The photo-induced ligand loss of the complexes [Ru(tpy)(AN)3]2+ and cis-[Ru(tpy)(AN)2Cl]+ (tpy = 2,2’:6’2’’-terpyridine) was studied in water and in CH2Cl2 in the presence of chloride from tetrabutylammonium chloride (TBACl). Both complexes photolyze to the same photoproduct. Photolysis in CH2Cl2 in the presence of chloride ions led to the photoproduct trans-[Ru(tpy)(AN)Cl2], and photolysis in water led to the photoproduct trans-[Ru(tpy)(AN)(H2O)2]2+. The two axial acetonitrile ligands were replaced, while the equatorial acetonitrile remained coordinated to the metal. For cis-[Ru(tpy)(AN)3]2+ the axial acetonitrile ligands were replaced in a step-wise fashion, forming an intermediate with one axial acetonitrile. Polypyridyl ruthenium complexes such as cis-[Ru(bpy)2(AN)2]2+ are known to form the diaqua species cis-[Ru(bpy)2(H2O)2]2+ and bind to DNA upon photolysis, but not in the absence of light. Since [Ru(tpy)(AN)3]2+ and cis-[Ru(tpy)(AN)2Cl]+ exhibit analogous photoreactivity in water, they are potential anti-tumor agents for use in photodynamic therapy (PDT). cis-[Ru(tpy)(AN)2Cl]+ has a lower energy metal to ligand charge transfer (MLCT) tranisiton and a higher quantum yield of ligand substitution, making it the better candidate for PDT.
The series of complexes cis-[Ru(bpy)2L2]2+ (bpy = 2,2’-dipyridly) (L = NCPh, 4-F-NCPh, 4-Me-NCPh, and 4-OMe-NCPh) were synthesized and their photolysis rate in water was studied. The complexes with more electron withdrawing substituents exhibited faster photolysis rates. Electronic absorption and electrochemical data showed π-back bonding in the complexes, with more π-back bonding correlating with a faster rate of photolysis. The electrochemical data showed similar degrees of π-back bonding for cis-[Ru(bpy)2(NCPh)2]2+ and cis-[Ru(bpy)2(4-Me-NCPh)2]2+, indicating that both amounts of σ bonding and π-back bonding play a role in the rate of photolysis. However, Cis-[Ru(bpy)2(4-OMe-NCPh)2]2+ did not fit the trend.