Tuning the Oxidation State, Nuclearity, and Chemistry of Uranium

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Tuning the Oxidation State, Nuclearity, and Chemistry of Uranium Hydrides with Phenylsilane and Temperature: The Case of the Classic Uranium(III) Hydride Complex [(C5Me5)2U(μ-H)]2 Justin K. Pagano,†,§ Jacquelyn M. Dorhout,†,∥ Kenneth R. Czerwinski,∥ David E. Morris,† Brian L. Scott,‡ Rory Waterman,§ and Jaqueline L. Kiplinger*,† †

Chemistry Division and ‡Materials Physics & Applications Division, Los Alamos National Laboratory, Mail Stop J514, Los Alamos, New Mexico 87545, United States § Department of Chemistry, University of Vermont, Cook Physical Sciences Building, Burlington, Vermont 05405, United States ∥ Department of Chemistry, University of NevadaLas Vegas, 4505 South Maryland Parkway, Box 454009, Las Vegas, Nevada 89154, United States S Supporting Information *

ABSTRACT: This work demonstrates that the oxidation state and chemistry of uranium hydrides can be tuned with temperature and the stoichiometry of phenylsilane. The trivalent uranium hydride [(C5Me5)2U−H]x (5) was found to be comprised of an equilibrium mixture of U(III) hydrides in solution at ambient temperature. A single U(III) species can be selectively prepared by treating (C5Me5)2UMe2 (4) with 2 equiv of phenylsilane at 50 °C. The U(III) system is a potent reducing agent and displayed chemistry distinct from the U(IV) system [(C5Me5)2U(H)(μ-H)]2 (2), which was harnessed to prepare a variety of organometallic complexes, including (C5Me5)2U(dmpe)(H) (6), and the novel uranium(IV) metallacyclopentadiene complex (C5Me5)2U(C4Me4) (11). n 1982, Marks and co-workers reported the first tetravalent organometallic actinide hydride complexes [(C5Me5)2An(H)(μ-H)]2 (An = Th (1), U (2)), which were prepared by treating the corresponding alkyl complexes (C5Me5)2AnMe2 (An = Th (3), U (4)) with an atmosphere of H2 at ambient temperature.1 Whereas the thorium complex 1 is stable for hours in toluene solution at 80 °C, the uranium derivative [(C5Me5)2U(H)(μ-H)]2 (2) only exists under a dihydrogen atmosphere, as it reversibly loses H2 at ambient temperature to form the trivalent uranium hydride complex [(C5Me5)2U(μH)]2 (5) (eq 1).

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uranium species in solution; however, the equilibrium can be shifted toward trivalent [(C5Me5)2U(μ-H)]2 (5) using a laborintensive multiple dissolution/evacuation protocol under argon2 or toward tetravalent [(C5Me5)2U(H)(μ-H)]2 (2) by using a high pressure of H2 (80 psi, solid state).5 Recently, we have been investigating the use of organosilanes as liquid surrogates for H2 gas and reported that the tetravalent uranium hydride complex [(C5Me5)2U(H)(μ-H)]2 (2) can be conveniently prepared in 30 min by treating (C5Me5)2UMe2 (4) with excess phenylsilane at 50 °C in toluene.6 These mild conditions provided the opportunity to study the chemistry of pure [(C5Me5)2U(H)(μ-H)]2 (2), which revealed reductive chemistry different from that reported for trivalent [(C5Me5)2U(μ-H)]2 (5).2 Encouraged by these results, we hypothesized that reducing the amount of phenylsilane would promote the formation of the trivalent uranium hydride complex. During our studies we discovered that, in the absence of excess phenylsilane, [(C5Me5)2U(μ-H)]2 (5) exists as a mixture of uranium(III) hydride complexes and that this equilibrium can be shifted with temperature. Herein, we report the first synthesis of the classic uranium(III) hydride [(C5Me5)2U(μ-H)]2 (5) that does not involve dihydrogen gas, and we show that the oxidation state and chemistry of uranium hydrides can be tuned with temperature and the stoichiometry of phenylsilane. This control is demonstrated

