Electronic Communication in the Mixed-valence States of Cyclobutadienecobalt Complexes having Two Ferrocenes and Two Anthraquinones

Two geometric isomers, Z and E forms of (η⁴-cyclobutadiene)(η⁵-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, α, 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.