Syntheses and redox properties of bis(hydroxoruthenium) complexes with quinone and bipyridine ligands. Water-oxidation catalysis

The novel bridging ligand 1,8-bis(2,2':6',2-terpyridyl)anthracene (btpyan) is synthesized by three reactions from 1,8-diformylanthracene to connect two [Ru(L)(OH)](+) units (L = 3,6-di-tert-butyl-1,2;benzoquinone (3,6-Bu-t(2)-qui) and 2,2'-bipyridine (bpy)). An addition of (BuOK)-Bu-t (2.0 equiv) to a methanolic solution of [Ru-2(II)(OH)(2)(3,6-(t)Bu(2)qui)(2)(btpyan)](SbF6)(2) ([1](SbF6)(2)) results in the generation of [Ru-2(II)(O)(2)(3,6-(t)Bu(2)sq)(2)(btpyan)](0) (3,6-(t)Bu(2)sq = 3,6 -di-tert-butyl-1,2-semiquinone) due to the reduction of quinone coupled with the dissociation of the hydroxo protons. The resultant complex [Ru-2(II)(O)(2)(3,6-(t)Bu(2)sq)(2)(btpyan)](0) undergoes ligand-localized oxidation at E-1/2 = +0.40 V (vs Ag/AgCl) to give [Ru-2(II)(O)(2)(3,6-(t)Bu(2)qui)(2)(btpyan)](2+) in MeOH solution: Furthermore, metal-localized oxidation of [Ru-2(II)(O)(2)(3, 6-(t)Bu(2)qui)(2)(btpyan)](2+) at E-p = + 1.2 V in CF3CH2OH/etheq or water gives [Ru-2(III)(O)(2)(3,6-(t)Bu(2)qui)(2)(btpyan)](4+), which catalyzes water oxidation. Controlled-potential electrolysis of [1](SbF6)(2) at +1.70 V in the presence of H2O in CF3CH2OH evolves dioxygen with a current efficiency of 91% (21 turnovers). The turnover number of O-2 evolution increases to 33 500 when the electrolysis is conducted in water (pH 4.0) by using a [1](SbF6)(2)-modified ITO electrode. On the other hand, the analogous complex [Ru-2(II)(OH)(2)(bpy)(2)(btpyan)](SbF6)(2) ([2](SbF6)(2)) shows neither dissociation of the hydroxo protons, even in the presence of a large excess of (BuOK)-Bu-t, nor activity for the oxidation of H2O under similar conditions.