Organometallic Polymers Based on S-S and Se-Se Linked n-Butylferrocenes

Polyferrocenylene persulfides, [(BuC₅H₃)(C₅H₄)FeS₂] _n, have been prepared by desulfurization of butyltrithiaferrocenophane (BuC₅H₃)(C₅H₄)FeS₃. The molecular precursor was prepared by dilithiation of butylferrocene followed by treatment with elemental sulfur and isolated as a 7:1 mixture of the 1,3,1′ and 1,2,1′ isomers. The trisulfide was converted to polymer by treatment with the desulfurization agent PBu₃. The rate of this desulfurization and the nature of the polymer depends critically on the reaction solvent. Desulfurization in DMF and THF afforded insoluble but swellable rubbers. Desulfurization of BufcS₃ in mixed CH₂Cl₂-THF solutions gave soluble polymers with M_n ranging from 12 000 to 359 000, with lower molecular weight products being favored at high [CH₂Cl₂]. The effect of high [CH₂Cl₂] was modeled by the reaction of [fcS₂]_n with a CH₂Cl₂ solution of PBu₃ to give fcS₂CH₂. Molecular weights for [BufcS₂]_n were confirmed by light scattering measurements. Soluble [BufcS₂]_n undergoes reversible electrochemical oxidation in two steps, centered at 780 V (vs Ag/AgCl) and separated by ∼300 mV. The first oxidation potential is similar to that for the trisulfide monomer and is thought to correspond to oxidation of alternating Fe centers, followed by conversion of the remaining Fe^II sites. The polymers photodegrade upon exposure to UV light in air. Deselenization of BufcSe₃ gave red soluble [BufcSe₂]_n with only moderate molecular weights and with high polydispersities. The microstructure of this polymer was established by ⁷⁷Se NMR studies which support the presence of head-to-head, head-to-tail, and tail-to-tail dyads.