Identification of the Formal +2 Oxidation State of Plutonium: Synthesis

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Identification of the Formal +2 Oxidation State of Plutonium: Synthesis and Characterization of {Pu[CH(SiMe)]} II

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Cory J Windorff, Guo P. Chen, Justin N. Cross, William J Evans, Filipp Furche, Andrew J. Gaunt, Michael T. Janicke, Stosh A Kozimor, and Brian L. Scott J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b00706 • Publication Date (Web): 24 Feb 2017 Downloaded from http://pubs.acs.org on February 24, 2017

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Journal of the American Chemical Society

Identification of the Formal +2 Oxidation State of Plutonium: Synthesis and Characterization of {PuII[C5H3(SiMe3)2]3}1– Cory J. Windorff,1,2 Guo P. Chen,1 Justin N. Cross,2 William J. Evans,1* Filipp Furche,1* Andrew J. Gaunt,2* Michael T. Janicke,2 Stosh A. Kozimor,2* and Brian L. Scott3 1

Department of Chemistry, University of California, Irvine, CA 92697-2025, United States. 2Chemistry Division, 3Materials Physics and Applications Division, Los Alamos National Laboratory, Los Alamos, NM 87545, United States.

Supporting Information Placeholder ABSTRACT: Over seventy years of chemical investigations have shown plutonium exhibits some of the most complicated chemistry in the periodic table. Six Pu oxidation states have been unambiguously confirmed (0, +3 to +7) and five different oxidation states can exist simultaneously in solution. We report a new formal oxidation state for plutonium, namely Pu2+ in [K(crypt)][PuIICp′′3], Cp′′ = C5H3(SiMe3)2. The synthetic precursor, PuIIICp"3 is also reported here, comprising the first structural characterization of a Pu–C bond. Absorption spectroscopy and DFT calculations indicate that the Pu2+ ion predominantly a 5f 6 electron configuration with some 6d-mixing.

One critical step in characterizing the chemical behavior of any element involves establishing its range of accessible oxidation states. Such understanding provides crucial information for predicting chemical behavior and physical properties. Oxidation state diversity is central to the chemistry and physics of an element. Studies defining accessible oxidation states have been pursued for over 100 years. Indeed, the boundaries of oxidation states are often presumed to be established, and new oxidation states are not expected. Since Seaborg, McMillan, Kennedy, and Wahl’s discovery of plutonium in 1940,1 Pu emerged as one of the most high profile elements in the periodic table. The recognition that Pu chemistry is pivotal in a wide range of long-term global challenges is reflected in a recent renaissance in actinide chemistry. Motivated by this realization, international efforts are underway to provide fundamental understanding that underlies actinide processing and applications.2 Unfortunately, advances in uncovering new properties for Pu have been slow compared to the 4f elements, Th, and U. This results from the high specific-radioactivity and limited accessibility of Pu. Consequently, chemical research with Pu needs to be conducted in specialized

radiological facilities. Usually, synthetic chemistry with Pu is performed on a small scale (milligrams) owing to the quantity limitations imposed for reasons of both safety and security.2a,3 These constraints render synthetic work and characterization methods technically challenging, especially when targeting molecules that are reactive towards air/moisture. In fact, it is rare to find laboratories equipped with modern structural tools for fundamental air and moisture-sensitive Pu chemistry. To date there are less than 25 structural records in the Cambridge Structural Database (CSD) that contain anhydrous molecular Pu compounds prepared under inert atmospheres.4 None contain Pu–C bonds. Recent advances in lanthanide (Ln) chemistry resulted in a new series of complexes that contained all of the 4felements (excluding Pm) in the formal +2 oxidation state.5 Among this series, eight elements (La, Ce, Pr, Gd, Tb, Ho, Er, and Lu) were reported to have unusual 4fn5d1 ground state electron configurations, as opposed to the typical 4fn+15d0. It was proposed that in these unusual compounds, the C3-symmetric tris(cyclopentadienyl) environment stabilized population of the 5d-orbitals (over the 4f-orbitals). Based on these results, efforts to use the same ligand system for stabilizing actinide(II) compounds through population of analogous 6d orbitals was investigated. Indeed, the first U2+ and Th2+ complexes were prepared6 and also contained the rare 5f36d1 (U2+) and 5f06d2 (Th2+) electron configurations. Inspired by these foundational compounds, we set out to explore (1st) if a formal +2 oxidation state is stable and isolable for transuranic elements (specifically for Pu), and (2nd) if the stable 5fn6d1 (as opposed to 5fn+16d0) configurations would continue across the actinide series. The new Ln2+, U2+, and Th2+ compounds were discovered by reduction of organometallic complexes containing metals in the +3 oxidation state. Success in preparing these compounds appeared to rely on encapsulation of the potassium cation with 2.2.2-

