Mechanisms and Factors Controlling Photoisomerization Equilibria, Ligand Exchange, and Water Oxidation Catalysis of Mononuclear Ruthenium(II) Complexes

Hirahara, M.; Hakamata, T.; League, A. B.; Ertem, M. Z.; Takahashi, K.; Nagai, S.; Inaba, K.; Yamazaki, H.; Saito, K.; Yui, T.; Cramer, C. J.; Yagi, M.
Eur. J. Inorg. Chem. in press 2015, 3892 (doi:10.1002/ejic.201500642)>

The photoisomerization equilibrium between distal- and proximal-[Ru(tpy)(pyqu)OH₂]²⁺ (d- and p-RuH₂O, tpy = 2,2',6',2"-terpyridine, pyqu = 2-(2-pyridyl)-quinoline) is characterized. The kinetic analysis of the pD-dependent photoisomerization reactions (monitored by ¹H NMR) of d-RuH₂O and p-RuH₂O show (1) that both hydroxo isomers, distal- and proximal-[Ru(tpy)(pyqu)OH]⁺, are inert to photoisomerization and (2) that the back reaction (distal to proximal) is 3.0 times faster than the forward reaction (proximal to distal). Isolation of distal- and proximal-[Ru(tpy)(pyqu)Cl]⁺ (d- and p-RuCl) as well as d- and p-RuH₂O isomers enabled comprehensive studies on geometric structures, ligand exchange and redox reactions, and water oxidation catalysis for these isomers. The observed aquation rate constant (9.2 x 10^-2 s^-1 at 40 μM) of p-RuCl to form p-RuH₂O is 1700 times higher than that (5.4 x 10^-5 s^-1 at 63 μM) of d-RuCl at 298 K owing to steric repulsion of a chloro ligand and the 8-proton of the quinoline moiety. The turnover frequency (TOF =1.7 X 10^-3 s^-1) of p-RuH₂O for catalytic water oxidation is 1.7 times greater than that (1.0 X 10^-3 s^-1) for d-RuH₂O, in contrast to the [Ru(tpy)(pynp)OH₂]²⁺ isomer system, in which the TOF of the distal-isomer is higher than that of the proximal-one by one order of magnitude. The mechanisms and factors controlling the photoisomerization equilibria and water oxidation catalysis of the d- and p-RuH₂O isomers are discussed based on the experimental and theoretical investigations.