Aminoalkylbis(phosphonates): Their complexation properties in solution and in the solid state

Five geminal bis(phosphonates) containing an amino group in the alpha, gamma or delta position of the carbon chain have been synthesised and their acid-base and complexation properties with Cu2+, Zn2+, Ca2+, Mg2+ and Na+ ions studied by potentiometry. Determination of the stability constants of the complexes with divalent metal ions is complicated by the formation of precipitates. The results of these studies show a negligible effect of the hydroxo group in the a position on the acid-base and complexation properties of the ligands. The presence of an amino group in the alpha position, however, decreases the basicity, which results in a lower complexation ability of the bis(phosphonates). The crystal structures of the three ligands with different degrees of protonation have also been determined. The structure of the triammonium salt of aminomethylbis(phosphonic acid) shows that the proton is bound to the nitrogen atom in monoprotonated species. The structure of the Cu2+ complex of pamidronate [{Cu-2(H(2)pam)(2)}center dot H2O](n) shows the presence of dimeric units with a relatively short distance (2.99 angstrom) between the metal centres. These units form a coordination polymeric chain. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)