Intermediates in nickel(0)-phosphine complex catalyzed dehydrogenative silylation of olefins

Article abstract of DOI:10.1016/j.ica.2005.12.067

Nickel(0) complexes 1-4 containing π-coordinated olefin and triphenylphosphine (tricyclohexylphosphine) (starting from Ni(cod)₂) were prepared and the X-ray structures of 1 and 2 were resolved. The complexes appeared as efficient catalysts in d

Full text of DOI:10.1016/j.ica.2005.12.067

Inorganica Chimica Acta 359 (2006) 2989–2997
www.elsevier.com/locate/ica
Intermediates in nickel(0)–phosphine complex catalyzed
dehydrogenative silylation of olefins
a
a
a,
a
Hieronim Maciejewski , Agnieszka Sydor , Bogdan Marciniec *, Maciej Kubicki ,
b
Peter B. Hitchcock
a
Faculty of Chemistry, Adam Mickiewicz University, Grunwaldzka 6, 60-780 Poznan´, Poland
b
The Chemical Laboratory, University of Sussex, Brighton BN1 9QJ, UK
Received 24 November 2005; accepted 31 December 2005
Available online 9 March 2006
Dedicated to Professor Brian James on the occasion of his 70th birthday in recognition of his great contribution to homogeneous catalysis.
Abstract
Nickel(0) complexes 1–4 containing p-coordinated olefin and triphenylphosphine (tricyclohexylphosphine) (starting from Ni(cod)2)
were prepared and the X-ray structures of 1 and 2 were resolved. The complexes appeared as efficient catalysts in dehydrogenative sily-
lation of styrene and vinyltris(trimethylsiloxy)silane, but only after prior oxygenation of phosphine ligand. Stoichiometric studies of
Ni(0) complexes with substrates showed that the bis(silyl)nickel(II) complex was a key intermediate in both reactions examined. A
scheme of catalysis by Ni(0) complex involving olefin insertion into Ni–Si bond, as a crucial step, is presented.
ꢀ 2006 Elsevier B.V. All rights reserved.
Keywords: Nickel–silyl complexes; Dehydrogenative silylation; Catalysis
1. Introduction
catalyzed not only by complexes of the iron and cobalt tri-
ads but also by nickel complexes as a result of the dehy-
drogenative silylation of olefins, proceeding competitively
with hydrosilylation [7–9]. A key step for the dehydrogena-
tive silylation catalyzed by late transition-metal complexes
is the insertion of alkene into the M–Si bond (for recent
review, see [9]). The first mechanism involving this step
was presented by Seitz and Wrighton, on the basis of the
photochemical study of the reaction with Co–Si complex
[8].
Our previous examination of the reaction between tri-
substituted silanes and vinylsilanes [10,11], styrene [11,12]
and other olefins [12] has shown that in the presence of
nickel complexes the dehydrogenative silylation proceeds
via three ways yielding the unsaturated product and the
respective hydrogenated product of direct dehydrogenation
(DC) and olefin hydrogenation (DS-1), as well as hydroge-
nated dimerization (DS-2), according to the following
equations:
Hydrosilylation is a versatile reaction in organosilicon
chemistry and apart from the ‘‘direct process,’’ the only
industrially used silicon–carbon bond formation method
[1–5]. The mechanism of catalysis of hydrosilylation by
platinum complexes, later generalized for other transition
metal complexes, was proposed by Chalk and Harrod [6].
It involves oxidative addition of trisubstituted silanes to a
metal–alkene complex, followed by migratory insertion of
alkene to M–H bond and reductive elimination of the
hydrosilylation product from the metal center. Although
the basic aspects of this mechanism still enjoy wide accep-
tance, it has failed to explain a number of phenomena, e.g.,
the formation of an unsaturated organosilicon product.
Such unsaturated compounds are formed in the reactions
*
Corresponding author. Tel.: +48 61 8291 366; fax: +48 61 8291 508.
E-mail address: marcinb@amu.edu.pl (B. Marciniec).
0020-1693/$ - see front matter ꢀ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2005.12.067

2990
H. Maciejewski et al. / Inorganica Chimica Acta 359 (2006) 2989–2997
R'
The aim of the study reported in this paper was to exam-
ine the initial species formed in the system
[Ni(cod)2] + PR3 + substrates in order to identify the
active intermediates in the reaction of H–Si species (i.e.,
hydro-silanes and -siloxanes) and olefins (styrene and
vinyl-siloxanes).
R'
+ HSiR3
+
H2
(DC)
SiR3
R'
R'
R'
2
+
+ HSiR3
+ HSiR3
(DS-1)
(DS-2)
SiR3
R'
R'
3
R'
+
R'
2. Experimental
SiR3
accompanied by the small amount of hydrosilylation
products:
2.1. Materials
All reagents were dried and purified before use by the
usual procedures. Triethoxysilane was obtained by alcohol-
ysis of HSiCl3 and thoroughly purified to avoid the pres-
ence of traces of HCl. [Ni(cod)2] was prepared as
described in the literature [27]. Other chemicals were pur-
chased from Aldrich.
