Catalytic hydrosilylation of the carbonyl functionality via a transient nickel hydride complex

Article abstract of DOI:10.1021/om801160j

We report the syntheses and characterization of two new sterically demanding chelate ligands, denoted PN^Me3 (N-(2-(diisopropylphosphino) -4-methylphenyl)-2,4,6-trimethylanilide) and PN^iPr3 (N-(2-(diisopro-pylphosphino)-4-methylphenyl)-2,4,6-triisopropylanilide), as well as the preparation of their nickel(II) halide complexes [(PN ^iPr3)Ni(μ₂-Br)]₂ (1) and [(PN ^Me3)Ni(μ₂-Cl)]₂ (2). The combination of complex 1 with KO^tBu and Et₃SiH as the hydride source can promote the hydrosilylation of a variety of carbonyl compounds in excellent yield and in a catalytic manner. The reactivity for a scope of substrates and a plausible mechanism along the hydrosilylation reaction are discussed in this work.

Full text of DOI:10.1021/om801160j

2234
Organometallics 2009, 28, 2234–2243
Catalytic Hydrosilylation of the Carbonyl Functionality via a
Transient Nickel Hydride Complex
Ba L. Tran, Maren Pink, and Daniel J. Mindiola*
Department of Chemistry, Indiana UniVersity, Bloomington, Indiana 47405
ReceiVed December 6, 2008
We report the syntheses and characterization of two new sterically demanding chelate ligands, denoted
PNMe3 (N-(2-(diisopropylphosphino)-4-methylphenyl)-2,4,6-trimethylanilide) and PNiPr3 (N-(2-(diisopro-
pylphosphino)-4-methylphenyl)-2,4,6-triisopropylanilide), as well as the preparation of their nickel(II)
halide complexes [(PNiPr3)Ni(µ2-Br)]2 (1) and [(PNMe3)Ni(µ2-Cl)]2 (2). The combination of complex 1
with KOtBu and Et3SiH as the hydride source can promote the hydrosilylation of a variety of carbonyl
compounds in excellent yield and in a catalytic manner. The reactivity for a scope of substrates and a
plausible mechanism along the hydrosilylation reaction are discussed in this work.
inasmuch as disposal concerns of the heavier congeners have
prompted the search for alternative hydrosilylation catalysts such
as Ti(IV), Cu(I), Fe(II), and Zn(II).15,16,19-35 To our knowledge,
analogous hydrosilylation catalysts involving well-defined Ni(II)
precursors are exceedingly rare; however, there have been earlier
reports of nickel-mediated hydrosilylation of olefins,36 alkynes,37
and R,ꢀ-unsaturated aldehydes.38 Specifically, the few reduction
studies of the carbonyl moiety using nickel complexes often
invoke the generation of a nickel hydride species as the active
species for converting the carbonyl motif to an alcohol.39,40
Recently, Guan and co-workers have spectroscopically observed
1. Introduction
The reduction of the carbonyl functionality, specifically the
aldehyde and ketone groups, to alcohol via hydride transfer is
an important transformation in organic synthesis.1-3 The
application of stoichiometric quantities of main-group-metal
hydrides composed of boron and aluminum for the reduction
of aldehydes and ketones to alcohol is ubiquitous but is of
concern, due to waste production being generated when these
reactions are performed on larger scales. More importantly,
reduction of the carbonyl group followed by protection of the
alcohol moiety would appeal from a synthetic standpoint if such
a substrate is being used for subsequent multistep synthesis.
Consequently, metal-catalyzed hydrosilylation reactions con-
stitute a practical protocol in synthetic organic chemistry,
because both the reduction and the protection steps are
performed in a single, atom-efficient fashion. Traditionally,
catalytic hydrosilylation of the carbonyl functionality has been
employed with precious, heavy metals ranging from Re, Rh,
and Ru to Ir.4-14 However, the cost affiliated with these metals
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* To whom correspondence should be addressed. E-mail: mindiola@
indiana.edu.
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10.1021/om801160j CCC: $40.75
2009 American Chemical Society
Publication on Web 03/10/2009

Hydrosilylation Via a Transient Ni Hydride Complex
Organometallics, Vol. 28, No. 7, 2009 2235
the insertion of the carbonyl functionality across the nickel-hy-
premise for nickel(II), specifically a nickel(II) hydride, being a
viable candidate for carbonyl reduction catalysis.
1
dride bond by H NMR spectroscopy, using the catalyst [2,6-
(iPr2PO)2C6H3]NiH.41 In 1995, the group of Iyer and co-workers
reported catalytic homogeneous hydrogenations of ketones and
aldehydes to the corresponding alcohol using NiCl2(PPh3)2 and
NaOH in isopropyl alcohol.40 However, the conditions for the
system are somewhat inefficient, considering the high catalyst
loading needed (∼15 mol % NiCl2(PPh3)2) as well as prolonged
reaction times required in refluxing isopropyl alcohol (12-36
h). These reactions also require basic conditions, plausibly due
to the deprotonation step of the alcohol for subsequent trans-
metalation to the nickel center. Whether the isopropoxide
nickel(II) system presented by Iyer could be amenable to
ꢀ-hydride elimination to generate a nickel hydride is speculative
at present, but such a step seems intuitive to propose, on the
basis of the fact that alcohols containing ꢀ-hydrogens appear
to be indispensable for catalytic turnover. In their studies, it
was also observed that exchanging PPh3 in NiCl2(PPh3)2 for a
bidentate phosphine such as dppe (dppe ) Ph2PCH2CH2PPh2)
resulted in loss of catalytic activity. On the basis of this
observation, it was inferred that dppe might be coordinatively
saturating the metal center by confining it to a square-planar d8
configuration, whereas a triphenylphosphine ligand could dis-
sociate from the metal, thus exposing a vacant site for
subsequent binding of the substrate. These sets of observations
suggest that a low-coordination Ni(II) environment, possibly
three-coordinate,42-53 is a prerequisite for the catalytic hydro-
genation of the organic substrate. Aside from high catalyst
loadings and extended reaction times required by the
NiCl2(PPh3)2 hydrogenation system,40 such work laid the
In recent years, Cu(I), in the form of (IPr)CuOtBu (IPr )
N,N′-bis(2,6-diisopropylphenyl)imidazol-2-ylidene), has been
also shown to catalyze the hydrosilylation of the carbonyl
functionality in the presence of various silanes.23-25 Moreover,
Sadighi and co-workers have recently isolated the product of
the alkoxide-silyl σ-bond metathesis reaction, in the form of a
[(IPr)Cu(µ2-H)]2 dimer, by reacting the precursor complex
(IPr)Cu(OtBu) with stoichiometric triethoxysilane, (EtO)3SiH.
1
In addition, the byproduct, (EtO)3SiOtBu, was observed by H
NMR spectroscopy when the reaction was conducted in a
stoichiometric fashion, therefore confirming the role of the silane
in the σ-bond metathesis step.54 The surprising dearth of Ni(II)
systems in carbonyl reduction chemistry coupled with the
precedent of the NiCl2(PPh3)2 hydrogenation catalysis prompted
us to investigate hydrosilylation chemistry using a nickel(II)
system supported by a bidentate and sterically encumbering
amido-phosphine ligand. Expanding on original work by
Stryker17,18 with [(Ph3P)CuH]6 and H2 for the catalytic reduction
of R,ꢀ-unsaturated ketones, as well as a similar strategy reported
by Lipshutz19-22 and Nolan,23-25,55 we report a Ni(II) precursor
capable of conducting the hydrosilylation of ketones and
aldehydes, utilizing Et3SiH as the hydride source. In these
studies we have also detected a transient nickel hydride as the
active species mediating the catalytic hydrosilylation reaction,
via a series of independent reactions using various hydride
sources.
