to two electrons).

Figure 5

Figure 5. Cyclic voltammograms of 4-Th(Mo5)2, 5-U(Mo5)2, and 7-Np(Mo5)2. Dashed traces highlight behavior at more oxidizing potentials. The data were acquired in MeCN with 0.1 M TBA(PF6) supporting electrolyte, 1 mM of cluster, and a scan rate of 200 mV s–1.

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As the M(IV) ion series has been extended, a trend in the difference between the E1/2 of the first and second reduction event can be observed. The gap is smallest for 2-Zr(Mo5)2/3-Hf(Mo5)2 and increases in the order 4-Th(Mo5)2 < 5-U(Mo5)2 < 7-Np(Mo5)2. An increased gap between the first and second reduction events corresponds to a larger energy difference between the one electron and two electron reduced states. It is hypothesized that the first and second reduction events correspond to addition of a single electron to each of the {Mo5} units. If these {Mo5} units were completely electronically separated, a single two electron reduction event would be expected, as the addition of an electron to one {Mo5} unit would have no impact on the energy required to add an electron to the other (and therefore these energies would be expected to be the same). However, the resolution of individual redox events at different energies means that the addition of an electron to one {Mo5} unit affects the energy required to reduce the other. The observations here indicate some level of electronic communication between the two {Mo5} units and, moreover, that the gap between the energy of the first and second reduction events can be considered a measure of the extent of this communication. A larger gap, which is observed for the actinide derivatives, corresponds to stronger electronic communication between the two halves of the sandwich complex (i.e., the addition of the first electron has a stronger influence on the energy required to add the second). This may initially seem counterintuitive as the large actinide ions create a larger spatial separation between the two {Mo5} units. However, this observation can be rationalized by also considering that the actinide centers have access to extended valence 5f orbitals. The ability of these orbitals to overlap with the cluster based LUMO could provide a pathway for electronic communication between the two halves of the complex that is not possible for the transition metal derivatives. This is corroborated by considering that the energy of these valence 5f orbitals drops across the period, making these orbitals more accessible for U(IV) and even more so for Np(IV). (37) This would lead to the expectation that the influence of the first reduction event on the energy of the second reduction event should increase as we move across the period, which is observed. This demonstrates that actinide substitution can be used to tune ligand based redox chemistry.
Examining the more positive region of the cyclic voltammogram of 7-Np(Mo5)2 (Figure 5, green dashed line) shows the presence of an irreversible oxidation event at 1.54 V (followed by another sharp irreversible oxidation). Previously, an irreversible oxidation event has also been observed for 1-NaMo5 (0.76 V vs Fc0/+ or 1.09 V vs SCE, both in MeCN), (TBA)3[Mo6O18NO] (0.83 V vs SCE in dimethylformamide, DMF), and (TBA)2[Mo6O17(OMe)NO] (1.25 V vs SCE in DMF). These oxidation events are proposed to be associated with the Mo–NO unit and are thought to shift to more positive potentials upon decreasing the overall negative charge of the system. To aid in the assignment of the oxidation events observed in the CV of 7-Np(Mo5)2, the behavior at more positive potentials was also investigated for 2-Zr(Mo5)2 (Figure S16), 4-Th(Mo5)2 (Figure 5, blue dashed line), and 5-U(Mo5)2 (Figure 5, brown dashed line). All these compounds display a single irreversible oxidation event with an onset potential of ca. 1.35 V. Given the consistency of this event, occurring at almost identical potential for the three compounds studied, this event can confidently be assigned to the same oxidation of the Mo–NO unit previously reported, with the more anodic potentials required to drive the oxidation in 2-Zr(Mo5)2, 4-Th(Mo5)2, and 2-U(Mo5)2 caused by the lower overall charge of the system.
This process also appears to be present in the CV of 7-Np(Mo5)2, being the cause of the continued rise in current observed after the oxidation event at 1.54 V. However, the additional irreversible process is more likely to be caused by Np(IV) → Np(V) oxidation. The event occurs approximately 0.8 V higher than the previously observed, reversible, U(V)/U(IV) couple of 5-U(Mo5)2. As discussed above, anodic shifting of neptunium redox couples compared to the corresponding uranium based redox couple has been commonly observed, with shifts of 0.7–0.9 V being typical, consistent with a neptunium oxidation event. Shifting of the redox couple to more positive potentials leads to a lack of stability for the oxidized species, evidenced by the irreversibility. It may be that the similarity in the potential required for Np(IV) → Np(V) oxidation to that required for oxidation of the Mo–NO unit provides a facile pathway for decomposition, with irreversible electron transfer from the Mo–NO unit to the Np(V) ion driving reformation of Np(IV) and decomposition (or rearrangement) of the sandwich complex.
To gain further insights into the chemical environments of the oxygen nuclei in our series of M(IV)/M(V) complexes and the influence of incorporation of paramagnetic ions on NMR chemical shifts, analysis by 17O NMR spectroscopy was pursued. 17O NMR spectroscopy is a powerful technique that provides detailed information about structure, bonding, and dynamic processes present in solution. (52) Typically, 17O NMR chemical shifts can be correlated with the degree of M–O π-bonding (M being the framework metal), with higher π-bond order (or decreased M–O bond length) resulting in more positive chemical shifts. (52) This leads to characteristic chemical shifts for various types of metal(VI) oxo groups, with terminal M═O groups occurring between 700 and 1000 ppm, bridging M–O–M groups at 300–600 ppm, and central oxo moieties (i.e., μ5-O or μ6-O) between 100 and −150 ppm. (53) These values are subject to change according to redox state, protonation, changing the framework metal, or incorporation of a heterometal, all of which can have drastic impacts on the observed chemical shifts.
One major barrier to obtaining high quality 17O NMR spectra is the ability to efficiently 17O enrich the compound of interest, with spectra obtained at natural 17O abundance requiring a very high concentration of the compound. This can lead to high viscosity solutions and very broad resonances. Previously, some POMs have been effectively 17O enriched by direct treatment with 17O enriched water. (52) If the M–O bonds of the POM are sufficiently labile, this leads to a dynamic equilibrium in which O atom exchange between the oxo-groups of the POM and the added water drives statistical enrichment of the cage. If the M–O bonds of the target POM are not labile with respect to O atom exchange, then alternative methods of 17O enrichment must be pursued. (54−56)
To probe the ability to directly enrich this family of complexes, 4-Th(Mo5)2 was stirred with 10 equiv of 40% 17O enriched H2O at 50 °C for 3 h. It was hypothesized that statistical exchange of the all oxo-groups present in the structure (i.e., Mo═O, Mo–O–Mo, Mo–O–Th, and the μ5-O) would lead to an overall 11% 17O enrichment, with the only groups not enriched being the bridging -OMe groups and the terminal NO unit. Unfortunately, inspection of the resulting 17O NMR spectrum (Figure S7) showed only unreacted H2O. This indicates that 4-Th(Mo5)2 undergoes no appreciable O atom exchange under these conditions, speaking to the chemical stability (with respect to rearrangement) of the [M{Mo5O13(OMe)4NO}2]2– sandwich complexes in a range of solvents. This contrasts 1-NaMo5, which has been shown to undergo facile rearrangement to (TBA)3[Mo6O18NO] in solvents other than MeOH. (33)
Given the inability to directly 17O enrich our Th(IV) complex, it was postulated that the observed instability of 1-NaMo5 may be indicative of the lability of this framework and therefore it might serve as a better entry point for enrichment chemistry. To test this, 1-NaMo5 was dissolved in MeOH and 5 equiv of 17O enriched H2O were added (targeting the same 11% enrichment if all oxo groups were statistically enriched). The mixture was stirred for 3 h at 50 °C before the volatiles were removed and an 17O NMR spectrum was obtained (Figure S8). The spectrum contained a total of six observed resonances, with the intense resonance at −16 ppm indicative of a significant amount of residual water. To minimize this, the crude material was recrystallized (see experimental section) and then extensively dried under vacuum. The spectrum was then rerecorded (Figure 3, M = Na). First, it is apparent the resonance at −16 ppm is less intense, suggesting that a significant amount of residual water was removed. The resonance observed at 18 ppm is assigned to the central μ5-oxo (1 O) group of the [Mo5O13(OMe)4NO]3– cluster. An additional resonance, observed at −39 ppm, was tentatively assigned to a H2O bound to the sodium cation present in the cavity of [Mo5O13(OMe)4NO]3–, suggesting the coordinated MeOH can be displaced or joined by H2O during the enrichment. This is consistent with previous reports, where the use of hydrated materials in the synthesis of {[Mo5O13(OMe)4NO][Na(L)]}2– led to structures where L could be both MeOH and H2O. (33) The upfield shifting of this resonance compared to free H2O is also consistent with the increase in the oxygen coordination number upon binding to sodium. (52)
The remaining three resonances in the spectrum are all equal intensity and therefore can be assigned to the terminal Mo═O’s (4 O), bridging Mo–O–Mo’s (4 O), and formally bridging Mo–O–Na’s (4 O). The most upfield resonance, present at 417 ppm, is in the region characteristic of bridging M–O–M oxygens and therefore is assigned as such. The two remaining resonances, at 801 and 838 ppm, respectively, are in the region characteristic of terminal M═O groups. The exact assignment of these resonances can be made based on the crystallographic determined bond lengths, which relate to the π-bond order. The average terminal Mo═O bond length in 1-NaMo5 is ca. 1.69 Å, whereas the average Mo–O–Na bond length is ca. 1.72 Å. (33) The slightly shorter bond lengths for the Mo═O groups should result in a slightly higher chemical shift and therefore these O atoms are assigned to the resonance at 838 ppm. As a result, the resonance at 801 ppm is attributed to the Mo–O–Na groups and, when combined with the relatively short Mo–O bond length, is indicative of a significant amount of π-character in the Mo–O bonds (i.e., could be described as Mo═O···Na).
These results show that 1-NaMo5 can be readily 17O enriched and the oxygen environments in the structure can be differentiated. This 17O enriched sample of 1-NaMo5 was used in the synthesis of the M(IV)/M(V) sandwich complexes discussed both here and previously (31) (Figure 6). The 17O NMR spectra of (TBA)2[Zr{Mo5O13(OMe)4NO}2] (2-Zr(Mo5)2) or (TBA)2[Hf{Mo5O13(OMe)4NO}2] (3-Hf(Mo5)2) (CD2Cl2) are very similar, mirroring the behavior observed in 1H NMR spectroscopy experiments. Compared to 1-NaMo5, the resonances for the terminal Mo═O groups (blue), the bridging Mo–O–Mo groups (green), and the central μ5-oxo (magenta) are all shifted downfield. This can be attributed to the effective drop in charge from 2– per {Mo5} cluster in 1-NaMo5 to 2– across two {Mo5} clusters in 2-Zr(Mo5)2/3-Hf(Mo5)2 (effectively “deshielding” the 17O nuclei). Contrary to this, the resonance assigned to the Mo–O–M groups is drastically shifted upfield, occurring more than 100 ppm lower than in 1-NaMo5. This is indicative of a decrease in the extent of π-bonding present in the Mo–O bonds, likely caused by competitive π-donation into the empty d-orbitals of the d0 transition metals now bound at the lacunary site of the {Mo5} units. (52) This observation is consistent with the extension of the Mo–O bond lengths noted in the crystal structures of the lacunary cluster and its Zr- and Hf-congeners, going from ca. 1.72 Å in 1-NaMo5 to ca. 1.77 Å in 2-Zr(Mo5)2/3-Hf(Mo5)2. (31,33) The observed 17 ppm difference in the position of the resonance assigned to the Mo–O–M groups in 2-Zr(Mo5)2 and 3-Hf(Mo5)2 indicates a minor change in the bonding between the M(IV) center and the {Mo5} clusters, with the lower observed chemical shift of 669 ppm signifying the Mo–O bonds of the Mo–O–Hf bridges have lower π-bond order compared to the corresponding bonds in 2-Zr(Mo5)2. This could be attributed to the increased π-donation to Hf(IV) compared to Zr(IV), which is reflected in the slight shortening (ca. 0.01 Å) of the Hf–O bonds of 3-Hf(Mo5)2 versus the Zr–O bonds of 2-Zr(Mo5)2, likely caused by better orbital overlap between filled O 2p orbitals and empty Hf 5d oribtals which are more extended than the valence 4d orbitals of Zr. (57)

Figure 6

Figure 6. 17O NMR spectra collected on 17O enriched samples of (TBA)2[Mo5O13(OMe)4NO][Na(H2O)] and (TBA)X[M{Mo5O13(OMe)4NO}2] sandwich complexes (when M = Zr(IV), Hf(IV), Th(IV), U(IV), or Np(IV), X = 2; when M = U(V), X = 1). Spectra obtained at 21 °C in CD3OD (M = Na), CDCl3 (M = Np), or CD2Cl2 (rest).

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The spectrum obtained for 4-Th(Mo5)2 (Figure 6) is similar to those obtained for 2-Zr(Mo5)2 and 3-Hf(Mo5)2. The resonances assigned to the terminal Mo═O groups (blue), bridging Mo–O–Mo groups (green), and μ5-oxo groups (magenta) vary only slightly from the M(IV) (M = Zr, Hf) containing derivatives, with these minor changes attributed to the installation of the larger Th(IV) cation (105 pm), which in turn leads to differences in the distances between the {Mo5} units of the complex. The biggest change is observed for the oxygen nuclei directly part of the thorium coordination sphere. The larger Th(IV) cation, which possesses relatively high energy (unoccupied) valence 5f orbitals, forms longer, more ionic bonds to oxygen (with π-back bonding relatively disfavored vs the Zr(IV)/Hf(IV) derivatives). Consequently, more π-electron density is available for donation to molybdenum, and therefore a downfield shift is observed for this peak, appearing now at 718 ppm for 4-Th(Mo5)2 compared to 686 and 669 ppm, respectively, for 2-Zr(Mo5)2 and 3-Hf(Mo5)2.
More severe differences in the 17O NMR spectra are observed for the remaining actinide complexes (Figure 6; M = U(V) (f1), U(IV) (f2), and Np(IV) (f3)), which is in part caused by the presence of unpaired f-electrons in these derivatives. Though the paramagnetism of the metal ion makes complete assignment of the spectra difficult, some trends can still be observed. In particular, the resonances assigned to the terminal Mo═O groups (blue) and the bridging Mo–O–Mo (green) groups are downfield shifted in 6-U(Mo5)2 (UV) compared to those in 5-U(Mo5)2 (UIV) and 7-Np(Mo5)2 (NpIV); this is most likely a result of the decrease in the overall negative charge present in the system after oxidation of U(IV) to U(V) (now only 1– per two {Mo5} units). The remaining resonances (i.e., Mo–O–M groups and central μ5-O groups) appear to be less ordered. In general it can be seen that the Mo–O–M groups and central μ5-O groups of 5-U(Mo5)2 (at 1178 and −34 ppm, respectively) appear significantly further downfield than for either 6-U(Mo5)2 and 7-Np(Mo5)2. This may indicate that the influence of the paramagnetic U(IV), f2 ion has on the surrounding 17O nuclei is greater than either the U(V), f1 or Np(IV), f3 ions. However, the observed paramagnetic shifting is impacted by the electronic configuration, coordination geometry, and distance of the 17O nuclei to the paramagnetic metal center, making rationalization of the observed shifts difficult. This is in line with the obtained 1H NMR spectra, where 6-U(Mo5)2 and 7-Np(Mo5)2 feature a single resonance at 4.79 and 4.73 ppm, respectively, assigned to the −OMe groups, which is close to the chemical shift observed for the diamagnetic complexes. Contrary to this, the same resonance is observed at 9.61 ppm for 5-U(Mo5)2, significantly shifted from all other sandwich complexes studied. These observations may be further rationalized by the fact that spin–orbit heavy atom effects on light atom shielding (SO-HALA effects) are known to vary depending on the oxidation state of the heavy atom, with both the magnitude and sign of shielding/deshielding effects varying (though it is difficult to separate these effects from the effects of paramagnetism). (58)

Conclusions

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In summary, the synthesis of a family of M(IV) ion-centered sandwich complexes has been demonstrated, with an interesting extension of the series to a rare example of a neptunium(IV) derivative supported by a molybdenum based polyoxoalkoxide described. Compound 7-Np(Mo5)2 behaves similarly to 5-U(Mo5)2 in many ways, possessing both broad and intense absorption in the visible region of its electronic spectrum, assigned to Np(5f) → Mo(4d) MLCT, and the ability to reversibly accept up to four additional electrons. However, incorporation of the later actinide leads to subtle changes in these processes, exemplifying the ability to fine-tune the electronic structure of the system by varying the M(IV) ion present. The facile 17O enrichment of 1-NaMo5 is also presented, with the dynamic nature of this structure in solution clearly allowing isotopic labeling of the oxo-groups under mild conditions. Using this 17O enriched material directly in the synthesis of target complexes allows isolation of actinide containing complexes which incorporate an additional spectroscopic handle. Analysis of the 17O NMR spectra of the series (TBA)2[M{Mo5O13(OMe)4NO}2] (M = Zr(IV), Hf(IV), Th(IV), U(IV), U(V), Np(IV)) produced clearly shows the ability to extract information about the influence of charge and M–O π-bond order on the chemical shifts of the oxygen nuclei present. Focusing on the U(V) (f1), U(IV) (f2), and Np(IV) (f3) derivatives shows a much bigger paramagnetic contribution to the observed chemical shifts for the U(IV) derivative than for either the U(V) or Np(IV) derivatives. This mimics what is observed in the 1H NMR data but gives a more complete picture. Overall, this further displays the utility of this system, with both a growing number of actinide derivatives in the series and an increasing number of analytical tools at our disposal to examine them. In particular, the growing number of accessible NMR spectroscopy handles available to us makes future studies focused on improving our understanding of paramagnetic shifting caused by the incorporation of heavy f-elements appealing, analogous to the detailed work already done for organic ligands. (59) These are ideal candidates for future systematic studies exploring both An–O bonding and the influence of the actinide present on the electronic and magnetic properties of the system.

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.4c04428.

  • NMR spectra, crystal data, CV, and E1/2 values (PDF)

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Author Information

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  • Corresponding Authors
  • Authors
    • Leyla R. Valerio - Department of Chemistry, University of Rochester, Rochester, New York 14627, United States
    • Dominic Shiels - Department of Chemistry, University of Rochester, Rochester, New York 14627, United States
    • Lauren M. Lopez - H. C. Brown Laboratory, James Tarpo Jr. and Margaret Tarpo, Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States
    • Andrew W. Mitchell - H. C. Brown Laboratory, James Tarpo Jr. and Margaret Tarpo, Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States
    • Matthias Zeller - H. C. Brown Laboratory, James Tarpo Jr. and Margaret Tarpo, Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States;  Orcidhttps://orcid.org/0000-0002-3305-852X
  • Author Contributions

    L.R.V. and D.S. contributed equally.

  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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L.R.V., D.S., and E.M.M. thank the Department of Energy for the financial support of this work, under award DE-SC0020436. This material is also based upon work supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Heavy Element Chemistry Program, under Award Number DE-SC0008479 (S.C.B.). L.R.V. and A.W.M. acknowledge support from the National Science Foundation Graduate Research Fellowship Program.

