Crystal Structures of the Hexafluoridophosphate Salts of the Isomeric 2-, 3-and 4-Cyano-1-Methylpyridinium Cations and Determination of Solid-State Interaction Energies

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Materials Research Institute, Pennsylvania State University, National Science Foundation

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Department of Chemistry

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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 [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.


DOI: 10.1107/S2056989018011003

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The support of NSF–MRI grant No. 1228232 for the purchase of the diffractometer and Tulane University for support of the Tulane Crystallography Laboratory are gratefully acknowledged. LVK acknowledges generous support from the Earl and Gertrude Vicknair Distinguished Professorship in Chemistry at Loyola University.