Electrochemistry of oxo-bridged ruthenium dimers with 4,4′-dichloro- and 5,5′-dichloro-2,2′-bipyridine and their catalytic properties towards water oxidation

Ruthenium μ-oxo dimers with chloro-substituted 2,2′-bipyridines [L2(H2O)RuIIIORuIII(OH2)L2]4+(L= 4,4′-dichloro- or 5,5′-dichloro-2,2′-bipyridine) were synthesized. Electrochemical studies on the complexes showed that the RuIII|O|RuIIIdimer undergoes a reversible one-electron oxidation to RuIII|O|RuIVfollowed by a reversible multi-electron oxidation assignable as the oxidation of RuIII|O|RuIVto RuIV|O|RuV. The RuIII|O|RuIVand RuIV|O|RVspecies were characterized by thin-layer spectroelectrochemistry. The RuIV|O|RuVdimer is an active catalyst for the chemical or electrochemical oxidation of water to dioxygen. Upon reduction, the RuIII|O|RuIIIdimer undergoes reductive cleavage to give [RuIIL2(H2O)2]2+. The chloro-substituted ruthenium dimers are more stable towards reductive cleavage in mildly acidic media than the unsubstituted [(bpy)2(H2O)RuIIIORuIII(OH2)(bpy)2]4+(bpy= 2,2′-bipyridine).