R'
R'
R'
+ HSiR3
+
(H)
ð1Þ
SiR3
SiR3
The Seitz and Wrighton mechanism does not explain the
possibility of formation of unsaturated product in the three
different reactions. On the other hand, this mechanism
involves insertion of alkene into the M–Si bond, but for
the nickel catalyzed reactions, a stable complex containing
one silyl group bonded to nickel atom has not been isolated
yet. However, a few stable bis(silyl)nickel complexes with
strong electron withdrawing substituents at silicon atoms
have been isolated and characterized [13–22]. Our recent
contribution to this field is the first example of nickel–silyl
complexes of the type [Ni(PR3)2(SiMenCl3 ꢀ n)2], containing
at least one alkyl group at silicon, which has been synthe-
sized and fully characterized [22]. Also our earlier study
of the reaction between triethoxysilane and olefins, cata-
lyzed by nickel equivalent of Karstedt catalyst [{Ni(g-
CH2@CHSiMe2)2O}2{l-(g-CH2@CHSiMe2)2O}], has led
to the isolation of a bis(silyl)nickel complex containing
divinyltetramethyldisiloxane as chelating ligands, besides
two triethoxysilyl groups [11]. The presence of these chelat-
ing ligands stabilizes this complex, so Ni(0) complex under
catalytic conditions does not undergo reduction to Ni
metal, which is commonly observed in the reaction with
hydrosilanes and other nickel complexes. Tertiary phos-
phines may also stabilize metal complexes and additionally
they play a crucial role in many industrially important TM-
catalyzed organic reactions [23]; therefore, they could be
used for the isolation of complexes forming during the cat-
alytic reactions.
Bis(cyclooctadiene)nickel(0) is a popular catalyst and a
good starting material for the synthesis of other nickel
complexes. Cyclooctadiene is a very labile ligand which
readily undergoes displacement by many olefinic ligands,
including vinyl-organosilicon compounds, e.g., vinylsilox-
ane [11,24], vinylsilazane [25] and vinylsilane [26], and also
tertiary phosphine. [Ni(cod)2] is also an effective precursor
of the dehydrogenative silylation reactions [11,12] but due
to its high reactivity, the isolation of the ‘‘real catalyst’’
(formed in the course of the catalytic reaction) was unsuc-
cessful. When [Ni(cod)2]-based catalyst was used, after the
completion of the reaction, a precipitate of nickel–metal
was observed. We used tertiary phosphine to prevent the
reduction of the nickel complex.
2.1.1. Synthesis of [Ni(PPh3)2(cod)] (1)
A solution of 1.05 g (4.0 mmol) PPh3 in 10 mL of diethyl
ether was added to 0.55 g (2 mmol) [Ni(cod)2] yielding a
red suspension. After a few minutes, a yellow solid precip-
itated. The mixture was stirred for 2 h at room tempera-
ture. Then, the solid was filtered and washed with diethyl
ether (3 · 5 mL) and dried under vacuum yielding complex
1 (yield 90%). Yellowish-gold crystals of 1 suitable for X-
ray diffraction were grown from benzene at ambient tem-
perature over a period of 24 h.
Anal. Calc. for C44H42NiP2: C, 76.43; H, 6.12. Found:
C, 76.21; H, 6.22%.
1H NMR (C6D6, 298 K, 300 MHz); d (ppm): 1.6 (s, 4H),
1.8 (s, 4H), 4.8 (s, 4H), 6.9–7.6 (m, 30H).
31P NMR (C6D6, 298 K, 101.2 MHz); d (ppm): 42.0
(s).
2.1.2. Synthesis of [Ni(PPh3)2(g2-CH2@CHC6H5)] (2)
To the suspension of 0.55 g (2 mmol) [Ni(cod)2] in
10 mL of diethyl ether, 1.05 g (4 mmol) of PPh3 was added.
Next, 1.5 mL of styrene was added to a stirred reaction
mixture. Suddenly, the suspension was dissolved and then
after 15 min an orange solid started to precipitate. The
reaction mixture was stirred for 24 h at room temperature
and the solid was filtered and washed by diethyl ether (95%
of yield). Yellow crystals of 2 suitable for X-ray diffraction
were grown from benzene at ambient temperature over a
period of 24 h.
Complex 2 was also independently synthesized at the
same conditions but using complexes 1 and 3, as starting
materials and yielded a desired complex 2 with 87% and
82% yield, respectively.
Anal. Calc. for C44H38NiP2: C, 76.88; H, 5.57. Found:
C, 76.65; H, 5.69%.
1H NMR (C6D6, 298 K, 300 MHz); d (ppm): 2.66 (m,
@CH2); 2.93 (m, @CH2); 4.13 (m, –CH@); 6,52, 7.15,
7.53 (Ph).
13C NMR (C6D6, 298 K, 125.8 MHz); d (ppm): 118.0 (s,
@CH2); 127 (m, Ph); 133.2 (d, –CH@).

H. Maciejewski et al. / Inorganica Chimica Acta 359 (2006) 2989–2997
2991
31P NMR (C6D6, 298 K, 101.2 MHz); d (ppm): 37.3 (d);
2.3. Structure determinations
32.1 (d).
