Syntheses and structures of and catalysis of hydrolysis by Zn(II) complexes of chelating pyridyl donor ligands

Zn(II) complexes of the ligands bis (2-pyridyl-2-ethyl) amine (bpea), bis(2-pyridylmethyl) amine (bpa), 2,2'-dipyridylamine (dipyam) and 2,2'- dipyridyl (dipy) [Zn2(bpea)2(μ-OH)](ClO4)3 (1), [Zn(bpa)(H2O)2](ClO4)2 (2), [Zn(dipyam)2](BF4)2 (3), [Zn(dipyam)2(DMF)2] (ClO4)2 (4) and [Zn(dipy)2(H2O)] (BF4)2 (5) have been prepared and characterized. The structures of complexes 1 to 4 have been determined by X-ray crystallography. In complexes 1 and 3, Zn(II) coordinates in slightly distorted tetrahedral geometry and in complexes 2 and 4 in a distorted octahedral geometry. The number and the pK(a) values of the zinc- coordinated water molecules in solutions in water/DMF mixed solvent were determined by potentiometric titrations against aqueous NaOH solution. The pK(a) values are 8.35, 9.85, 8.00 and 9.01 for complexes 1, 2, 3, and 5 respectively. The pK(a) value for the hydroxo-bridged dimeric complex 1 is about 0.8 pH units greater than the pK(a) value (7.55) for the analogous complex [Zn2(μ-OH)(m-xylbpea)](ClO4)3 of the ligand bis(bis(2-pyridyl-2- ethyl)amino) m-xylene (m-xylbpea) in which the two chelating bis (2-pyridyl- 2-ethyl) amine arms are tethered by a m-xylyl group. The lower pK(a) value in the latter complex is attributed to the entropic advantage gained as the two Zn(II) ions are held together by the dinucleating ligand. However, in the hydrolysis of bis (p-nitrophenyl) phosphate (BNPP) catalyzed by Zn (II) complexes, the observed rate constant above pH 8 for complex 1 is significantly higher than that shown by the analogous complex formed by the dinucleating ligand. This difference for the latter complex may be due to the formation of a catalytically less active, possibly a double hydroxy-bridged moiety at lower pH values. The rate constants are observed to increase and pass through a maximum at or around the pK(a) values of all the 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. Second-order rate constant of the order of 10-3-10-2 M-1 s-1 were calculated for the hydrolysis of BNPP at the optimum pH values for the complexes studied here.