Reactivity of group 4 benzamidinate complexes towards mono- and bis-substituted silanes and 1,5-hexadiene

Article abstract of DOI:10.1016/j.jorganchem.2007.01.002

Zirconium and titanium bis(benzamidinate) dimethyl complexes were found to be active catalytic precursors for the oligomerization of mono- and bis-substituted silanes. The activation of such complexes, by an excess of MAO or by an equimolar amount of B(C<

Full text of DOI:10.1016/j.jorganchem.2007.01.002

Journal of Organometallic Chemistry 692 (2007) 1940–1950
www.elsevier.com/locate/jorganchem
Reactivity of group 4 benzamidinate complexes towards mono- and
bis-substituted silanes and 1,5-hexadiene
*
Victoria Volkis, Claudia Averbuj, Moris S. Eisen
Schulich Faculty of Chemistry and Institute of Catalysis Science and Technology, Technion – Israel Institute of Technology, Haifa 32000, Israel
Received 3 December 2006; received in revised form 4 January 2007; accepted 5 January 2007
Available online 13 January 2007
Abstract
Zirconium and titanium bis(benzamidinate) dimethyl complexes were found to be active catalytic precursors for the oligomerization
of mono- and bis-substituted silanes. The activation of such complexes, by an excess of MAO or by an equimolar amount of B(C6F5)3,
increases the catalytic activity of the complexes and the molecular weight of the obtained oligosilanes. The maximum polymerization
index that was accomplished (n = 90) is by far the largest obtained with other organometallic complexes. In addition, the activated benz-
amidinate catalysts were also found to be active for the polymerization of 1,5-hexadiene producing poly(methyl-1,3-cyclopentane). For
the latter, the results obtained with C2-symmetry bis(benzamidinate) dimethyl complexes were compared with a chiral complex bearing a
myrtanyl group at the benzamidinate ligand.
ꢀ 2007 Elsevier B.V. All rights reserved.
Keywords: Benzamidinate complexes; Polysilanes; Dehydrogenative coupling of silanes; Polysilanes; Polymethylcyclopentane; Chiral complexes
1. Introduction
tane units and crosslinked moieties [14]. In addition,
methyl-, methoxy- and phenyl-substituted hexa- and hept-
adienes can be polymerized producing functionalized poly-
mers [15].
In the last three decades, the use of cyclopentadienyl
containing complexes [1] and other related carbon- [2], oxy-
gen- [3], and nitrogen-based ligands [4], such as chelating
diamido [5], salicylaldiminato [6], acetylacetonate [7], ami-
dinates [8] and some others, have been used as ancillary
ligands in the synthesis of potential Ziegler–Natta types
of catalysts for the oligomerization and polymerization of
olefins [9], and for several other catalytic reactions [10].
Since the early report of Marvel and Stille [11] regarding
the polymerization of 1,5-hexadiene to poly(methyl-1,3-
cyclopentane) (PMCP) by metallocene complexes, the cat-
alytic polymerization of non-conjugated dienes resulting in
polycyclic polymer has been under detailed investigation
[12,13]. One important future of these reactions is the
potential ability of the catalysts to co-polymerize 1,5-hex-
adiene with a-olefins, producing polymers with cyclopen-
PMCP
The homogeneous metallocene complexes, Cp2ZrCl2
and Cp2ZrMe2, were the first examples of organometallic
pre-catalysts to promote such polymerizations [13]. The
pre-catalysts were usually activated by an excess of
MAO, boron compounds [16] or diethyl aluminium chlo-
ride co-catalysts [17]. The mechanism for the cyclic poly-
merization was proposed first by Marvel et al. [11,18].
This mechanism entails the insertion of the double bond
into the M–Me bond of the catalyst followed by a cycliza-
tion resulting from the insertion of the second double bond
*
Corresponding author.
E-mail address: chmoris@tx.technion.ac.il (M.S. Eisen).
0022-328X/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2007.01.002

V. Volkis et al. / Journal of Organometallic Chemistry 692 (2007) 1940–1950
1941
of the substrate into the metal–carbon (from the growing
polymer chain). Chain termination usually takes place via
b-H elimination, although some examples of aluminium
transfer termination mechanisms have been also described
[19]. Due to the great importance of chiral polymers, the
catalytic properties of some chiral pre-catalysts in the poly-
merization of 1,5-hexadiene have been also studied [20]. It
was found that only the trans-isotactic polymer shows chi-
ral properties.
been investigated in detail, and intermediates have been
isolated [29].
