Synthesis and reactivity of a conveniently prepared two-coordinate bis(amido) nickel(ii) complex

Article abstract of DOI:10.1039/c2cc32974c

A strictly two-coordinate nickel(ii) bis(amido) complex has been prepared and its reactivity towards a variety of small molecules is described. Ni[N(SiMe₃)(DIPP)]₂ reacts with DMAP and acetonitrile to form T-shaped three-coordinate complexes, and preliminary results show that Ni[N(SiMe₃)(DIPP)]₂ is a catalyst for the hydrosilation of olefins with secondary silanes at ambient temperature.

Full text of DOI:10.1039/c2cc32974c

View Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
ChemComm
Cite this: Chem. Commun., 2012, 48, 7146–7148
www.rsc.org/chemcomm
COMMUNICATION
Synthesis and reactivity of a conveniently prepared two-coordinate
bis(amido) nickel(II) complexw
Michael I. Lipschutz and T. Don Tilley*
Received 26th April 2012, Accepted 23rd May 2012
DOI: 10.1039/c2cc32974c
A strictly two-coordinate nickel(II) bis(amido) complex has been
prepared and its reactivity towards a variety of small molecules is
described. Ni[N(SiMe3)(DIPP)]2 reacts with DMAP and acetonitrile
to form T-shaped three-coordinate complexes, and preliminary results
show that Ni[N(SiMe3)(DIPP)]2 is a catalyst for the hydrosilation of
olefins with secondary silanes at ambient temperature.
Compound 1 was synthesized by reaction of Ni(DME)Br2
with 1.9 equiv of Li[N(SiMe3)(2,6-iPr2C6H3)] in benzene over
6 h at ambient temperature. Workup of the reaction mixture
afforded dark purple crystals of 1 from hexanes in 86% yield.w
The X-ray structure of 1 (Fig. 1) reveals a perfectly linear
structure, with a crystallographically imposed 1801 N–Ni–N
bond angle. The 1.7987(11) A Ni–N bond length is the shortest
Ni–N(amido) distance yet observed. For comparison, related
compounds bearing bulky terphenyl groups, synthesized by
Cui and Power, possess Ni–N bond lengths of 1.818(3) and
1.8284(15) A, respectively.9a,b The two amido ligands adopt an
eclipsed configuration whereby both substituents on the two
trigonal planar nitrogen atoms occupy the same plane. This is
in contrast to the staggered, allene-like structure adopted by
the similar iron compound Fe[N(SiMe3)2]2 in its monomeric
state.8 The eclipsed conformation is also adopted by the Ni(II)
bis(amido) complexes of Cui and Power and appears to be the
most stable, even in the absence of secondary interactions
between the metal and pendent aryl groups on the ligand. The
1H NMR spectrum exhibits strongly paramagnetic shifts that
are assigned by integration (see Supporting Informationw).
The magnetic moment of 2.67 mB, determined by Evans’
method,10 is consistent with two unpaired electrons per nickel.
Compound 1 reacts with the nitrogen donor compounds
p-dimethylaminopyridine (DMAP) and acetonitrile to produce
Two-coordinate metal complexes are a relatively small class of
compounds with unusual structural, electronic, and chemical
properties.1 The low coordination number and inherently
electron deficient nature of these compounds suggest that they
should display a rich reaction chemistry associated with
activations and conversions of a wide array of substrates.
However, chemical reactions of two-coordinate complexes have
not been extensively investigated. Low-coordinate complexes
have been shown to mediate interesting chemical transformations
such as nitrene- and phosphinidine-group transfer to ethylene,2
and the activation of small molecules such as N2O.3 Two-
coordinate compounds have also been shown to activate CO2.4
Most importantly, two-coordinate bis(amido) M[N(SiMe3)2]2
complexes of first row metals have found wide utility as
convenient starting materials for inorganic and organometallic
compounds.5 Metal bis(amido) complexes are also used as
precursors in nanoparticle synthesis.6 In these laboratories, the
recent discovery that Fe[N(SiMe3)2]2 is a catalyst precursor for
the efficient hydrosilation of organic carbonyl compounds7
prompted further investigations of the reaction chemistry of
two-coordinate amido complexes of the first row metals.
While two-coordinate, bis(amido) complexes of iron, cobalt,
and manganese are well known,8 related nickel(II) complexes
have only recently been reported.9 One such compound,
Ni{HN[2,6-(2,6-diisopropylphenyl)]}2, may be described as
pseudo-four-coordinate, in that the solid-state structure contains
two additional close contacts between the nickel center and aryl
groups of the amido ligand.9a Herein we report the synthesis and
characterization of a strictly two-coordinate nickel(II) bis(amido)
complex, its reactivity towards small molecules and preliminary
results on its activity as a catalyst in the hydrosilation of olefins.
Department of Chemistry, University of California, Berkeley,
Berkeley, CA 94720-1460, USA. E-mail: tdtilley@berkeley.edu
w Electronic supplementary information (ESI) available. CCDC
879064–879068. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c2cc32974c
Fig. 1 ORTEP diagram of 1. Thermal ellipsoids shown at 50%
probability. Bond lengths (A) and angles (deg): Ni–N = 1.7987(11);
N–Ni–N = 180.00(8).
c
7146 Chem. Commun., 2012, 48, 7146–7148
This journal is The Royal Society of Chemistry 2012

