SYNTHESIS, STRUCTURE, MAGNETISM AND ELECTROCHEMISTRY OF BINUCLEAR NICKEL(II) AND COPPER(II) COMPLEXES OF AN UNSYMMETRICAL PENTADENTATE SCHIFF-BASE LIGAND

A new unsymmetrical binucleating ligand, H3L, containing salicylidene and 1-methyl-3-oxobut-1-enyl end groups attached to a 1,3-diaminopropan-2-ol backbone, has been synthesized. Binuclear nickel(II) and copper(II) complexes of this ligand, of the type [M2L(pz)], containing a mu-pyrazolato (pz) exogenous bridging group and a mu-alkoxo endogenous bridge, have been synthesized and characterized. Cyclic voltammetric studies on dimethylformamide solutions of these complexes revealed well resolved waves attributable to sequential one-electron metal-centred reduction processes to yield the species [M2L(pz)]-and [M2L(pz)]2-. Comparison to published electrochemical data on the compounds of analogous symmetrical ligands allows identification of the metal-salicylidene end as the site of the first electron transfer. Magnetic susceptibility studies on powdered samples of [Cu2L(pz)] show medium-strength antiferromagnetic coupling (J12 = -129 cm-1) within the binuclear molecules and weaker antiferromagnetic coupling (J(intermol) = -41 cm-1) between the molecules. Intermolecular Ni ... Ni contact distances of 3.442(5) and 5.100(4) angstrom are evident in the crystal packing of [Ni2L(pz)]. Crystals of the latter have Z = 2 in a triclinic cell with space group P1BAR and dimensions a = 9.084(2), b = 12.185(4), c = 8.269(1) angstrom, alpha = 98.01 (2), beta = 100.55(1), gamma = 99.03(2)degrees; R = 0.087 based on 1283 reflections with I greater-than-or-equal-to 3.0sigma(I). The structure shows each nickel(II) ion to have a square-planar trans-N2O2 environment and the whole binuclear molecule to be essentially planar. The Ni(I)...Ni(2) distance is 3.212(4) angstrom.