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-tBu2qui) and 2,2'-bipyridine (bpy)). An addition of tBuOK (2.0 equiv) to a methanolic solution of RuII2(OH)2(3,6-tBu2qui)2(btpyan)(SbF6)2 (1(SbF6)2) results in the generation of RuII2(O)2(3,6-tBu2sq)2(btpyan)0 (3,6-tBu2sq = 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 RuII2(O)2(3,6-tBu2sq)2(btpyan)0 undergoes ligand-localized oxidation at E1/2 = +0.40 V (vs Ag/AgCl) to give RuII2(O)2(3,6-tBu2qui)2(btpyan)2+ in MeOH solution. Furthermore, metal-localized oxidation of RuII2(O)2(3,6-tBu2qui)2(btpyan)2+ at Ep = +1.2 V in CF3CH2OH/ether or water gives RuIII2(O)2(3,6-tBu2qui)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 O2 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 RuII2(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 tBuOK, nor activity for the oxidation of H2O under similar conditions.