Hexamethylmelamine: a study of solvation and crystallisation

Abstract

Dielectric, electro-optic, and spectroscopic properties of hexamethylmelamine (HMM) are examined in various solvents. The study is an extension of earlier work relating the dielectric characteristics of solvent to the habit of HMM crystals, a factor of importance to the effectiveness of pulmonary administration of this drug in the treatment of human small-call lung carcinoma. In solution HMM is confirmed to be non-dipolar though abnormally large atomic polarisation is found. The optical polarisability anisotropy correlates with an extensively delocalised π-system involving the triazine ring and the exocyclic nitrogens. Interactions between HMM and cyclohexane, dioxane, and benzene are weak and not sterically defined—each of these solvents yields predominantly ‘compact’ crystal forms of HMM. In hexafluorobenzene definitive evidence is provided for strongly favoured stereospecific face-to-face HMM–C₆F₆ interactions. Needle-shaped crystals obtained from this solvent very rapidly become opaque and friable. Thermal analysis indicates an unstable 1 : 1 HMM–C₆F₆ adduct. This was confirmed by X-ray crystal structure determination which showed that HMM and C₆F₆ alternate in infinite columns, suggestive of a π–π interaction between the rings. The study provides an important correlation, based on experiment, between the statistically preferred HMM–C₆F₆ configuration in solution and that found for the crystalline adduct when isolated.