Linear trinuclear oximato-bridged complexes Mn^III,IVM^IIMn^III,IV (M = Zn, Cu or Mn): Synthesis, structure, redox behaviour and magnetism

Six trinuclear linear complexes [LMn^III{μ-(niox)₃M^II}Mn^IIIL] ²⁺ 1-3 and [LMn^IV{μ-(niox)₃M^II}Mn^IVL] 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 M^IIN₆ chromophore. The molecular structure of the compound [LMn^IV{μ-(niox)₃Zn^II}Mn ^IVL][ClO₄]₄ 4 has been established by X-ray diffraction. The structure consists of tris(nioximato)-bridged Mn^IVZn^IIMn^IV tetracations and non-co-ordinated perchlorate anions, with an intramolecular Mn^IV⋯Mn^IV distance of 6.97 Å. A crystal structure determination of Mn^IVCu^IIMn^IV 5 was not up to normal crystallographic 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-reversible one-electron redox processes, Mn^IIIM^IIMn^III⇌^Ox1Mn ^IVM^IIMn^III⇌^Ox2Mn ^IVM^IIMn^IV and Mn^IIIM^IIMn^III⇌^Red1Mn ^IIM^IIMn^III⇌^Red2Mn ^IIM^IIMn^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 between the adjacent metal centres and J_t between the terminal centres, separated by a large distance of ≈7 Å. 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 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.