Even–odd effect in the co-crystals of pyrazine and dicarboxylic acids

The co-crystals of pyrazine with dicarboxylic acids of the form (COOH)–(CH2)n–COOH: oxalic (n = 0), malonic (n = 1) succinic (n = 2), glutaric (n = 3), adipic (n = 4), and pimelic (n = 5) acid, were synthesized and structurally characterized by means of X-ray diffraction. There are two principal structural motifs, depending on the stoichiometry: for 1:1 co-crystals, there are chains of alternate pyrazine and acid molecules, connected by strong O–H···N hydrogen bonds (···pyr···acid···pyr···acid···) while for 2:1 stoichiometry the chains are made of pyrazine molecules and carboxylic acid centrosymmetric dimers (···pyr···acid···acid···pyr···). The first possibility is observed for all acids in the series, while the second one only for these with odd numbers of methylene groups. The geometrical features of the acids do not show the clear indication for a reason of such behavior, the most probable reason can be connected with the possible symmetry of the acid molecule, Ci for n even, and with the mutual orientation of terminal carbonyl and hydroxyl groups, defined by improper O=C···C=O torsion angles, which are close to 180° for even and have much smaller values for odd n values.