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Scheme 1. Synthesis of [K(crypt)][PuIICp′′3]

cryptand and f-element ligation by three sterically bulky cyclopentadienyl rings, namely the trimethylsilylsubstituted rings, Cp′ or Cp′′ [Cp′ = C5H4SiMe3 and Cp′′ = C5H3(SiMe3)2–1,3]. The successful identification of U2+ and Th2+ sparked a renewed interest in organometallic transuranic chemistry from ourselves and others. Indeed, while our own studies were underway, a publication appeared that suggested the possibility of Np2+ upon reduction of a Np3+ complex with the transcalix[2]benzene[2]pyrrole supporting ligand.7 These results build upon earlier claims of plutonium in the formal +2 oxidation state, i.e. PuH2, PuE (E = S, Se, Te), and identification of Pu2+ compounds in molten salts or the gas phase.8 However, the identities of these Pu(II) compounds have yet to be substantiated through single crystal X-ray diffraction, and molecular transuranic(II) compounds have not been isolated to date. Herein we describe the successful synthesis, isolation, and characterization of a Pu2+-containing complex, namely [K(crypt)][PuIICp′′3], which represents a new formal oxidation state for Pu. Furthermore, the result distinguishes plutonium as being able to access more verified formal oxidation states than the other actinides. To accomplish the aforementioned Pu chemistry, it was first necessary to develop a synthetic route to the parent PuIIICp′′3 complex. While analogous chemistry is well established for lanthanides, U, and Th, only a handful of organoplutonium complexes have been reported in the literature.2a,3 None were characterized by single-crystal X–ray diffraction studies. We discovered that using the established PuI3(py)4 precursor9 in a salt

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metathesis reaction with KCp′′ did not cleanly yield the PuCp′′3 target. Instead, we discovered that oxidation of Pu metal with iodine in diethyl ether to form a putative “PuIIII3(Et2O)x” product10 followed by in situ treatment with three equivalents of KCp′′, cleanly generates PuCp′′3 (by 1H NMR analysis). Crystallization of this compound from pentane at −35 °C generated single crystals in 20% yield that were confirmed by X–ray crystallography to be PuIIICp′′3, 1. The bulk product of 1 was isolated in 96% yield as a blue powder. Complex 1 crystallizes (C2/c) with unit cell constants similar to the other MCp′′3 complexes (M = La, Ce, Nd, and U).11 However, it is not isomorphous with other members of the MCp′′3 series. Although the X–ray data confirmed the structure of 1 as having a PuIII ion in the trigonal plane of the three cyclopentadienyl ring centroids – analogous to the other MCp′′3 complexes – disorder in the structure precludes detailed discussion of the metrical parameters. Complex 1 exhibits a paramagnetically-shifted 1H NMR spectrum consistent with the solid-state structure (−0.4 ppm, 54 H, ∆ν1/2 = 6 Hz, Si(CH3)3; 16.5 ppm, 3 H, and 15.0 ppm, 6 H, ∆ν1/2 = 100 Hz ppm, ring protons). The 29Si NMR spectrum contained a single resonance at +8.4 ppm, which was distinct from the −14.59 ppm signal from KCp′′.12 The bulk powdered form of 1 was used in the subsequent reaction step to form the Pu2+ complex (Scheme 1). Addition of potassium graphite, KC8, to blue Et2O solutions of 1 and 2.2.2–cryptand (crypt) resulted in consumption of KC8 and an immediate color change to dark purple. From this solution [K(crypt)][PuIICp′′3], 2, was isolated as a purple solid after (1st) removing the solid graphite byproduct, (2nd) evaporation of the solvent under reduced pressure, and (3rd) washing the resulting residue with pentane to remove unreacted 1. Et2O solutions of 2 layered with hexane and stored at −35 °C overnight generated very dark block-like single crystals in 65% crystalline yield that were suitable for X–ray diffraction studies. The 1H NMR spectrum of 2 in THF-d8 is distinct from that observed for 1, exhibiting paramagnetic signals for the SiMe3 groups at 1.6 ppm (54 H, ∆ν1/2 = 10 Hz ) and ring protons tentatively assigned to broad features at −5.5 (3 H, ∆ν1/2 = 80 Hz) and 17.5 (6 H, ∆ν1/2 = 40 Hz) ppm. Resonances for the [K(crypt)]+ cation are observed at 3.71, 3.65, and 2.62 ppm. As would be expected for a formal oxidation state of Pu2+, the complex is highly susceptible towards oxidation – solutions of 2 in PTFE NMR tubes decompose within minutes. Also, despite having solutions of 2 multiply contained (inside a glass capillary, nested within a doubly plugged PTFE NMR tube liner, and placed inside a glass NMR tube) the onset of decomposition was observed within 20 min of

