Catalysis of hydrosilylation: Part XXXIV. High catalytic efficiency of the nickel equivalent of Karstedt catalyst [{Ni(η-CH2=CHSiMe2)2O} 2{μ-(η-CH2=CHSiMe2)2O}]

Article abstract of DOI:10.1016/S0022-328X(99)00685-3

The nickel equivalent of Karstedt catalyst [{Ni(η-CH₂=CHSiMe₂)₂O} ₂{μ-(η-CH₂=CHSiMe₂)₂O}] (1) appeared to be a very efficient catalyst for dehydrogenative coupling of vinyl derivatives (styrene, vinylsilanes, vinylsiloxanes) with trisubstituted silanes HSi(OEt)₃, HSiMe₂Ph. The reaction occurs via three pathways of dehydrogenative coupling, involving formation of an unsaturated compound as the main product as well as a hydrogenated olefin (DS-1) pathway, hydrogenated dimeric olefin (DS-2) and dihydrogen (DC), respectively. The reaction is accompanied by side hydrosilylation. Stoichiometric reactions of 1 with styrene and triethoxysilane, in particular synthesis of the bis(triethoxysilyl) (divinyltetramethyldisiloxane) nickel complex 3 and the first documented insertion of olefin (styrene) into Ni-Si bond of complex 3, as well as all catalytic data have allowed us to propose a scheme of catalysis of this complex reaction by 1.

Full text of DOI:10.1016/S0022-328X(99)00685-3

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 597 (2000) 175–181
Catalysis of hydrosilylationꢀ
Part XXXIV. High catalytic efficiency of the nickel equivalent of
Karstedt catalyst
[{Ni(h-CH2ꢀCHSiMe2)2O}2{m-(h-CH2ꢀCHSiMe2)2O}]
H. Maciejewski, B. Marciniec *, I. Kownacki
Department of Organometallic Chemistry, Faculty of Chemistry, Adam Mickiewicz Uni6ersity, Grunwaldzka 6, PL-60780 Poznan´, Poland
Received 16 June 1999; received in revised form 29 October 1999
Dedicated to Professor Stanis*law Pasynkiewicz, in recognition of his great contribution to organometallic chemistry
Abstract
The nickel equivalent of Karstedt catalyst [{Ni(h-CH2ꢀCHSiMe2)2O}2{m-(h-CH2ꢀCHSiMe2)2O}] (1) appeared to be a very
efficient catalyst for dehydrogenative coupling of vinyl derivatives (styrene, vinylsilanes, vinylsiloxanes) with trisubstituted silanes
HSi(OEt)3, HSiMe2Ph. The reaction occurs via three pathways of dehydrogenative coupling, involving formation of an
unsaturated compound as the main product as well as a hydrogenated olefin (DS-1) pathway, hydrogenated dimeric olefin (DS-2)
and dihydrogen (DC), respectively. The reaction is accompanied by side hydrosilylation. Stoichiometric reactions of 1 with styrene
and triethoxysilane, in particular synthesis of the bis(triethoxysilyl) (divinyltetramethyldisiloxane) nickel complex 3 and the first
documented insertion of olefin (styrene) into NiꢁSi bond of complex 3, as well as all catalytic data have allowed us to propose
a scheme of catalysis of this complex reaction by 1. © 2000 Elsevier Science S.A. All rights reserved.
Keywords: Silicon; Dehydrogenative (coupling) silylation; Hydrosilylation; Nickel complexes; Silanes
pounds as potential hydrosilylation catalysts. By anal-
ogy to the Karstedt catalyst, the nickel complex
[{Ni(h-CH2ꢀCHSiMe2)2O}2{m-(h-CH2ꢀCHSiMe2)2O}]
was obtained in the reaction between (trans,trans,trans-
cyclododeca-1,5,9-triene)nickel(0) and LL [4,5]:
1. Introduction
The use of vinylsilicon compounds as ligands in
organometallic chemistry is a relatively recent develop-
ment. The silicone industry extensively uses highly
active platinum catalysts, e.g. the silicone-soluble
Karstedt catalyst, which is prepared by the reaction of
chloroplatinic acid with vinylsilicon-containing com-
pounds such as divinyltetramethyldisiloxane (LL) [1,2].
