Syntheses of Oligometalloles by Catalytic Dehydrocoupling

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 (ε = 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 (ε = 9600
L/mol·cm) and luminesces green at 506 nm (Φ = 0.007). The methyl-terminated dimer
absorbs at 370 nm (ε = 11 800 L/mol·cm) and luminesces blue at 468 nm (Φ = 0.004).