Synthesis of Poly(arylene cobaltacyclopentadienylene)s, a New Class of Organometallic π-Conjugated Polymers, by Metallacycling Polymerization and Their Physical Properties

Reaction of Cp‘Co(PPh3)2,
where Cp‘ = η5-cyclopentadienyl (Cp) or
η5-n-hexylcyclopentadienyl (hexCp), with conjugated diacetylenes, RC⋮CAC⋮CR, where
A = p-phenylene,
p-fluorophenylene, p-2,5-difluorophenylene, or
4,4‘-biphenylene and R = H or Me affords
new organometallic polymers,
[−C4R2{CoCp‘(PPh3)}−A−]n
(1−6) involving linearly π-conjugated chains. The polymers are insoluble when Cp‘ = Cp but
soluble in common organic
solvents when Cp‘ = hexCp. It is suggested by the results of
reactions of
(hexCp)Co(PPh3)2
with PhC⋮CH and PhC⋮CMe that stereoregularity favorable to
π-conjugation, i.e., connection of cobaltacyclopentadienylene moieties at 2,5 position is higher
when R = Me compared
to when R = H. The cobaltacyclopentadiene polymers undergo
quasi-reversible oxidation
and irreversible reduction at low temperatures. The oxidation
potential is not significantly
changed, but the reduction potential shifts considerably in the
positive direction for the
polymers compared to monomeric cobaltacyclopentadienes. The
polymers' bandgap energies
(as estimated from visible spectra) are between 2.1 and 2.3 eV.
Photoconductivity is observed
for
[−C4Me2{Co(hexCp)(PPh3)}−C6H4−]n
(6e).