Activations of all Bonds to Silicon (Si-H, Si-C) in a Silane with Extrusion of [CoSiCo] Silicide Cores

Article abstract of DOI:10.1021/jacs.9b04265

The [BP₃^iPr]Co(I) synthon Na(THF)₆{[BP₃^iPr]CoI} (1, [BP₃^iPr] = κ³-PhB(CH₂PⁱPr₂)₃^-) reacts with PhSiH₃ or SiH₄ to form unusual {[BP₂^iPr](SiH₂R)CoH₂}=Si={H₂Co[BP₃^iPr]} species (R = Ph, 2a; R = H, 2b; [BP₂^iPr] = κ²-PhB(CH₂PⁱPr₂)₂) that result from activation of all Si - H and Si - C bonds in the starting silanes. Solution-spectroscopic data (multinuclear NMR, IR) for 2a,b, and the solid-state structure of 2a, indicate substantial Co=Si=Co multiple bonding and minimal interaction of the core Si atom with nearby hydride ligands. In the presence of 4-dimethylaminopyridine (DMAP), 1 reacts with PhSiH₃ to give [BP₃^iPr](H)₂CoSiHPh(DMAP) (3). Complexes 2a,b eliminate RSiH₃ upon thermolysis in the presence of DMAP to generate {[BP₂^iPr]Co(NC₅H₃NMe₂)}=Si={H₂Co[BP₃^iPr]} (4).

Full text of DOI:10.1021/jacs.9b04265

Communication
pubs.acs.org/JACS
Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Activations of all Bonds to Silicon (Si−H, Si−C) in a Silane with
Extrusion of [CoSiCo] Silicide Cores
Rex C. Handford, Patrick W. Smith, and T. Don Tilley*
Department of Chemistry, University of California, Berkeley, Berkeley, California 94720-1460, United States
S
* Supporting Information
with Na/Hg in THF (eq 1).21 The Evans method measure-
ABSTRACT: The [BP3iPr]Co(I) synthon Na-
ment of the magnetic moment of 1 (3.0 μB) is consistent with
−
(THF)6{[BP3iPr]CoI} (1, [BP3iPr] = κ3-PhB(CH2PiPr2)3 )
reacts with PhSiH3 or SiH4 to form unusual {[BP2
an S = 1 configuration, and the structure of 1 was
crystallographically established (see Supporting Information).
iPr -
]
(SiH2R)CoH2}Si{H2Co[BP3iPr]} species (R = Ph,
2a; R = H, 2b; [BP2iPr] = κ2-PhB(CH2PiPr2)2) that result
from activation of all SiH and SiC bonds in the
starting silanes. Solution-spectroscopic data (multinuclear
NMR, IR) for 2a,b, and the solid-state structure of 2a,
indicate substantial CoSiCo multiple bonding and
minimal interaction of the core Si atom with nearby
hydride ligands. In the presence of 4-dimethylaminopyr-
Treatment of a THF solution of 2 equiv of 1 with 3 equiv of
RSiH3 (R = Ph, H) resulted in a rapid color change from
purple to deep red. Workup of the reaction mixtures gave
deposition of a white powder, presumably NaI, and 2a,b as
analytically pure, dark-red solids (eq 2).
iPr -
idine (DMAP), 1 reacts with PhSiH3 to give [BP3
]
(H)2CoSiHPh(DMAP) (3). Complexes 2a,b eliminate
RSiH3 upon thermolysis in the presence of DMAP to
generate {[BP2iPr]Co(NC5H3NMe2)}Si{H2Co-
[BP3iPr]} (4).
rystalline and amorphous transition-metal silicide
C
(MxSiy) phases are of immense technological importance
for their roles in electronic materials,1−6 and in processes that
form and break chemical bonds to silicon.7−9 Elemental silicon
is a reactant in the Direct Process, which is practiced on a large
industrial scale to convert silicon and MeCl to Me2SiCl2, via
intermediate copper silicide phases such as Cu3Si.7,8,10,11 In
addition, silicide phases mediate the reverse type of reaction, in
which silanes (e.g., SiH4, PhSiH3, and Ph2SiH2) are used as the
silicon source for producing nanoscaled silicon structures such
as nanowires and nanocrystals.12−15 The latter processes
involve the controlled crystallization of elemental Si at elevated
temperatures (>300 °C), from a metal silicide phase formed by
activation of SiH and/or SiC bonds.
