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

The title compound, C₈H₁₉NO₇P₂, is a member of the bis­phosphonate family of therapeutic compounds. PHPBP has inner‐salt character, consisting of a negatively charged PO₃ 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.