Multiple Solid Forms of 1,5-Bis(salicylidene)carbohydrazide: Polymorph-Modulated Thermal Reactivity

Abstract

1,5-Bis(salicylidene)carbohydrazide, prepared by a condensation reaction of salicylaldehyde and carbohydrazide, afforded a diversity of crystalline forms depending on crystallization conditions: three polymorphs (I, II, and 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 cross-polarization magic-angle spinning (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–visible 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.