One of the most active complexes for the polymerization
of mono-substituted silanes was obtained by Corey by the
reaction in situ between dichlorozirconocene, n-BuLi and
phenylsilane [30]. Oligomers with molecular weights
between 700 and 3000 Da were obtained and with 60–
99% conversion. Other similar complexes usually produced
mixtures of the low molecular weight oligomers, which
contain 12–20 silicon atoms (according to the GPC results)
[29]. Regarding late transition metals, a nickel indenyl com-
plex (1-MeInd)Ni(PPh3)Cl that was activated by either
MAO or LiAlH4 was found to be active in the oligomeriza-
tion of phenylsilane giving linear products with 90% con-
version [31].
Zirconocene and titanocene alkyne complexes are able
to oligomerize di-substituted silanes as methylphenylsilane,
giving a mixture of dimers, trimers and tetramers; whereas
for diphenylsilane, only dimers with low conversion have
been obtained [32]. Hence, it is indeed challenging to
research for new complexes that will polymerize bulky di-
substituted silanes.
Fascinatingly, the structure and molecular weight of some
polysilanes can be controlled by the structure of the organo-
metallic catalyst [29]. Some metallocene-type catalysts are
able to regulate the stereochemistry of the propagated chain,
producing syndiotactic [33] or isotactic [34] polymers.
Recently, we have investigated the polymerization of
ethylene and propylene promoted by racemic mixtures of
bis (benzamidinate) zirconium and titanium cis-octahedral
complexes [35]. Moreover, chiral benzamidinates com-
plexes have been also synthesized and utilized for the ste-
reoregular polymerization of propylene and styrene and
for the isomerization of a-olefins [36].
It has been only shown regarding the use of benzamidi-
nate complexes for the cyclic polymerization of 1,5-hexadi-
ene, that the tris-trimethylsilyl benzamidinate zirconium
monocloride complex can selectively polymerize 1,5-hex-
adiene producing short oligomers (Mw = 800, conversion
60–90%) [37].
In this paper we present the results of polymerization of
1,5-hexadiene with titanium and zirconium bis-benzamidi-
nate complexes and with a chiral zirconium tris-benzamidi-
nate complex. The polymerization conditions, the nature
of catalysts and ligands and their influence on the proper-
ties of the resulted PMCP will be discussed. The NMR
study of the polymer structure is shown as well. In addi-
tion, we also disclose here studies for the polymerization
of mono- and bis-substituted silanes initiated by the benz-
amidinate complexes of titanium and zirconium. The effect
of the polymerization conditions, the nature of catalysts
and different co-catalysts on the polymerization reaction
and on the properties of the obtained polysilanes is pre-
sented. These processes were studied conceptually to
expand the scope of reactivity of the benzamidinate neu-
tral and cationic complexes.
*
(+)
(-)
n
*
n
A second important process that has attained a large
industrial and academic interest regards the formation
of linear polysilanes. Being potential precursors for
ceramics, non-linear optics, deep-UV photo resists, con-
ductive and photoconductive polymers [21], and nano-
particle polycarbosilanes [22], linear polysilanes became
a target for many synthetic approaches. The simplest
and most industrially used synthetic method for silane
polymerization is via the Wurst reaction [23], which can
be thermal [24], sonic [25] or electrochemical [26]. Along
with such important advantages as high activity, com-
modity and high molecular weight of the resulting poly-
mers, this synthetic method has strong limitations in
polymer linearity, and the ability to control the process.
In addition, chain cyclization side-reactions on the early
steps of polymerization, and a wide molecular weight dis-
tribution strongly limit the fields of use for such polymers.
Therefore, development of an alternative organometallic
approach in the polymerization of silanes has been of
intense research [27]. Among new synthetic approaches,
hydrosilylation, transition metal-catalyzed dehydrocou-
pling of organic silanes, electrochemical routes to organo-
silicon polymers, and other metal complex-catalyzed
syntheses of organosilicon polymers, have been investi-
gated [21b,23].
For example, metallocene-type difluoro-, diphenoxy-
and diamino- catalysts were found to be active in the dehy-
drogenative coupling of mono-substituted silanes towards
oligomers, which are produced with high conversion (80–
90%), with molecular weights up to 4400 Da and the ratios
between linear and cyclic polymer are from 1:2 to 1:30 [28].
Interestingly, hafnium complexes provides the greatest
amount of the linear products, whereas titanium metallo-
cene complexes react about ten times faster than the corre-
sponding Zr and Hf complexes [29]. The mechanism of the
polymerization of silanes by metallocene complexes has

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V. Volkis et al. / Journal of Organometallic Chemistry 692 (2007) 1940–1950
2. Results and discussion
2.1. Synthesis of the catalysts
and the nature of complexes on the activity and the prop-
erties of the polymers were studied. In addition, we were
interested to compare the polymerization results using
complexes 1–2 which are obtained as racemic mixtures of
octahedral complexes with the results using complex 3 con-
taining chiral centers. The polymerization data are pre-
sented in Table 1.
