Nickel-catalyzed cyclopolymerization of hexyl- and phenylsilanes

Article abstract of DOI:10.1021/om301052f

[Ni(dmpe)₂] (dmpe = 1,2-bis(dimethylphosphino)ethane) catalyzed the dehydrogenative polymerization of hexylsilane in toluene at room temperature to produce a mixture of acyclic and cyclic poly(hexylsilanes). A simlar reaction at 70 C resulted in selective cyclopolymerization of hexylsilane to yield a cyclic polymer with an average molecular weight of M_n = 1450 (M_w/M_n = 1.01, GPC polystyrene standard). The ¹H NMR, ²⁹Si{¹H} DEPT NMR, and IR spectroscopic data indicated the presence or absence of the -SiH₂R end groups of the polymer and its acyclic or cyclic structure. Addition of hexylsilane to the solution of the poly(hexylsilanes) containing the acyclic polymer (M_n = 1330) and heating the mixture in the presence of 5 mol % of [Ni(dmpe) ₂] catalyst formed a polymer composed of the cyclic molecules without a change in the average molecular weight. The polymerization of phenylsilane catalyzed by [Ni(dmpe)₂] also yielded the cyclic poly(phenylsilane). The reaction using a mixture of [Ni(cod)₂] (cod = 1,5-cyclooctadiene) and PMe₃ as the catalyst produced acyclic and/or cyclic poly(phenylsilanes) depending on the conditions. 9,9-Dihydrosilafluorene reacted with [PdMe₂(dmpe)] to afford a persilylated palladacyclopentane, [Pd(SiC₁₂H₈)₄(dmpe)], with four 1,1-silafluorene units.

Full text of DOI:10.1021/om301052f

Article
pubs.acs.org/Organometallics
Nickel-Catalyzed Cyclopolymerization of Hexyl- and Phenylsilanes
Makoto Tanabe, Atsushi Takahashi, Tomoko Fukuta, and Kohtaro Osakada*
Chemical Resources Laboratory, Tokyo Institute of Technology, 4259-R1-3 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan
S
* Supporting Information
ABSTRA CT: [ N i(d m pe )2 ] (d mp e
= 1 , 2 - b i s-
(dimethylphosphino)ethane) catalyzed the dehydrogenative
polymerization of hexylsilane in toluene at room temperature
to produce a mixture of acyclic and cyclic poly(hexylsilanes). A
simlar reaction at 70 °C resulted in selective cyclopolymeriza-
tion of hexylsilane to yield a cyclic polymer with an average
molecular weight of Mn = 1450 (Mw/Mn = 1.01, GPC
polystyrene standard). The 1H NMR, 29Si{1H} DEPT NMR, and IR spectroscopic data indicated the presence or absence of the
−SiH2R end groups of the polymer and its acyclic or cyclic structure. Addition of hexylsilane to the solution of the
poly(hexylsilanes) containing the acyclic polymer (Mn = 1330) and heating the mixture in the presence of 5 mol % of
[Ni(dmpe)2] catalyst formed a polymer composed of the cyclic molecules without a change in the average molecular weight. The
polymerization of phenylsilane catalyzed by [Ni(dmpe)2] also yielded the cyclic poly(phenylsilane). The reaction using a mixture
of [Ni(cod)2] (cod = 1,5-cyclooctadiene) and PMe3 as the catalyst produced acyclic and/or cyclic poly(phenylsilanes) depending
on the conditions. 9,9-Dihydrosilafluorene reacted with [PdMe2(dmpe)] to afford a persilylated palladacyclopentane,
[Pd(SiC12H8)4(dmpe)], with four 1,1-silafluorene units.