Figure 1. Molecular structure of [K(2.2.2-cryptand)]{PuII[C5H3(SiMe3)2]3} at the 50% probability level with hydrogens omitted.


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removing the sample from the inert atmosphere glovebox. The rapid decomposition precluded characterizing 2 by 29Si NMR. The structural determination of 2 confirms a molecular formula of [K(crypt)][PuIICp′′3] consistent with formally Pu2+ based on charge balance. The salt crystallizes in the P 1 space group, is isomorphous with [K(crypt)][ThIICp′′3], 3, and shares similar metal-ligand coordination geometry and space group with [K(crypt)][LaIICp′′3], 4.5b,6b In 2, the three Pu-(ring centroid) distances (2.509, 2.521, and 2.536 Å) are similar to those in 3 (2.512, 2.519, and 2.533 Å) and 4 (2.606, 2.620, and 2.642 Å). Geometry optimizations using density functional theory (DFT) predicted trigonal planar structures for both PuIIICp′′3, 1, and the [PuIICp′′3]1− anion in 2. The computational results are in good agreement with the crystallographic data, e.g. the calculated metal−(ring centroid) average distance for [PuIICp′′3]1− is within 0.01 Å of the X-ray result. The calculated 0.05 Å difference in the metal-(ring centroid) distances between 1 and 2 is larger than that observed between ThCp′′3/UCp′3 and [ThCp′′3]1−/[UCp′3]1− for An = U, Th (~0.02 Å), but less than the anticipated ~0.1 Å change in ionic radius from Pu3+ to Pu2+. Mulliken population analysis suggests that the HOMO of [PuIICp′′3]1− (Figure S16) is predominantly a Pu–Cp′′ non-bonding fz3 orbital. However, the HOMO also possesses appreciable (7%) dz2 character. The mixing of 5f and 6d orbitals is consistent with the slight C3h→C3 pseudo-Jahn–Teller distortion of the complex, due to the near degeneracy of the Pu2+ 5f56d1 and 5f66d0 configurations. Thus, the

Figure 2. Solution phase UV/vis/NIR experimental data of 2 (black trace). The orange bars represent the energy and oscillator strength for TDDFT calculated UV/vis/NIR spectrum (orange dashed trace).