All data were measured using monochromatic Mo Ka
˚
radiation (k = 0.71073 A). For 2, the measurement was
2.1.3. Synthesis of [Ni(PPh3)2C2H4] (3)
Compound 3 was synthesized as described in the litera-
ture [28,29]. Analytical data of 3 are identical with those
from the literature.
performed at 173(1) K on an Enraf-Nonius CAD4 four-cir-
cle diffractometer [30] by the x–2h scan technique; for 1 the
same scan technique was used on a KUMA KM4 four-cir-
cle diffractometer [31], at 293 K. The temperature was con-
trolled by an Oxford Instruments Cryosystem cooling
device. The data were corrected for Lorentz-polarization
effects [31,32] as well as for absorption [31,33]. Accurate
unit-cell parameters were determined by a least-squares
fit of 25 (1) and 50 (2) reflections. The structures were
solved with SHELXS-97 [34] and refined with the full-matrix
least-squares procedure on F2 by SHELXL-97 [35]. Scattering
2.1.4. Synthesis of [Ni(PCy3)2(g2-CH2@CHC6H5)] (4)
Compound 4 was prepared in a similar manner as 2,
except that PCy3 was used instead of PPh3.
1H NMR (C6D6, 298 K, 300 MHz); d (ppm):1.1–2.2 (m,
66H), 2.4 (m, 2H), 4.04 (t, 1H), 7.00–7.05 (m, 2H), 7.09–
7.30 (m, 2H), 7.48 (d, 1H).
13C NMR (C6D6, 298 K, 125.8 MHz); d (ppm): 27.47–
37.62 (m, C6H11-c), 48.81 (s, @CH-), 52.28 (s, @CH2),
131.25, 128.37, 125.82, 121.77 (s, Ph).
factors incorporated in SHELXL-93 were used. The function
P
w(|Fo|2 ꢀ |Fc|2)2 was minimized, with wꢀ1 = [r2(Fo)2 +
(A Æ P)2 + BP ðP ¼ ½maxðF o2; 0Þ þ 2F 2cꢁ =3Þ. No empirical
extinction corrections were applied. The final values of A
and B are listed in Table 1. All non-hydrogen atoms were
refined anisotropically. In 1, hydrogen atoms were put in
idealized positions and refined as a ‘riding model’, with Uiso
set at 1.2 times Ueq of appropriate carrier atom. In 2,
31P NMR (C6D6, 298 K, 101.2 MHz); d (ppm): 37.36
(d).
2.2. Stoichiometric reactions
2.2.1. Reaction of 2 with triethoxysilane – synthesis of
[Ni(PPh3)2{Si(OEt)3}2] (5)
Table 1
Crystal data, data collection and structure refinement
Triethoxysilane (0.2 mL, 1 mmol) was added under
argon, to a rapidly stirred solution of [Ni(PPh3)2{g2-
(CH2@CHC6H5)}] (2) (0.30 g, 0.43 mmol) in benzene
(5 mL), in a Schlenk tube. The reaction mixture was stirred
for 6 h at 60 ꢁC. After this time, some amount of black-red
solid precipitated, was filtered off, washed with pentane
and dried under vacuum. The GC–MS analysis of filtrate
showed the presence of ethylbenzene. The obtained com-
plex 5 was analyzed by NMR method.
Compound
1
2
Formula
Ni(PPh3)2(C8H12
691.43
monoclinic
Cc
)
Ni(PPh3)2(C8H8)
687.39
orthorhombic
Pna21
24.657(3)
13.009(2)
10.955(2)
90
90
90
3514.0(9)
4
1.299
Formula weight
Crystal system
Space group
˚
a (A)
17.504(4)
11.370(2)
17.951(4)
90
102.82(3)
90
3483.6(13)
4
1.318
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
1H NMR (C6D6, 298 K, 300 MHz); d (ppm): 0.91–1.18
(m, 18H), 3.69 (q, 12H); 6.90–7.05 (m, 18H); 7.25–7.39
(m, 12H).
3
˚
V (A )
Z
Dcalc (g cmꢀ3
)
13C NMR (C6D6, 298 K, 125.8 MHz); d (ppm): 18.29 (s,
CH3), 56.40 (s, –OCH2); 130.10, 131.57, 132.38, 135.23
(s, –Ph).
l (mmꢀ1
)
0.680
0.4 · 0.2 · 0.15
3–26
0.674
0.3 · 0.2 · 0.1
3–25
Crystal size (mm)
h Range (ꢁ)
hkl Range
0 6 h 6 21,
0 6 k 6 14,
ꢀ22 6 l 6 21
3544
ꢀ29 6 h 6 29,
0 6 k 6 13,
0 6 l 6 139
6251
2.2.2. Reaction of [Ni(PCy3)2(g2-CH2@CHC6H5)] (4)
with heptamethyltrisiloxane
Number of reflections
collected
0.34 g (0.5 mmol) of 4 was dissolved under argon in
0.6 mL of C5D5CD3 in an NMR tube. A portion of
0.25 mL (1 mmol) heptamethyltrisiloxane was added to
the solution and the course of the reaction was monitored
Number of unique
3540
3086
424
3172 (0.0352)
2738
reflections (Rint
)
Number of reflections
with [I > 2r(I)]
Number of parameters
Weighting scheme
A
1
436
by H NMR at ꢀ20 ꢁC.