As can be concluded from the polymerization data, all
complexes were found to be active catalysts for the poly-
merization of 1,5-hexadiene, producing only polycyclic
polymers with a similar activity as compared to the metal-
locene complexes. The PMCP obtained with the bis-benz-
amidinate complexes 4–5 demonstrated elastomeric
properties, whereas the polymers obtained by the chiral
complex 7 were obtained as solid polymers, both polymers
were soluble in hot toluene. This fact indicates that no
apparent crosslinking, which is also characteristic for poly-
mers with high stereoregularity, occurred during the poly-
merization of this olefin [39].
The bis-benzamidinate derivatives of titanium and zirco-
nium were synthesized as described in literature [35k]. The
lithium salt of the ligand was obtained by the reaction of
benzonitrile and LiN(SiMe3)2 as described in a previous
publication [38]. The reaction of the latter lithium ligand
with ZrCl4 or TiCl4 resulted in the formation of the
bis(benzamidinate)dichloride complexes. Finally, dimethyl
complexes of zirconium (1) and titanium (2) were prepared
by the methylation of the dichloride complexes with methyl
lithium. The zirconium complex containing the chiral myr-
tanyl group attached to benzamidinate ligand (3) was syn-
thesized also as described in literature [36]. All
benzamidinate complexes were either used as neutral com-
plexes or activated with either methylalumoxane (MAO) or
tris(pentafluorophenyl)borane, as shown in Scheme 1.
The cationic complex 7 was found to be more active in
the polymerization, as compared to complexes 4 and 5,
respectively. It was found that during the activation of
complex 3, MAO removes one of the chiral benzamidinate
ligands making the resulted active species 7 [36]. Due to the
presence of the myrtanyl group that is attached to the benz-
amidinate ancillary ligand via a methylene group, the effective
2.2. Polymerization of 1,5-hexadiene
The polymerization of 1,5-hexadiene was carried out
using catalysts 1–3. MAO was used as the co-catalyst to
form the putative cationic active species 4, 5, and 7
(Scheme 1). The influence of the polymerization conditions
TMS
TMS
MeMAO
TMS
Me
N
Ph
M
N
N
N
TMS
Me
MAO
Me
N
TMS
Ph
M
TMS
N
TMS
N
4: M = Zr,
5: M = Ti
N
TMS
TMS
TMS
TMS
B(C6F5)3
Me
N
1: M = Zr,
2: M = Ti
Ph
M
B(C6F5)3Me
N
N
TMS
N
TMS
TMS
Ph
6: M = Zr
Ph
MAO
TMS
O
N
R
Al
N
R
N
N
R
Me
Zr
Ph
N
Zr
Me
N
MAO
N
R
TMS
Ph
N
TMS
N
TMS
N
N
TMS
N
Ph
R
TMS
R
3
Ph
R =
Ph
7
Scheme 1. Group IV neutral and putative cationic benzamidinate complexes.

V. Volkis et al. / Journal of Organometallic Chemistry 692 (2007) 1940–1950
1943
Table 1
Data for the cyclic polymerization of 1,5-hexadiene promoted by the benzamidinate complexes 4, 5 and 7a
c
Run
Catalyst
M:MAO
T (ꢁC)
A · 10ꢀ3b
cis:trans (%)
Type of polymer
mpd (ꢁC)
Mn
5000
5200
23,000
13,900
1560
Mw
MWDe
1
2
3
4
5
6
7
a
4
4
5
5
7
7
7
1:100
1:400
1:100
1:400
1:100
1:100
1:800
25
25
25
25
25
50
50
2.2
6.8
2.4
4.1
27
55:45
58:42
52:48
54:46
41:59
31:69
36:64
Ef
E
E
E
Sf
S
–
–
–
–
76
82
88
13,500
13,500
31,000
21,100
2180
2.72
2.62
1.35
1.52
1.4
35
110
2460
3120
3200
4680
1.3
1.5
S
6 mL toluene.
b
c
d
e
f
g polm./(mol cat h).
Calculated from the integration of 1,3-cis- and trans-carbons in 13C NMR.
From DSC measurements from the second thermogram.
Mw/Mn; Mn, Mw and MWD were measured by GPC.
E – elastomer, S – solid.
coordinative unsaturation of complex 7 is more open than
in complexes 4 and 5, and as a result, the entrance of the
monomer becomes less crowded and easier. The activity
of complex 7 increases with rising temperature, as well as
a Zr:Al ratio. Concerning the properties of the resulted
polymer, one can indicate two important effects: molecular
weight of oligomers growths with the increasing of the tem-
perature (entries 5 and 6) and the ratio between MAO and
the catalyst (entries 6 and 7).
