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.