Influence of chloro substituent on photoisomerization, redox reactions and water oxidation catalysis of mononuclear ruthenium complexes

• Mononuclear Ru complexes with 4′-chloro-2,2′;6′,2″-terpyridine is synthesized.
• Aquation of distal-[Ru(Cl-tpy)(pynp)Cl]+ is faster than that of the tpy derivative.
• Potoisomerization of distal-[Ru(Cl-tpy)(pynp)OH2]2+ to proximal-isomer occurs.
• distal-Aquo complex works more efficiently for catalysis relative to proximal-isomer.
• The catalytic activity of distal-complex is increased by Cl substitution on tpy.

distal-[Ru(Cl-tpy)(pynp)Cl]+ (d-2Cl) (Cl-tpy = 4′-chloro-2,2′;6′,2″-terpyridine, pynp = 2-(2-pyridyl)-1,8-naphthyridine), and distal- and proximal-[Ru(Cl-tpy)(pynp)OH2]2+ (d- and p-2H2O) complexes are newly synthesized and characterized to compare structures and physicochemical properties with a 2,2′;6′,2″-terpyridine (tpy) ligand derivatives of distal-[Ru(tpy)(pynp)Cl]+ (d-1Cl), distal- and proximal-[Ru(tpy)(pynp)OH2]2+ (d- and p-1H2O). The equilibrium turned out to be involved in the aquation reaction of d-2Cl to d-2H2O in contrast to observed irreversible aquation reaction of d-1Cl under the same conditions. The kinetic analysis showed that the aquation reaction of d-2Cl is slightly slower than that of d-1Cl. The stoichiometric photoisomerization of d-2H2O to p-2H2O occurs by visible light irradiation as it is for d-1H2O, and Φ (2.1%) at 520 nm for photoisomerization of d-2H2O was higher than that (1.5%) observed for d-1H2O. d-2H2O undergoes the two-step reaction involving the successive one-proton-coupled one-electron reactions of the RuIIOH2/RuIIIOH and RuIIIOH/RuIVO redox couples, whereas p-2H2O undergoes the one-step reaction involving the two-proton-coupled two-electron reaction of the RuIIOH2/RuIVO redox couple. These redox potentials of d- and p-2H2O are higher than those for d- and p-1H2O at pH 7.0 by 10 ∼ 50 mV due to the electron-withdrawing chloro substitution. The turnover frequency (kO2=6.3×10−3s−1)(kO2=6.3×10−3s−1) of d-2H2O for water oxidation was higher than that (3.9 × 10−4 s−1) of p-2H2O by a factor of 16. kO2 for d-2H2O was also 1.6 times higher than that (3.8 × 10−3 s−1) for d-1H2O, whereas kO2 for p-2H2O was 1.2 times lower than that (4.8 × 10−4 s−1) for p-1H2O.