Multinuclear NMR analyses of the crude reaction mixtures
containing 2a,b reveal the formation of 1 equiv of
iPr2PCH2SiH2R (R = Ph, H, respectively, by 1H NMR
iPr
spectroscopy), thereby accounting for the loss of a [BP3
]
ligand side arm from 1, as well as the requirement for 3 equiv
RSiH3 in affecting the complete consumption of 1. The crude
reaction mixture from 2a also contains 1 equiv of benzene,
apparently evolved from the Ph substituent of 1 equiv of
PhSiH3. Similarly, the crude reaction mixture for 2b also
A few studies have provided mechanistic information on the
Direct Process, which is thought to proceed through silylene
intermediates that engage in SiC and SiCl bond-forming
steps.7−11,16−19 However, detailed studies of the reverse
process, in which a silicide is produced by Sielement bond
activations in a silane, have not been well described.20 This
report describes a molecular model system in which the
generation of Co-silicide complexes results from activations of
all bonds in PhSiH3 or SiH4 by reaction with a Co(I) complex,
Na+(THF)6{[BP3iPr]CoI}− (1), to produce dicobalt species
containing unusual CoSiCo silicide cores. Notably,
discrete MSiM linkages are rare20 and their direct formation
from silane starting materials is unprecedented.
i
contains Pr2PCH2SiH3 and H2 (see Supporting Information).
The solid-state molecular structure of 2a (Figure 1a)
exhibits relatively long SiH (dave = 1.77 Å) and extremely
short CoSi distances (d(Co1Si1) = 2.0737(7) Å;
d(Co2Si1) = 2.0750(7) Å), indicating multiple bonding in
the central CoSiCo unit. The CoH distances (dave
=
1.44 Å) are similar to those of other crystallographically
characterized terminal CoH bond lengths (dave = 1.42 Å).22
The hydride positions, which were located in the difference
map and refined isotropically, indicate that the silicon and
hydrides do not form a tetrahedral SiH4 unit (Figure 1b,c),
Complex 1, which serves as a source of [BP3iPr]Co(I), was
readily prepared in good yields by reduction of [BP3iPr]CoI
Received: April 19, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/jacs.9b04265
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Journal of the American Chemical Society
Communication
interesting molecular models for further investigation. Though
solid-phase catalysts for these reactions require more forcing
conditions to “decompose” the silane precursors,9,12,13 and
presumably involve reactions of surface and bulk silicide
species,9 the formation of 2a,b shows that this process can
occur under mild conditions via metal-mediated reactions.
To gain further insight into the mechanism leading to the
formation of 2a, THF solutions of complex 1 were treated with
PhSiH3, followed rapidly by addition of 4-dimethylaminopyr-
idine (DMAP) as a trapping reagent. This led to isolation of
the base-stabilized silylene complex [BP3iPr](H)2CoSiHPh-
(DMAP) (3, eq 3).
Figure 1. (a) Solid-state molecular structure 2a; the isopropyl CH3
groups, non ipso-C atoms of the Ph rings, and most H atoms have
been omitted for clarity. (b) View of the core atoms of 2a along the
CoSiCo axis. (c) Newman projection of the core atoms of 2a,
viewed along the same axis as in panel b.
which would be expected in the case of limited SiH
activation, as in [(iPr3P)2(H)2Ru]2(μ-SiH4).23 These solid-
state data for 2a are consistent with its assignment as
possessing a CoSiCo core with minimal SiH bonding.
