Efficiency of Photodissociation of Carbonyl Ligand in Ruthenium Monocarbonyl Bipyridine Complexes: Effect of Isomeric Structures and Ligands

Photoactivated CO-releasing molecules have potential medical applications. trans(Cl,Cl)-[Ru(bpy)(CO)2Cl2] (bpy = 2,2'-bipyridine) efficiently releases the first CO ligand (∼100% quantum yield), but the second CO dissociation is inefficient, and its precise quantum yield remains undetermined. Photodissociation of trans(Cl,Cl)-[Ru(bpy)(CO)2Cl2] yielded two monocarbonyl complexes, cis(Cl,Cl),trans(Cl,CH3CN)-[Ru(bpy)(CO)(CH3CN)Cl2] and trans(Cl,Cl)-[Ru(bpy)(CO)(CH3CN)Cl2], as the major and minor stereoisomeric products, respectively. The apparent quantum yields for the second CO dissociations were 0.56% and 0.78% for cis(Cl,Cl),trans(Cl,CH3CN)- and trans(Cl,Cl)-isomers, respectively. Considering the efficient photoisomerization of the trans(Cl,Cl)- to cis(Cl,Cl),trans(Cl,CH3CN)-isomer, the trans(Cl,Cl)-isomer would have a higher quantum yield. We also isolated the monocarbonyl complex after the first CO dissociation of trans(Cl,Cl)-[Ru(6Mes-bpy)(CO)2Cl2] (6Mes-bpy = 6,6'-dimesityl-2,2'-bipyridine), which contains bulky substituents at the 6,6'-positions of the bpy ligand. X-ray crystal structure analysis revealed the selective formation of trans(Cl,Cl)-[Ru(6Mes-bpy)(CO)(CH3CN)Cl2]. The quantum yield for the second CO dissociation from this structure was found to be 2.0%, nearly four times larger than that of cis(Cl,Cl),trans(Cl,CH3CN)-[Ru(bpy)(CO)(CH3CN)Cl2]. Time-resolved IR (TRIR) measurements demonstrated that photoirradiation of cis(Cl,Cl),trans(Cl,CH3CN)-[Ru(bpy)(CO)(CH3CN)Cl2] produced a five-coordinate pyramidal square intermediate with a stronger Ru-CO bond than in the ground state. In contrast, no such intermediate was observed for trans(Cl,Cl)-[Ru(6Mes-bpy)(CO)(CH3CN)Cl2]. The presence of this intermediate is thought to be a key factor in inhibiting further CO dissociation.