Photochemical Resolution of a Thermally Inert Cyclometalated Ru(phbpy)(N-N)(Sulfoxide)⁺ Complex

In this work a photosubstitution strategy is presented that can be used for the isolation of chiral organometallic complexes. A series of five cyclometalated complexes Ru(phbpy)(N-N)(DMSO-κS)](PF₆) ([1]PF₆-[5]PF₆) were synthesized and characterized, where Hphbpy = 6'-phenyl-2,2'-bipyridyl, and N-N = bpy (2,2'-bipyridine), phen (1,10-phenanthroline), dpq (pyrazino[2,3- f][1,10]phenanthroline), dppz (dipyrido[3,2- a:2',3'- c]phenazine, or dppn (benzo[ i]dipyrido[3,2- a,2',3'- c]phenazine), respectively. Due to the asymmetry of the cyclometalated phbpy^- ligand, the corresponding [Ru(phbpy)(N-N)(DMSO-κS)]⁺complexes are chiral. The exceptional thermal inertness of the Ru-S bond made chiral resolution of these complexes by thermal ligand exchange impossible. However, photosubstitution by visible light irradiation in acetonitrile was possible for three of the five complexes ([1]PF₆-[3]PF₆). Further thermal coordination of the chiral sulfoxide ( R)-methyl p-tolylsulfoxide to the photoproduct [Ru(phbpy)(phen)(NCMe)]PF₆, followed by reverse phase HPLC, led to the separation and characterization of the two diastereoisomers of [Ru(phbpy)(phen)(MeSO(C₇H₇))]PF₆, thus providing a new photochemical approach toward the synthesis of chiral cyclometalated ruthenium(II) complexes. Full photochemical, electrochemical, and frontier orbital characterization of the cyclometalated complexes [1]PF₆-[5]PF₆ was performed to explain why [4]PF₆ and [5]PF₆ are photochemically inert while [1]PF₆-[3]PF₆ perform selective photosubstitution.