Metal complexes of an o,o'-dihydroxy azo dye. A spectroscopic study

Chromium(III) and cobalt(III) complexes of 4-(3',5'-dinitro-2'-hydroxy-phenylazo)-3-hydroxy-2-naphthanilide, known as TRH and Co-TRH respectively, were studied by NMR, IR, Raman and UV-Vis spectroscopy in order to determine the number of, and the structures of their structural isomers. NMR data for Co-TRH clearly indicated one major isomer, which was identified as the N_α isomer having the phenyl nitrogen chelating with the Co(III) ion. No useful NMR information was obtained for TRH, because it is paramagnetic; however, TLC results suggested a single isomer. IR and Raman spectra of the complexes suggested that the ligand in TRH and Co-TRH displayed both azo and hydrazone characteristics. These could be the resonance forms of the six-membered chelate ring adjacent to the naphthalene moiety. The UV-Vis absorption band for TRH showed a bathochromic shift with respect to that for Co-TRH. This was attributed to the relatively ionic nature of TRH.