30–45 ppm are related to the PMCP carbons and those
labeled by asterisks, characterize the chain end. The signals
obtained in Fig. 1 are in good agreement with the literature
data for a polymer that is obtained via a b-H elimination
mechanism [40,43]. The same spectrum was observed for
polymers produced by complexes 4 and 5.
Two factors influence the stereochemistry in the cyclic
polymerization of the non-conjugated dienes: the orienta-
tions between the rings in the polymer chain, and the
ring closing orientation. Hence, four kinds of different
stereoregular orientations can be found in the cyclic poly-
mer [11,15]: cis-isotactic (or meso-diisotactic), cis-syndio-
tactic (or meso-disyndiotactic), trans-syndiotactic (or
racemo-disyndiotactic) and trans-isotactic (or racemo-
diisotactic):
2.3. NMR determination of the microstructure of PMCP
The typical 13C NMR spectrum (in toluene d8) at 45 ꢁC
for PMCP produced by the benzamidinate complex 7 in
toluene is presented in Fig. 1. The signals in the region of
4
2
6
2
6
1
1
3
3
n
n
5
4
5
cis-
trans-
Fig. 1. 13C NMR spectrum for PMCP received using the bis-benzamidinate complex 7 as the catalyst (s = solvent, c = cis, t = trans). Carbons numbered 2
and 6 (cis and trans) emerge jointly.

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V. Volkis et al. / Journal of Organometallic Chemistry 692 (2007) 1940–1950
In the NMR analysis towards the tacticity of the PMCP,
we have used the same notation as described by Waymouth
et al. [20,43]. The capital letters M (meso) and R (racemic)
described the relative stereochemistry within the ring
whereas a small cap letters m and r are used to describe
the relative stereochemistry between the closed rings. The
small letter x detones either m or r stereochemistry.
Our results for the tacticity of the obtained polymers are
presented in Table 2. The results obtained for the complex
Cp2ZrCl2 [46], which produces a polymer with no stereo-
regular control, and the results obtained with the complex
(+)-(S,S)-(EBTI)ZrBINOL((S,S)-1) [15], which produces a
highly stereoregular polymer, are presented for compari-
son. On NMR basis, it was possible to assign and differen-
tiate between RrR and MmM types of rings for the
isotactic tetrads, whereas for dissimilar rings RmM,
RrM, MrR and MmR, a reliable assignment of the tetrads
was not possible.
M
m
M
m
M
m
M
cis-isotactic or meso-diisotactic
M
R
r
M
r
M
r
M
R
cis-sindiotactic or meso-disindiotactic
m
R
m
R
m
trans-sindiotactic or racemo-disindiotactic
R
r
R
r
R
r
R
trans-isotactic or racemo-diisotactic
As can be concluded from Table 2, the polymer pro-
duced by complexes 4 and 5, and the polymer obtained
from the chiral complex 7 are totally different as to poly-
mers obtained either by the non-chiral zirconocene or the
chiral complex (+)-(S,S)-(EBTI)ZrBINOL((S,S)-1) [15].
Interestingly, complex 7 produces a cyclic polymer with a
high amount of RmR moieties and with almost a total lack
MxR and MrM tetrads. The structure of PMCP produced
by complexes 4 and 5 are alike, and the amount of the
RmR tetrads for these polymers is similar to that obtained
with the achiral zirconocene. However, a high content of
MmM and MxR tetrads makes these polymers unique. In
conclusion, the use of racemic benzamidinate or chiral
benzamidinate group IV complexes allows to produce com-
plementary types of PMCP polymers as compared to other
achiral and chiral metallocenes. Important to point out
that the ratio between MAO and catalysts are not the same
as to the comparison complexes. For the cyclization pro-
cess, we have found very little effect of the MAO on the
polymerization stereoselectivity.
The stereochemistry of insertion of the first double bond
of the diene into the metal–carbon bond resembles the ste-
reochemistry as found for the polymerization of a-olefins
[41], which is strongly dependent on the symmetry of the
catalyst [42]. The orientation of the ring closure depends
normally on the bulkiness of the ligands, at the metal cen-
ter, and the polymerization conditions [43].
The percentage of cis and trans rings for the PMCP was
calculated for complexes 4, 5, and 7 from the integration
ratio between 1,3-cis and 1,3-trans carbons (Table 1). The
obtained ratios were corroborated by the results obtained
based on the integration ratio for 4,5-cis and 4,5-trans-
carbons.
