Mild protocol for the synthesis of stable nickel complexes having primary and secondary silyl ligands

Article abstract of DOI:10.1021/om801187b

A mild protocol for the synthesis of (PNP)Ni(SiH₂Ph) and (PNP)Ni(SiHPh₂) (PNP^- = N[2-P(CHMe₂) ₂-4-MeC₆H₃]₂) stemming from the hydride complex (PNP)NiH is presented along with their corresponding solid state structures. The combination of preliminary reactivity and theoretical studies suggests these silyl ligands to be robust species due to the square-planar environment enforced by the pincer ancillary.

Full text of DOI:10.1021/om801187b

2072
Organometallics 2009, 28, 2072–2077
Mild Protocol for the Synthesis of Stable Nickel Complexes Having
Primary and Secondary Silyl Ligands
Debashis Adhikari, Maren Pink, and Daniel J. Mindiola*
Department of Chemistry and the Molecular Structure Center, Indiana UniVersity,
Bloomington, Indiana 47405
ReceiVed December 15, 2008
A mild protocol for the synthesis of (PNP)Ni(SiH2Ph) and (PNP)Ni(SiHPh2) (PNP- ) N[2-P(CHMe2)2-
4-MeC6H3]2) stemming from the hydride complex (PNP)NiH is presented along with their corresponding
solid state structures. The combination of preliminary reactivity and theoretical studies suggests these
silyl ligands to be robust species due to the square-planar environment enforced by the pincer ancillary.
the three-component coupling of aldehydes, alkynes, and silanes
to produce protected allylic alcohols are also becoming an
attractive field of study.5
Introduction
Hydrosilylation is a very effective tool in the chemist’s arsenal
to form C-Si bonds, especially if this can be performed in a
catalytic manner. The C-Si bond in turn can have widespread
use to incorporate multiple functionalities and to produce fine
chemicals, chemical intermediates, and organosiloxane poly-
mers.1 For example, asymmetric hydrosilylation followed by
oxidative cleavage of the C-Si bond results in optically active
alcohols, avoiding the potentially difficult and sometimes
inconsistent step of asymmetric hydroboration.2 Moreover, biaryl
species can be easily furnished employing the Hiyama coupling,3
in which organosilanes are coupled with aryl and alkyl halo-
genides or triflates. Therefore, important organosilane precursors
can be readily prepared by hydrosilylation. It has been proposed
in a number of stoichiometric and catalytic silylation reactions
that transition metal complexes having a metal-silicon bond
might be playing an essential role in these processes.4 Under
this premise, transition metal-catalyzed hydrosilylation utilizing
a metal such as nickel is always attractive given the low cost
associated with this element when compared to its heavier
congeners or precious metal group of coinage metals such as
Pd, Pt, and Re. Quite justifiably, nickel-catalyzed reactions for
As noted before, in the nickel-catalyzed silylation reactions
it is believed that nickel-silicon-based complexes may be
playing pivotal roles in these transformations. Our current
interest in nickel-mediated group transfer reactions prompted
us to explore silylation chemistry using this transition metal.6
Surprisingly, a survey of the literature reveals that silyl com-
plexes of nickel are not very well documented, and isolable
examples of this functional group are relatively scarce.7 Argu-
ably, the low number of isolable and well-characterized nickel-
silyl complexes might stem from the lack of synthetic methods
to generate the silyl anion ligand. Indeed, this limitation likely
involves the difficulty associated with synthesizing lithium salts
of silanes (often restricted to sterically protected silyl salts),
which is necessary to perform salt metathesis with metal halides
to prepare the corresponding silyl complexes.8 In our attempted
synthesis we choose PNP (PNP- ) N[2-P(CHMe2)2-4-Me-
C6H3]2) as the ancillary ligand, as this has proven to be a very
robust scaffold in generating reactive complexes of both early
and late transition metals.9 Specifically, the meridional constraint
and chelation offered by the aryl-bridged PNP framework, along
with the steric protection provided by the phosphine groups,
results in a very rigid platform that likely prevents decomposi-
tion pathways, especially for square-planar d8 metal centers. One
clear example is the ability of the PNP ligand to stabilize the
* Corresponding author. E-mail: mindiola@indiana.edu.
(1) (a) Marciniec, B. ComprehensiVe Handbook on Hydrosilylation;
Pergamon Press: Oxford, NY, 1992. (b) Marciniec, B. Silicon Chem. 2002,
1, 155. (c) Brook, M. A. Silicon in Organic, Organometallic, and Polymer
Chemistry; Wiley: New York, 2000. (d) Lewis, L. N.; Stein, J.; Gao, Y.;
Colborn, R. E.; Hutchins, G. Platinum Met. ReV. 1997, 41, 66. (e) Roy,
A. K. AdV. Organomet. Chem. 2008, 55, 1.
(2) (a) Tamao, K. In AdVances in Silicon Chemistry; Larson, G. L., Eds.;
JAI Press: London, 1996; Vol. 3, pp 1-62. (b) Pelter, A.; Smith, K.; Brown,
H. C.; Borane Reagents; Academic Press: New York, 1988.
(3) (a) Hiyama, T. In Metal Catalyzed Cross-Coupling Reactions;
Diederich, F., Stang, P. J., Eds.; Wiley: New York, 1998; Chapter 10, p
421. (b) Denmark, S. E.; Choi, J. Y. J. Am. Chem. Soc. 1999, 121, 5821.