References

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    A review. Prior to the year 2000, nonaq. uranium chem. mainly involved metallocene and classical alkyl, amide, or alkoxide compds. as well as established carbene, imido, and oxo derivs. Since then, there was a resurgence of the area, and dramatic developments of supporting ligands and multiply bonded ligand types, small-mol. activation, and magnetism are reported. This Review (1) introduces the reader to some of the specialist theories of the area, (2) covers all-important starting materials, (3) surveys contemporary ligand classes installed at U, including alkyl, aryl, arene, carbene, amide, imide, nitride, alkoxide, aryloxide, and oxo compds., (4) describes advances in the area of single-mol. magnetism, and (5) summarizes the coordination and activation of small mols., including CO, CO2, nitric oxide, dinitrogen, white phosphorus, and alkanes.
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    Dam, H. H.; Reinhoudt, D. N.; Verboom, W. Multicoordinate ligands for actinide/lanthanide separations. Chem. Soc. Rev. 2007, 36, 367– 377,  DOI: 10.1039/B603847F
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    Multicoordinate ligands for actinide/lanthanide separations
    Dam, Henk H.; Reinhoudt, David N.; Verboom, Willem
    Chemical Society Reviews (2007), 36 (2), 367-377CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
    A review. In nuclear waste treatment processes there is a need for improved ligands for the sepn. of actinides (An(III)) and lanthanides (Ln(III)). Several research groups are involved in the design and synthesis of new An(III) ligands and in the confinement of these and existing An(III) ligands onto mol. platforms giving multicoordinate ligands. The preorganization of ligands considerably improves the An(III) extn. properties, which are largely dependent on the soly. and rigidity of the platform. This tutorial review summarizes the most important An(III) ligands with emphasis on the preorganization strategy using (macrocyclic) platforms.
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  5. 5
    Arnold, P. L.; Dutkiewicz, M. S.; Walter, O. Organometallic Neptunium Chemistry. Chem. Rev. 2017, 117, 11460– 11475,  DOI: 10.1021/acs.chemrev.7b00192
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    Organometallic Neptunium Chemistry
    Arnold, Polly L.; Dutkiewicz, Michal S.; Walter, Olaf
    Chemical Reviews (Washington, DC, United States) (2017), 117 (17), 11460-11475CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
    A review. Fifty years have passed since the foundation of organometallic neptunium chem., and yet only a handful of complexes have been reported, and even fewer have been fully characterized. Yet, increasingly, combined synthetic/spectroscopic/computational studies are demonstrating how covalently bonding, soft, carbocyclic organometallic ligands provide an excellent platform for advancing the fundamental understanding of the differences in orbital contributions and covalency in f-block metal-ligand bonding. Understanding the subtleties is the key to the safe handling and sepns. of the highly radioactive nuclei. This review describes the complexes that have been synthesized to date and presents a crit. assessment of the successes and difficulties in their anal. and the bonding information they have provided. Because of increasing recent efforts to start new Np-capable air-sensitive inorg. chem. labs., the importance of radioactivity, the basics of Np decay and its ramifications (including the radiochem. synthesis of one organometallic compd.), and the available anhyd. starting materials are also surveyed. The review also highlights a range of instances in which important differences in the chem. behavior between Np and its closest neighbors, uranium and plutonium, are found.
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    Sokolova, M. N.; Andreev, G. B.; Yusov, A. B. First transuranium mixed-ligand polyoxometalate complex. Inorg. Chem. Commun. 2011, 14, 1089– 1092,  DOI: 10.1016/j.inoche.2011.03.060
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    Tourne, C. M.; Tourne, G. F.; Brianso, M.-C. Bis(undecatungstogermanato)uranate(IV) de cesium: Cs12[U(GeW11O39)2]•13–14 H2O. Acta Crystallogr. Sect. B: Struct. Sci. 1980, 36, 2012– 2018,  DOI: 10.1107/S0567740880007832
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    There is no corresponding record for this reference.
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    Bion, L.; Moisy, P.; Vaufrey, F.; Méot-Reymond, S.; Madic, C. Coordination of U4+ in the Complex U(P2W17O61)216- in Solid State and in Aqueous Solution. Radiochim. Acta 1997, 78, 73– 82,  DOI: 10.1524/ract.1997.78.special-issue.73
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    Coordination of U4+ in the complex U(P2W17O61)216- in solid state and in aqueous solution
    Bion, L.; Moisy, P.; Vaufrey, F.; Meot-Reymond, S.; Simoni, E.; Madic, C.
    Radiochimica Acta (1997), 78 (), 73-82CODEN: RAACAP; ISSN:0033-8230. (R. Oldenbourg Verlag)
    The aim of this work is to understand the reasons for the selectivity shown in the complexation by unsatd. heteropoly-anions (HPA) of actinides (An) which are oxidized to the no. +IV. Different studies were carried out, both in soln. and in solid state, on P2W17O6110 and its complexes with Zn2+, UO22+, Ce4+, Th4+, and U4+ to characterize the nature of the complexation site offered to the cation. Among the actinides(IV), the U4+ ion was selected due to its singular spectroscopic and magnetic properties. An initial series of studies, in soln., using 31P-NMR has thus enabled us to characterize these complexes and to compare them to those formed with the complexing agent PW11O398-. This body of data allows to identify an identical complexation site for both ligands. An anal. of the 31P-NMR spectrum of U(P2W17O61)216- has shown a plane of symmetry passing through the actinide and enables us to confirm the hypothesis of a transfer of intermetallic charge on the U-O-W bonds, obsd. in the visible absorption spectrum of the complex. The second part of the work is the study of U(P2W17O61)216- in solid state. Following synthesis, elementary and thermogravimetric analyses have confirmed the formula K16[U(P2W17O61)2].38H2O of the compd. This solid, characterized by spectrophotometry of visible and IR absorption, shows absorption bands of energy identical to those of the coordination compd. in soln. We then measured magnetic susceptibility as a function of temp., characteristic of the Curie-Weiss law. The Curie const. is characteristic of a site occupied by the cation U4+ of D4d symmetry (Archimedes antiprism). This 8-coordinate form derived from the cube is entirely favorable to the stability of the actinide(IV) and, assocd. with a high charge d., tends towards selective complexation of these ions.
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    Erine, E. A.; Baranov, A. A.; Volkov, A. Y.; Chistyakov, V. M.; Timofeev, G. A. Thermodynamics of actinide redox reactions in potassium phosphotungstate solutions. J. Alloys Compd. 1998, 271–273, 782– 785,  DOI: 10.1016/S0925-8388(98)00207-2
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    Chiang, M.-H.; Williams, C. W.; Soderholm, L.; Antonio, M. R. Coordination of Actinide Ions in Wells–Dawson Heteropolyoxoanion Complexes. Eur. J. Inorg. Chem. 2003, 2003, 2663– 2669,  DOI: 10.1002/ejic.200300014
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    Jeannin, Y. Synthèse et étude cristallographique d’un nouveau composé de coordination asymétrique de l’uranium(IV) lié à deux ligands du type polytungstate [(H3SbIIIW17O59)UIV(HW5O18)]11–. Compt. Rend. Chim. 2005, 8, 999– 1004,  DOI: 10.1016/j.crci.2004.11.017
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    Duval, S.; Sobanska, S.; Roussel, P.; Loiseau, T. B-α-[AsW9O33]9– polyoxometalates incorporating hexanuclear uranium {U6O8}-like clusters bearing the UIV form or unprecedented mixed valence UIV/UVI involving direct UVI-O-UIV bonding. Dalton Trans. 2015, 44, 19772– 19776,  DOI: 10.1039/C5DT02932E
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    Dufaye, M.; Duval, S.; Hirsou, B.; Stoclet, G.; Loiseau, T. Complexation of tetravalent uranium cations by the As4W40O140 cryptand. CrystEngComm 2018, 20, 5500– 5509,  DOI: 10.1039/C8CE00873F
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    Complexation of tetravalent uranium cations by the As4W40O140 cryptand
    Dufaye, Maxime; Duval, Sylvain; Hirsou, Bastien; Stoclet, Gregory; Loiseau, Thierry
    CrystEngComm (2018), 20 (37), 5500-5509CODEN: CRECF4; ISSN:1466-8033. (Royal Society of Chemistry)
    The complexation properties of the anionic [As4W40O140]28- macrocycle were studied towards the trapping of the 5f tetravalent uranium tetrachloride precursor. In aq. soln., the increase of the uranium concn. (from one to six molar equivalents per [As4W40O140] unit) leads to the crystn. of three new isolated cryst. compds., [{U2(μ2-OH)3(μ2-O)}AsW40O140]25- (1), [{U2(μ2-O)(HCOO)3}As4W40O140]25- (2) and [{U4(μ2-O)2(HCOO)4}As4W40O140]20- (3), in which two to four uranium atoms were trapped within the cryptand-like mol. In compds. 1 and 3, the macrocycle keeps its architecture, while a rotation of the position of one {AsW9O33} sub-unit of the cryptand could be obsd. in compd. 2. All the inclusion complexes have been characterized by single-crystal X-ray diffraction, IR spectroscopy, TGA, SEM/EDX microscopy, SAXS and ICP analyses.
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  14. 14
    Dickman, M. H.; Gama, G. J.; Kim, K.-C.; Pope, M. T. The structures of europium(III)- and uranium(IV) derivatives of [P5W30O110]15-: Evidence for “cryptohydration. J. Cluster Sci. 1996, 7, 567– 583,  DOI: 10.1007/BF01165802
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    14
    The structures of europium(III)- and uranium(IV) derivatives of [P5W30O110]15-: Evidence for "cryptohydration"
    Dickman, Michael H.; Gama, Gennaro J.; Kim, Kee-Chan; Pope, Michael T.
    Journal of Cluster Science (1996), 7 (4), 567-583CODEN: JCSCEB; ISSN:1040-7278. (Plenum)
    The crystal structures of (NH4)11.5K0.5[Eu(OH2)P5W30O110]·24H2O, K5H5[Eu(OH2)P5W30O110]·31H2O, and (NH4)11[U(OH2)P5W30O110]·12H2O were detd. In each case, the anion has the overall virtual C5v symmetry previously obsd. for the sodium deriv., [NaP5W30O110]14-. The encrypted Eu3+ and U4+ cations lie on the C5 axis, but are displaced further than the Na+ from the equatorial plane defined by the five phosphorus atoms. Only minor differences are obsd. between the structures of the two salts of the europium deriv., although solns. of these display 31P NMR spectra with chem. shifts differing by 10 ppm, provisionally attributed to the effects of protonation of the anion. The most significant feature of the three new structures is the presence of a water mol. within the central cavity and coordinated to the Eu3+ or U4+ cation. The coordination spheres of the central cations can therefore be described as monocapped pentagonal antiprisms.
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  15. 15
    Griffith, W. P.; Morley-Smith, N.; Nogueira, H. I. S.; Shoair, A. G. F.; Suriaatmaja, M.; White, A. J. P.; Williams, D. J. Studies on polyoxo and polyperoxo-metalates: Part 7. Lanthano- and thoriopolyoxotungstates as catalytic oxidants with H2O2 and the X-ray crystal structure of Na8[ThW10O36]·28H2O. J. Organomet. Chem. 2000, 607, 146– 155,  DOI: 10.1016/S0022-328X(00)00308-9
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    15
    Studies on polyoxo and polyperoxo-metalates Part 7. Lanthano- and thoriopolyoxotungstates as catalytic oxidants with H2O2 and the X-ray crystal structure of Na8[ThW10O36]·28H2O
    Griffith, W. P.; Morley-Smith, N.; Nogueira, H. I. S.; Shoair, A. G. F.; Suriaatmaja, M.; White, A. J. P.; Williams, D. J.
    Journal of Organometallic Chemistry (2000), 607 (1-2), 146-155CODEN: JORCAI; ISSN:0022-328X. (Elsevier Science S.A.)
    The effectiveness of salts of [LnIIIW10O36]9- (Ln=Y, La, Ce, Pr, Sm, Eu, Gd, Dy, Er, Lu) and [MIVW10O36]8- (M=Ce, Th) as catalysts with H2O2 for alc. oxidns. and alkene epoxidns. has been studied. It appears that catalysis arises from the polyperoxotungstates formed from H2O2. The X-ray crystal structure of the title complex shows that in the [ThW10O36]8- anion the thorium has square antiprismatic geometry in which eight oxygen atoms from two W5O18 moieties form vertex-sharing bonds; Raman data suggest that the structure of the anion is retained in aq. soln. New 31P[1H]-NMR data for [LnIII{PW11O39}2]11- (Ln=Y, La, Ce, Pr, Nd, Sm, Eu, Tb, Ho, Er, Yb, Lu) and [CeIV{PW11O39}2]10- in the solid state, in water and in H2O2 soln. are presented; these species have also been used for oxidn. catalysis.
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  16. 16
    Ostuni, A.; Bachman, R. E.; Pope, M. T. Multiple Diastereomers of [Mn+(α-m-P2W17O61)2](20–n)– (M = UIV, ThIV, CeIII; m = 1, 2). Syn- and Anti-Conformations of the Polytungstate Ligands in α1α1, α1α2, and α2α2 Complexes. J. Cluster Sci. 2003, 14, 431– 446,  DOI: 10.1023/B:JOCL.0000005074.03104.e1
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    16
    Multiple Diastereomers of [Mn+(αm-P2W17O61)2](20-n)- (M = UIV, ThIV, CeIII; m = 1, 2). Syn- and Anti-Conformations of the Polytungstate Ligands in α1α1, α1α2, and α2α2 Complexes
    Ostuni, Angelo; Bachman, Robert E.; Pope, Michael T.
    Journal of Cluster Science (2003), 14 (3), 431-446CODEN: JCSCEB; ISSN:1040-7278. (Kluwer Academic/Plenum Publishers)
    Several examples of the complexes, [M(P2W17O61)2]n- (M = UIV, ThIV, CeIII), contg. both α1 (C1) and α2 (Cs) isomers of the tungstophosphate ligands, were synthesized and characterized by x-ray crystallog. and 31P NMR spectroscopy. In every case the heteroatom M has the anticipated square antiprism coordination. When both ligands are α2 (the UIV and CeIII complexes) the solid state structure adopts a syn-conformation of the ligands, as had been previously demonstrated for M = CeIV and LuIII; in soln. a single set of P-NMR lines is obsd., consistent with a single unique structure or rapid interconversion of syn- and anti-forms. When one or both ligands are α1 (Uα1α1, Uα1α2, Thα1α1, Ceα1α2) multiline P-NMR spectra reveal two major diastereomers in soln., presumably the syn- and anti-forms. In the solid state, crystals of ammonium or K salts of the U and Ce complexes contain anti-conformers, while the Th complex proves to be syn. Variable temp. (∼25-∼60°) P-NMR spectroscopy of solns. of the Uα1α1 complex reveals the onset of syn-anti interconversion together with the irreversible formation of minor amts. of other diastereomers generated by dissocn. and scrambling of the enantiomers of the polytungstate ligands.
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  17. 17
    Duval, S.; Béghin, S.; Falaise, C.; Trivelli, X.; Rabu, P.; Loiseau, T. Stabilization of Tetravalent 4f (Ce), 5d (Hf), or 5f (Th, U) Clusters by the [α-SiW9O34]10– Polyoxometalate. Inorg. Chem. 2015, 54, 8271– 8280,  DOI: 10.1021/acs.inorgchem.5b00786
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    Stabilization of Tetravalent 4f (Ce), 5d (Hf), or 5f (Th, U) Clusters by the [α-SiW9O34]10- Polyoxometalate
    Duval, Sylvain; Beghin, Sebastien; Falaise, Clement; Trivelli, Xavier; Rabu, Pierre; Loiseau, Thierry
    Inorganic Chemistry (2015), 54 (17), 8271-8280CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    The reaction of Na10[α-SiW9O34] with tetravalent metallic cations such as 4f ((NH4)2Ce(NO3)6), 5d (HfCl4), or 5f (UCl4 and Th(NO3)4) in a pH 4.7 NaOAc buffer soln. gives four sandwich-type polyoxometalates [Ce4(μ3-O)2(SiW9O34)2(CH3COO)2]10- (1), [U4(μ3-O)2(SiW9O34)2(CH3COO)2]10- (2), [Th3(μ3-O)(μ2-OH)3(SiW9O34)2]13- (3), and [Hf3(μ2-OH)3(SiW9O34)2]11- (4). All four compds. consist of a polynuclear cluster fragment stabilized by two [α-SiW9O34]10- polyanions. Compds. 1 and 2 are isostructural with a tetranuclear core (Ce4, U4), while compd. 3 presents a trinuclear Th3 core bearing a μ3-O-centered bridge. It is an unprecedented configuration in the case of the Th(IV) cluster. Compd. 4 also possesses a trinuclear Hf3 core but with the absence of the μ3-O bridge. The mols. were characterized by single-crystal x-ray diffraction, 183W and 29Si NMR spectroscopy, IR spectroscopy, TGA, and SEM/energy-dispersive X-ray (SEM/EDX) anal.
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  18. 18
    Sokolova, M. N.; Fedosseev, A. M.; Andreev, G. B.; Budantseva, N. A.; Yusov, A. B.; Moisy, P. Synthesis and Structural Examination of Complexes of Am(IV) and Other Tetravalent Actinides with Lacunary Heteropolyanion α2-P2W17O6110–. Inorg. Chem. 2009, 48, 9185– 9190,  DOI: 10.1021/ic900710c
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    18
    Synthesis and Structural Examination of Complexes of Am(IV) and Other Tetravalent Actinides with Lacunary Heteropolyanion α2-P2W17O6110-
    Sokolova, Marina N.; Fedosseev, Alexander M.; Andreev, Grigory B.; Budantseva, Nina A.; Yusov, Alexander B.; Moisy, Philippe
    Inorganic Chemistry (2009), 48 (19), 9185-9190CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    This study concerns the formation of complexes with actinides at oxidn. state +IV, from Th to Am, through a lacunary heteropolyanion ligand P2W17O6110-. The prepn. of original single crystals of complexes with 1:2 stoichiometry provided structural data through single-crystal x-ray diffraction for the entire An(IV) series. An(IV) atoms in these complexes have a coordination no. of 8, and their coordination polyhedron is a distorted square antiprism. The data were used to compute a mean (An-O) interat. distance for each actinide. When the interat. distance is plotted vs. the inverse ionic radius of these tetravalent actinides, it appears that the simple electrostatic model is not suitable for Am(IV) and Pu(IV), although this trend must be confirmed by further investigation with other examples from the An(IV) series.
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  19. 19
    Colliard, I.; Lee, J. R. I.; Colla, C. A.; Mason, H. E.; Sawvel, A. M.; Zavarin, M.; Nyman, M.; Deblonde, G. J. P. Polyoxometalates as ligands to synthesize, isolate and characterize compounds of rare isotopes on the microgram scale. Nat. Chem. 2022, 14, 1357– 1366,  DOI: 10.1038/s41557-022-01018-8
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    Polyoxometalates as ligands to synthesize, isolate and characterize compounds of rare isotopes on the microgram scale
    Colliard, Ian; Lee, Jonathan R. I.; Colla, Christopher A.; Mason, Harris E.; Sawvel, April M.; Zavarin, Mavrik; Nyman, May; Deblonde, Gauthier J.-P.
    Nature Chemistry (2022), 14 (12), 1357-1366CODEN: NCAHBB; ISSN:1755-4330. (Nature Portfolio)
    The synthesis and study of radioactive compds. are both inherently limited by their toxicity, cost and isotope scarcity. Traditional methods using small inorg. or org. complexes typically require milligrams of sample-per attempt-which for some isotopes is equiv. to the world's annual supply. Here we demonstrate that polyoxometalates (POMs) enable the facile formation, crystn., handling and detailed characterization of metal-ligand complexes from microgram quantities owing to their high mol. wt. and controllable soly. properties. Three curium-POM complexes were prepd., using just 1-10 μg per synthesis of the rare isotope 248Cm3+, and characterized by single-crystal X-ray diffraction, showing an eight-coordinated Cm3+ center. Moreover, spectrophotometric, fluorescence, NMR and Raman analyses of several f-block element-POM complexes, including 243Am3+ and 248Cm3+, showed otherwise unnoticeable differences between their soln. vs. solid-state chem., and actinide vs. lanthanide behavior. This POM-driven strategy represents a viable path to isolate even rarer complexes, notably with actinium or transcalifornium elements.
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  20. 20
    Korenev, V. S.; Abramov, P. A.; Gushchin, A. L.; Stass, D. V.; Babaev, V. M.; Rizvanov, I. K.; Sokolov, M. N. Uranyl Incorporation into the Polyoxometalate Cavity. Synthesis and Characterization of [(UO2)8P8W48O184]24–. Russ. J. Inorg. Chem. 2019, 64, 1105– 1114,  DOI: 10.1134/S0036023619090146
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    20
    Uranyl Incorporation into the Polyoxometalate Cavity. Synthesis and Characterization of [(UO2)8P8W48O184]24-
    Korenev, V. S.; Abramov, P. A.; Gushchin, A. L.; Stass, D. V.; Babaev, V. M.; Rizvanov, I. Kh.; Sokolov, M. N.
    Russian Journal of Inorganic Chemistry (2019), 64 (9), 1105-1114CODEN: RJICAQ; ISSN:0036-0236. (Pleiades Publishing, Ltd.)
    Abstr.: The reaction of uranyl nitrate with the [P8W48O184]40- polylacunary polyanion gives the [(UO2)7P8W48O184]26- (1) and [(UO2)8P8W48O184]24- (2) anions, which were isolated as potassium and ammonium salts. For K24[(UO2)8P8W48O184] · 50H2O · 3LiCl and (NH4)16H8[(UO2)8P8W48O184] · 50H2O, X-ray diffraction study was performed. The incorporation of uranyl ions into [P8W48O184]40- was confirmed by IR and Raman spectroscopy and by mass spectrometry. Electrochem. study of 2 indicated the possibility of uranium redn. to U(IV). X-ray luminescence of the potassium salts of 1 and 2 was investigated.
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  21. 21
    Gaunt, A. J.; May, I.; Copping, R.; Bhatt, A. I.; Collison, D.; Danny Fox, O.; Travis Holman, K.; Pope, M. T. A new structural family of heteropolytungstate lacunary complexes with the uranyl, UO22+, cation. Dalton Trans. 2003, 3009– 3014,  DOI: 10.1039/b302955g
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    A new structural family of heteropolytungstate lacunary complexes with the uranyl, UO22+, cation
    Gaunt, Andrew J.; May, Iain; Copping, Roy; Bhatt, Anand I.; Collison, David; Danny Fox, O.; Travis Holman, K.; Pope, Michael T.
    Dalton Transactions (2003), (15), 3009-3014CODEN: DTARAF; ISSN:1477-9226. (Royal Society of Chemistry)
    The reaction of UO22+ with the trivacant lacunary polyoxometalate anions, [SbW9O33]9- and [TeW9O33]8-, yields the novel isostructural complexes [(UO2)2(H2O)2(SbW9O33)2]14- (1) and [(UO2)2(H2O)2(TeW9O33)2]12- (2), resp. The complex anions contain two [XW9O33]n- (X = SbIII or TeIV) anions linked by two UO22+ cations. Each uranyl moiety bonds to two unsatd. O atoms of each lacunary anion in the complex. Each [XW9O33]n- anion has six unsatd. O atoms meaning that in 1 and 2 each [XW9O33]n- anion has two unsatd. O atoms which remain uncoordinated to U with the result being the formation of an 'open' sandwich structure. The fact that a 3rd UO22+ cation is not coordinated to form a 'closed' sandwich structure (as is obsd. for 1st row d-block transition metals) is attributed to the steric hindrance of the axially' O atoms of the uranyl group. The products, prepd. as NH4+ salts, were characterized by single crystal x-ray diffraction, elemental anal., TGA anal., IR, Raman and UV/visible spectroscopy, which indicate that the O donor atoms of the lacunary heteropolytungstate anions are strongly coordinating to U(VI) in the equatorial plane, weakening the uranyl U-O axial bonds.
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  22. 22
    Copping, R.; Talbot-Eeckelaers, C.; Collison, D.; Helliwell, M.; Gaunt, A. J.; May, I.; Reilly, S. D.; Scott, B. L.; McDonald, R. D.; Valenzula, O. A. Probing the 5f electrons in a plutonyl(vi) cluster complex. Dalton Trans. 2009, 5609– 5611,  DOI: 10.1039/b908648j
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    Probing the 5f electrons in a plutonyl(VI) cluster complex
