Abstract
Four ligands, which can be considered as di‐ [both symmetric (L^1^) and asymmetric (L^2^)], tri‐ (L^3^), and tetraacetate (L^4^) derivatives of bis(aminomethyl)phosphinic acid (L^0^) have been synthesized and their complexation equilibria involving Mg^2+^, Ca^2+^, Ni^2+^, Cu^2+^, Zn^2+^, Cd^2+^, and Pb^2+^ have been studied by pH potentiometry. The first two protonation constants of the ligands are lower than those of the analogous diaminopolycarboxylates due to the electron‐withdrawing effect of the phosphinate group. The asymmetric ligands L^2^ and L^3^ and their complexes are protonated first at the primary (NH~2~) and secondary (NH) amine groups, respectively. The ligands L^0^, L^1^, L^2^, and L^3^ form the species ML, MLH, ML~2~, and ML~2~H whereas heptadentate L^4^ forms very stable ML complexes, protonated (MLH and MLH~2~) species and, unlike the other ligands, it also forms dinuclear M~2~L complexes. The stability constants of the complexes of L^0^, L^1^, L^2^, and L^3^ are generally lower, while those of the L^4^ are similar or higher, than those of the analogous diaminocarboxylate derivatives. The phosphinate group is not directly coordinated to the metal ions but may contribute to the stability by increasing the electrostatic interaction, which is significant for the complexes of L^4^. The ^1^H NMR spectra of complexes ZnL^2^, ZnL^3^, and ZnL^4^ show an AB multiplet pattern for the protons of the iminodiacetate moiety of the ligands, which indicates the long half‐life of the Zn^2+^–N bond and the rigid structure of this segment of the complexes. The solid‐state structure of the ligand [H~5~L^3^]Cl and complex K[CuL^1^]·H~2~O have been determined by X‐ray diffraction techniques. The arrangement of the ligand L^1^ is square planar around Cu^2+^ with two nitrogen and carboxylate oxygen donor atoms, although the axial proximity of a carboxylate and a phosphinate oxygen atom in two neighboring molecules results in a distorted octahedral coordination.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)