2. Results and Discussion
2.1. Synthesis of the PNMe3 (N-(2-(Diisopropylphosphino)-
4-methylphenyl)-2,4,6-trimethylanilide) and PNiPr3 (N-(2-
(Diisopropylphosphino)-4-methylphenyl)-2,4,6-triisopropy-
lanilide) Scaffold. Recently, Liang and Peters reported
amido-phosphine ligands for the stabilization of late-transi-
tion-metal complexes composed of Ni(II), Zn(II), Cu(I), and
Ag(I).56-58 In their synthesis, the incorporation of the asym-
metric diarylamine skeleton involved a Buchwald-Hartwig
C-N cross-coupling step. Nucleophilic aromatic substitution
of the fluoride group on the diarylamine backbone with
potassium and/or lithium salts of diphenylphosphide (-PPh2)
and diisopropylphosphide (-PiPr2), respectively, elicited forma-
tion of the amidophosphine ligand. Unfortunately, it was
observed by us that incorporation of the phosphide group onto
the aryl moiety becomes problematic when the phosphide salt
in question was both electron rich and sterically demanding.
Hence, the caveat to their scaffold synthesis was restricted to
the installation of large peripheral groups on either the P or N
site, but not both. As a result, the latter limitation along with
the long reaction times needed to complete the assembly of these
interesting ligands led us to adopt a different synthetic approach
to prepare similar monoanionic amidophosphine ligands, but
having encumbering groups at both the N and P sites. Moreover,
a bulky scaffold is desirable for the purpose of generating low-
coordinate nickel complexes, on the basis of the observation
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2236 Organometallics, Vol. 28, No. 7, 2009
Scheme 1. Synthetic Outline of the PN Ancillary Liganda
Tran et al.
i
a
Legend: (a) Pd(OAc)2, 1,1′-bis(diphenylphosphino)ferrocene (DPPF), sodium t-pentoxide, toluene, reflux, 4 days (R ) Me, 72%; Pr, 72%); (b)
N-bromosuccinimide (NBS), acetonitrile, 12 h, 0-30 °C (R ) Me, 94%; Pr, 92%); (c) (1) BuLi (2 equiv), Et2O, -78 °C, 3 h; (2) ClPiPr2, 48 h;
i
n
(3) degassed water with Ar.
that the choice of ligand is critical for catalytic activity by a
Ni(II) precursor (vide supra).40
produced crude Li(PNAr) salts. To prepare pure lithium salts,
however, the crude product must be quenched slowly with
meticulously deoxygenated water under an inert atmosphere.
Subsequent workup of the mixture and drying of the viscous
orange oil allows for isolation of the crude, free base amino-
phosphines (PNMe3)H and (PNiPr3)H. Mass spectrometry of
(PNMe3)H and (PNiPr3)H yielded the parent ion peaks at 341.00
and 426.33 [M+], respectively. It is worth emphasizing that
during quenching of the lithio salts and workup, to afford the
aminophosphines (PNMe3)H and (PNiPr3H, the water must be
completely deoxygenated with an argon purge for at least 30-50
min to avoid formation of aminophosphine oxide ligand. Under
an inert atmosphere, deprotonation of the aminophosphines
(PNMe3)H and (PNiPr3)H, in thawing pentane, with 1 equiv of
nBuLi resulted in formation of the bright yellow lithio salts
Li(PNMe3) and Li(PNiPr3). Isolation of pure lithium salt can be
accomplished either by precipitation from pentane, in the case
of Li(PNMe3), or by formation of the THF adduct followed by
trituration with cold hexane, in the case of Li(PNiPr3), to form
(THF)2Li(PNiPr3) (see the Experimental Section, Scheme 2).
Compounds Li(PNMe3) and (THF)2Li(PNiPr3) have been fully
characterized by multinuclear (1H, 13C, 31P) NMR spectroscopy
and high-resolution mass spectrometry. The isopropylmethyl
The synthesis of the PNAr scaffold, where Ar ) 2,4,6-R3C6H2
i
(R ) CH3, Pr), involved a synthetic modification from the
pincer ligand PNP (PNP ) N[2-P(CHMe2)2-4-MeC6H3]2) pro-
cedure devised by Ozerov and co-workers (Scheme 1) and
utilized extensively in our laboratory.59-68 Our choice of Ar )
2,4,6-R3C6H2 is ideal for steric protection of the anilide moiety,
while the incorporation of the tolyl group allows for construction
of a rigid backbone. Preparing the free bases (PNMe3)H and
(PNiPr3)H involved first a Buchwald-Hartwig C-N coupling
reaction of commercially available aniline with the appropriate
aryl bromide (see the Experimental Section). Having the para
positions blocked at both aryl moieties results in smooth ortho
bromination utilizing N-bromosuccinimide (NBS), in acetoni-
trile, to afford the desired bromo-containing precursor N-(2-
bromo-4-methylphenyl)-2,4,6-triisopropylanilide and N-(2-
bromo-4-methylphenyl)-2,4,6-trimethylanilide in 92% and 94%
isolated yields, respectively (Scheme 1). Lithiation of the latter
n
substrates with 2 equiv of BuLi in a frozen ethereal solution
under N2, followed by quenching of the mixture with ClPiPr2,
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groups for both lithium salts of Li(PNMe3) and (THF)2Li(PNiPr3
)
are diastereotopic, as evinced by their 1H NMR spectra.
Moreover, the two coordinated THF molecules in
(THF)2Li(PNiPr3) are easily identified and integrated at 3.19 and
1.21 ppm. The 31P{1H} spectra of Li(PNMe3
)
and
(THF)2Li(PNiPr3) display chemical shifts at -13.2 and -4.5
ppm, respectively.
2.2. Synthesis of Dinuclear Nickel(II) Halide Precursors
Supported by the Sterically Encumbering PNAr Scaffold. The
treatment of 1.1 equiv of anhydrous NiBr2 with
(THF)2Li(PNiPr3) in THF initially produced a dark green
solution, which upon stirring overnight at 70 °C resulted in
the isolation of a dark red solid product subsequent to
filtration and removal of all volatiles. Crystallization of the

Hydrosilylation Via a Transient Ni Hydride Complex
Organometallics, Vol. 28, No. 7, 2009 2237
Scheme 2. Lithiation of the Free Ligand and Synthesis of the Nickel Complexes 1 and 2
crude material from slow evaporation of hexane at room
temperature under an inert atmosphere gave the complex
lanilide)reportedbyLiangandco-workers.57TheP(1)-Ni(1)-Br(1)
angle is 91.35(2)°, while the P(1)-Ni(1)-Br(1A) angle is
176.57(2)°, consistent with a Ni(II) system confined in a
square-planar environment. A complete description of the
crystallographic metric parameters for the structure of
compound 1 is presented in the Experimental Section. The
preparation of the closely related complex [(PNMe3)Ni(µ2-
Cl)]2 (2) was accomplished analogously to that of 1, in 75%
isolated yield, by reacting Li(PNMe3) with NiCl2(THF)1.5 in
THF over 12 h at room temperature. The purification of 2
involved the removal of the THF solvent from the mixture
via reduced pressure, and the red solids were extracted into
toluene and filtered through a Celite-filled medium-porosity
frit to remove unreactive NiCl2(THF)1.5 and salt side product.
Subsequently, the toluene solvent was evaporated from the
filtrate by reduced pressure, affording red solids, which were
extracted into hexane and stored at -37 °C to precipitate
1
[(PNiPr3)Ni(µ2-Br)]2 (1) in 70% yield (Scheme 2). The H
NMR spectrum of 1 in C6D6 was consistent with a square-
planar Ni(II) diamagnetic species likely arising from bridging
of the bromide ligand. The 31P NMR spectrum of 1 displayed
a broad resonance at 63.9 ppm (∆ν1/2 ) 95.8 Hz), which is
shifted significantly downfield in comparison to the sharp
resonance at -4.5 ppm for (THF)2Li(PNiPr3) and -16.6 ppm
for the free base (PNiPr3)H. Moreover, X-ray diffraction
analysis of single crystals of 1 grown from the evaporation
of toluene at ambient temperature confirmed the degree and
mode of aggregation as well as the geometry about each
nickel(II) center. Accordingly, the solid-state structure of 1
features two square-planar nickel centers bridged by the
bromide ligands (Figure 1). The phosphine and triisopropy-
laryl groups are oriented anti to one another in order to
minimize steric congestion. The Ni-Namido and Ni-P bond
distances are 1.8853(19) and 2.1391(6) Å, respectively, and
are within range of the expected distances for these donor
atoms in comparison to the complex [(PNiPr2)NiCl(PMe3)]
(PNipr2 ) N-(2-(diphenylphosphino)phenyl)-2,6-diisopropy-
1
pure material. The H and 31P NMR spectra of 2 are also
consistent with a square-planar complex essentially analogous
to 1. Due to the fact that complexes 1 and 2 are dimers in
the solid state, and since the focus of this study is to generate
unsaturated Ni(II) fragments, only the catalytic proficiency
of the more sterically congested complex, 1, will be presented
herein.