The physical properties of PMCP strongly depend on
the cis/trans rings ratio: the larger amount of the trans
rings, the higher the melting temperature that is observed
[44]. It was found that polymers contains more than 60%
of trans rings are solid with the melting temperature
above 180 ꢁC, whereas the polymers with 50–60% of cis
rings are waxes with melting temperatures below 70 ꢁC.
As can be concluded from Table 1, for bis-benzamidinate
catalysts 4, 5, the cis:trans rings ratio are very close to
50:50, corroborating with the elastomeric properties of
the obtained polymers. For complex 7, containing the chi-
ral myrtanyl group on the benzamidinate ligands, the
preference towards the trans ring closure (59–69%) was
observed.
The tacticity of PMCP is influenced by the enantioface
stereoregularity in the 1,5-hexadiene-insertion step. Regard-
ing the enantioselectivity, the performance of the benzamidi-
nate complexes in the polymerization of 1,5-hexadiene is
excepted to be alike to that encounter in the polymerization
of propylene. Hence, the zirconium complex 4 was more ster-
eoselective when compared to complex 5, in the polymeriza-
tion of propylene at high pressure [45] and for complex 7, a
highly isotactic polypropylene was obtained (mmmm >
99.9%) [36].
2.4. Dehydrogenative coupling of silanes
The bis-benzamidinate complexes 1–2 were found to be
active in the polymerization of some mono- and di-substi-
tuted silanes (Eq. (1)):
R
R
R
R
cat.
-H2
H
Si
R'
H
H
Si Si Si H
ð1Þ
R' R' R'
n
R = Ph, Et, Me
R' = H, Ph, Et
The formation of polysilanes was monitored by 29Si NMR
spectroscopy using the INEPTND method in combination
1
with H NMR spectroscopy. As the polymer evolves, the
quartet signal of the mono-substituted silane (RSiH3) is re-
placed by a triplet signal of the end-group of the formed
polymer chain, and a doublet signal of the internal

V. Volkis et al. / Journal of Organometallic Chemistry 692 (2007) 1940–1950
1945
Table 2
13C NMR data for carbons 4 and 5 of PMCP produced by complexes 4, 5 and 7
Entry
Catalyst
M:MAO
T (ꢁC)
MmM (%)
MrM (%)
RrR + RxM (%)
RmR (%)
MmR + MrR (%)
1
2
3
4
5
4
1:100
1:100
1:800
1:2300
1:1200
25
25
50
25
25
26.1
24.4
30.1
10.1
15.1
7.0
9.4
2.4
1.8
0.0
5.1
15.3
5.3
49.1
53.6
37.5
33.7
61.1
34.0
6.6
24.3
17.2
1.1
5.0
38.7
5
7
Cp2ZrCl2
(+)-(S,S)-(EBTI)ZrBINOL((S,S)-1)
(–SiHR–) moieties from the growing chain. For bis-substi-
tuted substrates, only the doublet signal of the end group
can be observed on the 29Si NMR spectrum (a polarization
transfer from H to Si is required). In addition, 29Si NMR is
useful for the determination of the polysilanes type. Hence,
linear polymers have signals between ꢀ60 and ꢀ70 ppm,
whereas cyclic polysilanes have signals in the range of
ꢀ20 to ꢀ30 ppm, and branched polysilanes appeared with
signals between 10 and 30 ppm [47]. The conversion or the
Monomer
1
end of the polymerization was determined by H NMR
spectroscopy, using the integration ratio between the Si–
H signal of the growing polymer chain and the Si–H signal
of the monomer.
1
Figs. 2 and 3 illustrate the H NMR and 29Si NMR
spectra of polysilanes obtained in the polymerization of
Oligomer
phenylsilane promoted by the neutral complex 1.
monomer
Fig. 3. 29Si NMR spectrum of the monomer and the obtained oligomer in
the dehydrogenative coupling of phenylsilane promoted by the neutral
complex 1 at 90 ꢁC.
Complexes 1 and 2 exhibit low oligomerization activity
in toluene at room temperature. However, when the reac-
tion is carried out at 90 ꢁC, most of the substrates can be
easily oligomerized/polymerized, yielding remarkable con-
versions (see Table 3).
All the dehydrogenative coupling studies were per-
formed both in NMR sealed J-Young tubes, and in large
scales in Schlenk glass reactors to obtain larger quantities
of the oligomers/polymers. The resulted compounds were
isolated, washed out, and analyzed by GPC (gel perme-
ation chromatography) and multinuclear NMR spectros-
copy. The polymerization results are presented in Table 3.
