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General Synthesis of Di-μ-oxo Dimanganese Complexes as Functional Models for the Oxygen Evolving Complex of Photosystem II

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posted on 17.10.2005, 00:00 by Hongyu Chen, Ranitendranath Tagore, Siddhartha Das, Christopher Incarvito, J. W. Faller, Robert H. Crabtree, Gary W. Brudvig
A series of complexes with the formula [MnIII/IV2(μ-O)2(L)2(X)2]3+ have been prepared in situ from MnIILCl2 precursors by a general preparative method (L = terpy, Cl-terpy, Br-terpy, Ph-terpy, tolyl-terpy, mesityl-terpy, t Bu3-terpy, EtO-terpy, py-phen, dpya, Me2N-terpy, or HO-terpy, and X = a labile ligand such as water, chloride, or sulfate). The parent complex, where L = terpy and X = water, is a functional model for the oxygen-evolving complex of photosystem II (Limburg, et al. J. Am. Chem. Soc. 2001, 123, 423−430). Crystals of MnII(dpya)Cl2, MnII(Ph-terpy)Cl2, MnII(mesityl-terpy)Cl2, and an organic-soluble di-μ-oxo di-aqua dimanganese complex, [MnIII/IV2(μ-O)2(mesityl-terpy)2(OH2)2](NO3)3, were obtained and characterized by X-ray crystallography. Solutions of the in situ-formed di-μ-oxo dimanganese complexes were characterized by electrospray mass spectrometry, EPR spectroscopy, and UV−visible spectroscopy, and the rates of catalytic oxygen-evolving activity were assayed. The use of MnIILCl2 precursors leads to higher product purity of the Mn dimers while achieving the 1:1 ligand to Mn stoichiometry appropriate for catalytic activity assay. These methods can be used to screen the catalytic activity of other di-μ-oxo dimanganese complexes generated by using a ligand library.