(c) Lee, J.-Y.; Fu, G. C. J. Am. Chem. Soc. 2003, 125, 5616. (d) Denmark,
S. E.; Tymonko, S. A. J. Am. Chem. Soc. 2005, 127, 8004. (e) Murata, M.;
Shimazaki, R.; Watanabe, S.; Masuda, Y. Synthesis 2001, 2231.
(4) (a) Tilley, T. D. In The Chemistry of Organic Silicon Compounds;
Patai, S., Rappoport, Z., Eds.; Wiley: New York, 1989; Chapter 24, p 1415.
(b) Tilley, T. D. In The Silicon-Heteroatom Bond; Patai, S., Rappoport, Z.,
Eds.; Wiley: New York, 1991; Chapters 9 and 10, p 245. (c) Sharma, H.;
Pannell, K. H. Chem. ReV. 1995, 95, 1351. (d) Eisen, M. S. In The Chemistry
of Organic Silicon Compounds; Apeloig, Y., Rappoport, Z., Eds.; Wiley:
New York, 1998; Vol. 2,Chapter 35, p 2037. (e) Corey, J. Y.; Braddock-
Wilking, J. Chem. ReV. 1999, 99, 175. (f) Reichl, J. A.; Berry, D. H. AdV.
Organomet. Chem. 1998, 43, 97. (g) Schneider, N.; Finger, M.; Hafer-
kemper, C.; Bellemin-Laponnaz, S.; Hofmann, P.; Gade, L. H. Angew.
Chem., Int. Ed. 2009, 48, 1609.
(5) (a) Herath, A.; Montgomery, J. J. Am. Chem. Soc. 2008, 130, 8132.
(b) Baxter, R.; Montgomery, J. J. Am. Chem. Soc. 2008, 130, 9662.
(6) Adhikari, D.; Huffman, J. C.; Mindiola, D. J. Chem. Commun. 2007,
4489.
(7) For a few examples of complexes containing the Ni-Si moiety: (a)
Iluc, V. M.; Hillhouse, G. L. Tetrahedron 2006, 62, 7577. (b) Shimada, S.;
Rao, M. L. N.; Tanaka, M. Organometallics 1999, 18, 292. (c) Avent, A. G.;
Gehrhus, B.; Hitchcock, P. B.; Lappert, M. F.; Maciejewski, H. J.
Organomet. Chem. 2003, 686, 292. (d) Shimada, S.; Rao, M. L. N.; Hayashi,
T.; Tanaka, M. Angew. Chem., Int. Ed. 2001, 40, 213. (e) Bierchenk, T. R.;
Guerra, M. A.; Juhlke, T. J.; Larson, S. B.; Lagow, R. J. J. Am. Chem. Soc.
1987, 109, 4855. (f) Nlate, S.; Herdtweek, E.; Fischer, R. A. Angew. Chem.,
Int. Ed. 1996, 35, 1861. (g) Maciejewski, H.; Marciniec, B.; Kownacki, I.
J. Organomet. Chem. 2000, 597, 175. (h) Gehrhus, B.; Hitchcock, P. B.;
Lappert, M. F.; Maciejewski, H. Organometallics 1998, 17, 5599. (i) Kang,
Y.; Lee, J.; Kong, Y. K.; Kang, S. O.; Ko, J. Chem. Commun. 1998, 2343.
(j) Schmedake, T. A.; Haaf, M.; Paradise, B. J.; Powell, D.; West, R.
Organometallics 2000, 19, 3263.
(8) (a) Roddick, D. M.; Heyn, R. H.; Tilley, T. D. Organometallics 1989,
8, 324. (b) Powell, S. A.; Tenenbaum, J. M.; Woerpel, K. A. J. Am. Chem.
Soc. 2002, 124, 12648. (c) Arnold, J.; Roddick, D. M.; Tilley, T. D.; Geib,
S. J. Inorg. Chem. 1988, 27, 3510.
10.1021/om801187b CCC: $40.75
2009 American Chemical Society
Publication on Web 03/10/2009

Synthesis of Stable Ni Complexes with Silyl Ligands
Organometallics, Vol. 28, No. 7, 2009 2073
Scheme 1. General Synthetic Scheme for the Preparation of
Compounds 2 and 3 from the Hydride Precursor 1
over 2 days. Complex 2 is stable for months when stored under
a nitrogen atmosphere, or upon heating in benzene at 150 °C
for prolonged periods of time (∼4 days). Even under these
forcing conditions, no decomposition products are detected by
31P and 1H NMR spectroscopy. In the case of a secondary silane
such as Ph2SiH2, the use of ZnI2 was ineffective in promoting
clean Si-H activation by 1. Instead, copious amounts of
(PNP)NiI (see Experimental Details and Supporting Information)
was observed as a side product when the mixture was heated
to 60 °C for 24 h in hexane. Other attempts employing MgI2,
AlMe3, or SmCl3 as a Lewis acid also failed to generate the
expected nickel-silyl product. To our surprise, it was found that
[PhNHMe2][B(C6F5)4] was the most suitable reagent to ac-
complish formation of the desired secondary silyl complex,
(PNP)Ni(SiHPh2) (3), isolated in 42% yield as a red-colored
material. In an independent experiment it was found that
[PhNHMe2][B(C6F5)4] can also promote the reaction of 1 and
PhSiH3 to engender quantitative formation of 2 (3 h, 65 °C)
when the reaction is monitored by 31P NMR spectroscopy.
