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 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.