Crystal Structures of the Hexafluoridophosphate Salts of the Isomeric 2-, 3-and 4-Cyano-1-Methylpyridinium Cations and Determination of Solid-State Interaction Energies
Materials Research Institute, Pennsylvania State University, National Science Foundation
Department of Chemistry
The synthesis and crystal structures of the isomeric molecular salts 2-, 3-and 4-cyano-1-methylpyridinium hexafluoridophosphate, C7 H7 N2+ ·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  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.
Mague, J. T.; Larrabee, E.; Olivier, D.; Vaccaro, F.; and Riley, K E., "Crystal Structures of the Hexafluoridophosphate Salts of the Isomeric 2-, 3-and 4-Cyano-1-Methylpyridinium Cations and Determination of Solid-State Interaction Energies" (2018). Faculty and Staff Publications. 102.