As can be concluded from Table 3, complex 1 is able to
polymerize only small and not bulky substrates, such as
phenylsilane and diethylsilane to form mixtures of oligo-
mers; whereas the titanium complex 2 is able to polymerize
only the smallest phenylsilane to high molecular weight
polymer but unexpectedly is unable to oligomerize any
oligomer
Fig. 2. 1H NMR spectrum of the monomer and the obtained oligomer in
the dehydrogenative coupling of phenylsilane promoted by the neutral
complex 1 at 90 ꢁC.

1946
V. Volkis et al. / Journal of Organometallic Chemistry 692 (2007) 1940–1950
Table 3
Oligomerization/polymerization data for the dehydrogenative coupling of silanes promoted by the neutral complexes 1 and 2a
Substrate
Complex
Reaction time (h)
Conversion (%)
Polymerization results
Dimer (%)
Trimer (%)
Tetramer (%)
Octamer (%)
Higher fractions
%
Mw
MWD
PhSiH3
Et2SiH2
PhSiH3
1
1
2
5
4
5
91
66
88
30
10
–
40
10
–
30
20
–
–
60
–
–
70
30
4800
13,500
1.60
1.45
a
Complex 1 (8.8 mmol), Complex 2 (7.7 mmol), 20 mL toluene, 0.1 mol of silane at 90 ꢁC.
other di-substituted substrates. Interestingly, for both neu-
tral complexes, attempts to polymerize challenging di-
substituted monomers like Ph2SiH2 or PhMeSiH2 were
unsuccessful. Indication on the sensitivity of this complex
towards the steric bulkiness of the substrates is obtained
by the fact that complex 2 was able to polymerize phenylsi-
lane but unable to react with diethylsilane or other di-
substituted silanes.
Remarkably, the single site complex 1 is able to produce
linear dimers, trimers, tetramers and octamers, but not pen-
tamer and hexamers. This fact is similar for metallocene
complexes when disilanes are polymerized with higher activ-
ity when compare to monosilanes [30]. Hence, a dimer
formed in the first polymerization cycle, reacts with the cat-
alyst faster than the monomer to form a trimer. However, the
tetramer is formed as the result of the reaction of two dimers
and the octamer is formed from two tetramers. The octamer
is the thermodynamic sink obtained with the highest conver-
sion presumably due to steric reasons. Regarding complex 2,
which produce a bimodal polymer, it was found recently that
the complex disproportionate in a toluene solution forming
two different species [45], each of them responsible to pro-
duce a different oligosilane fraction.
To overcome the steric hindrance of the mono- and di-
substituted silanes, and to increase the catalytic activity
of both complexes, we have introduced methylaluminoxane
(MAO) and B(C6F5)3 as a co-catalysts to obtain the cat-
ionic complexes. Interestingly, for complex 1, the reaction
with MAO produces the cationic complex 4, and a similar
reaction with B(C6F5)3 produces complex 5. Both com-
plexes exhibit remarkable activities and selectivities for
the polymerization of mono- and di-substituted silanes.
The conceptual reasoning of use of a co-catalyst for the
formation of a cationic complex is rendered by the fact that
the cationic complex has a more coordinative unsaturated
geometry, as compared to the neutral complex, and as a
result, bulky substrates will be able to approach the active
metal center of the complex. In addition, we also expected
that a positive charge on the metal will promote silane
insertions, inducing better polymerization activities. A
plausible mechanism for the formation of the oligomers/
polymers is presented in Scheme 2.
The polymerization results for the dehydrogenative cou-
pling of silanes promoted by the cationic complexes 4 and 5
are presented in Table 4. Small silanes were easily polymer-
ized to products contained between 15 and 40 silicon atoms
H3SiPh
MAO or B(C6F5)3
CH4
L2ZrMe2
L2Zr-Me
L2Zr-SiH2Ph
PhSiH3
PhH2Si-SiH2Ph
L2Zr-H
PhH2Si-SiH2Ph
H3SiPh
H2
PhH2Si-SiH2Ph
H3SiPh
PhH2Si-SihPh-SiH2Ph
PhH2Si-SiH2Ph
H2
L2Zr-SiH2Ph
L2Zr-SiHPh-SiH2Ph
Scheme 2. Mechanism for the oligomerization of silanes by the cationic benzamidinate complexes (only one catalytic site is presented for clarity).