2. Characterization of the Nickel-Silyl Complexes 2
and 3. Alternative Syntheses, Orbital Analysis, and Pre-
liminary Reactivity. In order to unambiguously ascertain the
geometry about the metal center in complex 2, we resorted to
single-crystal X-ray diffraction analysis. The crystal structure,
as depicted in Figure 1, unequivocally exposes the nickel center
confined in a square-planar environment and bearing a terminal
sp3-hybridized silicon atom. The phenyl ring associated with
the Si atom makes an angle of 90.05° with the imaginary plane
comprising the atoms Ni-P-P-N. The orthogonal orientation
of the ring is probably due to steric congestion imposed by the
isopropyl groups on phosphorus.
first crystallographically authenticated nickel boryl complex.6
In addition, other transition metal silyls having the same pincer-
PNP ligand have been explored recently by Tilley and Ozerov
et al.11 A mild protocol has been developed to synthesize nickel-
silyl complexes from both primary and secondary silanes and
both complexes have been fully characterized, including crystal-
lographic analysis. We also provide some theoretical studies
and preliminary reactivity addressing the nature of the Ni-Si
bond.
Results and Discussion
1. Synthesis of Nickel-Silyl Complexes. Very recently we
discovered that (PNP)NiH (1), when stirred with catechol
borane, generates a nickel boryl complex along with the
liberation of dihydrogen.6 In the pursuit of synthesizing nickel-
silyl complexes we speculated that the same strategy may
produce an analogous set of complexes having Ni-Si bonds.
Unfortunately, we observed that reaction of 1 with phenylsilane
required forcing conditions (120 °C, >14 days) for at least
50-60% conversion of complex 1 to the corresponding silyl
species (PNP)Ni(SiH2Ph) (2). Though it is expected that the
liberation of dihydrogen will create the entropic driving force
toward product formation, the sluggishness of the reaction may
stem from the inherent strength of the Ni-H bond coupled with
the inherent reactivity of the Ni-Si bond (formation of 2 from
1 is sluggish even if the reaction is conducted with periodical
removal of hydrogen gas). Inspired by the work of Liang and
co-workers, where use of a powerful Lewis acid such as AlMe3
triggered the C-H activation of benzene by a similar nickel
hydride species (PNP′)NiH (PNP′ ) N[2-PR2C6H4]2, R ) Cy,
iPr2), we hypothesized that an analogous Si-H activation
reaction could be promoted if the Ni-H bond were to be
polarized with a Lewis acid.10
If one assumes local symmetry of the -SiH2Ph group and
average planarity of PNP aryl residues, the molecular structure
of complex 2 grossly possesses C2V symmetry, which is
corroborated by 1H NMR spectroscopic data (vide infra).
Diamagnetic 2 evinces a triplet at 4.63 ppm for the silyl
3
hydrogens (1H NMR: JP-H ) 10 Hz), therefore proving the
equivalence of the hydrogens attached to silicon. Analogously,
the 31P NMR spectrum displays a singlet at 44.13 ppm, further
being consistent with the proposed C2V symmetry of the
complex. In the 31P NMR spectrum, satellites are observed due
to coupling with a 29Si nucleus (I ) 1/2, 4.68% abundant) with
a coupling constant 2JP-Si of 54 Hz. Most notably, the 29Si{1H}
NMR spectrum exposes a triplet at -40.45 ppm (Figure 2),
while the proton-coupled 29Si NMR spectrum portrays a triplet
of triplets due to phosphorus and hydrogen coupling (2JP-Si
)
54 Hz, 1JSi-H ) 160 Hz, Figure 2). The combination of the 29Si
NMR chemical shift value and large JSi-H are in accord with
the sp3-hybridized silicon nucleus and argues against the
possibility of the silylene moiety being present in solution by
virtue of a 1,2-hydrogen migration.12 Accordingly, the Ni-Si
bond length was found as 2.2346(5) Å, a distance comparable
with the few structurally characterized nickel-silyl complexes
reported in the literature.7 From X-ray crystallographic analysis
silyl-hydrogens were located distinctly in the Fourier difference
map (1.44(2) and 1.40(2) Å). Other details regarding data
collection and metrical parameters are summarized in Table 1.
Accordingly, complex 1 was treated with various Lewis acids
to facilitate silane activation to form the desired Ni-Si bond.
Gratifyingly it was found that the primary silane, PhSiH3, in
the presence of ZnI2 (anhydrous) generated the nickel-silyl
species 2 within 14 h (quantitatively) when the mixture was
heated at 60 °C (Scheme 1). The product was isolated in good
yield (82%) when crystallized from diethyl ether at -35 °C
(9) (a) Fan, L.; Foxman, B. M.; Ozerov, O. V. Organometallics 2004,
23, 326. (b) Fan, L.; Yang, L.; Guo, C.; Foxman, B. M.; Ozerov, O. V.
Organometallics 2004, 23, 4778. (c) Ozerov, O. V.; Guo, C.; Papkov, V. A.;
Foxman, B. M. J. Am. Chem. Soc. 2004, 126, 4792. (d) Bailey, B. C.; Fan,
H.; Huffman, J. C.; Baik, M.-H.; Mindiola, D. J. J. Am. Chem. Soc. 2007,
129, 8781. (e) Fafard, C. M.; Adhikari, D.; Foxman, B. M.; Mindiola, D. J.;
Ozerov, O. V. J. Am. Chem. Soc. 2007, 129, 10318.
