The solid state of anti-inflammatory morniflumate diniflumate: A cocrystalline salt

Morniflumate diniflumate, a molecular compound involving niflumic acid and its β-morpholino ethyl ester (morniflumate) in the mole ratio 2:1, is found to crystallize in a triclinic P - 1 space group with a unit-cell volume of 2203.4(5) ų. It is a cocrystal between a morniflumate⁺ niflumate^- salt and a neutral niflumic acid molecule. The co-crystalline salt forms endothermically with a positive excess volume and it melts incongruently at 382.3(8) K. Differential scanning calorimetry executed at heating rates above 20 K⋅min^-1, leads to congruent melting at 387.8(9)K with an enthalpy change of Δ_fusH = 80(2) J g^-1. The rare occurrence that incongruent and congruent melting can be observed for the same cocrystal may be due to the conformational versatility of the niflumic acid molecule and its slow conversion between the different conformations due to weak intramolecular hydrogen bonding.