Selective Photoinduced Ligand Exchange in a New Tris-Heteroleptic Ru(II) Complex

The complex cis-[Ru(biq)(phen)(CH3CN)(2)](2+) (1, biq = 2,2'-biquinoline, phen = 1,10-phenathroline) displays selective photosubstitution of only one CH3CN ligand with a solvent molecule upon irradiation with low energy light (lambda(irr) >= 550 nm), whereas both ligands exchange with lambda(irr) >= 420 nm. In contrast, [Ru(phen)(2)(CH3CN)(2)](2+) (2) and [Ru(biq)(2)(CH3CN)(2)](2+) (3) exchange both CH3CN ligands with similar rates upon irradiation with a broad range of wavelengths. The photolysis of 1 in the presence of pyridine (py) results in the formation of the intermediate cis-[Ru(biq)(phen)(py)(MeCN)](2+), which was isolated and characterized by X-ray crystallography, revealing that the CH3CN positioned trans to the phen ligand is more photolabile than that positioned trans to the biq ligand when irradiated with low energy light. These results are explained using the calculated stabilities of the two possible products, together with the molecular orbitals involved in the lowest energy excited state.