    Copping, Roy; Talbot-Eeckelaers, Catherine; Collison, David; Helliwell, Madeleine; Gaunt, Andrew J.; May, Iain; Reilly, Sean D.; Scott, Brian L.; McDonald, Ross D.; Valenzula, Oscar A.; Jones, Chris J.; Sarsfield, Mark J.
    Dalton Transactions (2009), (29), 5609-5611CODEN: DTARAF; ISSN:1477-9226. (Royal Society of Chemistry)
    The synthesis, structural, spectroscopic and preliminary magnetic characterization of a tri-metallic plutonyl(VI) polyoxometalate complex, K11[K3(PuO2)3(GeW9O34)2]·12H2O, are reported. The structure consists of three {PuO2}2+ moieties sandwiched between two A-α-[GeW9O34]10- anions. The magnetic properties of the complex were studied revealing an antiferromagnetic exchange interaction between the Pu centers.
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    Gaunt, A. J.; May, I.; Helliwell, M.; Richardson, S. The First Structural and Spectroscopic Characterization of a Neptunyl Polyoxometalate Complex. J. Am. Chem. Soc. 2002, 124, 13350– 13351,  DOI: 10.1021/ja028005e
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    The First Structural and Spectroscopic Characterization of a Neptunyl Polyoxometalate Complex
    Gaunt, Andrew J.; May, Iain; Helliwell, Madeleine; Richardson, Steve
    Journal of the American Chemical Society (2002), 124 (45), 13350-13351CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The reaction between PW9O349- and NpO2+ has yielded the 1st structurally characterized neptunyl(V) polyoxometalate complex, [Na2(NpO2)2(A-PW9O34)2]14-. This complex is isostructural with the uranyl(VI) analog, and there is also spectroscopic evidence for its existence in soln. The complex is readily extd. into toluene, and this may have significance in the sequestering and/or sepn. of the neptunyl ion in terms of nuclear waste management.
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    Kim, K.-C.; Pope, M. T. Cation-Directed Structure Changes in Polyoxometalate Chemistry. Equilibria between Isomers of Bis(9-tungstophosphatodioxouranate(VI)) Complexes. J. Am. Chem. Soc. 1999, 121, 8512– 8517,  DOI: 10.1021/ja9909125
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    Cation-Directed Structure Changes in Polyoxometalate Chemistry. Equilibria between Isomers of Bis(9-tungstophosphatodioxouranate(VI)) Complexes
    Kim, Kee-Chan; Pope, Michael T.
    Journal of the American Chemical Society (1999), 121 (37), 8512-8517CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The 1st examples of polyoxometalates incorporating uranyl hetero groups were synthesized by reaction of Na9[A-PW9O34] with UO2(NO3)2. Na, ammonium, and K salts of [M2(UO2)2(PW9O34)2]12- (M = Na, NH4, K) were characterized by P and W NMR spectroscopy and single-crystal structural anal. The Na salt (1) contains an anion of Ci symmetry in which two PW9O34 groups sandwich two UO22+ and two Na+ cations. The U atoms have pentagonal-bipyramidal coordination, achieved by three equatorial bonds to one PW9O34 and two bonds to the other. The Na cations have approx. tetrahedral coordination geometry. Single-line P and five-line W NMR spectra confirm that the anion structure is maintained in solns. contg. excess Na+. The ammonium and K salts contain anions in which the UO22+ cations (pentagonal bipyramids) are unsym. sandwiched between the PW9O34 groups, as are two closely assocd. cations, one internal and one external. In soln. both salts give two-line P and nine-line W NMR spectra that are consistent with the solid-state structures. Soln. equil. are rapidly established between the two structure types and are dependent upon the concns. and identities of the cations. Based on integrated P NMR spectra, the equil. consts. for [Na2(UO2)2(PW9O34)2]12- + 2M+ ↔ [M2(UO2)2(PW9O34)2]12- + 2Na+ are 128 ± 12 (M = K+) and 1.8 ± 0.8 (M = NH4+). The requirement of the 2nd cation to satisfy the mass-action expression for Keq, coupled with a W NMR spectrum that implies Cs symmetry, demonstrates that the external ion-paired cation undergoes rapid exchange between several surface sites of the heteropolyanion. Solns. of 1 are stable at pH 7 in the presence of excess Na cations, and the anion is transferable into toluene by phase-transfer techniques. Addn. of Ca ions to solns. of 1 generates a new 1-line P NMR spectrum of a Ca deriv. which is assumed to be isostructural with 1.
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    Baranov, A. A.; Simakin, G. A.; Kosyakov, V. N.; Erin, E. A.; Timofeev, G. A.; Kopytov, V. V.; Rykov, A. G. Redox potentials of pairs of Bk4+/Bk3+, Am4+/Am3+ and Ce4+/Ce3+ in K10P2W17O61 solution at different pH values. Radiokhimiya 1981, 23, 127– 129
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    Redox potentials of berkelium(IV)-berkelium(III), americium(IV)-americium(III) and cerium(IV)-cerium(III) pairs in potassium phosphotungstate (K10P2W17O6+1) solution at different pH levels
    Baranov, A. A.; Simakin, G. A.; Kosyakov, V. N.; Erin, E. A.; Kopytov, V. V.; Timofeev, G. A.; Rykov, A. G.
    Radiokhimiya (1981), 23 (1), 127-9CODEN: RADKAU; ISSN:0033-8311.
    The redox potentials Eop of the title elements increase with increase in activity of H+. Moreover, a region exists of acidity, in which Eop does not depend on pH. At pH 4-5, a shift of Eop in the neg. direction is obsd., which reaches 0.9-1.0 V compared with the Eop of the corresponding couple in 1M HClO4. An evaluation was made of the potentials of the Cm(IV)-Cm(III) and Cf(IV)-Cf(III) couples in the studied solns.
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    Saprykin, A. S.; Spitsyn, V. I.; Orlova, M. M.; Zhuravleva, O. P.; Krot, N. N. Preparation and properties of compounds of uranium and transuranium elements with non-saturated heteropolytungstates. Radiokhimiya 1978, 20, 247– 252
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    Preparation and properties of compounds of uranium and transuranium elements with unsaturated heteropolytungstates
    Saprykin, A. S.; Spitsyn, V. I.; Orlova, M. M.; Zhuravleva, O. P.; Krot, N. N.
    Radiokhimiya (1978), 20 (2), 247-52CODEN: RADKAU; ISSN:0033-8311.
    U(IV), Np(IV), Pu(IV), and Pu(III) form with unsatd. heteropolytungstates the complexes with the compn. M2pnM'(M''W11O39)2.nH2O and K20-nM'(P2W17O61)2.nH2O (M = Cs, K; M' = U, Np, Pu; M'' = P, As, Si, B; p and n are the valence of M and M'). These heteropolytungstates have a high thermal stability and with heating at 500°, decompn. was not obsd. The IR data indicate that the coordination sphere of the central atom in these heteropolytungstates and in decatungstates are similar.
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    Saprykin, A. S.; Shilov, V. P.; Spitsyn, V. I.; Krot, N. N. Stabilization of the americium, curium and terbium tetravalent state in aqueous solutions. Dokl. Akad. Nauk SSSR 1976, 226, 4
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    Dufaye, M.; Duval, S.; Loiseau, T. Trends and new directions in the crystal chemistry of actinide oxo-clusters incorporated in polyoxometalates. CrystEngComm 2020, 22, 3549– 3562,  DOI: 10.1039/D0CE00088D
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    Trends and new directions in the crystal chemistry of actinide oxo-clusters incorporated in polyoxometalates
    Dufaye, Maxime; Duval, Sylvain; Loiseau, Thierry
    CrystEngComm (2020), 22 (21), 3549-3562CODEN: CRECF4; ISSN:1466-8033. (Royal Society of Chemistry)
    The present highlight article deals with the incorporation of actinide cations into polyoxometalate (POM) moieties since the first example was structurally characterized and described in the literature at the beginning of the 70s. It illustrates the various structural types of topologies that can arise from the assocn. of different polyoxometalates (molybdates or tungstates, Keggin- or Wells-Dawson-based entities, etc.) with the light actinide elements (mainly thorium and uranium). Nevertheless, some rare investigations have also been reported, performed with transuranium elements such as neptunium, plutonium, americium, curium and californium. The synthetic strategies for obtaining such mol. species, their crystal structure arrangements and their behavior in aq. soln. are discussed. The diverse varieties of actinide-POM assemblies are classified on the basis of the nature of the metal (Mo or W) and the vacancy states occurring in the polyoxometallic precursors, starting with the polyoxomolybdate family. The second series of polyoxotungstates has been intensively studied and is described more widely.
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    Budantseva, N. A.; Grigor’ev, M. S.; Fedoseev, A. M. Synthesis and spectra of Np(V) γ-Octamolybdates of the composition M6[(NpO2)2(Mo8O28)]·2H2O (M = NH4, K, Rb, Cs, Tl). Radiochemistry 2015, 57, 225– 232,  DOI: 10.1134/S1066362215030017
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    Grigorev, M. S.; Charushnikova, I. A.; Fedoseev, A. M. Molybdate Complexes of Np(V) with Li+ and Na+ Cations in the Outer Sphere. Radiochemistry 2020, 62, 465– 473,  DOI: 10.1134/S1066362220040037
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    Shiels, D.; Brennessel, W. W.; Crawley, M. R.; Matson, E. M. Leveraging a reduced polyoxomolybdate-alkoxide cluster for the formation of a stable U(V) sandwich complex. Chem. Sci. 2024, 15, 11072– 11083,  DOI: 10.1039/D4SC02644F
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    Klemperer, W. G. Tetrabutylammonium Isopolyoxometalates. In Inorganic Syntheses; Ginsberg, A. P. Ed.; 1990; Vol. 27, pp 74– 85.
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    Proust, A.; Gouzerh, P.; Robert, F. Molybdenum oxo nitrosyl complexes. 1. Defect Lindqvist compounds of the type [Mo5O13(OR)4(NO)]3- (R = CH3, C2H5). Solid-state interactions with alkali-metal cations. Inorg. Chem. 1993, 32, 5291– 5298,  DOI: 10.1021/ic00075a056
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    Reilly, S. D.; Brown, J. L.; Scott, B. L.; Gaunt, A. J. Synthesis and characterization of NpCl4(DME)2 and PuCl4(DME)2 neutral transuranic An(IV) starting materials. Dalton Trans. 2014, 43, 1498– 1501,  DOI: 10.1039/C3DT53058B
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    Synthesis and characterization of NpCl4(DME)2 and PuCl4(DME)2 neutral transuranic An(iv) starting materials
    Reilly, Sean D.; Brown, Jessie L.; Scott, Brian L.; Gaunt, Andrew J.
    Dalton Transactions (2014), 43 (4), 1498-1501CODEN: DTARAF; ISSN:1477-9226. (Royal Society of Chemistry)
    The 1,2-dimethoxyethane (DME) solvento adducts of Np(iv) and Pu(iv) tetrachloride were prepd. and isolated in good and moderate yields, resp., along with single-crystal structural detns. These neutral mols. are expected to provide alternative synthetic pathways in the pursuit of nonaq. and organometallic complexes.
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    Shannon, R. Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides. Acta Crystallogr. Sect. A: Found. Crystallogr. 1976, 32, 751– 767,  DOI: 10.1107/S0567739476001551
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    Neidig, M. L.; Clark, D. L.; Martin, R. L. Covalency in f-element complexes. Coord. Chem. Rev. 2013, 257, 394– 406,  DOI: 10.1016/j.ccr.2012.04.029
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    Covalency in f-element complexes
    Neidig, Michael L.; Clark, David L.; Martin, Richard L.
    Coordination Chemistry Reviews (2013), 257 (2), 394-406CODEN: CCHRAM; ISSN:0010-8545. (Elsevier B.V.)
    A review. The presence of covalency in complexes of the 4f and 5f elements has been a source of intense research and controversy. In addn. to academic interest in this debate, there is an industrial motivation for better understanding of bonding in f-element complexes due to the need to sep. trivalent trans-plutonium elements from trivalent lanthanide fission products in advanced nuclear fuel cycles. This review discusses the key evidence for covalency in f-element bonds derived from structural, spectroscopic and theor. studies of some selected classes of mols., including octahedral hexahalides, linear actinyl and organometallic sandwich complexes. This evidence is supplemented by a discussion of covalency, including the possibility of both overlap and near-degeneracy driven covalency and the need to quantify their relative contributions in actinide metal-ligand bonds.
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    Su, J.; Batista, E. R.; Boland, K. S.; Bone, S. E.; Bradley, J. A.; Cary, S. K.; Clark, D. L.; Conradson, S. D.; Ditter, A. S.; Kaltsoyannis, N.; Keith, J. M.; Kerridge, A.; Kozimor, S. A.; Löble, M. W.; Martin, R. L.; Minasian, S. G.; Mocko, V.; La Pierre, H. S.; Seidler, G. T.; Shuh, D. K.; Wilkerson, M. P.; Wolfsberg, L. E.; Yang, P. Energy-Degeneracy-Driven Covalency in Actinide Bonding. J. Am. Chem. Soc. 2018, 140, 17977– 17984,  DOI: 10.1021/jacs.8b09436
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    Energy-Degeneracy-Driven Covalency in Actinide Bonding
    Su, Jing; Batista, Enrique R.; Boland, Kevin S.; Bone, Sharon E.; Bradley, Joseph A.; Cary, Samantha K.; Clark, David L.; Conradson, Steven D.; Ditter, Alex S.; Kaltsoyannis, Nikolas; Keith, Jason M.; Kerridge, Andrew; Kozimor, Stosh A.; Loble, Matthias W.; Martin, Richard L.; Minasian, Stefan G.; Mocko, Veronika; La Pierre, Henry S.; Seidler, Gerald T.; Shuh, David K.; Wilkerson, Marianne P.; Wolfsberg, Laura E.; Yang, Ping
    Journal of the American Chemical Society (2018), 140 (51), 17977-17984CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Evaluating the nature of chem. bonding for actinide elements represents one of the most important and long-standing problems in actinide science. We directly address this challenge and contribute a Cl K-edge X-ray absorption spectroscopy and relativistic d. functional theory study that quant. evaluates An-Cl covalency in AnCl62- (AnIV = Th, U, Np, Pu). The results showed significant mixing between Cl 3p- and AnIV 5f- and 6d-orbitals (t1u*/t2u* and t2g*/eg*), with the 6d-orbitals showing more pronounced covalent bonding than the 5f-orbitals. Moving from Th to U, Np, and Pu markedly changed the amt. of M-Cl orbital mixing, such that AnIV 6d- and Cl 3p-mixing decreased and metal 5f- and Cl 3p-orbital mixing increased across this series.
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    Liu, Y.; Wang, J.; Ji, K.; Meng, S.; Luo, Y.; Li, H.; Ma, P.; Niu, J.; Wang, J. Construction of polyoxometalate-based metal–organic frameworks through covalent bonds for enhanced visible light-driven coupling of alcohols with amines. J. Catal. 2022, 416, 149– 156,  DOI: 10.1016/j.jcat.2022.10.024
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    Construction of polyoxometalate-based metal-organic frameworks through covalent bonds for enhanced visible light-driven coupling of alcohols with amines
    Liu, Yanan; Wang, Jing; Ji, Kaihui; Meng, Sha; Luo, Yinghua; Li, Huafeng; Ma, Pengtao; Niu, Jingyang; Wang, Jingping
    Journal of Catalysis (2022), 416 (), 149-156CODEN: JCTLA5; ISSN:0021-9517. (Elsevier Inc.)
    Broad spectral response and rapid carrier transport are essential in making efficient photocatalysts. In this work, we utilized Keggin-type polyoxometalates (POMs) [CoIIW12O40]6- as metal-to-metal charge transfer (MMCT) chromophores and assembled them into metal-org. frameworks (MOFs) to construct visible-light-responsive, noble metal-free cryst. POM-based MOFs (POMOFs) (CoW-1 and CoW-2). The precise introduction of MMCT chromophores extending the light absorption of POMOFs to the visible region and improving the solar energy utilization efficiency, meanwhile, the Cu-O-W covalent bonds constructed in CoW-1 makes the framework structure more robust, reduces the interfacial contact resistance, facilitates the electron transfer, suppresses the recombination of the electron-hole pairs, improves the charge carriers sepn. efficiency, and boosts the quantum efficiency, thus achieving high catalytic activity in the coupling reaction of benzyl alc. with aniline under visible light irradn. (>400 nm). In the presence of CoW-1, the reaction conversion yield was measured as 92.6 % with turnover frequency reaching 374.4 h-1, and the apparent quantum yield at 595 nm was calcd. as 17.5 %, besides, CoW-1 also exhibited high catalytic stability and reusability. To the best of our knowledge, this is the first work describing the enhanced photocatalytic performance of POMOFs based on the synergistic effect of MMCT and covalent linkage.
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    Glass, E. N.; Fielden, J.; Kaledin, A. L.; Musaev, D. G.; Lian, T.; Hill, C. L. Extending Metal-to-Polyoxometalate Charge Transfer Lifetimes: The Effect of Heterometal Location. Chem.─Eur. J. 2014, 20, 4297– 4307,  DOI: 10.1002/chem.201304119
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    Extending Metal-to-Polyoxometalate Charge Transfer Lifetimes: The Effect of Heterometal Location
    Glass, Elliot N.; Fielden, John; Kaledin, Alexey L.; Musaev, Djamaladdin G.; Lian, Tianquan; Hill, Craig L.
    Chemistry - A European Journal (2014), 20 (15), 4297-4307CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
    In an effort to develop robust mol. sensitizers for solar fuel prodn., the electronic structure and photodynamics of transition-metal-substituted polyoxometalates (POMs), a novel class of compd. in this context, was examd. Exptl. and computational techniques including femtosecond (fs) transient absorption spectroscopy have been used to study the cobalt-contg. Keggin POMs, [CoIIW12O40]6- (1 a), [CoIIIW12O40]5- (2 a), [SiCoII(H2O)W11O39]6- (3 a), and [SiCoIII(H2O)W11O39]5- (4 a), finding the longest lived charge transfer excited state so far obsd. in a POM and elucidating the electronic structures and excited-state dynamics of these compds. at an unprecedented level. All species exhibit a bi-exponential decay in which early dynamic processes with time consts. in the fs domain yield longer lived excited states which decay with time consts. in the ps to ns domain. The initially formed states of 1 a and 3 a are considered to result from metal-to-polyoxometalate charge transfer (MPCT) from CoII to W, while the longer-lived excited state of 1 a is tentatively assigned to a localized intermediate MPCT state. The excited state formed by the tetrahedral cobalt(II) centered heteropolyanion (1 a) is far longer-lived (τ = 420 ps in H2O; τ = 1700 ps in MeCN) than that of 3 a (τ = 1.3 ps), in which the single CoII atom is located in a pseudo-octahedral addendum site. Short-lived states are obsd. for the two CoIII-contg. heteropolyanions 2 a (τ=4.4 ps) and 4 a (τ = 6.3 ps) and assigned solely to O → CoIII charge transfer. The dramatically extended lifetime for 1 a vs. 3 a is ascribed to a structural change permitted by the coordinatively flexible central site, weak orbital overlap of the central Co with the polytungstate framework, and putative transient valence trapping of the excited electron on a single W atom, a phenomenon not noted previously in POMs.
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    Zhao, C.; Huang, Z.; Rodríguez-Córdoba, W.; Kambara, C. S.; O’Halloran, K. P.; Hardcastle, K. I.; Musaev, D. G.; Lian, T.; Hill, C. L. Synthesis and Characterization of a Metal-to-Polyoxometalate Charge Transfer Molecular Chromophore. J. Am. Chem. Soc. 2011, 133, 20134– 20137,  DOI: 10.1021/ja209360x
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    Synthesis and Characterization of a Metal-to-Polyoxometalate Charge Transfer Molecular Chromophore
    Zhao, Chongchao; Huang, Zhuangqun; Rodriguez-Cordoba, William; Kambara, Choon Sung; O'Halloran, Kevin P.; Hardcastle, Ken I.; Musaev, Djamaladdin G.; Lian, Tianquan; Hill, Craig L.
    Journal of the American Chemical Society (2011), 133 (50), 20134-20137CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    [P4W35O124{Re(CO)3}2]16- (1), a Wells-Dawson [α2-P2W17O61]10- polyoxometalate (POM)-supported [Re(CO)3]+ complex contg. covalent WVI-O-ReI bonds was synthesized and characterized by several methods, including x-ray crystallog. This complex shows a high visible absorptivity (ε470 nm = 4000 M-1 cm-1 in H2O) due to the formation of a ReI-to-POM charge transfer (MPCT) band. The complex was studied by computational modeling and transient absorption measurements in the visible and mid-IR regions. Optical excitation of the MPCT transition results in instantaneous (<50 fs) electron transfer from the ReI center to the POM ligand.
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    Rusu, M.; Marcu, G.; Rusu, D.; Roşu, C.; Tomsa, A. R. Uranium(IV) polyoxotungstophosphates. J. Radioanal. Nucl. Chem. 1999, 242, 467– 472,  DOI: 10.1007/BF02345579
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    Uranium(IV) polyoxotungstophosphates
    Rusu, M.; Marcu, Gh.; Rusu, D.; Rosu, C.; Tomsa, A.-R.
    Journal of Radioanalytical and Nuclear Chemistry (1999), 242 (2), 467-472CODEN: JRNCDM; ISSN:0236-5731. (Elsevier Science B.V.)
    Two tris(oxouranium)-substituted Keggin and Dawson sandwich-type tungstophosphate heteropolyanions Na12[(UO)3(H2O)6(PW9O34)2]·21 H2O (1) and Na18[(UO)3(H2O)6(P2W15O56)2]·27 H2O (2) were prepd. by reaction of U sulfate with [PW9O34]9- and [P2W15O56]12-, resp., in aq. media at 4.7 pH. The products were characterized by elemental and thermal analyses, IR, UV-visible spectroscopy and magnetic susceptibility. The results of these studies suggest that the compds. obtained from Keggin and Dawson trilacunary anions are 2:3 sandwich-type complexes and both exhibit a square antiprismatic stereochem. for U(IV) with retention of polyoxometalate parent structure.
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  42. 42
    Staun, S. L.; Stevens, L. M.; Smiles, D. E.; Goodwin, C. A. P.; Billow, B. S.; Scott, B. L.; Wu, G.; Tondreau, A. M.; Gaunt, A. J.; Hayton, T. W. Expanding the Nonaqueous Chemistry of Neptunium: Synthesis and Structural Characterization of [Np(NR2)3Cl], [Np(NR2)3Cl]−, and [Np{N(R)(SiMe2CH2)}2(NR2)]− (R = SiMe3). Inorg. Chem. 2021, 60, 2740– 2748,  DOI: 10.1021/acs.inorgchem.0c03616
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    42
    Expanding the Nonaqueous Chemistry of Neptunium: Synthesis and Structural Characterization of [Np(NR2)3Cl], [Np(NR2)3Cl]-, and [Np{N(R)(SiMe2CH2)}2(NR2)]- (R = SiMe3)
    Staun, Selena L.; Stevens, Lauren M.; Smiles, Danil E.; Goodwin, Conrad A. P.; Billow, Brennan S.; Scott, Brian L.; Wu, Guang; Tondreau, Aaron M.; Gaunt, Andrew J.; Hayton, Trevor W.
    Inorganic Chemistry (2021), 60 (4), 2740-2748CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    Reaction of 3 equiv of NaNR2 (R = SiMe3) with NpCl4(DME)2 in THF afforded the Np(IV) silylamide complex, [Np(NR2)3Cl] (1), in good yield. Reaction of 1 with 1.5 equiv of KC8 in THF, in the presence of 1 equiv of dibenzo-18-crown-6, gave [{K(DB-18-C-6)(THF)}3(μ3-Cl)][Np(NR2)3Cl]2 (4), also in good yield. Complex 4 represents the first structurally characterized Np(III) amide. Finally, reaction of NpCl4(DME)2 with 5 equiv of NaNR2 and 1 equiv of dibenzo-18-crown-6 afforded the Np(IV) bis(metallacycle), [{Na(DB-18-C-6)(Et2O)0.62(κ1-DME)0.38}2(μ-DME)][Np{N(R)(SiMe2CH2)}2(NR2)]2 (8), in moderate yield. Complex 8 was characterized by 1H NMR spectroscopy and x-ray crystallog. and represents a rare example of a structurally characterized neptunium-hydrocarbyl complex. To support these studies, the authors also synthesized the uranium analogs of 4 and 8, namely, [K(2,2,2-cryptand)][U(NR2)3Cl] (2), [K(DB-18-C-6)(THF)2][U(NR2)3Cl] (3), [Na(DME)3][U{N(R)(SiMe2CH2)}2(NR2)] (6), and [{Na(DB-18-C-6)(Et2O)0.5(κ1-DME)0.5}2(μ-DME)][U{N(R)(SiMe2CH2)}2(NR2)]2 (7). Complexes 2, 3, 6, and 7 were characterized by a no. of techniques, including NMR spectroscopy and x-ray crystallog.
