Synthesis and characterization of tetrasodium salt of ethylenediphosphinetetraacetic acid, the phosphorus analogue of EDTA

Abstract

The phosphorus analogue of EDTA was prepared by alkylation of Cl2PCH2CH2PCl2 with ethyl bromozincacetate and hydrolysis of the ester formed. It was isolated as crystalline tetrasodium salt monohydrate, (NaOOCCH2)2PCH2CH2P(CH2CoONa)2 . H2O, stable on air and well soluble in water. The free acid was only obtained in solution. The structure of the new ligand was checked by spectral methods (UV, IR, and 1H, 13C and 31P-NMR). The acid behaves in solution as tetrabasic with relatively close dissociation constants in the range of pK 2.5-6.0, corresponding to protons bonded at the carboxyl groups. Unlike EDTA, the phosphorus atoms protonize only at pH 0, and, consequently, the ligand does not form zwitterions.