We are using single-crystal X-ray diffraction to examine how molecular structure influences crystal structure and solid-state reactivity. Our focus is on two families of nitrogenous organic compounds: the benzylideneanilines (R-CH=N-R’, R = aryl) and the benzonitrile oxides (R-CN+-O-, R = aryl). Among the benzylideneanilines we have identified pairs of molecules we have designated bridge-flipped isomers, molecules that differ only in the orientation of a bridge of atoms connecting two major parts of the molecule (R-CH=N-R’ vs. R-N=CH-R’). Bridge-flipped isomeric benzylideneanilines that are isomorphous (that assume identical molecular packing arrangements in the solid state) should be capable of solid solution formation over a wide range of component ratios; those that are not isomorphous may be capable of serving as seed crystals that could initiate the growth of a new polymorph of each isomeric compound in the pair. We have been investigating the role of the nitrile (CN) group in defining molecular packing patterns in crystals; its versatility in participating in a range of commonly occurring intermolecular interactions such as halogen-nitrile contacts and C-H…:NC contacts lends it significance in crystal engineering. We have thus prepared and determined the crystal structures of two nitrile-bearing benzylideneanilines: 4’-cyanobenzylidene-2-methylaniline and 2’-cyanobenzylidene-4-cyanoaniline. Although we have not yet obtained crystals of the bridge-flipped isomer of the methyl compound, our previous determination of the crystal structure of the bridge-flipped isomer of the dicyano compound shows that these two are not isomorphous. In contrast, the methyl compound is isomorphous with the corresponding iodo compound but not with the corresponding bromo and chloro compounds. In a parallel investigation, we are investigating the relationship between ring substitution and solid-state reactivity in benzonitrile oxides. To that end, we have prepared 4-bromobenzonitrile oxide and 3-nitrobenzonitrile oxide and have been investigating their potential solid-state dimerization to one of three possible dimers.

Benzylideneanilines and benzonitrile oxides:  Solid-state structure and reactivity studies

249th National Meeting (2015)

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