According to earlier by Lappert and co-workers, stud-
ies (at Sussex) the reaction mixture, obtained by mixing
these two components, contained a bridging LL and
each platinum had a chelating LL group [{Pt(LL)}2(m-
LL)] [3]. The cost of platinum compounds stimulated
much research to check other transition metal com-
[Ni(trans,trans,trans-CDT)]+3LL“
[{Ni(LL)2}(m-LL)]+2-trans,trans,trans-CDT
1
Complex 1 was an orange crystalline solid, extremely
soluble in both aliphatic and aromatic solvents, and
was initially isolated from the reaction mixture as an
oil. The solubility of 1 has thus far inhibited the
isolation of single crystals suitable for X-ray
diffraction.
Nickel complexes are well known as usually non-se-
lective catalysts for hydrosilylation reactions of olefins
[6]. The hydrosilylation is accompanied very often by
the reaction of dehydrogenative coupling [7]. The dehy-
ꢀ For Part XXXIII, see Ref. [22].
* Corresponding author. Tel.: +48-61-865-9651; fax: +48-61-865-
9568.
E-mail address: marcinb@main.amu.edu.pl (B. Marciniec)
0022-328X/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(99)00685-3

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H. Maciejewski et al. / Journal of Organometallic Chemistry 597 (2000) 175–181
drogenative coupling of hydrosilanes with olefins is
catalysed mainly by iron and cobalt triad members and
proceeds via two pathways yielding unsaturated
product (vinylsilane) and dihydrogen (dehydroconden-
sation) (Eq. (1)) and/or a hydrogenated olefin (dehy-
drogenative silylation) (Eq. (2)) as shown below:
2.1.1. Synthesis of [{Ni(p-CH2ꢀCHSiMe2)2O}2{v-
(p-CH2ꢀCHSiMe2)2O}] (1)
Divinyltetramethyldisiloxane (2 ml) was added slowly
to a stirred red suspension of [Ni(cod)2] (0.5 g, 1.8
mmol) in ether (10 ml) at ambient temperature. The
suspension was dissolved and a yellow–red solution
was obtained. The reaction mixture was stirred
overnight. The volatiles were removed in vacuo to give
an yellow–orange oil with the deposition of some col-
loidal nickel metal, which was taken up into pentane
and filtered through Celite (2×2 ml). After concentra-
tion of the solution, the red oil of 1 was obtained (1.15
g, 95% yield).
HSiR3+CH2ꢀCHR%“R%CHꢀCHSiR3+H2
HSiR3+2CH2ꢀCHR%“R%CHꢀCHSiR3+R%CH2CH3
(2)
(1)
Although much data on the competitive hydrosilyl-
ation–dehydrogenative coupling reactions have been
reported, examples of the exclusive (or at least highly
selective) formation of vinylsilanes are still limited [8–
10]. Vinylsilanes (alkenylsilanes) exhibit considerable
synthetic utility in organic synthesis [11].
Our recent contribution to this field is the first exam-
ple of the nickel-catalysed dehydrogenative silylation of
vinylsubstituted silanes and styrene by triethoxysilane
and triethylsilane occurring in the presence of [Ni-
(acac)2], [Ni(cod)2] and nickel phosphine complexes
[12–16]. Besides the two above-mentioned pathways
(Eqs. (1) and (2)), in the presence of nickel catalysts the
dehydrogenative coupling occurs also via another route
giving an unsaturated product and the product of hy-
drogenative dimerization of olefins, as follows:
1
Analytical data of 1: H-NMR (C6D6, 298 K, 500
MHz); l −0.35 (s, 6H, Me), −0.21 (s, 6H, Me),
−0.04 (s, 6H, Me), 0.10 (s, 6H, Me), 0.41 (s, 6H, Me),
0.42 (s, 6H, Me), 2.94–3.91 (m, 18H, CH2ꢀCH),
2J(1H1ꢁ1H2)=12.9, 2J(1H1ꢁ1H3)=16.8 Hz. 13C{1H}-
NMR (C6D6, 298 K, 75.5 MHz); l −3.0 (s, Me), −0.5
(s, Me), 0.0 (s, Me), 0.7 (s, Me), 65.5 (s, ꢁCHꢀ), 66.6 (s,
ꢀCH2). 29Si{1H}-NMR (C6D6, 231 K, 99.4 MHz); l 6.4
(s), 1.6 (s), 7.5 (s). EI (MS): m/z 186 [27%,
(CH2ꢀCHSiMe2)2O+], 229 [42%, Ni(CH2ꢀCHSiMeOSiꢁ
Me2CHꢀCH2)+], 244 [34%, Ni(CH2ꢀCHSiMe2)2O+],
430 [39%, {Ni(h-CH2ꢀCHSiMe2)2Oꢁ(m-CH2ꢀCHSi-
Me2)2O}+].
