Structural studies of arylazo and arylimino compounds. 15 N NMR and X-ray crystallographic studies of azo–hydrazo tautomerism

15 N NMR measurements on several diarylazo, arylhydrazonopyrazolone, diarylazomethine and arylaminomethylidenepyrazolone compounds are reported. The presence of hydrazo tautomers is recognised by NOE experiments. The crystal structures of 4-(2,6-dibromo-4-methylphenylazo)-3-hydroxy-N,N-diethylaniline (12) and 4-(2-bromophenylhydrazono)-3-methyl-phenyl-1H-pyrazol-5(4H)-one (15) have been determined from X-ray diffraction data. Both of these molecules contain a six-membered [graphic omitted] ring as a result of intramolecular hydrogen bonding. Whereas (12) exists as the azo-phenol tautomer, the pyrazolone (15) is in the hydrazo-ketone form. The implication of these results for copper-assisted nucleophilic substitution of the halogen in these aryl bromides is discussed.