The hierarchies of hydrogen bonds in salts/cocrystals of isoniazid and its Schiff base - a case study

A series of two salts of isoniazid and two cocrystals of its Schiff base have been synthesised and characterized using FT-IR, H-1-NMR, DSC, XRPD and SCXRD. The crystal structures with 2,5-dihydroxybenzoic acid, 2,6-dihydroxy benzoic acid, 3-aminobenzoic acid and o-phthalic acid have been deduced. In all complexes the involvement of pyridine N (proton transfer/robust synthon) is noticed. The unit cell similarity index was calculated between two cocrystals (N-(propan-2-ylidene) isonicotinohydrazide).(2,5-dihydroxybenzoic acid) and (3-aminobenozic acid).(N-(propan-2-ylidene) isonicotinohydrazide). Furthermore, the index was compared with the reported complexes and it was found to be close to zero indicating isostructurality. The correlation between the N-Pyr-H center dot center dot center dot O (acid) hydrogen bond distance for the reported cocrystals and their corresponding pK(a) values has been deduced and validated.