The performance of MnII/III-monosubstituted heteropolytungstates
[MnIII(H2O)GeW11O39]5– ([GT-MnIII-OH2]5–, where GT = GeW11O39) and [MnII(H2O)GeW11O39]6– ([GT-MnII-OH2]6–) as water
oxidation catalysts at pH 9 was explored using density functional
theory calculations. The counterion effect was fully considered, in
which five and six Na+ ions were included in the calculations
for water oxidation catalyzed by [GT-MnIII-OH2]5– and [GT-MnII-OH2]6–, respectively. The process of water oxidation catalysis
was divided into three elemental stages: (i) oxidative activation,
(ii) O–O bond formation, and (iii) O2 evolution.
In the oxidative activation stage, two electrons and two protons are
removed from [Na5-GT-MnIII-OH2] and
three electrons and two protons are removed from [Na6-GT-MnII-OH2]. Therefore, the MnIV-O• species [Na5-GT-MnIV-O•]
is obtained. Two mechanisms, (i) water nucleophilic attack and (ii)
oxo–oxo coupling, were demonstrated to be competitive in O–O
bond formation triggered from [Na5-GT-MnIV-O•]. In the last stage, the O2 molecule could
be readily evolved from the peroxo or dinuclear species and the catalyst
returns to the ground state after the coordination of a water molecule(s).