Dimer-of-Dimers Model for the Oxygen-Evolving Complex of Photosystem II. Synthesis and Properties of [Mn^IV₄O₅(terpy)₄(H₂O)₂](ClO₄)₆

A dimer-of-dimers model compound for the oxygen-evolving complex of photosystem II, [{(H2O)(terpy)MnIV(μ-O)2MnIV(terpy)}2(μ-O)](ClO4)6 (terpy = 2,2‘:6‘,2‘ ‘-terpyridine), has been prepared and characterized by X-ray crystallography and ESI-MS. Low pH was found to promote the disproportionation of [MnIII/IV2O2(terpy)2(OH2)2]3+ to Mn2+ and a MnIV/IV2O2(terpy)2 species; the latter complex slowly dimerizes to form the title complex. Protonation of a μ-oxo bridge is proposed to initiate the disproportionation, based on analogy with the [MnIII/IV2O2(bpy)4]3+ system.