(10) Liang, L.; Chien, P.; Huang, Y. J. Am. Chem. Soc. 2006, 128, 15562.
(11) (a) Calimano, E.; Tilley, T. D. J. Am. Chem. Soc. 2008, 130, 9226.
(b) Gatard, S.; Chen, S.-H.; Foxman, B. M.; Ozerov, O. V. Organometallics
2008, 27, 6257.
(12) (a) Mitchell, G. P.; Tilley, T. D. J. Am. Chem. Soc. 1998, 120,
7635. (b) Mitchell, G. P.; Tilley, T. D. Angew. Chem., Int. Ed. Engl. 1998,
37, 2524. (c) Schmedake, T. A.; Haaf, M.; Paradise, B. J.; Powell, D.; West,
R. Organometallics 2000, 19, 3263. (d) Glaser, P. B.; Wanandi, P. W.;
Tilley, T. D. Organometallics 2004, 23, 693. (e) Feldman, J. D.; Peters,
J. C.; Tilley, T. D. Organometallics 2002, 21, 4065. (f) Peters, J. C.;
Feldman, J. D.; Tilley, T. D. J. Am. Chem. Soc. 1999, 121, 9871.

2074 Organometallics, Vol. 28, No. 7, 2009
Adhikari et al.
Figure 1. Solid state structures of 2 (left) and 3. All hydrogens except those on silicon have been omitted for clarity, as well as solvents
and isopropyl methyls on phosphorus. Thermal ellipsoids are displayed at the 50% probability level. Selected metrical parameters (distances
in Å and angles are in deg): for 2: Ni-Si, 2.2346(5); Ni-P1, 2.1765(4); Ni-P2, 2.1776(4); Ni-N, 1.9362(12); Si-H1, 1.44(2); Si-H2,
1.40(2); N-Ni-Si, 174.24(4); P1-Ni-P2, 168.127(17); Ni-Si-H1, 113.2(9); Ni-Si-H2, 115.4(9); for 3: Ni-Si, 2.2607(7); Ni-P1,
2.1792(6); Ni-P2, 2.1857(6); Ni-N, 1.9375(17); Si-H, 1.42(3); N-Ni-Si, 175.06(6); P1-Ni-P2, 171.57(2); Ni-Si-H, 112.1(10).
Figure 2. Expanded regions of the 29Si{1H} and 29Si NMR spectra for complexes 2 (A and B) and 3 (C and D).
Likewise, the molecular structure of 3 presents grossly the
same features as found in 2. Salient features include a square-
planar nickel center coordinating a terminal and secondary silyl
ligand with a bond distance of 2.2607(7) Å. The hydrogen atom
on silicon is oriented away from the metal center, thus excluding
the possibility of an R-H agostic interaction taking place in the
electron density around the silicon atom in 3 is considerably
depleted due to the presence of an additional delocalizing phenyl
ring, hence shifting the resonance for 3 more downfield than
1
the primary phenylsilyl observed in complex 2. When the H-
coupled 29Si NMR spectrum is collected for 3, the signal resolves
1
as a doublet of triplets with a JSi-H ) 161 Hz (Figure 2).
solid state.13 The H NMR spectrum of complex 3 shows the
Alternatively, complexes 2 and 3 can also be synthesized by
a metalloradical-assisted reductive cleavage of the Si-H bond
of the corresponding organosilane. Accordingly, the reaction
1
silyl hydrogen centered at 5.42 ppm (3JP-H ) 13 Hz), a value
that is expected for most transition metal silyl complexes.4e
Equivalence of phosphorus due to average C2V symmetry is
evident from a singlet at 42.59 ppm, along with the observation
of Si satellites (2JSi-P ) 52 Hz). To our surprise, however, the
29Si{1H}NMR spectrum exhibits a doublet at -11.08 ppm,
which is significantly deshielded when compared to the silyl
chemical shift observed in 2 (Figure 2). We surmise that the
14
of [Ni(µ2-PNP)]2 (4) with PhSiH3 rapidly produces 1 and 2
in equal molar amounts (Scheme 2). In the case of the secondary
silane, Ph2SiH2, binuclear oxidative addition of the Si-H bond
also occurs to form 1 and 3, but the reaction requires moderate
heating and an extended period of time (65 °C, 20 h),
presumably due to increased steric encumbrance surrounding
the Si-H bond of the organosilane. Recently, our group has
(13) (a) Hermann, W. A.; Eppinger, J.; Spiegler, M.; Runte, O.;
Anwander, R. Organometallics 1997, 16, 1813. (b) Ohff, A.; Kosse, P.;
Baumann, W.; Tillack, A.; Kempe, R.; Gorls, H.; Burlakov, V. V.;
Rosenthal, U. J. Am. Chem. Soc. 1995, 117. (c) Schubert, U.; Schwarz,
M.; Moller, F. Organometallics 1994, 13, 1554.
(14) (a) Adhikari, D.; Mossin, S.; Basuli, F.; Dible, B. R.; Chipara, M.;
Fan, H.; Huffman, J. C.; Meyer, K.; Mindiola, D. J. Inorg. Chem. 2008,
47, 10479. (b) Binuclear oxidative addition by a (PNP)Pd(I) dimer has been
also reported in ref 9e.

