Synthesis of π-conjugated cobaltadithiolene cyclotrimers and significant effects of electrolyte cation and solvent on their electrochemical, optical and magnetic properties†
A new class of cobalt cyclic trinuclear complexes with four fused aromatic rings, [Co3(η-C5H5)3(S6C6)] and [Co3(η-C5H4Me)3(S6C6)], was synthesized by a condensation reaction of [Co(η-C5H5)(S2C2H2)] and [Co(η-C5H4Me)(S2C2H2)], respectively, in aqueous halogenoacid solution. The complexes undergo reversible three-step 1e– reductions in aprotic media forming two mixed-valence states, with overall charges 1– and 2–. The thermodynamic stability of the mixed-valence complexes estimated from the redox potential differences and the degree of internuclear electronic interaction evaluated from the intervalence-transfer bands depends markedly on the size of the electrolyte cation and the solvent polarity; larger cations and more polar solvents increase the stability and the electronic interaction. The EPR spectra of [Co3(η-C5H5)3(S6C6)] reduced by [Co(η-C5Me5)2] in propylene carbonate (pc) indicate that the 1– and 3– species are paramagnetic with S = ½, whereas the 3– species formed by reduction with Na in thf gives a spectrum of the quartet state (S = ) denoting ferromagnetic interaction. This difference is attributed to a weaker internuclear interaction caused by a stronger electrostatic effect of the smaller counter ion, Na+ in thf, compared to [Co(η-C5Me5)2]+ in pc.