Configurationally Nonselective Aquation of a Mononuclear Ru(II) Chloro Complex to Aquo Complex Isomers with Distinctive Aspects in Photoisomerization, Redox, and Catalytic Water Oxidation

Abstract

distal-[Ru(EtOtpy)(pynp)Cl] + (d-EtO1Cl) (EtOtpy = 4′-ethoxy-2,2′:6′,2″-terpyridine, pynp = 2-(2-pyridyl)-1,8-naphthyridine), and distal/proximal-[Ru(EtOtpy)(pynp)OH 2 ] 2+ (d/p-EtO1H 2 O) complexes were newly synthesized to investigate the synergistic influence of the geometric configuration coupled with substituent introduction of an ethoxy (EtO) group on the physicochemical properties and reactions of the Ru(II) complexes. Configurationally nonselective aquation of d-EtO1Cl was uniquely observed to form d/p-EtO1H 2 O isomers in water, in contrast to configurationally selective aquation of distal-[Ru(tpy)(pynp)Cl] + (d-1Cl, tpy = 2,2′:6′,2″-terpyridine) without the EtO group [Yamazaki, H. et al.. J. Am. Chem. Soc. 2011, 133, 8846−8849].The kinetic profiles of the aquation reactions of d-EtO1Cl were well analyzed using a sequential reversible reaction model assuming the reversible interconversion between d/p-EtO1H 2 O isomers via d-EtO1Cl. The observed equilibrium constant (K iso ) of isomerization between p/d-EtO1H 2 O was calculated from the kinetic analysis as K iso = 0.45, which is consistent with the final concentration ratio (1:0.43) of p/d-EtO1H 2 O generated in the aquation reaction of d-EtO1Cl. The irreversible photoisomerization from d-EtO1H 2 O to p-EtO1H 2 O was observed in water with an internal quantum yield (Φ) of 0.44% at 520 nm. Electrochemical measurements showed that d-EtO1H 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 1.3− 9.7, whereas p-EtO1H 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 1.8−11.5. Any redox potential of d/p-EtO1H 2 O isomers was decreased by the electro-donating EtO substitution, compared with distal/proximal-[Ru(tpy)(pynp)OH 2 ] 2+ (d/p-1H 2 O). The turnover frequency (k Od 2 = 1.7 × 10 −2 s −1 ) of d-EtO1H 2 O for water oxidation catalysis is higher than that (3.5 × 10 −4 s −1 ) of p-EtO1H 2 O by a factor of 48.6. The k Od 2 value (1.7 × 10 −2 s −1 ) for d-EtO1H 2 O is 4.5-fold higher than those of d-1H 2 O (3.8 × 10 −3 s −1 ). The higher k Od 2 value of d-EtO1H 2 O compared with d-1H 2 O could be explained by the fast oxidation rate from Ru IV �O to Ru V �O involved in the rate-determining step due to the electrondonating EtO group.