Kinetics of Bis(p-nitrophenyl)phosphate (BNPP) Hydrolysis Reactions with Trivalent Lanthanide Complexes of N-Hydroxyethyl(ethylenediamine)-N,N ', N '-triacetate (HEDTA)

Abstract

Kinetic studies of hydrolysis reactions of BNPP [sodium bis(p-nitrophenyl) phosphate] with trivalent lanthanide (Ln(3+)) complexes of HEDTA [HEDTA = N-hydroxyethyl(ethylenediamine)-N,N',N'-triacetate] were performed at pH 6.96-11.34 and 25 degrees C by a spectrophotometric method and by HPLC analysis. The reaction rates increase with increasing atomic number of lanthanide and solution pH from PrHEDTA to EuHEDTA and then decrease for heavier LnHEDTA complexes. Plots of pseudo-first-order rate constants (k(obs)) vs. pH could be fitted to the equation k(obs) = k(LnL(OH))[LnL](T)/{1+exp-[-2.303(pH-pK(h))]} where K(LnL(OH)) is the rate constant for the reaction of LnHEDTA(OH)(-) with BNPP, K(h) is the hydrolysis constant of LnHEDTA, and [LnL](T) is the total concentration of LnHEDTA. The pK(h) values obtained by the kinetic method are in the range 8.2-10.3 and are similar to those measured by potentiometric methods. At [LnL](T) = 10-70 mM and pH 10.5, most of the observed pseudo-first-order rate constants could be fitted to a simple saturation kinetic model, k(obs) = K(1)K[LnHEDTA(OH)(-)]/[1 + K[LnHEDTA(OH)(-)]}, where K is the equilibrium constant for the formation for LnHEDTA(OH)(-)BNPP and is in the range 2-147 M(-1). The k(1) values are in the range 1.12X10(-5)-2.71 X 10(-3) s(-1). The k(obs) data for TbHEDTA and HoHEDTA were fitted to a quadratic equation. It was observed that the dinuclear species are more reactive. ESI mass spectrometry confirmed that the reaction between BNPP and EuHEDTA is a simple hydrolysis but not a transesterification, presumably because the three inner-sphere coordinated water molecules are far away from the coordinated hydroxyethyl group. Hydrolysis is likely to occur by proton transfer from one inner-sphere coordinated water molecule to the deprotonated ethyl oxide group followed by nucleophilic attack of the resulting hydroxide ion on the bonded BNPP anion, ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)