Syntheses, characterization and catalytic properties of Zn(II) and Ni(II) complexes: Modeling the active sites of zinc and nickel enzymes

The Zn(II) complexes of the ligands bis(2-pyridylmethyl)amine (bpma) [Zn(bpma)(H2O)2](ClO4)2] (1) and that of bis-(2-pyridyl-2-ethyl)amine (bpea) [Zn 2( m -OH)(bpeah)2](ClO4)3] (2) have been synthesized and characterized. The structures of both zinc complexes have been determined by X-ray crystallography. Zn(II) ions are coordinated in a distorted octahedral geometry in complex 1 and in a slightly distorted tetrahedral geometry in complex 2. In water/DMF (3/5, v/v) solution, the coordinated waters in complexes 1 and 2 have pKa values of 8.35 +/- 0.02 and 9.85 +/- 0.02 respectively. The pKa value for the hydroxo-bridged dimeric complex 2 is about 0.8 pH units greater than the pKa value (7.55) for the analogous Zn(II) complex of the ligand bis(bis(2-pyridyl-2-ethyl)amino)m-xylene (m-xyl-bpea) [Zn2( m -OH)(m-xyl-bpea)] (ClO4)3 (4) in which the two chelating bis(2-pyridyl-2-ethyl)amine arms are tethered by a m-xylene group. However, in the hydrolysis of bis(p-nitrophenyl)phosphate (BNPP) catalyzed by Zn(II) complexes, the observed rate constant above pH 8 for complex 2 is significantly higher than that shown by the analogous complex 4 formed by the dinucleating ligand. The rate constants are observed to increase and pass through a maximum at or around the pKa values of both the two zinc complexes reported here. This is consistent with the widely accepted mechanism in many hydrolytic reactions catalyzed by metal complexes in which the metal-coordinated hydroxide is the reactive nucleophile.;A dinuclear complex of the ligand bis(bis(2-(2-pyridyl2-ethyl)amino)methyl)phenol (m-xyl-bpeap) [Ni(II)2(m-xyl-bpeap)( m -OH)](ClO4)2 (3) has been synthesized and characterized. The structure of the cation of complex 3 shows two doubly bridged, square pyramidal Ni(II) ions. The UV-Visible absorption spectrum shows maxima at 362, 607 and 820 mn. The room temperature magnetic moment, corrected for diamagnetism, is 2.67 BM indicating an antiferromagnetic coupling of the two Ni(II) ions. The nickel-coordinated water molecule in water/DMF (3/5, v/v) solution of the complex 3 has a pK a value of 8.50 +/- 0.03 which coincides with the pH of the maximum catalytic efficiency of the complex 3 for the hydrolysis of BNPP. Cyclic voltammetry in dry dimethylsulfoxide solution of complex 3 showed irreversible reduction peaks at -1.25 V and -1.45 V (vs. Ag/AgCl) and also irreversible oxidation waves at +0.13 V and +0.87 V (vs. Ag/AgCl).