Phenylsilane as a safe, versatile alternative to hydrogen for the synthesis of actinide hydrides

Article abstract of DOI:10.1039/c5cc06856h

The thorium and uranium dihydride dimer complexes [(C₅Me₅)₂An(H)(μ-H)]₂ (An = Th, U) have been easily prepared using phenylsilane, which is an efficient and safer alternative to hydrogen gas. The synthetic utili

Full text of DOI:10.1039/c5cc06856h

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Phenylsilane as a safe, versatile alternative to
hydrogen for the synthesis of actinide hydrides†
Cite this:DOI: 10.1039/c5cc06856h
Justin K. Pagano,ab Jacquelyn M. Dorhout,ac Rory Waterman,b
Kenneth R. Czerwinskic and Jaqueline L. Kiplinger*a
Received 14th August 2015,
Accepted 13th October 2015
DOI: 10.1039/c5cc06856h
www.rsc.org/chemcomm
The thorium and uranium dihydride dimer complexes [(C5Me5)2- convenient, thereby enabling future progress in the field of actinide
An(H)(l-H)]2 (An = Th, U) have been easily prepared using phenylsilane, hydride chemistry.28
which is an efficient and safer alternative to hydrogen gas. The synthetic
Using the [(C5Me5)2An(H)(m-H)]2 (An = Th (1), U (2)) systems
utility of this new hydriding method has been demonstrated by the as a platform, we now demonstrate that phenylsilane (PhSiH3)
preparation of a variety of organometallic complexes, including, for is a safe, versatile alternative to H2 for the synthesis of actinide
the first time, (C5Me5)2U(SMe)2, (C5Me5)2Th(C4Ph4), (C5Me5)2U(C4Ph4), hydride complexes from the corresponding alkyl precursors.
(C5Me5)2ThS5, and (C5Me5)2U(bipy) using [(C5Me5)2An(H)(l-H)]2 (An = Th, This operationally simple method gives rise to synthetically
U) as multi-electron reductants.
useful hydrides and importantly only produces volatile, easily
separable byproducts. To further add to the scope of the
Compared to the hundreds of reported transition metal and reaction, the method was applied to the synthesis of a variety
lanthanide hydrides, there are only a limited number of thorium of thorium and uranium complexes – some for the first time –
and uranium hydride complexes.1–14 The bis(pentamethylcyclo- using 1 or 2 as multi-electron reductants.
pentadienyl) complexes [(C5Me5)2An(H)(m-H)]2 (An = Th (1), U (2))
As illustrated in eqn (1), quantitative conversion of (C5Me5)2-
were the first isolable organoactinide hydride complexes;12 and AnMe2 (An = Th (3), U (4)) into the corresponding dihydride dimer
since their introduction in 1982, they have displayed a palette of complexes [(C5Me5)2An(H)(m-H)]2 (An = Th (1), U (2)) was conveni-
chemical reactivity such as the stoichiometric and catalytic hydro- ently achieved in 30 minutes by heating 3 and 4, respectively, with
genation of unsaturated substrates,9,12,15–17 the catalytic dimeriza- 5 equiv. of PhSiH3 at 50 1C in toluene solution. When the reactions
tion of propylene,18,19 coupling reactions,20–22 and multi-electron were monitored by 1H NMR spectroscopy in C6D6 using an internal
reductions (following liberation of H2).13,14,23–25
standard (Cp2Fe for Th; C6Me6 for U), the complete loss of complexes
The chemistry of actinide hydrides is still not well understood. 3 (dTh–Me = ꢀ0.19)12 and 4 (dU–Me = ꢀ124),12 and the quantitative
One reason may be that the primary synthetic route for accessing formation of PhMeSiH2 (dSi–H = q, 4.46)29 and complexes 1 (dTh–H
=
actinide hydride complexes is the hydrogenolysis of alkyl precur- 19.43)12 and 2 (dU–H = ꢀ343),12 were easily confirmed by their
sors.26 For example, the classic route to the dihydride dimer com- respective diagnostic 1H NMR An–Me, Si–H and An–H chemical
plexes 1 and 2 involves treating the known alkyl complexes shifts.12 Interestingly, under these reaction conditions, the tetra-
(C5Me5)2AnMe2 (An = Th (3), U (4)) with H2; the uranium derivative valent hydrides [(C5Me5)2An(H)(m-H)]2 (An = Th (1), U (2)) are the only
(2) must be stored under H2 as it is prone to loss of H2 and formation species observed in solution with no evidence for the formation of
of [(C5Me5)2U(m-H)]2 (5).12,27 Having stable and easy-to-handle liquid the trivalent [(C5Me5)2U(m-H)]2 (5) (dC5Me5 = ꢀ9.37)12 from 2.