View Online
three-coordinate, nearly T-shaped complexes Ni[N(SiMe3)-
(DIPP)]2(DMAP) (2) and Ni[N(SiMe3)(DIPP)]2(MeCN) (3),
respectively (Fig. 2). Reaction of 1 with 1 equivalent of DMAP
in pentane for 20 min at ambient temperature, and subsequent
evaporation of the solvent results in quantitative formation
of the analytically pure compound 2. Recrystallization from
pentane afforded 2 as red crystals in 89% yield.
Reaction of compound 1 with 2 equivalents of 2,6-dimethyl-
phenylisocyanide in benzene at ambient temperature over 30 min
resulted in formation of the diamagnetic insertion product 4 in
96% yield, as determined by NMR spectroscopy (Fig. 4). Reaction
of 1 with 1 equivalent of 2,6-dimethylphenylisocyanide resulted in
formation of 0.5 equivalents of 4 and unreacted starting material.
Interestingly, the product arises from insertion of an isonitrile
into an Ni–N bond, accompanied by cleavage of a Ni–Si bond
via 1,3-migration of the trimethylsilyl group from the amido
nitrogen to a nitrogen atom originating from the isonitrile
reactant (Scheme 1). The diamagnetic nature of 4 suggests that
the structure is best regarded as a distorted, square planar
Ni(II) d8 species. The coordinated C–N bond distance is
1.278(2) A, which is slightly longer than expected for a
CQN double bond, but somewhat shorter than similar, less
sterically crowded ligands in related insertion products of
zirconium (1.299–1.315 A).11 While there is significant double
bond character between the carbon and nitrogen bound to the
metal center, the p-system of that bond is oriented perpendicular
to the plane formed by the carbon, nitrogen and nickel atoms,
resulting in no interaction between the p-system and the metal
center. Rather, the ligand is a bidentate XL-type donor, and
isoelectronic with an acyl group.
The structures of both 2 and 3 exhibit significant distortions
from an ideal trigonal planar geometry. The N–Ni–N bond
angles in these compounds are 155.07(7)1 and 161.26(6)1,
respectively, resulting in a nearly T-shaped coordination geometry
(Fig. 3). Interestingly, despite the added steric hindrance, the
eclipsed geometry of the two amido substituents in compound 1 is
almost entirely conserved in both 2 and 3. Furthermore, the
plane occupied by the substituents of both amido ligands
contains the ligand field, which would seem to maximize steric
interactions between the three ligands on the metal center and
their substituents.
Reactions of bis(silylamido) complexes of Co and Fe with
oxygen have been utilized in effective preparations of metal
oxide nanoparticles.6 In this context, the stoichiometric reaction
of 1 with dioxygen, which also results in N–Si bond cleavage,
was examined. Reaction of 1 under 1 atm of O2 (99.99% pure)
produced a mixture of the amine HN(SiMe3)(DIPP) and the
Fig. 2 Reactions of Ni[N(SiMe3)(DIPP)]2 (1).
Fig. 4 ORTEP diagram of 4. Thermal ellipsoids shown at 50% prob-
ability. Bond lengths (A) and angles (deg): Ni1–N2 = 1.9699(16);
Ni1–C24 = 1.8181(19); N2–C24 = 1.278(2); N2–Ni1–C24 = 39.16(7).
Fig. 3 ORTEP diagram of 2. Thermal ellipsoids shown at 50%
probability. Bond lengths (A) and angles (deg): Ni–N1 = 1.8614(15);
Ni–N2 = 1.8717(15); Ni–N3 = 2.0187(16); N1–Ni–N2 = 155.07(7);
N1–Ni1–N3 = 103.71(6); N2–Ni1–N3 = 100.47(6).
Scheme 1 Formation of insertion product 4.
Chem. Commun., 2012, 48, 7146–7148 7147
c
This journal is The Royal Society of Chemistry 2012

