Syntheses of oligometalloles by catalytic dehydrocoupling

Article abstract of DOI:10.1021/om0490886

The dehydrocoupling polycondensation of dihydro(tetraphenyl)metalloles, where every silicon or germanium atom of the oligomer backbone is part of a silole or germole rink was presented. Slightly less reactive catalyst systems of mol % of Wilkinson's catalyst, ((RhCl(PPh₃)₄ yield the oligometallole in good yield. It is observed that under less vigorous reflux conditions, the (tetraphenyl)silole dimer may be isolated in 40% yield. X-ray structural characterization of the dimer reveals a Si-Si bond length of 2.363 A and an H-Si-Si-H torsion angle of 90(2)°.

Full text of DOI:10.1021/om0490886

Organometallics 2005, 24, 3081-3087
3081
Syntheses of Oligometalloles by Catalytic
Dehydrocoupling
Sarah J. Toal, Honglae Sohn, Lev N. Zakarov, W. Scott Kassel,†
James A. Golen,§ Arnold L. Rheingold,* and William C. Trogler*
Department of Chemistry and Biochemistry, University of California at San Diego,
9500 Gilman Drive, La Jolla, California 92093-0358
Received November 23, 2004
The dehydrocoupling polycondensation of dihydro(tetraphenyl)metalloles (M ) Si or Ge)
with 0.2 mol % H2PtCl6‚xH2O and excess cyclohexene produces the respective oligometallole
in high yield (>80%), where every silicon or germanium atom of the oligomer backbone is
part of a silole or germole ring. Slightly less reactive catalyst systems of 1 mol % of
Wilkinson’s catalyst, (RhCl(PPh3)3), or Pd(PPh3)4 yield the respective oligometallole in good
yield (∼60%). With these latter systems, and under less vigorous reflux conditions, the
(tetraphenyl)silole dimer may be isolated in 40% yield. X-ray structural characterization of
the dimer reveals a Si-Si bond length of 2.363(2) Å and an H-Si-Si-H torsion angle of
90(2)°. Using excess cyclohexene as a coreagent with RhCl(PPh3)3 increases the yield of
oligomer and also eliminates dimer byproduct. The methyl-terminated dimer forms in the
reaction between H2PtCl6‚xH2O and methylhydro(tetraphenyl)silole, but not in the similar
reaction with the Rh and Pd catalysts. The methyl-terminated dimer has a Si-Si bond length
of 2.375(1) Å and an H-Si-Si-H torsion angle of 92.3(1)°. Additionally, the oligosilole may
be prepared by placing 1:2 dihydrosilole/cyclohexene and 1 mol % Wilkinson’s catalyst in an
Emrys Optimizer microwave synthesizer for 2 h. The molecular weight (Mw) of the
oligometalloles ranges from 3000 to 7000. Even though the metallole possesses a secondary
metalloid atom and contains bulky phenyl groups, polymerization may occur because the
tetraphenylmetallole monomers have small angles at C-M-C (93.21° on C-Si-C and 90.14°
on C-Ge-C), resulting in less steric hindrance at the metalloid center. Oligo(tetraphenyl)-
germole exhibits an absorption at 378 nm (ꢀ ) 5400 L/mol Ge‚cm). The germole is
photoluminescent in toluene solution, emitting blue-green light (498 nm, Φ ) 0.01). The
hydrogen-terminated silole dimer shows a similar UV-vis absorption at 372 nm (ꢀ ) 9600
L/mol‚cm) and luminesces green at 506 nm (Φ ) 0.007). The methyl-terminated dimer
absorbs at 370 nm (ꢀ ) 11 800 L/mol‚cm) and luminesces blue at 468 nm (Φ ) 0.004).
Introduction
due to σ*-π* conjugation from the interaction between
the σ* orbital of the Si-Si or Ge-Ge chains and the π*
orbital of the butadiene moiety of the ring.8,9 In addition,
polymetalloles exhibit σ-σ* delocalization10 of the con-
jugated electrons along the M-M backbone.
Siloles and germoles have attracted attention in
recent years, because of their unusual electronic proper-
ties1,2 and their applications as electron-transporting
materials in electronic devices,3 polymer light-emitting
diodes (PLEDs),4-6 and inorganic polymer sensors.7
Polymetalloles (M ) Si or Ge) are unique in having both
a M-M backbone and an unsaturated five-membered
ring system. Characteristic features of the metallole unit
are a low reduction potential and a low-lying LUMO,
Potential applications for polymetalloles have stimu-
lated the search for more efficient synthetic methods.
Current preparative routes use hazardous reagents and
are of low efficiency. For example, poly(tetraphenyl)-
silole has been synthesized by Wurtz-type polyconden-
sation of dichloro(tetraphenyl)silole; however, the reac-
tion yields are low (ca. ∼30%).4 Catalytic dehydrocoupling
of dihydrosilole is an attractive alternative, since Wurtz-
type coupling is also problematic in large-scale synthe-
† Permanent address: Department of Chemistry, Villanova Uni-
versity, Villanova, PA 19085.
§ Permanent address: Department of Chemistry and Biochemistry,
University of Massachusetts, Dartmouth, North Dartmouth, MA
02747.