The 1H NMR spectrum of 2a exhibits hydride resonances at
−13.0 and −10.5 ppm as featureless singlets, which belong to
the [BP3iPr]Co and [BP2iPr]Co fragments, respectively (assign-
ments by 31P−1H HMBC NMR). The 29Si−1H HMBC NMR
spectrum of 2a reveals an extremely downfield-shifted Si
nucleus at 350 ppm, which couples only to the hydride
resonance at −13.0 ppm with a very small coupling constant of
1JSiH = 8 Hz. The lack of strong SiH coupling suggests little
direct SiH bonding character. The IR spectrum of 2a
Figure 2. Solid-state molecular structure of 3; the isopropyl CH3
groups, non ipso-C atoms of the Ph rings, and most H atoms have
been omitted for clarity.
contains bands at 1812 and 1720 cm−1, resulting from CoH
iPr
stretches of the [BP2iPr](SiH2Ph)CoH2 and H2Co[BP3
]
fragments, respectively. The decreased energy of the 1720
cm−1 band results from a weak bridging interaction with the
central Si atom, consistent with the NMR data. The
spectroscopic properties of 2b are very similar to those of
2a, though the 29Si−1H HMBC NMR spectrum exhibits very
weak coupling constants for both sets of hydride ligands to the
The solid-state molecular structure of 3 (Figure 2) reveals a
distorted octahedral coordination environment about the Co
atom with a CoSi distance of 2.1524(8) Å, which is
significantly longer than that observed in 2a. Complex 3 bears
close structural similarities to [BP3Ph]Ru(μ-H)(η3-H2SiMePh-
(DMAP)), though the coordination environments about Si are
different in each case.25,26 The formation of 3 suggests that the
unstabilized silylene dihydride, [BP3iPr]Co(H)2SiHPh, may
be an intermediate in the mechanistic pathway resulting in 2a.
This assumption is consistent with a possible mechanism
involving condensation of [BP3iPr]Co(H)2SiHR (R = H, Ph)
with a hydride species, e.g. [BP2iPr]Co(H)(THF)x, to form a
1
central Si (δSi = 354 ppm; JSiH = 5, 7 Hz).
The CoSiCo cores of 2a and 2b result from exhaustive
SiH and SiC bond activations in PhSiH3 and SiH4, and
analysis of THF-d8 solutions of the crude reaction mixtures
reveal no detectable quantities of silane redistribution
products, indicating that the formation of 2a,b is mechanis-
tically distinct from the pathway giving rise to
[(iPr3P)2(H)2Ru]2(μ-SiH4).23,24 Complexes 2a,b are the first
silicide complexes to be prepared directly from silane
precursors; a related species, [Tp′(CO)2Mo]Si[Mo-
iPr
-
bimetallic species with a bridging silyne ligand, {[BP2
]
CoH2}Si(R){H2Co[BP3iPr]}. This species would then
react further to provide 2a,b (Scheme S1). Interestingly,
treatment of toluene solutions of 1 with PhSiH3 resulted in
formation of [BP2iPr]Co(H)2(κ2-Si,P-iPr2PCH2SiHPh) (see
Supporting Information) rather than 2a, suggesting that THF
is important in stabilizing an intermediate that leads to the
silicide product.