These actinide hydride complexes are unique in comparison to their transition-metal counterparts: depending on the substrate, they can function as traditional electrophilic hydrides and also as reductants in reactions involving the H− → e− + 1/2 H2 half-reaction.2−5 By combining this hydride ligand-based reduction with uranium metal-based reductions, Evans and coworkers showed that both 2 and 5 perform multielectron reduction chemistry with a variety of substrates.2−4 Due to the equilibrium between 2 and 5 (eq 1), a key challenge with this chemistry has been controlling the oxidation state of the © XXXX American Chemical Society

Received: February 3, 2016

A

DOI: 10.1021/acs.organomet.6b00091 Organometallics XXXX, XXX, XXX−XXX

Communication

Organometallics

species 5a at δ = −9.37 is favored at ambient temperature and the species 5b at δ = 3.03 is favored at higher temperature (eq 2). However, the involvement of a uranium(III) hydride trimer cannot be ruled out. As such, we propose that complex 5 is best formulated as [(C5Me5)2U−H]x. This hypothesis was initially probed by variable-temperature (VT) 1H NMR spectroscopic studies (Figure 1). When a C6D6 solution of (C5Me5)2UMe2 (4) with 2 equiv of PhSiH3 was heated to 50 °C for 15 min, the 1 H NMR spectrum revealed a single uranium product at δ = 3.03 in addition to the resonances for PhMeSiH2 (δSi−Ph = 7.52 (m), 7.43 (m); δSi−H = 4.46 (q); δSi−Me = 0.15 (t)). As this solution was cooled to ambient temperature, the peaks at δ = −2.59 and −9.37 appeared; as the solution was reheated, the 1 H NMR spectrum again revealed a single product at δ = 3.03. These data show that the nuclearity of the hydride complex can be tuned by temperature, with a single uranium(III) hydride species, [(C5Me5)2U−H]x (5), at 50 °C. Additional evidence supporting the assertion that [(C5Me5)2U−H]x (5) is a uranium(III) hydride species can be seen in the UV−vis−NIR spectral data obtained for 5 and the structurally characterized uranium(III) hydride complex (C5Me5)2U(dmpe)(H) (6) (Figure 2).7 There is excellent

through the synthesis of a variety of trivalent, tetravalent, and hexavalent uranium complexes, including the novel metallacyclopentadiene complex (C5Me5)2U(C4Me4) (11). As shown in eq 2, treatment of (C5Me5)2UMe2 (4) with 2 equiv of PhSiH3 in toluene at 50 °C for 10 min affords the

uranium(III) hydride complex [(C5Me5)2U(μ-H)]2 (5), with PhMeSiH2 as the byproduct. Upon addition of PhSiH3 there is an immediate color change from orange to dark brown. 1H NMR spectroscopy shows that, under these conditions, the trivalent hydride complex 5 (δC5Me5 = 3.03) is the only uranium species observed in solution with no evidence for the formation of the tetravalent [(C5Me5)2U(H)(μ-H)]2 (2) (δC5Me5 = −2.59, δU−H = −343) or remaining (C5Me5)2UMe2 (4) (δC5Me5 = 5.15, δU−Me = −124).1 The volatile byproducts make workup simple, and [(C5Me5)2U(μ-H)]2 (5) can be isolated in quantitative yields as a brown solid. The solid-state IR spectrum of the isolated solid displays a single broad band at 1378 cm−1 assignable to a bridging U−H−U stretching mode, which is consistent with the data previously reported for complex 5.1,2 Interestingly, the 1H NMR spectrum of 5 at ambient temperature is quite different from that obtained at 50 °C. At ambient temperature, resonances are displayed at δ = 3.03 (s, ν1/2 = 10 Hz), −2.59 (s, ν1/2 = 6 Hz), and −9.37 (s, ν1/2 = 20 Hz) in a 10:1:1 ratio; only a single resonance at δ = 3.04 (s, ν1/2 = 8 Hz) is observed at 50 °C (Figure 1). The minor resonance

Figure 2. UV−vis−NIR electronic absorption spectra of the trivalent uranium hydride complexes [(C5Me5)2U−H]x (5) and (C5Me5)2U(dmpe)(H) (6) in toluene solution at ambient temperature.