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10.1021/om0490886 CCC: $30.25 © 2005 American Chemical Society
Publication on Web 05/18/2005

3082 Organometallics, Vol. 24, No. 13, 2005
Toal et al.
in 88% and 81% isolated yields, respectively. An alter-
native catalyst system of 1 mol % of Wilkinson’s
catalyst, RhCl(PPh3)3, or Pd(PPh3)4 catalyst yields the
respective oligosilole 3b,c and oligogermole 4b,c in good
yield (∼60%). Adding cyclohexene along with Wilkin-
son’s catalyst improves the yield of oligosilole 3d to 82%,
although the yield of oligogermole 4d does not change.
The dihydrometallole was completely consumed in the
1
reaction for each catalyst system, as observed by H
Figure 1. Catalytic dehydrocoupling of dihydrometalloles.
NMR spectroscopy, and Si-29 NMR spectra are consis-
tent with those previously reported for poly(tetra-
phenyl)silole.23 Catalytic dehydrocoupling significantly
improves isolated oligomer yields over traditional Wurtz-
type coupling.
ses. Bis(cyclopentadienyl) complexes of group 4 have
been extensively studied and are shown to catalyze the
dehydrocoupling of hydrosilanes to polysilanes by the
formation of Si-Si bonds;11-13 however, only primary
organosilanes are polymerized. Secondary and tertiary
silanes afford dimers or oligomers in low yields.14,15 It
has been reported that the reactivity decreases dra-
matically with increasing substitution at the silicon
atom, since reactions catalyzed by metallocenes are
typically very sensitive to steric effects.15,16 Mechanisms
for dehydrocoupling of silanes with metallocenes have
also been extensively investigated, which involve σ-bond
metathesis.17,18 Studies using Wilkinson’s catalyst for
dehydrogenative coupling of primary silanes show that
only short chain oligomers are formed.19,20 A platinum-
complex-catalyzed dehydrocoupling polymerization of a
silafluorene has been reported21 using 4 mol % catalyst,
yielding a bimodal weight distribution, in equal propor-
tions, as determined by GPC (Mw ) 8100 and 3000,
relative to polystyrene). Silafluorene has also been
polymerized via dehydrocoupling with Cp2ZrMe2, Cp2-
TiMe2, Cp2ZrCl2/n-BuLi, and Wilkinson’s catalyst. How-
ever, only the dimer and trimer were formed in signifi-
cant yields, along with minor amounts of short chain
oligomers.22 Inorganic hydrides have also been used to
effect the dehydrocoupling of dihydrosilole to form
polysilole (Mw ) 4000-6000).23
If less vigorous reflux conditions are used with only
the RhCl(PPh3)3 or Pd(PPh3)4 catalyst, or if the solvent
is not sufficiently degassed, the silole dimer (5) (n ) 2)
is observed to form in ∼40% yield. This product is
slightly soluble in hot toluene and precipitates from the
reaction mixture during reflux, and even more so upon
cooling. Addition of 2 equiv of cyclohexene with the
RhCl(PPh3)3 catalyst, however, prevents dimer forma-
tion and aids in the further polymerization of the
shorter chain oligomers. Wilkinson’s catalyst is known
to be a hydrogenation catalyst,24 and it is likely that
addition of the alkene provides a means for more
efficient hydrogen removal by the catalysts. When
1-hexene or 1-dodecene is used as the alkene coreagent,
however, competing reactions of polymerization and
hydrosilation are observed, with hydrosilation being
dominant. This is not surprising since Wilkinson’s
reagent is also a hydrosilation catalyst.25 The steric bulk
of the internal alkene, cyclohexene, favors hydrogena-
tion over hydrosilation and dehydrocoupling predomi-
nates, although a minor amount of hydrosilation is
1
sometimes visible by H NMR.
Similar results are observed with H2PtCl6‚xH2O. In
the absence of an alkene coreagent, large amounts of
dimer and lesser amounts of oligomer exist after 24 h
reflux. The addition of cyclohexene produces oligomers
of higher molecular weights than the syntheses with
Wilkinson’s catalyst, and no hydrosilation products are
observed. Both hydrosilation and polymerization prod-
ucts are observed when 1-hexene is used with H2PtCl6‚
xH2O, as observed with Wilkinson’s catalyst.25 One
advantage of the Pt catalyst is that polymerization
proceeds much more quickly (1 day versus 3 days for
Wilkinson’s catalyst). Additionally, removal of the het-
erogeneous catalyst formed from the H2PtCl6 is simpler.
After reflux, black particles are seen in solution and are
easily removed by filtration. Most likely the particles
are Pt colloids, the proposed active catalytic species for
H2PtCl6-catalyzed hydrosilation.26,27
Results and Discussion
Herein is reported the dehydrocoupling polyconden-
sation of dihydro(tetraphenyl)metalloles, where every
silicon or germanium atom of the oligomer backbone is
part of a silole or germole ring (Figure 1). Dehydrocou-
pling of dihydro(tetraphenyl)silole (1) or dihydro(tet-
raphenyl)germole (2) with 0.2 mol % H2PtCl6‚xH2O and
2 equiv of cyclohexene produces the oligomer 3a or 4a
(11) Tilley, T. D. Acc. Chem. Res. 1993, 26, 22.
(12) Harrod, J. F. Coord. Chem. Rev. 2000, 206-207, 493.
(13) Grimmond, B. J.; Corey, J. Y. Organometallics 2000, 19, 3776.
