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

Article abstract of DOI:10.1021/acs.organomet.6b00091

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 t

Full text of DOI:10.1021/acs.organomet.6b00091

Communication
pubs.acs.org/Organometallics
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
uranium species in solution; however, the equilibrium can be
Iorganometallic actinide hydride complexes [(C Me ) An- shifted toward trivalent [(C5Me5)2U(μ-H)]2 (5) using a labor-
5
5 2
intensive 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
(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(μ-
H)]2 (5) (eq 1).
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 co-
workers 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
Received: February 3, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.6b00091
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Organometallics
Communication
through the synthesis of a variety of trivalent, tetravalent, and
hexavalent uranium complexes, including the novel metal-
lacyclopentadiene 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
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
1
probed by variable-temperature (VT) H 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
1H 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
1H 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
uranium(III) hydride complex [(C5Me5)2U(μ-H)]2 (5), with
PhMeSiH2 as the byproduct. Upon addition of PhSiH3 there is
1
an immediate color change from orange to dark brown. H
NMR spectroscopy shows that, under these conditions, the
trivalent hydride complex 5 (δC Me = 3.03) is the only uranium
5
5
species observed in solution with no evidence for the formation
of the tetravalent [(C5Me5)2U(H)(μ-H)]2 (2) (δC Me = −2.59,
5
5
δU−H = −343) or remaining (C5Me5)2UMe2 (4) (δC Me = 5.15,
5
5
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 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).
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.
correlation between the spectra of 5 and 6 across the entire
spectral range in the band profiles, most notably in the near-
infrared 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
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Communication
and uranium(V)11−16 metallocene complexes, providing
compelling evidence for the uranium oxidation state assignment
of [(C5Me5)2U−H]x (5).
compound 10 has been previously reported,1 the synthesis of
11 represents only the second example of a uranium
metallacyclopentadiene complex. The H 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
1
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
a
Scheme 1.
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 = 1,2-
bis(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
the metallacyclopentadiene ligand contained within the metal-
locene 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 phenyl-
silane,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.
1
evolution of gas when the substrate was added to 5 and a H
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
C
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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.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.6b00091.
■
S
Full experimental details, including details of crystallo-
graphic collection for compound 11 (CCDC 1436949)
(PDF)
Crystallographic data for compound 11 (CIF)
(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.
AUTHOR INFORMATION
Corresponding Author
*E-mail for J.L.K.: kiplinger@lanl.gov.
Notes
■
(19) Mehdoui, T.; Berthet, J.-C.; Thuer
Ephritikhine, M. Chem. - Eur. J. 2005, 11, 6994.
́ ̀
y, P.; Salmon, L.; Riviere, E.;
The authors declare no competing financial interest.
(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.
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 Fellow-
ship 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).
(25) See the Supporting Information for details.
(26) Evans, W. J.; Kozimor, S. A.; Ziller, J. W. Chem. Commun. 2005,
4681.
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DOI: 10.1021/acs.organomet.6b00091
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