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, distal- / proximal -[Ru(tpy)(dpda)Cl] + ( d- / p - RuCl , 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 d- / p - RuCl 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 d - RuCl 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 p - RuCl . The aquation reaction (1.5 × 10 -3 s -1 ) of p - RuCl to yield proximal -[Ru(tpy)(dpda)(OH 2 )] 2+ ( p - RuH 2 O ) is faster than that (5.3 × 10 -6 s -1 ) of d - RuCl in D 2 O/CD 3 OD (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 p - RuCl . Electrochemical measurements showed that d - RuH 2 O undergoes a 2-step oxidation reaction of 1H + -coupled 1e - processes of Ru II -OH 2 /Ru III -OH and Ru III -OH/Ru IV ═O at pH 1-9, whereas p - RuH 2 O undergoes a 1-step oxidation reaction of a 2H + -coupled 2e - process of Ru II -OH 2 /Ru IV ═O in the pH range of pH 1-10. The irreversible photoisomerization from d - RuH 2 O to p - RuH 2 O was observed in aqueous solution with an internal quantum yield (Φ) of 5.4 × 10 -3 % 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 3 MLCT state to the ground state for d - RuH 2 O due to the lower π* level of dpda ligands according to the energy-gap law: the rate decreases exponentially with the increasing energy gap.