View Online
diazo compound (2,6-iPr2C6H3)2N2 (5) in an approximately
4 to 1 molar ratio (by 1H NMR spectroscopy). The diazo
compound was characterized by X-ray diffraction, NMR
spectroscopy and high-resolution mass spectrometry.
the scope, utility and mechanism of this reaction type in various
hydrosilations.
Formation of the diazo compound is accompanied by loss
of ꢀSiMe3 groups to form 0.5 equivalent of (Me3Si)2O per
equivalent of 5 (as determined by 1H and 13C NMR spectroscopy)
and an insoluble brown powder. This powder has proven difficult
to characterize, due to its insolubility in all solvents including 6M
HCl. Infrared spectroscopy revealed no notable features and the
material was determined to be amorphous by powder XRD.
Elemental analysis (CHN) showed that the powder contained
carbon, nitrogen and hydrogen in a ratio which closely matches
that of a ꢀN(SiMe3)(DIPP) group (C, 39.08%; H, 6.30%; N,
3.29%) and the observed stoichiometry is consistent with the
incorporation of one amido group per Ni into this powder.
Reaction of 1 with 1 equivalent of O2 per Ni center resulted
in incomplete consumption of the starting material and a
mixture of the free amine and 5 as before, but in an approxi-
mately 7 to 1 ratio. Presumably, the nitrogen-bound proton of
the observed HN(SiMe3)(DIPP) product arises from the amido
moiety which becomes incorporated into the insoluble powder.
Further, based on the increase in the production of the amine
relative to the diazo product when using sub-stoichiometric
amounts of oxygen, it seems likely that the amine is formed initially,
and is subsequently oxidized to form the diazo compound 5.
This oxidation may be catalyzed by the transient Ni species
remaining from initial reaction with oxygen before formation
of the insoluble Ni-containing material occurs.
ð1Þ
In summary, 1 represents a stable, conveniently prepared
bis(amido) complex containing two-coordinate nickel(II). Initial
reactivity studies indicate that 1 mediates several interesting
chemical transformations. These results should be of use in further
investigations of 1 as a precursor to a variety of nickel-based
compounds, catalysts and materials.
Notes and references
1 (a) P. P. Power, Comments Inorg. Chem., 1989, 8, 177; (b) H. Chen,
R. A. Bartlett, M. M. Olmstead, P. P. Power and S. C. Shoner, J. Am.
Chem. Soc., 1990, 112, 1048; (c) D. L. Kays and A. R. Cowley, Chem.
Commun., 2007, 10, 1053; (d) P. L. Holland, Angew. Chem., Int. Ed.,
2011, 50, 6213.
2 R. Waterman and G. L. Hillhouse, J. Am. Chem. Soc., 2003,
125, 13350.
3 N. D. Harrold, R. Waterman, G. L. Hillhouse and T. R. Cundari,
J. Am. Chem. Soc., 2009, 131, 12872.
4 L. R. Sita, J. R. Babcock and R. Xi, J. Am. Chem. Soc., 1996,
118, 10912.
5 (a) B. A. Frazier, P. T. Wolczanski, E. B. Lobkovsky and T. R. Cundari,
J. Am. Chem. Soc., 2009, 131, 3428; (b) K. R. Nikolaides,
S. D. Hoffmann and J. J. Eppinger, J. Organomet. Chem., 2008,
693, 2223; (c) H. Gosink, H. W. Roesky, H. Schmidt, M. Noltemeyer,
E. Irmer and R. Herbst-Inner, Organometallics, 1994, 13, 3420;
(d) C. Weymann, A. A. Danapolous and G. Wilkinson, Polyhedron,
1996, 15, 3605; (e) T. Deschner, K. W. Tornroos and R. Anwander,
Inorg. Chem., 2011, 50, 7217; (f) R. Anwander, Chem. Mater., 2011,
13, 4419.
6 M. L. Kahn, A. Glaria, C. Pages, M. Monge, L. S. Macary,
A. Maisonnat and B. Chaudret, J. Mater. Chem., 2009, 19, 4044.
7 J. Yang and T. D. Tilley, Angew. Chem., Int. Ed., 2010, 49, 10186.
8 R. A. Andersen, K. Faegri, J. C. Green, A. Haaland, M. F. Lappert,
W. P. Leung and K. Rypdal, Inorg. Chem., 1988, 27, 1782.
9 (a) J. Li, H. Song, C. Cui and J. Cheng, Inorg. Chem., 2008,
47, 3468; (b) A. M. Bryan, W. A. Merrill, W. M. Reiff,
J. C. Fettinger and P. P. Power, Inorg. Chem., 2012, 51, 3366.
10 D. F. Evans, J. Chem. Soc., 1957, 2003.
Given the observed activity of Fe[N(SiMe3)2]2 as a catalyst
for the hydrosilation of carbonyl compounds,7 complex 1 has
been investigated as a catalyst for the hydrosilation of olefins
with secondary silanes. Reaction of 1 with 55 equivalents of
1-octene and 50 equivalents of diphenylsilane in d6-benzene
at room temperature over 2 h resulted in formation of the
anti-Markovnikov addition product (n-octyl)diphenylsilane in
495% yield with no observed isomerization of the olefin to
internal isomers (eqn (1)). Ongoing investigations are addressing
11 Z. Wu, J. Diminnie and Z. Xue, Organometallics, 1999, 18, 1002.
c
7148 Chem. Commun., 2012, 48, 7146–7148
This journal is The Royal Society of Chemistry 2012