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    Grödler, D.; Sperling, J. M.; Rotermund, B. M.; Scheibe, B.; Beck, N. B.; Mathur, S.; Albrecht-Schönzart, T. E. Neptunium Alkoxide Chemistry: Expanding Alkoxides to the Transuranium Elements. Inorg. Chem. 2023, 62, 2513– 2517,  DOI: 10.1021/acs.inorgchem.2c04338
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    Pattenaude, S. A.; Anderson, N. H.; Bart, S. C.; Gaunt, A. J.; Scott, B. L. Non-aqueous neptunium and plutonium redox behaviour in THF–access to a rare Np(iii) synthetic precursor. Chem. Commun. 2018, 54, 6113– 6116,  DOI: 10.1039/C8CC02611D
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    Dutkiewicz, M. S.; Farnaby, J. H.; Apostolidis, C.; Colineau, E.; Walter, O.; Magnani, N.; Gardiner, M. G.; Love, J. B.; Kaltsoyannis, N.; Caciuffo, R.; Arnold, P. L. Organometallic neptunium(III) complexes. Nat. Chem. 2016, 8, 797– 802,  DOI: 10.1038/nchem.2520
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    Organometallic neptunium(iii) complexes
    Dutkiewicz, Michal S.; Farnaby, Joy H.; Apostolidis, Christos; Colineau, Eric; Walter, Olaf; Magnani, Nicola; Gardiner, Michael G.; Love, Jason B.; Kaltsoyannis, Nikolas; Caciuffo, Roberto; Arnold, Polly L.
    Nature Chemistry (2016), 8 (8), 797-802CODEN: NCAHBB; ISSN:1755-4330. (Nature Publishing Group)
    Studies of transuranic organometallic complexes provide a particularly valuable insight into covalent contributions to the metal-ligand bonding, in which the subtle differences between the transuranium actinide ions and their lighter lanthanide counterparts are of fundamental importance for the effective remediation of nuclear waste. Unlike the organometallic chem. of uranium, which has focused strongly on U(iii) and has seen some spectacular advances, that of the transuranics is significantly tech. more challenging and has remained dormant. In the case of neptunium, it is limited mainly to Np(iv). Here we report the synthesis of three new Np(iii) organometallic compds. and the characterization of their mol. and electronic structures. These studies suggest that Np(iii) complexes could act as single-mol. magnets, and that the lower oxidn. state of Np(ii) is chem. accessible. In comparison with lanthanide analogs, significant d- and f-electron contributions to key Np(iii) orbitals are obsd., which shows that fundamental neptunium organometallic chem. can provide new insights into the behavior of f-elements.
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  46. 46
    Su, J.; Cheisson, T.; McSkimming, A.; Goodwin, C. A. P.; DiMucci, I. M.; Albrecht-Schönzart, T.; Scott, B. L.; Batista, E. R.; Gaunt, A. J.; Kozimor, S. A. Complexation and redox chemistry of neptunium, plutonium and americium with a hydroxylaminato ligand. Chem. Sci. 2021, 12, 13343– 13359,  DOI: 10.1039/D1SC03905A
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    46
    Complexation and redox chemistry of neptunium, plutonium and americium with a hydroxylaminato ligand
    Su, Jing; Cheisson, Thibault; McSkimming, Alex; Goodwin, Conrad A. P.; DiMucci, Ida M.; Albrecht-Schonzart, Thomas; Scott, Brian L.; Batista, Enrique R.; Gaunt, Andrew J.; Kozimor, Stosh A.; Yang, Ping; Schelter, Eric J.
    Chemical Science (2021), 12 (40), 13343-13359CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
    There is significant interest in ligands that can stabilize actinide ions in oxidn. states that can be exploited to chem. differentiate 5f and 4f elements. Applications range from developing large-scale actinide sepn. strategies for nuclear industry processing to carrying out anal. studies that support environmental monitoring and remediation efforts. Here, we report syntheses and characterization of Np(iv), Pu(iv) and Am(iii) complexes with N-tert-butyl-N-(pyridin-2-yl)hydroxylaminato, [2-(tBuNO)py]-(interchangeable hereafter with [(tBuNO)py]-), a ligand which was previously found to impart remarkable stability to cerium in the +4 oxidn. state. An[(tBuNO)py]4 An = Pu, 1; Np, 2 have been synthesized, characterized by X-ray diffraction, X-ray absorption, 1H NMR and UV-vis-NIR spectroscopies, and cyclic voltammetry, along with computational modeling and anal. In the case of Pu, oxidn. of Pu(iii) to Pu(iv) was obsd. upon complexation with the [(tBuNO)py]- ligand. The Pu complex 1 and Np complex 2 were also isolated directly from Pu(iv) and Np(iv) precursors. Electrochem. measurements indicate that a Pu(iii) species can be accessed upon one-electron redn. of 1 with a large neg. redn. potential (E1/2 = -2.26 V vs. Applying oxidn. potentials to 1 and 2 resulted in ligand-centered electron transfer reactions, which is different from the previously reported redox chem. of UIV[(tBuNO)py]4 that revealed a stable U(v) product. Treatment of an anhyd. Am(iii) precursor with the [(tBuNO)py]- ligand did not result in oxidn. to Am(iv). Instead, the dimeric complex [AmIII(μ2-(tBuNO)py)((tBuNO)py)2]2 (3) was isolated. Complex 3 is a rare example of a structurally characterized non-aq. Am-contg. mol. complex prepd. using inert atm. techniques. Predicted redox potentials from d. functional theory calcns. show a trivalent accessibility trend of U(iii) < Np(iii) < Pu(iii) and that the higher oxidn. states of actinides i.e., +5 for Np and Pu and +4 for Am are not stabilized by [2-(tBuNO)py]-, in good agreement with exptl. observations.
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  47. 47
    Dutkiewicz, M. S.; Apostolidis, C.; Walter, O.; Arnold, P. L. Reduction chemistry of neptunium cyclopentadienide complexes: from structure to understanding. Chem. Sci. 2017, 8, 2553– 2561,  DOI: 10.1039/C7SC00034K
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    Reduction chemistry of neptunium cyclopentadienide complexes: from structure to understanding
    Dutkiewicz, Michal S.; Apostolidis, Christos; Walter, Olaf; Arnold, Polly L.
    Chemical Science (2017), 8 (4), 2553-2561CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
    Neptunium complexes in the formal oxidn. states II, III, and IV supported by cyclopentadienyl ligands are explored, and significant differences between Np and U highlighted as a result. A series of neptunium(III) cyclopentadienyl (Cp) complexes [Np(Cp)3], its bis-acetonitrile adduct [Np(Cp)3(NCMe)2], and its KCp adduct K[Np(Cp)4] and [Np(Cp')3] (Cp' = C5H4SiMe3) have been made and characterized providing the first single crystal X-ray analyses of NpIII Cp complexes. In all NpCp3 derivs. there are three Cp rings in η5-coordination around the NpIII center; addnl. in [Np(Cp)3] and K[Np(Cp)4] one Cp ring establishes a μ-η1-interaction to one C atom of a neighboring Np(Cp)3 unit. The solid state structure of K[Np(Cp)4] is unique in contg. two different types of metal-Cp coordination geometries in the same crystal. NpIII(Cp)4 units are found exhibiting four units of η5-coordinated Cp rings like in the known complex [NpIV(Cp)4], the structure of which is now reported. A detailed comparison of the structures gives evidence for the change of ionic radii of ca. -8 pm assocd. with change in oxidn. state between NpIII and NpIV. The rich redox chem. assocd. with the syntheses is augmented by the redn. of [Np(Cp')3] by KC8 in the presence of 2.2.2-cryptand to afford a neptunium(II) complex that is thermally unstable above -10 °C like the UII and ThII complexes K(2.2.2-cryptand)[Th/U(Cp')3]. Together, these spontaneous and controlled redox reactions of organo-neptunium complexes, along with information from structural characterization, show the relevance of organometallic Np chem. to understanding fundamental structure and bonding in the minor actinides.
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    Chiang, M.-H.; Soderholm, L.; Antonio, M. R. Redox Chemistry of Actinide Ions in Wells–Dawson Heteropolyoxoanion Complexes. Eur. J. Inorg. Chem. 2003, 2003, 2929– 2936,  DOI: 10.1002/ejic.200300225
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    Sonnenberger, D. C.; Gaudiello, J. G. Cyclic voltammetric study of organoactinide compounds of uranium(IV) and neptunium(IV). Ligand effects on the M(IV)/M(III) couple. Inorg. Chem. 1988, 27, 2747– 2748,  DOI: 10.1021/ic00288a036
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    Cyclic voltammetric study of organoactinide compounds of uranium(IV) and neptunium(IV). Ligand effects on the M(IV)/M(III) couple
    Sonnenberger, David C.; Gaudiello, John G.
    Inorganic Chemistry (1988), 27 (15), 2747-8CODEN: INOCAJ; ISSN:0020-1669.
    The electrochem. redn. of a series of organoactinide complexes [Cp4M (Cp = η5-C5H5; M = U, Np], Cp3MCl (M = U, Np), and Cp2*MCl2 [Cp* = η5-C5Me5; M = U, Np]) were investigated by cyclic voltammetry. The ease with which these complexes are reduced varies with the nature of the ligand environment. The redox potential of the U(IV)/U(III) couple is more sensitive to ligand environment than the Np(IV)/Np(III) couple.
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    Sonnenberger, D. C.; Gaudiello, J. Synthesis and cyclic voltammetric study of bis(pentamethylcyclopentadienyl)neptunium dichloride. J. Less-Common Met. 1986, 126, 411– 414,  DOI: 10.1016/0022-5088(86)90350-4
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    Synthesis and cyclic voltammetric study of bis(pentamethylcyclopentadienyl)neptunium dichloride
    Sonnenberger, David C.; Gaudiello, John
    Journal of the Less-Common Metals (1986), 126 (), 411-14CODEN: JCOMAH; ISSN:0022-5088.
    Cp'2NpCl2 (Cp' = pentamethylcyclopentadienyl) was prepd. by the reaction of NpCl4 with Cp'MgCl·Et2O. The title compd. exhibits a one-electron reversible redn. in acetonitrile at E1/2 = -0.68 V vs. SCE.
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    Otte, K. S.; Niklas, J. E.; Studvick, C. M.; Boggiano, A. C.; Bacsa, J.; Popov, I. A.; La Pierre, H. S. Divergent Stabilities of Tetravalent Cerium, Uranium, and Neptunium Imidophosphorane Complexes. Angew. Chem., Int. Ed. 2023, 62, e202306580  DOI: 10.1002/anie.202306580
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    Filowitz, M.; Ho, R. K. C.; Klemperer, W. G.; Shum, W. Oxygen-17 nuclear magnetic resonance spectroscopy of polyoxometalates. 1. Sensitivity and resolution. Inorg. Chem. 1979, 18, 93– 103,  DOI: 10.1021/ic50191a021
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    Oxygen-17 nuclear magnetic resonance spectroscopy of polyoxometalates. 1. Sensitivity and resolution
    Filowitz, M.; Ho, R. K. C.; Klemperer, W. G.; Shum, W.
    Inorganic Chemistry (1979), 18 (1), 93-103CODEN: INOCAJ; ISSN:0020-1669.
    Over 100 17O NMR chem. shifts are reported for 27 diamagnetic polyoxoanions of the early transition metals. Efficient procedures for obtaining 17O-enriched compds. are described, and the factors which control sensitivity and spectral resoln. are examd. and discussed in detail. Comparisons of chem. shift values with structural data show that chem. shifts are detd. largely by metal-O bond strengths.
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    Pascual-Borràs, M.; López, X.; Rodríguez-Fortea, A.; Errington, R. J.; Poblet, J. M. 17O NMR chemical shifts in oxometalates: from the simplest monometallic species to mixed-metal polyoxometalates. Chem. Sci. 2014, 5, 2031– 2042,  DOI: 10.1039/c4sc00083h
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    17O NMR chemical shifts in oxometalates: from the simplest monometallic species to mixed-metal polyoxometalates
    Pascual-Borras, Magda; Lopez, Xavier; Rodriguez-Fortea, Antonio; Errington, R. John; Poblet, Josep M.
    Chemical Science (2014), 5 (5), 2031-2042CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
    We report a theor. anal. on 17O NMR chem. shifts for a family of prototypical polyoxometalate anions. The huge diversity of structures and compns. in this family of oxometalates provides a unique resource for evaluating the influence of the metal type and connectivity over the resonance of 17O nuclei. For a set of 75 signals, we show that DFT calcns. performed with the GGA-type OPBE functional, including spin-orbit and scaling corrections, provide a mean abs. error <30 ppm, a small value considering that the range of δ(17O) values in these systems is ∼1200 ppm. For terminal M:O oxygens, the chem. shifts primarily depend on the energy gap between π*M-O and σM-O orbitals. When M is in its highest oxidn. state, the energy of π*M-O increases as we replace M going to the left and down in the periodic table. Consequently, we must expect large energy gaps and upfield shifts for O atoms linked to more electropos. ions. Although there is not a direct relationship between δ(17O) and the neg. charge of the oxygen, it is not entirely wrong to correlate at. charge and chem. shift because the ionicity of the M-O bond, the orbital energy gap and the charge d. of oxygen are related. The 17O NMR chem. shifts move upfield with an increasing no. of bound metal ions because of the larger energy gap in the involved orbitals. Finally, we explored the effect of protonation on δ(17O) in oxometalates and demonstrated that 17O NMR can be a powerful tool to identify the site(s) of protonation at low pH.
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    Clegg, W.; Elsegood, M. R. J.; Errington, R. J.; Havelock, J. Alkoxide hydrolysis as a route to early transition-metal polyoxometalates: synthesis and crystal structures of heteronuclear hexametalate derivatives. J. Chem. Soc., Dalton Trans. 1996, 681– 690,  DOI: 10.1039/dt9960000681
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    54
    Alkoxide hydrolysis as a route to early transition-metal polyoxometalates: synthesis and crystal structures of heteronuclear hexametalate derivatives
    Clegg, William; Elsegood, Mark R. J.; Errington, R. John; Havelock, Joanne
    Journal of the Chemical Society, Dalton Transactions: Inorganic Chemistry (1996), (5), 681-90CODEN: JCDTBI; ISSN:0300-9246. (Royal Society of Chemistry)
    O-17 NMR studies indicated that the hexametalates [MW5O19]n- or their derivs. [(MeO)MW5O18](n-1)- (M = Ti, Zr, V, Nb, Ta, Mo or W) can be obtained by hydrolysis of the appropriate mixt. of metal alkoxides in the presence of M'O42- (M' = W or Mo) with remarkable selectivity in some cases, giving a very efficient method of prepg. 17O-enriched polyoxometalates. The crystal structure of [NBu4]3[(MeO)TiW5O18].0.5MeCN shows the terminal Ti-OMe bond in the anion to have Ti-O 1.760(10) Å and Ti-O-C 150.1(12)° and also reveals W-O bridging bond length alternations due to the substitution of Ti(OMe)3+ for WO4+ in the [W6O19]2- structure. Hydrolysis of [NBu4]2[(MeO)NbW5O18] gives [NBu4]4[(NbW5O18)2O], and the crystal structure revealed two eclipsed NbW5O18 oxide fragments joined by a strictly linear Nb-O-Nb linkage with Nb-O 2.264(8) Å.
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  55. 55
    Errington, R. J.; Petkar, S. S.; Middleton, P. S.; McFarlane, W.; Clegg, W.; Coxall, R. A.; Harrington, R. W. Synthesis and Reactivity of the Methoxozirconium Pentatungstate (nBu4N)6[{(μ-MeO)ZrW5O18}2]: Insights into Proton-Transfer Reactions, Solution Dynamics, and Assembly of {ZrW5O18}2- Building Blocks. J. Am. Chem. Soc. 2007, 129, 12181– 12196,  DOI: 10.1021/ja0725495
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    Synthesis and reactivity of the methoxozirconium pentatungstate and an efficient route to 17O-enriched samples for 17O NMR studies. 1H NMR provided no evidence for dissociation of 1 in solution, although exchange with MeOH is slow by an EXSY study. (nBu4N)6[{(μ-MeO)ZrW5O18}2]: Insights into proton-transfer reactions, solution dynamics, and assembly of {ZrW5O18}2- building blocks
    Errington, R. John; Petkar, Sagar S.; Middleton, Paul S.; McFarlane, William; Clegg, William; Coxall, Robert A.; Harrington, Ross W.
    Journal of the American Chemical Society (2007), 129 (40), 12181-12196CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The methoxo-bridged, dimeric, ZrIV-substituted Lindqvist-type polyoxometalate (POM) (Bu4N)6[{(μ-MeO)ZrW5O18}2], (TBA)61, was synthesized by stoichiometric hydrolysis of Zr(OPr)4, [{Zr(OiPr)3(μ-OPr)(iPrOH)}2], or [{Zr(OiPr)4(iPrOH)}2] and [{WO(OMe)4}2] in the presence of (Bu4N)2WO4, providing access to the systematic nonaq. chem. of ZrW5 POMs for the 1st ti. 1H NMR provided no evidence for dissocn. of 1 in soln., although exchange with MeOH is slow by an EXSY study. Reactions with HX at elevated temps. gave a range of [{XZrW5O18}n]3n- (X = OH, 3; OPh, 4; OC6H4Me-4, 5; OC6H4(CHO)-2, 6; acac, 7; OAc, 8), where n = 2 for 3 and n = 1 for 4-8, while 1H and 17O NMR studies of hydrolysis of 1 revealed the formation of an intermediate [(μ-MeO)(μ-HO)(ZrW5O18)2]6-. Electrospray ionization mass spectrometry of 1 and 3 illustrated the robust nature of the ZrW5O18 framework, and x-ray crystal structure detns. showed that steric interactions between ligands X and the ZrW5O18 surface are important. The coordination no. of Zr is restricted to six in aryloxides 4 and 5, while seven-coordination is achieved in the chelate complexes 6-8. Given the inert nature of the methoxo bridges in 1, protonation of ZrOW sites is proposed as a possible step in reactions with HX. The diphenylphosphinate ligand in [(Ph2PO2)ZrW5O18]3- is labile and upon attempted recrystn. the aggregate [(μ3-HO)2(ZrW5O18)3H]7- 9 was formed, which is protonated at ZrOZr and ZrOW sites. This work demonstrates the flexibility of the {ZrW5O18}2- core as a mol. platform for modeling catalysis by tungstated zirconia surfaces.
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  56. 56
    Kandasamy, B.; Wills, C.; McFarlane, W.; Clegg, W.; Harrington, R. W.; Rodríguez-Fortea, A.; Poblet, J. M.; Bruce, P. G.; Errington, R. J. An Alkoxido-Tin-Substituted Polyoxometalate [(MeO)SnW5O18]3–: The First Member of a New Family of Reactive {SnW5} Lindqvist-Type Anions. Chem.─Eur. J. 2012, 18, 59– 62,  DOI: 10.1002/chem.201103544
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    An Alkoxido-Tin-Substituted Polyoxometalate [(MeO)SnW5O18]3-: The First Member of a New Family of Reactive {SnW5} Lindqvist-Type Anions
    Kandasamy, Balamurugan; Wills, Corinne; McFarlane, William; Clegg, William; Harrington, Ross W.; Rodriguez-Fortea, Antonio; Poblet, Josep M.; Bruce, Peter G.; Errington, R. John
    Chemistry - A European Journal (2012), 18 (1), 59-62, S59/1-S59/8CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The synthesis of (Bu4N)3[(MeO)SnW5O18] (1) starting from (Bu4N)2WO4, WO(OMe)4 and Sn(OBut)4 is described. Polyoxometalate 1 was characterized by 1H, 119Sn, 17O, and 183W NMR and by FTIR spectral data. Cyclic voltammetry of 1 shows a single quasi-reversible one-electron redn. at -1.84 V vs. Ag/AgCl. The crystal structure of 1 was detd.
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    Martins, C.; Aichhorn, M.; Biermann, S. Coulomb correlations in 4d and 5d oxides from first principles─or how spin–orbit materials choose their effective orbital degeneracies. J. Phys.: Condens. Matter 2017, 29, 263001  DOI: 10.1088/1361-648X/aa648f
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    Vicha, J.; Novotný, J.; Komorovsky, S.; Straka, M.; Kaupp, M.; Marek, R. Relativistic Heavy-Neighbor-Atom Effects on NMR Shifts: Concepts and Trends Across the Periodic Table. Chem. Rev. 2020, 120, 7065– 7103,  DOI: 10.1021/acs.chemrev.9b00785
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    Relativistic Heavy-Neighbor-Atom Effects on NMR Shifts: Concepts and Trends Across the Periodic Table
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    A review. Chem. shifts present crucial information about an NMR spectrum. They show the influence of the chem. environment on the nuclei being probed. Relativistic effects caused by the presence of an atom of a heavy element in a compd. can appreciably, even drastically, alter the NMR shifts of the nearby nuclei. A fundamental understanding of such relativistic effects on NMR shifts is important in many branches of chem. and phys. science. This review provides a comprehensive overview of the tools, concepts, and periodic trends pertaining to the shielding effects by a neighboring heavy atom in diamagnetic systems, with particular emphasis on the "spin-orbit heavy-atom effect on the light-atom" NMR shift (SO-HALA effect). The analyses and tools described in this review provide guidelines to help NMR spectroscopists and computational chemists est. the ranges of the NMR shifts for an unknown compd., identify intermediates in catalytic and other processes, analyze conformational aspects and intermol. interactions, and predict trends in series of compds. throughout the Periodic Table. The present review provides a current snapshot of this important subfield of NMR spectroscopy and a basis and framework for including future findings in the field.
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    How the Ligand Field in Lanthanide Coordination Complexes Determines Magnetic Susceptibility Anisotropy, Paramagnetic NMR Shift, and Relaxation Behavior
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    Accounts of Chemical Research (2020), 53 (8), 1520-1534CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
    A review. Conspectus: Complexes of lanthanide(III) ions are being actively studied because of their unique ground and excited state properties and the assocd. optical and magnetic behavior. In particular, they are used as emissive probes in optical spectroscopy and microscopy and as contrast agents in magnetic resonance imaging (MRI). However, the design of new complexes with specific optical and magnetic properties requires a thorough understanding of the correlation between mol. structure and elec. and magnetic susceptibilities, as well as their anisotropies. The traditional Judd-Ofelt-Mason theory has failed to offer useful guidelines for systematic design of emissive lanthanide optical probes. Similarly, Bleaney's theory of magnetic anisotropy and its modifications fail to provide accurate detail that permits new paramagnetic shift reagents to be designed rather than discovered. A key determinant of optical and magnetic behavior in f-element compds. is the ligand field, often considered as an electrostatic field at the lanthanide created by the ligands. The resulting energy level splitting is a sensitive function of several factors: the nature and polarizability of the whole ligand and its donor atoms; the geometric details of the coordination polyhedron; the presence and extent of solvent interactions; specific hydrogen bonding effects on donor atoms and the degree of supramol. order in the system. The relative importance of these factors can vary widely for different lanthanide ions and ligands. For nuclear magnetic properties, it is both the ligand field splitting and the magnetic susceptibility tensor, notably its anisotropy, that det. paramagnetic shifts and nuclear relaxation enhancement. The authors review the factors that control the ligand field in lanthanide complexes and link these to aspects of their utility in magnetic resonance and optical emission spectroscopy and imaging. The authors examine recent progress in this area particularly in the theory of paramagnetic chem. shift and relaxation enhancement, where some long-neglected effects of zero-field splitting, magnetic susceptibility anisotropy, and spatial distribution of lanthanide tags have been accommodated in an elegant way.
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  1. Jun Wang, Yanli Li, Ruihong Yao, Bin Yang, Wangbo Qu, Jiao Lu, Zhou Wu, Yong Li, Zhihao Lu, Zhirong Geng, Zhilin Wang. Selective Crystallization Separation of Uranium(VI) Complexes from Lanthanides. Inorganic Chemistry 2025, 64 (1) , 202-212. https://doi.org/10.1021/acs.inorgchem.4c04459
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  • Abstract