(14) Peulecke, N.; Thomas, D.; Baumann, W.; Fischer, C.; Rosenthal,
U. Tetrahedron Lett. 1997, 38, 6655.
(15) Imori, T.; Woo, H.-G.; Walzer, J. F.; Tilley, T. D. Chem. Mater.
1993, 5, 1487-1492.
(16) Banovetz, J. P.; Suzuki, H.; Waymouth, R. M. Organometallics
1993, 12, 4700.
The synthesis of oligosilole, under each catalytic
system, was also carried out with the use of an Emrys
Optimizer microwave synthesizer. Polymerization oc-
curs in only 2 h at 170 °C with Wilkinson’s catalyst
(17) Woo, H.-G.; Walzer, J. F.; Tilley, T. D. J. Am. Chem. Soc. 1992,
114, 7047.
(18) Woo, H.-G.; Heyn, R. H.; Tilley, T. D. J. Am. Chem. Soc. 1992,
114, 5698.
(19) Rosenberg, L.; Kobus, D. N. J. Organomet. Chem. 2003, 685,
107.
(24) Principles and Applications of Organotransition Metal Chem-
istry; Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G., Eds.;
University Science Books: Mill Valley, CA, 1987; pp 530-535.
(25) Comprehensive Handbook on Hydrosilylation; Marciniec, B.,
Ed.; Pergamon Press: Oxford, 1992; pp 77-93, 178-184.
(26) Lewis, L. N. J. Am. Chem. Soc. 1990, 112, 5998.
(27) Whitesides, G. M.; Hackett, M.; Brainard, R. L.; Lavalleye, J.
P. M.; Sowinski, A. F.; Izumi, A. N.; Moore, S. S.; Brown, D. W.; Staudt,
E. M. Organometallics 1985, 4, 1819.
(20) Rosenberg, L.; Davis, C. W.; Yao, J. J. Am. Chem. Soc. 2001,
123, 5120.
(21) Chauhan, B. P. S.; Shimizu, T.; Tanaka, M. Chem. Lett. 1997,
785.
(22) (a) Chang, L. S.; Corey, J. Y. Organometallics 1989, 8, 1885.
(b) Corey, J. Y.; Zhu, X. H.; Bedard, T. C.; Lang, L. D. Organometallics
1991, 10, 924.
(23) Kim, B. H.; Woo, H. G. Organometallics 2002, 21, 2796.

Syntheses of Oligometalloles
Organometallics, Vol. 24, No. 13, 2005 3083
Table 1. Results of Dehydrocoupling Syntheses of Oligo(tetraphenyl)metalloles
catalyst
(1 mol %)
alkene
(2 equiv)
oligomer
method
yield %
Mn (IR)
Mn (NMR)
Mw (Mw/Mn) (GPC)
a
3a
3b
3c
3d
3e
4a
4b
4c
4d
H2PtCl6
C6H12
none
none
C6H12
C6H12
C6H12
none
reflux 24 h
reflux 72 h
reflux 72 h
reflux 72 h
microwave 2 h
reflux 24 h
reflux 72 h
reflux 72 h
reflux 72 h
88
62
58
82
78
81
50
67
49
6500
3500
3500
4000
2700
3600
3400
5500
5600
6500
4100
4800
5200
2800
4000
3800
6900
5700
5600 (1.1), 2400 (1.1), 1300 (1.0)
4800 (1.2), 1800 (1.0), 1200 (1.0)
4800 (1.2), 1700 (1.0), 1200 (1.0)
5100 (1.1), 2100 (1.0), 1300 (1.0)
3300 (1.3), 1200 (1.0)
3400 (1.1), 1300 (1.0)
4000 (3.3), 1200 (1.0)
RhCl(PPh3)3
Pd(PPh3)4
RhCl(PPh3)3
RhCl(PPh3)3
a
H2PtCl6
RhCl(PPh3)3
Pd(PPh3)4
RhCl(PPh3)3
none
C6H12
4800 (1.9), 1200 (1.0)
3600 (1.3), 1200 (1.0)
a Catalytic concentration of 0.2 mol % used.
when cyclohexene is present (3e). When no alkene is
present, only limited polymerization takes place, and
mostly monomer and dimer are observed by NMR after
2 h reaction time. Microwave synthesis offers the
advantage of more rapid polymerizations, although
lower Mw’s are obtained. Microwave syntheses with Pd-
(PPh3)4 and H2PtCl6 catalysts were not successful. A
summary of reaction conditions and results is shown
in Table 1.
