COMPETING EXCHANGE INTERACTIONS AND GROUND-STATE VARIABILITY - LINEARHOMOTRINUCLEAR AND HETEROTRINUCLEAR MANGANESE(III), MANGANESE(IV) COMPLEXES WITH TRIS(DIMETHYLGLYOXIMATO)METALATE(II) TETRAANIONS AS BRIDGING LIGANDS

Two series of linear homo-and heterotrinuclear complexes, (MnMMnIII)-M -III-Mn-II and (MnMMnIV)-M-IV-Mn-II, where Mn represents Mn-II (1 and 5), N-II (2 and 6), Cu-II (3 and 7), or Zn-II (4 and 8), containing th ree 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 ha ve been characterized on the basis of elemental analyses, IR, UV-vis, and EPR spectroscopy, and variable-temperature (2-295 K) magnetic susc eptibility measurements. The trinuclear complexes are quasi-isostructu ral with the terminal manganese ions in a distorted octahedral environ ment, (MnN3O3)-N-III/IV, and the divalent metal ions M are six-coordin ate with the (MN6)-N-II chromophore. The molecular structures of the c ompounds [LMnIII{(mu-dmg)(3)Mn-II}(MnL)-L-III](ClO4)(2) (1) and [LMnII I{(mu-dmg)(3)Cu-II}(MnL)-L-III](ClO4)(2) (3) have been established by X-ray diffraction. 1 crystallizes in the monoclinic system, space grou p C2/c, with cell constants a = 32.472(6) Angstrom, b = 9.058(2) Angst rom, c = 16.729(3) Angstrom, beta = 107.60(3)degrees, V = 4690.2(16) A ngstrom(3), and Z = 4. The crystal data for 3 are as follows: monoclin ic, space group C2/c, a 32.686(5) Angstrom, b = 8.870(1) Angstrom c = 16.867(2) Angstrom, beta = 108.65(1), V = 4633(1) Angstrom(3), and Z = 4. Analyses of the susceptibility data indicate the presence of weak to moderate exchange interactions, both ferro-and antiferromagnetic, b etween 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 similar to 7 Angstrom. The effect o f J(13) On the energy-splitting pattern has been demonstrated by the v ariability of the ground states. A qualitative rationale has been prov ided for the difference in magnetic behaviors. The cyclic voltammogram s of complexes 1-8 reveal two reversible and two quasireversible one-e lectron redox processes. The central divalent metal ion in these compl exes is redox-inactive.