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 [M3(HL)2] and [M3(HL)3]3− with Zn2+ or [M2(H2L)2] with Ca2+ and Mg2+. 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.