Polymerization may take place for the disubstituted
silicon or germanium since the tetraphenylmetalloles
have small angles at C-M-C in the metallacyclopen-
tadiene ring, which results in less steric hindrance at
the metalloid center. The angles of C-M-C of dihydro-
(tetraphenyl)silole (1) (Figure 2) and dihydro(tetra-
phenyl)germole (2) (Figure 3) are 93.21° and 90.14°,
respectively. In addition, no polymer is observed using
an H atom abstracting reagent, such as AIBN, which
suggests that the monomers are not simply undergoing
free radical polymerization. The bulky phenyl groups
of the silole might reduce the formation of a cyclic
hexamer, which is often problematic in polysilane
syntheses.28 Nevertheless, the GPC profiles of the
The molecular weight (Mw) of the oligosilole is in the
range 3000-7000, even though the silole possesses a
secondary silicon atom and contains bulky phenyl
groups (Table 1). The oligomers obtained are slightly
lower in Mw than those obtained from Wurtz-coupling
polycondensation. A similar result is also obtained from
the catalytic dehydrocoupling of dihydro(tetraphenyl)-
germole. The molecular weights of the oligometalloles
were estimated in three ways. First, the Mn were
calculated using infrared spectroscopy by integrating
the phenyl C-H stretch region relative to the Si-H or
Ge-H stretch, compared to the ratio in the dihydro
monomers. Similarly, NMR was used to calculate Mn,
by integrating the phenyl protons relative to the Si-H
or Ge-H end-groups. Finally the Mw was determined
by gel permeation chromatography (GPC). For the
siloles, higher molecular weights are obtained using the
H2PtCl6 catalyst, followed by Wilkinson’s catalyst with
the alkene coreagent, Pd(PPh3)4, and then Wilkinson’s
catalyst alone. For the germoles, highest molecular
weights are obtained using Pd(PPh3)4, followed by
Wilkinson’s catalyst with the alkene coreagent, H2PtCl6,
and then Wilkinson’s catalyst alone. Molecular weights
determined by IR were slightly higher than those
determined by GPC, and the NMR measurements
yielded the highest molecular weights.
Figure 2. Thermal ellipsoid plot of 1 at the 50% prob-
ability level. Selected bond lengths (Å): Si1-C1 1.856(2),
Si1-C4 1.866(1), C1-C2 1.358(2), C2-C3 1.506(2), C3-
C4 1.359(2), Si1-H1 1.40(2), Si1-H2 1.37(2). Delected bond
angles (deg): C1-Si1-C4 93.21(6), Si1-C1-C2 107.4(1),
Si1-C4-C3 107.0(1), C1-C2-C3 116.1(1), C2-C3-C4
116.3(1), H1-Si1-H2 109(1).
NMR is the most reliable method for determining
actual molecular weights because it does not, as GPC
does, calculate Mw based on the size exclusion of
individual polymer strands relative to an external
standard. The Si-H end-group of the dimer appears at
δ ) 5.3 ppm, and end-groups of the oligomers appear
to shift increasingly downfield as chain length increases,
arising near δ ) 5.5 ppm for 3b,e, near 5.6 for 3c,d,
and 5.7 for 3a. For the oligogermoles, the end-group
Ge-H resonance appears at δ ) 5.37 ppm for 3a and
5.45 for 3b,c,d, shifting downfield with increasing
molecular weight. GPC, however, reveals the size
distribution of the polymer. Polysiloles 3a-d are tri-
modal in weight distribution, and 3e and polygermoles
4a-d are bimodal, each in roughly equal proportion,
plus or minus 5%.
Figure 3. Thermal ellipsoid plot for 2 at the 30% prob-
ability level. Selected bond lengths (Å): Ge1-C1 1.949(2),
Ge1-C4 1.935(2), C1-C2 1.361(3), C2-C3 1.506(3), C3-
C4 1.358(3), Ge1-H1 1.40(3), Ge1-H2 1.43(3). Selected
bond angles (deg): C1-Ge1-C4 90.14(9), Ge1-C1-C2
107.4(2), Ge1-C4-C3 108.0(2), C1-C2-C3 117.2(2), C2-
C3-C4 117.2(2), H1-Ge1-H2 106(2).

3084 Organometallics, Vol. 24, No. 13, 2005
Toal et al.
Scheme 1. Proposed Dehydrocoupling Mechanism
for Rh- and Pd-Catalyzed Oligosilole Synthesis
Figure 4. Thermal ellipsoid plot for 5 at the 50% prob-
ability level. Selected bond lengths (Å): Si1-Si2 2.363(2),
Si1-H1 1.47(4), Si2-H2 1.48(4), Si1-C1 1.870(5), C1-C2
1.363(7), C2-C3 1.503(7). Selected bond angles (deg): H1-
Si1-Si2 106(2), Si1-Si2-H2 107(2), C1-Si1-C4 92.8(2).
Selected torsion angles (deg): H1-Si1-Si2-H2 90(2), C1-
Si1-Si2-C32 94.6(2).
Scheme 2. Proposed Dehydrocoupling Mechanism
of Methylhydrosilole on Pt Colloids Formed from
H2PtCl6
oligomers prepared by each method show a peak cen-
tered around Mw ) 1200 with a low polydispersity of
1.0. This may indicate the formation of a cyclic species
or a low molecular weight oligomer. In addition to the
Si-29 resonances of polysilole at δ ) -34.2 (terminal
Si) and -40.1 (internal Si), a small resonance at δ )
-32.0 is sometimes visible, which may indicate a cyclic
species or low Mw oligomer. The NMR spectra contain
a broad phenyl region with very small Si-H resonances,
suggesting that the low molecular weight fraction is
probably cyclic.
Figure 5. Thermal ellipsoid plot for 6 at the 50% prob-
ability level. Selected bond lengths (Å): Si1-Si2 2.375(1),
Si1-C5 1.867(2), Si2-C10 1.873(2), Si1-C1 1.881(2), C1-
C2 1.355(2), C2-C3 1.502(2). Selected bond angles (deg):
C5-Si1-Si2 109.32(6), Si1-Si2-C10 108.80(6), C1-Si1-
C4 91.99(7). Selected torsion angles (deg): C5-Si1-Si2-
C10-92.3(1), C1-Si1-Si2-C9 164.88(8), C4-Si2-Si1-C6
10.09(8).