0.0511
1.3389
0.0300
0.0758
0.0452
0.0834
1.114
0.0238
0.0
B
2.2.3. Reaction of 5 with styrene
R(F) [I > 2r(I)]
wR(F2) [I > 2r(I)]
R(F) [all data]
wR(F2) [all data]
Goodness-of-fit
0.0303
0.0601
0.0410
0.0640
1.035
1 mL of styrene was added to the solution of 5 (0.2 g,
0.22 mmol) in benzene (5 mL). The reaction mixture was
stirred for 2 h at 60 ꢁC. After this time, the mixture was
analyzed by GC–MS method, which showed the presence
of 1-phenyl-2-silylethene and phosphine oxide.
ꢀ3
˚
Maximum/minimum Dq (e A
)
0.70/ꢀ0.37
0.34/ꢀ0.21

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H. Maciejewski et al. / Inorganica Chimica Acta 359 (2006) 2989–2997
hydrogen atoms from terminal CH2 group were refined,
others were treated as in 1. Relevant crystal data are listed
in Table 1, together with refinement details.
[Ni(cod)2]
+ 2 PPh3
- cod
+ 2 PPh3
+ 2 PPh3
- 2 cod
+ C2H4
+
Ph
2.4. Equipment and analytical measurements
- 2 cod
The NMR spectra (1H, 13C, 31P, 29Si) were recorded on
a Varian XL 300 spectrometer. In all cases, C6D6 was used
as a solvent; GC–MS analyses were carried out with a Var-
ian 3300 chromatograph (equipped with a DB-1, 30 m cap-
illary column), connected to a Finnigan Mat 700 mass
detector. GC analysis was also carried out with a Varian
3800 chromatograph with Megabore column (30 m, DB-1).
Ph3P
Ph3P
Ph3P
+
+
Ph3P
Ph3P
Ph
- cod
Ph
Ni
Ni
Ni
- C2H4
Ph3P
Ph
2
4
3
1
or (contains PCy instead of PPh )
3
3
Scheme 1.
2.5. General procedure for catalytic test
The catalyst was placed in a Schlenk tube, equipped
with a condenser, and filled with a mixture of styrene and
triethoxysilane (or heptamethyltrisiloxane and vinyl-
tris(trimethylsiloxy)silane). All manipulations were carried
out using the standard Schlenk and high vacuum line tech-
niques. The Schlenk tubes were heated at a given tempera-
ture, under argon (or in the presence of air). The
distribution of substrates and products, conversion of sub-
strates and the yield of products were determined by GC–
MS and GC analyses.
3. Results and discussion
3.1. Synthesis and structure of nickel(0) complexes
Fig. 1. Anisotropic ellipsoid representation of complex 1. The ellipsoids
are drawn at 33% probability level, hydrogen atoms are depicted as
spheres with arbitrary radii. X1a, X1b are the middlepoints of C@C
bonds.
The addition of triphenylphosphine to [Ni(cod)2] (in
the ratio 2:1) leads to the synthesis of [Ni(PPh3)2(cod)]
(1) with a high yield. The excess of PPh3 or the presence
of other olefins (e.g., styrene, ethylene) in this system
causes a displacement of the second molecule of cyclooct-
adiene yielding [Ni(PPh3)4], [Ni(PPh3)2(g-CH2@CHC6H5)]
(2) and [Ni(PPh3)2C2H4] (3), respectively. Due to a stron-
ger coordination of styrene to the metal center, when
compared to the other olefinic ligands, the formation of
complex [Ni(PPh3)2(g-CH2@CHC6H5)] (2) was observed
in all cases when styrene was present in the system. Com-
plexes 1, 2 and 3 were synthesized and fully characterized.
Complex 2 was obtained in the direct reaction from
[Ni(cod)2] and PPh3 or from complex 1 or 3 with a high
yield (Scheme 1). The replacement of PPh3 by PCy3 per-
mitted the synthesis of the counterpart of complex 2,
[Ni(PCy3)2CH2@CHC6H5] (4), in an analogous reaction.
The synthetic routes to complexes 1–4 are illustrated in
Scheme 1.
Complex 3 is well known but the others have not been
fully characterized yet, so the crystals of 1 and 2 suitable
for X-ray diffraction were used to resolve their crystal
structure.
Anisotropic ellipsoid representations of molecules 1 and
2 are presented in Figs. 1 and 2, respectively. Table 2 lists
the relevant geometrical parameters.
Fig. 2. Anisotropic ellipsoid representation of complex 2. The ellipsoids
are drawn at 50% probability level, hydrogen atoms are depicted as
spheres with arbitrary radii. X1 is the middlepoint of C@C bonds.