2.3. Catalytic Hydrosilylation Studies with Complex 1.
When complex 1 is combined with 2 equiv of KOtBu, the
mixture can mediate the catalytic hydrosilylation of carbonyl
functionalities such as aldehydes and ketones to the silyl ether
products at 100 °C when Et3SiH is used as a coreagent. No
catalysis is observed in the absence of silane. Hydrosilylation
1
reactions were monitored by H NMR spectroscopy in C6D6
by following the decay of the aldehyde resonance with
concomitant growth of the methylene protons of the silyl ether
product. In the case of ketones, the reaction was monitored by
the growth of the proton located on the R-carbon with respect
to the silyl ether moiety.
We examined hydrosilylation catalysis of benzaldehyde with
2 mol % of 1 in benzene or toluene at 100 °C and in the presence
of KOtBu (4 mol %) and triethylsilane (1.2 equiv with respect
to the carbonyl compound) to afford the desired silyl ether
product PhCH2OSiEt3. Alternatively, complex 1 can be treated
with 2 equiv of KOtBu and utilized for subsequent catalysis by
addition of the silane and aldehyde. The hydrosilylation reaction
was also performed in d5-bromobenzene, with benzaldehyde
under the same catalytic conditions, producing results identical
with those observed in benzene as a solvent, thus indicating
that the solvent polarity essentially did not play a significant
role in this transformation. A compiled list of substrates along
with the reaction time, yield of products, turnover number, and
turnover frequencies is presented in Table 1. The library of
substrates was chosen to probe for degree of tolerance, for
halogen and amino groups on the aryl, for heteroatom-containing
heterocycles, as well as differences in electronic and steric
Figure 1. Molecular structure of 1 with 50% thermal ellipsoids.
The structure of 1 is depicted in its entirety, with hydrogen omitted
for clarity (top). The bottom drawing displays a simplified version
of 1 showing only first- and second-coordination-sphere atoms.
Selected bond lengths (Å) and angles (deg) for 1: Ni(1)-N(1),
1.8853(19); Ni(1)-P(1), 2.1391(6); Ni(1)-Br(1), 2.3337(4);
Ni(1)-Br(1A),2.3949(4);N(1)-Ni(1)-P(1),85.98(6);N(1)-Ni(1)-Br(1),
176.85(6); N(1)-Ni(1)-Br(1A), 96.44(6); P(1)-Ni(1)-Br(1),
91.35(2); P(1)-Ni(1)-Br(1A), 176.57(2).

2238 Organometallics, Vol. 28, No. 7, 2009
Tran et al.
Table 1. Catalytic Hydrosilylation Conditions using Complex 1 as
the Precatalyst
for reaction mixtures diluted in solvents such as benzene and
toluene. For most substrates, the reaction is rapid and complete
within 15 min to 4 h. The successful hydrosilylations of
2-fluorobenzaldehyde (entry 2) and 4-chlorobenzaldehyde (entry
3) indicate that the catalyst, under our reaction conditions, can
tolerate these functional groups on the aromatic ring. However,
placing an electron-donating group in the para position of the
aryl ring slowed the reduction significantly. For instance, the
electron-rich 4-(dimethylamino)benzaldehyde substrate (entry
4) required 31 h for completion, in comparison to the values
observed in entries 2 and 3. This difference in reactivity may
be due to competitive binding to the metal center from the
dimethylamino and aldehyde groups. Despite this, the donating
nature of the NMe2 moiety in the para position can allow for a
resonance structure whereby the aldehyde oxygen becomes more
nucleophilic than it generally is. This feature might explain why
the dimethylamino group does not appear to poison the catalyst.
A similar observation was previously reported for copper(I)-
catalyzed hydrosilylations.55 The substrate 2,6-dimethylbenzal-
dehyde (entry 5) and benzophenone (entry 6) were both
examined to probe the effect of steric about the carbonyl
functionality on the rate of the reaction. Not surprisingly, both
substrates required longer reaction times, ∼2 h, thereby produc-
ing a moderate yield of the silyl ether product. When an
aliphatic-based aldehyde, trimethylacetaldehyde (entry 7), was
subjected to the established catalytic conditions, the expected
silyl ether along with the ether (tBuCH2)2O were formed in the
1
reaction mixture (inferred by H NMR spectroscopy and GC-
MS). Similar products have been previously obtained in the
hydrosilylation of isopropylaldehyde with cationic mono-
oxo-rhenium(V) salen complexes utilizing Et3SiH as the
hydride source, in which the iPrCH2OCH2Pri ether product was
obtained as the major product.5 In their studies, it was found
i
that PrCH2OSiMePh2 could be produced free of the ether
i
product, PrCH2OCH2Pri, by varying the silane source from
Et3SiH to the bulkier Ph2MeSiH compound. In our system, it
was determined that one of the byproducts, (tBuCH2)2O, could
be suppressed by performing the reaction at room temperature
t
for 4 h to afford the silyl ether product, BuCH2OSiEt3, in
quantitative yield. The mechanism of formation of the ether is
presently unclear; however, Olah and co-workers have observed
the reductive condensation of carbonyl compounds and alkox-
ysilanes to produce ether products.69 Unfortunately, substrates
containing good donor motifs such as 2-pyridinecarboxaldehyde
(entry 8) and 3-thiophenecarboxaldehyde (entry 9) inhibit the
catalytic reaction, whereby no traces of product are detected
by NMR spectroscopy. It is very likely that these substrates
coordinatively saturate the catalyst by forming a square-planar
nickel complex upon migratory insertion of the hydride into
the carbonyl group. Consistent with our observations, Kuhl and
co-workers have also noted unsuccessful hydrogenation reac-
tions of the pyridyl-containing substrate N-benzylidene-1-
(pyridin-2-yl)methanamine, with a Ni(0)/IMes (IMes ) N,N′-
bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) system utilizing
Et2CHONa as a hydrogen donor.70 As our final substrate, the
reduction of a aliphatic-substituted ketone such as cyclohex-
anone (entry 10) required a longer reaction time of 2.5 h and
produced product in moderate yield (70%). Unfortunately,
attempts to perform the selective hydrosilylation of a vinyl
ketone such as 2-cyclohexen-1-one yielded a mixture of
a All products were analyzed and confirmed by 1H NMR
spectroscopy and GC-MS. Unless otherwise noted, yields are from
isolated product from column chromatography employing pentane as the
eluent. All yields are an average of two runs. b The reaction was
performed at room temperature. The yield was determined by H NMR
spectroscopy. TON ) turnover number ((mol of product) (mol of
cat.)-1), TOF ) turnover frequency ((mol of product) (mol of cat.)-1
h-1), and NR ) no reaction.
c
1
(69) Sassaman, M. B.; Kotian, K. D.; Prakash, G. K. S.; Olah, G. A. J.
Org. Chem. 1987, 52, 4314–4319.