The reaction of 1-methyl-1-hydro(tetraphenyl)silole
with 1 mol % of Wilkinson’s catalyst, RhCl(PPh3)3, or
Pd(PPh3)4 catalyst yielded no dimer, C4Ph4Si(CH3)-
(CH3)SiC4Ph4, with or without added cyclohexene. How-
ever, the same reaction with H2PtCl6‚xH2O catalyst does
yield the methyl-terminated dimer (6) in 40% isolated
yield. This demonstrates the higher reactivity of the Pt
catalyst, as do the higher Mw silole oligomers obtained.
Scheme 1 shows a proposed reaction mechanism for the
homogeneous catalysts (Rh and Pd catalysts) with a
putative silylene intermediate formed after oxidative
addition of the Si-H bond to the metal center. Elec-
tronic stabilization of the silylene intermediate by
π-conjugation with the silole ring may contribute to the
unexpected observation of catalytic dehydrocoupling for
a secondary silane. This scheme is consistent with the
fact that no dimer is observed with the methylhy-
drosilole because an Si-Me bond would not oxidatively
add to the metal center to form the silylene intermedi-
ate. However, dimerization of the methylhydrosilole is
observed using the Pt catalyst. This suggests that the
surface-catalyzed reaction on the Pt colloid proceeds by
oxidative addition of the Si-H of the silole, followed by
reductive elimination of H2 and dimer (Scheme 2).
Crystal structures for both the hydrogen-terminated
and methyl-terminated dimers were obtained. The
H-terminated dimer (5) has a H1-Si1-Si2-H2 torsion
angle of 90(2)° (Figure 4), and the Me-terminated dimer
(6) has a similar torsion angle of 92.3(1)° (Figure 5). This
torsion angle is larger than that of the 51.2° observed
for the chlorosilole dimer.29 This deviation is obviously
not due to sterics, as Me and Cl ligands are of similar
size, but perhaps arises from electronic effects of the
interaction of the π system with the halide. The torsion
angles suggest that the structure of the oligomers is
helical, as previously proposed,7b and as established for
several disubstituted polysilanes.30
The hydrogen-terminated dimer has a UV-vis ab-
sorption at 372 nm, similar to that of the dihydro
monomer at 368 nm, which is assigned to the π-π*
transition of the silole ring. The dimeric compound is
strongly luminescent, much more so than the monomer,
emitting green light at 506 nm when excited at 360 nm
(Figure 6). Similarly the Me-terminated dimer is much
more luminescent than both methylhydrosilole and
dimethylsilole, emitting at 468 nm. Fluorescence spectra
of 20 mg/L metallole samples in toluene are shown in
Figure 6. The broad emission spectra could possibly be
(29) Sohn, H.; Huddleston, R. R.; Powell, D. R.; West, R. J. Am.
Chem. Soc. 1999, 121, 2935.
(30) Furukawa, S.; Takeuchi, K.; Shimana, M. J. Phys.: Condens.
Matter 1994, 6, 11007.
(28) Bratton, D.; Holder, S. J.; Jones, R. G.; Wong, W. K. C. J.
Organomet. Chem. 2003, 685, 60.

Syntheses of Oligometalloles
Organometallics, Vol. 24, No. 13, 2005 3085
siloles are dramatically more luminescent in the solid
state than in fluid solution.7cd,33
The UV-vis absorption spectrum of oligo(tetra-
phenyl)germole has an absorption at 378 nm, which is
slightly red shifted to that of the corresponding oli-
gosilole (370 nm), and is assigned to both the σ-σ*
transition of the germanium-germanium backbone
chain and the π-π* transition of the germole ring. Like
oligosilole, the oligogermole is highly photoluminescent,
emitting blue-green light at 498 nm when excited at 360
nm, which is 12 nm blue shifted to that of the analogous
oligosilole (510 nm). Germole species are much more
intensely photoluminescent than the corresponding
siloles. The wavelengths of fluorescence red shift with
increasing chain length, which is consistent with the
lowering of the LUMO energy due to σ-σ* conjugation
along the metalloid backbone.
Figure 6. Fluorescence spectra of 20 mg/L toluene solu-
tions of (a) methyl dimer, (b) oligogermole, (c) hydrogen
dimer, (d) oligosilole, (e) dihydrogermole (THF), (f) meth-
ylhydrosilole, and (g) dihydrosilole on excitation at 360 nm.
Experimental Section
due to the overlapping of emission from the π-π* and
σ-π* transitions. Absorbance and fluorescence data are
summarized in Table 2.