calculations suggest that [PuIICp′′3]1− is a borderline case between the traditional 4fn+15d0 Ln2+ compounds and the new Ln2+, U2+, and Th2+ compounds with manifest d occupation. Our calculations on [AnIICp′′3]1− (An = Th– Cm, Figure S20 and Table S1) and earlier results on [AnIICp′3]1− (An = Th–Am)13 also suggest the 5fn6d1 to 5fn+16d0 crossover occurs near Pu. The UV/vis/NIR absorption spectrum of PuIIICp′′3, 1, in hexane contains a broad and intense band around 17,153 cm–1 (~600 M–1cm–1), which is not typical in the visible spectra of complexes containing Pu3+ ions.1b,2a,9 Our time-dependent DFT (TDDFT) calculations suggest that this band predominantly originates from a 5f→6d transition. The band is observable in the visible region due to the strong stabilization of the 6dz2 orbital in the trigonal planar ligand field. Numerous weak absorptions between 20,000 and 7,700 cm–1 are assigned to Laporte forbidden 5f→5f transitions characteristic of Pu3+.1b Reduction of 1 to 2 imparts substantial changes in the UV/vis/NIR spectra. The 5f→5f transitions characteristic of 1 show up as an impurity only in the solution phase UV/vis/NIR spectrum of 2 (discussed in SI) and were not detected in solid-state spectrum. Both the solution (Figure 2) and solid-state spectra of 2 are dominated by very broad bands with maxima ~21,300 cm−1 extending past 12,500 cm−1. With an approximate molar absorptivity of 2700 M−1cm−1, this band is considerably more intense than 5f→5f transitions typically observed in this region. TDDFT calculations on [PuIICp′′3]1− attribute these strong absorptions to metal-to-ligand charge transfer (MLCT) excitations originating from Pu 5f orbitals (Table S2). The unusually high intensity of these transitions compared to 1 may be rationalized by an increase in 5f orbital energy and radial extent, likely caused by electron repulsion in the 5f6 configuration in [PuIICp′′3]1− compared to the 5f5 Pu3+ configuration. These factors red-shift the MLCT transitions into the visible region and lead to larger transition dipole moments involving coupling between 5f and ligand orbitals, ultimately providing a mechanism to increase absorption intensities. In contrast, the 4f orbitals in traditional Ln2+ compounds with 4fn+15d0 configurations, such as [SmIICp′3]1−, are considerably lower in energy and more contracted than the 5f orbitals in 2. Hence, these compounds do not exhibit strong MLCT transitions in the visible spectrum.5 The calculations also suggest that the high energy of the 5f orbitals and stabilized 6dz2 orbital results in low energy 5f→6d transitions in [PuIICp′′3]1−. The calculated energy of this transition is near 2,600 cm-1, outside the range of the



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conventional UV/vis/NIR spectrum provided in Figure 2. While some of the intensity of the visible transitions in 2 may be attributable to configuration mixing in the ground state and/or thermal population of the low-lying 5f56d1 excited state, more sophisticated measurements and computations are needed for verification. The initial discovery and characterization of a Pu2+containing molecule is a starting point for full understanding of the electronic structure and reactivity of this new formal oxidation state. However, we have conducted a few preliminary reactivity studies (see SI) to aid in demonstrating that 2 reacts as one would expect for a Pu2+ complex. For example, treatment of 2 with an equivalent of the one-electron oxidant AgBPh4 regenerates the parent trivalent PuIIICp′′3, 1, in high yield, which was unambiguously identified by 1H NMR spectroscopy. Analogous behavior has been observed in the oxidation of the related [UIICp′3]1– complex to reform its trivalent parent UIIICp′.6a,14 In summary, organoplutonium complexes have been isolated and characterized, allowing a Pu–C bond to be measured for the first time by single-crystal X-ray diffraction. PuIIICp′′3, 1, provided a synthetic pathway towards a Pu2+ analog of Ln2+, Th2+ and U2+ molecules. Specifically, reduction of 1 to form [K(crypt)][PuIICp′′3], 2, is a demonstration that Pu2+ complexes are now accessible after over 70 years of chemical research. In contrast to the U2+ and Th2+ analogues, the analysis of 2 suggests a predominant 5f66d0 ground state with some 5f56d1 configuration-mixing. Note: While this manuscript was in review, a report on attempts to make a Np2+ complex with the Cp′ ligand appeared, but the reaction product was too unstable to allow structural or spectroscopic verification.15 ASSOCIATED CONTENT

Supporting Information Complete experimental details, NMR, UV/vis/NIR, IR spectra, Xray crystallographic details (CIF), reactivity studies, and computational details. This material is available free of charge via the Internet at http:/pubs.acs.org. AUTHOR INFORMATION

Corresponding Author To whom correspondence should be addressed. [email protected], [email protected], [email protected], [email protected]

ACKNOWLEDGMENT The computational studies are based on work supported by the U.S. National Science Foundation under contract CHE-1464828 (FF). Experimental studies, were supported by the U.S. Department of Energy (DOE), Chemical Sciences, Geosciences, and Biosciences

