Electronic Structure of Oxidized Complexes Derived from cis-RuII(bpy)2(H2O)2]2+ and Its Photoisomerization Mechanism

The geometry and electronic structure of cis-[Ru-II(bpy)(2)(H2O)(2)](2+) and its higher oxidation state species up formally to Ru-VI have been studied by means of UV-vis, EPR, XAS, and DFT and CASSCF/CASPT2 calculations. DFT calculations of the molecular structures of these species show that, as the oxidation state increases, the Ru-O bond distance decreases, indicating increased degrees of Ru-O multiple bonding. In addition, the O-Ru-O valence bond angle increases as the oxidation state increases. EPR spectroscopy and quantum chemical calculations indicate that low-spin configurations are favored for all oxidation states. Thus, cis-[Ru-IV(bpy)(2)(OH)(0)](2+) (d(4)) has a singlet ground state and is EPR-silent at low temperatures, while cis-[Ru-V(bpy)(2)(O)(OH)](2+) (d(3)) has a doublet ground state. XAS spectroscopy of higher oxidation state species and DFT calculations further illuminate the electronic structures of these complexes, particularly with respect to the covalent character of the O-Ru-O fragment. In addition, the photochemical isomerization of cis-[Ru-II(bpy)(2)(H2O)(2)](2+) to its trans-[Ru-II(bpy)(2)(H2O)(2)](2+) isomer has been fully characterized through quantum chemical calculations. The excited-state process is predicted to involve decoordination of one aqua ligand, which leads to a coordinatively unsaturated complex that undergoes structural rearrangement followed by recoordination of water to yield the trans isomer.