LINEAR TRINUCLEAR OXIMATO-BRIDGED COMPLEXES MN-III,MN-IV,M-II,MN-III,MN-IV (M=ZN, CU OR MN) - SYNTHESIS, STRUCTURE, REDOX BEHAVIOR AND MAGNETISM

Six trinuclear linear complexes [LMnIII{mu-(niox)(3)Mn-II}(MnL)-L-III] (2+) 1-3 and [LMnIV{mu-(niox)(3)Mn-II}(MnL)-L-IV](4+) 4-6 where M repr esents either Zn (1 and 4), Cu (2 and 5) or Mn (3 and 6), containing t hree cyclohexane-1,2-dione dioximate dianions (niox) as bridging ligan ds and 1,4,7-trimethyl-1,4,7-triazacyclononane (L) as the capping liga nd for the terminal manganese-(III) or -(IV) ions, have been synthesiz ed. They have been characterized by elemental analysis, IR, UV/VIS and EPR spectroscopy, cyclic voltammetry, and by variable-temperature (2- 295 K) magnetic susceptibility and isothermal magnetization measuremen ts. The trinuclear complexes are quasi-isostructural with the terminal manganese ions in a distorted octahedral environment, MnN3O3, and the divalent metal ions M are six-co-ordinate with a (MN6)-N-II chromopho re. The molecular structure of the compound [LMnIV{mu-(niox)(3)Zn-II}( MnL)-L-IV][ClO4](4) 4 has been established by X-ray diffraction. The s tructure consists of tris(nioximato)-bridged (MnZnMnIV)-Zn-IV-Mn-II te tracations and non-co-ordinated perchlorate anions, with an intramolec ular Mn-IV... Mn-IV distance of 6.97 Angstrom. A crystal structure det ermination of (MnCuMnIV)-Cu-IV-Mn-II 5 was not up to normal crystallog raphic standards. Nevertheless, the atom connectivities together with the heterotrinuclear nature with Mn-IV as the terminal ions are clear. The cyclic voltammograms of 1-6 reveal two reversible and two quasi-r eversible one-electron redox processes, Mn(III)M(II)Mn(III)reversible arrow(Ox1) Mn(IV)M(II)Mn(III)reversible arrow(Ox2) (MnMMnIV)-M-IV-Mn-I I and Mn(III)M(II)Mn(III)reversible arrow(Red1)Mn(II)M(II)Mn(III)rever sible arrow(Red2)Mn(II)M(II)Mn(II). The central divalent metal ion is redox inactive. Analyses of susceptibility data showed the occurrence of intramolecular ferro-and antiferro-magnetic exchange interactions. There are indeed two different coupling constants, J(a) operating betw een the adjacent metal centres and J(t) between the terminal centres, separated by a large distance of approximate to 7 Angstrom. The effect of J(t) on the splitting pattern has been shown by the variability of the ground states. That the assumption commonly made of no coupling b etween the terminal ions in trinuclear linear complexes may lead to a wrong spin ground state has been conclusively demonstrated. An analysi s of the interacting magnetic orbitals in complexes containing three m etal centres is presented.