The synthesis and crystal structures of the isomeric mol-ecular salts 2-, 3- and 4-cyano-1-methyl-pyridinium hexa-fluorido-phosphate, C7H7N2+·PF6-, are reported. In 2-cyano-1-methyl-pyridinium hexa-fluorido-phosphate, 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) inter-actions into stepped layers. For 3-cyano-1-methyl-pyridinium hexa-fluorido-phosphate, corrugated sheets parallel to [001] are generated by C-H⋯F hydrogen bonds and P-F⋯π(ring) inter-actions. The sheets are weakly associated by a weak inter-action of the cyano group with the six-membered ring of the cation. In 4-cyano-1-methyl-pyridinium hexa-fluorido-phosphate, 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 mol-ecular ionic cation can play a large role in determining the strength of a cation-anion inter-action within a crystal structure. Crystals of 2-cyano-1-methyl-pyridinium hexa-fluorido-phosphate are twinned by a 180° rotation about the c* axis. The anion in 3-cyano-1-methyl-pyridinium hexa-fluorido-phosphate is rotationally disordered by 38.2 (1)° in an 0.848 (3):0.152 (3) ratio.
Keywords: DFT calculations; crystal structure; cyanopyridinium salts; hexafluoridophosphate; hydrogen bonds.