Complexation Properties of the Di-, Tri-, and Tetraacetate Derivatives of Bis(aminomethyl)phosphinic Acid

Four ligands, which can be considered as di‐ [both symmetric (L1) and asymmetric (L2)], tri‐ (L3), and tetraacetate (L4) derivatives of bis(aminomethyl)phosphinic acid (L0) have been synthesized and their complexation equilibria involving Mg2+, Ca2+, Ni2+, Cu2+, Zn2+, Cd2+, and Pb2+ 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 L2 and L3 and their complexes are protonated first at the primary (NH2) and secondary (NH) amine groups, respectively. The ligands L0, L1, L2, and L3 form the species ML, MLH, ML2, and ML2H whereas heptadentate L4 forms very stable ML complexes, protonated (MLH and MLH2) species and, unlike the other ligands, it also forms dinuclear M2L complexes. The stability constants of the complexes of L0, L1, L2, and L3 are generally lower, while those of the L4 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 L4. The 1H NMR spectra of complexes ZnL2, ZnL3, and ZnL4 show an AB multiplet pattern for the protons of the iminodiacetate moiety of the ligands, which indicates the long half‐life of the Zn2+–N bond and the rigid structure of this segment of the complexes. The solid‐state structure of the ligand [H5L3]Cl and complex K[CuL1]·H2O have been determined by X‐ray diffraction techniques. The arrangement of the ligand L1 is square planar around Cu2+ 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)