General Procedures. All synthetic manipulations were
carried out under an atmosphere of dry argon gas using
standard Schlenk techniques. Solvents were purchased from
Aldrich Chemical Co. Inc. and distilled from sodium/benzophe-
none ketyl. NMR data were collected with Varian Unity 300
Fluorescence quantum yields (Φ) of the metalloles
were measured relative to a diphenylanthracene (DPA)
standard and are listed in Table 2.31 The Φrel of the
oligogermole is roughly 10 times greater than dihydro-
germole, and the quantum yields of the siloles follow
the order oligosilole > H-dimer > dihydrosilole. One
possible explanation of this series order is the fact that
coupling of the excited state to high frequency (Si-H
or Ge-H) vibrations of the ground state increases the
efficiency of nonradiative decay processes.32 To test this
hypothesis, the dideuterometallole monomers were pre-
pared, along with dimethylsilole and methylhydrosilole,
in order to vary the vibrational frequencies directly
coupled to the metallole chromophore. Infrared spectra
of dideuterosilole (7) and dideuterogermole (8) show the
decrease in vibrational frequencies of the metal-
deuterium stretches (ν ) 1543 cm-1 for Si-D and 1476
cm-1 for Ge-D) relative to the metal-hydrogen ana-
logues (ν ) 2140 cm-1 for Si-H and 2053 cm-1 for Ge-
H), which is consistent with the increase in the reduced
mass. Dideuterogermole has a slightly higher Φrel than
the dihydrogermole, but dihydrosilole has a very slightly
higher Φrel than dideutersilole. The dimethyl- and
methylhydrosiloles have quantum yields comparable to
the other silole monomers. Because emission intensity
and quantum yield are not significantly affected by the
varying vibrational frequencies coupled to the chro-
mophore, vibrational relaxation does not appear to be
the dominant pathway in determining the variation in
metallole fluorescence quantum yields. Interestingly,
the hydrogen-terminated dimer is more than 8 times
more luminescent than monomer, while the oligomer
is about 13 times more luminescent. The enhanced
quantum yields of the dimer and oligomer may possibly
be attributed to restricted conformations about the Si-
Si bond. It may also result from shielding of the
metallole excited state from solvent. Polysilole and
1
or 500 MHz spectrometers (300.1 MHz for H, 77.5 MHz for
13C, and 99.4 MHz for 29Si NMR). Infrared spectra were
obtained with the use of a Nicolet Magna-IR 550 spectrometer.
GPC data were obtained with the use of a Viscotek GPCmax
VE 2001 GPC and a Viscotek VE 3580 refractive index
detector. A calibration curve was obtained using 1, 5, and three
polysilole samples calibrated by Viscotek using RI, viscosity,
and light-scattering detectors (this triple detection method
yields absolute molecular weights). Fluorescence emission and
excitation spectra were recorded with the use of a Perkin-
Elmer LS 50B luminescence spectrometer. UV-vis spectra
were obtained with the use of a Hewlett-Packard 8452A diode
array spectrometer.
X-ray Crystal Structure Determinations. Diffraction
intensity data were collected with a Bruker P4/CCD diffrac-
tometer at 213 K (1, 6) and 218 K (5) and a Bruker P4/CCD
Smart Apex CCD diffractometer at 100 K (2). Crystal, data
collection, and refinement parameters are given in Table 3.
The space groups were chosen on the basis of the systematic
absences (1, 2, 6) and intensity statistics (5). The structures
were solved by direct methods, completed by subsequent
difference Fourier syntheses, and refined by full matrix least-
squares procedures on F2. SADABS absorption corrections
were applied to all data. All non-hydrogen atoms were refined
with anisotropic displacement coefficients. All H atoms in 1,
2, and 6 and the H atoms bonded to the Si atoms in 5 were
found on the difference maps and refined with isotropic
thermal parameters. Other H atoms in 5 were treated as
idealized contributions. The structures of 7 and 8 were also
determined and are isomorphous with their protio analogues.
Data for 7 and 8 are included in the Supporting Information
only. All software and sources of scattering factors are
contained in the SHELXTL (5.10) program package (G. Sheld-
rick, Bruker XRD, Madison, WI).
1,1-Dihydro-2,3,4,5-tetraphenyl-1-silacyclopenta-2,4-
diene (Dihydrosilole) (1). Diphenylacetylene (18 g, 0.10 mol)
and Li (0.25 mol) in dry ether (130 mL) were stirred for 2.5 h
and then frozen with liquid nitrogen. Dichlorosilane (25% in
xylenes, 20 mL, 0.20 mol) was added, and the solution was
thawed and stirred for 4 h at room temperature. The solvent
was evaporated, and the solid was extracted with toluene,
(31) Morris, J. V.; Mahaney, M. A.; Huber, J. R. J. Phys. Chem. 1976,
80, 969.
(32) (a) Modern Molecular Photochemistry; Turro, N. J., Ed.; The
Benjamin/Cummings Publishing Co., Inc.: Menlo Park, CA, 1978; pp
170-176. (b) Photophysics of Aromatic Moleucles; Birks, J. B., Ed.;
Wiley-Interscience: New York, 1970.
(33) Luo, J.; Xie, Z.; Lam, J. W. Y.; Cheng, L.; Chen, H.; Qui, C.;
Kwok, H. S.; Zhan, X.; Liu, Y.; Zhu, D.; Tang, B. Z. Chem. Commun.
2001, 1740.

3086 Organometallics, Vol. 24, No. 13, 2005
Toal et al.
