Iron(III) chloride complexation in hydrothermal solutions: A combined spectrophotometric and density functional theory study

Abstract

Iron(III) hydrolysis and chloride complexation in hydrothermal solutions was studied experimentally using UV–Vis spectrophotometry and based on density functional theory (DFT) and time-dependent density functional theory (TD-DFT) techniques. The strong ligand-to-metal charge transitions (LMCT) at wavelengths below 400 nm were used to obtain the number of absorbing species, molar absorptivities (ε), and equilibrium formation constants, using principle component analysis of the spectra. CAM-B3LYP calculations of molecular geometries, energies and electronic absorption spectra have been undertaken for microsolvated iron(III) hydroxo and chloro complexes with up to four Cl atoms, showing that a combination of explicit first and second shell water solvation, together with PCM models, is required to successfully simulate aqueous UV–Vis spectra. With increasing chloride concentration and temperature, iron(III) chloride complexes become important with FeCl2+, FeCl2+, FeCl3(aq) and FeCl4− forming. Upon the progressive addition of Cl− to the aquated Fe3+ ion the complex geometry changes from octahedral to tetrahedral coordination of the FeCl4− ion. The respective formation constants were derived experimentally between 25° and 200 °C and were in the range: log β0,1 = 1.42–3.24 (25–150 °C), log β0,2 = 1.98–7.37 (25–200 °C), log β0,3 = 2.98–9.03 (100–200 °C) and log β0,4 = 5.27–9.88 (150–200 °C). With increasing chloride concentration and temperature iron(III) chloride complexes become increasingly important species in acid solutions.