Crystal structures of 1:1 complexes of meclofenamic acid with choline and ethanolamine

Abstract

The hydrated 1:1 complex of meclofenamic acid with choline crystallites in the orthorhombic acid space group Pna2₁ with a = 9.637(1), b = 12.962(2), c = 33.099(4) Å and Z = 8 . Crystals of the corresponding anhydrous complex with ethanolamine are triclinic, space group P1̅, with a = 9.232(3), b = 12.287(5), c = 17.033(3) Å, α = 70.21(2), β = 76.72(2), γ = 68.21(3)° and Z = 4. The structures have been solved by direct methods and refined to R value of 0.062 and 0.079, respectively for 1942 and 2852 observed reflections. The four crystallographically independent meclofenamate anions in the complexes have nearly the same molecular geometry which in turn is very similar to that found in the crystal structure of free meclofenamic acid. The choline and ethanolamine molecules assume a gauche conformation with respect to the central C---C bond. The invariant structural features observed in the crystals of the free fenamates are retained by the meclofenamate ions in the complexes. These features are the rigid coplanar geometry of the six-membered ring carrying the carboxyl group, the carboxyl group and the imino nitrogen atom, and the internal hydrogen bond connecting the imino and the carboxyl groups. The crystal structures are stabilised by ionic interactions between the carboxylate groups of meclofenamate ions and choline or ethanolamine cations, and hydrogen bonds. The choline complex exhibits pseudosymmetry and the distribution of molecules in it is nearly centrosymmetric although the space group is noncentrosymmetric. The packing of molecules in the crystals is such that the polar columns are surrounded by non-polar regions. The core of each column in the choline complex is made up of water molecules connected by hydrogen bonds involving disordered protons. The results of the X-ray structure analysis of fenamates and their crystalline complexes provide some insights into structure-function relationships in this family of drugs.