AN EXAFS STUDY OF BIS(MU-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 catal ytic site of photosynthetic water oxidation. The structure and mechani sm of action of this complex are largely unknown. Mn-coordination comp lexes can be used as models for the manganese complex of Photosystem I I (PS II). We report here an X-ray Absorption Spectroscopic study of s even different manganese coordination compounds, six of which are binu clear and contain the structural bis (mu-oxo) unit. In all the compoun ds, manganese is ligated by 3,3'-17-crown-6-SALPN, providing one crown ether moiety per Mn centre. Alkali or alkaline-earth cations were int roduced into this crown-ether moiety in binuclear complexes, bringing the cations (Na+, K+, Ca2+ and Ba2+) into the vicinity (< 4 Angstrom) of the manganese. The results are discussed in terms of their relevanc e to the PS II manganese complex.