Figure 1. VT 1H NMR spectra of C6D6 solutions of [(C5Me5)2U−H]x (5) generated from (C5Me5)2UMe2 (4) with 2 equiv of PhSiH3. Note that the peak for [(C5Me5)2U(H)(μ-H)]2 (2) disappears upon heating, leaving only [(C5Me5)2U−H]x (5).

correlation between the spectra of 5 and 6 across the entire spectral range in the band profiles, most notably in the nearinfrared region, where the diagnostic f−f transitions dominate. A very similar near-infrared spectral motif has also been observed for the structurally characterized trivalent uranium complex [(C5Me5)2UI(THF)].8,9 The near-infrared spectral signatures for these three uranium(III) metallocene complexes are distinctly different from those reported for uranium(IV)10

at δ = −2.59 corresponds to the tetravalent hydride [(C5Me5)2U(H)(μ-H)]2 (2), due to the equilibrium between 2 and 5 at this temperature.1 Previously, the peak located at δ = −9.37 was assigned to the C5Me5 group resonance of the complex [(C5Me5)2U(μ-H)]x (5).1 To reconcile these data, we propose that the resonances at δ = 3.03 and −9.37 represent two uranium(III) hydride species in an equilibrium, where the B

DOI: 10.1021/acs.organomet.6b00091 Organometallics XXXX, XXX, XXX−XXX

Communication

Organometallics and uranium(V)11−16 metallocene complexes, providing compelling evidence for the uranium oxidation state assignment of [(C5Me5)2U−H]x (5). Finally, the identity of [(C5Me5)2U−H]x (5) as a trivalent uranium hydride was borne out by its reaction chemistry with a range of substrates, as outlined in Scheme 1. In all reactions, the Scheme 1.

compound 10 has been previously reported,1 the synthesis of 11 represents only the second example of a uranium metallacyclopentadiene complex. The 1H NMR spectrum of 11 displays only three singlets: δ = 1.52 corresponding to the C5Me5 group resonance and δ = −3.62 and −4.56 assigned to the methyl groups on the metallacyclopentadiene ring. The UV−vis−NIR spectrum of 11 features weak Laporte-forbidden f−f transitions (ε < 150 M−1 cm−1) in the low-energy region, which is fully consistent with the presence of a uranium(IV) center.10,24,25 The molecular structure of 11 is presented in Figure 3 and reveals a typical bent-metallocene framework with

a

a

Reagents and conditions: (i) 1 equiv of dmpe, room temperature, 15 min, 94%; (ii) 1 equiv of terpy, 50 °C, 24 h, 94%; (iii) 1 equiv of Ph2CNNCPh2, 50 °C, 24 h, 83%; (iv) 1 equiv of PhNNPh, 50 °C, 1 h, 95%; (v) 2 equiv of RCCR (R = Ph, Me), 30 min, 50 °C, 100%.

Figure 3. Molecular structure of 11 with thermal ellipsoids displayed at the 50% probability level. Hydrogen atoms have been omitted for clarity.

complex [(C5Me5)2U−H]x (5) was generated in toluene solution from (C5Me5)2UMe2 (4) and 2 equiv of PhSiH3 for 10 min at 50 °C, followed by addition of the substrate. Additionally, we tested the reactivity of these compounds with PhMeSiH2, a byproduct of this reaction.17 In all instances, these products were unreactive toward PhMeSiH2 under the conditions screened, which is expected due to the generally higher reactivity of primary versus secondary organosilanes.18 Addition of the bidentate ligand dmpe (dmpe = 1,2bis(dimethylphosphino)ethane) to 5 readily afforded the known trivalent complex (C5Me5)2U(dmpe)(H) (6)7 in 94% isolated yield. The yield of 6 is noteworthy, as it is an improvement over the previously reported 75% yield produced from the hydrogenolysis of (C5Me5)2UMe2 and dmpe at −20 °C.7 Complex 5 is also a competent multielectron reductant in reactions with 2,2′:6′,2″-terpyridine (terpy), benzophenone azine, and azobenzene to form the known trivalent (C5Me5)2U(terpy) (7),19,20 tetravalent (C5Me5)2U(−NCPh2)2 (8),21 and hexavalent (C5Me5)2U(NPh)2 (9)22 in 94%, 85%, and 95% isolated yields, respectively. In all reactions, there is evolution of gas when the substrate was added to 5 and a 1H NMR resonance at δ = 4.46 consistent with H2 was observed.23 Attempts to synthesize these compounds from tetravalent [(C5Me5)2U(H)(μ-H)]2 (2) using the phenylsilane method6 were unsuccessful, acutely demonstrating distinct reductive chemistry for tetravalent [(C5Me5)2U(H)(μ-H)]2 (2) versus trivalent [(C5Me5)2U−H]x (5) hydride species. Reductive coupling chemistry between [(C5Me5)2U−H]x (5) and 2 equiv of of diphenylacetylene or 2-butyne afforded the uranium(IV) metallacyclopentadiene complexes (C5Me5)2U(C4R4) (R = Ph (10), Me (11)) in quantitative yields. While

