Abstract

To probe photoinduced water oxidation catalyzed by the Mn4O4L6 cubane clusters, we have computationally studied the mechanism and controlling factors of the O-2 formation from the [Mn4O4L6] catalyst, 6. It was demonstrated that dissociation of an L = H2PO2- ligand from 6 facilitates the direct O-O bond formation that proceeds with a 28.3 (33.4) kcal/mol rate-determining energy barrier at the transition state TS1. This step (the O-O single bond formation) of the reaction is a two-electron oxidation/reduction process, during which two oxo ligands are transformed into to mu(2):eta(2)-O-2(2-) unit, and two ("distal") Mn centers are reduced from the 4+ to the 3+ oxidation state. Next two-electron oxidation/reduction occurs by "dancing" of the resulted O-2(2-) fragment between the Mn-1 and Mn-2/Mn-2'-centers, keeping its strong coordination to the Mn-1'-center. As a result of this four-electron oxidation/reduction process Mn centers of the Mn-4-core of I transform from {Mn-1(III)-Mn-1'(III)-Mn-2(IV)-Mn-2'(IV)} to {Mn-1(II)-Mn-1'(II)-Mn-2(III)-Mn-2'(III)} in IV. In other words, upon O-2 formation in cationic complex [Mn4O4L5](+), I, all four Mn-centers are reduced by one electron each. The overall reaction I -> TS1 -> II -> III -> TS2 -> IV -> TS3 -> V -> VI + O-2 is found to be exothermic by 15.4 (10.5) kcal/mol. We analyze the lowest spin states and geometries of all reactants, intermediates, transition states, and products of the targeted reaction.