New Reaction Model for
O–O Bond Formation and O<sub>2</sub> Evolution Catalyzed by
Dinuclear Manganese Complex
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Abstract
A new mechanism of the oxygen evolving reaction catalyzed
by [H<sub>2</sub>O(terpy)Mn(μ-O)<sub>2</sub>Mn(terpy)OH<sub>2</sub>]<sup>3+</sup> is proposed by using density functional theory.
This proton coupled electron transfer (PCET) model shows reasonable
barriers. Because in experiments excess oxidants (OCl<sup>–</sup> or HSO<sub>5</sub><sup>–</sup>) are required to evolve oxygen
from water, we considered the Mn<sub>2</sub> complex neutralized by
three counterions. Structure optimization made the coordinated OCl<sup>–</sup> withdraw a H<sup>+</sup> from the water ligand and
produces the reaction space for H<sub>2</sub>O<sub>2</sub> formation
with the deprotonated OH<sup>–</sup> ligand. The reaction barrier
for the H<sub>2</sub>O<sub>2</sub> formation from OH<sup>–</sup> and protonated OCl<sup>–</sup> depends significantly on the
system charge and is 14.0 kcal/mol when the system is neutralized.
The H<sub>2</sub>O<sub>2</sub> decomposes to O<sub>2</sub> during
two PCET processes to the Mn<sub>2</sub> complex, both with barriers
lower than 12.0 kcal/mol. In both PCET processes the spin moment of
transferred electrons prefers to be parallel to that of Mn 3d electrons
because of the exchange interaction. This model thus explains how
the triplet O<sub>2</sub> molecule is produced