Substituent effects on the electrochemistry and electronic coupling of terphenyl-bridged cyclometalated ruthenium-amine conjugated complexes

Abstract

Six terphenyl-bridged cyclometalated ruthenium-amine conjugated complexes 4(PF6)-9(PF6) were synthesized and studied. Three different substituents, methoxy, methyl, and chloro, were used to vary the electronic nature of the amine unit, and two terminal ligands 2,2′:6′,2″-terpyridine (tpy) and trimethyl-4,4′,4″-tricarboxylate-2,2′:6′,2″-terpyridine (Me3tctpy) were used to tune the electronic nature of the ruthenium component. All complexes, except 7(PF6) with the methoxy substituent and Me3tctpy ligand, display two well-separated redox waves in the potential range of +0.5 to +1.1 V versus Ag/AgCl. The regular electrochemical changes of these complexes help to establish the oxidation order of ruthenium and amine and hence of the direction of the electron transfer in odd-electron state. The degree of electronic coupling was estimated by analyzing the donor-to-acceptor charge transfer band in the near-infrared region obtained by oxidative spectroelectrochemical measurements. Electron paramagnetic resonance analyses and density functional theory calculations were performed on the one-electron oxidized forms to obtain information on the spin distribution of these complexes.