surrogates for H2 as hydriding reagents would be synthetically more Presumably, this is due to the presence of the excess phenylsilane
in the reaction mixture. If preferred, the dihydride complexes 1 and 2
can be isolated in 96% and 97% yields, respectively, by simply
removing the volatiles from the reaction mixture.
a Chemistry Division, Los Alamos National Laboratory, Mail Stop J514, Los Alamos,
NM 87545, USA. E-mail: kiplinger@lanl.gov; Fax: +1 505-667-9905;
Tel: +1 505-665-9553
b Department of Chemistry, University of Vermont, Cook Physical Sciences Building,
Burlington, VT 05405, USA
c Department of Chemistry, University of Nevada – Las Vegas,
(1)
4505 S. Maryland Parkway, Box 454009, Las Vegas, NV 89154, USA
† Electronic supplementary information (ESI) available: Experimental procedures
and characterization details, including modified preparations of 3 and 4. See DOI:
10.1039/c5cc06856h
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uranium equivalents. We targeted four thorium and uranium
molecules that have not been previously prepared from complexes
1 and 2: (C5Me5)2An(C4Ph4) (An = Th (11), U (12)), (C5Me5)2ThS5 (13)
and (C5Me5)2U(bipy) (14).
First, the coupling of PhCRCPh to (C4Ph4)2ꢀ by complexes
1 and 2 was examined. Reaction of (C5Me5)2AnMe2 (An = Th (3),
U (4)) with 5 equiv. PhSiH3 and 2 equiv. of PhCRCPh at 50 1C in
toluene afforded PhMeSiH2, H2, and the previously reported metal-
lacylopentadiene complexes (C5Me5)2An(C4Ph4) (An = Th (11), U (12))
in 68% and 61% isolated yields, respectively (Scheme 1). In these
reactions, both complexes 1 and 2 are functioning as four-electron
reductants. The formation of complexes 11 and 12 establishes that
the (C5Me5)2AnMe2/phenylsilane reduction combination provides an
attractive synthetic strategy for accessing chemistry previously
reported for very reactive low-valent uranium and thorium
species. For example, the reductive coupling of PhCRCPh to
(C4Ph4)2ꢀ to prepare complex 12 has been achieved in the past, but
requires either sterically crowded complexes such as (C5Me5)3U,31
[(C5Me5)2U][(m-Ph)2BPh2],31 and [(C5Me5)2U]2(m-Z6:Z6-C6H6)31 (which
are capable of transferring multiple electrons to a substrate), or the
reaction of (C5Me5)2UCl2 combined with excess reducing equivalents
provided by Na/Hg,27 and KC8,27 or salt metathesis between
(C5Me5)2UCl2 and 1,4-dilithiotetraphenylbutadiene.12 Although
complex 12 has been known for nearly 30 years, only very
recently has the synthesis of the thorium analogue (11) been
realized. Walter and co-workers reported that (C5Me5)2Th(C4Ph4)
(11) can be prepared from the reaction of (C5Me5)2ThCl2 and
Scheme 1 Reagents and conditions: (i) 1 equiv. REER, 5 equiv. PhSiH3, toluene,
50 1C, 0.25–5 h, 6 (100% yield), 7 (69% yield), 8 (81% yield), 9 (95% yield), 10 (77%
yield); (ii) 2 equiv. PhCRCPh, 5 equiv. PhSiH3, toluene, 50 1C, 3 h, 11 (68% yield),
12 (61% yield); (iii) 0.625 equiv. S8, 5 equiv. PhSiH3, toluene 50 1C, 0.25 h, 80%
yield; (iv) 1 equiv. 2,20-bipyridine, 5 equiv. PhSiH3, toluene, 50 1C, 15 h, 69% yield. 2 equiv. of PhCRCPh in the presence of 5 equiv. of KC8.32
The synthesis of complexes (C5Me5)2ThS5 (13) and (C5Me5)2-
Previously, Evans and co-workers reported that [(C5Me5)2An(H)- U(bipy) (14) were initially reported by Ryan in 1986,33 and Ephritikhine
(m-H)]2 (An = Th (1), U (2)), prepared using the traditional H2 in 2005,34 respectively. Complex 14 is unique as it contains a
hydriding route, function as multi-electron reductants. To confirm trivalent uranium metal center coordinated to a redox-active bipy
that the chemical integrity of 1 and 2 produced from phenylsilane ligand (bipy = 2,20-bipyridine). Previously, it was found that
