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

Abstract

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) [Zn 2(bpea) 2(μ-OH)](ClO 4) 3 ( 1), [Zn(bpa)(H 2O) 2](ClO 4) 2 ( 2), [Zn(dipyam) 2](BF 4) 2 ( 3), [Zn(dipyam) 2(DMF) 2] (ClO 4) 2 (4) and [Zn(dipy) 2(H 2O)] (BF 4) 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 p K 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 p K a values are 8.35, 9.85, 8.00 and 9.01 for complexes 1, 2, 3, and 5 respectively. The p K a value for the hydroxo-bridged dimeric complex 1 is about 0.8 pH units greater than the p K a value (7.55) for the analogous complex [Zn 2(μ-OH)( m-xylbpea)](ClO 4) 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 p K 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 p K 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 constants 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.