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    Figure 1

    Figure 1. General schematic for the synthesis of sandwich complexes from 1-NaMo5, extended to Np(IV) in this work. (31)

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    Figure 2

    Figure 2. 1H NMR spectra of 4-Th(Mo5)2, 5-U(Mo5)2, 6-U(Mo5)2, and 7-Np(Mo5)2 ordered by increasing 5f electron count. Spectra recorded in CDCl3 at room temperature.

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    Figure 3

    Figure 3. Single crystal X-ray diffraction structure of 7-Np(Mo5)2 with probability ellipsoids set at 50%. The tetrabutylammonium cations and some disorder has been masked for clarity.

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    Figure 4

    Figure 4. Electronic absorption spectrum of 7-Np(Mo5)2 collected at room temperature in acetonitrile. The inset shows the near-infrared region of the spectrum to highlight f–f transitions of the Np(IV) center.

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    Figure 5

    Figure 5. Cyclic voltammograms of 4-Th(Mo5)2, 5-U(Mo5)2, and 7-Np(Mo5)2. Dashed traces highlight behavior at more oxidizing potentials. The data were acquired in MeCN with 0.1 M TBA(PF6) supporting electrolyte, 1 mM of cluster, and a scan rate of 200 mV s–1.

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    Figure 6

    Figure 6. 17O NMR spectra collected on 17O enriched samples of (TBA)2[Mo5O13(OMe)4NO][Na(H2O)] and (TBA)X[M{Mo5O13(OMe)4NO}2] sandwich complexes (when M = Zr(IV), Hf(IV), Th(IV), U(IV), or Np(IV), X = 2; when M = U(V), X = 1). Spectra obtained at 21 °C in CD3OD (M = Na), CDCl3 (M = Np), or CD2Cl2 (rest).

