Coordination abilities of piperyd-1-yl-methane-1,1-diphosphonic acids towards zinc(II), magnesium(II) and calcium(II): potentiometric and NMR studies.

The combination of the pH-metric and NMR studies is used to examine the stabilities and coordination modes as well as related structural aspects of zinc(II), magnesium(II) and calcium(II) complexation to piperyd-1-yl-methane-1,1-diphosphonic acid (1) and its derivatives containing a topologically modified piperidine ring (2-7). The studied compounds coordinate metal ions exclusively via the phosphonate functions with a nitrogen atom remaining protonated over the whole range of studied pH. Compounds 1-6 readily form soluble multinuclear complexes of type [M(3)(HL)(2)] and [M(3)(HL)(3)](3-) with Zn(2+) or [M(2)(H(2)L)(2)] with Ca(2+) and Mg(2+). These species are formed based on dimers consisting of two head-to-head arranged molecules linked by strong symmetrical hydrogen bonds. The placement of the two methyl groups at 2- and 6-positions on the piperidine ring precludes the molecular recognition via similar hydrogen bonds and accounts for different complexation properties of 7 compared to 1-6. The role that the metal coordination plays on conformation dynamics in 1-7 is also discussed.