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)}-C₆H ₄-]_n (6e).