Cryst. Res. Technol. 35 2000 8 1001–1009 The structures of two solid inclusion compounds with the 2’,6’-dimethoxyflavone host molecule, 1, were investigated by single-crystal X-ray analysis. Both compounds, 1·3-bromo-2,6-dimethoxybenzoic acid (1 :1) and 1·3-chloro-2,6-dimethoxybenzoic acid (1 :1) crystallize in the centrosymmetric group P1. The complexation involves an intermolecular hydrogen bond between the carbonyl group of the flavone and the acidic hydrogen of the acid. Keywords: crystal structure, pK a values, hydrogen bond (Received June 9, 1999; Accepted February 5, 2000) 1 Introduction The 2’,6’-dimethoxyflavone, 1, has a benzopyrone portion and a dimethoxyphenyl ring that are placed approximatively perpendicular to form cavities that can include guest molecules. That flavone forms a variety of complexes with many acidic compounds (both Brönsted and Lewis acids). With aliphatic (WALLET et al., 1989 ; TINANT et al., 1991 ; ESPINOSA et al., 1999) and aryl carboxylic acids (WALLET et al., 1998) it gives inclusion compounds of 1 :1 stoichiometry. The two title complexes crystallize in the triclinic space group P1 as do complexes with acetic and propionic acid. On the other hand, the complex with 2,6- dimethoxybenzoic acid crystallizes in the monoclinic space group P2 1 /c. With stronger acids (formic, chloroacetic and trichloroacetic acid), the complexes crystallize in non- centrosymmetric space groups. The present investigation was carried out in order to try to understand whether the strength of the acid could overcome the tendency for these complexes to crystallize in centrosymmetric space groups which is not favourable to the second harmonic generation phenomenon (RUSSEL et al., 1994). Moreover, this work provides structural and conformational details about both title acids in the complexes. These acids have been used in the synthesis of penicillins (DOYLE et al ; 1963) and 3-bromo-2,6- dimethoxybenzoic acid in the synthesis of remoxipride, a promising antischizophrenia pharmaceutical (AUERBACH et al., 1993). To our knowledge, X-ray crystal structures of these two acids have not been reported. 2 Experimental 2’,6’-Dimethoxyflavone has been synthesized as previously reported (WALLET et al., 1989). 3-Bromo-2,6-dimethoxybenzoic and 3-chloro-2,6-dimethoxybenzoic acid were synthesized following the method given by DOYLE et al. (1963). J.-C. WALLET, E. M. GAYDOU, H. LACHEKAR*, V. PICHON-PESME* Faculté des Sciences et Techniques de Saint-Jérôme, Marseille, France *Faculté des Sciences, Vandoeuvre les Nancy, France 1:1 Crystal Complexes of 2’,6’-Dimethoxyflavone with 3- Bromo and 3-Chloro-2,6-dimethoxybenzoic Acid