Conformational richness and multiple Z′ in salt co-crystal of N-methylpiperidine betaine with N-methylpiperidine betaine hexafluorosilicate

The co-crystal structure of N-methylpiperidine betaine with N-methylpiperidine betaine hexafluorosilicate represents an unusual case of a salt co-crystal with a high Z' value (3), unexpected conformational variability, and with nearly 50% of its contents disordered. The betaine units from the salt and co-crystal formers are paired into several homoconjugated dimers via very short, linear O-center dot center dot center dot H+center dot center dot center dot O- bridges. These hydrogen bonds are the dominating interactions in the co-crystal structure, in variance with the simple hexafluorosilicate salt, which has a structure governed by COOH center dot center dot center dot F hydrogen bonds. The SiF62- anion in the co-crystal structure has only C-H center dot center dot center dot F interactions with the betaine units. The zwitterion:cation:anion stoichiometry is 3:3:1.5. Some of the betaine units display disorder, but each case is different. One of the SiF62- anions is ordered while possessing exact crystallographic symmetry. The other one is disordered in a general position. In addition, there are three water molecules in the crystal structure. One is fully ordered, one has an H atom disordered in two positions and the third one occupies two alternative positions with unequal populations.