2.2. Stoichiometric reactions
3CH2ꢀCHR+HSiR%
3
“RCH2CH2CH2CH2R+R%SiCHꢀCHR
(3)
2.2.1. Reaction of 1 with styrene — synthesis of
[Ni{(p-CH2ꢀCHSiMe2)2O}(p-CH2ꢀCHPh)] (2)
To a stirred solution of 1 (0.67 g, 1 mmol) in 10 ml
of benzene (under argon, at room temperature), 0.23 ml
(2 mmol) of styrene was added. After 4 h the reaction
mixture was analysed by GC–MS and divinylte-
tramethyldisiloxane was detected. The volatiles were
removed from the reaction mixture under reduced pres-
sure and a red oil of 2 was obtained.
3
where R=SiR%, Ph; R%=OC2H5, Et.
3
The aim of this work is to examine the catalytic
system based on 1 in the reaction of trisubstituted
silanes with various vinyl derivatives. We shall also
describe a new, very useful method for the synthesis of
1.
1
Analytical data of 2: H-NMR (C6D6, 298 K, 300
2. Experimental
MHz); l −0.76 (s, 3H, Me), −0.34 (s, 3H, Me), 0.24
(s, 3H, Me), 0.33 (s, 3H, Me), 2.85–4.25 (m, 6H,
CH2ꢀCH), 6.81–7.20 (m, Ph). 13C{1H}-NMR (C6D6,
298 K, 75.5 MHz); l −2.29 (s, Me), −1.28 (s, Me),
69.43 (s, ꢁCHꢀ), 70.28 (s, CH2ꢀ), 114.06 (s, CH2ꢀ),
137.8 (s, ꢁCHꢀ), 126.21–129.28 (m, Ph).
2.1. Materials
All reagents were dried and purified before use by the
usual procedures. Triethoxysilane was obtained by alco-
holysis of SiHCl3 and purified to avoid the presence of
traces of HCl. [Ni(cod)2] was prepared as described in the
literature [17]. Other chemicals were purchased as spe-
cified: divinyltetramethyldisiloxane (CH2ꢀCHSiMe2)2O,
dimethylphenylsilane HSiMe2Ph, vinyltrimethylsilane
CH2ꢀCHSiMe3, vinyltriethoxysilane CH2ꢀCHSi(OC2-
H5)3 and styrene CH2ꢀCHPh from Fluka, vinylmethyl-
bis(trimethylsilyloxy)silane CH2ꢀCHSiMe(OSiMe3)2 and
vinyltris(trimethylsilyloxy)silane, CH2ꢀCHSi(OSiMe3)3
from ABCR. Benzene (Fluka), n-pentane and diethyl
ether (Merck) were purified by standard methods and
distilled prior to use.
2.2.2. Reaction of 2 with triethoxysilane — synthesis
of [Ni{(p-CH2ꢀCHSiMe2)2O}{Si(OC2H5)3}2] (3)
Triethoxysilane (1 ml) was added to a rapidly stirred
red solution of 2 in benzene (15 ml). The reaction
mixture was stirred for 4 h at 40°C. The volatiles were
removed under reduced pressure. The residual deep-red
oil was dissolved in pentane (10 ml) and filtered
through Celite (2×2.5 ml), then washed with pentane
(2×5 ml). The combined filtrate and washings were
concentrated under reduced pressure and dried in
vacuo for 8 h yielding a deep-red oil of 3.