(70) Kuhl, S.; Schneider, R.; Fort, Y. Organometallics 2003, 22, 4184–
4186.
factors. On the basis of our compiled data, it was observed that
substrates which are liquids can be hydrosilylated in the absence
of solvent media to afford product in yields comparable to those

Hydrosilylation Via a Transient Ni Hydride Complex
Organometallics, Vol. 28, No. 7, 2009 2239
intractable materials. We have also determined the turnover
number (TON) and the turnover frequency (TOF) of the
hydrosilylation reaction mediated by complex 1, which is
presented in Table 1. The TON and TOF values obtained are
moderate in comparison to those for the reported Cu and Rh
systems.24 Furthermore, the high temperature (100 °C) utilized
in our system renders it a marginal catalyst when considering
the Cu and Rh systems.
and R ) Ph, iPr, Cy.75 Attempts to isolate and fully characterize
3 have been hampered by its gradual decomposition as well as
the presence of other impurities in the reaction mixture. Hence,
our inability to isolate 3 might stem from its lack of stability.
A recent report by Limberg describes the isolation of [(ꢀ-
diketiminato)Ni(µ2-H)]2 ([ArNC(CH3)]2CH-, Ar
) 2,6-
iPr2C6H3) and the crystal structure along with the innate
reactivity of the compound.76 The complex is extremely
sensitive to external stimulus, including heat, solvent, and Lewis
base (4-dimethylaminopyridine, propionitrile), leading to the
reduction of the Ni(II) center to Ni(I) via elimination of H2,
2.4. Proposed Mechanism for Catalytic Hydrosilylation
of Aldehydes and Ketones by Precursor 1. It was assessed
that no catalytic activity was observed under similar conditions
in the absence of 1, therefore implying that the nickel complex
must be necessary for hydrosilylation and catalytic turnover.
The use of KOtBu as an alkoxide source is also critical in the
reaction medium, since compound 1 alone failed to promote
hydrosilylation of the carbonyl group (i.e., benzaldehyde) in
the presence of triethylsilane. In fact, treatment of 1 with
stoichiometric triethylsilane in the absence of carbonyl substrate
does not result in a reaction. Therefore, we propose that the
transient “(PNiPr3)Ni(OtBu) (A)” must be generated along the
reaction coordinate, followed by exchange of the alkoxide for
hydride with triethylsilane, a process that is now readily
observed in copper-based hydrosilylation catalysis.19-26,54,55
This proposal is further corroborated by the observation that
red 1 undergoes a rapid color change to green when treated
with 2 equiv of KOtBu. Unfortunately, we have been unable to
isolate the putative A from independent and stoichiometric
reactions involving 1 and 2 equiv of KOtBu. Likewise, we have
also been unable to cleanly isolate the hypothetical nickel
hydride species “(PNiPr3)NiH” presumably being generated in
the course of the reaction. In fact, attempts to produce the
nickel(II) hydride by independent routes such as oxidative
addition of the N-H bond in PNiPr3H by Ni(COD)2 have been
hampered by formation of multiple products. We do, however,
observe formation of a nickel hydride in solution (albeit not in
1
which was detected by H NMR spectroscopy. Furthermore,
the subjection of [(ꢀ-diketiminato)Ni(µ2-H)]2 to heat in hexane
led to the formation of a nickel mirror with detectable protonated
ligand and unidentified paramagnetic products.76
To probe for whether a hydride was the active species along
the catalytic cycle, we explored an independent synthesis to such
a complex while subjecting the mixture to the appropriate
carbonyl substrate. Accordingly, it was found that treatment of
complex 1, with 2 equiv of NaHB(OMe)3 or NaHBEt3 and in
the presence of benzaldehyde and triethylsilane, effectively
promoted the catalytic hydrosilylation of the aldehyde. Further-
more, the combination of NaHB(OMe)3 and triethylsilane with
benzaldehyde, in the absence of 1, failed to yield hydrosilylated
product, thus implying that a mild Lewis acid such as B(OMe)3
could not be responsible for the observed catalysis or reduction
of the carbonyl group. As a result, our control reactions strongly
endorse the possibility of a transient nickel hydride complex,
3, being the responsible species for the reduction the carbonyl
group. At present, we cannot discard the possibility of 3
dissociating into the more reactive three-coordinate monomer
“(PNiPr3)NiH” during the catalytic cycle. Whether the carbonyl
substrate or solvent promotes dissociation in 3 is speculative at
present and we have been unable to trap the monomer with
Lewis bases such as phosphines and pyridines.
1
pure form), on the basis of a combination of H and 31P NMR
The unsuccessful reduction of the substrates in entries 8 and
9 (Table 1) and the hypothesis that a low-coordinate nickel
hydride might be responsible for catalytic activity prompted us
to investigate if the vacant site was necessary for hydrosilylation
to proceed. Accordingly, we turned our attention to the known
complex (PNP)NiH, originally reported by Ozerov and co-work-
ers,60,74 since this species is a d8 square-planar species and,
therefore, a coordinatively saturated hydride. The treatment of
2 mol % of (PNP)NiH with benzaldehyde and Et3SiH did not
result in formation of the silyl ether product, PhCH2OSiEt3,
despite heating the mixture at 100 °C for 12 h. Monitoring the
stoichiometric reaction of (PNP)NiH and benzaldehyde by 31P
NMR spectroscopy revealed the starting material (PNP)NiH (31P
NMR 56.7 ppm) along with traces of a minor byproduct at 33.7
spectral data for reactions conducted in situ using the same types
of reagents involved in the catalytic reactions. Accordingly,
treating 1 with 2 equiv of KOtBu (15 min) followed by addition
of a stoichiometric amount of triethylsilane in C6D6 over 1 h at
room temperature revealed a diagnostic triplet resonance in the
1H NMR spectrum at -20.66 ppm (JP-H ) 18 Hz). The 31P{1H}
NMR spectrum of the reaction mixture also evinced a new
singlet at 54.7 ppm, recorded in d6-benzene at 25 °C. The small
JP-H value suggests a hydride bridge dimer where the hydride
is likely oriented cis with respect to the two phosphine
residues.71 We therefore tentatively assign this resonance to the
hydride dimer [(PNiPr3)Ni(µ2-H)]2 (3). Slightly larger JP-H values
of 22-24 Hz have been observed in the hydride-bridged Ni(I)
dimers [(R2PCH2CH2PR2)Ni(µ2-H)]2 (R ) iPr, tBu) reported by
Jones72 and Porschke.73 Likewise, our JP-H value and multiplic-
ity negate the possibility of 3 being a monomer in solution,
since the hydride monomer, (PNP)NiH,60,74 displays a much
higher JP-H value (triplet, 63.3 Hz). In addition, Liang has
reported a series of analogous (R-PNP)NiH complexes with
similar JP-H values, where the PNP core is [N(o-C6H4PR2)2]-
1
ppm. Interestingly, the H NMR spectrum of this mixture did
not display the expected methylene protons for the anticipated
insertion product (PNP)Ni(OCH2Ph) (4). The distribution of
products did not change, even with heating at 100 °C overnight,
as the majority of (PNP)NiH remained unreacted. To assess if
the minor product formed from the reaction of (PNP)NiH and
benzaldehyde was the insertion product 4, we independently
prepared such a species from NaOCH2Ph and (PNP)NiCl in
diethyl ether at room temperature in 90% isolated yield. The
1H NMR spectrum for complex 4 reveals salient spectroscopic
features for the methylene protons at 4.55 ppm (2 H) as well as
(71) Crabtree, R. H. The Organometallic Chemistry of the Transition
Metals; Wiley: Hoboken, NJ, 2005.
(72) Vicic, D. A.; Jones, W. D. J. Am. Chem. Soc. 1997, 119, 10855–
10856.
(73) Bach, I.; Goddard, R.; Kopiske, C.; Seevogel, K.; Prschke, K.-R.
Organometallics 1999, 18, 10–20.
(74) Adhikari, D.; Mossin, S.; Basuli, F.; Dible, B. R.; Chipara, M.;
Fan, H.; Huffman, J. C.; Baik, M.-H.; Meyer, K.; Mindiola, D. J. Inorg.
Chem. 2008, 47, 10479–10490.
(75) Liang, L.-C.; Chien, P.-S.; Lee, P.-Y. Organometallics 2008, 27,
3082–3093.
