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.

Automated summary

Geometric isomers of mononuclear ruthenium(II) complexes were synthesized and comprehensively studied to investigate the geometric and electronic structures, as well as unique aspects in various reactions between isomers.