V. Volkis et al. / Journal of Organometallic Chemistry 692 (2007) 1940–1950
1947
Table 4
Dehydrogenative coupling of silanes promoted by the cationic complexes 4 and 6 at 90 ꢁC
Substrate
Reaction time (h)
Catalyst
Conversiona (%)
Polymerization results
Dimerb (%)
Trimerb (%)
Tetramerb (%)
Polymer
b
%
100
90
–
Mn
Mw
4800
2400
–
MWD
1.95
1.42
–
4
PhSiH3
4
4
4
4
4
4
Zr:Al = 1:50
98
92
67
70
62
88
–
–
–
2650
1700
–
6
PhSiH3
Zr:B = 1:1
3
4
3
4
Ph2SiH2
Ph2SiH2
PhMeSiH2
Zr:Al = 1:50
20
20
20
20
22
35
20
–
58
45
40
–
6
Zr:B = 1:1
–
–
–
–
6
Zr:B = 1:1
20
80
600
3600
1350
5300
2.25
1.46
6
Et2SiH2
Zr:B = 1:1
a
Mass% of the total resulting mixture of oligo-/polymers from the started amount of substrate.
Mass% of the polymeric fraction from the total amount of the converted product.
b
in the chain, whereas for bulky silanes, only oligomers were
the polymer chain). These cationic complexes were also
able to polymerize bulky di-substituted silanes, which have
not been observed previously for any other organometallic
complex. Regarding the effect of the co-catalysts, we have
found no major discrepancies among MAO and the
tris(pentafluorophenyl)borane indicating that similar cat-
ionic motif are formed. Our attempts to perform the cou-
pling of silanes with a cationic titanium complex 5 were
unsuccessful presumably due to the formation of Ti (III)
observed by the reaction of Ti(IV) and MAO [45].
observed. According to the NMR analysis, all the polysil-
anes and oligosilanes were characterized having a linear
structure. It is important to point out, as expected, for sim-
ilar silanes the cationic complexes were better catalysts
than the corresponding neutral complexes.
To obtain the optimal time of performance in the dehy-
drogenative coupling, we have study the progress of the
reaction as a function of time, as shown for phenylsilane
catalyzed by complex 4 (Fig. 4). Fig. 4 shows that during
the first 3 h of the reaction, the chain grows linearly with
a monomer insertion rate of 7–8 units/h. After that period,
the reaction rate starts to decelerate, due to monomer star-
vation (the reaction runs to 98% yield, forming 25 mono-
meric units on the chain).
When comparing under the same reaction conditions the
dehydrogenative coupling of phenylsilane with the cationic
complexes 4 or 6 (Table 4) and the neutral complex 1 it can
be concluded that the former complexes are able to poly-
merize the silane in better yields (100%) and better molec-
ular weights (compare octamer with 25 monomeric units in
3. Conclusions
Complexes 4, 5 and 7 were found to be active complexes
in the polymerization of 1,5-hexadienes. For the cationic
bis-benzamidinate complexes 4 and 5 the cis to trans ring
closure ratio was found to be close to 50:50%. For complex
7, containing the chiral myrtanyl moiety on the ancillary
ligand, a slight preference towards the trans closure was
observed. Complex 7 produces the polymer with higher
amount of RmR tetrad moieties and with almost full
absence of MxR and MrM tetrads, as compared to the
other chiral zirconium complexes, described in literature.
The structure of PMCP produced by complexes 4 or 5 is
quite similar. The percentage of RmR tetrads for these
polymers is comparable to those obtained with a simple
zirconocene. However, a high content of MmM and
MxR tetrads makes these polymers different from all types
of PMCP described previously.
Complexes 1 and 2 were found to be active in the poly-
merization of non-bulky phenylsilane. The zirconium com-
plex 1 produced oligomers only, whereas titanium complex
2, forms a bimodal polymer with 28–88 monomeric units.
To our knowledge, this is the longest linear polysilanes-
obtained using an organometallic complex. The large
Fig. 4. Polymerization of phenylsilane by complex 4 activated by MAO in
toluene at 90 ꢁC. n = the number of monomeric units in the polysilane.

1948
V. Volkis et al. / Journal of Organometallic Chemistry 692 (2007) 1940–1950
coordinative unsaturation of the cationic complexes 4 and
6 allows the polymerization of bulky mono- and di-substi-
tuted silanes. This is the first example in which cationic
octahedral group 4 complexes are used for the dehydroge-
native coupling of silanes. The degree of polymerization
was strongly dependent on the bulkiness of the silane
substrates.
(10ꢀ6 mmHg), and a certain amount of a dry deuterated
solvent was transferred into the tube, to dissolve the cata-
lyst. An exact amount of silane was vacuum-transferred to
the frozen tube. The tube was place into a glycerol/oil bath
(according to the reaction temperature), and the reaction
was started. Monitoring of the reaction was performed
1
by H NMR spectroscopy. At the end of polymerization,
the solvent and the non-reacted silane were removed by
vacuum transfer to another NMR tube, and the contents
of the latter was analyzed by NMR spectroscopy in order
to confirm the presence of dimerization product and a
non-reacted monomer. The reactions were finally quenched
by the addition of water. The resulted polymer (or oligo-
mer) was dissolved in toluene at 90 ꢁC and the toluene frac-
tion was evaporated, and the resulted poly-/oligosilane was
dried in vacuum.