−
(PMe3)(CO)2Tp′] (Tp′ = HB(3,5-Me2C3N2)3 ), was gen-
erated by a salt-metathesis reaction between Na[Tp′Mo-
(CO)2(PMe3)] and 0.5 equiv of Br2Si(SIPr) (SIPr =
C[N(2,6-iPr2C6H3)CH2]2). The corresponding symmetrical
silicide, {[Tp′(CO)2Mo]Si[Mo(CO)2Tp′]}2−, was pre-
pared by KC8 reduction of the MoSiMo starting
material.20
To examine further the bonding in the CoSiCo cores of
2a and 2b, density functional theory (DFT) computations
(ωB97X-D3; def2-TSVP (Co, Si, P, hydride), def2-SVP (C,H)
basis sets) were carried out using a model complex,
{[BP2Me](SiH2Ph)CoH2}Si{H2Co[BP3Me]} (2*;
In the context of silicides as intermediates in bond-forming
and -cleaving reactions of silanes, complexes 2a,b represent
B
DOI: 10.1021/jacs.9b04265
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Journal of the American Chemical Society
Communication
[BPxMe] = PhB(CH2PMe2)x; x = 2, 3). The computed
Mulliken charges for the two Co atoms, as well as their
respective hydride ligands, are essentially neutral, while the
central Si atom bears a partial negative charge of −0.80. This is
in contrast to monometallic silylene complexes, which possess
substantial silylium (R3Si+) character.25,27,28 The computed
CoSi distances of 2.0252 and 2.0280 Å are somewhat
of 2a,b in the presence of DMAP (for 65 °C for 4 and 10 h,
respectively) affected their conversion to {[BP2iPr]Co-
(NC5H3NMe2)}Si{H2Co[BP3iPr]} (4; eq 5). Crystals of
shorter than those measured experimentally for 2a, likely
Me
resulting from the diminished steric demands of the [BPx
]
ligands. The SiH and CoH distances in the core of 2*
average 1.82 and 1.56 Å respectively, in agreement with the X-
ray data for 2a which indicate that the H atoms are closer to
Co than to Si.
The MOs of 2* include two CoSiCo σ-bonding
orbitals derived from Si 3s and 3pz orbitals, which interact with
two Co 3dz2 group orbitals to form two 3c−2e bonding levels
(Figure 3, 1σ and 2σ). Two π-bonding orbitals result from
4 suitable for X-ray diffraction analysis were grown from a
saturated THF solution layered with (Me3Si)2O and
maintained at −30 °C, and its solid-state molecular structure
is shown in Figure 4. Due to disorder of the central Si atom,
the remaining hydride ligands could not be located in the
difference map. Complex 4 results from the CH activation of
DMAP and eliminations of RSiH3 and H2 from the
[BP2iPr](RSiH2)CoH2 units of 2a,2b.
Figure 4. Solid-state molecular structure of 4; the isopropyl CH3
groups, non ipso-C atoms of the Ph rings and most H atoms, have
been omitted for clarity.
The CoSi bond lengths of 4 are similar to analogous
distances in 2a, and the 29Si−1H HMBC NMR spectrum
reveals a Si resonance at 294 ppm, with evidence of weak
coupling to the remaining CoH hydrogens of the H2Co-
[BP3iPr] unit (1JSiH = 5 Hz).
In conclusion, complex 1 facilitates extreme degrees of Si
H and SiC bond activations in PhSiH3 and SiH4 to generate
species (2a,b) bearing rare MSiM cores, and demonstrates a
new reactivity mode for such silanes. Though the full
mechanism for the formation of 2a,b remains unclear, trapping
studies have resulted in the isolation of 3, which suggests that a
silylene dihydride intermediate is involved. With respect to
related processes that generate silicon nanowires from silane
precursors, the facile formation of silicides 2a,b implies a
greater role of the metal catalyst than simply seeding nanowire
growth. Current efforts are focused toward developing novel
MSiM silicide complexes bearing more reactive Si cores with
chemically innocent and less sterically demanding supporting
ligands. Such systems should be amenable to the systematic
exploration of the reactivity and behavior of the core Si atom
with small molecules and elements.
Figure 3. Simplified depiction of the bonding molecular orbitals of 2*
(left), and visualization of the corresponding computed molecular
orbitals of 2* (right).
interaction of Si 3px and 3py orbitals with sets of Co 3dxz and
3dyz orbitals, respectively (Figure 3, 1π and 2π). A Mulliken
population analysis indicates that only the 2σ and 1π MOs
include partial hydride 1s orbital character. These features are
consistent with the structural and spectroscopic features of
2a,b, which also indicate weak silicide SiH interactions and
substantial CoSiCo bonding.
