A crystallographic and thermal study of pridinol mesylate and its monohydrated solvate

Herein are reported the crystal and molecular structures of the pridinol mesylate salt (C₂₀H₂₅NO⁺·CH₃O₃S^-) (I) and its monohydrated solvate form (C₂₀H₂₅NO⁺·CH₃O₃S^-·H₂O) (II). A comparison of both with the already reported structure of pure pridinol [1,1-diphenyl-3-piperidino-1-propanol, C₂₀H₂₅NO; Tacke et al. (1980). Chem. Ber. 113, 1962-1980] is made. Molecular structures (I) and (II) are alike in bond distances and bond angles, but differ in their spatial conformation, and, more relevant still, in their hydrogen-bonding motifs. This gives rise to quite different packing schemes, in the form of simple dimers in (I) but water-mediated hydrogen-bonded chains in (II). The dehydration behaviour of form (II) is highly dependent on the heating rate, with slow rates leading to a clear endothermic dehydration step, towards anhydrous (I), with subsequent melting of this latter phase. Increased heating rates result in a more unclear behaviour ending in a structural collapse (melting of the hydrated phase), at temperatures significantly lower than the melting point of the anhydrous phase. The eventual relevance of the water link in the structure of (II) is discussed in regard to this behaviour.