Electronic Structures of Divinylchalcogenophene-Bridged Biruthenium Complexes: Exploring Trends from O to Te

An homologous series of divinylchalcogenophene-bridged binuclear ruthenium complexes [(PMe3)3Cl(CO)Ru]2(μ-CH=CHC4H2XCH=CH) (4a-4d, X = O, S, Se, Te) has been synthesized and fully characterized by X-ray crystallography and various spectroscopic techniques. Single-crystal X-ray diffraction results show a distinct short-long bond length alternation along the polyene-like hydrocarbon backbone, with the geometric constraints imposed by the chalcogenophene leading to an increasing distance between two metal centres (dRu-Ru) in complexes 4a-4d as the heteroatom in the five-membered ring is changed from oxygen (9.980 A in 4a) to tellurium (11.063 A in 4d). The complexes undergo two sequential one-electron oxidation processes, the half-wave potential and separation of which appear to be sensitive to a range of factors including aromatic stabilization and reorganization energies. Analysis of [4a-4d]n+ (n = 0, 1, 2) by UV-Vis-NIR and IR spectroelectrochemical methods, supported by DFT calculations (n = 0, 1) revealed that the redox character of the complexes is dominated by the polyene-like backbone, with the chalcogenide playing a subtle but influential, structural rather than electronic, role. In the radical cations [4a-4d]+, charge is rather effectively delocalized over the 10-atom Ru-{4-s-cis-all-trans-(CH=CH)4}-Ru chain, giving rise to a species with spectroscopic properties not dissimilar to oxidized polyaceylene.