Structure and electrochemical properties of directly bound dinuclear cobaltadithiolene complexes. Substituent effect on reduction potentials and mixed-valence states

The X-ray structures and electrochemical properties of the dinuclear cobaltadithiolene complexes, mu-(1,4-disubstituted 1,3-butadiene- 1,2,3,4-tetrathiolato)-bis[(eta(5)-cyclopentadienyl)cobalt(III)], have been investigated. In the diphenyl derivative, two benzene rings in the same molecule are located face to face with the distance of 3.5 Angstrom, while in the dimethyl derivative, two methyl groups are in remote positions. However, in the electrically neutral dinuclear complexes, there is no evidence for the intramolecular interaction between two aryl groups in solutions. Electrochemical and spectrochemical investigations revealed that in electrically neutral state, the interaction between two cobaltadithiolene moieties is small, while in monoanioic state it becomes greater. The Hammett's plot for Delta E-1/2(r) (Delta E-1/2(r) = E-1/2(r) (red.1) -E-1/2(r)(red.2)) shows that the electron-attracting substituents can stabilize the mixed-valence state, [Co(II)-Co(III)](-). The smaller Delta E-1/2(r) values for the complexes having aryl substituents are due to the smaller interactions between the anioic and neutral moieties, because of the steric hindrance of aryl groups which inhibit the conjugation of the two cobaltadithiolene rings.