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  • References

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      Addn. of 2,6-bis(5,6-dialkyl-1,2,4-triazin-3-yl)pyridines (btp) to UI3 and CeI3 in anhyd. pyridine gave the 1:3 [M(btp)3]I3, the terdentate N ligand being completely selective for UIII over CeIII, as shown by 1H NMR competition expts. Crystal structures of analogous btp complexes of UIII and CeIII revealed that the U-N bond distances are significantly shorter than the corresponding Ce-N distances.
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      The Renaissance of Non-Aqueous Uranium Chemistry
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      A review. Prior to the year 2000, nonaq. uranium chem. mainly involved metallocene and classical alkyl, amide, or alkoxide compds. as well as established carbene, imido, and oxo derivs. Since then, there was a resurgence of the area, and dramatic developments of supporting ligands and multiply bonded ligand types, small-mol. activation, and magnetism are reported. This Review (1) introduces the reader to some of the specialist theories of the area, (2) covers all-important starting materials, (3) surveys contemporary ligand classes installed at U, including alkyl, aryl, arene, carbene, amide, imide, nitride, alkoxide, aryloxide, and oxo compds., (4) describes advances in the area of single-mol. magnetism, and (5) summarizes the coordination and activation of small mols., including CO, CO2, nitric oxide, dinitrogen, white phosphorus, and alkanes.
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      A review. In nuclear waste treatment processes there is a need for improved ligands for the sepn. of actinides (An(III)) and lanthanides (Ln(III)). Several research groups are involved in the design and synthesis of new An(III) ligands and in the confinement of these and existing An(III) ligands onto mol. platforms giving multicoordinate ligands. The preorganization of ligands considerably improves the An(III) extn. properties, which are largely dependent on the soly. and rigidity of the platform. This tutorial review summarizes the most important An(III) ligands with emphasis on the preorganization strategy using (macrocyclic) platforms.
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      Organometallic Neptunium Chemistry
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      Chemical Reviews (Washington, DC, United States) (2017), 117 (17), 11460-11475CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
      A review. Fifty years have passed since the foundation of organometallic neptunium chem., and yet only a handful of complexes have been reported, and even fewer have been fully characterized. Yet, increasingly, combined synthetic/spectroscopic/computational studies are demonstrating how covalently bonding, soft, carbocyclic organometallic ligands provide an excellent platform for advancing the fundamental understanding of the differences in orbital contributions and covalency in f-block metal-ligand bonding. Understanding the subtleties is the key to the safe handling and sepns. of the highly radioactive nuclei. This review describes the complexes that have been synthesized to date and presents a crit. assessment of the successes and difficulties in their anal. and the bonding information they have provided. Because of increasing recent efforts to start new Np-capable air-sensitive inorg. chem. labs., the importance of radioactivity, the basics of Np decay and its ramifications (including the radiochem. synthesis of one organometallic compd.), and the available anhyd. starting materials are also surveyed. The review also highlights a range of instances in which important differences in the chem. behavior between Np and its closest neighbors, uranium and plutonium, are found.
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      Bion, L.; Moisy, P.; Vaufrey, F.; Méot-Reymond, S.; Madic, C. Coordination of U4+ in the Complex U(P2W17O61)216- in Solid State and in Aqueous Solution. Radiochim. Acta 1997, 78, 73– 82,  DOI: 10.1524/ract.1997.78.special-issue.73
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      Coordination of U4+ in the complex U(P2W17O61)216- in solid state and in aqueous solution
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      Radiochimica Acta (1997), 78 (), 73-82CODEN: RAACAP; ISSN:0033-8230. (R. Oldenbourg Verlag)
      The aim of this work is to understand the reasons for the selectivity shown in the complexation by unsatd. heteropoly-anions (HPA) of actinides (An) which are oxidized to the no. +IV. Different studies were carried out, both in soln. and in solid state, on P2W17O6110 and its complexes with Zn2+, UO22+, Ce4+, Th4+, and U4+ to characterize the nature of the complexation site offered to the cation. Among the actinides(IV), the U4+ ion was selected due to its singular spectroscopic and magnetic properties. An initial series of studies, in soln., using 31P-NMR has thus enabled us to characterize these complexes and to compare them to those formed with the complexing agent PW11O398-. This body of data allows to identify an identical complexation site for both ligands. An anal. of the 31P-NMR spectrum of U(P2W17O61)216- has shown a plane of symmetry passing through the actinide and enables us to confirm the hypothesis of a transfer of intermetallic charge on the U-O-W bonds, obsd. in the visible absorption spectrum of the complex. The second part of the work is the study of U(P2W17O61)216- in solid state. Following synthesis, elementary and thermogravimetric analyses have confirmed the formula K16[U(P2W17O61)2].38H2O of the compd. This solid, characterized by spectrophotometry of visible and IR absorption, shows absorption bands of energy identical to those of the coordination compd. in soln. We then measured magnetic susceptibility as a function of temp., characteristic of the Curie-Weiss law. The Curie const. is characteristic of a site occupied by the cation U4+ of D4d symmetry (Archimedes antiprism). This 8-coordinate form derived from the cube is entirely favorable to the stability of the actinide(IV) and, assocd. with a high charge d., tends towards selective complexation of these ions.
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      Jeannin, Y. Synthèse et étude cristallographique d’un nouveau composé de coordination asymétrique de l’uranium(IV) lié à deux ligands du type polytungstate [(H3SbIIIW17O59)UIV(HW5O18)]11–. Compt. Rend. Chim. 2005, 8, 999– 1004,  DOI: 10.1016/j.crci.2004.11.017
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      Duval, S.; Sobanska, S.; Roussel, P.; Loiseau, T. B-α-[AsW9O33]9– polyoxometalates incorporating hexanuclear uranium {U6O8}-like clusters bearing the UIV form or unprecedented mixed valence UIV/UVI involving direct UVI-O-UIV bonding. Dalton Trans. 2015, 44, 19772– 19776,  DOI: 10.1039/C5DT02932E
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      Dufaye, M.; Duval, S.; Hirsou, B.; Stoclet, G.; Loiseau, T. Complexation of tetravalent uranium cations by the As4W40O140 cryptand. CrystEngComm 2018, 20, 5500– 5509,  DOI: 10.1039/C8CE00873F
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      Complexation of tetravalent uranium cations by the As4W40O140 cryptand
      Dufaye, Maxime; Duval, Sylvain; Hirsou, Bastien; Stoclet, Gregory; Loiseau, Thierry
      CrystEngComm (2018), 20 (37), 5500-5509CODEN: CRECF4; ISSN:1466-8033. (Royal Society of Chemistry)
      The complexation properties of the anionic [As4W40O140]28- macrocycle were studied towards the trapping of the 5f tetravalent uranium tetrachloride precursor. In aq. soln., the increase of the uranium concn. (from one to six molar equivalents per [As4W40O140] unit) leads to the crystn. of three new isolated cryst. compds., [{U2(μ2-OH)3(μ2-O)}AsW40O140]25- (1), [{U2(μ2-O)(HCOO)3}As4W40O140]25- (2) and [{U4(μ2-O)2(HCOO)4}As4W40O140]20- (3), in which two to four uranium atoms were trapped within the cryptand-like mol. In compds. 1 and 3, the macrocycle keeps its architecture, while a rotation of the position of one {AsW9O33} sub-unit of the cryptand could be obsd. in compd. 2. All the inclusion complexes have been characterized by single-crystal X-ray diffraction, IR spectroscopy, TGA, SEM/EDX microscopy, SAXS and ICP analyses.
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      Dickman, M. H.; Gama, G. J.; Kim, K.-C.; Pope, M. T. The structures of europium(III)- and uranium(IV) derivatives of [P5W30O110]15-: Evidence for “cryptohydration. J. Cluster Sci. 1996, 7, 567– 583,  DOI: 10.1007/BF01165802
      14
      The structures of europium(III)- and uranium(IV) derivatives of [P5W30O110]15-: Evidence for "cryptohydration"
      Dickman, Michael H.; Gama, Gennaro J.; Kim, Kee-Chan; Pope, Michael T.
      Journal of Cluster Science (1996), 7 (4), 567-583CODEN: JCSCEB; ISSN:1040-7278. (Plenum)
      The crystal structures of (NH4)11.5K0.5[Eu(OH2)P5W30O110]·24H2O, K5H5[Eu(OH2)P5W30O110]·31H2O, and (NH4)11[U(OH2)P5W30O110]·12H2O were detd. In each case, the anion has the overall virtual C5v symmetry previously obsd. for the sodium deriv., [NaP5W30O110]14-. The encrypted Eu3+ and U4+ cations lie on the C5 axis, but are displaced further than the Na+ from the equatorial plane defined by the five phosphorus atoms. Only minor differences are obsd. between the structures of the two salts of the europium deriv., although solns. of these display 31P NMR spectra with chem. shifts differing by 10 ppm, provisionally attributed to the effects of protonation of the anion. The most significant feature of the three new structures is the presence of a water mol. within the central cavity and coordinated to the Eu3+ or U4+ cation. The coordination spheres of the central cations can therefore be described as monocapped pentagonal antiprisms.
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      Griffith, W. P.; Morley-Smith, N.; Nogueira, H. I. S.; Shoair, A. G. F.; Suriaatmaja, M.; White, A. J. P.; Williams, D. J. Studies on polyoxo and polyperoxo-metalates: Part 7. Lanthano- and thoriopolyoxotungstates as catalytic oxidants with H2O2 and the X-ray crystal structure of Na8[ThW10O36]·28H2O. J. Organomet. Chem. 2000, 607, 146– 155,  DOI: 10.1016/S0022-328X(00)00308-9
      15
      Studies on polyoxo and polyperoxo-metalates Part 7. Lanthano- and thoriopolyoxotungstates as catalytic oxidants with H2O2 and the X-ray crystal structure of Na8[ThW10O36]·28H2O
      Griffith, W. P.; Morley-Smith, N.; Nogueira, H. I. S.; Shoair, A. G. F.; Suriaatmaja, M.; White, A. J. P.; Williams, D. J.
      Journal of Organometallic Chemistry (2000), 607 (1-2), 146-155CODEN: JORCAI; ISSN:0022-328X. (Elsevier Science S.A.)
      The effectiveness of salts of [LnIIIW10O36]9- (Ln=Y, La, Ce, Pr, Sm, Eu, Gd, Dy, Er, Lu) and [MIVW10O36]8- (M=Ce, Th) as catalysts with H2O2 for alc. oxidns. and alkene epoxidns. has been studied. It appears that catalysis arises from the polyperoxotungstates formed from H2O2. The X-ray crystal structure of the title complex shows that in the [ThW10O36]8- anion the thorium has square antiprismatic geometry in which eight oxygen atoms from two W5O18 moieties form vertex-sharing bonds; Raman data suggest that the structure of the anion is retained in aq. soln. New 31P[1H]-NMR data for [LnIII{PW11O39}2]11- (Ln=Y, La, Ce, Pr, Nd, Sm, Eu, Tb, Ho, Er, Yb, Lu) and [CeIV{PW11O39}2]10- in the solid state, in water and in H2O2 soln. are presented; these species have also been used for oxidn. catalysis.
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      Ostuni, A.; Bachman, R. E.; Pope, M. T. Multiple Diastereomers of [Mn+(α-m-P2W17O61)2](20–n)– (M = UIV, ThIV, CeIII; m = 1, 2). Syn- and Anti-Conformations of the Polytungstate Ligands in α1α1, α1α2, and α2α2 Complexes. J. Cluster Sci. 2003, 14, 431– 446,  DOI: 10.1023/B:JOCL.0000005074.03104.e1
      16
      Multiple Diastereomers of [Mn+(αm-P2W17O61)2](20-n)- (M = UIV, ThIV, CeIII; m = 1, 2). Syn- and Anti-Conformations of the Polytungstate Ligands in α1α1, α1α2, and α2α2 Complexes
      Ostuni, Angelo; Bachman, Robert E.; Pope, Michael T.
      Journal of Cluster Science (2003), 14 (3), 431-446CODEN: JCSCEB; ISSN:1040-7278. (Kluwer Academic/Plenum Publishers)
      Several examples of the complexes, [M(P2W17O61)2]n- (M = UIV, ThIV, CeIII), contg. both α1 (C1) and α2 (Cs) isomers of the tungstophosphate ligands, were synthesized and characterized by x-ray crystallog. and 31P NMR spectroscopy. In every case the heteroatom M has the anticipated square antiprism coordination. When both ligands are α2 (the UIV and CeIII complexes) the solid state structure adopts a syn-conformation of the ligands, as had been previously demonstrated for M = CeIV and LuIII; in soln. a single set of P-NMR lines is obsd., consistent with a single unique structure or rapid interconversion of syn- and anti-forms. When one or both ligands are α1 (Uα1α1, Uα1α2, Thα1α1, Ceα1α2) multiline P-NMR spectra reveal two major diastereomers in soln., presumably the syn- and anti-forms. In the solid state, crystals of ammonium or K salts of the U and Ce complexes contain anti-conformers, while the Th complex proves to be syn. Variable temp. (∼25-∼60°) P-NMR spectroscopy of solns. of the Uα1α1 complex reveals the onset of syn-anti interconversion together with the irreversible formation of minor amts. of other diastereomers generated by dissocn. and scrambling of the enantiomers of the polytungstate ligands.
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    17. 17
      Duval, S.; Béghin, S.; Falaise, C.; Trivelli, X.; Rabu, P.; Loiseau, T. Stabilization of Tetravalent 4f (Ce), 5d (Hf), or 5f (Th, U) Clusters by the [α-SiW9O34]10– Polyoxometalate. Inorg. Chem. 2015, 54, 8271– 8280,  DOI: 10.1021/acs.inorgchem.5b00786
      17
      Stabilization of Tetravalent 4f (Ce), 5d (Hf), or 5f (Th, U) Clusters by the [α-SiW9O34]10- Polyoxometalate
      Duval, Sylvain; Beghin, Sebastien; Falaise, Clement; Trivelli, Xavier; Rabu, Pierre; Loiseau, Thierry
      Inorganic Chemistry (2015), 54 (17), 8271-8280CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      The reaction of Na10[α-SiW9O34] with tetravalent metallic cations such as 4f ((NH4)2Ce(NO3)6), 5d (HfCl4), or 5f (UCl4 and Th(NO3)4) in a pH 4.7 NaOAc buffer soln. gives four sandwich-type polyoxometalates [Ce4(μ3-O)2(SiW9O34)2(CH3COO)2]10- (1), [U4(μ3-O)2(SiW9O34)2(CH3COO)2]10- (2), [Th3(μ3-O)(μ2-OH)3(SiW9O34)2]13- (3), and [Hf3(μ2-OH)3(SiW9O34)2]11- (4). All four compds. consist of a polynuclear cluster fragment stabilized by two [α-SiW9O34]10- polyanions. Compds. 1 and 2 are isostructural with a tetranuclear core (Ce4, U4), while compd. 3 presents a trinuclear Th3 core bearing a μ3-O-centered bridge. It is an unprecedented configuration in the case of the Th(IV) cluster. Compd. 4 also possesses a trinuclear Hf3 core but with the absence of the μ3-O bridge. The mols. were characterized by single-crystal x-ray diffraction, 183W and 29Si NMR spectroscopy, IR spectroscopy, TGA, and SEM/energy-dispersive X-ray (SEM/EDX) anal.
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    18. 18
      Sokolova, M. N.; Fedosseev, A. M.; Andreev, G. B.; Budantseva, N. A.; Yusov, A. B.; Moisy, P. Synthesis and Structural Examination of Complexes of Am(IV) and Other Tetravalent Actinides with Lacunary Heteropolyanion α2-P2W17O6110–. Inorg. Chem. 2009, 48, 9185– 9190,  DOI: 10.1021/ic900710c
      18
      Synthesis and Structural Examination of Complexes of Am(IV) and Other Tetravalent Actinides with Lacunary Heteropolyanion α2-P2W17O6110-
      Sokolova, Marina N.; Fedosseev, Alexander M.; Andreev, Grigory B.; Budantseva, Nina A.; Yusov, Alexander B.; Moisy, Philippe
      Inorganic Chemistry (2009), 48 (19), 9185-9190CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      This study concerns the formation of complexes with actinides at oxidn. state +IV, from Th to Am, through a lacunary heteropolyanion ligand P2W17O6110-. The prepn. of original single crystals of complexes with 1:2 stoichiometry provided structural data through single-crystal x-ray diffraction for the entire An(IV) series. An(IV) atoms in these complexes have a coordination no. of 8, and their coordination polyhedron is a distorted square antiprism. The data were used to compute a mean (An-O) interat. distance for each actinide. When the interat. distance is plotted vs. the inverse ionic radius of these tetravalent actinides, it appears that the simple electrostatic model is not suitable for Am(IV) and Pu(IV), although this trend must be confirmed by further investigation with other examples from the An(IV) series.
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    19. 19
      Colliard, I.; Lee, J. R. I.; Colla, C. A.; Mason, H. E.; Sawvel, A. M.; Zavarin, M.; Nyman, M.; Deblonde, G. J. P. Polyoxometalates as ligands to synthesize, isolate and characterize compounds of rare isotopes on the microgram scale. Nat. Chem. 2022, 14, 1357– 1366,  DOI: 10.1038/s41557-022-01018-8
      19
      Polyoxometalates as ligands to synthesize, isolate and characterize compounds of rare isotopes on the microgram scale
      Colliard, Ian; Lee, Jonathan R. I.; Colla, Christopher A.; Mason, Harris E.; Sawvel, April M.; Zavarin, Mavrik; Nyman, May; Deblonde, Gauthier J.-P.
      Nature Chemistry (2022), 14 (12), 1357-1366CODEN: NCAHBB; ISSN:1755-4330. (Nature Portfolio)
      The synthesis and study of radioactive compds. are both inherently limited by their toxicity, cost and isotope scarcity. Traditional methods using small inorg. or org. complexes typically require milligrams of sample-per attempt-which for some isotopes is equiv. to the world's annual supply. Here we demonstrate that polyoxometalates (POMs) enable the facile formation, crystn., handling and detailed characterization of metal-ligand complexes from microgram quantities owing to their high mol. wt. and controllable soly. properties. Three curium-POM complexes were prepd., using just 1-10 μg per synthesis of the rare isotope 248Cm3+, and characterized by single-crystal X-ray diffraction, showing an eight-coordinated Cm3+ center. Moreover, spectrophotometric, fluorescence, NMR and Raman analyses of several f-block element-POM complexes, including 243Am3+ and 248Cm3+, showed otherwise unnoticeable differences between their soln. vs. solid-state chem., and actinide vs. lanthanide behavior. This POM-driven strategy represents a viable path to isolate even rarer complexes, notably with actinium or transcalifornium elements.
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    20. 20
      Korenev, V. S.; Abramov, P. A.; Gushchin, A. L.; Stass, D. V.; Babaev, V. M.; Rizvanov, I. K.; Sokolov, M. N. Uranyl Incorporation into the Polyoxometalate Cavity. Synthesis and Characterization of [(UO2)8P8W48O184]24–. Russ. J. Inorg. Chem. 2019, 64, 1105– 1114,  DOI: 10.1134/S0036023619090146
      20
      Uranyl Incorporation into the Polyoxometalate Cavity. Synthesis and Characterization of [(UO2)8P8W48O184]24-
      Korenev, V. S.; Abramov, P. A.; Gushchin, A. L.; Stass, D. V.; Babaev, V. M.; Rizvanov, I. Kh.; Sokolov, M. N.
      Russian Journal of Inorganic Chemistry (2019), 64 (9), 1105-1114CODEN: RJICAQ; ISSN:0036-0236. (Pleiades Publishing, Ltd.)
      Abstr.: The reaction of uranyl nitrate with the [P8W48O184]40- polylacunary polyanion gives the [(UO2)7P8W48O184]26- (1) and [(UO2)8P8W48O184]24- (2) anions, which were isolated as potassium and ammonium salts. For K24[(UO2)8P8W48O184] · 50H2O · 3LiCl and (NH4)16H8[(UO2)8P8W48O184] · 50H2O, X-ray diffraction study was performed. The incorporation of uranyl ions into [P8W48O184]40- was confirmed by IR and Raman spectroscopy and by mass spectrometry. Electrochem. study of 2 indicated the possibility of uranium redn. to U(IV). X-ray luminescence of the potassium salts of 1 and 2 was investigated.
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    21. 21
      Gaunt, A. J.; May, I.; Copping, R.; Bhatt, A. I.; Collison, D.; Danny Fox, O.; Travis Holman, K.; Pope, M. T. A new structural family of heteropolytungstate lacunary complexes with the uranyl, UO22+, cation. Dalton Trans. 2003, 3009– 3014,  DOI: 10.1039/b302955g
      21
      A new structural family of heteropolytungstate lacunary complexes with the uranyl, UO22+, cation
      Gaunt, Andrew J.; May, Iain; Copping, Roy; Bhatt, Anand I.; Collison, David; Danny Fox, O.; Travis Holman, K.; Pope, Michael T.
      Dalton Transactions (2003), (15), 3009-3014CODEN: DTARAF; ISSN:1477-9226. (Royal Society of Chemistry)
      The reaction of UO22+ with the trivacant lacunary polyoxometalate anions, [SbW9O33]9- and [TeW9O33]8-, yields the novel isostructural complexes [(UO2)2(H2O)2(SbW9O33)2]14- (1) and [(UO2)2(H2O)2(TeW9O33)2]12- (2), resp. The complex anions contain two [XW9O33]n- (X = SbIII or TeIV) anions linked by two UO22+ cations. Each uranyl moiety bonds to two unsatd. O atoms of each lacunary anion in the complex. Each [XW9O33]n- anion has six unsatd. O atoms meaning that in 1 and 2 each [XW9O33]n- anion has two unsatd. O atoms which remain uncoordinated to U with the result being the formation of an 'open' sandwich structure. The fact that a 3rd UO22+ cation is not coordinated to form a 'closed' sandwich structure (as is obsd. for 1st row d-block transition metals) is attributed to the steric hindrance of the axially' O atoms of the uranyl group. The products, prepd. as NH4+ salts, were characterized by single crystal x-ray diffraction, elemental anal., TGA anal., IR, Raman and UV/visible spectroscopy, which indicate that the O donor atoms of the lacunary heteropolytungstate anions are strongly coordinating to U(VI) in the equatorial plane, weakening the uranyl U-O axial bonds.
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    22. 22
      Copping, R.; Talbot-Eeckelaers, C.; Collison, D.; Helliwell, M.; Gaunt, A. J.; May, I.; Reilly, S. D.; Scott, B. L.; McDonald, R. D.; Valenzula, O. A. Probing the 5f electrons in a plutonyl(vi) cluster complex. Dalton Trans. 2009, 5609– 5611,  DOI: 10.1039/b908648j
      22
      Probing the 5f electrons in a plutonyl(VI) cluster complex
      Copping, Roy; Talbot-Eeckelaers, Catherine; Collison, David; Helliwell, Madeleine; Gaunt, Andrew J.; May, Iain; Reilly, Sean D.; Scott, Brian L.; McDonald, Ross D.; Valenzula, Oscar A.; Jones, Chris J.; Sarsfield, Mark J.
      Dalton Transactions (2009), (29), 5609-5611CODEN: DTARAF; ISSN:1477-9226. (Royal Society of Chemistry)
      The synthesis, structural, spectroscopic and preliminary magnetic characterization of a tri-metallic plutonyl(VI) polyoxometalate complex, K11[K3(PuO2)3(GeW9O34)2]·12H2O, are reported. The structure consists of three {PuO2}2+ moieties sandwiched between two A-α-[GeW9O34]10- anions. The magnetic properties of the complex were studied revealing an antiferromagnetic exchange interaction between the Pu centers.
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    23. 23
      Gaunt, A. J.; May, I.; Helliwell, M.; Richardson, S. The First Structural and Spectroscopic Characterization of a Neptunyl Polyoxometalate Complex. J. Am. Chem. Soc. 2002, 124, 13350– 13351,  DOI: 10.1021/ja028005e
      23