H. Maciejewski et al. / Journal of Organometallic Chemistry 597 (2000) 175–181
177
1
Analytical data of 3: H-NMR (C6D6, 298 K, 300
with vinyl derivatives, i.e. styrene and vinyltrisubsti-
tuted silanes and vinylsiloxanes.
The complex reaction proceeds according to Eqs. (5)
and (6):
MHz); l 0.21 (s, 6H, Me), 0.35 (s, 6H, Me), 1.15 (t,
18H, Me), 2.26–2.72 (m, 6H), 3.83 (q, 12H, ꢁOCH2).
13C{1H}-NMR (C6D6, 298 K, 75.5 MHz); l −0.24 (s,
Me), 1.52 (s, Me), 18.51 (s, Me), 58.78 (s, ꢁOCH2), 71.0
(s, ꢀCH2), 72.26 (s, ꢁCHꢀ).
2.3. Equipment and analytical measurements
The NMR spectra (1H, 13C, 29Si) were recorded on a
Varian XL 300 spectrometer. In all cases C6D6 was
used as solvent; GC–MS analyses were carried out with
a Varian 3300 chromatograph (equipped with a DB-1,
30 m capillary column), connected to a Finnigan Mat
700 mass detector. GC analysis was also carried out
with a Varian 3800 chromatograph with a Megabore
column (30 m, DB-1).
(5)
(6)
Thus, the three reactions of dehydrogenative coupling
DC, DS-1 and DS-2 compete with regular hydrosilyl-
ation H. The amount of DS (RCHꢀCHSiR3, mainly
E-isomer) is equal to the sum of the second products of
hydrogenative silylation of olefin DS-1, hydrogenative
dimerization of olefin DS-2 and hydrogen evolved (not
measured but calculated from DC=DS−(DS-1+DS-
2)). Besides, the presence of small amounts of tetrasub-
stituted silanes SiR4 and siloxanes, as by-products
coming from disproportionation of HSi(OEt)3, and
their condensation, are also observed.
2.4. General procedure for the catalytic test
Catalyst 1 was placed in a glass ampoule (or Schlenk
tube) and filled with a mixture of vinyl derivatives and
triethoxysilane or dimethylphenylsilane. All manipula-
tions were carried out using standard Schlenk and high
vacuum line techniques.
Sealed ampoules (or Schlenk tubes) were heated at a
given temperature. The distribution of substrates and
products, conversion of substrates and the yield of
products were determined by GC–MS and GLC
analyses.
Effects of temperature, reaction time and the sub-
strate ratio on the yield and selectivity of this complex
competitive process occurring in the presence of 1 and
[Ni(cod)2] (for comparison) are compiled in Table 1 (for
3. Results and discussion
triethoxysilane) and in Table
phenylsilane).
2
(for dimethyl-
3.1. Synthesis of 1
Our previous experience in the reaction of tri-
ethoxysilane with vinyltrisubstituted silanes (e.g.
(EtO)3SiCHꢀCH2 and Me3SiCHꢀCH2) [12,15] and
styrene [16] catalysed by [Ni(cod)2] and [Ni(acac)2]
showed the preference of dehydrogenative silylation
DS-1 and DS-2 over dehydrogenative coupling DC as
well as over the hydrosilylation H.
However [Ni(cod)2], [Ni(acac)2] and particularly the
latter activated by triethoxysilane complex, character-
ised as [(acac)Ni(C2H5)PPh3]-intermediate, appeared to
be active but not selective catalysts in the dehydrogena-
tive silylation of vinylsilanes, yielding the majority of
side reactions such as hydrosilylation, redistribution
and oxidation of hydrosilanes and condensation prod-
ucts [14,15].
On the other hand, dehydrogenative coupling of
styrene with triethoxysilane occurs in the presence of
[Ni(acac)2] and [Ni(cod)2] exclusively via DS-1 and
DS-2 pathways and with relatively high selectivity com-
pared to the hydrosilylation H [16]. Yet, application of
1 finds a much more efficient catalyst for dehydro-
The new method for synthesis of 1 is based on the
reaction of [Ni(cod)2] with vinylsiloxane occurring as
follows:
[Ni(cod)2]+3(CH2ꢀCHSiMe2)2O
“[{Ni(h-CH2ꢀCHSiMe2)2O}2-
{m-(h-CH2ꢀCHSiMe2)2O}]+4 cod
(4)
Cyclooctadiene, which is formed as a co-product, may
be very easily removed under vacuum from the reaction
mixture, thus a practically pure product is obtained.
