Arrested 1,2-hydrogen migration from silicon to nickel upon oxidation of a three-coordinate Ni(I) silyl complex

Article abstract of DOI:10.1021/ja1052329

Reaction of the dimeric Ni(I) chloride complex [(dtbpe)NiCl]₂ (1) with dimesitylsilyl potassium affords the three-coordinate Ni(I) silyl complex (dtbpe)Ni(SiHMes₂) (2). Alternatively, 2 can be prepared by an oxidative-addition reacti

Full text of DOI:10.1021/ja1052329

Published on Web 08/06/2010
Arrested 1,2-Hydrogen Migration from Silicon to Nickel upon Oxidation of a
Three-Coordinate Ni(I) Silyl Complex
Vlad M. Iluc and Gregory L. Hillhouse*
Gordon Center for IntegratiVe Science, Department of Chemistry, UniVersity of Chicago, Chicago, Illinois 60637
Received June 15, 2010; E-mail: g-hillhouse@uchicago.edu
Abstract: Reaction of the dimeric Ni(I) chloride complex [(dtbpe)-
NiCl]2 (1) with dimesitylsilyl potassium affords the three-coordinate
Ni(I) silyl complex (dtbpe)Ni(SiHMes2) (2). Alternatively, 2 can be
prepared by an oxidative-addition reaction of Mes2Si(H)OTf (Tf
) CF3SO3) with the nickel(0) complex [(dtbpe)Ni]2(µ-C6H6) (3),
with (dtbpe)Ni(OTf) (4) formed as an easily separable byproduct.
The one-electron oxidation of 2 by ferrocenium affords diamag-
netic [(dtbpe)Ni(µ-H)SiMes2][BArF4] (5), a Ni(II) complex formed
by partial 1,2-H migration from silicon to nickel and featuring an
unusual 3-center, 2-electron bonding motif between Ni, Si, and
the bridging H. Complex 5 was also obtained from Mes2SiH2
activation by the neopentyl complex salt [(dtbpe)Ni(CH2CMe3)]-
[BArF ] (6) with elimination of neopentane.
4
Silylene complexes1 have received considerable attention because
Figure 1. A perspective view of complex 2 (35% probability ellipsoids; H
atoms except on Si omitted for clarity). Selected metrical parameters (distance,
Å; angle, deg): Ni-Si ) 2.3731(10), Ni-P(1) ) 2.2502(10), Ni-P(2) )
2.2507(9), Si-H ) 1.53(3); P(1)-Ni-P(2) ) 90.60(3), P(1)-Ni-Si )
126.68(4), P(2)-Ni-Si ) 138.75(4), Ni-Si-H ) 110.0(12), Ni-Si-C(71)
) 124.35(12), Ni-Si-C(81) ) 112.88(11), C(71)-Si-C(81) ) 104.65(15).
of their participation in various transformations of organosilicon
compounds, such as dehydrogenative coupling of hydrosilanes,2
redistribution of substituents on silicon atoms,3 Rochow’s direct
process,4 and silylene transfer to unsaturated organic compounds.5
Several synthetic strategies have been employed in the formation of
silylene complexes, relying on abstraction of triflate6 or chloride7 from
silicon to produce cationic complexes, coordination of photochemically
generated silylenes,8 and, most interestingly, 1,2-hydrogen migrations.9
Intramolecular hydrogen migration from silicon to a late-transition
metal is significant in understanding the rearrangements taking place
in metal silyl complexes involved in catalysis,1b,10,11 but only one
discrete example has been reported for a group 10 metal, [(1,2-
iPr2PCH2CH2PiPr2)Pt(H)(SiMes2)][MeB(C6F5)3].12a DFT studies on a
simplified model were inconclusive on whether hydrogen bridges Pt
and Si or is present as a standard Pt hydride.12b Herein we describe
an analogous, structurally characterized Ni complex in which the
H atom unambiguously bridges the metal and silicon. This report offers
insight into processes taking place during Si-H activation at metals and
characterizes the bonding motif in this nonclassical nickel-silyl cation.
A Ni(I) silyl complex, in which one of the silyl substituents is
hydrogen, was targeted as a precursor to a possible nickel-silylene
fragment. The reaction between [(dtbpe)Ni(µ-Cl)]2 (1, dtbpe ) 1,2-
tBu2PCH2CH2PtBu2)13 and Mes2SiHK (Mes ) 2,4,6-Me3C6H2) al-
lowed for the isolation of (dtbpe)Ni(SiHMes2) (2) as green dichroic
crystals in 86% yield (Scheme 1). This method was higher yielding
and had better reproducibility than the oxidative addition of
Mes2Si(H)OTf (Tf ) CF3SO3) to a nickel(0) complex, [(dtbpe)Ni]2(µ-
C6H6) (3, Scheme 1).14 Although other silanes give arrested oxidative
addition of the Si-H bond to the Ni(0) center,15 two products were
formed in the reaction of 3 with Mes2Si(H)OTf: (dtbpe)Ni(OTf) (4)13b
and 2.
Scheme 1
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11890 J. AM. CHEM. SOC. 2010, 132, 11890–11892
10.1021/ja1052329  2010 American Chemical Society