      The First Structural and Spectroscopic Characterization of a Neptunyl Polyoxometalate Complex
      Gaunt, Andrew J.; May, Iain; Helliwell, Madeleine; Richardson, Steve
      Journal of the American Chemical Society (2002), 124 (45), 13350-13351CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      The reaction between PW9O349- and NpO2+ has yielded the 1st structurally characterized neptunyl(V) polyoxometalate complex, [Na2(NpO2)2(A-PW9O34)2]14-. This complex is isostructural with the uranyl(VI) analog, and there is also spectroscopic evidence for its existence in soln. The complex is readily extd. into toluene, and this may have significance in the sequestering and/or sepn. of the neptunyl ion in terms of nuclear waste management.
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    24. 24
      Kim, K.-C.; Pope, M. T. Cation-Directed Structure Changes in Polyoxometalate Chemistry. Equilibria between Isomers of Bis(9-tungstophosphatodioxouranate(VI)) Complexes. J. Am. Chem. Soc. 1999, 121, 8512– 8517,  DOI: 10.1021/ja9909125
      24
      Cation-Directed Structure Changes in Polyoxometalate Chemistry. Equilibria between Isomers of Bis(9-tungstophosphatodioxouranate(VI)) Complexes
      Kim, Kee-Chan; Pope, Michael T.
      Journal of the American Chemical Society (1999), 121 (37), 8512-8517CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      The 1st examples of polyoxometalates incorporating uranyl hetero groups were synthesized by reaction of Na9[A-PW9O34] with UO2(NO3)2. Na, ammonium, and K salts of [M2(UO2)2(PW9O34)2]12- (M = Na, NH4, K) were characterized by P and W NMR spectroscopy and single-crystal structural anal. The Na salt (1) contains an anion of Ci symmetry in which two PW9O34 groups sandwich two UO22+ and two Na+ cations. The U atoms have pentagonal-bipyramidal coordination, achieved by three equatorial bonds to one PW9O34 and two bonds to the other. The Na cations have approx. tetrahedral coordination geometry. Single-line P and five-line W NMR spectra confirm that the anion structure is maintained in solns. contg. excess Na+. The ammonium and K salts contain anions in which the UO22+ cations (pentagonal bipyramids) are unsym. sandwiched between the PW9O34 groups, as are two closely assocd. cations, one internal and one external. In soln. both salts give two-line P and nine-line W NMR spectra that are consistent with the solid-state structures. Soln. equil. are rapidly established between the two structure types and are dependent upon the concns. and identities of the cations. Based on integrated P NMR spectra, the equil. consts. for [Na2(UO2)2(PW9O34)2]12- + 2M+ ↔ [M2(UO2)2(PW9O34)2]12- + 2Na+ are 128 ± 12 (M = K+) and 1.8 ± 0.8 (M = NH4+). The requirement of the 2nd cation to satisfy the mass-action expression for Keq, coupled with a W NMR spectrum that implies Cs symmetry, demonstrates that the external ion-paired cation undergoes rapid exchange between several surface sites of the heteropolyanion. Solns. of 1 are stable at pH 7 in the presence of excess Na cations, and the anion is transferable into toluene by phase-transfer techniques. Addn. of Ca ions to solns. of 1 generates a new 1-line P NMR spectrum of a Ca deriv. which is assumed to be isostructural with 1.
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    25. 25
      Baranov, A. A.; Simakin, G. A.; Kosyakov, V. N.; Erin, E. A.; Timofeev, G. A.; Kopytov, V. V.; Rykov, A. G. Redox potentials of pairs of Bk4+/Bk3+, Am4+/Am3+ and Ce4+/Ce3+ in K10P2W17O61 solution at different pH values. Radiokhimiya 1981, 23, 127– 129
      25
      Redox potentials of berkelium(IV)-berkelium(III), americium(IV)-americium(III) and cerium(IV)-cerium(III) pairs in potassium phosphotungstate (K10P2W17O6+1) solution at different pH levels
      Baranov, A. A.; Simakin, G. A.; Kosyakov, V. N.; Erin, E. A.; Kopytov, V. V.; Timofeev, G. A.; Rykov, A. G.
      Radiokhimiya (1981), 23 (1), 127-9CODEN: RADKAU; ISSN:0033-8311.
      The redox potentials Eop of the title elements increase with increase in activity of H+. Moreover, a region exists of acidity, in which Eop does not depend on pH. At pH 4-5, a shift of Eop in the neg. direction is obsd., which reaches 0.9-1.0 V compared with the Eop of the corresponding couple in 1M HClO4. An evaluation was made of the potentials of the Cm(IV)-Cm(III) and Cf(IV)-Cf(III) couples in the studied solns.
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    26. 26
      Saprykin, A. S.; Spitsyn, V. I.; Orlova, M. M.; Zhuravleva, O. P.; Krot, N. N. Preparation and properties of compounds of uranium and transuranium elements with non-saturated heteropolytungstates. Radiokhimiya 1978, 20, 247– 252
      26
      Preparation and properties of compounds of uranium and transuranium elements with unsaturated heteropolytungstates
      Saprykin, A. S.; Spitsyn, V. I.; Orlova, M. M.; Zhuravleva, O. P.; Krot, N. N.
      Radiokhimiya (1978), 20 (2), 247-52CODEN: RADKAU; ISSN:0033-8311.
      U(IV), Np(IV), Pu(IV), and Pu(III) form with unsatd. heteropolytungstates the complexes with the compn. M2pnM'(M''W11O39)2.nH2O and K20-nM'(P2W17O61)2.nH2O (M = Cs, K; M' = U, Np, Pu; M'' = P, As, Si, B; p and n are the valence of M and M'). These heteropolytungstates have a high thermal stability and with heating at 500°, decompn. was not obsd. The IR data indicate that the coordination sphere of the central atom in these heteropolytungstates and in decatungstates are similar.
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    27. 27
      Saprykin, A. S.; Shilov, V. P.; Spitsyn, V. I.; Krot, N. N. Stabilization of the americium, curium and terbium tetravalent state in aqueous solutions. Dokl. Akad. Nauk SSSR 1976, 226, 4
      There is no corresponding record for this reference.
    28. 28
      Dufaye, M.; Duval, S.; Loiseau, T. Trends and new directions in the crystal chemistry of actinide oxo-clusters incorporated in polyoxometalates. CrystEngComm 2020, 22, 3549– 3562,  DOI: 10.1039/D0CE00088D
      28
      Trends and new directions in the crystal chemistry of actinide oxo-clusters incorporated in polyoxometalates
      Dufaye, Maxime; Duval, Sylvain; Loiseau, Thierry
      CrystEngComm (2020), 22 (21), 3549-3562CODEN: CRECF4; ISSN:1466-8033. (Royal Society of Chemistry)
      The present highlight article deals with the incorporation of actinide cations into polyoxometalate (POM) moieties since the first example was structurally characterized and described in the literature at the beginning of the 70s. It illustrates the various structural types of topologies that can arise from the assocn. of different polyoxometalates (molybdates or tungstates, Keggin- or Wells-Dawson-based entities, etc.) with the light actinide elements (mainly thorium and uranium). Nevertheless, some rare investigations have also been reported, performed with transuranium elements such as neptunium, plutonium, americium, curium and californium. The synthetic strategies for obtaining such mol. species, their crystal structure arrangements and their behavior in aq. soln. are discussed. The diverse varieties of actinide-POM assemblies are classified on the basis of the nature of the metal (Mo or W) and the vacancy states occurring in the polyoxometallic precursors, starting with the polyoxomolybdate family. The second series of polyoxotungstates has been intensively studied and is described more widely.
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    29. 29
      Budantseva, N. A.; Grigor’ev, M. S.; Fedoseev, A. M. Synthesis and spectra of Np(V) γ-Octamolybdates of the composition M6[(NpO2)2(Mo8O28)]·2H2O (M = NH4, K, Rb, Cs, Tl). Radiochemistry 2015, 57, 225– 232,  DOI: 10.1134/S1066362215030017
      There is no corresponding record for this reference.
    30. 30
      Grigorev, M. S.; Charushnikova, I. A.; Fedoseev, A. M. Molybdate Complexes of Np(V) with Li+ and Na+ Cations in the Outer Sphere. Radiochemistry 2020, 62, 465– 473,  DOI: 10.1134/S1066362220040037
      There is no corresponding record for this reference.
    31. 31
      Shiels, D.; Brennessel, W. W.; Crawley, M. R.; Matson, E. M. Leveraging a reduced polyoxomolybdate-alkoxide cluster for the formation of a stable U(V) sandwich complex. Chem. Sci. 2024, 15, 11072– 11083,  DOI: 10.1039/D4SC02644F
      There is no corresponding record for this reference.
    32. 32
      Klemperer, W. G. Tetrabutylammonium Isopolyoxometalates. In Inorganic Syntheses; Ginsberg, A. P. Ed.; 1990; Vol. 27, pp 74– 85.
      There is no corresponding record for this reference.
    33. 33
      Proust, A.; Gouzerh, P.; Robert, F. Molybdenum oxo nitrosyl complexes. 1. Defect Lindqvist compounds of the type [Mo5O13(OR)4(NO)]3- (R = CH3, C2H5). Solid-state interactions with alkali-metal cations. Inorg. Chem. 1993, 32, 5291– 5298,  DOI: 10.1021/ic00075a056
      There is no corresponding record for this reference.
    34. 34
      Reilly, S. D.; Brown, J. L.; Scott, B. L.; Gaunt, A. J. Synthesis and characterization of NpCl4(DME)2 and PuCl4(DME)2 neutral transuranic An(IV) starting materials. Dalton Trans. 2014, 43, 1498– 1501,  DOI: 10.1039/C3DT53058B
      34
      Synthesis and characterization of NpCl4(DME)2 and PuCl4(DME)2 neutral transuranic An(iv) starting materials
      Reilly, Sean D.; Brown, Jessie L.; Scott, Brian L.; Gaunt, Andrew J.
      Dalton Transactions (2014), 43 (4), 1498-1501CODEN: DTARAF; ISSN:1477-9226. (Royal Society of Chemistry)
      The 1,2-dimethoxyethane (DME) solvento adducts of Np(iv) and Pu(iv) tetrachloride were prepd. and isolated in good and moderate yields, resp., along with single-crystal structural detns. These neutral mols. are expected to provide alternative synthetic pathways in the pursuit of nonaq. and organometallic complexes.
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    35. 35
      Shannon, R. Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides. Acta Crystallogr. Sect. A: Found. Crystallogr. 1976, 32, 751– 767,  DOI: 10.1107/S0567739476001551
      There is no corresponding record for this reference.
    36. 36
      Neidig, M. L.; Clark, D. L.; Martin, R. L. Covalency in f-element complexes. Coord. Chem. Rev. 2013, 257, 394– 406,  DOI: 10.1016/j.ccr.2012.04.029
      36
      Covalency in f-element complexes
      Neidig, Michael L.; Clark, David L.; Martin, Richard L.
      Coordination Chemistry Reviews (2013), 257 (2), 394-406CODEN: CCHRAM; ISSN:0010-8545. (Elsevier B.V.)
      A review. The presence of covalency in complexes of the 4f and 5f elements has been a source of intense research and controversy. In addn. to academic interest in this debate, there is an industrial motivation for better understanding of bonding in f-element complexes due to the need to sep. trivalent trans-plutonium elements from trivalent lanthanide fission products in advanced nuclear fuel cycles. This review discusses the key evidence for covalency in f-element bonds derived from structural, spectroscopic and theor. studies of some selected classes of mols., including octahedral hexahalides, linear actinyl and organometallic sandwich complexes. This evidence is supplemented by a discussion of covalency, including the possibility of both overlap and near-degeneracy driven covalency and the need to quantify their relative contributions in actinide metal-ligand bonds.
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    37. 37
      Su, J.; Batista, E. R.; Boland, K. S.; Bone, S. E.; Bradley, J. A.; Cary, S. K.; Clark, D. L.; Conradson, S. D.; Ditter, A. S.; Kaltsoyannis, N.; Keith, J. M.; Kerridge, A.; Kozimor, S. A.; Löble, M. W.; Martin, R. L.; Minasian, S. G.; Mocko, V.; La Pierre, H. S.; Seidler, G. T.; Shuh, D. K.; Wilkerson, M. P.; Wolfsberg, L. E.; Yang, P. Energy-Degeneracy-Driven Covalency in Actinide Bonding. J. Am. Chem. Soc. 2018, 140, 17977– 17984,  DOI: 10.1021/jacs.8b09436
      37
      Energy-Degeneracy-Driven Covalency in Actinide Bonding
      Su, Jing; Batista, Enrique R.; Boland, Kevin S.; Bone, Sharon E.; Bradley, Joseph A.; Cary, Samantha K.; Clark, David L.; Conradson, Steven D.; Ditter, Alex S.; Kaltsoyannis, Nikolas; Keith, Jason M.; Kerridge, Andrew; Kozimor, Stosh A.; Loble, Matthias W.; Martin, Richard L.; Minasian, Stefan G.; Mocko, Veronika; La Pierre, Henry S.; Seidler, Gerald T.; Shuh, David K.; Wilkerson, Marianne P.; Wolfsberg, Laura E.; Yang, Ping
      Journal of the American Chemical Society (2018), 140 (51), 17977-17984CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Evaluating the nature of chem. bonding for actinide elements represents one of the most important and long-standing problems in actinide science. We directly address this challenge and contribute a Cl K-edge X-ray absorption spectroscopy and relativistic d. functional theory study that quant. evaluates An-Cl covalency in AnCl62- (AnIV = Th, U, Np, Pu). The results showed significant mixing between Cl 3p- and AnIV 5f- and 6d-orbitals (t1u*/t2u* and t2g*/eg*), with the 6d-orbitals showing more pronounced covalent bonding than the 5f-orbitals. Moving from Th to U, Np, and Pu markedly changed the amt. of M-Cl orbital mixing, such that AnIV 6d- and Cl 3p-mixing decreased and metal 5f- and Cl 3p-orbital mixing increased across this series.
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    38. 38
      Liu, Y.; Wang, J.; Ji, K.; Meng, S.; Luo, Y.; Li, H.; Ma, P.; Niu, J.; Wang, J. Construction of polyoxometalate-based metal–organic frameworks through covalent bonds for enhanced visible light-driven coupling of alcohols with amines. J. Catal. 2022, 416, 149– 156,  DOI: 10.1016/j.jcat.2022.10.024
      38
      Construction of polyoxometalate-based metal-organic frameworks through covalent bonds for enhanced visible light-driven coupling of alcohols with amines
      Liu, Yanan; Wang, Jing; Ji, Kaihui; Meng, Sha; Luo, Yinghua; Li, Huafeng; Ma, Pengtao; Niu, Jingyang; Wang, Jingping
      Journal of Catalysis (2022), 416 (), 149-156CODEN: JCTLA5; ISSN:0021-9517. (Elsevier Inc.)
      Broad spectral response and rapid carrier transport are essential in making efficient photocatalysts. In this work, we utilized Keggin-type polyoxometalates (POMs) [CoIIW12O40]6- as metal-to-metal charge transfer (MMCT) chromophores and assembled them into metal-org. frameworks (MOFs) to construct visible-light-responsive, noble metal-free cryst. POM-based MOFs (POMOFs) (CoW-1 and CoW-2). The precise introduction of MMCT chromophores extending the light absorption of POMOFs to the visible region and improving the solar energy utilization efficiency, meanwhile, the Cu-O-W covalent bonds constructed in CoW-1 makes the framework structure more robust, reduces the interfacial contact resistance, facilitates the electron transfer, suppresses the recombination of the electron-hole pairs, improves the charge carriers sepn. efficiency, and boosts the quantum efficiency, thus achieving high catalytic activity in the coupling reaction of benzyl alc. with aniline under visible light irradn. (>400 nm). In the presence of CoW-1, the reaction conversion yield was measured as 92.6 % with turnover frequency reaching 374.4 h-1, and the apparent quantum yield at 595 nm was calcd. as 17.5 %, besides, CoW-1 also exhibited high catalytic stability and reusability. To the best of our knowledge, this is the first work describing the enhanced photocatalytic performance of POMOFs based on the synergistic effect of MMCT and covalent linkage.
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    39. 39
      Glass, E. N.; Fielden, J.; Kaledin, A. L.; Musaev, D. G.; Lian, T.; Hill, C. L. Extending Metal-to-Polyoxometalate Charge Transfer Lifetimes: The Effect of Heterometal Location. Chem.─Eur. J. 2014, 20, 4297– 4307,  DOI: 10.1002/chem.201304119
      39
      Extending Metal-to-Polyoxometalate Charge Transfer Lifetimes: The Effect of Heterometal Location
      Glass, Elliot N.; Fielden, John; Kaledin, Alexey L.; Musaev, Djamaladdin G.; Lian, Tianquan; Hill, Craig L.
      Chemistry - A European Journal (2014), 20 (15), 4297-4307CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
      In an effort to develop robust mol. sensitizers for solar fuel prodn., the electronic structure and photodynamics of transition-metal-substituted polyoxometalates (POMs), a novel class of compd. in this context, was examd. Exptl. and computational techniques including femtosecond (fs) transient absorption spectroscopy have been used to study the cobalt-contg. Keggin POMs, [CoIIW12O40]6- (1 a), [CoIIIW12O40]5- (2 a), [SiCoII(H2O)W11O39]6- (3 a), and [SiCoIII(H2O)W11O39]5- (4 a), finding the longest lived charge transfer excited state so far obsd. in a POM and elucidating the electronic structures and excited-state dynamics of these compds. at an unprecedented level. All species exhibit a bi-exponential decay in which early dynamic processes with time consts. in the fs domain yield longer lived excited states which decay with time consts. in the ps to ns domain. The initially formed states of 1 a and 3 a are considered to result from metal-to-polyoxometalate charge transfer (MPCT) from CoII to W, while the longer-lived excited state of 1 a is tentatively assigned to a localized intermediate MPCT state. The excited state formed by the tetrahedral cobalt(II) centered heteropolyanion (1 a) is far longer-lived (τ = 420 ps in H2O; τ = 1700 ps in MeCN) than that of 3 a (τ = 1.3 ps), in which the single CoII atom is located in a pseudo-octahedral addendum site. Short-lived states are obsd. for the two CoIII-contg. heteropolyanions 2 a (τ=4.4 ps) and 4 a (τ = 6.3 ps) and assigned solely to O → CoIII charge transfer. The dramatically extended lifetime for 1 a vs. 3 a is ascribed to a structural change permitted by the coordinatively flexible central site, weak orbital overlap of the central Co with the polytungstate framework, and putative transient valence trapping of the excited electron on a single W atom, a phenomenon not noted previously in POMs.
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    40. 40
      Zhao, C.; Huang, Z.; Rodríguez-Córdoba, W.; Kambara, C. S.; O’Halloran, K. P.; Hardcastle, K. I.; Musaev, D. G.; Lian, T.; Hill, C. L. Synthesis and Characterization of a Metal-to-Polyoxometalate Charge Transfer Molecular Chromophore. J. Am. Chem. Soc. 2011, 133, 20134– 20137,  DOI: 10.1021/ja209360x
      40
      Synthesis and Characterization of a Metal-to-Polyoxometalate Charge Transfer Molecular Chromophore
      Zhao, Chongchao; Huang, Zhuangqun; Rodriguez-Cordoba, William; Kambara, Choon Sung; O'Halloran, Kevin P.; Hardcastle, Ken I.; Musaev, Djamaladdin G.; Lian, Tianquan; Hill, Craig L.
      Journal of the American Chemical Society (2011), 133 (50), 20134-20137CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      [P4W35O124{Re(CO)3}2]16- (1), a Wells-Dawson [α2-P2W17O61]10- polyoxometalate (POM)-supported [Re(CO)3]+ complex contg. covalent WVI-O-ReI bonds was synthesized and characterized by several methods, including x-ray crystallog. This complex shows a high visible absorptivity (ε470 nm = 4000 M-1 cm-1 in H2O) due to the formation of a ReI-to-POM charge transfer (MPCT) band. The complex was studied by computational modeling and transient absorption measurements in the visible and mid-IR regions. Optical excitation of the MPCT transition results in instantaneous (<50 fs) electron transfer from the ReI center to the POM ligand.
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    41. 41
      Rusu, M.; Marcu, G.; Rusu, D.; Roşu, C.; Tomsa, A. R. Uranium(IV) polyoxotungstophosphates. J. Radioanal. Nucl. Chem. 1999, 242, 467– 472,  DOI: 10.1007/BF02345579
      41
      Uranium(IV) polyoxotungstophosphates
      Rusu, M.; Marcu, Gh.; Rusu, D.; Rosu, C.; Tomsa, A.-R.
      Journal of Radioanalytical and Nuclear Chemistry (1999), 242 (2), 467-472CODEN: JRNCDM; ISSN:0236-5731. (Elsevier Science B.V.)
      Two tris(oxouranium)-substituted Keggin and Dawson sandwich-type tungstophosphate heteropolyanions Na12[(UO)3(H2O)6(PW9O34)2]·21 H2O (1) and Na18[(UO)3(H2O)6(P2W15O56)2]·27 H2O (2) were prepd. by reaction of U sulfate with [PW9O34]9- and [P2W15O56]12-, resp., in aq. media at 4.7 pH. The products were characterized by elemental and thermal analyses, IR, UV-visible spectroscopy and magnetic susceptibility. The results of these studies suggest that the compds. obtained from Keggin and Dawson trilacunary anions are 2:3 sandwich-type complexes and both exhibit a square antiprismatic stereochem. for U(IV) with retention of polyoxometalate parent structure.
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    42. 42
      Staun, S. L.; Stevens, L. M.; Smiles, D. E.; Goodwin, C. A. P.; Billow, B. S.; Scott, B. L.; Wu, G.; Tondreau, A. M.; Gaunt, A. J.; Hayton, T. W. Expanding the Nonaqueous Chemistry of Neptunium: Synthesis and Structural Characterization of [Np(NR2)3Cl], [Np(NR2)3Cl]−, and [Np{N(R)(SiMe2CH2)}2(NR2)]− (R = SiMe3). Inorg. Chem. 2021, 60, 2740– 2748,  DOI: 10.1021/acs.inorgchem.0c03616
      42
      Expanding the Nonaqueous Chemistry of Neptunium: Synthesis and Structural Characterization of [Np(NR2)3Cl], [Np(NR2)3Cl]-, and [Np{N(R)(SiMe2CH2)}2(NR2)]- (R = SiMe3)
      Staun, Selena L.; Stevens, Lauren M.; Smiles, Danil E.; Goodwin, Conrad A. P.; Billow, Brennan S.; Scott, Brian L.; Wu, Guang; Tondreau, Aaron M.; Gaunt, Andrew J.; Hayton, Trevor W.
      Inorganic Chemistry (2021), 60 (4), 2740-2748CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      Reaction of 3 equiv of NaNR2 (R = SiMe3) with NpCl4(DME)2 in THF afforded the Np(IV) silylamide complex, [Np(NR2)3Cl] (1), in good yield. Reaction of 1 with 1.5 equiv of KC8 in THF, in the presence of 1 equiv of dibenzo-18-crown-6, gave [{K(DB-18-C-6)(THF)}3(μ3-Cl)][Np(NR2)3Cl]2 (4), also in good yield. Complex 4 represents the first structurally characterized Np(III) amide. Finally, reaction of NpCl4(DME)2 with 5 equiv of NaNR2 and 1 equiv of dibenzo-18-crown-6 afforded the Np(IV) bis(metallacycle), [{Na(DB-18-C-6)(Et2O)0.62(κ1-DME)0.38}2(μ-DME)][Np{N(R)(SiMe2CH2)}2(NR2)]2 (8), in moderate yield. Complex 8 was characterized by 1H NMR spectroscopy and x-ray crystallog. and represents a rare example of a structurally characterized neptunium-hydrocarbyl complex. To support these studies, the authors also synthesized the uranium analogs of 4 and 8, namely, [K(2,2,2-cryptand)][U(NR2)3Cl] (2), [K(DB-18-C-6)(THF)2][U(NR2)3Cl] (3), [Na(DME)3][U{N(R)(SiMe2CH2)}2(NR2)] (6), and [{Na(DB-18-C-6)(Et2O)0.5(κ1-DME)0.5}2(μ-DME)][U{N(R)(SiMe2CH2)}2(NR2)]2 (7). Complexes 2, 3, 6, and 7 were characterized by a no. of techniques, including NMR spectroscopy and x-ray crystallog.
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    43. 43
      Grödler, D.; Sperling, J. M.; Rotermund, B. M.; Scheibe, B.; Beck, N. B.; Mathur, S.; Albrecht-Schönzart, T. E. Neptunium Alkoxide Chemistry: Expanding Alkoxides to the Transuranium Elements. Inorg. Chem. 2023, 62, 2513– 2517,  DOI: 10.1021/acs.inorgchem.2c04338
      There is no corresponding record for this reference.
    44. 44
      Pattenaude, S. A.; Anderson, N. H.; Bart, S. C.; Gaunt, A. J.; Scott, B. L. Non-aqueous neptunium and plutonium redox behaviour in THF–access to a rare Np(iii) synthetic precursor. Chem. Commun. 2018, 54, 6113– 6116,  DOI: 10.1039/C8CC02611D
      There is no corresponding record for this reference.