Moreover, the synthesis of [Ni(cod)2] as a starting
substance is relatively simple as compared with the
trans,trans,trans - cyclododeca - 1,5,9 - trienenickel(0) —
the starting material for synthesis of 1 in the previous
method [4,5].
3.2. Catalytic examinations
Complex 1 was used as a catalyst in the reactions of
some hydrosilanes, mainly HSiMe2Ph and HSi(OEt)3

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H. Maciejewski et al. / Journal of Organometallic Chemistry 597 (2000) 175–181
genative coupling (the yield and selectivity) than any
of the reported earlier based on [Ni(acac)2] and
[Ni(cod)2].
Similarly to [Ni(acac)2] and [Ni(cod)2] catalysts, the
dehydrogenative coupling of vinylsubstituted silanes
and siloxanes by triethoxysilane is based on dehydro-
genative silylation (DS-1 and DS-2) and there is no
direct dehydrocondensation process involved. The de-
hydrogenative silylation (DS) pathway dominates over
hydrosilylation (H) in the case of all vinylsubstituted
silanes and particularly siloxanes. The latter results are
very promising for possible synthesis of unsaturated
(poly)siloxanes, so supposedly under the optimum con-
ditions it would be possible to elaborate procedures for
Table 1
Dehydrogenative coupling versus hydrosilylation of vinyl derivatives CH2ꢀCHR with triethoxysilane catalysed by 1 and [Ni(cod)2] a
R
T (°C)
t (h)
Yield (%)
Product distribution (%)
Ratio DC:DS-1:DS-2
DS
H
Others
Ph
120
0.5
0.5
0.5
50
50
120
92
100
52
49
32
85
88
79
79
83
81
13
10
10
19
8
2
2
2
2
2
9
0.55:0.32:0.13
0.26:0.50:0.24
0.20:0.46:0.34
0.31:0.50:0.19
0.35:0.46:0.19
0.32:0.49:0.19
120 b
120 d
r.t.
r.t. d
r.t.
68
17
Si(OC2H5)3
SiMe3
120
120
0.5
2
56
95
98
81
0
12
2
7
0.01:0.53:0.46
0.16:0.60:0.24
120
2
2
100
61
91
70
4.5
21
4.5
9
0:0.80:0.20
0:0.63:0.37
120 d
Si(OSiMe3)3
120
120 d
r.t.
0.5
0.5
24
100
40
18
96
72
100
93
4
19
0
Trace
9
Trace
Trace
0.10:0.80:0.10
0.17:0.45:0.38
0:077:0.23
r.t.
120
74
7
0:0.69:0.31
SiMe(OSiMe3)2
120
0.5
0.5
88
43
92
85
6
13
2
2
0.12:088:0
0.34:0.66:0
120 c
a [HSiꢂ]:[CH2ꢀCHR]:[cat]=1:2:10−3, argon, glass ampoules.
b [HSiꢂ]:[CH2ꢀCHR]=1:3.
c [HSiꢂ]:[CH2ꢀCHR]=1:1.
d [Ni(cod)2].
Table 2
Dehydrogenative coupling versus hydrosilylation of vinyl derivatives CH2ꢀCHR with dimethylphenylsilane catalysed by 1 and [Ni(cod)2] a
R
T (°C)
t (h)
Yield (%)
Product distribution (%)
Ratio DC:DS-1:DS-2
DS
H
Ph
120
1
1
96
96
0.5
83
65
18
Trace
37
91
84
79
Trace
84
9
16
21
–
0.45:0.55:0
0.24:0.45:0.31
0.56:0.44:0
–
120 d
r.t.
r.t. d
120 b
16
0.71:0.29:0
Si(OC2H5)3
SiMe3
120
2
63
37
63
0:0.75:0.25
120
24
24
0.5
100
55
61
58
36
79
42
57
21
0.39:0.33:0.28
0.25:0.45:0.30
0.43:0.42:0.15
120 d
120 c
Si(OSiMe3)3
120
60
0.5
24
75
95
72
89
28
11
0:0.16:0.84
0.02:0.14:0.84
a [HSiꢂ]:[CH2ꢀCHR]:[cat]=1:2:10−3, argon, glass ampoules.
b [HSiꢂ]:[CH2ꢀCHR]=1:3.
c [HSiꢂ]:[CH2ꢀCHR]=1:1.
d [Ni(cod)2].