C O M M U N I C A T I O N S
Figure 3. Frontier molecular π-symmetry orbitals for a hypothetical nickel
silylene (dmpe)NidSiPh2 (left) and for the three-center bond of (dmpe)Ni(µ-
H)SiPh2+ (right).
Figure 2. A perspective view of the complex cation of 5 (35% probability
ellipsoids; H atoms except on Si omitted for clarity). Select metrical data
(distance, Å; angle, deg): Ni-Si ) 2.147(2), Ni-H ) 1.70(7), Ni-P(1) )
2.189(2), Ni-P(2) ) 2.254(2), Si-H ) 1.64(7); P(1)-Ni-P(2) ) 92.56(6),
P(1)-Ni-Si ) 115.17(7), P(2)-Ni-H ) 103(2), Si-Ni-H ) 49(2),
Ni-Si-C(31) ) 121.2(2), Ni-Si-C(41) ) 124.5(2), C(31)-Si-C(41) )
114.3(3).
by silane coordination to the electrophilic Ni center,15 replacing a
weak C-H agostic interaction,13c to give an intermediate that
undergoes intramolecular H-abstraction with neopentane elimination
to generate 5. This metathesis route is attractive as it uses a
secondary silane that is more accessible than Mes2SiHK and would
appear to be the more general synthetic approach.
Complex 2 represents the first example of a three-coordinate Ni(I)
silyl complex. The solid-state structure of 2 (Figure 1) features a
trigonal-planar nickel center and tetrahedral silicon. The Ni-Si
distance of 2.3731(10) Å is slightly longer than other reported
Ni-Si bonds (2.21-2.30 Å),15,16 presumably due to sterics. The
hydrogen connected to the silicon atom was located in the electron-
density map at 1.50 Å from Si and refined isotropically. The
assignment of a nickel(I) center is supported by the effective
magnetic moment of 2.1 µB, determined in solution (22 °C, C6D6),
corresponding to one unpaired electron and consistent with a d9
electronic configuration.
DFT calculations (B3LYP, LANL2DZ basis sets)20 were carried out
using a (dmpe)Ni(µ-H)(SiPh2)+ model (dmpe ) Me2PCH2CH2PMe2) to
understand the unusual Ni-H-Si bonding motif in 5 relative to
the hypothetical parent silylene (dmpe)Ni(SiPh2) (Figure 3). The
calculations indicate that the bridging hydrogen participates in a
3-center, 2e- bond using the 1s H orbital and the π orbital of the
NidSi core to effectively give a “protonated” NidSi double bond,
and an NBO analysis is consistent with this picture. Optimized
metrical parameters for the (dmpe)Ni(µ-H)(SiPh2)+ model (e.g.,
Ni-H ) 1.731 Å, Si-H ) 1.616 Å, Ni-Si ) 2.177 Å) agree well
with actual values observed in the structure of 5 and converge to
the bridging structure from either initial hydrido silylene or σ-silyl
models (see Supporting Information).
A cyclic voltammogram of 2 shows a quasi-reversible wave for
the Ni(I)/Ni(II) couple at E1/2 ) -0.48 V (THF; vs Cp2Fe/Cp2Fe+).
Oxidation of 2 with [Cp2Fe][B(ArF)4] (Scheme 1, ArF ) 3,5-
(CF3)2C6H3) allowed for the isolation of diamagnetic [(dtbpe)Ni(µ-
H)SiMes2][BArF ] (5) in 85% yield. X-ray crystallography (Figure
In conclusion, a new three-coordinate nickel(I) silyl complex was
isolated and characterized. Its oxidation leads to a 1,2-hydrogen
migration from silicon to nickel and gives an unusual cationic
H-bridging species that features 3-center, 2e- bonding.
4
2) reveals that a hydrogen atom is located in the P2NiSi plane of 5,
bridging nickel and silicon (Ni-H ) 1.70(7) Å, Si-H ) 1.64(7)
Å) and resulting in distorted square-planar coordination geometry
at nickel. The Ni-Si distance (2.147(2) Å) is 9% shorter than the
corresponding distance in 2 and is close in value to that reported
for Ni silylene complexes (∼2.14 Å).16d The {C(31), C(41), Si}
plane is perpendicular to the {P(1), P(2), Ni} plane (∠87.75°). There
are similarities in the structures of Ni(0) boryl complexes17a and a
Mo hydrosilylene complex17b and 2.
Acknowledgment. This work was supported by the National
Science Foundation through Grant CHE-0957816 to G.L.H.
Supporting Information Available: Crystallographic data of 2 and
5 (CIF). Synthetic and spectroscopic characterization of all complexes
and computational information (PDF). This material is available free
of charge via the Internet at http://pubs.acs.org.
In agreement with its solid-state structure, features of the µ-H
resonance in the 1H NMR spectrum of 5 are indicative of hydridic
character. It appears at δ -8.64 (dd, JHP ) 4.8, 47.1 Hz) with a
JHP smaller than those found in other d8 square-planar nickel
hydrides.18 In addition, the JHSi at 43.4 Hz is smaller than those in
conventional hydrosilyls and hydrosilanes (∼60-150 Hz).19 The
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