Ru water oxidation catalysts based on py5 ligands

Ru complexes containing the pentapyridyl ligand 6,6''-(methoxy(pyridin-2-yl)methylene)di-2,2'-bipyridine (L-OMe) of general formula trans-[RuII(X)(L-OMe-k-N5)]n+ (X = Cl, n = 1, trans-1+; X = H2O, n = 2, trans-22+) have been isolated and characterized in solution (NMR, UV-Vis) and in the solid state by XRD. Both complexes suffer a series of substitution reactions at oxidation state II and III, when dissolved in aqueous triflic acid-trifluroethanol solutions that have been monitored by UV-vis spectroscopy and their corresponding rate constants reported. In particular, aqueous solutions of the RuIII-Cl complex trans-[RuIII(Cl)(L-OMe-k-N5)]2+ (trans-12+) generates a family of Ru-aqua complexes namely trans-[RuIII(H2O)(L-OMe-k-N5)]3+ (trans-23+), [RuIII(H2O)2(L-OMe-k-N4)]3+ (trans-33+) and [RuIII(Cl)(H2O)(L-OMe-k-N4)]3+ (trans-42+). While complex trans-42+ is a powerful water oxidation catalyst, complex trans-23+ has only a moderate activity and trans-33+ is not a catalyst. Further, a parallel work has been carried out with related complexes but containing the methyl substituted ligand 6,6''-(1-pyridin-2-yl)ethane-1,1-diyl)di-2,2'-bipyridine (L-Me). The behavior of all these catalysts has been rationalized based on substitution kinetics, oxygen evolution kinetics, electrochemical properties and DFT calculations. The best catalysts, trans-42+, reach turnover frequencies of 0.71 s-1 using Ce(IV) as a sacrificial chemical oxidant with oxidative efficiencies above 95%