Electronic communication in the mixed-valence states of cyclobutadienecobalt complexes having two ferrocenes and two anthraquinones

Two geometric isomers, Z and E forms of (eta(4)-cyclobutadiene)(eta(5)-cyclopentadienyl) cobalt simultaneously containing two ferrocene moieties and two anthraquinone moieties, 1 and 2, respectively, were synthesized, and their crystal structures were determined. These molecules formed double mixed-valence states caused by ferrocene-ferrocenium and anthraquinone-anthrasemiquinone electronic communications. In both 1 and 2, the thermodynamic stability of the mixed-valence state for the ferrocene moieties was higher than that of the anthraquinone moieties because of the orbital coupling of the CB-Co bridging unit. The comproportionation constant, K (c), for the ferrocene moieties was similar between 1 and 2, whereas the mixing coefficient, alpha, and the off-diagonal matrix-coupling element in the Marcus-Hush two-state model, H (AB), evaluated from analysis of the intervalence charge transfer (IVCT) band were larger for 1 than for 2. These results suggest the existence of higher electrostatic repulsion between two ferrocenium cations in 1 than in 2 and the occurrence of through-bond electronic communication. K (c) for the anthraquinone moieties in the E form is smaller than that of the Z form, probably due to the through-space electronic interaction.