Head-to-head dimers in the zwitterion of 1-hydroxy-1-phosphono-3-(1-piperidino)propylidene-1-phosphonate (PHPBP).

The title compound, C(8)H(19)NO(7)P(2), is a member of the bisphosphonate family of therapeutic compounds. PHPBP has inner-salt character, consisting of a negatively charged PO(3) group and a positively charged N atom. The six-membered piperidine ring adopts an almost-perfect chair conformation. The hydroxyl group and the N atom have gauche and trans conformations in relation to the O-C-C-C-N backbone, respectively. Hydrogen bonding is the main contributor to the packing in the crystal, which consists of head-to-head dimers formed through phosphonyl-phosphonyl hydrogen bonds, while O-H.O and N-H.O interactions join the dimers into a plane parallel to crystallographic b and c axes.