Proton Magnetic Resonance Studies of 4-(2-Pyridylazo)resorcinol (PAR) and Its Cobalt(III) Complexes

The 1H NMR spectra of PAR, [CoIII(par)2]−, and [CoIII(par)(dien)]+ were measured in D2O solutions. The signals were assigned. The deuteration of H7 was observed in D2O solutions of free ligand PAR around pD 9–10, where PAR exists in the form of LH− having an intramolecular hydrogen-bond. The significant up-field shift for the resonance of H1 was observed in [CoIII(par)2]− but not in [CoIII(par)(dien)]+. The shift was interpreted in terms of the magnetic anisotropy due to the π-electron system of the neighboring PAR molecule in the complex. The down-field shift for the resonance of the H5 signal of the cobalt(III) complexes reflects the structural change of PAR upon chelate-ring formation. PAR coordinates to cobalt(III) ion in planar terdentate fashion, viz., in a mer configuration.