Redox and photochemical behaviour of ruthenium(II) complexes with H2dcbpy ligand (H2dcbpy = 2,2′-bipyridine-4,4′-dicarboxylic acid)

Abstract

The electrochemical redox properties of the ruthenium(II) complexes Ru(H₂dcbpy)(CO)₂Cl₂1, Ru(H₂dcbpy)₂Cl₂2, Ru(H₂dcbpy)₂Br₂3, Ru(H₂dcbpy)₂(dcbpy) 4 and [Ru(H₂dcbpy)₃]Cl₂5 [H₂dcbpy = 2,2′-bipyridine-4,4′-dicarboxylic acid] were studied in CH₃CN and DMSO by cyclic voltammetry. The metal-based Ru^II–Ru^III oxidations are observed at variable positive potentials (E_1/2 = 0.15–1.62 V vs. Ag⁺–Ag). The H₂dcbpy ligands withdraw electron density from the ruthenium centre and make the oxidation more difficult (seen as positive shifts in E_1/2 values). At negative potentials the first irreversible ligand-based reductions (E_pc = −1.37 to −1.57 V) are due to the deprotonation of carboxylic acid groups whereas the subsequent reversible reactions at high negative potentials are assigned as bipyridine ring based reductions. Based on CV and UV-Vis studies, photoirradiation in the co-ordinating solvent (CH₃CN) revealed the tendency to light-induced CH₃CN co-ordination for complexes 1–5. The photochemical products were also identified by using Lever’s E(L) parameters. For example, upon photoirradiation of 1, the principal product is Ru(H₂dcbpy)(CO)(CH₃CN)Cl₂. The structures of 1 and 3–5 were determined by single-crystal X-ray crystallography.