Multi-electron reduction from alkyl/hydride ligand combinations in U 4+ complexes

Article abstract of DOI:10.1021/ja805107v

The U⁴⁺ mixed alkyl hydride complex (C₅Me₅)U[μ-C₅Me₃(CH₂)₂](μ-H)₂U(C₅Me₅)₂, 1, which contains a cyclopentadienyl ligand with two metalated methylene substituents, can effect four, six, and eight-electron reductions in which the combination of the two H^1- ligands and the [C₅Me₃(CH₂)₂]^3- moiety delivers four electrons and forms (C₅Me₅)^1-. The reaction is formally equivalent to an alkyl hydride reductive elimination, a transformation common with transition metals not previously observed with f element compounds. This type of alkyl hydride reduction reactivity is also observed with a combination of U⁴⁺ alkyl and hydride complexes, (C₅Me₅)₂UMe₂/[(C₅Me₅)₂UH₂]₂, which reduces benzene to make [(C₅Me₅)₂U]₂(C₆H₆), a U³⁺ complex formally containing a (C₆H₆)^2- ligand. Copyright

Full text of DOI:10.1021/ja805107v

Published on Web 08/23/2008
Multi-Electron Reduction from Alkyl/Hydride Ligand Combinations in U4+
Complexes
William J. Evans,* Elizabeth Montalvo, Stosh A. Kozimor, and Kevin A. Miller
Department of Chemistry, UniVersity of California, IrVine, California 92697-2025
Received July 2, 2008; E-mail: wevans@uci.edu
Multielectron reduction chemistry has been recently expanded
in the organoactinide area by the realization that traditional metal-
based redox couples can be combined with a growing list of ligand-
based redox processes.1-7 For example, in sterically crowded
complexes such as (C5Me5)3U,8 the (C5Me5)1- ligand becomes
activated for reduction and 2 equiv of this U3+ complex can effect
a six-electron reduction of 3 equiv of C8H8 to form the U4+ product
[(C5Me5)(C8H8)U]2(C8H8), eq 1.1 Multielectron reduction can also
involves a bimetallic reductive elimination and oxidative addition
of dihydrogen.9
(RCH2)1- + H1- f RCH3 + 2e1-
(4)
The alkyl hydride reductive reactivity was observed in the course
of characterizing the product of heating [(C5Me5)2UH]2 shown in
eq 5. This complex, (C5Me5)U[µ-η5:η1:η1-C5Me3(CH2)2](µ-H)2U(C5-
Me5)2, 1,17 has a [C5Me3(CH2)2]3- ligand that has one methylene
group ”tucked-in” to bind to the uranium of its ring and one ”tucked-
over” to the uranium of another metallocene unit. Since 1 is a U4+
hydride, like [(C5Me5)2UH2]2 in eq 2, it could have hydride
reduction chemistry that would generate new tuck-in and tuck-over
derivatives beyond 1, the sole example in the literature. Accordingly,
the reductive chemistry of 1 was examined.
be achieved with ligands such as (BPh4)1- and H1- 2,5
For example,
.
in eq 2 [(C5Me5)2UH2]29 and [(C5Me5)2ThH2]29 act as six-electron
reductants with C8H8 by using the hydride ligands as reductants in
combination with (C5Me5)1- reduction.5
Scheme 1
In contrast to these tetravalent actinide hydrides, the U4+ methyl
complex, (C5Me5)2UMe2,9 does not act as an analogous reductant.
With PhSSPh, a reagent more easily reduced than C8H8, the alkyl
ligands reacted via σ bond metathesis, a reaction more precedented
for f element hydride and alkyl complexes, eq 3.10-15
As shown in Scheme 1, complex 1 quantitatively reduces 2 equiv
of PhSSPh in benzene in a four electron reduction process, but the
product (C5Me5)2U(SPh)2,11,18 2, contains neither tuck-in nor tuck-
over ligands. In fact, the cyclopentadienide ligands in the product
contain no metalated methylene groups. Since the starting material
and product both contain U4+ and there are only two hydride
ligands, two additional electrons in the four electron reduction must
come from some other source. Since the methylene groups in 1
have regained hydrogen to become methyl groups and there is no
other obvious source of hydrogen in the reaction other than the
hydride ligands, the four electron reduction can be explained by
the formal half-reaction shown in eq 6. This is a variation of eq 4
involving two hydrides and R ) “(C5Me3CH2)2-”. Reactions in
toluene-d8 give the fully protonated (C5Me5)2U(SPh)2. Mechanistic
studies with a deuterium analog of 1 were not possible due to
deuterium exchange with the (C5Me5)1- ring in 1 and the
precursors.17
We now report that the combination of one alkyl and one hydride
ligand in U4+ complexes can effectively function as a two-electron
reductant to make U4+ products. The redox couple in eq 4 shows
only the net reaction and does not have any mechanistic implica-
tions. This alkyl hydride reduction chemistry is formally related to
reductive elimination reactions in transition metal chemistry in
which two ligands eliminate and the metal oxidation state is reduced
by two.16 The reactions reported here were unexpected since two
anions are eliminated and the electrons are passed directly onto a
substrate instead of forming a reduced metal complex. This type
of reaction has not been observed before in f element chemistry.
The reaction in the literature that is closest to the results reported
here is the reversible interconversion between the U4+ and U3+
hydrides, [(C5Me5)2UH2]2 and [(C5Me5)2UH]2, which formally
[C5Me3(CH2)2]3- + 2H1- f (C5Me5)1- + 4e1-
(6)
Complex 1 can also function as a six-electron reductant in the
reduction of C8H8 to [(C5Me5)(C8H8)U]2(C8H8), 3,1 in benzene,
9
12258 J. AM. CHEM. SOC. 2008, 130, 12258–12259
10.1021/ja805107v CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
Scheme 4
Scheme 2. Again, in this case, there is no obvious source of
hydrogen for the intact (C5Me5)1- ligands in the product except
Scheme 2
transition metal chemistry, but in this case, the electrons reduce
substrates directly rather than providing metal complexes in lower
oxidation states. Neither reductive elimination of two anions nor
regeneration of a (C5Me5)1- ring from a (C5Me4CH2)2- and a
hydride involve reaction pathways seen before with organoactinides.
The general applicability of this approach to accessing multielectron
reduction from mixed ligand complexes is under investigation.
the hydride ligands in 1. The quantitative six electron reduction
formally can be accounted for by two two-electron alkyl hydride
reductions and two one-electron (C5Me5)1--based reductions which
give the (C5Me5)2 byproduct observed.
Complex 1 can also quantitatively reduce 2 equiv of PhNdNPh
in benzene to form the previously characterized U6+ imido complex
(C5Me5)2U(dNPh)2, 4,19 Scheme 3. In this case, an eight-electron
reduction occurs involving two two-electron alkyl hydride reduc-
tions and two two-electron U4+ to U6+ processes.
Acknowledgment. We thank the National Science Foundation
for support of this research. This research was facilitated in part
by a National Physical Science Consortium Fellowship and stipend
support from Los Alamos National Laboratory (to E.M.).
Scheme 3
Supporting Information Available: Synthetic and spectroscopic
details (PDF). This material is available free of charge via the Internet
at http://pubs.acs.org.
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to the unusual tuck-in tuck-over complex, 1, reactions of combina-
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Combinations of (C5Me5)2UMe2 and [(C5Me5)2UH2]2 could not be
examined with C8H8 and PhNNPh, since the hydride complex alone
reduces these substrates.5 However, the combination of
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complex of (C6H6)2-, that is, the previously characterized [(C5Me5)2-
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formal half-reaction is as shown in eq 4.
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