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Division of the Office of Basic Energy Sciences, Heavy Element Chemistry program (WJE; contract DESC0004739 and AJG, SAK, JNC; contract DE-AC5206NA25396), and the Office of Workforce Development for Teachers and Scientists, Office of Science Graduate Student Research (SCGSR) program (administered by the Oak Ridge Institute for Science and Education (ORISE) for the DOE) (CJW; contract DE-AC0506OR23100). We are grateful to Hyungjoon Choi (computational assistance) and Benjamin Stein, Maryline Ferrier, and Samantha Cary (technical support). REFERENCES (1) (a) Seaborg, G. T.; McMillan, E. M.; Kennedy, J. W.; Wahl, A. C. Phys. Rev. 1946, 69, 366–367. (b) The Chemistry of the Actinide and Transactinide Elements, 3rd ed.; Morss, L. R., Edelstein, N. M., Fuger, J., Katz, J. J., Eds.; Springer: Dordrecht, The Netherlands, 2006. (2) (a) Jones, M. B.; Gaunt, A. J. Chem. Rev. 2013, 4, 1137–1198. (b) Hayton, T. W. Chem. Commun. 2013, 49, 2956–2973. (c) Johnson, S. A.; Bart, S. C. Dalton Trans. 2015, 44, 7710–7726. (d) Liddle, S. T. Angew. Chem. Int. Ed. 2015, 54, 8604–8641. (e) Ephritikhine, M. Dalton Trans. 2006, 2501–2516. (3) Gaunt, A. J.; Neu, M. P. C. R. Chim. 2010, 13, 821–831. (4) Information about the Cambridge Structural Database can be found at http://ccdc.cam.ac.uk/products/csd/. Citation based on depositions up to and including the November 2016 update. (5) (a) Evans, W. J. Organometallics 2016, 35, 3088–3100. (b) Hitchcock, P. B.; Lappert, M. F.; Maron, L.; Protchenko, A. V. Angew. Chem. Int. Ed. 2008, 47, 1488–1491. (6) (a) MacDonald, M. R.; Fieser, M. E.; Bates, J. E.; Ziller, J. W.; Furche, F.; Evans, W. J. J. Am. Chem. Soc. 2013, 135, 13310–13313. (b) Langeslay, R. R.; Fieser, M. E.; Ziller, J. W.; Furche, F.; Evans, W. J. Chem. Sci. 2015, 6, 517–521. (7) Dutkiewicz, M. S.; Farnaby, J. H.; Apostilidis, C.; Colineau, E.; Walter, O.; Magnani, N.; Gardiner, M. G.; Love, J. B.; Kaltsoyannis, N.; Caciuffo, R.; Arnold, P. L. Nat. Chem. 2016, 8, 797–802. (8) (a) Ward, J. W. J. Less-Common Met. 1983, 93, 279–292. (b) Mikheev, N. B.; Inorg. Chim. Acta 1987, 140, 177–180. (c) Gibson, J. K.; Haire, R. G.; Santos, M.; Morçalo, J.; Pires de Matos, A. J. Phys. Chem. A 2005, 109, 2768–2781. (9) Avens, L. R.; Bott, S. G.; Clark, D. L.; Sattelberger, A. P.; Watkin, J. G.; Zwick, B. D. Inorg. Chem. 1994, 3, 2248–2256. (10) Gaunt, A. J.; Enriquez, A. E.; Reilly, S. D.; Scott, B. L.; Neu, M. P. Inorg. Chem. 2008, 47, 26–28. (11) (a) Stults, S. D.; Andersen, R. A.; Zalkin, A. Organometallics 1990, 9, 115–122. (b) Xie, Z.; Chui, K.; Liu, Z.; Zhang, Z.; Mak, T. C. W.; Sun, J. J. Organomet. Chem. 1997, 549, 239–244. (c) del Mar Conejo, M.; Parry, J. S.; Carmona, E.; Schultz, M.; Brennann, J. G.; Beshouri, S. M.; Andersen, R. A.; Rogers, R. D.; Coles, S.; Hursthouse, M. Chem. Eur. J. 1999, 5, 3000–3009. (12) Windorff, C. W.; Evans, W. J. Organometallics 2014, 33, 3786– 3791. (13) Wu, Q. Y.; Lan, J. H.; Wang, C. Z.; Cheng, Z. P.; Chai, Z. F.; Gibson, J. K.; Shi, W. Q.; Dalton Trans 2016, 45, 3102-3110. (14) Windorff, C. J.; MacDonald, M. R.; Meihaus, K. R.; Ziller, J. W.; Long, J. R.; Evans, W. J. Chem. Eur. J. 2016, 22, 772–782. (15) Dutkiewicz, M. S.; Apostolidis, C.; Walter, O.; Arnold, P. L. Chem. Sci. DOI: 10.1039/C7SC00034.


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Identification of the Formal +2 Oxidation State of Plutonium: Synthesis

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