Tautomerism in the [1,2,4]triazolo[3,2-c][1,2,4]triazole fused aromatic system has been investigated by single-crystal Xray analysis, UV/Vis spectroscopy and theoretical calculations on selected new heterobicycle derivatives. The reactions of 3,4-diamino-1,2,4-triazoles with acyl chlorides or acetic anhydride in pyridine at reflux led to ring closure and the fused aromatic system was obtained as the secondary amide at N2 instead of at N5 as previously reported in the literature. For the [1,2,4]triazolo[3,2-c][1,2,4]triazole system, three different tautomers can be present. Crystal structure analysis showed the presence of only one tautomer or, depending on the packing, a mixture of two; theoretical calculations supported this finding, predicting that for all the investigated compounds one tautomer is energetically disfavoured with respect to the other two. The electronic absorption spectra of some compounds show a marked solvent dependence, which has been interpreted, with the help of theoretical calculations, in terms of acid/base equilibria, which are critically dependent upon the substituents at the heterobicycle.
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