is maintained, we surveyed their capacity to serve as multi-electron reaction of (C5Me5)2ThCl2 with Li2S5 formed the rare thorium
reductants with a wide range of substrates as outlined in Scheme 1. pentasulfide complex 13; whereas, 14 was synthesized from the
Importantly, control experiments showed that phenylsilane does reaction of (C5Me5)2U(I)(bipy) with excess Na/Hg or KC8.34,35 We now
not reduce any of the substrates used as reagents in Scheme 1. As a demonstrate that complexes 13 and 14 can be easily prepared from
starting point, the reductive chemistry of complexes 1 and 2, the corresponding hydrides [(C5Me5)2An(H)(m-H)]2 (An = Th (1), U (2))
generated in situ from phenylsilane, was examined with several (Scheme 1). For example, reaction of (C5Me5)2ThMe2 (3) with 5 equiv.
diaryl and dialkyl dichalcogenides. The reaction of (C5Me5)2AnMe2 PhSiH3 and 0.625 equiv. of S8 at 50 1C in toluene resulted in an
(An = Th (3), U (4)) with 5 equiv. of PhSiH3 and 1 equiv. of PhEEPh immediate color change from red-orange to bright yellow, along with
(E = S, Se, Te) or MeSSMe at 50 1C in toluene afforded PhMeSiH2, the formation of PhMeSiH2, H2, and (C5Me5)2ThS5 (13) (dC Me
=
5
5
H2, and the known compounds (C5Me5)2An(EPh)2 (An = Th, E = S 2.04)33 in 80% isolated yield. Similarly, the reaction of 4 with 5 equiv.
(6); An = U, E = S (7); An = U, E = Se (8); An = U, E = Te (9)) and of PhSiH3 and 2,20-bipyridine at 50 1C in toluene for 15 h gave
(C5Me5)2U(SMe)2 (10). Reaction times and temperatures varied (C5Me5)2U(bipy) (14) in 69% isolated yield after workup.34,35
between compounds 6–10, as did yields. However, this chemistry
In conclusion, we have shown that the tetravalent thorium and
using the (C5Me5)2AnMe2/phenylsilane reduction combination pro- uranium hydrides [(C5Me5)2An(H)(m-H)]2 (An = Th (1), U (2)) can be
vided comparable, and in some cases superior, yields to those easily and efficiently prepared from the corresponding alkyl precur-
previously reported for the syntheses of 6–10.13,23,24,30 Although sors (C5Me5)2AnMe2 and PhSiH3. This new organosilane method
complex 10 has not previously been prepared by hydride-based obviates the need for H2 gas or pyrophoric reductants (e.g., sodium
four-electron reduction chemistry, collectively, these results parallel amalgam, potassium graphite, etc.) and provides a potent synthetic
the chemistry reported by Evans and co-workers for the hydride tool for preparing actinide hydride complexes. Using this approach,
complexes 1, 2 and 5 prepared and isolated from the hydro- we have expanded the synthetic profile of the hydride complexes
genolysis of (C5Me5)2AnMe2 (An = Th (3), U (4)).13,23,24,30
[(C5Me5)2An(H)(m-H)]2 (An = Th, U) by using them to prepare, for the
Using this new hydriding method, we were interested in the first time, (C5Me5)2U(SMe)2, (C5Me5)2Th(C4Ph4), (C5Me5)2U(C4Ph4),
ability of complexes 1 and 2 to serve as low-valent thorium and (C5Me5)2ThS5, and (C5Me5)2U(bipy). Further studies examining the
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For financial support of this work, we acknowledge the U.S.
Department of Energy through the Office of Workforce Devel-
opment 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., 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 (LANL) for performing preliminary experi-
ments and Dr Louis A. Silks III (LANL) for helpful discussions. The
views and conclusions contained in this document are those of the
authors and should not be interpreted as representing the official
policies, either expressed or implied, of the U.S. Department of
Homeland Security. 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-
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