Synthesis of Stable Ni Complexes with Silyl Ligands
Organometallics, Vol. 28, No. 7, 2009 2075
Table 1. Summary of Crystallographic Data and Structure
Refinement Details for Compounds 2 and 3
primarily on the silyl moiety with significant contribution from
the Si sp3 hybrid orbital. Not surprisingly, the DFT analysis of
complex 3 exposes essentially the same features as those
observed for 2.17 Additional calculations to obtain a quantitative
understanding of the orbital composition using the ADF18
package are also fully consistent with the above result.17
2
3
empirical formula
fw
C32H47NNiP2Si · 0.5Et2O C38H51NNiP2Si · C5H12
631.51
706.63
cryst syst
space group
a (Å)
b (Å)
c (Å)
R (deg)
ꢀ (deg)
γ (deg)
V (Å3)
triclinic
P1
monoclinic
C2/c
24.9191(13)
10.3235(5)
31.4006(16)
90.00
110.577(3)
90.00
7562.5(7)
8
j
10.0074(7)
10.0249(7)
17.2061(12)
84.918(1)
77.723(1)
83.292(1)
1671.5(2)
2
The reactivity of compounds 2 and 3 was examined with
1-hexene. Unfortunately, no insertion was observed even upon
heating the mixture at 120 °C for 24 h. This phenomenon
suggests that the square-planar Ni(II) environment (as well as
sterically protected metal center) prevents the substrate from
reacting at the Ni-Si bond. Likewise, treatment of 2 or 3 with
benzaldehyde also failed to react even under extended periods
(2 days) at 70 °C, presumably due to the same kinetic factors.
Moreover, addition of 1 equiv of PhSiH3 to a hexane solution
of 3 did not result in an equilibrium mixture of 2 and Ph2SiH2
even under thermolytic conditions (120 °C, 12 h), therefore
rendering the nickel-silyl ligand rather inert toward substitution
chemistry. A similar substitution attempt involving 2 and
Ph2SiH2 also failed to show exchange. Silyl group transfer also
failed to occur with bromobenzene over a period of 3 days at
120 °C. Therefore, our ability to kinetically stabilize nickel-
silyl complexes in a square-planar environment compromises
its reactivity, presumably due to the sterically protected nature
of the only orbital, d(x2-y2), available to coordinate the
substrate.
Z
Dcalc (g · cm-3
cryst size (mm)
crystallization solvent,
cryst color
)
1.255
1.241
0.21 × 0.13 × 0.11
0.20 × 0.20 × 0.10
Et2O, orange
pentane, reddish-orange
(h, k, l)
-12 e h e 12,
-13 e k e 13,
-22 e l e 21
678
-31 e h e 31,
-12 e k e 12,
-39 e s e 39
3032
F(000)
θ range (deg)
linear abs coeff (mm-1
total reflcns collected
indep reflcns
unique reflcns
Rint
2.05 to 27.53
0.737
24 624
7674
6947
2.16 to 26.42
0.658
50 579
7751
5952
)
0.0251
0.0811
data/params
7674/387
7751/403
0.0351, 0.0964
0.860
R1, wR2 (for I > 2σ(I))a 0.0312, 0.0846
GoF
1.050
0.698/-0.729
peak/hole (e/Å-3
)
0.555/-0.525
3. Investigation of the Mechansim of Si-H Activation.
Work by Liang and co-workers has elegantly demonstrated that
a powerful Lewis acid such as Al(CH3)3 can promote the
activation of the C-H bond in benzene or toluene when
combined with a nickel hydride or methyl complex (PNP′)NiR
a
2
Goodness-of-fit ) [∑[w(Fo - Fc2)2]/Nobservns - Nparams)]1/2, all data.
R1 ) ∑(|Fo| - |Fc|)/∑|Fo|. wR2 ) [∑[w(Fo - Fc2)2]/∑[w(Fo )2]]1/2
.
2
2
Scheme 2. Si-H Bond Cleavage Promoted by Dimeric
Complex 4 to Prepare Equal Molar Mixtures of Hydride 1
and Silyl 2 or 3
10
i
(PNP′ ) N[2-PR′2-phenyl]2, R ) H, Me; R′ ) Cy, Pr2).
Although not addressed, it is speculated that hydride or methide
abstraction by Al(CH3)3, followed by binding of benzene to the
electrophilic Ni(II) cation, and subsequent acid-base chemistry,
might be transpiring along this process. For us, the use of
Al(CH3)3 to promote Si-H activation resulted in formation of
a myriad of uncharacterized products along with a minor amount
of the expected nickel-silyl complexes. However, the use of
anhydrous ZnI2 proved fortuitous since it is a much milder Lewis
acid than Al(CH3)3. On the other hand, wet or “crude” ZnI2 did
not yield the same result, therefore arguing against adventitious
HI being involved for the aforementioned reaction. In fact, no
HI is likely generated in this reaction since addition of a base
such as Et3N does not inhibit the formation of 2 from 1 when
PhSiH3 is treated with anhydrous ZnI2. To partially understand
the process leading to Si-H activation, we investigated the
reaction of ZnI2 with 1 in benzene and in the absence of silane.
