The oxidation of water to form alternative sources of clean and renewable energy has been the subject of significant fundamental research in the last several years. Much of our current understanding of the mechanistic details of water oxidation has been derived from studies of Ru- or Ir-based catalysts; such catalysts are effective but the transition metals employed are not earth-abundant. Given the expense of such metals, efforts to develop new catalysts integrating less rare metals will be critical to progress in this field. Recent advances have been made on this front with the discovery of copper-based water oxidation catalysts. The elegance of these systems lies in their simplicity: one system being comprised of a copper bicarbonate solution and another being simply a copper coordinated by a polypeptide. The mechanism of water oxidation active in the copper bicarbonate system has been characterized to be fundamentally different from canonical water oxidation mechanisms.<br/>In this work, we utilize quantum chemical models to propose an unconventional mechanism for water oxidation with a copper polypeptide catalyst. Density functional computations on microscopic models are used to characterize a catalytic cycle for the generation of O2 from the copper(II) tetraglycine system in water that is consistent with experimentally observed cyclic voltammetry and reaction rates under catalytic conditions. This characterization has revealed the non-innocent nature of the tetraglycine coordinating ligand during oxidation. In contrast with canonical water oxidation mechanisms, significant oxidation of the catalyst is required before water favorably coordinates to the active metal site. The non-innocent nature of the ligand is highlighted by the formation of a six-membered ring intermediate where hydroxide has inserted into a Cu-N bond. We anticipate that the proposed mechanism can contribute to next-generation catalyst design by highlighting factors that are essential for ligand stability and overall catalytic activity.

Mechanistic analysis of water oxidation catalyzed by a mononuclear copper(II) polypeptide in aqueous solutions

249th National Meeting (2015)

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