EPR investigation of TiCl3 dissolved in polar solvents – implications for the understanding of active Ti(iii) species in heterogeneous Ziegler–Natta catalysts

Abstract

Multi-frequency continuous-wave and pulsed EPR techniques are employed to investigate Ti(III)-chloro complexes obtained by dissolving TiCl₃ in anhydrous and hydrated methanol. Two distinctly different species, characterized by different g matrices are observed in the two cases. Hyperfine sublevel correlation (HYSCORE) spectroscopy is found to be a powerful method to identify the type of nuclei surrounding the Ti³⁺ ion. For the first time, the hyperfine and nuclear quadrupole data of Ti(III)-bound ^35/37Cl nuclei are reported together with ¹H and ¹³C hyperfine data of the coordinated methanol molecules. DFT modelling allows interpreting the measured spin Hamiltonian parameters in terms of microscopic models of the solvated species. The theoretical observable properties (g matrix, ^35/37Cl, ¹H and ¹³C hyperfine tensors) are in quantitative agreement with the experiments for two families of complexes: [TiCl_n(CH₃OH)_6−n]⁽³⁻ⁿ⁾⁺ (with n ranging from 1 to 3) and [Ti(CH₃OH)₅(OH)]²⁺ or [Ti(CH₃OH)₅(OCH₃)]²⁺. The first complex is observed in anhydrous methanol, while the second type of complex is observed when water is added to the solution, the presence of OH⁻ and/or CH₃O⁻ species being promoted by water hydrolysis. The results obtained for the frozen solutions are critically compared to EPR spectra recorded for a MgCl₂-supported Ti-based Ziegler–Natta model catalyst.