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

The synthesis and crystal structures of the isomeric mol­ecular salts 2-, 3- and 4-cyano-1-methyl­pyridinium hexa­fluorido­phosphate, C(7)H(7)N(2) (+)·PF(6) (−), 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.