Oxygen Evolution Catalyzed by a Mononuclear Ruthenium Complex Bearing Pendant SO₃^- Groups

Rational molecular design of catalytic systems capable of smooth O-O bond formation is critical to the development of efficient catalysts for water oxidation. A new ruthenium complex was developed, which bears pendant SO₃^- groups in the secondary coordination sphere: [Ru(terpy)(bpyms)(OH₂)] (terpy=2,2′:6′,2′′-terpyridine, bpyms=2,2′-bipyridine-5,5′-bis(methanesulfonate)). Water oxidation driven by a Ce⁴⁺ oxidant is distinctly accelerated upon introduction of the pendant SO₃^- groups in comparisons to the parent catalyst, [Ru(terpy)(bpy)(OH₂)]²⁺ (bpy=2,2′-bipyridine). Spectroscopic, electrochemical, and crystallographic investigations concluded that the pendant SO₃^- groups promote the formation of an OO bond via the secondary coordination sphere on the catalyst, whereas the influence of the pendant SO₃^- groups on the electronic structure of the [Ru(terpy)(bpy)(OH₂)]²⁺ core is negligible. The results of this work indicate that modification of the secondary coordination sphere is a valuable strategy for the design of water oxidation catalysts.