Theory of chemical bonds in metalloenzymes XVIII. Importance of mixed-valence configurations for Mn₅O₅, CaMn ₄O₅ and Ca₂Mn₃O₅ clusters revealed by UB3LYP computations. A bio-inspired strategy for artificial photosynthesis

Full geometry optimizations of mixed-valence (MV) Mn(II) _RMn(II)_aMn(IV)_b Mn(IV)_cMn(III) _d (1), Ca(II)_RMn(III)_aMn(III) _bMn(IV)_cMn(III)_d (2) and Ca(II) _RCa(II)_a Mn(IV)_bMn(IV)_cMn(III) _d (3) clusters by UB3LYP have been performed to elucidate possible roles of substitutions of Mn(II) with Ca(II) in parent manganese oxides. The optimized Mn-Mn and Mn-Ca distances for 1, 2 and 3 have been compared with the EXAFS and XRD experimental structures of the oxygen evolving complex (OEC) of photosystem II (PSII) to elucidate variations of geometrical structures and valence fluctuations by the substitutions. The optimized Mn-O distances of 1, 2 and 3 have been examined to elucidate Jahn-Teller distortions induced by the Mn(III) ions. The computational results have illuminated possible origins of the elongated Mn-Mn distances and Mn-O distances in the high-resolution XRD structure by Umena et al. Implications of the computational results have been discussed in relation to chemical modifications of multi-nuclear manganese complexes with substitutions of Mn(II) with Ca(II) for rational design of artificial catalysts for water oxidation. A new bio-inspired strategy for artificial photosynthesis is also proposed based on a guiding principle, namely use of hole- and electron-doped strongly correlated electron systems (SCES) for oxidation and reduction reactions instead of conventional semiconductor materials.