Linear trinuclear oximato-bridged complexes MnIII,IVMIIMnIII,IV (M = Zn, Cu or Mn): synthesis, structure, redox behaviour and magnetism ‡

Abstract

Six trinuclear linear complexes [LMnIII{µ-(niox)3MII}MnIIIL]2+ 1–3 and [LMnIV{µ-(niox)3MII}MnIVL]4+ 4–6 where M represents either Zn (1 and 4), Cu (2 and 5) or Mn (3 and 6), containing three cyclohexane-1,2-dione dioximate dianions (niox) as bridging ligands and 1,4,7-trimethyl-1,4,7-triazacyclononane (L) as the capping ligand for the terminal manganese-(III) or -(IV) ions, have been synthesized. 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 measurements. 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 MIIN6 chromophore. The molecular structure of the compound [LMnIV{µ-(niox)3ZnII}MnIVL][ClO4]4 4 has been established by X-ray diffraction. The structure consists of tris(nioximato)-bridged MnIVZnIIMnIV tetracations and non-co-ordinated perchlorate anions, with an intramolecular MnIV· · ·MnIV distance of 6.97 A. A crystal structure determination of MnIVCuIIMnIV 5 was not up to normal crystallographic standards. Nevertheless, the atom connectivities together with the heterotrinuclear nature with MnIV as the terminal ions are clear. The cyclic voltammograms of 1–6 reveal two reversible and two quasi-reversible one-electron redox processes, MnIIIMIIMnIII ⇌Ox1 MnIVMIIMnIII ⇌Ox2 MnIVMIIMnIV and MnIIIMIIMnIII ⇌Red1 MnIIMIIMnIII ⇌Red2 MnIIMIIMnII. 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, Ja operating between the adjacent metal centres and Jt between the terminal centres, separated by a large distance of ≈7 A. The effect of Jt on the splitting pattern has been shown by the variability of the ground states. That the assumption commonly made of no coupling between the terminal ions in trinuclear linear complexes may lead to a wrong spin ground state has been conclusively demonstrated. An analysis of the interacting magnetic orbitals in complexes containing three metal centres is presented.