Redox-induced formation and cleavage of O-O sigma and pi bonds in a peroxo-bridged manganese dimer: A density functional study

The structural and electronic consequences of reduction and oxidation of a peroxo-bridged Mn-2(IV/IV) dimer, Mn-2(mu-O)(2)(mu-O-2)(NH3)(6)(2+), are ex amined using approximate density functional theory. In both cases, the init ial electron-transfer step is localized on the metal centers, but subsequen t structural. rearrangement results in transfer of the excess charge to the mu-O-2 unit, with concomitant regeneration of the Mn-2(IV/IV) core. Two-el ectron reduction results in population of the O-O sigma* orbital and comple te cleavage of the O-O bond, whereas two-electron oxidation depopulates the O-O pi* orbital, forming molecular oxygen. The coupling between the metal centers (antiferromagnetic or ferromagnetic) affects the stability of the i ntermediate species, in which the redox process is metal based, and hence i nfluences the kinetic barrier to bond formation or cleavage. Reductive clea vage of the O-O sigma bond is favored when the metal centers are antiferrom agnetically coupled, whereas oxidative formation of the pi component of the O-O bond is favored by ferromagnetic coupling. The possible implications f or the relationship between structure and function in the oxygen-evolving c omplex found in photosynthetic organisms are discussed.