Synthesis of new 'end-off' μ-phenoxo and bis-μ-acetato tri-bridged copper(II), nickel(II) and zinc(II) complexes:: Spectral, magnetic, electrochemical and catalytic studies

A new end-off type acyclic ligand with two N-methyl piperazine arms, 2,6-bis[(4-methyl piperazin-1-yl)]-4-formyl phenol [L], has been synthesized by a simple Mannich reaction. The mono and binuclear complexes of Cu(II), Ni(II) and Zn(II) have been prepared. The complexes were characterized by elemental and spectral analysis. The EPR spectrum of the mononuclear copper complex shows four hyperfine splittings and the binuclear complex shows a broad signal due to an anti-ferromagnetic interaction. The room temperature magnetic moment of the mono and binuclear copper complexes are found to be 1.72 mu(B) (mu(eff)) and 1.58 mu(B) (mu(eff)), respectively. The mononuclear Ni(II) complex is square planar and diamagnetic, whereas the six-coordinated binuclear Ni(II) complex shows a magnetic moment value of 2.98 mu(B) (mu(eff)). Cyclic voltammetric studies evidenced one irreversible reduction wave for all the mononuclear complexes and two irreversible one-electron reduction waves for the binuclear complexes are obtained in the cathodic region. In the anodic region, one irreversible oxidation wave for the mononuclear nickel(II) complex and two irreversible oxidation waves for the binuclear nickel(II) complexes are obtained. Kinetic studies on the oxidation of pyrocatechol to o-quinone using the mono and binuclear copper(II) complexes as catalysts and on the hydrolysis of 4-nitrophenylphosphate using the mono and binuclear copper(II), nickel(II) and zinc(II) complexes as catalysts were carried out. The binuclear complexes have higher rate constant values than those of the corresponding mononuclear complexes. The rate constant values for the complexes for the hydrolysis are in the order nickel(II) > copper(II) > zinc(II). (C) 2006 Elsevier Ltd. All rights reserved.