Crystal engineering via non-bonded interactions involving oxygen. X-ray crystal structures of 3,4-methylenedioxycinnamic acid and 3,4-dimethoxycinnamic acid

It has been shown that the presence of a methylenedioxy substituent in a planer aromatic molecule tends to favour its crystallisation in highly overlapped structures. Thus, in a series of methylenedioxycinnamic acid derivatives, there is a preference for the β-structure over the α-and γ-forms. It is suggested that this substituent is a good steering device towards the β-form since, in this form, the oxygen atoms of the substituent can participate in stabilising non-bonded interactions involving p electrons and further, the compactness of the substituent allows the molecules to crystallise with a 4 Å repeat. The crystal-structure determination of 3,4-methylenedioxycinnamic acid, (1), illustrates these concepts. The crystals of (1) are triclinic, the space group is P, Z= 2, a= 3.804(3),b= 10.502(5), c= 11.112(4)Å, α = 77.84(4), β= 84.26(9),γ= 80.17(5)°. The structure was solved, not without difficulty, and has been refined to an R-factor of 0.067 on 742 non-zero reflections. Molecules related by translation along [100] are within photoreactive distance and the β-structure leads to the formation of a mirror-symmetry cyclobutane on solid-state irradiation. If the oxygenated substituents are bulky, crystallisation in the β -form is not easy. The crystal structure of 3,4-dimethoxycinnamic acid, (2), demonstrates an alternative packing. Acid (2) is triclinic, the space group is P, Z= 4, a= 8.448(3), b= 15.072(8),c= 8.437(6)Å, α= 99.44(5),β = 94.71(5),γ = 101.59(4)°. The structure solution was non-trivial and only possible with the use of the program YZARC 80. The structure has been refined to an R-factor of 0.107 on 725 non-zero reflections. Pairs of pseudo-centrosymmetric molecules in the asymmetric unit are hydrogen bonded to form cyclic dimers. Nearest neighbours are related by a pseudo-translation (γ) and centres of inversion (α). Only half the molecules in the structure are potentially reactive; for each these there are two inversion-related near neighbours. However, only one of these is within photoreactive distance to give the topochemical inversion-symmetry cyclobutane on irradiation