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^IIIM^IIMn^III and Mn^IVM^IIMn^IV, where M^II represents Mn^II (1 and 5). Ni^II (2 and 6). Cu^II (3 and 7), or Zn^III (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/IVN₃O₃, and the divalent metal ions M are six-coordinate with the M^IIN₆ chromophore. The molecular structures of the compounds [LMn^III{(μ-dmg)₃Mn^II}Mn ^IIIL](ClO₄)₂ (1) and [LMn^III{(μ-dmg)₃-Cu^II}Mn ^IIIL](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) ų 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) ų, and Z = 4. Analyses of the susceptibility data indicate the presence of weak to moderate exchange interactions, both terro- 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.