(76) Pfirrmann, S.; Limberg, C.; Ziemer, B. Dalton Trans. 2008,
Advance Articles.

2240 Organometallics, Vol. 28, No. 7, 2009
Tran et al.
1H and 31P NMR spectra for hydride 3 suggests this species to
retain a dimeric form in solution (unless equilibration to a
monomer results in a paramagnetic species that we cannot
detect), therefore hinting that the substrate (carbonyl moiety)
might be responsible for breaking of the dimer.
Scheme 3. Proposed Catalytic Cycle for the Hydrosilylation
of the Carbonyl Functionality Mediated by [(PNiPr3)Ni(µ2-Br)]2
(1) with KOtBu and Et3SiH
3. Conclusions
In addition to developing a convenient route to sterically
demanding amidophosphine ligands, we have shown that a
nickel(II) complex supported by the monoanionic bidentate
PNiPr3 scaffold is capable of mediating catalytic hydrosilylation
on a variety of carbonyl-containing substrates, when combined
with common reagents such as KOtBu and Et3SiH. Our studies
indicate that a nickel(II) hydride is likely generated, which then
undergoes migratory insertion of the carbonyl group to generate
the corresponding alkoxide. The metathesis of the alkoxide
group for the hydride of the silane results in the expulsion of
the silyl ether product, thereby closing the hydrosilylation cycle.
Reactivity studies indicated that aromatic aldehydes are more
efficiently reduced than the aliphatic analogues or aliphatic and
aromatic ketones. Our catalyst is also tolerant of the aryl halide
containing functionality; however, donor groups such as (dim-
ethylamino)aryl reduce the rate of conversion, while pyridine-
and thiophene-containing substrates completely inhibit catalytic
turnover. Interestingly, when the square-planar complex (PN-
P)NiH was employed as the catalyst, no turnover was observed,
even under forcing conditions. Through a series of control
experiments we demonstrated that the migratory insertion step
of the carbonyl functionality can be inhibited when the nickel(II)
hydride is confined to a chelate-enforced square-planar environ-
ment. We are currently exploring the chemistry of the reactive
intermediate 3, since such a species might undergo other
insertion chemistry at the hydride ligand as well as reductive
elimination reactions to generate transient “(PNiPr3)Ni” fragments.
a singlet resonance at 25.80 ppm in the 31P{1H} NMR spectrum.
Therefore, the spectroscopic characterization data for 4 exclude
this species as the minor resonance observed at 33.74 ppm (vide
supra). Surprisingly, it was found that treatment of complex 4
with Et3SiH at 100 °C over 12 h afforded the compound
(PNP)NiH quantitatively. Formation of (PNP)NiH from 4 may
result from alkoxide for hydride metathesis; however, a ꢀ-hy-
dride elimination pathway cannot be ruled out, since thermolysis
of 4 at 100 °C over 12 h revealed the presence of (PNP)NiH
and 4. Moreover, quantitative conversion 4 to (PNP)NiH was
accomplished after extended periods (72 h at 100 °C). These
combination of results suggest that both σ-bond metathesis as
well as ꢀ-hydride elimination, to generate (PNP)NiH, can occur
in this type of system. On the basis of these results, we propose
that the migratory insertion step is likely forbidden in a complex
such as (PNP)NiH, due to the lack of a vacant site for binding
of the carbonyl group in a substrate such as benzaldehyde (i.e.,
filled d(z2)). However, Liang and co-workers have observed the
insertion of unactivated olefins (ethylene, 1-hexene, and nor-
bornene) into an analogous (PNPPh)NiH complex (PNPPh ) N[2-
PPh2-phenyl]2-) is influenced by the substituents on the phos-
phine.71 For example, all unactivated olefins reacted with
(PNPPh)NiH, but no reaction was detected with the more
sterically congested and electron-rich PNP derivative (PN-
PiPr)NiH (PNPiPr ) N[2-PiPr2-phenyl]2), even at elevated tem-
peratures. Therefore, the lack of reactivity between (PNP)NiH
and benzaldehyde is not simply due to the availability of a
vacant site but also a combination of steric congestion and/or
lability of the Ni-P bond.
On the basis of all these results, the most plausible catalytic
cycle for the Ni(II)-mediated hydrosilylation of aldehydes and
ketones using 1 as a precatalyst, and in the presence of KOtBu
and Et3SiH, involves first the in situ generation of a nickel
alkoxide intermediate, A. Regardless of whether A is a dimer
or monomer, this complex converts to the hydride 3 by σ-bond
metathesis with Et3SiH concurrent with elimination of the silyl
ether Et3SiOtBu, which has been observed via a stoichiometric
reaction. Our 1H NMR spectroscopic data are in accord with 3
being a dimer in solution, and we currently do not know if such
a species is necessarily the active state of the catalyst. Coordina-
tion of the carbonyl group of OdCRR′ into the nickel-hydride
bond in 3 forms the transient adduct (PNiPr3)Ni(H)(OdCRR′),
which then undergoes migratory insertion to produce a hypo-
thetical alkoxide intermediate complex (PNiPr3)Ni(OCHRR′).
The latter would then undergo a σ-bond metathesis reaction with
triethylsilane to close the catalytic hydrosilylation cycle by re-
forming 3 and the silyl ether product (Scheme 3). Our solution
4. Experimental Section
4.1. General Considerations. Unless otherwise stated, all
operations were performed in an M. Braun Laboratory Master
double-drybox under an atmosphere of purified nitrogen or using
high-vacuum standard Schlenk techniques under a nitrogen atmo-
n
n
sphere. Anhydrous hexane, pentane, toluene, and benzene were
purchased from Aldrich in Sure-Seal reservoirs (18 L) and dried
by passage through two columns of activated alumina and Q-5.
Diethyl ether and CH2Cl2 were dried by passage through a column
of activated alumina. THF was distilled, under nitrogen, from purple
sodium benzophenone ketyl and stored under sodium metal.
Distilled THF was transferred into an inert atmosphere using
vacuum reservoirs. C6D6 and CDCl3 were purchased from Cam-
bridge Isotope Laboratory (CIL), degassed, and vacuum-transferred
to 4 Å molecular sieves. Celite, alumina, and 4 Å molecular sieves
were activated under vacuum overnight at 200 °C. All other
1
chemicals were used as received unless otherwise stated. H, 13C,
and 31P NMR spectra were recorded on Varian 300 and 400 MHz
NMR spectrometers. 1H and 13C NMR chemical shifts are reported
with reference to solvent resonances of C6D6 at 7.15 and 128.0
ppm, respectively. 31P NMR chemical shifts are reported with
respect to external H3PO4 (aqueous solution, δ 0.0 ppm). Mass
spectrometry experiments were performed on an Agilent 6890N
gas chromatograph equipped with a 5973 inert mass selective
detector under PC control with ChemStation software at the Indiana
University Chemistry Department Mass Spectrometry Facility.
X-ray diffraction data were collected on an APEX II Kappa Duo
(Bruker) system under a stream of N2(g) at low temperatures.

