Experimental and Hirshfeld Surface Investigations for Unexpected Aminophenazone Cocrystal Formation under Thiourea Reaction Conditions via Possible Enamine Assisted Rearrangement

Considering the astounding biomedicine properties of pharmaceutically active drug, 4-aminophenazone, also known as 4-aminoantipyrine, the work reported in this manuscript details the formation of novel cocrystals of rearranged 4-aminophenazone and 4-nitro-N-(4-nitrobenzoyl) benzamide in 1:1 stoichiometry under employed conditions for thiourea synthesis by exploiting the use of its active amino component. However, detailed analysis via various characterization techniques such as FT-IR, nuclear magnetic resonance spectroscopy and single crystal XRD, for this unforeseen, but useful cocrystalline synthetic adduct (4 and 5) prompted us to delve into its mechanistic pathway under provided reaction conditions. The coformer 4-nitro-N-(4-nitrobenzoyl) benzamide originates via nucleophilic addition reaction following tetrahedral mechanism between para-nitro substituted benzoyl amide and its acid halide (1). While the enamine nucleophilic addition reaction by 4-aminophenazone on 4-nitrosubstituted aroyl isothiocyanates under reflux temperature suggests the emergence of rearranged counterpart of cocrystal named N-(1,5-dimethyl-3-oxo-2-phenyl-2,3-dihydro-1H-pyrazole-4-carbonothioyl)-4-nitrobenzamide. Crystallographic studies reveal triclinic system P-1 space group for cocrystal (4 and 5) and depicts two different crystallographically independent molecules with prominent C-H center dot center dot center dot O and N-H center dot center dot center dot O hydrogen bonding effective for structure stabilization. Hirshfeld surface analysis also displays hydrogen bonding and van der Waals interactions as dominant interactions in crystal packing. Further insight into the cocrystal synthetic methodologies supported the occurrence of solution-based evaporation/cocrystallization methodology in our case during purification step, promoting the synthesis of this first-ever reported novel cocrystal of 4-aminophenazone with promising future application in medicinal industry.