Abstract
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, C i 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.
Highlights
The self-organization of molecules in order to form welldefined supramolecular structures under the influence of weak forces such as hydrogen bonds and van der Waals interaction is still being explored extensively
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–HÁÁÁN hydrogen bonds (ÁÁÁpyrÁÁÁacidÁÁÁpyrÁÁÁacidÁÁÁ) while for 2:1 stoichiometry the chains are made of pyrazine molecules and carboxylic acid centrosymmetric dimers (ÁÁÁpyrÁÁÁacidÁÁÁacidÁÁÁpyrÁÁÁ)
We present the crystallographic structures of hydrogen-bonded adducts obtained by the co-crystallization of pyrazine with dicarboxylic acids of the form (COOH)–(CH2)n–COOH (Scheme 1): oxalic (n = 0), Scheme 1 The studied co-crystals malonic (n = 1) succinic (n = 2), glutaric (n = 3), adipic (n = 4), and pimelic (n = 5)
Summary
The self-organization of molecules in order to form welldefined supramolecular structures under the influence of weak forces such as hydrogen bonds and van der Waals interaction is still being explored extensively. The parallels between such supramolecular synthesis and chemical, molecular synthesis were stressed by a number of authors Preparation of the co-crystals is the supramolecular synthesis: using the weak interactions to obtain new materials with properties different from those of the component molecules. For the reasons of this paper, we will use the term ‘‘cocrystal’’ in its widely accepted sense (e.g. as used by Stahly [10] ): as the solid which consists of two or more components that form a unique crystalline structure having unique properties
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