Hydrosilylation Via a Transient Ni Hydride Complex
Organometallics, Vol. 28, No. 7, 2009 2241
NiCl2(THF)1.5,77 N-(2,4,6-trimethylphenyl)-4-methylaniline, N-(2,4,6-
trimethylphenyl)-2-bromo-4-methylaniline, and 1-bromo-2,4,6-tri-
isopropylbenzene were prepared according to published proce-
dures.78,79 The syntheses of (PNP)NiCl and (PNP)NiH were
reported elsewhere.60,74 NaOCH2Ph was prepared from sodium
metal and benzyl alcohol.80
300 MHz, CDCl3): δ 7.28 (s, 1H, Ar-H), 7.04 (s, 2H, Ar-H), 6.80
(d, JH-H ) 8 Hz, 1H, Ar-H), 6.07 (d, JH-H ) 8 Hz, 1H, Ar-H),
5.46 (br s, 1H, NH), 3.05 (m, 2H, ArCH(CH3)2), 2.91 (m, 1H,
ArCH(CH3)2), 2.20 (s, 3H, ArCH3), 1.28 (d, JH-H ) 7 Hz, 6H,
ArCH(CH3)2)), 1.17 (d, JH-H ) 7 Hz, 6H, ArCH(CH3)2), 1.10 (d,
JH-H ) 7 Hz, 6H, ArCH(CH3)2). 13C{1H} NMR (25 °C, 100 MHz,
CDCl3): δ 148.4 (Ar-C), 147.8 (Ar-C), 143.4 (Ar-C), 133.2 (Ar-
C), 129.4 (Ar-C), 128.0 (Ar-C), 122.4 (Ar-C), 113.12 (Ar-C), 109.1
(Ar-C), 34.9 (ArCH(CH3)2), 28.9 (ArCH(CH3)2), 25.3 (ArCH-
(CH3)2), 24.7 (ArCH(CH3)2), 23.8 (ArCH(CH3)2), 20.6 (ArCH3).
GC-MS (m/z): calcd for C22H30BrN 387.17, found 387.00.
4.2. N-(2,4,6-Triisopropylphenyl)-4-methylaniline. Inside the
glovebox, a mixture of Pd(OAc)2 (80.0 mg, 0.36 mmol) and DPPF
(391 mg, 0.71 mmol) were stirred in 300 mL of toluene in a 500
mL two-neck round-bottom flask for 0.5 h, generating an orange
solution. The round-bottom flask was removed from the glovebox
and capped with a rubber septum. Under a high-pressure flow of
nitrogen, to this flask was added 1-bromo-2,4,6-triisopropylbenzene
(10.0 g, 35.3 mmol), p-toluidine (4.54 g, 42.4 mmol) in toluene
via syringe, and solid sodium t-pentoxide (6.54 g, 59.4 mmol),
respectively. The reaction flask was connected to a reflux condenser,
and the mixture was heated to reflux at 120 °C for 4 days under a
nitrogen atmosphere. After the reaction mixture was cooled to room
temperature, the mixture was filtered through a tall silica pad (4
cm) in a medium-porosity frit. The solvent from the filtrate was
removed under reduced pressure to obtain a crude dark brown solid.
The resulting crude product was extracted into 200 mL of
dichloromethane and washed three times with 250 mL of water
and once with concentrated brine solution. The CH2Cl2 solution
was dried with anhydrous sodium sulfate, the solution was filtered,
and the filtrate was dried under reduced pressure to form an orange
oil. To the oil was added 75 mL of hexane, and the solution was
filtered through a tall silica pad (4 cm) in a medium-porosity frit
and the silica was washed with copious amounts of hexane. All
volatiles were removed under reduced pressure to afford beige
crystalline solids. Yield: 72% (7.87 g, 25.4 mol). 1H NMR (25 °C,
300 MHz, CDCl3): δ 7.04 (s, 2H, Ar-H), 6.95 (d, 2H, JH-H ) 8
Hz, Ar-H), 6.41 (d, JH-H ) 8 Hz, 2H, Ar-H), 4.93 (br s, 1H, NH),
3.17 (m, 2H, ArCH(CH3)2), 2.92 (m, 1H, ArCH(CH3)2), 2.23 (s,
3H, ArCH3), 1.29 (d, JH-H ) 7 Hz, 6H, ArCH(CH3)2), 1.14 (d,
JH-H ) 7 Hz, 12H, ArCH(CH3)2). 13C{1H} NMR (25 °C, 100 MHz,
CDCl3): δ 147.5 (Ar-C), 147.3 (Ar-C), 146.3 (Ar-C), 133.4 (Ar-
C), 129.9 (Ar-C), 126.7 (Ar-C), 121.9 (Ar-C), 113.17 (Ar-C), 34.5
(ArCH(CH3)2), 28.5 (ArCH(CH3)2), 24.4 (ArCH(CH3)2), 24.2
(ArCH(CH3)2), 20.7 (ArCH3). GC-MS (m/z): calcd for C22H31N
309.25, found 309.00.
4.4. Lithium N-(2-(Diisopropylphosphino)-4-methylphenyl)-
2,4,6-trimethylaniline (Li(PNMe3)). To a solution of N-(2,4,6-
trimethylphenyl)-2-bromo-4-methylaniline (10.0 g, 32.8 mmol) in
150 mL of diethyl ether in a 250 mL round-bottom flask was added
nBuLi (41.0 mL, 65.7 mmol) dropwise via syringe at -190 °C.
The mixture was warmed to room temperature over a period of
3 h. After this time, the solution was frozen at -190 °C, and to the
thawing solution was added chlorodiisopropylphosphine (5.20 mL,
32.8 mmol) via syringe, resulting in a red solution that was stirred
for 48 h. Upon completion, the flask was sealed with a rubber
stopper and removed from the glovebox, whereupon the mixture
was quenched with 5 mL of carefully degassed water via syringe
and stirred at room temperature for 0.5 h, generating a bright yellow
solution. The solution was diluted with 50 mL of diethyl ether and
washed twice with water and once with concentrated brine solution.
The ethereal layer was removed under reduced pressure to afford
an orange oil, which was dissolved in 100 mL of hexane and dried
with Na2SO4 for 0.5 h. The hexane solution was filtered through a
tall silica bed (4 cm) in a medium-porosity frit to obtain a faint
yellow filtrate. Removal of all volatiles produced a yellow oil, which
was taken into the glovebox for subsequent deprotonation. Ac-
cordingly, treating the yellow oil with nBuLi (20.5 mL, 2.8 mmol)
at -37 °C in 100 mL of pentane and stirring the solution at room
temperature over 8 h resulted in formation of a yellow precipitate.
The product was collected via a medium-porosity frit and washed
with 25 mL of cold pentane to isolate Li(PNMe3). Yield: 65% (7.43
1
g, 21.3 mmol). H NMR (25 °C, 300 MHz, C6D6): δ 6.98 (dd,
JH-H ) 8 Hz, 1H, Ar-H), 6.90 (s, 2H, Ar-H), 6.78 (dd, JH-H ) 8
Hz, 1H, Ar-H), 5.58 (dd, JP-H ) 8 Hz 1H, Ar-H), 2.24 (s, 3H,
ArCH3), 2.22 (s, 3H, ArCH3), 2.09 (s, 6H, ArCH3), 1.98 (m, 2H,
PCH(CH3)2), 1.12 (dd, JP-H ) 7 Hz, 6H, PCH(CH3)2), 1.03 (dd,
JP-H ) 7 Hz, 6H, PCH(CH3)2). 31P{1H} NMR (25 °C, 121 MHz,
C6D6): δ -13.1 (s). GC-MS (m/z): calcd for C22H32NP 341.23,
found for C22H32NP 341.00.
4.3. N-(2,4,6-Triisopropylphenyl)-2-bromo-4-methylaniline.
N-(2,4,6-triisopropylphenyl)-4-methylaniline (10.0 g, 32.6 mmol)
was dissolved in 150 mL of acetonitrile in a 0 °C ice bath. To the
cold solution was added N-bromosuccinimide (NBS) (5.08 g, 32.6
mmol) in a solid portion over 5 min in air. The yellow solution
turned orange immediately upon addition of NBS. The mixture was
warmed to room temperature after stirring for 12 h. The reaction
was quenched with 10 mL of a saturated aqueous solution
containing sodium bisulfite. After 0.5 h of stirring, all volatiles were
removed, giving yellow solids. The crude product was extracted
into 200 mL of dichloromethane and washed three times with 200
mL of water and once with 200 mL of concentrated brine solution.
The dichloromethane layer was removed in vacuo, the resulting
orange solids were extracted into 100 mL of hexane and dried with
anhydrous sodium sulfate, and the extract was filtered through a
tall silica pad (4 cm) in a medium-porosity frit. The silica pad was
washed with copious amounts of hexane to obtain a colorless filtrate.
