An EXAFS study of bis(μ-oxo) bridged dinuclear manganese coordination compounds as models for the manganese complex of photosystem II

A complex containing four manganese centres is thought to be the catalytic site of photosynthetic water oxidation. The structure and mechanism of action of this complex are largely unknown. Mn-coordination complexes can be used as models for the manganese complex of Photosystem II(PS II). We report here an X-ray Absorption Spectroscopic study of seven different manganese coordination compounds, six of which are binuclear and contain the structural bis (μ-oxo) unit. In all the compounds, manganese is ligated by 3,3′-17-crown-6-SALPN, providing one crown ether moiety per Mn centre. Alkali or alkaline-earth cations were introduced into this crown-ether moiety in binuclear complexes, bringing the cations (Na⁺, K⁺, Ca²⁺ and Ba²⁺) into the vicinity (< 4Å) of the manganese. The results are discussed in terms of their relevance to the PSII manganese complex.