the metallacyclopentadiene ligand contained within the metallocene wedge. Compound 11 has M−Cα (2.337(2) Å), Cα−Cβ (1.354(4) Å), Cβ−Cβ· (1.521(6) Å), and M−Ccent (2.465 Å) bond distances and U−Cα−Cβ (103.8(2)°), Cα−Cβ−Cβ′ (124.4(2)°), and Ccent−M−Ccent (140.2°) bond angles that compare favorably with those observed for (C5Me5)2U(C4Ph4) (10) (2.395(2), 1.365(3), 1.509(4), 2.467 Å; 108.5(2), 122.8(2), 142.6°).26 To conclude, we have reported that the classic trivalent uranium hydride complex 5 can be easily and selectively prepared at 50 °C from (C5Me5)2UMe2 (4) and 2 equiv of PhSiH3. A combination of spectroscopic and reactivity studies were used to characterize 5 and showed that it exists as an equilibrium mixture of uranium(III) hydride complexes with the general formula [(C5Me5)2U−H]x (5) and that this equilibrium can be shifted with temperature. Coupled with our previous work for the selective preparation of the tetravalent [(C5Me5)2U(H)(μ-H)]2 (2) using excess phenylsilane,6 the present study shows that the oxidation state, nuclearity, and chemistry of uranium hydrides can be tuned with temperature and stoichiometry of phenylsilane. We demonstrated this unprecedented control by preparing several complexes using the trivalent hydride complex [(C5Me5)2U− H]x (5), most of which were not accessible from the tetravalent hydride [(C5Me5) 2U(H)(μ-H)]2 (2). We are currently exploring methodologies for preparing actinide hydrides as well as expanding this work to include lanthanides and transuranic compounds, to access new areas of f-element chemistry. C

