Rearranging from 6- to 7-coordination initiates the catalytic activity: An EPR study on a Ru-bda water oxidation catalyst

Abstract

The coordination of a substrate water molecule on a metal centered catalyst for water oxidation is a crucial step involving the reorganization of the ligand sphere. This process can occur by substituting a coordinated ligand with a water molecule or via a direct coordination of water onto an open site. In 2009, we reported an efficient ruthenium-based molecular catalyst, Ru-bda, for water oxidation. Despite the impressive improvement in catalytic activity of this type of catalyst over the past years, a lack of understanding of the water coordination still remains. Herein, we report our EPR and DFT studies on Ru-bda(triethylammonium 3-pyridine sulfonate)2 (1) at its RuIII oxidation state, which is the initial state in the catalytic cycle for the O–O bond formation. Our investigation suggests that at this III-state, there is already a rearrangement in the ligand sphere where the coordination of a water molecule at the 7th position (open site) takes place under acidic conditions (pH=1.0) to form a rare 7-coordinated RuIII species.