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

The electrochemical redox properties of the ruthenium(II) complexes Ru(H(2)dcbpy)(CO)(2)Cl-2 1, Ru(H(2)dcbpy)(2)Cl-2 2, Ru(H(2)dcbpy)(2)Br-2 3, Ru(H(2)dcbpy)(2)(dcbpy) 4 and [Ru(H(2)dcbpy)(3)]Cl-2 5 [H(2)dcbpy=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 H(2)dcbpy 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(H(2)dcbpy)(CO)(CH3CN)Cl-2. The structures of 1 and 3-5 were determined by single-crystal X-ray crystallography.