Studies of carbon monoxide release from ruthenium(II) bipyridine carbonyl complexes upon UV light exposure

The UV light-induced CO release characteristics of a series of ruthenium(II) carbonyl complexes of the form trans-(Cl) [RuLCl2(CO)2] (L = 4,4’-dimethyl-2,2’-bipyridine, 4’-methyl-2,2’-bipyridine-4-carboxylic acid or 4,4’-dicarboxylic acid-2,2’-bipyridine) have been elucidated using a combination of UV-Vis absorbance and Fourier transform infrared (FTIR) spectroscopies, multivariate curve resolution-alternating least squares (MCR-ALS) analysis, and density functional theory calculations. In acetonitrile, photolysis appears to proceed via a serial three-step mechanism involving sequential formation of [RuL(CO)(CH3CN)Cl2], [RuL(CH3CN)2Cl2], and [RuL(CH3CN)3Cl]+. Release of the first CO molecule occurs quickly (k1 >> 3 min-1), while release of the second CO proceeds at a much more modest rate (k2 = 0.099–0.17 min-1) and is slowed by the presence of electron-withdrawing carboxyl substituents on the bipyridine ligand. In aqueous media (1% DMSO in H2O), the two photo-decarbonylation steps proceed much more slowly (k1 = 0.46–1.3 min-1 and k2 = 0.026–0.035 min-1, respectively) and the influence of the carboxyl groups is less pronounced. These results have implications for the design of new light-responsive CO-releasing molecules (“photoCORMs”) intended for future medical use.