Scheme 1
INTRODUCTION
■
Polysilanes have been known as σ-conjugated polymers whose
backbone is composed of Si−Si linkages. The photochemical
and thermal properties of the polymers are influenced
significantly by the conformation of the polymer chains.1 One
of the common preparations of polysilanes is the dehydrogen-
ative polycondensation reaction of primary and secondary
organosilanes catalyzed by transition-metal complexes.2 This
reaction has attracted recent attention because of the mild
reaction conditions and high selectivity. Complexes of early
transition metals such as Ti and Zr catalyze efficient
dehydrogenative polycondensation to produce linear polymers
as the main product and cyclic polymers with lower molecular
weight as the byproducts. The electrophilic metals bonded to
the silyl ligands undergo σ-bond metathesis with oligosilanes to
cause activation of the M−Si bond and formation of a new Si−
Si bond (Scheme 1(i)).3 Scheme 1(ii) shows the polymer
growth via a back-biting reaction during the polymer growth to
yield the cyclic polymer.2a,4 Formation of the cyclic polysilanes
was reported in the polymerization of primary alkylsilanes
catalyzed by early-transition-metal complexes.5
Ni complexes also catalyze polycondensation of organo-
silanes to produce polysilanes. The catalytic activity is high
among late-transition-metal complexes, although it is lower
than that for the early-transition-metal complexes. In 1991,
Parker reported that the Ni complex formed from NiI2 and Li
in the presence of PPh3 catalyzed the condensation of
secondary silanes to yield oligosilanes up to tetramers.6
Zargarian et al. studied the dehydrogenative oligomerization
of phenylsilane catalyzed by methylnickel complexes with
indenyl and PR3 supporting ligands.7 Recently, Abu-Omar
reported the synthesis of phenylsilane oligomers using
[(dippe)Ni(μ-H)]2 (dippe = 1,2-bis(diisopropylphosphino)-
ethane) as the catalyst.8 The products of these two reactions are
composed of a mixture of the cyclic and linear (acyclic)
polysilanes. Coupling of two silyl (SiHnR3−n) ligands bonded to
a late transition metal and migration of a silyl ligand to the Si
atom of a silylene (SiHnR2−n) ligand bonded to the metal were
proposed to occur rapidly, forming a new Si−Si bond.9 Tanaka
reported that the dinickel complexes with bridging silylene and
silyl ligands have a Si···Si interaction between the ligands.10 σ-
Bond metathesis is not usually suggested as the valid
mechanism for the Si−Si bond formation by late transition
metals, although it is discussed as a mechanism of the Si−H
Received: November 5, 2012
Published: February 6, 2013
© 2013 American Chemical Society
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bond activation in dinuclear Rh complexes with Si ligands.11 In
this paper, we present polycondensation of primary silanes
using Ni catalysts to form the cyclic oligosilanes selectively.
RESULTS AND DISCUSSION
■
Stirring a mixture of hexylsilane (232 mg, 2.0 mmol) and
toluene (0.15 mL) in the presence of 1 mol % of [Ni(dmpe)2]
(dmpe = 1,2-bis(dimethylphosphino)ethane) at room temper-
ature caused a polycondensation reaction, yielding a mixture of
acyclic and cyclic poly(hexylsilane)s (poly-Ia and poly-Ic, 61%
yield), according to eq 1. The composition of the polymers
changed depending on the polymerization conditions, as
described below in detail.
Figure 2. 29Si{1H} DEPT NMR spectra (θ = 135°) of (a) poly-Ia (or
a mixture of poly-Ia and poly-Ic) and (b) poly-Ic.
The GPC trace of the poly(hexylsilane), obtained under the
with an opposite phase (δ −54). Thus, the polymerization with
a very highly concentrated solution at room temperature forms
a mixture of the acyclic and cyclic polysilanes containing a
significant amount of the acyclic polymer.