H. Maciejewski et al. / Inorganica Chimica Acta 359 (2006) 2989–2997
2993
In all cases the middlepoints of the double bonds were
assumed as coordination centers. In 1, the coordination of
the Ni atom is close to tetrahedral (cf. Table 2), slightly
distorted due to the different ligands (a very similar geom-
etry was found in a closely related bis-ethyldiphenylphos-
phine complex [36]). On the other hand, in complex 2 the
coordination number of the central Ni atom is 3, and
The amount of DS (PhCH@CHSi(OEt)3) is equal to the
sum of the second products of hydrogenative silylation
(C2H5Ph), hydrogenative dimerization (Ph(CH2)4Ph) and
hydrogen evolved.
The reaction that occurred in the presence of air showed
the preference of dehydrogenative coupling DC over dehy-
drogenative silylation DS-1 and particularly DS-2. In all
cases, the dehydrogenative silylation DS pathway domi-
nates over hydrosilylation H.
The nickel complexes 1, 2, and 4 were also used as cat-
alysts in the reaction of vinyltris(trimethylsiloxy)silane with
heptamethyltrisiloxane, which is a model reaction for
cross-linking of polysiloxane via hydrosilylation:
˚
nickel lies very close (0.044(1) A) to the plane defined
by P1, P2 and X (middlepoint of the double C@C bond)
– in this case, the coordination is therefore trigonal. It
was found that Ni–P distances in the crystal structures
show bimodal distribution with the mean values around
˚
2.187 and 2.321 A, without apparent chemical distinction
between these two groups [37]. The Ni–P distances in 1
and 2 belong to the group with shorter bond lengths
(2.16–2.19 A).
OSiMe3
OSiMe3
OSiMe3
H2C=HC Si
Me Si
H
+
˚
OSiMe3
OSiMe3
The phenyl rings are almost ideally planar (maximum
˚
deviation from the least squares plane is 0.020(2) A).
OSiMe3
OSiMe3
OSiMe3
OSiMe3
OSiMe3
Their mutual disposition may be described by the dihedral
angles between their planes or by the dihedral angles
between the phenyl ring and the P–Ni–P plane. The latter
values in 1 are all large, almost close to 90ꢁ (65–89ꢁ; mean
value is 80ꢁ), while in 2 these values are significantly smal-
ler (mean values are 64ꢁ). In 1, the cycloocta-1,5-diene
fragment adopts a nearly ideal boat conformation. This
is confirmed by the values of torsion angles (Table 1)
and of DC2 asymmetry parameter of 1.35ꢁ. The bond
lengths pattern clearly shows a distinction between single
and double bonds.
+ C2H5 Si
Me Si CH=CHSi
OSiMe3 OSiMe3
OSiMe3
B
A
OSiMe3
OSiMe3
+
OSiMe3
Me Si CH2CH2Si
OSiMe3
OSiMe3
C
ð3Þ
1
The H, 13C and 31P NMR spectra of complexes 1–4,
The obtained results are presented in Table 4.
show the signals in the expected region, typical of the phos-
All the obtained results show that tertiary phosphines
strongly stabilize nickel complexes; therefore, they are
inactive in the atmosphere of argon. However, after step-
wise elimination of phosphine via oxygenation in air,
these complexes are as active as [Ni(cod)2], and they cat-
alyzed mainly dehydrogenative silylation of olefins. In the
phine nickel p-complexes.
3.2. Catalytic examinations
The above complexes 1–3 were used as catalysts in the
reaction between styrene and triethoxysilane:
Si(OEt)3
+
(DS-1)
Si(OEt)3
(DC)
+
H2
ð2Þ
HSi(OEt)3
+
Si(OEt)3
Si(OEt)3
+
(DS-2)
(H)
Besides, the presence of small amounts of tetrasubsti-
tuted silanes Si(OEt)4 and siloxanes, as by-products com-
ing from disproportionation of HSi(OEt)3, and their
condensation, is also observed. The yields of these reac-
tions and product distributions are presented in Table 3.
dehydrogenative silylation of styrene by triethoxysilane in
air, complex 2 containing styrene as a ligand was the most
active.
It is worthy to say that phosphines stabilize Ni com-
plexes (contrary to complexes without phosphines, i.e.,

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H. Maciejewski et al. / Inorganica Chimica Acta 359 (2006) 2989–2997
Table 2
[Ni(cod)2]) preferring the dehydrogenative silylation via
˚
Relevant geometrical parameters (A, ꢁ)
Compound 1
direct dehydrogenation step (DC) over DS-1 and DS-2.
On the other hand, in the dehydrogenative silylation of
vinyltris(trimethylsiloxy)silane, no hydrogenated dimeriza-
tion step (DS-2) was observed. The catalytic activity of the
complexes examined depends on the kind of phosphine, as
follows from the observations of the hydrosilylation of
vinyltris(trimethylsiloxy)silane by heptamethyltrisiloxane.
Oxidation of tricyclohexylphosphine proceeds easier than
that of triphenylphosphine; therefore, the complex [Ni(P-
Cy3)2(CH2@CHC6H5)] (4) is more active than [Ni(PPh3)2-
(CH2@CHC6H5)] (2) in the examined reaction proceeding
in the air.
