ASSEMBLY OF HIGH-VALENT OXOMANGANESE CLUSTERS IN AQUEOUS-SOLUTION - REDOX EQUILIBRIUM OF WATER-STABLE MN3O44+ AND MN2O23+ COMPLEXES

The aqueous chemistry of a binuclear mixed-valence cluster, [(bpy)2Mn(0)2Mn(bpy)2](ClO4)3 (1), has been investigated. The cluster exists in a pH-dependent equilibrium with a higher valent trinuclear species, [Mn3O4(bpy)4(OH2)2]4+ (2). The trinuclear cluster is favored at low pH, conditions that favor disproportionation of 1. Evaporation of a pH 1.9 solution of 1 leads to the precipitation of red crystals of 2 in the space group Pī with a = 14.902 (5) Å, b = 15.787 (5) Å, c = 12.676 (4) Å, α = 100.22 (2)°, β = 93.45 (3)°, and γ = 73.98 (3)°. A total of 3718 unique data with I ≥(σ) were refined to values of R and Rw of 8.3 and 11.3%, respectively. The temperature-dependent magnetic susceptibility shows strong antiferromagnetic coupling with (ℋ = J(S1S2) + J'(S1S3 + S2S3)) g = 1.97, J = 182, cm-1, and J' = 98 cm-1. The 5 = 1/2 ground state gives a 35-line EPR spectrum with (from simulation) g = 1.965, A3 = 0.0066 cm-1, and A12 = 0.0061 cm-1. Electrochemistry and EPR show that the concentrations of 1 and 2 are governed by a pH-dependent equilibrium. At pH 2.5, 2 is the only species present in solution; at pH 6.5 only 1 is observable. Further, electrochemical reduction of 2 leads to the formation of 1 in aqueous solution and also in CH3CN. The relevance of this quantitative equilibrium to the photoactivated assembly of the manganese tetramer in photosystem II is discussed. © 1990, American Chemical Society. All rights reserved.