Award Address (ACS Award in Pure Chemistry sponsored by the Alpha Chi Sigma Fraternity and the Alpha Chi Sigma Educational Foundation). Heterometallic models of the oxygen evolving complex of photosystem II

Abstract

Redox-inactive metals are found in biol. and heterogeneous water oxidn. catalysts, but their roles are currently not well understood. Targeting synthetic model clusters of these catalysts, triphenylbenzene moieties appended with pyridine and alkoxide donors were employed as multinucleating ligands for first-row transition metals. Complexes of Mn^(2+), Fe^(2+), Co^(2+), Ni^(2+), Cu^(2+), Zn^(2+) display trinuclear cores supported by bridging alkoxides. Trimanganese complexes were utilized as precursors to more elaborate metal oxide clusters. Tetranuclear complexes displaying Mn_4, Mn_3Ca and other Mn_3M motifs (M = Na^+, Sr^(2+), Zn^(2+), Sc^(3+), Y^(3+)) with varied no. of bridging oxo ligands were synthesized and studied. The redn. potentials of these clusters span a window of over 1 V. With the pKa of the redox-inactive metal-aqua complex as a measure of Lewis acidity, structurally analogous series of clusters display linear dependence between redn. potential and acidity. Implications for the function of oxygen evolving catalysts will be discussed.