H. Maciejewski et al. / Journal of Organometallic Chemistry 597 (2000) 175–181
179
highly selective dehydrogenative coupling (silylation) of
all vinylsubstituted silanes and siloxanes with quantita-
tive conversion of hydrosilanes (e.g. triethoxysilane).
Dehydrogenative coupling of styrene by HSi(OEt)3
occurs in the presence of 1 and in excess of styrene with
very high efficiency (0.5 h, 120°C, 100% yield) and also
high selectivity (see compared with [Ni(cod)2] catalyst)
but apparently in this case E-1-phenyl-2-silyl-ethene is
also formed via dehydrocondensation. In the case of all
vinylsubstituted silanes and siloxanes, the process is
also regioselective (1,2-derivatives of ethene are noted
exclusively) but the stereoselectivity E/Z exceeds ten.
Contrary to triethoxysilane, dimethylphenylsilane re-
acts with all vinylsubstituted silicon compounds in the
presence of 1 with low selectivity, yielding a mixture of
dehydrogenative silylation (coupling) and hydrosilyla-
tion products, yet, with higher efficiency than those
reported earlier in the presence of other nickel catalysts.
Interestingly, dehydrogenative coupling of styrene by
HSiMe2Ph is efficiently catalysed (at 120°C) with high
selectivity of DS products, but the process proceeds
by the pathways DS-1 and DC. There is no 1,3-
diphenylbutane (product of hydrogenated dimerization
of styrene) observed in the reaction mixture.
between 2.85 and 4.25 ppm, which is evidence of coor-
dination of all these groups to the metal centre (see
Section 2.2.1).
Complex 2 was subjected to the reaction in excess of
triethoxysilane for 4 h at 40°C. In the reaction mixture,
ethylbenzene was detected by GC–MS analysis. After
removal of volatiles the product was isolated and char-
1
acterised by H- and 13C-NMR. In the NMR spectra,
the signals from two triethoxysilyl groups and one
molecule of divinyltetramethyldisiloxane were observed
(see Section 2.2.2). This experiment permits us to pro-
pose Eq. (8) for initiation of active intermediate
unisolable under the catalytic conditions.
Finally, it is worth emphasising that contrary to the
case when [Ni(acac)2]- and [Ni(cod)2]-based catalysts
are used, there is no precipitation of nickel products
after completion of the reaction catalysed by 1. Appar-
ently, divinylsiloxane chelate keeps Ni(0) molecule in
solution, being still active in reactions examined and
can be regenerated.
(8)
Additionally, the reaction of 3 with excess of styrene, at
40°C for 4 h, was carried out. After the reaction, the
product of DS (1-phenyl-2-silylethene) was detected by
the GC–MS method, which evidences the insertion of a
styrene molecule into the NiꢁSi bond
3.3. Reactions of 1 with substrates under the conditions
of catalysis
3.4. Mechanistic implications
Although the Chalk–Harrod mechanism for hydrosi-
lylation has been widely accepted, the fact that it does
not explain the formation of alkenylsilanes led to pro-
posals of alternative mechanisms, e.g. Seitz and
Wrighton mechanism [18,19]. The key step of this
mechanism involves insertion of an alkene into a
metal–silicon bond (or migration of silyl group, to a
coordinated alkene molecule). The elimination of vinyl-
silane and regeneration of the MꢁH complex may
support the dehydrogenative pathway of the hydro-
silylation processes [6,19].
In order to explain the role of 1 in the complex
dehydrogenative coupling–hydrosilylation process, a
separate study on the reaction of 1 with styrene and
HSi(OEt)3 at ambient temperature was undertaken,
under oxygen-free conditions; 1 reacts with styrene
according to Eq. (7):
However, in the case of nickel-catalysed reactions, a
stable complex containing one silyl group bonded to
nickel atom has not been isolated yet.