The result was a considerable amount of (PNP)NiI formation
under 1 h heating at 60 °C, therefore hinting that the hydride in
1 is likely reacting with the Lewis acid. Unfortunately, we have
been unable to characterize the Zn product formed from this
control experiment. Addition of ZnI2 to silane resulted in no
detectable reaction when the mixture was monitored by 1H NMR
spectroscopy. Examination of the volatiles from the reaction
mixture leading to 2 revealed formation of H2 as a byproduct,
thus hinting that ZnI2 is likely polarizing the hydride moiety in
1 to facilitate elimination with the H-Si bond in H3SiPh. We
cannot refute, however, the possibility of ZnI2 polarizing the
hydridic Si-H bond especially since a similar reaction has been
reported several other bond cleavages facilitated by the highly
reducing Ni2N2 diamond core in 4.14
In order to understand the electronic structure of the nickel-
silyl complexes 2 and 3, as well as the nature of the Ni-Si
bond, we performed high-level DFT calculations on the full
model for both complexes. Structural optimization using the
B3LYP/6-31G** level of theory reproduced the geometrical
features of the two molecules within the expected accuracy
range. The Ni-Si bond order obtained from NBO15 calculations
reveals a value of 0.91 for both 2 and 3, thus unequivocally
supporting the presence of a single Ni-Si bond. As portrayed
in Figure 3, the frontier orbitals of complex 2 have a filled d(z2)
HOMO-1 orbital and unoccupied d(x2-y2)LUMO orbital,
which is anticipated for a square-planar d8 system. The HOMO
contains a great amount of the pincer amide nitrogen lone pair
character as well as alternating carbons composing the PNP aryl
rings.16 Further analysis reveals the LUMO to be localized
(15) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. ReV. 1988, 88,
899.
(16) Adhikari, D.; Mossin, S.; Basuli, F.; Huffman, J. C.; Szilagyi, R. K.;
Meyer, K.; Mindiola, D. J. J. Am. Chem. Soc. 2008, 130, 3676.
(17) See Supporting Information.
(18) Velde, G. T.; Bickelhaupt, F. M.; Baerends, E. J.; Guerra, C. F.;
VanGisbergen, S. J. A.; Snijders, J. G.; Ziegler, T. J. Comput. Chem. 2001,
22, 931.

2076 Organometallics, Vol. 28, No. 7, 2009
Adhikari et al.
Figure 3. Most important molecular orbitals for the full model of complex 2 shown with isodensity ) 0.05 au.
proposed and reported in the literature.19 Regardless, ZnI2 must
be used as a stoichiometric reagent in order for complete
conversion of 1 to 2 to take place. The latter fact suggests that
ZnI2 is likely being transformed along the Si-H activation
pathway since substoichiometric amounts of the Lewis acid
resulted in lower yields of 2.
and secondary silanes. The nickel-silyl complexes reported in
this work have been thoroughly characterized by multinuclear
NMR and high-resolution mass spectroscopies, as well as single-
crystal X-ray diffraction analyses. DFT calculations on both the
primary and secondary nickel silyls have provided information
regarding the electronic structure of the molecules. Structural
and theoretical studies predict stable Ni-Si bonds confined in
a square-planar environment, which has been corroborated by
the chemical inertness of the silyl moiety toward alkenes,
aldehydes, and bromobenzene. The role of the activator is
unclear at present, but such reagents must be used stoichiomet-
rically in order for reactions to reach near completion. As a
result, the mechanism for this interesting reaction is still under
scrutiny. Our efforts are currently aimed at preparing low-
coordinate nickel silyls in order to facilitate group transfer
reactions.
Unfortunately, the use of ZnI2 failed to promote the activation
of the Si-H bond in the secondary silane H2SiPh2. Instead, the
Brønsted acid [PhNHMe2][B(C6F5)4] facilitated the conversion
when combined with 1 and the primary or secondary silane.
Examination of the volatiles for the latter process also reveals
H2 to be produced as a byproduct. The use of 1 in the Si-H
activation is essential since [PhNHMe2][B(C6F5)4] alone fails
to show any reactivity with the secondary silane. It was found
that addition of NEt3 to 1 and [PhNHMe2][B(C6F5)4]/silane
completely ceased formation of 3, therefore implicating that
acid-base chemistry likely takes place in this type of reaction.
This observation contrasts the Lewis acid-promoted Si-H
activation to produce 2 (vide supra) and thus implies a different
mechanism to formation of 3. Therefore, we propose for the
latter reaction that the nickel hydride (PNP)NiH is likely being
protonated to a transient cation [(PNP)Ni]+ (complex 1 does
react with [PhNHMe2][B(C6F5)4] to form a new diamagnetic
nickel species, which we have been unable to fully characterize),
which engages in activation of the silane, presumably via the
base NMe2Ph. Unfortunately, substoichiometric amounts of
[PhNHMe2][B(C6F5)4] (20 mol %) resulted in incomplete
reactions, hence hinting that the salt is likely involved as a
promoter apart from just a simple acid-base transformation.