    45. 45
      Dutkiewicz, M. S.; Farnaby, J. H.; Apostolidis, C.; Colineau, E.; Walter, O.; Magnani, N.; Gardiner, M. G.; Love, J. B.; Kaltsoyannis, N.; Caciuffo, R.; Arnold, P. L. Organometallic neptunium(III) complexes. Nat. Chem. 2016, 8, 797– 802,  DOI: 10.1038/nchem.2520
      45
      Organometallic neptunium(iii) complexes
      Dutkiewicz, Michal S.; Farnaby, Joy H.; Apostolidis, Christos; Colineau, Eric; Walter, Olaf; Magnani, Nicola; Gardiner, Michael G.; Love, Jason B.; Kaltsoyannis, Nikolas; Caciuffo, Roberto; Arnold, Polly L.
      Nature Chemistry (2016), 8 (8), 797-802CODEN: NCAHBB; ISSN:1755-4330. (Nature Publishing Group)
      Studies of transuranic organometallic complexes provide a particularly valuable insight into covalent contributions to the metal-ligand bonding, in which the subtle differences between the transuranium actinide ions and their lighter lanthanide counterparts are of fundamental importance for the effective remediation of nuclear waste. Unlike the organometallic chem. of uranium, which has focused strongly on U(iii) and has seen some spectacular advances, that of the transuranics is significantly tech. more challenging and has remained dormant. In the case of neptunium, it is limited mainly to Np(iv). Here we report the synthesis of three new Np(iii) organometallic compds. and the characterization of their mol. and electronic structures. These studies suggest that Np(iii) complexes could act as single-mol. magnets, and that the lower oxidn. state of Np(ii) is chem. accessible. In comparison with lanthanide analogs, significant d- and f-electron contributions to key Np(iii) orbitals are obsd., which shows that fundamental neptunium organometallic chem. can provide new insights into the behavior of f-elements.
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    46. 46
      Su, J.; Cheisson, T.; McSkimming, A.; Goodwin, C. A. P.; DiMucci, I. M.; Albrecht-Schönzart, T.; Scott, B. L.; Batista, E. R.; Gaunt, A. J.; Kozimor, S. A. Complexation and redox chemistry of neptunium, plutonium and americium with a hydroxylaminato ligand. Chem. Sci. 2021, 12, 13343– 13359,  DOI: 10.1039/D1SC03905A
      46
      Complexation and redox chemistry of neptunium, plutonium and americium with a hydroxylaminato ligand
      Su, Jing; Cheisson, Thibault; McSkimming, Alex; Goodwin, Conrad A. P.; DiMucci, Ida M.; Albrecht-Schonzart, Thomas; Scott, Brian L.; Batista, Enrique R.; Gaunt, Andrew J.; Kozimor, Stosh A.; Yang, Ping; Schelter, Eric J.
      Chemical Science (2021), 12 (40), 13343-13359CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
      There is significant interest in ligands that can stabilize actinide ions in oxidn. states that can be exploited to chem. differentiate 5f and 4f elements. Applications range from developing large-scale actinide sepn. strategies for nuclear industry processing to carrying out anal. studies that support environmental monitoring and remediation efforts. Here, we report syntheses and characterization of Np(iv), Pu(iv) and Am(iii) complexes with N-tert-butyl-N-(pyridin-2-yl)hydroxylaminato, [2-(tBuNO)py]-(interchangeable hereafter with [(tBuNO)py]-), a ligand which was previously found to impart remarkable stability to cerium in the +4 oxidn. state. An[(tBuNO)py]4 An = Pu, 1; Np, 2 have been synthesized, characterized by X-ray diffraction, X-ray absorption, 1H NMR and UV-vis-NIR spectroscopies, and cyclic voltammetry, along with computational modeling and anal. In the case of Pu, oxidn. of Pu(iii) to Pu(iv) was obsd. upon complexation with the [(tBuNO)py]- ligand. The Pu complex 1 and Np complex 2 were also isolated directly from Pu(iv) and Np(iv) precursors. Electrochem. measurements indicate that a Pu(iii) species can be accessed upon one-electron redn. of 1 with a large neg. redn. potential (E1/2 = -2.26 V vs. Applying oxidn. potentials to 1 and 2 resulted in ligand-centered electron transfer reactions, which is different from the previously reported redox chem. of UIV[(tBuNO)py]4 that revealed a stable U(v) product. Treatment of an anhyd. Am(iii) precursor with the [(tBuNO)py]- ligand did not result in oxidn. to Am(iv). Instead, the dimeric complex [AmIII(μ2-(tBuNO)py)((tBuNO)py)2]2 (3) was isolated. Complex 3 is a rare example of a structurally characterized non-aq. Am-contg. mol. complex prepd. using inert atm. techniques. Predicted redox potentials from d. functional theory calcns. show a trivalent accessibility trend of U(iii) < Np(iii) < Pu(iii) and that the higher oxidn. states of actinides i.e., +5 for Np and Pu and +4 for Am are not stabilized by [2-(tBuNO)py]-, in good agreement with exptl. observations.
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    47. 47
      Dutkiewicz, M. S.; Apostolidis, C.; Walter, O.; Arnold, P. L. Reduction chemistry of neptunium cyclopentadienide complexes: from structure to understanding. Chem. Sci. 2017, 8, 2553– 2561,  DOI: 10.1039/C7SC00034K
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      Reduction chemistry of neptunium cyclopentadienide complexes: from structure to understanding
      Dutkiewicz, Michal S.; Apostolidis, Christos; Walter, Olaf; Arnold, Polly L.
      Chemical Science (2017), 8 (4), 2553-2561CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
      Neptunium complexes in the formal oxidn. states II, III, and IV supported by cyclopentadienyl ligands are explored, and significant differences between Np and U highlighted as a result. A series of neptunium(III) cyclopentadienyl (Cp) complexes [Np(Cp)3], its bis-acetonitrile adduct [Np(Cp)3(NCMe)2], and its KCp adduct K[Np(Cp)4] and [Np(Cp')3] (Cp' = C5H4SiMe3) have been made and characterized providing the first single crystal X-ray analyses of NpIII Cp complexes. In all NpCp3 derivs. there are three Cp rings in η5-coordination around the NpIII center; addnl. in [Np(Cp)3] and K[Np(Cp)4] one Cp ring establishes a μ-η1-interaction to one C atom of a neighboring Np(Cp)3 unit. The solid state structure of K[Np(Cp)4] is unique in contg. two different types of metal-Cp coordination geometries in the same crystal. NpIII(Cp)4 units are found exhibiting four units of η5-coordinated Cp rings like in the known complex [NpIV(Cp)4], the structure of which is now reported. A detailed comparison of the structures gives evidence for the change of ionic radii of ca. -8 pm assocd. with change in oxidn. state between NpIII and NpIV. The rich redox chem. assocd. with the syntheses is augmented by the redn. of [Np(Cp')3] by KC8 in the presence of 2.2.2-cryptand to afford a neptunium(II) complex that is thermally unstable above -10 °C like the UII and ThII complexes K(2.2.2-cryptand)[Th/U(Cp')3]. Together, these spontaneous and controlled redox reactions of organo-neptunium complexes, along with information from structural characterization, show the relevance of organometallic Np chem. to understanding fundamental structure and bonding in the minor actinides.
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    48. 48
      Chiang, M.-H.; Soderholm, L.; Antonio, M. R. Redox Chemistry of Actinide Ions in Wells–Dawson Heteropolyoxoanion Complexes. Eur. J. Inorg. Chem. 2003, 2003, 2929– 2936,  DOI: 10.1002/ejic.200300225
      There is no corresponding record for this reference.
    49. 49
      Sonnenberger, D. C.; Gaudiello, J. G. Cyclic voltammetric study of organoactinide compounds of uranium(IV) and neptunium(IV). Ligand effects on the M(IV)/M(III) couple. Inorg. Chem. 1988, 27, 2747– 2748,  DOI: 10.1021/ic00288a036
      49
      Cyclic voltammetric study of organoactinide compounds of uranium(IV) and neptunium(IV). Ligand effects on the M(IV)/M(III) couple
      Sonnenberger, David C.; Gaudiello, John G.
      Inorganic Chemistry (1988), 27 (15), 2747-8CODEN: INOCAJ; ISSN:0020-1669.
      The electrochem. redn. of a series of organoactinide complexes [Cp4M (Cp = η5-C5H5; M = U, Np], Cp3MCl (M = U, Np), and Cp2*MCl2 [Cp* = η5-C5Me5; M = U, Np]) were investigated by cyclic voltammetry. The ease with which these complexes are reduced varies with the nature of the ligand environment. The redox potential of the U(IV)/U(III) couple is more sensitive to ligand environment than the Np(IV)/Np(III) couple.
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    50. 50
      Sonnenberger, D. C.; Gaudiello, J. Synthesis and cyclic voltammetric study of bis(pentamethylcyclopentadienyl)neptunium dichloride. J. Less-Common Met. 1986, 126, 411– 414,  DOI: 10.1016/0022-5088(86)90350-4
      50
      Synthesis and cyclic voltammetric study of bis(pentamethylcyclopentadienyl)neptunium dichloride
      Sonnenberger, David C.; Gaudiello, John
      Journal of the Less-Common Metals (1986), 126 (), 411-14CODEN: JCOMAH; ISSN:0022-5088.
      Cp'2NpCl2 (Cp' = pentamethylcyclopentadienyl) was prepd. by the reaction of NpCl4 with Cp'MgCl·Et2O. The title compd. exhibits a one-electron reversible redn. in acetonitrile at E1/2 = -0.68 V vs. SCE.
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    51. 51
      Otte, K. S.; Niklas, J. E.; Studvick, C. M.; Boggiano, A. C.; Bacsa, J.; Popov, I. A.; La Pierre, H. S. Divergent Stabilities of Tetravalent Cerium, Uranium, and Neptunium Imidophosphorane Complexes. Angew. Chem., Int. Ed. 2023, 62, e202306580  DOI: 10.1002/anie.202306580
      There is no corresponding record for this reference.
    52. 52
      Filowitz, M.; Ho, R. K. C.; Klemperer, W. G.; Shum, W. Oxygen-17 nuclear magnetic resonance spectroscopy of polyoxometalates. 1. Sensitivity and resolution. Inorg. Chem. 1979, 18, 93– 103,  DOI: 10.1021/ic50191a021
      52
      Oxygen-17 nuclear magnetic resonance spectroscopy of polyoxometalates. 1. Sensitivity and resolution
      Filowitz, M.; Ho, R. K. C.; Klemperer, W. G.; Shum, W.
      Inorganic Chemistry (1979), 18 (1), 93-103CODEN: INOCAJ; ISSN:0020-1669.
      Over 100 17O NMR chem. shifts are reported for 27 diamagnetic polyoxoanions of the early transition metals. Efficient procedures for obtaining 17O-enriched compds. are described, and the factors which control sensitivity and spectral resoln. are examd. and discussed in detail. Comparisons of chem. shift values with structural data show that chem. shifts are detd. largely by metal-O bond strengths.
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    53. 53
      Pascual-Borràs, M.; López, X.; Rodríguez-Fortea, A.; Errington, R. J.; Poblet, J. M. 17O NMR chemical shifts in oxometalates: from the simplest monometallic species to mixed-metal polyoxometalates. Chem. Sci. 2014, 5, 2031– 2042,  DOI: 10.1039/c4sc00083h
      53
      17O NMR chemical shifts in oxometalates: from the simplest monometallic species to mixed-metal polyoxometalates
      Pascual-Borras, Magda; Lopez, Xavier; Rodriguez-Fortea, Antonio; Errington, R. John; Poblet, Josep M.
      Chemical Science (2014), 5 (5), 2031-2042CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
      We report a theor. anal. on 17O NMR chem. shifts for a family of prototypical polyoxometalate anions. The huge diversity of structures and compns. in this family of oxometalates provides a unique resource for evaluating the influence of the metal type and connectivity over the resonance of 17O nuclei. For a set of 75 signals, we show that DFT calcns. performed with the GGA-type OPBE functional, including spin-orbit and scaling corrections, provide a mean abs. error <30 ppm, a small value considering that the range of δ(17O) values in these systems is ∼1200 ppm. For terminal M:O oxygens, the chem. shifts primarily depend on the energy gap between π*M-O and σM-O orbitals. When M is in its highest oxidn. state, the energy of π*M-O increases as we replace M going to the left and down in the periodic table. Consequently, we must expect large energy gaps and upfield shifts for O atoms linked to more electropos. ions. Although there is not a direct relationship between δ(17O) and the neg. charge of the oxygen, it is not entirely wrong to correlate at. charge and chem. shift because the ionicity of the M-O bond, the orbital energy gap and the charge d. of oxygen are related. The 17O NMR chem. shifts move upfield with an increasing no. of bound metal ions because of the larger energy gap in the involved orbitals. Finally, we explored the effect of protonation on δ(17O) in oxometalates and demonstrated that 17O NMR can be a powerful tool to identify the site(s) of protonation at low pH.
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    54. 54
      Clegg, W.; Elsegood, M. R. J.; Errington, R. J.; Havelock, J. Alkoxide hydrolysis as a route to early transition-metal polyoxometalates: synthesis and crystal structures of heteronuclear hexametalate derivatives. J. Chem. Soc., Dalton Trans. 1996, 681– 690,  DOI: 10.1039/dt9960000681
      54
      Alkoxide hydrolysis as a route to early transition-metal polyoxometalates: synthesis and crystal structures of heteronuclear hexametalate derivatives
      Clegg, William; Elsegood, Mark R. J.; Errington, R. John; Havelock, Joanne
      Journal of the Chemical Society, Dalton Transactions: Inorganic Chemistry (1996), (5), 681-90CODEN: JCDTBI; ISSN:0300-9246. (Royal Society of Chemistry)
      O-17 NMR studies indicated that the hexametalates [MW5O19]n- or their derivs. [(MeO)MW5O18](n-1)- (M = Ti, Zr, V, Nb, Ta, Mo or W) can be obtained by hydrolysis of the appropriate mixt. of metal alkoxides in the presence of M'O42- (M' = W or Mo) with remarkable selectivity in some cases, giving a very efficient method of prepg. 17O-enriched polyoxometalates. The crystal structure of [NBu4]3[(MeO)TiW5O18].0.5MeCN shows the terminal Ti-OMe bond in the anion to have Ti-O 1.760(10) Å and Ti-O-C 150.1(12)° and also reveals W-O bridging bond length alternations due to the substitution of Ti(OMe)3+ for WO4+ in the [W6O19]2- structure. Hydrolysis of [NBu4]2[(MeO)NbW5O18] gives [NBu4]4[(NbW5O18)2O], and the crystal structure revealed two eclipsed NbW5O18 oxide fragments joined by a strictly linear Nb-O-Nb linkage with Nb-O 2.264(8) Å.
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    55. 55
      Errington, R. J.; Petkar, S. S.; Middleton, P. S.; McFarlane, W.; Clegg, W.; Coxall, R. A.; Harrington, R. W. Synthesis and Reactivity of the Methoxozirconium Pentatungstate (nBu4N)6[{(μ-MeO)ZrW5O18}2]: Insights into Proton-Transfer Reactions, Solution Dynamics, and Assembly of {ZrW5O18}2- Building Blocks. J. Am. Chem. Soc. 2007, 129, 12181– 12196,  DOI: 10.1021/ja0725495
      55
      Synthesis and reactivity of the methoxozirconium pentatungstate and an efficient route to 17O-enriched samples for 17O NMR studies. 1H NMR provided no evidence for dissociation of 1 in solution, although exchange with MeOH is slow by an EXSY study. (nBu4N)6[{(μ-MeO)ZrW5O18}2]: Insights into proton-transfer reactions, solution dynamics, and assembly of {ZrW5O18}2- building blocks
      Errington, R. John; Petkar, Sagar S.; Middleton, Paul S.; McFarlane, William; Clegg, William; Coxall, Robert A.; Harrington, Ross W.
      Journal of the American Chemical Society (2007), 129 (40), 12181-12196CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      The methoxo-bridged, dimeric, ZrIV-substituted Lindqvist-type polyoxometalate (POM) (Bu4N)6[{(μ-MeO)ZrW5O18}2], (TBA)61, was synthesized by stoichiometric hydrolysis of Zr(OPr)4, [{Zr(OiPr)3(μ-OPr)(iPrOH)}2], or [{Zr(OiPr)4(iPrOH)}2] and [{WO(OMe)4}2] in the presence of (Bu4N)2WO4, providing access to the systematic nonaq. chem. of ZrW5 POMs for the 1st ti. 1H NMR provided no evidence for dissocn. of 1 in soln., although exchange with MeOH is slow by an EXSY study. Reactions with HX at elevated temps. gave a range of [{XZrW5O18}n]3n- (X = OH, 3; OPh, 4; OC6H4Me-4, 5; OC6H4(CHO)-2, 6; acac, 7; OAc, 8), where n = 2 for 3 and n = 1 for 4-8, while 1H and 17O NMR studies of hydrolysis of 1 revealed the formation of an intermediate [(μ-MeO)(μ-HO)(ZrW5O18)2]6-. Electrospray ionization mass spectrometry of 1 and 3 illustrated the robust nature of the ZrW5O18 framework, and x-ray crystal structure detns. showed that steric interactions between ligands X and the ZrW5O18 surface are important. The coordination no. of Zr is restricted to six in aryloxides 4 and 5, while seven-coordination is achieved in the chelate complexes 6-8. Given the inert nature of the methoxo bridges in 1, protonation of ZrOW sites is proposed as a possible step in reactions with HX. The diphenylphosphinate ligand in [(Ph2PO2)ZrW5O18]3- is labile and upon attempted recrystn. the aggregate [(μ3-HO)2(ZrW5O18)3H]7- 9 was formed, which is protonated at ZrOZr and ZrOW sites. This work demonstrates the flexibility of the {ZrW5O18}2- core as a mol. platform for modeling catalysis by tungstated zirconia surfaces.
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    56. 56
      Kandasamy, B.; Wills, C.; McFarlane, W.; Clegg, W.; Harrington, R. W.; Rodríguez-Fortea, A.; Poblet, J. M.; Bruce, P. G.; Errington, R. J. An Alkoxido-Tin-Substituted Polyoxometalate [(MeO)SnW5O18]3–: The First Member of a New Family of Reactive {SnW5} Lindqvist-Type Anions. Chem.─Eur. J. 2012, 18, 59– 62,  DOI: 10.1002/chem.201103544
      56
      An Alkoxido-Tin-Substituted Polyoxometalate [(MeO)SnW5O18]3-: The First Member of a New Family of Reactive {SnW5} Lindqvist-Type Anions
      Kandasamy, Balamurugan; Wills, Corinne; McFarlane, William; Clegg, William; Harrington, Ross W.; Rodriguez-Fortea, Antonio; Poblet, Josep M.; Bruce, Peter G.; Errington, R. John
      Chemistry - A European Journal (2012), 18 (1), 59-62, S59/1-S59/8CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The synthesis of (Bu4N)3[(MeO)SnW5O18] (1) starting from (Bu4N)2WO4, WO(OMe)4 and Sn(OBut)4 is described. Polyoxometalate 1 was characterized by 1H, 119Sn, 17O, and 183W NMR and by FTIR spectral data. Cyclic voltammetry of 1 shows a single quasi-reversible one-electron redn. at -1.84 V vs. Ag/AgCl. The crystal structure of 1 was detd.
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    57. 57
      Martins, C.; Aichhorn, M.; Biermann, S. Coulomb correlations in 4d and 5d oxides from first principles─or how spin–orbit materials choose their effective orbital degeneracies. J. Phys.: Condens. Matter 2017, 29, 263001  DOI: 10.1088/1361-648X/aa648f
      There is no corresponding record for this reference.
    58. 58
      Vicha, J.; Novotný, J.; Komorovsky, S.; Straka, M.; Kaupp, M.; Marek, R. Relativistic Heavy-Neighbor-Atom Effects on NMR Shifts: Concepts and Trends Across the Periodic Table. Chem. Rev. 2020, 120, 7065– 7103,  DOI: 10.1021/acs.chemrev.9b00785
      58
      Relativistic Heavy-Neighbor-Atom Effects on NMR Shifts: Concepts and Trends Across the Periodic Table
      Vicha, Jan; Novotny, Jan; Komorovsky, Stanislav; Straka, Michal; Kaupp, Martin; Marek, Radek
      Chemical Reviews (Washington, DC, United States) (2020), 120 (15), 7065-7103CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
      A review. Chem. shifts present crucial information about an NMR spectrum. They show the influence of the chem. environment on the nuclei being probed. Relativistic effects caused by the presence of an atom of a heavy element in a compd. can appreciably, even drastically, alter the NMR shifts of the nearby nuclei. A fundamental understanding of such relativistic effects on NMR shifts is important in many branches of chem. and phys. science. This review provides a comprehensive overview of the tools, concepts, and periodic trends pertaining to the shielding effects by a neighboring heavy atom in diamagnetic systems, with particular emphasis on the "spin-orbit heavy-atom effect on the light-atom" NMR shift (SO-HALA effect). The analyses and tools described in this review provide guidelines to help NMR spectroscopists and computational chemists est. the ranges of the NMR shifts for an unknown compd., identify intermediates in catalytic and other processes, analyze conformational aspects and intermol. interactions, and predict trends in series of compds. throughout the Periodic Table. The present review provides a current snapshot of this important subfield of NMR spectroscopy and a basis and framework for including future findings in the field.
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    59. 59
      Parker, D.; Suturina, E. A.; Kuprov, I.; Chilton, N. F. How the ligand field in lanthanide coordination complexes determines magnetic susceptibility anisotropy, paramagnetic NMR shift, and relaxation behavior. Acc. Chem. Res. 2020, 53, 1520– 1534,  DOI: 10.1021/acs.accounts.0c00275
      59
      How the Ligand Field in Lanthanide Coordination Complexes Determines Magnetic Susceptibility Anisotropy, Paramagnetic NMR Shift, and Relaxation Behavior
      Parker, David; Suturina, Elizaveta A.; Kuprov, Ilya; Chilton, Nicholas F.
      Accounts of Chemical Research (2020), 53 (8), 1520-1534CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
      A review. Conspectus: Complexes of lanthanide(III) ions are being actively studied because of their unique ground and excited state properties and the assocd. optical and magnetic behavior. In particular, they are used as emissive probes in optical spectroscopy and microscopy and as contrast agents in magnetic resonance imaging (MRI). However, the design of new complexes with specific optical and magnetic properties requires a thorough understanding of the correlation between mol. structure and elec. and magnetic susceptibilities, as well as their anisotropies. The traditional Judd-Ofelt-Mason theory has failed to offer useful guidelines for systematic design of emissive lanthanide optical probes. Similarly, Bleaney's theory of magnetic anisotropy and its modifications fail to provide accurate detail that permits new paramagnetic shift reagents to be designed rather than discovered. A key determinant of optical and magnetic behavior in f-element compds. is the ligand field, often considered as an electrostatic field at the lanthanide created by the ligands. The resulting energy level splitting is a sensitive function of several factors: the nature and polarizability of the whole ligand and its donor atoms; the geometric details of the coordination polyhedron; the presence and extent of solvent interactions; specific hydrogen bonding effects on donor atoms and the degree of supramol. order in the system. The relative importance of these factors can vary widely for different lanthanide ions and ligands. For nuclear magnetic properties, it is both the ligand field splitting and the magnetic susceptibility tensor, notably its anisotropy, that det. paramagnetic shifts and nuclear relaxation enhancement. The authors review the factors that control the ligand field in lanthanide complexes and link these to aspects of their utility in magnetic resonance and optical emission spectroscopy and imaging. The authors examine recent progress in this area particularly in the theory of paramagnetic chem. shift and relaxation enhancement, where some long-neglected effects of zero-field splitting, magnetic susceptibility anisotropy, and spatial distribution of lanthanide tags have been accommodated in an elegant way.
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  • Supporting Information

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