Abstract
Strategies for co-crystal synthesis tend to employ either hydrogen- or halogen-bonds between different molecules. However, when both interactions are present, the structural influence that they may exert on the resulting assembly is difficult to predict a priori. To shed some light on this supramolecular challenge, we attempted to co-crystallize ten aliphatic dicarboxylic acids (co-formers) with three groups of target molecules; N-(pyridin-2-yl)picolinamides (2Pyr-X), N-(pyridin-2-yl)nicotinamides (3Pyr-X), N-(pyridin-2-yl)isonicotinamides (4Pyr-X); X=Cl/ Br/ I. The structural outcomes were compared with co-crystals prepared from the non-halogenated targets. As expected, none of the reactions with 2Pyr-X produced co-crystals due to the presence of a very stable intramolecular N-H···N hydrogen bond. In the 3Pyr series, all six structures obtained showed the same synthons, –COOH···N(py) and –COOH···N(py)-NH, that were found in the non-halogenated parent 3Pyr and were additionally accompanied by structure directing X···O(OH) interactions (X=Br/I). The co-crystals of the unhalogenated parent 4Pyr co-crystals assembled via intermolecular –COOH···N(py) and –COOH···N(py)-NH synthons. Three of the analogues 4Pyr-X co-crystals displayed only COOH···N(py) and –COOH···N(py)-NH interactions. The three co-crystals of 4Pyr-X with fumaric acid (for which no analogues structures with 4Pyr are known) formed –COOH···N(py)-NH and –NH···O=C hydrogen bonds and showed no structure-directing halogen bonds. In three co-crystals of 4Pyr-I in which –COOH···N(py)-NH hydrogen bond was present, a halogen-bond based –I···N(py) synthon replaced the –COOH···N(py) motif observed in the parent structures. The structural influence of the halogen atoms increased in the order of Cl < Br < I, as the size of σ-holes increased. Finally, it is noteworthy that isostructurality among structures of the homomeric targets was not translated to structural similarities between their respective co-crystals.
Highlights
Practical crystal engineering is driven by the use of intermolecular forces, primarily hydrogen-bonding [1], for connecting discrete molecular building blocks or coordination complexes into extended crystalline architectures [2,3,4,5]
In order to learn more about competing molecular-recognition events in supramolecular assembly, we previously examined the outcomes of co-crystallization of N-(pyridin-2-yl)nicotinamide and about competing molecular-recognition events in supramolecular assembly, we previously examined the outcomes of co-crystallization of N-(pyridin-2-yl)nicotinami2doef 1a5nd N
No co-crystals were obtained with the 2Pyr series since the omnipresent intramolecular hydrogen bonding prevented any opportunities for co-crystal synthesis via intermolecular interactions with any co-former
Summary
Practical crystal engineering is driven by the use of intermolecular forces, primarily hydrogen-bonding [1], for connecting discrete molecular building blocks or coordination complexes into extended crystalline architectures [2,3,4,5]. A “second-phase” of research in this area has added (i) synthetic complexity by taking advantage of additional structuredirecting interactions such as halogen [6] and chalcogen bonds [7] and (ii) compositional complexity by building multi-component architectures such as binary, ternary [8,9,10], and higher-order co-crystals [11]. Most of these efforts have been in the area of basic science, with the goal of establishing guidelines for how different molecules and functional groups recognize and bind to each other via non-covalent interactions [12]. Fbounrtdhinergmore, wesiatelsoonwthaenstuepdratmo oelsetcaublalirsfhraimf eitwiosrkreianstohneasbamleetsoethoyfptaortgheetsciozme pthoautnddsi.ffFeurretnhceerms obreet,ween crywstealaslstoruwctaunrtesdotfoheasltoabgleisnhatief ditains drenasoonn-ahballeogtoenhaytpeodthheosmizeomtheartidciaffsesreemncbelsiebsewtwoeuelnd lead to scirmysitlaalrstdriufcfteurreenscoefshianlocgoe-ncartyesdtaalnsdonfotnh-ehaslaomgeensaetetdohf otamrogmetesr.icTahsesewmobrlkiespwreosuelndteledadherein utiltiozseidmailasrydstifefmereanticcesstirnuccot-ucrraylstsatlusdoyf tohfe csoam-creyssettaolsf toafrgcelotss.eTlyherewlaotrekdprceosmenpteodunhedrse,inFigure 1, iuntoilirzdeedratosylsetaermnamticosrteruacbtuoruatl tshtuedbyaolafncoce-cbryesttwaleseonf ccloomselpyerteinlagteidntceormapctoiuonndssi,nFhigeutreero1,meric moimlneocoluredclauerrlaatroraclrehcaihtrientcemtcutourrreeessa..bout the balance between competing interactions in heteromeric
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