Crystal structures of the hexafluoridophosphate salts of the isomeric 2-, 3- and 4-cyano-1-methylpyridinium cations and determination of solid-state interaction energies

Abstract

The synthesis and crystal structures of the isomeric molecular salts 2-, 3- and 4-cyano-1-methylpyridinium hexafluoridophosphate, C7H7N2 +·PF6 −, are reported. In 2-cyano-1-methylpyridinium hexafluoridophosphate, C—H...F hydrogen bonds form chains extending along the c-axis direction, which are associated through C—H...F hydrogen bonds and P—F...π(ring) interactions into stepped layers. For 3-cyano-1-methylpyridinium hexafluoridophosphate, corrugated sheets parallel to [001] are generated by C—H...F hydrogen bonds and P—F...π(ring) interactions. The sheets are weakly associated by a weak interaction of the cyano group with the six-membered ring of the cation. In 4-cyano-1-methylpyridinium hexafluoridophosphate, C—H...F hydrogen bonds form a more open three-dimensional network in which stacks of cations and of anions are aligned with the b-axis direction. Dispersion-corrected density functional theory (DFT-D) calculations were carried out in order to elucidate some of the energetic aspects of the solid-state structures. The results indicate that the distribution of charge within a molecular ionic cation can play a large role in determining the strength of a cation–anion interaction within a crystal structure. Crystals of 2-cyano-1-methylpyridinium hexafluoridophosphate are twinned by a 180° rotation about the c* axis. The anion in 3-cyano-1-methylpyridinium hexafluoridophosphate is rotationally disordered by 38.2 (1)° in an 0.848 (3):0.152 (3) ratio.