Structure and Magnetic Properties of a Mixed-Valence Heptanuclear Manganese Cluster

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Abstract
Two novel polynuclear manganese(II,III) complexes have been synthesized by exploiting controlled methanolysis. A one-pot reaction of MnCl2, NaOMe, dibenzoylmethane (Hdbm), and O2 in anhydrous methanol, followed by recrystallization from MeOH/CHCl3 mixtures, afforded the alkoxomanganese complexes [Mn7(OMe)12(dbm)6]·CHCl3·14MeOH (2) and [Mn2(OMe)2(dbm)4] (3). Complex 2 crystallizes in trigonal space group R3̄ with a = 14.439(2) Å, α = 86.34(1)°, and Z = 1. Complex 3 crystallizes in triclinic space group P1̄ with a = 9.612(1) Å, b = 10.740(1) Å, c = 13.168(1) Å, α = 80.39(1)°, β = 87.66(1)°, γ = 83.57(1)°, and Z = 1. The solid-state structure of 2 comprises a [Mn6(OMe)12(dbm)6] “crown” with crystallographically imposed 6-fold symmetry plus a central manganese ion. The layered Mn/O core mimics a fragment of the manganese oxide mineral lithiophorite. Conductivity measurements confirmed the nonionic character of 2 and suggested a mixed-valence MnII3MnIII4 formulation. The metrical parameters of the core were analyzed with the aid of bond-valence sum calculations. The central ion is essentially a valence-trapped MnII ion, whereas the average Mn−O distances for the manganese ions of the “crown” are consistent with the presence of two MnII and four MnIII ions. However, 1H NMR spectra in solution strongly support valence localization and suggest that the observed solid-state structure may be a result of static disorder effects. Magnetic susceptibility vs T and magnetization vs field data at low temperature are consistent with an S = 17/2 ground state. Complex 3 is a symmetric alkoxo-bridged dimer. The two high-spin MnIII ions are antiferromagnetically coupled with J = 0.28(4) cm-1, g = 1.983(2), and D = −2.5(4) cm-1.

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