Supramolecular Mn−Ca Aggregates as Models for the Photosynthetic Water Oxidation Complex

Mn4/(AE)2 complexes, AE = Ca, 1 or Ba, 2, display MnIII3MnII valence-trapped states and Jahn−Teller compressed MnIII ions, consistent with the S0 (or S-2) oxidation level of PSII. Ba2+ and Ca2+ ligate Mn-bonded carboxylate oxygens either directly or via H2O molecules, respectively. The (O−H−O)3- ligand, symmetric, μ4, in 1, is asymmetric, μ5, in 2, part of a μ3-oxo Mn2Ba unit. AE dependent acetate ligation to Mn is also observed. 1 and 2 have biomimetic catalase activity.