Distinctive Aspects in Aquation, Proton-Coupled Redox, and Photoisomerization Reactions between Geometric Isomers of Mononuclear Ruthenium Complexes with a Large-p-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 pi* orbital of the dpda ligand in contrast to mixing of the pi* orbitals of the dpda and tpy ligands for p-RuCl. The aquation reaction (1.5 x 10(-3) s(-1)) of p-RuCl to yield proximal-[Ru(tpy)(dpda)(OH2)](2+) (p-RuH2O) is faster than that (5.3 x 10(-6) s(-1)) of d-RuCl in D2O/CD3OD (4:1 v/v) by three orders of magnitude, which resulted from the longer Ru-Cl bond by 0.017 & Aring; and the distorted angle (100.2(3)degrees) 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-RuH2O undergoes a 2-step oxidation reaction of 1H(+)-coupled 1e(-) processes of Ru-II-OH2/Ru-III-OH and Ru-III-OH/Ru-IV-O at pH 1-9, whereas p-RuH2O undergoes a 1-step oxidation reaction of a 2H(+)-coupled 2e(-) process of Ru-II-OH2/Ru-IV-O in the pH range of pH 1-10. The irreversible photoisomerization from d-RuH2O to p-RuH2O was observed in aqueous solution with an internal quantum yield (Phi) of 5.4 x 10(-3)% at 520 nm, which is lower compared with Phi = 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)-M-3 state to the ground state for d-RuH2O due to the lower pi* level of dpda ligands according to the energy-gap law: the rate decreases exponentially with the increasing energy gap.