Extended triple-bridged Ni(II)- and Co(II)-hydroxamate trinuclear complexes:: Synthesis, crystal structures, and magnetic properties

Crystal structures of new trinuclear complexes [Ni(3)(mu-OAC(F))(4)(mu-AA)(2)(tmen)(2)], [Ni(3)(mu-OAC(F))(4)(mu-BA)(2)(tmen)(2), and [Co(3)(mu-OAC(F))(4)(mu-BA)(2)(tmen)(2)] have been determined (OAC(F) = CF(3)COO(-), AA = acetohydroxamate anion, BA = benzohydroxamate, anion, tmen = N,N,N',N'-tetramethylethylenediamine). In each structure, the metal ions have distorted octahedral coordination and are triply bridged by one hydroxamate and two trifluoroacetate bridges. Magnetic properties of these compounds and of relative [Co(3)(mu-OAC(F))(4)(mu-AA)(2)(tmen)(2)] were studied by susceptibility and magnetization measurements. It was shown that for nickel trimers the intramolecular magnetic coupling is weak ferromagnetic in the case of the complex with the AA group, and there is nearly no coupling in the case with BA group. Rather large zero field splitting was obtained for the distorted octahedral environments of the terminal nickel ions. The cobalt trimers were additionally studied by magnetic circular dichroism (MCD) measurements. The exchange interaction of the cobalt complexes is antiferromagnetic.