Study of Proton Coupled Electron Transfer in a Biomimetic Dimanganese Water Oxidation Catalyst with Terminal Water Ligands.

The oxomanganese complex [H(2)O(terpy)Mn(III)(μ-O)(2)Mn(IV)(terpy)H(2)O](3+) (1, terpy = 2,2':6-2″-terpyridine) is a biomimetic model of the oxygen evolving complex of photosystem II with terminal water ligands. When bound to TiO(2) surfaces, 1 is activated by primary oxidants (e.g., Ce(4+)(aq), or oxone in acetate buffers) to catalyze the oxidation of water yielding O(2) evolution [G. Li et al. Energy Environ. Sci. 2, 230-238 (2009)]. The activation is thought to involve oxidation of the inorganic core [Mn(III)(μ-O)(2)Mn(IV)](3+) to generate the [Mn(IV)(μ-O)(2)Mn(IV)](4+) state 1(ox) first and then the highly reactive Mn oxyl species Mn(IV)O(•) through proton coupled electron transfer (PCET). Here, we investigate the step 1 → 1(ox) as compared to the analogous conversion in an oxomanganese complex without terminal water ligands, the [(bpy)(2) Mn (III) (μ-O)(2) Mn (IV) (bpy)(2)](3+) complex (2, bpy = 2,2'-bipyridyl). We characterize the oxidation in terms of free energy calculations of redox potentials and pKa's as directly compared to cyclic voltammogram measurements. We find that the pKa's of terminal water ligands depend strongly on the oxidation states of the Mn centers, changing by ~13 pH units (i.e., from 14 to 1) during the III, IV→IV, IV transition. Furthermore, we find that the oxidation potential of 1 is strongly dependent on pH (in contrast to the pH-independent redox potential of 2) as well as by coordination of Lewis base moieties (e.g., carboxylate groups) that competitively bind to Mn by exchange with terminal water ligands. The reported analysis of ligand binding free energies, pKa's and redox potentials indicates that the III, IV→IV, IV oxidation of 1 in the presence of acetate (AcO(-)) involves the following PCET: [H(2)O(terpy)Mn(III)(μ-O)(2)Mn(IV)(terpy)AcO](2+) → [HO(terpy)Mn(IV)(μ-O)(2)Mn(IV)(terpy)AcO](2+) + H(+) + e(-).