Polymorphs of tamoxifen citrate: detailed structural characterization of the stable form.

The structure of the stable polymorph of tamoxifen citrate (form B) has been analyzed by single crystal X-ray diffraction [monoclinic, a = 15.998 (4), b = 21.964 (3), c = 8.624 (5) A, beta = 96.28 (3) degrees, space group P2(1)/c]. The results include conformational details of the molecular structure as well as a description of the observed packing arrangement in the solid which is dominated by hydrogen-bonding interactions between the ionic entities. The proton transfer to the nitrogen atom of tamoxifen is from the central carboxyl group of the citric acid. There is a satisfactory correlation between the solid infrared spectrum of the carboxylic functions and the asymmetric environment observed around them in the crystal lattice. Additional results obtained from thermal and spectral analyses suggest that the second polymorph of tamoxifen citrate (form A) represents a metastable form of this compound. The two polymorphs are characterized by their powder diffraction spectra.