We cannot ignore the possibility of the pincer PNP nitrogen
being involved in a deprotonation step since recent studies by
Caulton and co-workers have demonstrated the role of the
R-nitrogen in C-H and N2 activation processes.20
Experimental Details
General Considerations. Unless otherwise stated, all operations
were performed in an M. Braun Lab Master double-drybox under
an atmosphere of purified nitrogen or using high-vacuum standard
Schlenk techniques under an argon atmosphere.21 Anhydrous
n-hexane, pentane, toluene, and benzene were purchased from
Aldrich in Sure-sealed reservoirs (18 L) and further dried by passage
through one column of activated alumina and one of Q-5. Diethyl
ether was dried by passage through two columns of activated
alumina. THF was distilled, under nitrogen, from purple sodium
benzophenone ketyl and stored over sodium metal. Distilled THF
was collected under argon or nitrogen into thick-walled vessels
before being transferred into a drybox. 1 and 4 were prepared
following the literature procedure.14 Hydrosilanes used for this work
were bought commercially from Aldrich and Alfa Aesar and used
without further purification. AlMe3 was purchased from Aldrich
and used as is (Caution should be exercised when handling this
reagent, as it is highly pyrophoric and combusts spontaneously once
it comes into contact with air or moisture). C6D6 was purchased
from Cambridge Isotope Laboratory (CIL), degassed, and dried over
4 Å molecular sieves. Celite, alumina, and 4 Å molecular sieves
were activated under vacuum overnight at 200 °C. All other
Obviously, the nature of the silane reagent is critical in these
reactions, because tertiary silanes such as triphenylsilane or
triethylsilane failed to react with 1 in the presence of ZnI2 or
[PhNHMe2][B(C6F5)4]. As a result, we are presently unsure of
what mechanism is governing this set of reactions, especially
given the divergent nature of the activators (ZnI2 versus
[PhNHMe2][B(C6F5)4]), as well as the selectivity for Si-H
activation of a primary versus a secondary silane.
1
chemicals were used as received. H, 31P, and 13C spectra were
recorded on Varian 400 or 300 MHz NMR spectrometers. 29Si NMR
1
spectra were recorded in a Varian 500 MHz spectrometer. H and
Conclusions
13C NMR spectra are reported with reference to solvent resonances
(residual C6D5H in C6D6, 7.16 and 128.0 ppm). 31P NMR chemical
shifts are reported with respect to external H3PO4 (aqueous solution,
In this report we propose a mild protocol for the synthesis of
pincer-based nickel-silyl complexes prepared from both primary
(19) (a) Parks, D. J.; Piers, W. E. J. Am. Chem. Soc. 1996, 118, 9440.
(b) Blackwell, J. M.; Foster, K. L.; Beck, V. H. J. Org. Chem. 1999, 64,
4887.
(20) Walstrom, A.; Pink, M.; Tsvetkov, N. P.; Fan, H.; Ingleson, M.;
Caulton, K. G. J. Am. Chem. Soc. 2005, 127, 16780.
(21) For a general description of the equipment and techniques used in
carrying out this chemistry see: (a) Burger, B. J.; Bercaw, J. E. In
Experimental Organometallic Chemistry; Wayda, A. L., Darensbourg, M. Y.,
Eds.;ACS Symposium Series 357; American Chemical Society: Washington
D.C., 1987; p 79.

Synthesis of Stable Ni Complexes with Silyl Ligands
Organometallics, Vol. 28, No. 7, 2009 2077
2
0.0 ppm). 29Si NMR chemical shifts are reported with respect to
external SiMe4 set at 0.0 ppm. UV-vis spectra were recorded on
a Cary 5000 UV-vis-NIR spectrophotometer. X-ray diffraction
data were collected on a SMART6000 (Bruker) system under a
stream of N2 (g) at low temperatures.22,23
δ -11.08 (triplet, JSi-P ) 54 Hz). UV-vis (hexane, 25 °C): 427
nm (ꢀ ) 3200 M-1 cm-1). MS-CI: calcd 669.2614, found 669.2605.
Mp: 106 °C.
Si-H Bond Cleavage Reaction with 4 to Prepare 2 and 1. In
a J-Young tube 4 was dissolved (30 mg, 0.031 mmol) in hexane,
and the calculated amount of PhSiH3 (3.32 mg, 0.031 mmol) was
added to that. The brown color of the solution changed to orange
immediately. The reaction mixture was stirred at room temperature
for 5 min. By 1H and 31P NMR spectra it was observed that 1 and
2 were formed in equal molar amount.
Si-H Bond Cleavage Reaction with 4 to Prepare 3 and 1. In
a J-Young tube 4 was dissolved (30 mg, 0.031 mmol) in hexane,
and the calculated amount of Ph2SiH2 (5.67 mg, 0.031 mmol) was
added to that. No immediate color change was observed after the
addition. The reaction mixture was stirred for 20 h at 65 °C. By
1H and 31P NMR spectra it was observed that 1 and 3 were formed
in approximately equal molar amount.
Independent Synthesis of (PNP)NiI. In a thick-walled reaction
vessel (PNP)H (50 mg, 0.116 mmol) was charged with anhydrous
NiI2 (36.4 mg, 0.116 mmol) in benzene and heated at 110 °C for
5 h. During the course of the reaction, the solution color changed
to deep green. After the completion of the reaction two drops of
triethylamine was added, and the reaction mixture was filtered
through a pad of silica gel. The green filtrate was dried in vacuo,
and the solid mass was found to be spectroscopically pure (PNP)NiI
(50 mg, 0.081 mmol, 70% yield).
