Si-H and Si-C bond cleavage reactions of silane and phenylsilanes with Mo(PMe3)6: Silyl, hypervalent silyl, silane, and disilane complexes

Article abstract of DOI:10.1021/ja503368j

Mo(PMe₃)₆ cleaves the Si-H bonds of SiH₄, PhSiH₃, and Ph₂SiH₂ to afford a variety of novel silyl, hypervalent silyl, silane, and disilane complexes, as respectively illustrated by Mo(PMe₃)₄(SiH₃) ₂H₂, Mo(PMe₃)₄(κ²- H₂-H₂SiPh₂H)H, Mo(PMe₃) ₃(σ-HSiHPh₂)H₄, and Mo(PMe ₃)₃(κ²-H₂-H₂Si ₂Ph₄)H₂. Mo(PMe₃) ₄(κ²-H₂-H₂SiPh₂H)H and Mo(PMe₃)₃(κ²-H₂-H ₂Si₂Ph₄)H₂ are respectively the first examples of complexes that feature a hypervalent κ²- H₂-H₂SiPh₂H silyl ligand and a chelating disilane ligand, and both compounds convert to the diphenylsilane adduct, Mo(PMe₃)₃(σ-HSiHPh₂)H₄, in the presence of H₂. Mo(PMe₃)₄(SiH ₃)₂H₂ undergoes isotope exchange with SiD ₄, and NMR spectroscopic analysis of the SiH_xD _4-x isotopologues released indicates that the reaction does not occur via initial reductive elimination of SiH₄, but rather by a metathesis pathway.

Full text of DOI:10.1021/ja503368j

Communication
pubs.acs.org/JACS
Si−H and Si−C Bond Cleavage Reactions of Silane and Phenylsilanes
with Mo(PMe3)6: Silyl, Hypervalent Silyl, Silane, and Disilane
Complexes
Ashley A. Zuzek and Gerard Parkin*
Department of Chemistry, Columbia University, New York, New York 10027, United States
S
* Supporting Information
Scheme 1
ABSTRACT: Mo(PMe3)6 cleaves the Si−H bonds of
SiH4, PhSiH3, and Ph2SiH2 to afford a variety of novel silyl,
hypervalent silyl, silane, and disilane complexes, as
respectively illustrated by Mo(PMe3)4(SiH3)2H2, Mo-
(PMe3)4(κ2-H2-H2SiPh2H)H, Mo(PMe3)3(σ-HSiHPh2)-
H4, and Mo(PMe3)3(κ2-H2-H2Si2Ph4)H2. Mo(PMe3)4(κ2-
H2-H2SiPh2H)H and Mo(PMe3)3(κ2-H2-H2Si2Ph4)H2 are
respectively the first examples of complexes that feature a
hypervalent κ2-H2-H2SiPh2H silyl ligand and a chelating
disilane ligand, and both compounds convert to the
diphenylsilane adduct, Mo(PMe3)3(σ-HSiHPh2)H4, in the
presence of H2. Mo(PMe3)4(SiH3)2H2 undergoes isotope
exchange with SiD4, and NMR spectroscopic analysis of
the SiHxD4−x isotopologues released indicates that the
reaction does not occur via initial reductive elimination of
SiH4, but rather by a metathesis pathway.
he interaction of Si−H bonds with transition metal
(SiH3)H.3 In this regard, evidence that Mo(PMe3)4(SiH3)2H2
T
compounds is of fundamental interest,1 not only because
is a silyl-hydride and not a silane complex is provided by the
observation of distinct quintet signals in the 1H NMR spectrum
at δ −4.80 and 4.02 in a 1:3 ratio, of which the former has a
it is a key step in hydrosilylation, dehydrogenative Si−H/O−H
coupling, and dehydrogenative polymerization of silanes,2 but
also because it provides a model for the corresponding
interactions of C−H bonds with metal centers.3,4 By
comparison to substituted silanes, however, the reactivity of
SiH4 towards transition metal compounds has received
relatively little attention.3,4a,5 Therefore, we report here the
first example of the oxidative addition of 2 equiv of SiH4 to a
molybdenum center and also describe the corresponding
reactivity of the series of phenylsilanes, PhxSiH4−x (x = 1−4),
which affords silyl, hypervalent silyl, silane, and disilane
complexes.
