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'-dipyrid yl (dipy) [Zn-2(bpea)(2)(mu-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 s tructures of complexes 1 to 4 have been determined by X-ray crystallography . In complexes 1 and 3, Zn(II) coordinates in slightly distorted tetrahedra l 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 sol utions in water/DMF mixed solvent were determined by potentiometric titrati ons against aqueous NaOH solution. The pK(a) values are 8.35, 9.85, 8.00 an d 9.01 for complexes 1, 2, 3, and 5 respectively. The pK(a) value for the h ydroxo-bridged dimeric complex 1 is about 0.8 pH units greater than the pK( a) value (7.55) for the analogous complex [Zn-2(mu-OH) (m-xylbpea)](ClO4)(3 ) of the ligand bis(bis(2-pyridyl-2-ethyl) amino) m-xylene (m-xylbpea) in w hich 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 t o 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 an alogous 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 const ants 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 wit h the widely accepted mechanism in many hydrolytic reactions catalyzed by m etal complexes in which the metal-coordinated hydroxide is the reactive nuc leophile. 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. (C) 1999 Elsevier Science Inc. All rights reser ved.