Compound 2
Ni–P
Ni–X
ÆP–Cæ
2.182(1)
2.189(1)
2.156(1)
2.168(1)
1.9984(6)
2.0160(5)
1.849(7)
1.839(4)
1.835(4)
ÆC–Siæ
P–Ni–P
107.56(4)
116.67(4)
P–Ni–X
117.13(4)
111.59(3)
112.83(4)
116.99(3)
119.18(3)
124.02(4)
3.3. Stoichiometric reactions
X–Ni–X
90.35(2)
X denotes the middlepoint of the double bond, regarded as the coordi-
nation site.
It is obvious that the first step in the catalysis of
dehydrogenative silylation of olefins by Ni(0) complexes
is the coordination of olefin to nickel (generally by the
displacement of ligands) but in order to explain the next
steps, a separate study of the reaction of 2 (and 4) in
excess of triethoxysilane at 60 ꢁC was undertaken (see
Scheme 2).
Table 3
Dehydrogenative silylation vs. hydrosilylation of styrene with triethox-
ysilane catalyzed by nickel(0) complexes
Catalyst
Yield (%)
Product distribution
Ratio DC:DS-1:DS-2
DS
H
Other
After 6 h, in the reaction mixture, ethylbenzene was
detected by GC–MS analysis. After the removal of vola-
tiles, the product was characterized by NMR analysis.
The location and intensity of signals in NMR spectra con-
firmed the presence of two triethoxysilyl groups and two
molecules of triphenylphosphine at the nickel center. The
formation of the same type of bis(silyl)nickel complex
was reported previously by us; however, this complex was
stabilized by divinyltetramethyldisiloxane [11]. The reac-
tion between 2 and triethoxysilane was very fast, so it
was not possible to observe the first complex 5a – product
of oxidative addition of one molecule of triethoxysilane;
however, it was possible to observe it in the reaction of 4
with heptamethyltrisiloxane. Low-temperature 1H NMR
spectra of the mixture of 4 and trisiloxane showed signals
at ꢀ15.9 ppm (see Fig. 3), which is the evidence for the
presence of hydrido-nickel complex 5a.
Ni(cod)2
1
1a
83
5
76
7
90
4
79
85
80
78
81
90
76
80
11
5
10
8
8
4
4
15
12
11
2
0.20:0.46:0.34
0.43:0.38:0.19
0.64:0.26:0.10
0.43:0.43:0.14
0.64:0.33:0.02
0.41:0.33:0.26
0.56:0.38:0.06
2
2a
3
20
5
3a
15
[HSi„]: [CH2@CHPh]:[cat] = 1:2:10ꢀ3; T = 120 ꢁC, t = 2 h; in argon.
a
In air.
Table 4
The reaction of vinyltris(trimethylsiloxy)silane with heptamethyltrisilox-
ane catalyzed by nickel(0) complexes
Catalyst
Conversion (%)
Yield of
product (%)
Ratio DC:DS:H
HSi„ CH2@CHSi
A
B
C
[Ni(cod)2] 88
91
6
57
3
42
8
90
64 19
5
0.65:0.28:0.07
0.55:0.39:0.06
0.60:0.36:0.04
0.69:0.26:0.05
1
5
54
2
39
6
3
2
trace
2
trace
1
trace
3
1a
2
36 15
The addition of the next molecule of trisiloxane led to
obtaining the bis(silyl)nickel complex with simultaneous
elimination of ethylbenzene (observed by GC–MS
method).
1
1
2a
4
27 10
4
2
4a
87
65 18
[HSi„]:[CH2@CHSi]:[cat] = 1:2:10ꢀ3; T = 120 ꢁC; t = 24 h; in argon.
The bis(silyl)nickel complexes seem to be the real
catalyst (the active intermediates) in all reactions between
a
In air.
Ph
Ph
R3P
R3P
R3P
R3P
R3P
R3P
R3P
R3P
Si
Si
+ HSi
+ HSi
Ni
Ni
Ni
Ni
Si
Si
-
Ph
Ph
H
2, 4
5a
5
where
R = Ph, Cy
=
HSi
HSi(OEt)3, HSiMe(OSiMe3)2
Scheme 2.

H. Maciejewski et al. / Inorganica Chimica Acta 359 (2006) 2989–2997
2995
Fig. 3. Low-temperature 1H NMR spectra of the mixture of 4 and heptamethyltrisiloxane.
trisubstituted silanes and olefins catalyzed by nickel com-
plexes. Confirmation of this fact is the reaction (in air) of
complex 5 with excess of styrene at 60 ꢁC. After the reac-
tion, similarly as in the previously reported results [11],
the product of dehydrogenative silylation (1-phenyl-2-sily-
lethene) was detected by the GC–MS analysis, which evi-
denced the insertion of styrene molecule into the Ni–Si.
Additionally, phosphine oxide was observed.
nickel(II) ones in the dehydrogenative silylation processes,
because in most cases the formation of bis(silyl)nickel com-
plex proceeds easier from nickel(0) precursor. Regardless of
the nickel precursor used, the formed bis(silyl)nickel com-
plex is an active intermediate responsible for the formation
of the dehydrogenative silylation products, according to
Scheme 3 (side hydrosilylation process is omitted for clarity).
