Aminomethylenephosphinic acids and their complexing properties

Abstract

A study of the dependence of the ¹H, ³¹P and ¹³C NMR spectra of D₂O solutions of piperidinomethylphosphinic acid (HL¹) and piperazine-1,4-diylbis(methylene)bis(phosphinic acid)(H₂L²) on the pH has indicated a strong acidity of the phosphinic groups and a low basicity of the nitrogen atoms. It is confirmed that the acids from zwitterions in aqueous solution and that the HL¹ ring behaves similarly to the N-methylpiperidine ring. The stability constants of complexes formed with Mg²⁺, Ca²⁺, Pb²⁺, Co²⁺, Ni²⁺, Cu²⁺, Cd²⁺ and Zn²⁺ are lower than those of their acetic or phosphonic acid analogues. In the solid state, HL¹·H₂O exists as a zwitterion, monoclinic, space group P2₁/n, a= 6.884(2), b= 11.485(2), c= 11.994(2)Å and β= 97.87(2)°. Two molecules are linked by hydrogen bonds into dimers which are connected by water molecules to form endless chains.