DOI: 10.1021/acs.organomet.6b00091 Organometallics XXXX, XXX, XXX−XXX

Communication

Organometallics

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(12) Graves, C. R.; Vaughn, A. E.; Schelter, E. J.; Scott, B. L.; Thompson, J. D.; Morris, D. E.; Kiplinger, J. L. Inorg. Chem. 2008, 47, 11879. (13) Graves, C. R.; Scott, B. L.; Morris, D. E.; Kiplinger, J. L. Organometallics 2008, 27, 3335. (14) Graves, C. R.; Yang, P.; Kozimor, S. A.; Vaughn, A. E.; Clark, D. L.; Conradson, S. D.; Schelter, E. J.; Scott, B. L.; Thompson, J. D.; Hay, P. J.; Morris, D. E.; Kiplinger, J. L. J. Am. Chem. Soc. 2008, 130, 5272. (15) Graves, C. R.; Scott, B. L.; Morris, D. E.; Kiplinger, J. L. Chem. Commun. 2009, 776. (16) Thomson, R. K.; Cantat, T.; Scott, B. L.; Morris, D. E.; Batista, E. R.; Kiplinger, J. L. Nat. Chem. 2010, 2, 723. (17) Xu, S.; Magoon, Y.; Reinig, R. R.; Schmidt, B. M.; Ellern, A.; Sadow, A. D. Organometallics 2015, 34, 3508. (18) Mucha, N. T.; Waterman, R. Organometallics 2015, 34, 3865. (19) Mehdoui, T.; Berthet, J.-C.; Thuéry, P.; Salmon, L.; Rivière, E.; Ephritikhine, M. Chem. - Eur. J. 2005, 11, 6994. (20) Schelter, E. J.; Wu, R.; Scott, B. L.; Thompson, J. D.; Morris, D. E.; Kiplinger, J. L. Angew. Chem., Int. Ed. 2008, 47, 2993. (21) Kiplinger, J. L.; Morris, D. E.; Scott, B. L.; Burns, C. J. Organometallics 2002, 21, 3073. (22) Arney, D. S. J.; Burns, C. J.; Smith, D. C. J. Am. Chem. Soc. 1992, 114, 10068. (23) Fulmer, G. R.; Miller, A. J. M.; Sherden, N. H.; Gottlieb, H. E.; Nudelman, A.; Stoltz, B. M.; Bercaw, J. E.; Goldberg, K. I. Organometallics 2010, 29, 2176. (24) Thomson, R. K.; Scott, B. L.; Morris, D. E.; Kiplinger, J. L. C. R. Chim. 2010, 13, 790. (25) See the Supporting Information for details. (26) Evans, W. J.; Kozimor, S. A.; Ziller, J. W. Chem. Commun. 2005, 4681.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00091. Full experimental details, including details of crystallographic collection for compound 11 (CCDC 1436949) (PDF) Crystallographic data for compound 11 (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail for J.L.K.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS For financial support of this work, we acknowledge the U.S. Department of Energy through the Office of Workforce Development for Teachers and Scientists, Office of Science Graduate Student Research (SCGSR) program (GRA Fellowship to J.K.P.), the LANL LDRD Program and the LANL G. T. Seaborg Institute for Transactinium Science (GRA Fellowship to J.M.D.), and the Office of Basic Energy Sciences, Heavy Element Chemistry program (J.L.K., B.L.S., materials & supplies). We also acknowledge the U.S. Department of Homeland Security (GRA fellowship to J.M.D. under grant 2012-DN-130-NF0001) and the U.S. National Science Foundation (grant CHE-1265608 to R.W.). Finally, we thank Dr. Nicholas E. Travia for performing preliminary experiments and Drs. Karla A. Erickson and Marisa J. Monreal (all LANL) for helpful discussions. The SCGSR program is administered by the Oak Ridge Institute for Science and Education for the DOE (contract DE-AC05-06OR23100). Los Alamos National Laboratory is operated by Los Alamos National Security, LLC, for the National Nuclear Security Administration of U.S. Department of Energy (contract DE-AC52-06NA25396).

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REFERENCES

(1) Fagan, P. J.; Manriquez, J. M.; Maatta, E. A.; Seyam, A. M.; Marks, T. J. J. Am. Chem. Soc. 1981, 103, 6650. (2) Evans, W. J.; Miller, K. A.; Kozimor, S. A.; Ziller, J. W.; DiPasquale, A. G.; Rheingold, A. L. Organometallics 2007, 26, 3568. (3) Evans, W. J.; Montalvo, E.; Kozimor, S. A.; Miller, K. A. J. Am. Chem. Soc. 2008, 130, 12258. (4) Grant, D. J.; Stewart, T. J.; Bau, R.; Miller, K. A.; Mason, S. A.; Gutmann, M.; McIntyre, G. J.; Gagliardi, L.; Evans, W. J. Inorg. Chem. 2012, 51, 3613. (5) Webster, C. L.; Ziller, J. W.; Evans, W. J. Organometallics 2014, 33, 433. (6) Pagano, J. K.; Dorhout, J. M.; Waterman, R.; Czerwinski, K. R.; Kiplinger, J. L. Chem. Commun. 2015, 51, 17379. (7) Duttera, M. R.; Fagan, P. J.; Marks, T. J.; Day, V. W. J. Am. Chem. Soc. 1982, 104, 865. (8) Cantat, T.; Scott, B. L.; Morris, D. E.; Kiplinger, J. L. Inorg. Chem. 2009, 48, 2114. (9) Avens, L. R.; Burns, C. J.; Butcher, R. J.; Clark, D. L.; Gordon, J. C.; Schake, A. R.; Scott, B. L.; Watkin, J. G.; Zwick, B. D. Organometallics 2000, 19, 451. (10) Morris, D. E.; Da Re, R. E.; Jantunen, K. C.; Castro-Rodriguez, I.; Kiplinger, J. L. Organometallics 2004, 23, 5142. (11) Graves, C. R.; Scott, B. L.; Morris, D. E.; Kiplinger, J. L. J. Am. Chem. Soc. 2007, 129, 11914. D

DOI: 10.1021/acs.organomet.6b00091 Organometallics XXXX, XXX, XXX−XXX