above conditions, indicates an average molecular weight of Mn
1
= 1420 (Mw/Mn = 1.02). The H NMR spectrum of the
polymer contains multiple peaks due to the Si−H hydrogens at
δ 4.08−4.38, as shown in Figure 1a. The polysilanes composed
of −(SiHR)− units were reported to show such signals because
of the relative stereochemistry of the neighboring repeating
units along the polymer chain.12 The precise ratio of poly-Ia
and poly-Ic could not be estimated due to severe overlapping of
these Si−H hydrogen signals. 29Si NMR spectroscopy using
DEPT technique was employed to determine the acyclic or
cyclic structures of the polysilanes, because the Si atoms
bonded to two hydrogens and to one hydrogen can be
distinguished clearly.5,13 The 29Si{1H} DEPT NMR spectrum
(θ = 135°), shown in Figure 2a, indicates the presence of
acyclic poly-Ia having −SiH2R end groups. The numerous
peaks with high intensity in the range from δ −58 to −68 are
assigned to the −(SiHR)− group of the main chain, while the
−SiH2R end groups of the polymers are observed as the peaks
The dehydrocoupling polymerization catalyzed by the same
Ni complex, at 70 °C in toluene (2 mL), produced the cyclo-
poly(hexylsilane) (poly-Ic) in 93% yield. Its average molecular
weight (Mn = 1450 and Mw/Mn = 1.01) is similar to that of the
polymer obtained under the above conditions. The FAB-MS
result of the poly-Ic indicates formation from octamer to
undecamer with the repeating −{SiH(C6H13)}− units of M =
114 and decrease of the C6H13 fragment (M = 85) to each
unit.14 No inverse 29Si DEPT NMR signal of poly-Ic is
observed in the spectra measured by a 135° pulse wave (Figure
2b), suggesting the absence of −SiH2R end groups. IR spectra
of poly-Ia and poly-Ic show a clear band of the Si−H groups at
2074 and 2068 cm−1, and the −SiH2R bending vibration of
poly-Ic, due to end groups, is not observed around 930 cm−1
(poly-Ia: ν 925 cm−1). Hilty reported that their cyclic
1
Figure 1. H NMR spectra in the Si−H region of (a) poly-Ia (or a mixture of poly-Ia and poly-Ic) and (b) poly-Ic.
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oligosilanes showed a lower wavenumber shift of the Si−H
stretching and absence of the −SiH2R bending vibration
peaks.15 The NMR spectroscopic data in the Si−H region differ
distinctively between the acyclic and cyclic polysilanes. Figure
1b shows that major Si−H hydrogen signals of poly-Ic are
observed at δ 4.25−4.58, which are shifted downfield in
comparison to the corresponding peaks of poly-Ia (δ 4.08−
4.38). Thus, the polymerization using the same Ni catalyst with
higher temperature produces the cyclic polymer exclusively.
In order to obtain further insights into the mechanism for
formation of the cyclic polymer, the following experiments
were conducted. Heating a mixture of acyclic and cyclic
poly(hexylsilanes) at 70 °C did not change the polymer
structure. The reaction with added hexylsilane monomer (0.2
equiv) and 5 mol % [Ni(dmpe)2] catalyst at the same
temperature for 144 h converted the polymer with the
−SiH2R end groups to the cyclic polymer without the end
group (eq 2). Figure 3 shows changes in the 1H NMR signals in
1
Figure 4. H NMR spectra of (a) poly-IIa and (b) poly-IIc.
peaks due to the −SiH2R end groups. Phenylsilane undergoes
cyclopolymerization under conditions milder than those for
hexylsilane. Table 1 summarizes the results of dehydrocoupling
Table 1. Dehydrocoupling Polymerization of Phenylsilane
a
Catalyzed by Ni Complexes
cyclic/
c
acyclic
amt of
toluene
(mL)
(poly-
IIc/poly-
IIa)
temp
(°C)
yield
(%)
b
b
cat.
Mn
Mw/Mn
1
[Ni(dmpe)2]
0.15
room
temp
64
49
33
610
1.02
100/0
89/11
5/95
Figure 3. Change of the H NMR spectra in the Si−H region during
conversion of poly-Ia to poly-Ic.
[Ni(cod)2]/
PMe3 (1/4)
2.0
70
780
1.09
1.16
the Si−H region during the reaction. A decrease of the
broadened signals at δ 4.05−4.28 and relative increase of the
signals at δ 4.45−4.60 indicate conversion of the polymer with
−SiH2R end groups to the cyclic polymer. The GPC
measurements of each sample show the Mn values of the
polymer within the range Mn = 1330−1350 during the
conversion.
[Ni(cod)2]/
PMe3 (1/0.5)
0.15
room
temp
1300
a
Reaction conditions: Ni complex 0.020 mmol, [monomer]/[Ni] =
b
100 at room temperature or 70 °C. Determined by GPC
c
measurements on the basis of polystyrene standards. Measured by
1H NMR spectroscopy (Si−H hydrogen region).