All volatiles were removed from the filtrate, affording pure white
4.5. N-(2-(Diisopropylphosphino)-4-methylphenyl)-2,4,6-tri-
isopropylaniline ((THF)2LiPNiPr3). To a chilled 100 mL ethereal
solution containing N-(2,4,6-triisopropylphenyl)-2-bromo-4-methy-
laniline (5.03 g, 12.9 mmol) in a 250 mL round-bottom flask was
added n-BuLi (16.2 mL, 25.9 mmol) with a syringe and the mixture
was stirred at room temperature. After 3 h, the resulting solution
was again chilled in the cold well and chlorodiisopropylphosphine
(2.06 mL, 12.9 mmol) was added, generating an orange suspension
that was stirred for 48 h. Upon completion of the reaction, the
round-bottom flask was sealed with a rubber stopper and removed
from the glovebox, and the reaction mixture was quenched with
0.50 mL of rigorously degassed water and stirred at room
temperature for 0.5 h. The orange suspension gradually became a
soluble yellow solution with a white precipitate attributed to LiOH.
All volatiles were removed, and the resulting orange oil was
dissolved in hexane or ether and sodium sulfate or magnesium
sulfate was added, respectively, to remove trace water. The mixture
was filtered through a silica pad (4 cm), and the solvent from the
filtrate was removed by reduced pressure and dried under vacuum.
The crude oil was taken into the glovebox, dissolved in 50 mL of
THF in a 250 mL round-bottom flask, and treated with nBuLi (8.10
mL, 12.9 mmol). Removal of the THF and addition of cold hexane
generated the product (THF)2LiPNiPr3, which was stored at -37
1
solid product. Yield: 92% (11.5 g, 29.7 mmol). H NMR (25 °C,
(77) Eckert, N. A.; Bones, E. M.; Lachicotte, R. J.; Holland, P. L. Inorg.
Chem. 2003, 42, 1720.
(78) MacLachlan, E. A.; Fryzuk, M. D. Organometallics 2005, 24, 1112–
1118.
(79) Miller, A. R.; Curtin, D. Y. J. Am. Chem. Soc. 1976, 98, 1860–
1865.
(80) Caine, D. In e-EROS Encyclopedia of Reagents for Organic
Synthesis; Wiley: London, 2001.

2242 Organometallics, Vol. 28, No. 7, 2009
Tran et al.
°C for 8 h and collected by vacuum filtration with a medium-
porosity frit. Yield: 50% (3.71 g, 6.45 mmol). H NMR (25 °C,
4.8. (PNP)Ni(OCH2Ph) (4). In a 20 mL vial, (PNP)NiCl (0.15
g, 0.28 mmol) was dissolved in 10 mL of diethyl ether to give
a green solution. To the vial was added dropwise Na(OCH2Ph)
(0.037 mg, 0.28 mmol) in an 3 mL ethereal solution at room
temperature. The reaction mixture was stirred for 12 h, affording
a red solution. All volatiles were removed under reduced
pressure, and the crude product was extracted into pentane and
filtered through a fiberglass plug containing Celite. The resulting
dark red filtrate was reduced in volume and placed in the freezer
at -30 °C for 12 h to afford red crystals. The crystalline product
was collected on a medium-porosity frit by vacuum filtration.
1
400 MHz, C6D6): δ 7.25 (s, 2H, Ar-H), 6.97 (d, JH-H ) 6 Hz, 1H,
Ar-H), 6.85 (d, JH-H ) 8 Hz, 1H, Ar-H), 6.16 (dd, JP-H ) 14 Hz,
1H, Ar-H), 3.63 (m, 2H, ArCH(CH3)2), 3.19 (br s, 8H, OCH2CH2)
2.97 (m, 1H, ArCH(CH3)2), 2.28 (s, 3H, ArCH3), 2.10 (m, 2H,
PCH(CH3)2), 1.37 (dd, JH-H ) 7 Hz, 12H, ArCH(CH3)2)), 1.25 (d,
JH-H ) 7 Hz, 6H, ArCH(CH3)2)), 1.20 (br s, 8H, OCH2CH2) 1.06
(m, 12H, PCH(CH3)2). 31P{1H} NMR (25 °C, 121 MHz, C6D6): δ
-4.52 (s). 13C{1H} NMR (25 °C, 75 MHz, C6D6): δ 165.0 (d, JP-C
) 21 Hz, Ar-C), 150.5 (Ar-C), 144.1 (Ar-C), 140.4 (Ar-C), 133.0
(Ar-C), 132.3 (Ar-C), 121.2 (Ar-C), 115.5 (Ar-C), 113.1 (Ar-C),
111.1 (d, JP-C ) 12 Hz, Ar-C), 68.3 (OCH2CH2), 34.7 (ArC-
H(CH3)2), 28.2 (ArCH(CH3)2), 25.4 (OCH2CH2), 25.0 (PCH(CH3)2),
24.9 (ArCH(CH3)2), 23.5 (ArCH(CH3)2), 21.0 (ArCH3), 20.5 (d,
JP-C ) 14 Hz, PCH(CH3)2), 20.2 (d, JP-C ) 14 Hz, PCH(CH3)2).
EI-HRMS (m/z): calcd for C28H44NP 425.321 12, found 426.3268
[M + H]+.
1
Yield: 90% (0.15 g, 0.25 mmol). H NMR (25 °C, 300 MHz,
C6D6): δ 7.55 (d, JH-H ) 7 Hz, 2H, Ar-H), 7.42 (d, JH-H ) 9
Hz, 2H, Ar-H), 7.03-7.27 (m, 5H, Ar-H), 6.89 (s, 2H, Ar-H),
4.55 (s, 2H, OCH2Ar), 2.17 (m, 4H, PCH(CH3)2), 2.13 (s, 6H,
ArCH3), 1.46 (m, 12H, PCH(CH3)2), 1.25 (m, 12H, PCH(CH3)2).
31P{1H} NMR (25 °C, 121 MHz, C6D6): δ 25.82 (s). 13C{1H}
NMR (25 °C, 75 MHz, C6D6): δ 162.2 (Ar-C), 147.4 (Ar-C),
132.1 (Ar-C), 127.1 (Ar-C), 127.0 (Ar-C), 126.7 (Ar-C), 126.0
(Ar-C), 120.1 (Ar-C), 116.9 (Ar-C), 71.3 (OCH2Ar), 24.1
(PCH(CH3)2), 20.6 (ArCH3), 18.7 (PCH(CH3)2), 17.8
(PCH(CH3)2). CI-HRMS: Anal. Calcd for C33H47NNiOP2:
593.2486. Found: 593.2471. Anal. Calcd. for C33H47NNiOP2 ·
C13H32O2: C, 67.81; H, 9.77; N, 1.72. Found: C, 67.97; H, 9.40;
N, 1.54.
