Distinctive Aspects in Aquation, Proton-Coupled Redox, and Photoisomerization Reactions between Geometric Isomers of Mononuclear Ruthenium Complexes with a Large-π-Conjugated Tetradentate Ligand.

Geometric isomers of mononuclear ruthenium(II) complexes, /-[Ru(tpy)(dpda)Cl] (/-, tpy = 2,2':6',2″-terpyridine, dpda = 2,7-bis(2-pyridyl)-1,8-diazaanthracene), were newly synthesized to comprehensively investigate the geometric and electronic structures and distinctive aspects in various reactions between isomers. The ultraviolet (UV)-visible absorption spectra of /- isomers show intense bands for metal-to-ligand charge transfer (MLCT) at close wavelengths of 576 and 573 nm, respectively. However, time-dependent density functional theory (TD-DFT) calculations suggest that the MLCT transition of - involves mainly single transitions to the π* orbital of the dpda ligand in contrast to mixing of the π* orbitals of the dpda and tpy ligands for -. The aquation reaction (1.5 × 10 s) of - to yield -[Ru(tpy)(dpda)(OH)] (-) is faster than that (5.3 × 10 s) of - in DO/CDOD (4:1 v/v) by three orders of magnitude, which resulted from the longer Ru-Cl bond by 0.017 Å and the distorted angle (100.2(3)°) of Cl-Ru-N (a nitrogen of dpda, being on a tpy plane) due to the steric repulsion between Cl and dpda for -. Electrochemical measurements showed that - undergoes a 2-step oxidation reaction of 1H-coupled 1e processes of Ru-OH/Ru-OH and Ru-OH/Ru═O at pH 1-9, whereas - undergoes a 1-step oxidation reaction of a 2H-coupled 2e process of Ru-OH/Ru═O in the pH range of pH 1-10. The irreversible photoisomerization from - to - was observed in aqueous solution with an internal quantum yield (Φ) of 5.4 × 10% at 520 nm, which is lower compared with Φ = 1.1-2.1% of mononuclear Ru(II) aquo complexes with similar bidentate ligands instead of dpda by three orders of magnitude. This is possibly ascribed to the faster nonradiative decay rate from the excited MLCT state to the ground state for - due to the lower π* level of dpda ligands according to the energy-gap law: the rate decreases exponentially with the increasing energy gap.