Dehydrocoupling polymerization of phenylsilane catalyzed
by [Ni(dmpe)2] at room temperature produced cyclo-poly-
(phenylsilane) (poly-IIc) in 64% yield (eq 1). The H NMR
spectrum of poly-IIc displays multiple Si−H hydrogen signals
in the range δ 4.8−6.0, as shown in Figure 4b. The broad
signals in a magnetic field lower than δ 5.0 indicate formation of
the cyclic polymers with different stereochemistries of the
neighboring −(SiHR)− groups and different ring sizes.5 The
29Si DEPT NMR (θ = 135°) spectrum of poly-IIc shows many
peaks between δ −44 and −67 but does not contain inverse
reactions of phenylsilane catalyzed by the Ni complexes with
mono- or bidentate phosphine ligands. The phosphine ligands
of the catalysts influence the polymer structures. Dehydrocou-
pling of phenylsilane catalyzed by [Ni(cod)2] and PMe3 in 1/4
ratio in toluene at 70 °C gave a mixture of poly-IIc with the
cyclic structure (49% yield) and H2SiPh2 formed via
redistribution of H3SiPh.16 In contrast, a similar polymerization
with a smaller amount of PMe3 to [Ni(cod)2] (0.5/1) at room
temperature produced the polymer containing acyclic poly-
(phenylsilane) (poly-IIa). In the 1H NMR spectra, broad Si−H
1
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resonances of poly-IIa are observed in the range δ 4.3−4.8
(Figure 4a), which is consistent with the data found in the
literature.5,7b Waterman reported on a similar cyclodehydro-
polymerization of phenylsilane catalyzed by an Ir complex with
a pincer ligand.17
white solid in 88% yield.22 The persilylated palladacyclopentane
[Pd(SiC12H8)4(dmpe)] (1) having four 1,1-silafluorene units
was successfully obtained from the dehydrocoupling of 9,9-
dihydrosilafluorene with [PdMe2(dmpe)] in a 4.5/1 ratio (eq
3). Heating of the reaction mixture in toluene at 80 °C for 120
Complexes of early transition metals have electron-deficient
metal centers, and they often undergo σ-bond metathesis
reactions. Thus, formation of the cyclic polysilane by
polycondensation of primary silanes catalyzed by Ti complexes
is attributed to a back-biting reaction via σ-bond metathesis
reactions (Scheme 1(ii)).2a,4 σ-Bond metathesis is much less
common in the reactions of late-transition-metal complexes.
The formation of Si−Si bonds from bis(silyl) complexes of late-
transition-metal complexes was reported to involve reductive
elimination by several research groups.18 Scheme 2 depicts the
h gradually generated yellow crystals of 1 in 44% yield. The
reaction mixture contained a soluble bis(silyl)palladium
precursor of 1, [Pd(SiHC12H8)2(dmpe)] (2). Complex 2 was
formed also by the silyl ligand exchange of [Pd-
(SiHPh2)2(dmpe)] with 9,9-dihydrosilafluorene in a 1/2 ratio.
The reactions, however, did not give the four-membered
trisilapalladacyclobutane as a product.
