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

Two novel polynuclear Mn(II,III) complexes were synthesized by exploiting controlled methanolysis. A 1-pot reaction of MnCl2, NaOMe, dibenzoylmethane (Hdbm), and O2 in anhyd. MeOH, followed by recrystn. from MeOH/CHCl3 mixts., afforded the alkoxomanganese complexes [Mn7(OMe)12(dbm)6]·CHCl3·14MeOH (2) and [Mn2(OMe)2(dbm)4] (3). 2 Crystallizes in trigonal space group R-3 with a = 14.439(2) Å, α = 86.34(1), and Z = 1. 3 Crystallizes in triclinic space group P-1 with a = 9.612(1), b = 10.740(1), 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 crystallog. imposed 6-fold symmetry plus a central Mn ion. The layered Mn/O core mimics a fragment of the Mn oxide mineral lithiophorite. Cond. measurements confirmed the nonionic character of 2 and suggested a mixed-valence MnII3MnIII4 formulation and the metrical parameters of the core were analyzed with the aid of bond-valence sum calcns. The central ion is essentially a valence-trapped MnII ion, whereas the av. Mn-O distances for the Mn ions of the crown are consistent with the presence of two MnII and four MnIII ions. However, 1H NMR spectra in soln. strongly support valence localization and suggest that the obsd. solid-state structure may be a result of static disorder effects. Magnetic susceptibility vs. T and magnetization vs. field data at low temp. are consistent with an S = 17/2 ground state. 3 Is a sym. alkoxo-bridged dimer and 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.