Modeling the Photosynthetic Water Oxidation Center: Synthesis, Structure, and Magnetic Properties of [Mn < inf > 2 < /inf > (μ-O) < inf > 2 < /inf > (μ-OAc)(H < inf > 2 < /inf > O) < inf > 2 < /inf > (bipy) < inf > 2 < /inf > ](ClO < inf > 4 < /inf > ) < inf > 3 < /inf > ·H < inf > 2 < /inf > O

Abstract

[Mn2(μ-O)2(μ-OAc)(H2O)2(bipy)2](ClO4)3·H2O crystallizes in the monoclinic system, space group P21/n (No. 14) with Z = 4 and a = 13.619(1) Å, b = 16.213(2) Å, c = 16.266(1) Å, and β = 113.08(1)°. The structure was solved by direct methods and refined to conventional agreement indices R = 0.021 and Rw = 0.024 with 5159 unique reflections for which I > 3σ(I). The structure consists of binuclear [MnIV2(μ-O)2(μ-OAc)(H2O)2(bipy)2]3+ cations separated by ClO4− anions and a molecule of water of crystallization. This dimanganese(IV) compound is the first to include simultaneously a bridging acetato ligand, a bis(μ-oxo) bridge and two water molecules, each directly coordinated to one manganese(IV) ion. This high-valent bis(μ-oxo)-μ-acetato-bridged binuclear species has been studied with optical and EPR spectroscopy. The analysis of the thermal variation of its magnetic susceptibility affords one of the weakest antiferromagnetic interaction (J = −43.7 cm−1, H = −2JS1S2) determined for a bis- (μ-oxo)-bridged MnIV2 species. The weakening of the antiferromagnetic interaction is tentatively correlated to the departure of the MnIV(μ-O)2MnIV ring from planarity (18.3°) as a result of the constraint exerted by the μ-acetato bridge. © 1994, American Chemical Society. All rights reserved.