Table 2. Summary of Absorbance (THF) and Fluorescence (toluene) Data of Various Metalloles
a
λabs
(nm)
ꢀmax
(L/mol‚cm)
fluor. quantum
metallole
λflue (nm)
yield (Φ)f
relative Φ
dihydrosilole
368
370
368
362
372
370
370
360
362
378
9500
10 500
10 250
9500
495
484
495
473
506
468
510
469
470
498
0.83 × 10-3
0.79 × 10-3
0.75 × 10-3
1.0 × 10-3
6.9 × 10-3
3.6 × 10-3
11 × 10-3
1.4 × 10-3
2.1 × 10-3
13 × 10-3
1.0
0.95
0.90
1.2
8.3
4.3
13
1.7
2.5
16
dideuterosilole
methylhydrosilolea
dimethylsilolea
hydrogen dimerb
methyl dimerb,c
oligosilole
4800
5900
2230
11 200
9200
5400
dihydrogermoled
dideuterogermolea
oligogermolea
a UV-vis taken in toluene. b Absorptivities are reported per mole of metalloid; actual molar absorptivities of dimers are twice the
reported value. c UV-vis data taken in CHCl3. d Fluorescence taken in THF. λex ) 360 nm. f (30%, relative to 9,10-diphenylanthracene31
,
e
λex ) 360 nm.
Table 3. Summary of X-ray Crystallographic Data
1
2
5
6
formula
C
28H22Si
C28H22Ge
C56H42Si2
C58H46Si2
fw
386.55
431.05
771.08
799.13
space group
a, Å
I41/a
I41/a
P1h
P21/c
26.5859(7)
26.5859(7)
11.9092(7)
26.457(2)
26.457(2)
11.846(2)
10.023(2)
12.806(2)
17.695(3)
89.440(3)
88.558(3)
71.737(3)
2156.1(6)
2, 1
12.4024(9)
19.9497(14)
18.2523(13)
b, Å
c, Å
R, deg
â, deg
104.951(1)
γ, deg
V, Å3
8417.5(6)
16, 1
8292(2)
16, 1
4363.2(5)
4, 1
Z, Z′
cryst color, habit
F(calc), g cm-3
µ(Mo KR), mm-1
temp, K
no. of reflns measd
no. of reflns indep
R(F) (I > 2σ(I))a
R(wF2) (I > 2σ(I))b
yellow, block
1.220
colorless, plate
1.381
green, block
1.188
light yellow, block
1.217
0.123
1.489
0.120
0.121
213(2)
29 964
5040 [Rint ) 0.0338]
0.0423
0.1111
100(2)
26 461
5045 [Rint ) 0.0495]
0.0390
0.0855
218(2)
10 251
6627 [Rint ) 0.0291]
0.0860
0.2774
213(2)
26 441
9645 [Rint ) 0.0303]
0.0493
0.1195
a R )∑||Fo| - |Fc||/∑|Fo|. b R(wF2) ) {∑[w(Fo - Fc )2]/∑[w(Fo2)2]}1/2; w ) 1/[σ2(Fo2) + (aP)2 + bP], P ) [2Fc + max(Fo,0)]/3.
2
2
2
washed with water, and recrystallized from toluene to afford
an Ar atmosphere and degassed through freeze-pump-thaw
cycles. The reaction mixture was then vigorously refluxed for
72 h and then evaporated to dryness. THF (1 mL) was added
to the reaction mixture, and the resulting solution was then
poured into 10 mL of methanol. Polysilole was obtained as a
pale yellow powder after filtration and freeze-drying. Addition
of 2 equiv of cyclohexene with RhCl(PPh3)3 (3d) eliminates
dimer precipitation and increases yield. 3b: 62%, 3c: 58%,
3d: 82%. 3: 1H NMR (300.133 MHz, CDCl3): δ 6.60-7.40 (br,
m, Ph). 13C{H} NMR (75.403 MHz, CDCl3 (δ 77.00)): δ 125-
132 (br m, Ph), 137-147 (silole carbons). 29Si NMR (99.37
MHz, INEPT, CDCl3, TMS (δ 0.0)): δ -31.8 (end-groups),
-40.0 (backbone). IR (KBr): νSi-H 2146 cm-1. CHN Anal.
Calc: C: 87.4, H: 5.24. Found: C: 86.9, H: 5.19.
3e. 1 (0.5 g, 1.3 mmol), 1 mol % RhCl(PPh3)3, cyclohexene
(2.6 mmol), and 5 mL of toluene were placed in an Emrys
Optimizer microwave synthesizer for 2 h at 170 °C and 120
W. The solution was then evaporated to dryness. Dissolution
with THF (1 mL) and precipitation with methanol (10 mL)
yielded the oligsilole (78%).
Oligo(tetraphenyl)germoles 4a,b,c,d. Reaction condi-
tions for preparing the oligogermole are the same as those for
oligosilole. 4a: 81%, 4b: 50%, 4c: 67%, 4d: 49%. 4: 1H NMR
(300.133 MHz, CDCl3): δ 6.30-7.90 (br, m, Ph). 13C{H} NMR
(75.403 MHz, CDCl3 (δ 77.00)): δ 124-130 (br m, Ph), 131-
139 (germole carbons). IR (KBr): νGe-H 2063 cm-1. CHN Anal.
Calc: C: 78.4, H: 4.70. Found: C: 78.2, H: 4.60.
Dihydrosilole Dimer (5). 1 (1.0 g, 2.59 mmol) and 1 mol
% of RhCl(PPh3)3 in toluene (10 mL) were placed under an Ar
atmosphere and degassed through freeze-pump-thaw cycles.
