Crystal engineering and solid state chemistry of some β -nitrostyrenes

Abstract

The unusual solid state photodimerisation of (E)-β -nitrostyrene to yield "topochemical" and "non-topochemical" cyclobutanes is accounted for by its disordered, photoreactive crystal structure which is monoclinic, P2₁/c, Z= 4, a= 8.097(6)Å, b= 5.768(5)Å, c= 18.647(2)Å, β = 117.71(5)° . This structure permits a trans→cis isomerisation which facilitates the formation of the anomalous product. However, 4-methyl-β -nitrostyrene which has a very similar disordered structure is photostable in the solid state since the potentially "reactive" double bonds are beyond the topochemical threshold. C-H ··· O and C-H ··· Cl interactions are important to the exclusion of Cl ··· Cl interactions in the layered structure of the 4-chloro derivative, but this nitrostyrene forms mixed crystals with the 4-methyl compound in the disordered structure of the latter showing that the role of the Cl atom in the 4-chloro derivative is at best marginal. In contrast, the higher Cl stoichiometry in the 2,4-dichloro analogue results in a layered, photoreactive β structure characterised by Cl ··· Cl and C-H ··· O interactions. The structures of three layered alkoxy-β -nitrostyrenes are very similar to each other and are held by strong, directional C-H ··· O contacts. The 4-methoxy compound has a photostable crystal structure while the 3,4-methylenedioxy and 3,4-dimethoxy crystals appear to be capable of topochemical 2 + 2 cycloaddition. However, only the latter is photoreactive because of optimal double-bond-to-double-bond overlap in the crystal. The 4-bromo derivative is distinct from the 4-chloro compound and there are two molecules in the asymmetric unit because of conflicting packing requirements of the C-H ··· O and Br ··· O interactions. A survey of 84 intermolecular Br ··· O contacts retrieved from 39 nitro-bromo crystal structures has revealed that at least some of these arise due to halogen polarisability. Atomic motion analysis in this crystal structure indicates the importance of lateral C-H ··· O interactions.