Role of redox-inactive metals in controlling the redox potential of heterometallic manganese-oxido clusters.

Photosystem II (PSII) contains Ca, which is essential to the oxygen-evolving activity of the catalytic MnCaO complex. Replacement of Ca with other redox-inactive metals results in a loss/decrease of oxygen-evolving activity. To investigate the role of Ca in this catalytic reaction, we investigate artificial Mn[M]O clusters redox-inactive metals  [M] ([M]  = Mg, Ca, Zn, Sr, and Y), which were synthesized by Tsui et al. (Nat Chem 5:293, 2013). The experimentally measured redox potentials (E) of these clusters are best described by the energy of their highest occupied molecular orbitals. Quantum chemical calculations showed that the valence of metals predominantly affects E(Mn), whereas the ionic radius of metals affects E(Mn) only slightly.