On the other hand, only a few stable bis(silyl)nickel
complexes, e.g. [Ni(SiCl3)2(bipy)] [19,20] and bis(si-
lyl)nickel complexes with o-carboranyl units [21] were
evidenced to be intermediates in facile double silylation
of alkynes. Our previous study on the reaction of
[Ni(acac)2] with triethoxysilane in the presence of PPh3
under oxygen-free conditions permitted isolation of the
(7)
1H- and 13C-NMR characterisation of the isolated
product 2 reveals a formation of trigonal p-complex of
nickel(0). The signals from vinyl groups (from styrene
and (CH2ꢀCHSiMe2)2O) in 1H-NMR, were observed

180
H. Maciejewski et al. / Journal of Organometallic Chemistry 597 (2000) 175–181
Scheme 1.
intermediate containing no NiꢁSi bond (i.e. [Ni(a-
cac)Et(PPh3)]) [15], which is active in the dehydrogena-
tive silylation but only after prior oxygenation of PPh3.
Present examination of a new chelating system of
nickel, i.e. 1 with triethoxysilane, afforded a double silyl
complex 3, which appeared to be an active intermediate
just in the dehydrogenative coupling (silylation) of
olefins. In view of the data from the catalytic and
stoichiometric examinations, we propose a mechanistic
scheme (Scheme 1) for this complex reaction catalysed
by 1, using the exemplary system of reagents styrene–
triethoxysilane.
the stoichiometric studies, 6“3 is proposed with elimi-
nation of ethylbenzene (DS-1 pathway). Alternatively
(competitively) an elimination of 1,2-phenylsilylethane,
as the hydrosilylation by-product, can lead to complex
8. The latter is formed via the main pathway of dehy-
drogenative silylation 3“8 involving a classical inser-
tion of styrene into one of NiꢁSi bonds and yielding the
main product — silylstyrene. Contrary to the reported
data on the insertion of alkenes [18] into such a com-
plex, and in agreement with the results concerning
olefin insertion [19], we do not observe any products of
bis(silylation).
Initiation occurs via the transformation 1“2 and
after the next oxidative addition, complex 4 starts the
catalytic cycle. The insertion of styrene into the NiꢁH
bond gives the s-complex 5. The two subsequent com-
petitive reactions can proceed: the oxidative addition of
the next hydrosilane molecule to yield 6 and/or coordi-
nation of the next molecule of styrene followed by its
insertion into NiꢁC in 5¦ giving finally (after the oxida-
tive addition of HSiꢂ) the product of hydrogenative
dimerization (DS-2). That the insertion of styrene into
NiꢁC in (5%) is favoured over that into the NiꢁSi bond
is evidenced by the absence of silylbutanes (or silylbu-
tenes) among the products, which if present should be
eliminated after the latter rearrangement. According to
Coordination of styrene to complex 8 closes the
catalytic cycle discussed. Dehydrocondensation (DC)
observed in the case of the reaction with styrene can be
accounted for by the side cycle involving the preferred
insertion of styrene into NiꢁSi bond of 8 to give
dihydrido-intermediate 9, very easily decomposed to
evolve dihydrogen and regenerate complex 2.
In the reactions catalysed by other Ni complexes, the
appearance of the products of redistribution of trisub-
stituted silanes, in particular those containing alkoxy
groups, and their condensation have been commonly
observed as competing to the main reactions, while in
the process catalysed by 1, these side reactions are
negligible.

H. Maciejewski et al. / Journal of Organometallic Chemistry 597 (2000) 175–181
181
4. Conclusions
Committee for Scientific Research, Poland, Project No.
13T09 A074.
1. [{Ni(h - CH2ꢀCHSiMe2)2O}2{m - ( - h - CH2ꢀCHSi -
Me2)2O}] (1) after prior dissociation and olefin coor-
dination appeared to be a very efficient (high yield
and selectivity and stability, i.e. non-reduced to the
metal under catalysis conditions) catalyst for dehy-
drogenative coupling of vinyl derivatives (styrene,
vinylsilanes, vinylsiloxane) particularly with tri-
ethoxysilane.