Scheme 2
The obtained palladacycle 1 was characterized by X-ray
crystallography and solid-state NMR spectroscopy due to its
poor solubility in any organic solvents. Figure 5 shows the
mechanism of cyclic polysilane formation involving reductive
elimination as the ring-forming step. The growing polymer (A)
contains the Ni center coordinated to an end of the acyclic
polysilane. Insertion of a zerovalent Ni complex into the Si−H
group of the other end groups in A results in the formation of
the dinuclear intermediate B. Intramolecular exchange of the
Ni−Si bond with the Ni−Y bond yields the cyclic intermediate
C, and the subsequent coupling of the silyl ligands affords the
cyclic polysilanes D as the product.19 Another possible
mechanism involves an intramolecular cyclization of A to give
the similar Ni(IV) intermediate C′18e and formation of the
same polymer D. The detailed ring-closing pathway of the
reaction in this study, however, is not clear at present. Abu-
Omar also discussed two possible mechanisms of σ-bond
metathesis or oxidative addition for dehydrogenative coupling
of primary silanes catalyzed by a similar Ni complex.8
Persilylated and pergermylated metallacycles were suggested
as possible intermediates for catalytic dehydrocoupling
reactions of organosilanes and -germanes.8,20 We found that
the four-membered germaplatinacycle underwent ring enlarge-
ment to the five-membered ring by insertion of a
diorganogermylene unit.21 The ring-expanding reactions of
the persilylated and pergermylated metallacycles would provide
another kind of formation reaction of the cyclic polysilanes and
polygermanes. Therefore, isolation of nickel-containing metal-
lacycles was examined by using secondary organosilanes as the
substrate. The dehydrocoupling reaction of dihexylsilane with
[Ni(dmpe)2] catalyst at room temperature or 70 °C did not
form any products, while 9,9-dihydrosilafluorene reacted with
[Ni(dmpe)2] to produce insoluble poly(1,1-silafluorenes) as a
Figure 5. Thermal ellipsoid diagram of 1 (probability 50%). Selected
bond distances (Å) and angles (deg): Pd−P = 2.3508(8), Pd−Si1 =
2.3610(8), Si1−Si2 = 2.375(1), Si2−Si2* = 2.362(1); P−Pd−P* =
84.43(3), P−Pd−Si1 = 95.50(3), Si1−Pd−Si1* = 84.58(3), Pd−Si1−
Si2 = 125.96(4), Si1−Si2−Si2* = 98.16(4).
ORTEP drawing of complex 1, which has C2 crystallographic
symmetry around the axis containing the Pd center and a
midpoint of the Si2−Si2* bond. The Pd−Si bond distance
(2.3610(8) Å) is slightly longer than that of a bis(silyl)
palladium complex with a dmpe ligand, [Pd(SiHPh2)2(dmpe)]
(2.3548(9) and 2.3550(7) Å).23 The Si−Si bond distances
(2.362(1) and 2.375(1) Å) are within the range of normal Si−
Si bonds (2.33−2.37 Å)24 and are similar to those of a
platinacycle with an analogous oligosilane ring, [Pt-
(SiC12H8)4(dppe)] (2.327(3)−2.365(3) Å; dppe = 1,2-bis-
(diphenylphosphino)ethane), reported by Braddock-Wilking et
al.20 The solid-state 29Si{1H} NMR spectrum of 1 shows two
d
peaks at δ 8.88 and −27.4. The former doublet (δ 8.88, 2JP−Si
=
146 Hz), assigned as the silicon atom close to the Pd, is
2
comparable to the 29Si NMR signal of 2 (δ −14.6, JPtrans−Si
=
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separate the low-molecular-weight oligomers. The polymer was dried
165 Hz). Further studies on the properties of 1 did not provide
information on the plausibility of the polysilane formation
mechanism involving silylene insertion into the M−Si bond of
the metallacyclic intermediate.
in vacuo to afford a mixture of acyclic and cyclic poly(hexylsilanes)
1
(poly-Ia and poly-Ic) as a pale yellow oil (140 mg, 61%). H NMR
(400 MHz, C6D6, room temperature): δ 4.08−4.38 (br, 1H, acyclic
SiH), 1.74 (br, 2H, SiCH2), 1.33−1.48 (br, 8H, Si(CH2)n (n = 2−5)),
0.91 (br, 3H, Si(CH2)5CH3). 29Si{1H} DEPT NMR (79 MHz, C6D6,
room temperature): δ −53.8 to −57.9 (SiH2), −58.3 to −68.0 (SiH).
IR (KBr): 2074 (νSi−H), 925 (δSi−H) cm−1. GPC (THF): Mn = 1420,
Mw/Mn = 1.02.
CONCLUSIONS
■
Dehydrogenative polycondensation of hexylsilane and phenyl-
silane catalyzed by nickel complexes yielded the cyclic
polysilanes by choices of the catalysts and reaction conditions.
