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)₂]²⁺ (d⁴) has a singlet ground state and is EPR-silent at low temperatures, while cis-[Ru^V(bpy)₂(O)(OH)]²⁺ (d³) 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.