4. Experimental
4.1. General procedure
All manipulations with air-sensitive materials were car-
ried out with the rigorous exclusion of oxygen and mois-
ture in a dual manifold Schlenk line, or interfaced to a
high vacuum (10ꢀ6 Torr) line, or in a nitrogen filled ‘Vac-
uum Atmospheres’ glove box with a medium capacity
recirculator (1–2 ppm O2). Argon and nitrogen were puri-
fied by passage through a MnO oxygen-removal column
4.3. Large scale polymerization of silanes
˚
and a Davison 4 A molecular sieve column. Deuterated sol-
Polymerization was carried out in a 50 ml Schlenk vessel
equipped with magnetic stirrer. The Schlenk vessel was
charged in glove-box by 5 mg of the catalyst. Then the
Schlenk vessel was connected to a vacuum line, and 10–
15 ml of the dry solvent was added by syringe. After dis-
solving the catalyst, 10 ml of pure dry silane was added,
the Schlenk vessel was placed into the oil bath at 90 ꢁC
and the polymerization started. At the end of polymeriza-
tion time the reaction was quenched by HCl/methanol
(1:1 mixture), and the toluene fraction was isolated. In
the case of monosilanes, toluene and dimeric products were
transferred to a Schlenk vessel, and the dimeric products
were separated by distillation. The solid/oily fraction was
dried under vacuum.
vents were stored under nitrogen over Na/K alloy. All sol-
vents for vacuum line manipulations were stored in
vacuum over Na/K alloy. All silanes (Aldrich) were distil-
˚
lated and dried using a Davison 4 A molecular sieve, and
transferred into calibrated storage tubes connected to high
vacuum line. Solid methylalumoxane (MAO) was prepared
from a 30% suspension in toluene (Witco) by vacuum evap-
oration of the solvent at 25 ꢁC/10ꢀ5 mmHg and was stored
in the glove box. 1,5-Hexadiene was distilled and dried
˚
using a Davison 4 A molecular sieve.
The synthesis of the bis-benzamidinate dimethyl com-
plexes of Zr and Ti were performed as described previously
[35k].
NMR spectra were recorded on a Bruker AM 200 and
Bruker AM 400 spectrometers. Chemical shifts for 1H
NMR, 13C NMR and 29Si NMR are referenced to internal
4.4. Polymerization of 1,5-hexadiene
solvent
tetramethylsilane.
resonance,
and
reported
relative
to
Polymerization was carried out in a 50 ml Schlenk vessel
equipped with magnetic stirrer. The Schlenk vessel was
charged in a glove-box with 5 mg of the catalyst and the
calculated amount of MAO. Then the Schlenk vessel was
connected to a high vacuum line, and 10–15 ml of dry tol-
uene was added by syringe. After dissolving the catalyst
and the formation of the active species, 10 ml of pure dry
1,5-hexadiene was added, and the polymerization started.
At the end of the polymerization time, the reaction was
quenched by the addition of methanol. Solid polymer
was filtered, washed with a large amount of methanol
and dried. The toluene fraction was isolated and toluene
was removed by vacuum. The toluene soluble polymer
was obtained and dried.
Melting points were determined on a Differential Scan-
ning Calorimeter (DSC). Three runs (first heating, cooling,
second heating) were performed at a heating rate of 10 ꢁC/
min in the temperature range 30–190 ꢁC. The second-heat-
ing melting peak temperature was taken as a melting point.
GPC measurements were carried out on Water’s Alli-
ance 2000 instrument: for polysilanes at 90 ꢁC, using tolu-
ene (HPLC) as a mobile phase and Water’s Styragel HT
columns, and for poly-1,5-hexadienes in triclorobenzene
at 150 ꢁC, using the same type of columns. Molecular
weights are referenced to polystyrene standards (Aldrich).
4.2. Polymerization of silanes in NMR tubes
4.5. Calculation of the number of monomeric units in the
resulted silane oligomer (n)
All polymerization experiments with silanes were per-
formed in Teflon valve-sealed tubes. A certain amount of
catalyst (and co-catalyst in part of experiments) was
weighted in a glove box, and then added to an NMR tube.
The closed tube was attached to a high vacuum line
At the end of reaction, the number of monomeric units
in the formed oligomer (n) was calculated as Mn/106, where
106 is Mw of one monomeric silane repeating unit. For

V. Volkis et al. / Journal of Organometallic Chemistry 692 (2007) 1940–1950
1949
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1
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units at the time ‘‘i’’ and n is a number of monomeric units
in the obtained oligomer.
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