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-C₄H₂E-CH=CH)] (4a–4d, E = O, S, Se, Te) have been synthesised and fully characterised by X-ray crystallography and various spectroscopic techniques. The single-crystal X-ray diffraction results reveal a distinct short/long bond-length alternation along the polyene-like hydrocarbon backbone, with geometric constraints imposed by the chalcogenophene leading to an increasing distance between the two metal centres (d_Ru–Ru) in complexes 4a–4d as the heteroatom in the five-membered ring is changed from oxygen (9.980 Å in 4a) to tellurium (11.063 Å 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 stabilisation and re-organisation 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]⁺, the charge is rather effectively delocalised 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 oxidised polyaceylene.