Synthesis, structure and electrochemistry of tetranuclear oxygen-centered copper(II) clusters with acetylacetone and benz-pyrazole hydrolyzed derivatives as ligand

Two copper(II) clusters Cu₄OCl₆(pyrazole)₄ (1) and Cu₄OBr₆(Br-pyrazole)₄ (2) have been synthesized by reacting acetylacetone and benzohydrazide (1:1 ratio) with CuX₂ (X = Cl for 1 and X = Br for 2) in methanol solution. The structures of both clusters have been established by X-ray crystallography. The clusters contain four Cu, one O, six μ₂-X atoms, and four pyrazole ligands. The pyrazole was prepared in situ by the reaction of acetylacetone with benzohydrazide in methanol under reflux. In 2, the methine hydrogen of the pyrazole ligand has been replaced by bromine atom. The four copper atoms encapsulate the central O atom in a tetrahedral arrangement. All copper atoms are five-coordinate and have similar coordination environments with slightly distorted trigonal bipyramidal geometry. The cyclic voltammogram of the clusters 1 and 2 show a one-electron quasi-reversible reduction wave in the region 0.485 to 0.731 V, and a one-electron quasi-reversible oxidation wave in the region 0.767 to 0.898 V. In 1, one irreversible oxidative response is observed on the positive of side of the voltammogram at 1.512 V and this can be assigned to Cu(II) to Cu(III) oxidation.