Mixed crystals of 6-chloro-3,4-methylenedioxycinnamic acid with 2,4- and 3,4-dichlorocinnamic acids; structure, topochemistry, and intermolecular interactions

Abstract

The presence of a chloro substituent in a planar aromatic molecule tends to favour its crystallisation in the 4 Å short axis β-structure. This structure is stabilised by inter-stack Cl ⋯ Cl interactions which are weakly attractive yet directional in nature, and by intra-stack dispersive C ⋯ C interactions. The balance between intra- and inter-sheet interactions is revealed in the crystal structures of 6-chloro-3,4-methylenedioxycinnamic acid (2a), its 2:1 complex (1a) with 2,4-dichlorocinnamic acid, and its 1:1 complex (1b) with 3,4-dichlorocinnamic acid. The acid (2a) is dimorphic with both forms triclinic and with short axes ca. 4 Å; the structure of one form has been determined by X-ray diffraction methods. Solid-state u.v. irradiation of the complexes (1a and b) affords mixtures of mirror and pseudomirror symmetrical cyclobutanes; the formation of these products is consistent only with an ordered sheet-disordered stack structure, derived from the completely ordered structure of (2a), for these complexes. The structures of (1a and b) have been deduced thus on the basis of topochemical, kinetic, and crystallographic evidence, and provide important corroboration of inter-stack stabilisation as a prerequisite for β-structure adoption. Mixed crystalline complexes such as (1a) permit a comparative kinetic study of topochemical reactions, a procedure normally difficult in solid-state chemistry.