Bis(alquilaril, aril)piridina-2,6-dicarboxiamidas: estrutura molecular, supramolecular e polimorfismo

Abstract

This work presents the study of intermolecular interactions and the molecular association process of bis (alkylaryl, aryl) pyridine-2,6-dicarboxamides, where the alkylaryl substitutes are (1) 2,2-diphenylethyl, (2) phenylethyl and (3) benzyl, and the aryl substitutes are (4) 4-fluorophenyl, (5) 4-chlorophenyl and (6) 4-bromophenyl, as well as the compound bis(2-2,diphenylethyl)isophthalamide (7). The supramolecular cluster, consisting of a central molecule and the molecules that make up the first sphere of molecular coordination, was used as a study demarcation. The pyridine-2,6-dicarboxamides 1-6 adopted a curved conformation, with the N-H groups facing inwards, while compound 7 presented a linear conformation. Bis(2,2-diphenylethyl)pyridine-2,6-dicarboxamide (1) crystallized as three conformational polymorphs. X-ray diffraction data showed that compound 2 is also a hydrate. The molecular overlays of the central part (CH – CH2 – NH – C (O) –2 – py – 6 – C (O) –NH – CH2 – CH) between the polymorphs of compound 1 indicated a greater molecular similarity between 1II and 1III. Calculations of the molecular stabilization energy carried out for polymorphs 1I-III indicated that polymorph 1II has a conformation of almost 22 Kcal mol-1 less stable than polymorph 1I, while polymorph 1III is 6 Kcal mol-1 less stable than polymorph 1I. Analyzing supramolecular stabilization energy data for the polymorphs, the highest total stabilization energy value was presented for compound 1II. Regarding the polymorphs, a crystalline packaging efficiency of 0.864 was observed for compound 1I and 0.874 for compound 1II. Compound 1III has the lowest packaging efficiency (0.811). According to the crystallization mechanisms proposed for compounds 1I-II and 3-6, there is a preference for stacking through amides, which form hydrogen bonds. Compounds 1III and 7 form blocks with preferential growth in two directions. Compound 2 forms dimers in the first crystallization stage, presenting the sum of the normalized energy contributions and contact area (NCG%) equal to 19 in this stage. Compound 7 stands out for presenting NCG% = 78 in the first stage. For compounds 1I-III and 3-6, the first crystallization stage has NCG% around 50. The alkylaryl and aryl substitutes are relevant at the end of the crystallization process, where the geometric parameter usually dominates. The ¹H NMR experiments in solution with concentration variation performed for compounds 1 and 7 detected intermolecular interactions N-H ∙∙∙ O = C and C-H ∙∙∙ π, which can be correlated with the respective crystallization mechanisms. From the analysis of the topological and energetic data, the formation of hydrates 1I-III and 2 was attributed to the presence of the amide groups together with the pyridinic nitrogen, interacting with the solvent molecule through hydrogen bonds. The occurrence of polymorphs for compound 1 was attributed to the possibility of different conformations for the molecule, enabling the formation of different intermolecular interactions. For compounds 1I-III and 2, it was observed that the water is positioned in the cavity formed by the other molecules and, therefore, they should not be evaluated in isolation.