The FAB-MS and NMR measurement results confirmed the
cyclic structure of the products unequivocally. The acyclic
poly(hexylsilane) was converted into the cyclic polymer in the
presence of a small amount of the monomer and the nickel
catalyst, which suggests formation of the cyclic products via
acyclic intermediates. In the polycondensation of phenylsilane,
nickel catalysts with PMe3 ligands produce the cyclic or acyclic
polymer depending on the ratio of phosphine to the Ni
complex. Another proposed mechanism for the cyclic
polysilanes involves initial formation of oligosilametallacycles
and their ring enlargement. Formation of persilylated
palladacyclopentane 1, prepared from dehydrocoupling of 9,9-
dihydrosilafluorene with a Pd complex, provided analogues of
the intermediates of such reaction pathways. The study did not
provide evidence for a ring-enlargement of the metallacycles,
although it was established in the reaction of pergermametalla-
cycles.21b,d
Polymerization of Hexylsilane at 70 °C. The procedure was
similar to the above polymerization. In a glovebox, to a toluene
solution (2 mL) of hexylsilane (232 mg, 2.0 mmol) in an open small
vial was added 1 mol % of [Ni(dmpe)2] (7.2 mg, 0.020 mmol). The
reaction mixture was stirred for 168 h at 70 °C. The cyclo-
poly(hexylsilane) poly-Ic was obtained as a pale yellow oil (212 mg,
1
93%). H NMR (400 MHz, C6D6, room temperature): δ 4.25−4.58
(br, 1H, cyclic SiH), 1.77 (br, 2H, SiCH2), 1.10−1.35 (br, 8H,
Si(CH2)n (n = 2−5)), 0.91 (br, 3H, Si(CH2)5CH3). 29Si{1H} DEPT
NMR (79 MHz, C6D6, room temperature): δ −47.7 to −68.1 (SiH).
IR (KBr): 2068 (νSi−H) cm−1. GPC (THF): Mn = 1450, Mw/Mn =
1.01.
Conversion of poly-Ia to poly-Ic. In a glovebox, to a toluene
solution (14 mL) of a mixture of poly-Ia and poly-Ic (210 mg, 1.84
mmol) was added 0.2 equiv of hexylsilane (43 mg, 0.37 mmol) and 5
mol % of [Ni(dmpe)2] (33 mg, 0.092 mmol). During the thermal
reactions at 70 °C, 2 mL portions of the solution were collected in
small vials after 3, 6, 9, 12, 24, 48, and 144 h. Each solution was passed
through Florisil (1.3 g) to remove the catalyst. The solvents of each
sample were evaporated under reduced pressure, and the crude
products were dried in vacuo to yield the polymers as colorless oils. All
samples were analyzed by 1H NMR spectroscopy and GPC
measurements. The Si−H hydrogen signals of the products were
shifted downfield, as shown in Figure 3, while the GPC traces of each
sample provide the average molecular weight within a constant range
(Mn = 1330−1350).
Polymerization of Phenylsilane Catalyzed by [Ni(dmpe)2]. In
a glovebox, to a toluene solution (150 μL) of phenylsilane (216 mg,
2.0 mmol) was added 1 mol % [Ni(dmpe)2] (7.2 mg, 0.020 mmol).
Rapid H2 gas evolution was observed, which was followed by stirring
the reaction mixture for 168 h at room temperature. The resulting
solution was diluted with THF (3 mL) and filtered through Florisil
(1.3 g) to remove the catalyst. The solvent was evaporated under
vacuum to give an oily residue. The crude product was washed with
acetonitrile (5 mL) at 0 °C to separate the low-molecular-weight
oligomers, which was dried in vacuo to afford cyclic poly(phenylsilane)
(poly-IIc) as a pale yellow solid (136 mg, 64%). 1H NMR (400 MHz,
C6D6, room temperature): δ 7.2−8.2 (br, C6H5), 6.7−8.2 (br, C6H5),
4.8−6.0 (m, SiH). 29Si{1H} DEPT NMR (79 MHz, C6D6, room
temperature): δ −43.7 to −67.0 (SiH). IR (KBr): 2086 (νSi−H) cm−1.
GPC (THF): Mn = 610, Mw/Mn = 1.02.
EXPERIMENTAL SECTION
■
General Procedures. All manipulations of the reactions were
carried out using standard Schlenk techniques under an argon or
nitrogen atmosphere or in a nitrogen-filled glovebox (Miwa MFG).