Previous studies have shown that zerovalent Mo(PMe3)6 is a
highly reactive molecule that is subject to oxidative addition
reactions with, for example, H−H, C−H, O−H, and C−S
bonds.6,7 Significantly, we now demonstrate that Mo(PMe3)6
also cleaves the Si−H bond of SiH4 at room temperature to
give the bis(silyl) compound Mo(PMe3)4(SiH3)2H2 (1)
(Scheme 1). This transformation is of particular note because
related zerovalent molybdenum complexes, namely Mo-
(R2PC2H4PR2)2(CO) (R = Ph, Bui), do not cleave the Si−H
bond of SiH4, but rather coordinate it to form σ‑silane adducts,
Mo(R2PC2H4PR2)2(CO)(σ-SiH4).3 Mo(Et2PC2H4PEt2)2(CO)
reacts similarly to Mo(R2PC2H4PR2)2(CO) (R = Ph, Bui),
although the silane adduct was shown to exist in equilibrium
with the silyl-hydride complex Mo(Et2PC2H4PEt2)2(CO)-
2
3
value of JP−H = 26 Hz and the latter JP−H = 8 Hz. In accord
with the silyl-hydride assignment, the signal attributable to the
SiH3 groups exhibits coupling to silicon (1JSi−H = 157 Hz),
2
whereas no coupling is observed (i.e., JSi−H < 15 Hz) for the
hydride signal.
In addition to the bis(silyl) complex, the mono(silyl)
counterpart, Mo(PMe3)4(SiH3)H3 (2), has been obtained by
both (i) reaction of the dihydride Mo(PMe3)5H2 with SiH4 and
(ii) addition of H2 to Mo(PMe3)4(SiH3)2H2 (Scheme 1). The
latter reaction is reversible, such that treatment of the
mono(silyl) complex with SiH4 regenerates the bis(silyl)
compound, Mo(PMe3)4(SiH3)2H2 (Scheme 1).8 The molecular
structures of Mo(PMe3)4(SiH3)H3 and Mo(PMe3)4(SiH3)2H2
have been determined by X-ray diffraction, as illustrated for the
latter in Figure 1a.
While a simple mechanism for formation of Mo(PMe3)4-
(SiH3)H3 upon treatment of Mo(PMe3)4(SiH3)2H2 with H2
could involve reductive elimination of SiH4 followed by
oxidative addition of H2, isotope labeling studies indicate that
such a mechanism, which is commonly invoked for non-d0
Received: April 4, 2014
© XXXX American Chemical Society
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Journal of the American Chemical Society
Communication
are formed during the course of the experiment. The absence of
SiH4 provides conclusive evidence that Mo(PMe3)4(SiH3)2H2
does not undergo reductive elimination of the silyl and hydride
ligands, and thus this transformation cannot be the first step in
the reaction of Mo(PMe3)4(SiH3)2H2 with H2.9 A plausible
mechanism for the exchange is, therefore, proposed to involve
metathesis between the Mo−H and D−SiD3 bonds (possibly
accompanied by reversible dissociation of PMe3), which would
form initially Mo(PMe3)4(SiH3)2HD and SiHD3 (Scheme 2).10
Subsequent incorporation of deuterium into the molybdenum
silyl groups can be rationalized on the basis of Mo(PMe3)4
(SiH3)2HD accessing a fluxional silane adduct, Mo(PMe3)4
(SiH3)(σ-SiH3D)H, which would allow for scrambling (Scheme
2).11
Further evidence that Si−H reductive elimination does not
operate is provided by examination of the corresponding
exchange reaction between Mo(PMe3)4(SiD3)2D2 and SiH4.
1
Specifically, H NMR spectroscopic analysis reveals the initial
formation of Mo−SiHD2 groups rather than Mo−SiH3 groups,
with the latter being generated as the reaction progresses.
Furthermore, the silane isotopologue that is released is
primarily SiH3D, which is consistent with a mechanism that
involves metathesis of the Mo−D bond with the H−SiH3 bond.
Mo(PMe3)6 also undergoes facile oxidative addition of Si−H
bonds of PhSiH3 at room temperature to give the bis-
(phenylsilyl) compound, Mo(PMe3)4(SiH2Ph)2H2 (3), which
can be isolated if the reaction mixture is immediately cooled to
−15 °C (Scheme 3). However, Mo(PMe3)4(SiH2Ph)2H2
Figure 1. Molecular structures of (a) Mo(PMe3)4(SiH3)2H2, (b)
Mo(PMe3)4(κ2-H2-H2SiPh2H)H, (c) Mo(PMe3)3(κ2-H2-H2Si2Ph4)H2,
and (d) Mo(PMe3)3(σ-HSiHPh2)H4.
metal phosphine hydride compounds, does not operate. For
example, treatment of Mo(PMe3)4(SiH3)2H2 with D2 results
primarily in the formation of SiHxD4−x rather than SiH4.