4.6. [(PNiPr3)Ni(µ2-Br)]2 (1). In a 100 mL round-bottom flask,
to a cold slurry of NiBr2 (0.042 g, 0.19 mmol) in THF (10 mL) at
-37 °C was added (THF)2Li(PNiPr3) (0.100 g, 0.17 mmol) dissolved
in 5 mL of THF at -37 °C. The yellow suspension gradually turned
to dark red-brown. After the mixture was stirred for 12 h at 70 °C,
all volatiles were removed under reduced pressure. The crude
product was extracted into pentane and the extract filtered through
a fiberglass plug to remove salt residues. The filtrate was removed
of solvent to generate dark red solids. Crystalline materials were
obtained from slow evaporation of a concentrated hexane solution
in a 20 mL vial at room temperature. Yield: 70% (0.067 g, 0.059
4.9. General Conditions for Nickel-Complex-Mediated Hy-
drosilylation of Carbonyl-Containing Substrates. In a glovebox,
a J-Young NMR tube equipped with a Teflon screw cap was
charged with 1.0 mL of C6D6 solution containing 2 mol % of 1
(30.8 mg, 0.027 mmol), 4 mol % of KOtBu (6.15 mg, 0.055 mmol),
and triethylsilane (262 µL, 1.64 mmol). The J-Young tube was
removed from the glovebox, and the carbonyl compound (1.37
mmol) was added to the J-Young via syringe. The sample was
placed into a preheated 100 °C oil bath. The reaction was monitored
1
mmol). H NMR (25 °C, 300 MHz, C6D6): δ 7.11 (s, 4H, Ar-H),
6.60 (s, 2H, Ar-H), 6.46 (d, JH-H ) 9 Hz, 2H, Ar-H), 5.69 (d,
JH-H ) 9 Hz, 2H, Ar-H), 4.10 (m, 4H, ArCH(CH3)2), 2.83 (m, 2H,
ArCH(CH3)2), 1.97 (s, 6H, ArCH3), 1.92 (d, JH-H ) 7 Hz, 12H,
ArCH(CH3)2), 1.83 (m, 4H, PCH(CH3)2), 1.48 (m, 24H, ArCH-
(CH3)2), 1.26 (m, 24H, PCH(CH3)2). 31P{1H} NMR (25 °C, 121
MHz, C6D6): δ 63.91 (br s). 13C{1H} NMR (25 °C, 75 MHz, C6D6):
δ 168.6 (Ar-C), 147.5 (Ar-C), 145.2 (d, JP-C ) 18 Hz, Ar-C), 134.4
(Ar-C), 130.5 (Ar-C), 121.6 (Ar-C), 120.4 (Ar-C), 115.3 (Ar-C),
34.9 (ArCH(CH3)2), 29.0 (ArCH(CH3)2), 26.1 (ArCH(CH3)2), 24.9
(ArCH(CH3)2), 24.5 (d, JP-C ) 9 Hz, PCH(CH3)2), 20.0 (ArCH3),
18.4 (PCH(CH3)2), 17.5 (PCH(CH3)2). Mp: 196-200 °C. Anal.
Calcd for C56H86Br2N2P2Ni2: C, 59.71; H, 7.70; N, 2.49. Found:
C, 59.45; H, 7.55; N, 2.30.
1
by H NMR spectroscopy. Upon completion, all volatiles were
removed under reduced pressure and the crude product was
subjected to flash chromatography on a thin, long silica gel column
(Mallinckrodt Baker 60-200 mesh) with pentane as eluent. The
oily product was obtained upon removal of all pentane via a rotary
evaporator.
4.10. X-ray Crystallography: General Parameters for Data
Collection and Refinement. Single crystals of 1 were grown at
room temperature from concentrated solutions of toluene via a
slow -evaporation technique. Inert-atmosphere techniques were
used to place the crystal onto the tip of a glass capillary
(0.06-0.20 mm diameter) mounted on a SMART6000 instrument
(Bruker) at 113(2) K. A preliminary set of cell constants was
calculated from reflections obtained from three nearly orthogonal
sets of 20-30 frames. The data collection was carried out using
graphite-monochromated Mo KR radiation with a frame time
of 3 s and a detector distance of 5.0 cm. A randomly oriented
region of a sphere in reciprocal space was surveyed. Three
sections of 606 frames were collected with 0.30° steps in ω at
different φ settings with the detector set at -43° in 2θ. Final
cell constants were calculated from the xyz centroids of strong
reflections from the actual data collection after integration
(SAINT).81 The structure was solved using SHELXS-97 and
refined with SHELXL-97.82 A direct-methods solution was
calculated that provided most non-hydrogen atoms from the E
map. Full-matrix least-squares/difference Fourier cycles were
performed that located the remaining non-hydrogen atoms. All
non-hydrogen atoms were refined with anisotropic displace-
ment parameters, and all hydrogen atoms were refined with
4.7. [(PNMe3)Ni(µ2-Cl)]2 (2). In a 20 mL vial, a suspension of
NiCl2(THF)1.5 (0.202 g, 0.76 mmol) in 5 mL of THF was added
dropwise to Li(PNMe3) (0.264 g, 0.76 mmol) in 5 mL of THF at
-37 °C. The yellow slurry gradually transformed into a homoge-
neous dark reddish brown solution within 2 h. After the mixture
was stirred for 12 h, all volatiles were removed under reduced
pressure. The crude product was extracted into toluene and filtered
through a Celite-filled medium-porosity frit to remove unreactive
NiCl2(THF)1.5 and salt side product. The filtrate was evaporated
under reduced pressure, triturated with 10 mL of hexane, and stored
at -37 °C for 12 h, and subsequent vacuum filtration with a
medium-porosity frit afforded red solids. Yield: 75% (0.331 g, 0.380
1
mmol). H NMR (25 °C, 300 MHz, C6D6): δ 6.85 (s, 4H, Ar-H),
6.64 (s, 2H, Ar-H), 6.56 (d, JH-H ) 9 Hz, 2H, Ar-H), 5.70 (d,
JH-H ) 9 Hz, 2H, Ar-H), 2.61 (s, 12H, ArCH3), 2.17 (s, 6H,
ArCH3), 2.09 (s, 6H, ArCH3), 1.67 (m, 4H, PCH(CH3)2), 1.41 (m,
12H, PCH(CH3)2), 1.27 (m, 12H, PCH(CH3)2). 31P{1H} NMR (25
°C, 121 MHz, C6D6): δ 63.4 (br s, ∆ν1/2 ) 57 Hz). 13C{1H} NMR
(25 °C, 75 MHz, C6D6): δ 150.3 (Ar-C), 141.7 (Ar-C), 133.6 (Ar-
C), 131.8 (Ar-C), 131.0 (Ar-C), 129.3 (Ar-C), 121.0 (Ar-C), 117.1
(Ar-C), 109.5 (Ar-C), 30.8 (Ar-CH3), 22.5 (PCH(CH3)2), 20.7 (d,
JP-C ) 4 Hz, PCH(CH3)2), 19.8 (Ar-CH3), 19.6 (Ar-CH3). Mp:
222-225 °C. Anal. Calcd for C44H62Cl2N2P2Ni2: C, 60.80; H, 7.19;
N, 3.22. Found: C, 60.65; H, 7.25; N, 3.15.
(81) SAINT, version 6.1; Bruker Analytical X-ray Systems, Madison,
WI, 1999.
(82) SHELXL-Plus, version 5.10; Bruker Analytical X-ray Systems,
Madison, WI, 1998.

Hydrosilylation Via a Transient Ni Hydride Complex
Organometallics, Vol. 28, No. 7, 2009 2243
isotropic displacement parameters (unless otherwise specifi-
ed). Some intensity data were corrected for absorption (SAD-
ABS).83
Acknowledgment. We thank Indiana UniversitysBloom-
ington and the Chemical Sciences, Geosciences and Bio-
sciences Division, Office of Basic Energy Science, Office
of Science, U.S. Department of Energy (No. DE-FG02-
07ER15893), for support of this research. Ms. Meghan R.
Mulcrone is thanked for crystallographic assistance.
Crystal data for [(PNiPr3)Ni(µ2-Br)]2 (1): C56H86Br2N2P2Ni2, Mw
) 1126.44, monoclinic, space group P21/c, a ) 11.0631(4) Å, b
) 14.6234(5) Å, c ) 18.2594(7) Å, ꢀ ) 94.1570(10)°, Z ) 2, µ )
2.084 mm-1, λ(Mo KR) ) 0.710 73 Å, V ) 2946.24(19) Å3, Fcalcd
) 1.270 Mg/m3, R(F) ) 0.0356, Rw(F2) ) 0.0856, and GOF on F2
) 1.048 (I > 2σ(I)), dark red platelike crystals with dimensions of
0.20 × 0.20 × 0.10 mm3, 27.48° g θ g 1.79°. Out of a total of
29 529 reflections collected, 6756 were unique and 5206 were
observed (Rint ) 2.99%).
Supporting Information Available: A CIF file giving crystal
data for complex1, text detailing control experiments, and figures
1
giving H NMR and GC-MS data of the hydrosilylated products.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(83) Blessing, R. Acta Crystallogr. 1995, A51, 33.
OM801160J