Photochemical reactivity of halogen-containing ruthenium–dcbpy (dcbpy = 4,4′-dicarboxylic acid-2,2′-bipyridine) compounds, trans(Br)-[Ru(dcbpy)(CO)2Br2] and trans(I)-[Ru(dcbpy)(CO)2I2]

Abstract

The ruthenium mono(dcbpy) (dcbpy = 4,4′-dicarboxylic acid-2,2′-bipyridine) complexes, trans(Br)-[Ru(dcbpy)(CO)₂Br₂] and trans(I)-[Ru(dcbpy)(CO)₂I₂], have been synthesized and structurally characterized. Both compounds show strong photochemical activity. Under illumination in acetonitrile the colour of the dye solutions changes and infrared spectra indicate an irreversible change, with two CO stretching bands disappearing from the spectra with concomitant appearance of a new CO stretching band, an indication of a loss of a CO ligand. Changes in the proton NMR spectra suggest that the release of the CO ligand is followed by reorganization of the halogen ligands and attachment of one solvent molecule leading to formation of the cis(X)-Ru(dcbpy)(CO)(CH₃CN)X₂] (X = Br, I) isomer. In the visible spectra, a new absorption band appears under illumination at 510 nm allowing observation of the kinetics of the reaction. The quantum yields of the reactions are 0.68 and 0.34 for the bromine and iodine complexes, respectively. The trans(I)-[Ru(dcbpy)(CO)₂I₂] compound shows a temperature dependent luminescence spectrum in the temperature range 77 to 116 K, with an activation energy of 850 cm⁻¹. We assign this dependence to a thermal population of a secondary triplet size, slightly above the emitting state, that is capable of initiating the photochemical reaction and/or a non-radiative relaxation to the ground state. The redox properties of the starting materials and the photoreaction products were studied with cyclic voltammetry and the results are discussed with reference to the reaction mechanisms.