Hexane, toluene, and tetrahydrofuran (THF) were purified by using a
Grubbs-type solvent purification system (Glass Contour).25 Dehy-
drated acetonitrile (Kanto Chemical) was used as received. 1H,
13C{1H}, 29Si{1H}, and 31P{1H} NMR spectra were recorded on a
Bruker Biospin Avance III 400 MHz NMR spectrometer. The pulse
sequence for 29Si{1H} DEPT NMR spectroscopy was set up according
to the literature.26 Chemical shifts in H and 13C{1H} NMR spectra
1
were referenced to the residual peaks of the solvents used.27 The 29Si
and 31P chemical shifts were referenced to external SiMe4 (δ 0) and
85% H3PO4 (δ 0) in C6D6 or C4D8O, respectively. Solid-state CPMAS
29Si{1H} and 31P{1H} NMR measurements were carried out on a
JEOL-ECA 600 spectrometer using a spinning speed of 15 kHz. The
29Si and 31P chemical shifts were referenced to external poly-
(dimethylsilane) (δ −33.8) and (NH4)H2PO4 (δ 1.6), respectively.
IR spectra were recorded on a JASCO FTIR-4100 spectrometer. FAB-
MS spectra (matrix 2-nitrophenyl octyl ether (NPOE)) were obtained
using a JEOL JMS-700 mass spectrometer. Elemental analyses were
performed using a LECO CHNS-932 or Yanaco MT-5 CHN
autorecorder. Gel permeation chromatography (GPC) measurements
were performed at 40 °C on a TOSOH HLC-8220 GPC/HT
equipped with a UV−vis detector, using THF as eluent at a flow rate
of 0.6 mL/min. Average molecular weights were calculated relative to
polystyrene standards. The compounds [Ni(dmpe)2],28 hexylsilane,29
9,9-dihydrosilafluorene,30 and [Pd(SiHPh2)2(dmpe)]23 were prepared
according to the reported procedure. Phenylsilane (Tokyo Chemical
Industry) was used as received.
Polymerization of Hexylsilane at Room Temperature. In a
glovebox, to a toluene solution (150 μL) of hexylsilane (232 mg, 2.0
mmol) in an open small vial was added 1 mol % of [Ni(dmpe)2] (7.2
mg, 0.020 mmol). Rapid H2 gas evolution was observed, which was
followed by stirring the reaction mixture for 168 h at room
temperature. The resulting solution was diluted with THF (3 mL)
and filtered through Florisil (1.3 g) to remove the catalyst. The solvent
was evaporated under vacuum to give an oily residue. Acetonitrile (5
mL) was added to the crude product, which was decanted at 0 °C to
Polymerization of Phenylsilane Catalyzed by a Ni-PMe3
Complex. The polymerization of phenylsilane (216 mg, 2.0 mmol)
in toluene (150 μL) in the presence of 1 mol % [Ni(cod)2] (5.4 mg,
0.020 mmol) and PMe3 (6.1 mg, 0.080 mmol) in a 1/4 ratio for 168 h
at 70 °C was conducted to give the cyclic poly(phenylsilane) poly-IIc
as a major product (106 mg, 49%). The cyclic and acyclic polysilanes
in a 89/11 ratio were determined by the Si−H signals according to the
literature.5 GPC (THF): Mn = 780, Mw/Mn = 1.09. The acyclic
poly(phenylsilane) poly-IIa was obtained as a brown paste (71 mg,
33%) from a similar polymerization of phenylsilane (216 mg, 2.0
mmol) catalyzed by 1 mol % of [Ni(cod)2] (5.4 mg, 0.020 mmol) and
PMe3 (0.7 mg, 0.010 mmol) in a 1/0.5 ratio at room temperature. The
cyclic and acyclic polysilanes were estimated similarly to be formed in
1
a 5/95 ratio. H NMR (400 MHz, C6D6, room temperature): δ 7.2−
7.5 (br, C6H5 ortho), 6.7−7.1 (br, C6H5 meta and para), 4.3−4.8 (br,
SiH). 29Si{1H} DEPT NMR (79 MHz, C6D6, room temperature): δ
−55.6 to −56.5 (SiH2), −58.6 (SiH2), −60.0 to −65.0 (SiH). IR
(KBr): 2118 (νSi−H) cm−1. GPC (THF): Mn = 1330, Mw/Mn = 1.16.
