Study of interaction of metal ions with methylthymol blue by chemometrics and quantum chemical calculations

In this study, we determine the acidity constants of methylthymol blue (MTB) and association constants of its complexes with the Zn(II), Cu(II), and Fe(II) metal ions (MIs), through theoretical and experimental means. The complexes were characterized using UV–Visible absorption spectroscopy combined with soft/hard chemometrics methods and quantum chemical calculations. Quantum chemical calculations revealed that electronic transitions in the UV–Visible spectra of MTB have mixed n → π* and π → π* characters. The results of molar ratio and multivariate curve resolution alternating least squares (MCR-ALS) revealed the formation of successive 1:2 and 1:1 complexes (MI:MTB) for the Zn(II) and Cu(II) systems. However, the formation of successive 1:1 and 2:1 complexes are suggested for Fe(II) by the molar ratio and MCR-ALS. The majority of transitions observed in the UV–Visible spectra of the Zn(MTB) and Cu(MTB) complexes have ligand-to-ligand charge transfer (LLCT) characters. However, the transitions in the UV–Visible spectrum of the Fe(MTB) complex have LLCT and metal-to-ligand charge transfer (MLCT) characters. For the Fe(2)(MTB) complex, the lowest energy transition of has an LLCT character. However, its higher energy transitions are a mixture of LLCT, MLCT, and metal-to-metal charge transfer (MMCT) characters. The correlation between experimental and computed wavelengths revealed that the 1:1 complexes of Zn(II) and Cu(II) prefer square pyramidal geometries. However, the Fe(II) complexes always show octahedral geometry.