Understanding the crystalline formation of triazene N -oxides and the role of halogen⋯π interactions

The crystallization of a series of 1-(4-halophenyl)-3-phenyltriazenide N 1-oxides ( 1–4 ) and 1-(phenyl)-3-phenyltriazenide N 1-oxide ( 5 ) was evaluated using the supramolecular cluster approach. This method is an efficient tool to assess the crystallization mechanism of compounds and, consequently, the steps involved in crystal formation. Compounds 1 and 4 show crystallization in two main steps while compounds 2 and 3 present three main steps, in which column formation occurs in the first step. The crystallization process for 5 occurs in 3 main steps, starting from a robust dimer formation (−16.82 kcal mol −1 ). Two new parameters – N CG % (topological and energetic contribution percentage) and NG/NC (energetic parameter/topological parameter ratio) – assisted in the interpretation of crystal growth. Compounds 1–4 showed N CG % = 50 in the first step while compound 5 reached only 50% of the contribution in the second step. The differences in N CG % were attributed to strong hydrogen bonds in the non-halogenated compound. The dominant parameter in each step of the crystallization process was indicated by the NG/NC parameter. The crystallization mechanism in all compounds was initially driven by an energetic process followed by a topological process. The existence of X⋯π interactions was shown and was observed to be a consequence of a topological process and without any major contributions to crystal formation. Thermal analysis and UV-vis spectral data were also discussed regarding the properties of these compounds.