Attempt To Polymerize Dihexylsilane. In a glovebox, to a
toluene solution (4 mL) of dihexylsilane (125 mg, 0.62 mmol) was
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Article
added 1 mol % of [Ni(dmpe)2] (2.2 mg, 0.0061 mmol). After the
mixture was stirred for 96 h at 70 °C, no polymerization was observed.
Reaction of 9,9-Dihydrosilafluorene with [Ni(dmpe)2]. In a
glovebox, to a C6D6 solution (1.5 mL) of 9,9-dihydrosilafluorene (82
mg, 0.45 mmol) was added [Ni(dmpe)2] (35.9 mg, 0.10 mmol). A
white solid immediately precipitated from the reaction mixtures, which
was collected by filtration, washed with toluene (1 mL × 3), and dried
in vacuo to give insoluble poly(1,1-silafluorene) (71 mg, 88%). Solid-
state CPMAS 29Si{1H} NMR (119 MHz, room temperature): δ −32.8,
−41.1, −49.9. IR (KBr): 2216 (νSi−H) cm−1.
ASSOCIATED CONTENT
* Supporting Information
Tables, figures, and a CIF file giving crystallographic data for 1,
NMR and IR spectra for poly-I, poly-II, 1, and 2, and additional
polymerization data. This material is available free of charge via
the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
*E-mail: kosakada@res.titech.ac.jp.
Notes
■
Preparation of [Pd(SiC12H8)4(dmpe)] (1). To a toluene solution
(5 mL) of [PdMe2(dmpe)] (666 mg, 2.32 mmol) was added a toluene
solution (15 mL) of 9,9-dihydrosilafluorene (1.91 g, 10.5 mmol). The
reaction mixture was heated at 80 °C with stirring for 120 h, resulting
in precipitation of a yellow crystalline solid. The solid was collected by
filtration, washed with THF (3 mL × 7), and dried in vacuo to give 1
as a pale yellow solid (1.01 g, 44%). The reaction mixture was
confirmed to contain a bis(silyl)palladium precursor of 1, [Pd-
(SiHC12H8)2(dmpe)] (2), which was prepared by the silyl ligand
exchange of [Pd(SiHPh2)2(dmpe)] (vide infra). Crystals of 1 suitable
for X-ray crystallography were obtained from the reaction mixture
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by Grants-in-Aid for
Scientific Research (B) (No. 24350027) and for Young
Chemists (No. 23750059) from the Ministry of Education,
Culture, Sports, Science, and Technology of Japan. We thank
our colleagues at the Center for Advanced Materials Analysis,
Technical Department, Tokyo Institute of Technology, for
elemental analysis. We thank Dr. Yoshiyuki Nakamura (Tokyo
Institute of Technology) for assistance in the measurement of
solid-state NMR spectra.
1
without stirring for a few days. H NMR (400 MHz, C4D8O, room
temperature): δ 7.82 (d, 4H, C12H8, JH−H = 7.2 Hz), 7.72 (d, 4H,
C12H8, JH−H = 7.6 Hz), 7.54 (d, 4H, C12H8, JH−H = 7.2 Hz), 7.49 (d,
4H, C12H8, JH−H = 8.0 Hz), 7.17 (t, 4H, C12H8, JH−H = 7.2 Hz), 7.10 (t,
4H, C12H8, JH−H = 8.0 Hz), 7.03 (t, 4H, C12H8, JH−H = 7.6 Hz), 6.87 (t,
4H, C12H8, JH−H = 7.2 Hz), 1.47 (d, 4H, PCH2, JP−H = 18 Hz), 0.53 (d,
12H, PCH3, JP−H = 9.2 Hz). Solid-state CPMAS 29Si{1H} NMR (119
MHz, room temperature): δ 8.88 (d, JP−Si = 146 Hz), −27.4 (s). Solid-
state CPMAS 31P{1H} NMR (243 MHz, room temperature): δ 21.8.
The solid-state 13C NMR spectrum showed multiple signals in the
aromatic region. Anal. Calcd for C54H48P2PdSi4: C, 66.34; H, 4.95.
Found: C, 62.51; H, 5.07. It was difficult to obtain satisfactory data
from the elemental analysis because purification by recrystallization
was not feasible due to the low solubility of 1.
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