Phosphate ester hydrolysis by yttrium(iii) complexes with Bis-Tris propane and Tris ligands†

Abstract

Unusual second-order, in metal, kinetics were observed for the hydrolysis of bis(4-nitrophenyl) phosphate (BNPP) and 4-nitrophenyl diphenylphosphate (NPDPP) catalyzed by Y(III) in the presence of Bis-Tris propane (BTP) and Tris ligands at 25 °C in weakly basic aqueous solutions. Potentiometric and ¹H-NMR titrations of BTP in the presence of Y(III) indicate formation of the mononuclear Y(BTP)³⁺ complex and dinuclear hydroxo complexes Y₂(BTP)(OH)_n ^6 − n where n = 2, 4, 5 or 6. Titrations of Tris in the presence of Y(III) were limited by the low stability of the system and were fitted to a tentative model involving formation of Y(Tris)³⁺ and a single dinuclear complex Y₂(Tris)₂(OH)₅ ⁺. Comparison of concentration and pH-dependences of the reaction rates with the species distribution diagrams show that the catalytic hydrolysis of BNPP involves a simultaneous interaction of the substrate with Y₂(BTP)(OH)₄ ²⁺ and Y₂(BTP)(OH)₅ ⁺ species in the Y(III)/BTP system and interaction with two Y₂(Tris)₂(OH)₅ ⁺ species in the Y(III)/Tris system. The half-life for the hydrolysis of BNPP is only ca. 5 min at 25 °C and pH 8.5 in the presence of 4 mM Y(III) and 20 mM BTP. A rate enhancement by Y(III) in the hydrolysis of triester NPDDP is much smaller than that for diester BNPP. The hydrolysis of mono 4-nitrophenyl phosphate, the intermediate in BNPP hydrolysis, is zero-order in Y(III) indicating Michaelis–Menten type “saturation” kinetics.