Synthesis of (PNP)Ni(SiH2Ph) (2). In a thick-walled reaction
vessel 1 (75 mg, 0.154 mmoml), phenylsilane (16.6 mg, 0.154
mmol), and zinc iodide (49.2 mg, 0.154 mmol) were charged in 2
mL of hexane. The reaction mixture was heated at 60 °C for 14 h,
and during the course of the reaction the solution color changed to
orange. After the completion of the reaction, the reaction mixture
was filtered, dried, dissolved in ether, and kept at -45 °C. Within
2 days bright orange-colored crystals were obtained (74.3 mg, 0.118
mmol, 82% yield). 1H NMR (25 °C, 399.8 MHz, C6D6): δ 8.04 (d,
2H, C6H5, JH-H ) 8 Hz), 7.78 (d, 2H, C6H3, JH-H ) 8 Hz), 7.22
(m, 3H, C6H5), 6.96 (s, 2H, C6H3), 6.88 (d, 2H, C6H3, JH-H ) 8
Hz), 4.63 (t, 2H, SiH2Ph) (3JH-P ) 10 Hz), 2.34 (septet, 4H,
CHMe2), 2.19 (s, 6H, ArCH3), 1.21 (dd, 12H, CHMe2), 1.06 (dd,
12H, CHMe2). 13C NMR (25 °C, 75.46 MHz, C6D6): δ 161.05 (aryl),
140.65 (aryl), 137.13 (aryl), 132.49 (aryl), 132.14 (aryl), 127.61
(aryl), 124.39 (aryl), 121.86 (aryl), 115.14 (aryl), 24.84 (CHMe2),
20.66(MeAr), 19.28 (CHMe2), 17.88 (CHMe2), one aromatic
resonance was not located, which might be buried under the residual
solvent peak. 31P NMR (25 °C, 121.5 MHz, C6D6): δ 44.13. 29Si
2
NMR (25 °C, 99.5 MHz, C6D6): δ -40.45 (triplet, JSi-P ) 54
1H NMR (25 °C, 399.8 MHz, C6D6): δ 7.56 (d, C6H3, 2H, JH-H
) 8 Hz), 6.94 (s, C6H3, 2H), 6.75 (d, C6H3, 2H, JH-H ) 8 Hz),
2.40 (septet, CHMe2, 4H), 2.13 (s, MeAr, 6H), 1.50 (dd, CHMe2,
12H), 1.22 (dd, CHMe2, 12H). 13C NMR (25 °C, 100.6 MHz, C6D6):
δ 162.37 (aryl), 132.47 (aryl), 132.02 (aryl), 125.46 (aryl), 121.36
(aryl), 116.53 (aryl), 25.53 (CHMe2), 20.47(MeAr), 19.20 (CHMe2),
18.06 (CHMe2). 31P NMR (25 °C, 121.5 MHz, C6D6): δ 44.74.
MS-CI: calcd 613.1029, found 613.1002.
Hz). UV-vis (hexane, 25 °C): 441 nm (ꢀ ) 1471 M-1 cm-1). MS-
CI: calcd 593.230, found 593.229. Mp: 68 °C.
Synthesis of (PNP)Ni(SiHPh2) (3). In a thick-walled reaction
vessel 1 was taken (60 mg, 0.123 mmol) in 2 mL of hexane.
Diphenylsilane (22.65 mg, 0.123 mmol) and [HNMe2Ph][B(C6F5)4]
(102.7 mg, 0.123 mmol) were added, and the mixture was heated
in an oil bath at 65 °C for 12 h. During the reaction the yellow
color of the solution changed to deep orange. After the completion
of the reaction, the hexane solution was filtered and dried. The dried
mass was redissolved in ether and kept at -45 °C to afford bright
Acknowledgment. We thank Indiana University-
Bloomington, the Dreyfus Foundation, the Sloan Foundation,
and the Chemical Sciences, Geosciences and Biosciences
Division, Office of Basic Energy Science, Office of Science,
U.S. Department of Energy (DE-FG02-07ER15893), for
financial support of this research. We deeply appreciate the
computational facility offered by Indiana University and
Montana State University. The authors also thank Alison R.
Fout for crystallographic data collection of complex 3.
1
orange crystals (36.7 mg, 0.051 mmol, 42% yield). H NMR (25
°C, 399.8 MHz, C6D6): δ 7.96 (m, 4H, C6H5), 7.78 (d, 2H, C6H3,
JH-H ) 8 Hz), 7.21 (m, 6H, C6H5), 6.97 (s, 2H, C6H3), 6.87 (d,
2H, C6H3, JH-H ) 8 Hz), 5.42 (t, 1H, SiHPh2), (3JH-P ) 13 Hz),
2.11-2.19 (m, 10H, MeAr and CHMe2 resonances overlapped),
1.17 (dd, 12H, CHMe2), 1.04 (dd, 12H, CHMe2). 13C NMR (25
°C, 75.46 MHz, C6D6): δ 160.89 (aryl), 142.92 (aryl), 137.34 (aryl),
132.38 (aryl), 132.29 (aryl), 127.47 (aryl), 124.15 (aryl), 121.96
(aryl), 121.73 (aryl), 115.33 (aryl), 24.83 (CHMe2), 20.66(MeAr),
19.98 (CHMe2), 18.16 (CHMe2). 31P NMR (25 °C, 121.5 MHz,
C6D6): 42.59 (2JSi-P ) 52 Hz). 29Si NMR (25 °C, 99.5 MHz, C6D6):
Supporting Information Available: Complete crystallographic
data, structure, solution, and refinement details of complexes 2 · Et2O
and 3 · pentane, and other important molecular orbitals and control
reactions for these complexes. This material is available free of
charge via the Internet at http://pubs.acs.org.
(22) SAINT 6.1; Bruker Analytical X-Ray Systems: Madison, WI.
(23) SHELXTL-Plus V5.10; Bruker Analytical X-Ray Systems: Madison,
WI.
OM801187B