The nature of chemical bonds of the CaMn4O5 cluster in oxygen evolving complex of photosystem II: Jahn‐Teller distortion and its suppression by Ca doping in cubane structures

Full geometry optimizations of mixed-valence (MV) cubane Mn(III)4-ωMn(IV)ωO4 cluster (ω = 0–4) by UB3LYP have been performed to elucidate Jahn-Teller (JT) effects for the Mn(III) ions. The optimized geometries have elucidated acute triangle for Mn(III)2 Mn(X) (X = III,IV) core in the cubane, indicating the JT effect. The JT effect is not remakable for Mn(IV)2Mn(III) cores, showing obtuse triangle and it diminishes in Mn(IV)3 core with equilateral triangle. The replacement of one of Mn ions with Ca (or Sr) ion in the cubane structure has also been examined to elucidate multiple roles of Ca(II) ion. The Ca-doping suppresses the JT effect even for Mn(III)2Mn(X) (X = III,IV) core because acute CaMnMn triangles with longer Ca-Mn distances than the Mn-Mn distance are newly formed. The suppression of the JT effect for Mn(IV)2 Mn(III) by Ca doping is also found because of the same reason. The Ca-doped cubane in the CaMn4O5 cluster (1) of oxygen evolving complex (OEC) of photosystem II (PSII) exhibits two acute CaMnMn triangles and one almost equilateral CaMnMn triangle because of the elongation of the Mn1(d)-Mn3(b) distance with the coordination of the extra Mn4(a) ion to the cubane. The Mn1(d)Mn2(c)Mn3(b) triangle becomes obtuse because of the suppression of the JT effect, indicating the Mn1(d)(III)Mn2(c)(IV)Mn3(b)(IV) valence state in the S1 state of 1. The JT distortion for Mn(III) ions modified by the Ca(or Sr) doping is coupled with the labile structural deformation for intracluster electron transfers via double exchange mechanism in 1. The structural anomalies of 1 revealed by the new XRD experiment are also crucial for derivation of several guiding principles for theoretical modeling of water splitting reaction at OEC of PSII. The hybrid DFT computational results for 1 have also supported these guiding principles that provide possible orbital and spin correlation diagrams for water splitting reactions. The Huckel-Hubbard-Hund (HHH) model has been used for qualitative understanding of these diagrams. Implications of the present computational results are discussed in relation to electronic and spin states of redox active Ca-doped Mott insulators constructed with magnetic transition metals such as Mn, Fe, Co, Ni, etc. Strongly correlated electron systems (SCES) modified by doping of Ca, Sr, Zn, etc. have been elucidated as possible candidates of bio-inspired artificial catalysts for water oxidation. © 2012 Wiley Periodicals, Inc.