Furthermore, treatment of Mo(PMe3)4(SiH3)2H2 with SiD4
demonstrates that H/D exchange involving both the silyl and
hydride ligands occurs without reductive elimination of SiH4
(Scheme 2). Specifically, 1H NMR spectroscopic analysis
reveals that SiHD3, and not SiH4, is the initially formed
isotopologue. SiH2D2 and SiH3D are also observed as the
exchange reaction progresses, but negligible quantities of SiH4
Scheme 3
Scheme 2
exhibits limited stability at room temperature and undergoes
a series of transformations in the presence of excess PhSiH3 to
form the silyl (SiH3) compounds Mo(PMe3)4(SiH2Ph)(SiH3)-
H2 (4), Mo(PMe3)4(SiH3)2H2, and Mo(PMe3)4(SiH3)H3, of
which the lattermost ultimately dominates (Scheme 3). The
generation of these Mo−SiH3 compounds upon treatment with
PhSiH3 is of significance not only because it requires the
cleavage of Si−C bonds, but also because there is no precedent
for the isolation of a metal complex with a terminal SiH3 ligand
from the reactions of PhSiH3.12,13
Although the mechanistic details are unknown, formation of
Mo(PMe3)4(SiH2Ph)(SiH3)H2 from Mo(PMe3)4(SiH2Ph)2H2
can be conceptually rationalized by an overall metathesis of
Mo−SiH2Ph and Ph−SiH3 bonds.14,15 In support of this
suggestion, Ph2SiH2 is also formed during the course of the
reaction. Furthermore, SiH4 is observed, thereby making it
evident that the system is capable of effecting the redistribution
of PhSiH3 into Ph2SiH2 and SiH4. In this regard, the catalytic
redistribution of PhSiH3 is well-known, although it is typically
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Journal of the American Chemical Society
Communication
observed for d0 transition metals16 and lanthanides,12a,b,17 for
which σ-bond metathesis mechanisms are generally invoked.
Interestingly, the reactivity of Ph2SiH2 towards Mo(PMe3)6
is quite distinct from that of either PhSiH3 or SiH4. Therefore,
rather than simply cleaving the Si−H bond, the reaction of
Mo(PMe3)6 with Ph2SiH2 results in the formation of, inter alia,
Mo(PMe3)4(κ2-H2-H2SiPh2H)H (5) and Mo(PMe3)3(κ2-H2-
H2Si2Ph4)H2 (6) (Scheme 4).18 The formation of the Si−Si
hypervalent [H2SiPh2H] silyl derivative than as a silyl-hydride
complex.27
Finally, in contrast to the reactions of Mo(PMe3)6 with SiH4,
PhSiH3, and Ph2SiH2, each of which involves Si−H bond
cleavage, the corresponding reaction of Ph3SiH results in the
formation of the η6-arene complex (η6-C6H5SiPh2H)Mo-
(PMe3)3 (8).28 Complexes that feature Ph3SiH as an η6-arene
ligand are rare, with there being only one other structurally
characterized example, namely (η6-C6H5SiPh2H)W(CO)3,29
listed in the Cambridge Structural Database.
Scheme 4
In summary, Mo(PMe3)6 exhibits diverse reactivity towards
SiH4, PhSiH3, and Ph2SiH2 to afford a variety of novel silyl,
hypervalent silyl, silane, and disilane complexes, whereas
Ph3SiH simply forms the η6-arene complex (η6-C6H5SiPh2H)-
Mo(PMe3)3. While the reactions of non-d0 metal phosphine
hydride compounds are often interpreted in terms of sequences
that involve oxidative addition and reductive elimination, NMR
spectroscopic analysis of the isotope exchange reaction between
Mo(PMe3)4(SiH3)2H2 and SiD4 indicates that the reaction does
not occur via initial reductive elimination of SiH4, but rather by
a metathesis pathway.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental details, crystallographic data for compounds 1−8
(CIFs), and Cartesian coordinates for geometry optimized
structures. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
parkin@columbia.edu
■
bond to yield the latter compound is, however, reversible, such
that the disilane complex reacts rapidly with H2 to give the
monosilane adduct, Mo(PMe3)3(σ-HSiHPh2)H4 (7) (Scheme
4). Furthermore, the latter compound is also formed upon
treatment of Mo(PMe3)4(κ2-H2-H2SiPh2H)H with H2 (Scheme
4).19
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the U.S. Department of Energy, Office of Basic
Energy Sciences (DE-FG02-93ER14339) for support of this
research. Aaron Sattler and Kaylyn Shen are thanked for helpful
contributions.
The molecular structures of Mo(PMe3)4(κ2-H2-H2SiPh2H)H,
Mo(PMe3)3(κ2-H2-H2Si2Ph4)H2, and Mo(PMe3)3(σ-
HSiHPh2)H4 have been determined by X-ray diffraction, as
illustrated in Figure 1b−d. The three-membered [Mo,H,Si]
moiety of Mo(PMe3)3(σ-HSiHPh2)H4 is characterized by Mo−
Si [2.500(1) Å], Mo−H [1.64(5) Å ], and Si−H [1.74(4) Å]
bond lengths that are in accord with its formulation as a silane
adduct.1,20,21 For example, the Si−H bond length associated
with the bridging hydrogen is within the range accepted for σ-
complexes (1.7−1.8 Å).1a
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1H NMR spectrum at δ 6.48 (3JP−H = 8 Hz) and −4.21 (2JP−H = 30
Hz), attributable to a terminal silicon hydride and five hydrogen atoms
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D
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