Spectroscopic studies of 4-(4,6-dimethylpyrimidin-2-ylazo) benzene-1,3-diol and its Cu(II) complexes

The electronic absorption spectra of 4-(4,6-dimethylpyrimidin-2-ylazo)benzene-1,3-diol have been studied in organic solvents of different polarities as well as in buffer solutions of varying pH. The observed UV-vis absorption bands are assigned to the corresponding electronic transitions. The effect of methanol ratio on the pK-value is discussed. Also, semiemperical molecular orbital calculations at the AM1 level have been performed to investigate the molecular and electronic structures of the free ligand in the ground state. According to these calculations, an intramolecular hydrogen bond leads to increasing of the molecular stability. The important bands in the IR spectrum as well as the main signals in the (1)H NMR spectrum are also assigned. The interaction of Cu(II) ion with the titled azo-dye in solution is studied spectrophotometrically and conductometrically. Optimization of the various experimental conditions is also described. Beer's law is obeyed in the range 0-11.43 ppm while that obtained applying Ringbom is 1.26-6.61 ppm. The use of the titled azo-dye as an indicator for determination of Cu(II) is considered. The solid Cu(II) complexes are synthesized and characterized by spectral, magnetic, conductance and thermal studies (TGA and DTA). The results indicate the formation of 1:1 and 1:2 (M:L) complexes. The kinetic parameters (n, E, A, DeltaS, DeltaH and DeltaG) of the thermal decomposition stages were computed and discussed.