The electrophilic Si centers of silylene complexes readily
coordinate Lewis bases (L) at Si to form base-stabilized M =
SiR2(L) adducts.27−29 In light of the calculated charge of the Si
atom in 2*, it was anticipated that 2a,b may exhibit
substantially divergent reactivity. Heating of benzene solutions
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.9b04265.
C
DOI: 10.1021/jacs.9b04265
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Journal of the American Chemical Society
Communication
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P-iPr2PCH2SiHPh); characterization data; and details
of X-ray crystallography (PDF)
X-ray crystallography structure data (CIF)
Structure model of 2* (XYZ)
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AUTHOR INFORMATION
Corresponding Author
*tdtilley@berkeley.edu
ORCID
Rex C. Handford: 0000-0002-3693-1697
Patrick W. Smith: 0000-0001-5575-4895
T. Don Tilley: 0000-0002-6671-9099
(14) Ramanujam, J.; Shiri, D.; Verma, A. Silicon Nanowire Growth
and Properties: A Review. Mater. Express 2011, 1, 105−126.
(15) Hasan, M.; Huq, M. F.; Mahmood, Z. H. A Review on
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Different Growth Techniques. SpringerPlus 2013, 2, 151−160.
(16) Okamoto, M.; Onodera, S.; Yamamoto, Y. Direct Synthesis of
Organochlorosilanes by the Reaction of Metallic Silicon with
Hydrogen Chloride and Alkene/Alkyne. Chem. Commun. 1998,
1275−1276.
■
Notes
(17) Wanandi, P. W.; Glaser, P. B.; Tilley, T. D. Reactivity of an
Osmium Silylene Complex toward Chlorocarbons: Promotion of
Metal Redox Chemistry by a Silylene Ligand and Relevance to the
Mechanism of the Direct Process. J. Am. Chem. Soc. 2000, 122, 972−
973.
(18) Lewis, L. N.; Whitney, J. M.; Bui, P. Direct Reaction of Silicon
with α−ω Dichloroalkanes: Direct Formation of Dichlorosilacyclo-
pentane. Organometallics 2005, 24, 2141−2146.
(19) Okamoto, M. Intermediacy of Silylene and Germylene in Direct
Synthesis of Organosilanes and Organogermanes. Res. Chem. Intermed.
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The authors declare no competing financial interest.
CCDC 1910274−1910279 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
ACKNOWLEDGMENTS
■
(20) The activation of an Si(II) source, Br2Si(SIPr), by a molybdate
complex resulting in a MoSiMo core has been reported: Ghana,
P.; Arz, M. I.; Chakraborty, U.; Schnakenburg, G.; Filippou, A. C.
Linearly Two-Coordinated Silicon: Transition Metal Complexes with
the Functional Groups M≡SiM and M = Si = M. J. Am. Chem. Soc.
2018, 140, 7187−7198.
This work was funded by the National Science Foundation
under grant no. CHE-1566538. R.C.H. is grateful to the
NSERC of Canada for a PGS-D fellowship. UC Berkeley
ChexRay is funded by the National Institutes of Health under
grant no. S10-RR027172. We thank Simon J. Teat and Laura J.
McCormick of the Advanced Light Source for their expertise
and assistance. We thank the reviewers for their thoughtful
comments and suggestions. This research used resources of the
Advanced Light Source, which is a DOE Office of Science User
Facility under contract no. DE-AC02-05CH11231.
(21) Note that the Na/Hg reduction of [BP3iPr]CoI under these
conditions has been previously reported to give {[BP3iPr]Co}2(μ-N2).
See: Betley, T. A.; Peters, J. C. Dinitrogen Chemistry from Trigonally
Coordinated Iron and Cobalt Platforms. J. Am. Chem. Soc. 2003, 125,
10782−10783 The synthesis of 1 with the current synthetic protocol
has been reproducible in our hands (see Supporting Information). .
(22) Groom, C. R.; Bruno, I. J.; Lightfoot, M. P.; Ward, S. C. The
Cambridge Structural Database. Acta Crystallogr., Sect. B: Struct. Sci.,
Cryst. Eng. Mater. 2016, 72, 171−179.
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