C–H group acidity and the nature of C–H···N interactions: crystal structural analysis of pyrazine and methyl substituted pyrazines

Abstract

Crystal structures of methyl-, 2,3-, 2,5- and 2,6-dimethyl-, and trimethylpyrazine (2–6) have been determined by X-ray diffraction analysis. Four of these compounds (2–4 and 6) are liquids and their single crystals have been grown in situ on the diffractometer using a miniature zone melting procedure. These structures are analysed together with those of pyrazine (1) and tetramethylpyrazine (7) in the context of C–H···N interactions. Compounds 1–7 were chosen because they contain only C, H and N atoms, and also C–H groups of variable acidity. This facilitates the comparison of C–H···N geometries with respect to the C–H group acidity. Compounds 1 and 2 are dominated by sp² C–H groups in their molecules and their supramolecular structures are generated by sp² C–H···N. On the other hand the domination of sp³ C–H groups at the molecular level in 6 and 7 results in crystal structures that are governed by sp³ C–H···N. A balance of sp² and sp³ C–H groups in the dimethyl derivatives 3–5 leads to a situation where both kinds of C–H groups play a structure directing role. While the sp² C–H groups form C–H···N, the sp³ C–H groups are involved in C–H···π interactions. Thus the C–H group interactions follow a property unique to hydrogen bonds—stronger donors interact with stronger acceptors while weaker donors approaching weaker acceptors. Inter-structural comparison revealed that H···N distances decrease with increasing C–H acidity and therefore C–H···N interactions could be considered as weak hydrogen bonds.