STRUCTURE AND MAGNETIC-PROPERTIES OF A MIXED-VALENCE HEPTANUCLEAR MANGANESE CLUSTER

Two novel polynuclear manganese(II,III) complexes have been synthesize d by exploiting controlled methanolysis. A one-pot reaction of MnCl2, NaOMe, dibenzoylmethane (Hdbm), and O-2 in anhydrous methanol, followe d by recrystallization from MeOH/CHCl3 mixtures, afforded the alkoxoma nganese complexes [Mn-7(OMe)(12)(dbm)(6)]. CHCl3. 14MeOH (2) and [Mn-2 (OMe)(2)(dbm)(4)] (3). Complex 2 crystallizes in trigonal space group <R(3)over bar> with a = 14.439(2) Angstrom, alpha = 86.34(1)degrees, a nd Z = 1. Complex 3 crystallizes in triclinic space group <P(1)over ba r> with a = 9.612(1) Angstrom, b = 10.740(1) Angstrom, c = 13.168(1) A ngstrom, alpha = 80.39(1)degrees, beta = 87.66(1)degrees, gamma = 83.5 7(1)degrees, and Z = 1. The solid-state structure of 2 comprises a [Mn -6(OMe)(12)(dbm)(6)] ''crown'' with crystallographically imposed 6-fol d 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 mix ed-valence (Mn3Mn4III)-Mn-II formulation. The metrical parameters of t he core were analyzed with the aid of bond-valence sum calculations. T he central ion is essentially a valence-trapped Mn-II ion, whereas the average Mn-O distances for the manganese ions of the ''crown'' are co nsistent with the presence of two Mn-II and four Mn-III ions. However, H-1 NMR spectra in solution strongly support valence localization and suggest that the observed solid-state structure may be a result of st atic disorder effects. Magnetic susceptibility vs T and magnetization vs field data at low,temperature are consistent with an S = 17/2 groun d state. Complex 3 is a symmetric alkoxo-bridged dimer. The two high-s pin Mn-III ions are antiferromagnetically coupled with J = 0.28(4) cm( -1), g = 1.983(2), and D = -2.5(4) cm(-1).