engleski

Multiple solid forms of 1, 5-bis(salicylidene)carbohydrazide : polymorph-modulated thermal reactivity

1, 5-Bis(salicylidene)carbohydrazide, prepared by a condensation reaction of salicylaldehyde and carbohydrazide, afforded a diversity of crystalline forms depending on the crystallization conditions: three polymorphs (I, II, III) and two solvates (IV and V). All solid phases comprise the same tautomer of the title compound, which bears the salicylidene residues in the enol-imino form and the carbamide fragment in the keto form, as established via X-ray crystallography, IR spectroscopy and CP MAS NMR (13C and 1H). Conformations of 1, 5-bis(salicylidene)carbohydrazide molecules and their hydrogen bonding patterns are similar for two polymorphs (II and III), while the third one (I) in this context differs significantly. Thermal analyses revealed that all phases undergo complicated degradation upon heating yielding salicylideneazine, CO2 and N2 either directly or via intermediates. Relative phase stability was inspected mechanochemically and by solvent mediated experiments, establishing polymorph II as the stable one at ambient conditions. NMR and UV-Vis solution studies revealed that the same tautomer of 1, 5-bis(salicylidene)carbohydrazide present in the solid state is also the dominant one in solution.

carbohydrazide; polymorphs; thermogravimetric analyses; temperature-resolved powder X-ray diffraction

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