2. Three pathways of dehydrogenative coupling DS-1,
DS-2 and DC of selected olefins (e.g. styrene) are
documented, involving the formation of unsaturated
product (e.g. E-1-phenyl-2-silylethene) as well as
hydrogenated olefin (e.g. ethylbenzene) (DS-1), hy-
drogenated dimeric product (e.g. 1,3-diphenylbu-
tane) (DS-2) and dihydrogen (as a product of
dehydrocondensation) (DC), respectively.
References
[1] D.N. Willing, US Patent No. 3 491 593 (1968).
[2] B.D. Karstedt, US Patent No. 3 775 452 (1973).
[3] P.B. Hitchcock, M.F. Lappert, N.J.W. Warhurst, Angew. Chem.
Int. Ed. Engl. 30 (1991) 438.
[4] B. Prost, Ph.D. Thesis, University of Dusseldorf, Germany,
1993.
[5] P.B. Hitchcock, M.F. Lappert, C. MacBeath, F.P.E. Scott,
N.J.W. Warhurst, J. Organomet. Chem. 528 (1997) 185.
[6] B. Marciniec (Ed.), Comprehensive Handbook on Hydrosilyla-
tion, Pergamon, Oxford, 1992.
[7] B. Marciniec, New J. Chem. 21 (1997) 815.
[8] F. Kakiuchi, Y. Tanaka, N. Chatani, S. Murai, J. Organomet.
Chem. 456 (1993) 45.
[9] Y. Seki, K. Takeshita, K. Kawamoto, J. Organomet. Chem. 369
(1989) 17.
3. Catalytic examinations and stoichiometric reactions
of 1 with styrene and triethoxysilane have made
grounds for the mechanistic scheme proposed.
4. The insertion of styrene into NiꢁSi of the bis(si-
lyl)divinyltetramethyldisiloxane) nickel complex is
documented as the first evidence for insertion of
olefins into a nickel–silicon bond.
[10] M.L. Christ, S. Sabo-Etienne, B. Chaudret, Organometallics 14
(1995) 1082.
[11] E.W. Colvin, Silicon Reagents in Organic Synthesis, Academic
Press, London, 1988.
[12] B. Marciniec, H. Maciejewski, J. Organomet. Chem. 454 (1993)
45.
[13] B. Marciniec, H. Maciejewski, U. Rosenthal, J. Organomet.
Chem. 484 (1994) 147.
5. The first evidence of high catalytic activity of nickel
complex 1 in the hydrosilylation and related reac-
tions (dehydrogenative coupling especially of vinyl
derivatives) raises hopes that the Karstedt platinum
complex might be replaced by nickel complexes of
type 1, used as a catalyst for various reactions, e.g.
cross-linking of vinylsubstituted silicones, synthesis
of alkenylsilanes and reagents in organic synthesis.
[14] B. Marciniec, H. Maciejewski, J. Gulin´ski, J. Chem. Soc. Chem.
Commun. (1995) 717.
[15] B. Marciniec, H. Maciejewski, J. Gulin´ski, B. Maciejewska, W.
Duczmal, J. Organomet. Chem. 521 (1996) 245.
[16] B. Marciniec, H. Maciejewski, I. Kownacki, J. Mol. Catal. A:
Chemical 135 (1998) 223.
[17] R.A. Shunn, Inorg. Synth. 15 (1974) 5.
[18] F. Seitz, M.S. Wrighton, Angew. Chem. Int. Ed. Engl. 27 (1988)
289.
[19] P. Braunstein, M. Knorr, J. Organomet. Chem. 500 (1995) 21.
[20] Y. Kiso, K. Tamao, M. Kumada, J. Organomet. Chem. 76
(1974) 105.
[21] Y. Kang, J. Lee, Y.K. Kong, S.O. Kang, J. Ko, Chem. Com-
mun. (Cambridge) (1998) 2343.
Acknowledgements
[22] B. Marciniec, P. Krzyz
; anowski, E. Walczuk-Gus´ciora, W. Ducz-
This work was supported by funds from the State
mal, J. Mol. Catal. A: Chemical 144 (1999) 263.
.