The evidence for this is the insertion of styrene into the
Ni–Si bond in complex
5
and [Ni{(CH2@CHSi-
3.4. Mechanistic implications
Me2)2O}(CH2@CHC6H5)] [11] and also ethylene into the
complex [Ni(PBu3)2(SiMeCl2)2] [22]. In all of these reac-
tions, the unsaturated products of dehydrogenative silyla-
tion were formed. As follows from analysis of the
catalytic data, DC competes with DS-1 cycle in both dehy-
drogenative silylation processes studied. The insertion of
olefin into M–Si bond (silyl migration) proceeds more
readily than into M–H bond (hydrid migration), which is
a characteristic feature of general Seitz–Wrighton mecha-
nism of dehydrogenative silylation. In the silylation of
vinyltris(trimethylsiloxy)silane, the latter molecule can sta-
bilize 8 even in the absence of phosphine. In this reaction,
the DS-2 process is not observed, presumably due to a ste-
ric hindrance of vinylsiloxane, preventing its insertion into
Ni–C bond (complex 5).
The isolation and also spectroscopic and crystallo-
graphic characterization of bis(silyl)platinum complex have
been recently described [38]. On the basis of the results
from the catalytic and stoichiometric reactions, the authors
postulate that bis(silyl)platinum complex is the truly cata-
lytic species responsible for Pt-catalyzed dehydrogenative
silylation processes.
The key step of catalysis by nickel complexes is the for-
mation of bis(silyl) species depending on the starting nickel
precursor. In the case of Ni(0) precursor, at the beginning
the coordination of olefins to metal is observed. This coor-
dinated olefin plays the role of a hydrogen acceptor from
hydrosilanes; therefore, after elimination of alkane (prod-
uct of hydrogenation of olefins) the bis(silyl)nickel complex
is formed, as a product of oxidative addition of two mole-
cules of silane to the nickel. The formation of bis(silyl)-
nickel complex from Ni(II) precursors occurs via the
oxidative addition of silane followed by the replacement
of an ionic ligand from Ni(II) precursor [22]. Therefore,
the differences in the catalytic activity of nickel(0) and nick-
el(II) precursors are observed. Due to high coordination of
tertiary phosphines to nickel center, the complexes
obtained are inactive; however, the use of phosphine per-
mitted isolation of the intermediates formed in the course
of the catalytic reaction.
On the basis of our experience, we may say that generally
the nickel(0) complexes are more active catalysts than the

2996
H. Maciejewski et al. / Inorganica Chimica Acta 359 (2006) 2989–2997
R'
H
R'
R'
R'
[Ni]
SiR3
SiR3
(11)
HSiR3
R'
R'
SiR3
SiR3
[Ni]
DS-2
(1)
SiR3
R'
H
R'
R'
R'
[Ni]
[Ni]
SiR3
[Ni]
(2)
(3)
(6)
SiR3
SiR3
SiR3
(10)
DS-1
R'
SiR3
R'
HSiR3
H
[Ni]
[Ni]
SiR3
SiR3
R'
(5)
R'
R'
H
[Ni]
SiR3
(4)
HSiR3
H
(7)
SiR3
[Ni]
[Ni]
(9)
DC
R'
R'
H2
(8)
H
H
[Ni]
R'
R'
SiR3
Scheme 3.
4. Conclusions
5. Supplementary material
1. Nickel(0) complexes 1–4 containing p coordinated ole-
fins have been prepared starting from Ni(cod)2, olefins
and triphenylphosphine (tricyclohexylphosphine) as
substrates. The X-ray crystal structures were resolved
for two of them 1 and 2.
2. Complexes 1–4 were used as catalysts of dehydrogena-
tive silylation of styrene and vinyltris(trimethylsil-
oxy)silane, showing high efficiency only after prior
elimination of phosphine via oxygenation in air.
3. Equimolar reactions of 2 and 4 with substrates evi-
denced that bis(silyl)nickel(II) complex is a key interme-
diate in the reactions examined. A scheme of catalysis of
dehydrogenative silylation by all Ni(0) complexes (1–4)
including an insertion of olefin into Ni–Si bonds as a
crucial step, is proposed.
Crystallographic data for the structural analysis have
been deposited with the Cambridge Crystallographic
Data Centre, Nos. CCDC 232292 (1) and CCDC
232293 (2). Copies of the information may be obtained
free of charge from The Director, CCDC, 12 Union
Road, Cambridge, CB2 1EZ, UK. (fax: +44 1223 336
033, e-mail: deposit@ccdc.cam.ac.uk or www: http://
www.ccdc.cam.ac.uk).
Acknowledgment
This work was supported by the State Committee for
Scientific Research (Poland), Project No.
121 26.
3 T09B

H. Maciejewski et al. / Inorganica Chimica Acta 359 (2006) 2989–2997
2997
´
[20] M. Brzezinski, J. Schneider, L.J. Radonovich, K.J. Klabunde, Inorg.
