SYNTHESIS AND CHARACTERIZATION OF A (MU-OXO)(MU-CARBOXYLATO)DIMANGANESE(III) POLYIMIDAZOLE COMPLEX

A novel mu-oxo- and mu-carboxylato-bridged dimanganese(III) complex, [Mn2O(TMIMA)2(OAc)](ClO4)3.2CH3CN (1), where TMIMA is tris(I-methylimidazol-2-yl)methyl)amine and OAc is the acetate ion, has been synthesized and characterized by X-ray crystallography, visible spectroscopy, variable-temperature magnetism, and electrochemical methods. X-ray crystallographic parameters are as follows for compound 1: C36H51N16O15Cl3Mn2, 1164.14 g mol-1, triclinic space group (P1BAR), a = 14.909(3) angstrom, b = 15.544(3) angstrom, c = 12.530(3) angstrom, alpha = 104.74(1)-degrees, beta = 112.98(1)-degrees, gamma = 87.46(2)-degrees, V = 2580.5 angstrom3, Z = 2; 5334 independent reflections with I > 3sigma\I\ to max 2theta of 45-degrees, R = 0.045, and R(W) = 0.049. The structure consists of a discrete mu-oxo- and mu-acetato-bridged dimanganese(III) cation and the three perchlorate counterions as well as two acetonitrile solvate molecules. The two acetonitrile solvate molecules and one perchlorate anion were found to be ordered, while the other perchlorate ions were found to be disordered. The coordination environment around each Mn(III) ion is best described as a distorted octahedron as expected for Jahn-Teller d4 metal ions. Imidazole donors bond trans to the bridging oxo ligand and have short Mn-N bond lengths. The remaining imidazoles and amine ligands are bonded cis to the oxo bridge. The electronic spectral properties of the complex are consistent with other high-spin Mn(III) complexes. Variable-temperature magnetic susceptibility data indicate that the two Mn(III) ions are ferromagnetically coupled. Diagonalization of the full Hamiltonian matrix including zero-field interactions (DS(z2)) and isotropic Zeeman interactions of the Mn(III) ions gives g = 2.05, D1 = D2 = 0.40 cm-1, and J = 1.33 cm-1. Compound 1 displays two one-electron redox processes in acetonitrile at a glassy-carbon electrode corresponding to the Mn(III)2 <-- --> Mn(IV)2 interconversion. The first step is electrochemically irreversible, while the second step is quasireversible (DELTAE = 70 mV).