The 2-aminopyridinium-carboxylate supramolecular heterosynthon: A robust motif for generation of multiple-component crystals

An analysis of the Cambridge Structural Database reveals that 77% of compounds that contain both 2-aminopyridine and carboxylic acid moieties generate 2-aminopyridine-carboxylic acid supramolecular heterosynthons rather than carboxylic acid or 2-aminopyridine supramolecular homosynthons. In the absence of other competing functionalities, the occurrence of heterosynthons increases to 97%. This observation is supported by the determination of single-crystal structures of 10 new compounds that contain both a 2-aminopyridine and a carboxylic acid moiety: 2-aminopyridinium 4-aminobenzoate, 1; 2-aminopyridinium isophthalate, 2; bis(2-aminopyridinium) terephthalate, 3; 2-amino-5-methylpyridinium benzoate, 4; bis(2-amino-5-methylpyridinium) 5-tertbutylisophthalate, 5; 2-amino5-methylpyridinium terephthalate, 6; bis(2-amino-5-methylpyridinium) 2,6-naphthalenedicarboxylate, 7; bis(2-amino5-methylpyridinium) adipate adipic acid, 8; bis(2-amino-5-methylpyridinium) 2,5-thiophenedicarboxylate 2,5-thiophenedicarboxylic acid, 9; and indomethacin 2-amino-5-methylpyridinium, 10. All 10 compounds are ionic, with proton transfer occurring to the aromatic nitrogen of the 2-aminopyridine moiety. Analysis of the supramolecular synthons and their effect upon crystal packing is presented in the context of crystal engineering and host-guest chemistry.