Synthesis, structure, magnetism and electrochemistry of binuclear nickel(II) and copper(II) complexes of an unsymmetrical pentadentate Schiff-base ligand

Abstract

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 µ-pyrazolato (pz) exogenous bridging group and a µ-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)Å are evident in the crystal packing of [Ni₂L(pz)]. Crystals of the latter have Z= 2 in a triclinic cell with space group P and dimensions a= 9.084(2), b= 12.185(4), c= 8.269(1)Å, α= 98.01(2), β= 100.55(1), γ= 99.03(2)°; R= 0.087 based on 1283 reflections with l 3.0σ(l). 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(1)⋯ Ni(2) distance is 3.212(4)Å.