Competing Exchange Interactions and Ground-State Variability: Linear Homo- and Heterotrinuclear Manganese(III, IV) Complexes with Tris(dimethylglyoximato)metalate(II) Tetraanions as Bridging Ligands

Abstract

Two series of linear homo- and heterotrinuclear complexes, MnIIIMIIMnIII and MnIVMIIMnIV, where MII represents MnII (1 and 5), NiII (2 and 6), CuII (3 and 7), or ZnII (4 and 8), containing three dimethylglyoximato dianions (dmg2-) as bridging ligands and 1,4,7-trimethyl-1,4,7-triazacyclononane (L) as the capping ligand for the terminal MnIII or MnIV ions, have been synthesized. Compounds 1−8 have been characterized on the basis of elemental analyses, IR, UV−vis, and EPR spectroscopy, and variable-temperature (2−295 K) magnetic susceptibility measurements. The trinuclear complexes are quasi-isostructural with the terminal manganese ions in a distorted octahedral environment, MnIII/IVN3O3, and the divalent metal ions M are six-coordinate with the MIIN6 chromophore. The molecular structures of the compounds [LMnIII{(μ-dmg)3MnII}MnIIIL](ClO4)2 (1) and [LMnIII{(μ-dmg)3CuII}MnIIIL](ClO4)2 (3) have been established by X-ray diffraction. 1 crystallizes in the monoclinic system, space group C2/c, with cell constants a = 32.472(6) Å, b = 9.058(2) Å, c = 16.729(3) Å, β = 107.60(3)°, V = 4690.2(16) Å3, and Z = 4. The crystal data for 3 are as follows: monoclinic, space group C2/c, a = 32.686(5) Å, b = 8.870(1) Å, c = 16.867(2) Å, β = 108.65(1), V = 4633(1) Å3, and Z = 4. Analyses of the susceptibility data indicate the presence of weak to moderate exchange interactions, both ferro- and antiferromagnetic, between the paramagnetic centers. It has been conclusively demonstrated that there are indeed two different coupling constants, J = J12 = J23 and J13, operative in these linear trinuclear complexes. J13 represents the exchange interaction between two terminal paramagnetic centers separated by a distance of ∼7 Å. The effect of J13 on the energy-splitting pattern has been demonstrated by the variability of the ground states. A qualitative rationale has been provided for the difference in magnetic behaviors. The cyclic voltammograms of complexes 1−8 reveal two reversible and two quasireversible one-electron redox processes. The central divalent metal ion in these complexes is redox-inactive.