Influence of fluorine side-group substitution on the crystal structure formation of benzene-1,3,5-trisamides†
By a combination of powder X-ray diffraction, multidimensional and multinuclear solid-state NMR spectroscopy and quantum chemical calculations, we were able to determine the crystal structure of 1,3,5-tris(2-fluoro-2-methylpropionylamino)benzene. Solid-state NMR experiments guided the structure solution by predicting the content of the asymmetric unit and the presence of a NH⋯OC hydrogen bond network. In addition to real-space structure solution and Rietveld refinement, quantitative symmetry-based 19F19F double-quantum recoupling experiments provided a cost function to determine the positions of the methyl groups and fluorine atoms. The structure solution of this particular fluorine-substituted trisamide illustrates the impact of fluorine side-group substitution on the common columnar packing motif of benzene-1,3,5-tricarboxamides. As also in the case 1,3,5-tris(2,2-dimethylpropionylamino)benzene, the supramolecular aggregation is then guided by the formation of triple helical NH⋯OC hydrogen bond networks within the individual columns. In contrast, the substitution of one methyl group by a fluorine atom in each side chain results in a two-dimensional NH⋯OC hydrogen bond pattern, leading to a lamellar crystal structure with only van der Waals interactions between the layers. Since fluorine is not involved in the hydrogen bond network and both chemical units exhibit a similar steric demand, the fundamental differences of the packing are most probably caused by changes in the molecular polarity.