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

Two series of linear homo- and heterotrinuclear complexes, Mn III M II Mn III and Mn IV M II Mn IV, where M II represents Mn II ( 1 and 5), Ni II ( 2 and 6), Cu II ( 3 and 7), or Zn II ( 4 and 8), containing three dimethylglyoximato dianions (dmg 2 - ) as bridging ligands and 1,4,7-trimethyl-1,4,7-triazacyclononane (L) as the capping ligand for the terminal Mn III or Mn IV 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, Mn III/IV N 3 O 3, and the divalent metal ions M are six-coordinate with the M II N 6 chromophore. The molecular structures of the compounds [LMn III {(μ-dmg) 3 Mn II }Mn III L](ClO 4 ) 2 ( 1 ) and [LMn III {(μ-dmg) 3 Cu II }Mn III L](ClO 4 ) 2 ( 3 ) have been established by X-ray diffraction. 1 crystallizes in the monoclinic system, space group C 2/ 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) Å 3, and Z = 4. The crystal data for 3 are as follows: monoclinic, space group C 2/ c, a = 32.686(5) Å, b = 8.870(1) Å, c = 16.867(2) Å, β = 108.65(1), V = 4633(1) Å 3, 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 = J 12 = J 23 and J 13, operative in these linear trinuclear complexes. J 13 represents the exchange interaction between two terminal paramagnetic centers separated by a distance of ∼7 Å. The effect of J 13 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.