References
Chem. 28 (1989) 2414.
[21] S. Schimada, M.L. Rao, M. Tanaka, Organometallics 18 (1999) 291.
[1] B. Marciniec (Ed.), Comprehensive Handbook on Hydrosilylation,
Pergamon Press, Oxford, 1992.
´
[22] H. Maciejewski, B. Marciniec, J. Gulinski, A. Karolak, N.K.
Skvortsov, Inorg. Chem. Commun. 5 (2002) 464.
[23] B. Cornils, W.A. Herrmann, Applied Homogeneous Catalysis with
Organometallic Compounds, VCH, Weinheim, 1996.
[24] P.B. Hitchcock, M.F. Lappert, H. Maciejewski, J. Organomet. Chem.
605 (2000) 221.
[25] H. Maciejewski, M. Kubicki, B. Marciniec, A. Sydor, Polyhedron 21
(2002) 1261.
[26] H. Maciejewski, A. Sydor, M. Kubicki, J. Organomet. Chem. 689
(2004) 3075.
[27] D.J. Krysan, P.B. Mackenzie, J. Org. Chem. 55 (1990) 4229.
[28] C.A Tolman, W.M. Riggs, W.J. Linn, C.M. King, R.C. Wendt,
Inorg. Chem. 12 (1973) 2770.
[29] P. Giannoccaro, A. Sacco, G. Vasapollo, Inorg. Chim. Acta 37
(1979) L455.
[30] CAD-4 Software, Version 5, Enraf-Nonius, Delft, The Netherlands,
1989.
[31] KUMA, KUMA KM4 Software, Version 5.0, KUMA Diffraction,
Wrocław, Poland, 1992.
[32] CRYSALISRED: Program for Reduction of the Data from Kuma CCD
Diffractometer, Kuma Diffraction, Wrocław, Poland, 1999.
[33] R.H. Blessing, J. Appl. Crystallogr. 22 (1989) 396.
[34] G.M. Sheldrick, Acta Crystallogr., Sect. A 46 (1990) 467.
[35] G.M. Sheldrick, SHELXL-97: Program for the Refinement of
Crystal Structures, University of Go¨ttingen, Go¨ttingen, Germany,
1997.
[36] R. Kempe, J. Sieler, D. Walther, Z. Kristallogr. 211 (1996) 565.
[37] F.H. Allen, O. Kennard, D.G. Watson, L. Brammer, G. Orpen, R.
Taylor, J. Chem. Soc., Dalton Trans. (1987) S1.
[38] A.K. Roy, R.B. Taylor, J. Am. Chem. Soc. 124 (2002) 951.
[2] B. Marciniec, in: B. Cornils, W.A. Herrman (Eds.), Applied
Homogenous Catalysis with Organometallic Compounds, Verlag
Chemie, Weinheim, 2002 (Chapter 2.6).
[3] I. Ojima, Z. Li, J. Zhu, in: Z. Rapport, Y. Apehoig (Eds.), The
Chemistry of Organic Silicon Compounds, Wiley, New York, 1998.
[4] M.A. Brook, Organic, Organometallic and Polymer Chemistry,
Wiley, New York, 2000.
[5] B. Marciniec, J. Gulin´ski, H. Maciejewski, in: I.T. Horvath (Ed.),
Encyclopedia of Catalysis, Wiley, New York, 2003, pp. 107–152.
[6] A.J. Chalk, J.F. Harrod, J. Am. Chem. Soc. 87 (1965) 16.
[7] M.A. Schroeder, M.S. Wrighton, J. Organomet. Chem. 128 (1977)
345.
[8] F. Seitz, M.S. Wrighton, Angew. Chem., Int. Ed. Engl. 27 (1988) 289.
[9] B. Marciniec, Coord. Chem. Rev. 249 (2005) 2374.
[10] B. Marciniec, H. Maciejewski, J. Organomet. Chem. 454 (1993) 45.
[11] H. Maciejewski, B. Marciniec, I. Kownacki, J. Organomet. Chem.
597 (2000) 175.
[12] B. Marciniec, H. Maciejewski, I. Kownacki, J. Mol. Catal. A 135
(1998) 223.
[13] S.K. Janikowski, L.J. Radonowich, T.J. Groshens, K.J. Klabunde,
Organometallics 4 (1985) 396.
[14] T.J. Groshens, K.J. Klabunde, J. Organomet. Chem. 259 (1983) 337.
[15] S.T. Lin, T.J. Groshens, K.J. Klabunde, Inorg. Chem. 23 (1984) 1.
[16] T.J. Groshens, K.J. Klabunde, Organometallics 1 (1982) 564.
[17] T.R. Bierschenk, M.A. Guerra, T.J. Juhlka, S.B. Larson, R.J. Lagow,
J. Am. Chem. Soc. 109 (1987) 4855.
[18] S.-B. Choe, K.J. Klabunde, J. Organomet. Chem. 359 (1989) 409.
[19] S.-B. Choe, J. Schneider, K.J. Klabunde, L.J. Radonovich, J.
Organomet. Chem. 376 (1989) 419.