The reaction mixture was vigorously refluxed for 72 h. The
solid that precipitates during cooling was filtered and recrys-
bright yellow crystals (10.3 g, 53%). Mp: 205-206 °C (lit. 209-
1
210 °C34). H NMR (300.133 MHz, CDCl3): δ 6.80-6.85 and
7.00-7.24 (br m, 20H, Ph), 4.90 (s, 2H, SiH2). 13C{H} NMR
(75.403 MHz, CDCl3): δ 157.50, 138.74, 135.20, 131.63, 129.62,
129.36, 128.09, 127.72, 126.63, 126.23. 29Si NMR (99.36 MHz,
INEPT, CDCl3, TMS (δ 0.0)): δ -35.03; IR (KBr): νSi-H 2140
cm-1
.
1,1-Dihydro-2,3,4,5-tetraphenyl-1-germaclopenta-2,4-
diene (Dihydrogermole) (2). The dichlorogermole starting
material was prepared according to the literature.7b To a dry
THF (100 mL) solution of dichlorogermole (1.0 g, 2 mmol) was
added LiAlH4 (1 M in THF, 2 mL, 2 mmol). The solution was
quenched with methanol and evaporated to dryness, and the
product was extracted and recrystallized from hexanes to
afford pale yellow crystals (0.44 g, 51%). Mp: 190-192 °C (lit.
1
192-193 °C35). H NMR (300.133 MHz, CDCl3): δ 6.80-6.84
and 6.95-7.07 (br m, 20H, Ph), 5.36 (s, 2H, GeH2). 13C{H}
NMR (75.403 MHz, CDCl3): δ 153.54, 139.33, 138.99, 138.01,
129.72, 129.43, 127.85, 127.55, 126.28, 126.01. IR (KBr): νGe-H
2053 cm-1 (lit. 206035).
Oligo(tetraphenyl)silole 3a. 1 (1.0 g, 2.59 mmol), 0.2 mol
% H2PtCl6‚xH2O, and 2 molar equiv of cyclohexene in toluene
(10 mL) were vigorously refluxed for 24 h. The solution was
passed through a sintered glass frit and evaporated to dryness
under an Ar atmosphere. Dissolution with THF (1 mL) and
precipitation with methanol (10 mL) yielded the oligosilole in
88%.
3b,c,d. 1 (1.0 g, 2.59 mmol) and 1 mol % of RhCl(PPh3)3
(3b) or Pd(PPh3)4 (3c) in toluene (10 mL) were placed under
(34) Mu¨ller, R. Z. Chem. 1968, 8, 262.
(35) Curtis, D. M. J. Am. Chem. Soc. 1969, 91, 6011.

Syntheses of Oligometalloles
Organometallics, Vol. 24, No. 13, 2005 3087
tallized from xylenes to afford bright yellow crystals (0.38 g,
38%). Mp g 250 °C. 1H NMR (300 MHz, CDCl3 (δ 7.26)): δ
6.4-7.4 (m, 40H, Ph), 5.30 (s, 2H). 13C NMR (99.40 MHz, CD2-
Cl2 (δ 54.00)): δ 157.13, 139.27, 139.22, 137.99, 130.23, 129.95,
128.41, 127.76, 126.81, 126.52. 29Si NMR (100 MHz, CDCl3,
TMS (δ 0.0)): δ -33.81 (dd, JSi1-H1 ) 200 Hz, JSi1-H2 ) 8.4
Hz). CHN Anal. Calc: C: 87.22, H: 5.49. Found: C: 86.67,
1,1-Dideuterometalloles (7, 8). Methods for synthesizing
7 and 8 are identical to the procedure described for preparing
2, beginning with either dichlorosilole or dichlorogermole and
reducing with LiAlD4, and recrystallizing from toluene (7) or
hexanes (8). 7: 58%. CHN Anal. Calc: C: 86.55, H: 6.22.
Found: C: 86.35, H: 6.37. Mp ) 204-206 °C. IR (KBr): νSi-D
1543 cm-1. 8: 52%. CHN Anal. Calc: C: 77.65, H: 5.58.
Found: C: 77.62, H: 5.74. Mp ) 191-193 °C. IR (KBr): νGe-D
H: 6.18. IR (KBr): νSi-H 2112 cm-1
.
Methylsilole Dimer (6). Methylhydrosilole36 (1.0 g, 2.5
mmol), 0.2 mol % H2PtCl6‚xH2O, and 2 molar equiv of cyclo-
hexene in toluene (10 mL) were vigorously refluxed for 24 h.
The solid that precipitates during cooling was filtered and
recrystallized from toluene to afford bright yellow crystals (0.41
g, 40%). Mp g 250 °C. 1H NMR (300 MHz, CDCl3 (δ 7.26)): δ
6.7-6.8 and 6.9-7.2 (m, 40H, Ph), 0.19 (s, 6H). 13C NMR (100
MHz, CD2Cl3 (δ 77.00)): δ 154.6, 143.4, 139.9, 138.8, 130.3,
129.4, 127.9, 127.4, 126.3, 125.6, -6.0. 29Si NMR (99.37 MHz,
INEPT, CDCl3, TMS (δ 0.0)): δ -9.34. CHN Anal. Calc: C:
87.16, H: 5.80. Found: C: 86.84, H: 5.87.
1476 cm-1
.
Acknowledgment. Support of this research by the
National Science Foundation (Grant CHE-0111376) is
gratefully acknowledged.
Supporting Information Available: Crystallographic
information files (CIF) for compound 1, 2, 5, 6, 7, and 8. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(36) Ruhlmann, K. Z. Chem. 1965, 5, 354.
OM0490886