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 ->pi* and pi ->pi* 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-toligand 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 metalto-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.

Automated summary

In this study, the acidity constants of methylthymol blue (MTB) and the association constants of its complexes with Zn-II, Cu-II, and Fe-II metal ions (MIs) were determined through theoretical and experimental methods. The results showed a mixed n->pi* and pi->pi* electronic transitions in the UV-Visible spectra of MTB, as well as the formation of 1:2 and 1:1 complexes for Zn-II and Cu-II, and 1:1 and 2:1 complexes for Fe-II. The UV-Visible spectra of the Zn(MTB) and Cu(MTB) complexes exhibited ligand-to-ligand charge transfer (LLCT) characters, while the Fe(MTB) complex showed LLCT and metal-to-ligand charge transfer (MLCT) characters.