EPR investigation of TiCl₃dissolved in polar solvents – implications for the understanding of active Ti(iii) species in heterogeneous Ziegler–Natta catalysts

Abstract

5Multi-frequency continuous-wave and pulsed EPR techniques are employed to investigate Ti(III)-chloro complexes obtained by dissolving TiCl 3 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 Ti 3+ ion. For the first time, the hyperfine and nuclear quadrupole data of Ti(III)-bound 10 35/37 Cl nuclei are reported together with 1 H and 13 C 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, 35/37 Cl, 1 H and 13 C hyperfine tensors) are in quantitative agreement with the experiments for two families of complexes: [TiCl n (CH 3 OH) 6-n )] (3-n)+ (with n ranging from 0 to 2) and [Ti(CH 3 OH) 5 (OH)] 2+ or 15 [Ti(CH 3 OH) 5 (OCH 3 )] 2+ . The first complex is observed in anhydrous methanol, while the second type of complexes is observed when water is added to the solution; the presence of OH -and/or CH 3 O -species being promoted by water hydrolysis. The results obtained for the frozen solutions are critically compared to EPR spectra recorded for a MgCl 2 -supported Ti-based Ziegler-Natta model catalyst.

Probing the Coordinative Unsaturation and Local Environment of Ti3+ Sites in an Activated High‐Yield Ziegler–Natta Catalyst

“…[24] Thec omputed g tensors for the two models are consistent with the single electron occupying ad xy or d x 2 Ày 2 orbital (Figure 2). Distinctly different g tensor elements are computed for the two models,with the case of the tetra-coordinated Ti 3+ located at the MgCl 2 (110) surface (Figure 2a)b eing in better agreement with the experimental values relative to the most abundant Ti 3+ species (S1).…”

“…These values are in line with experimental and computed values for molecular complexes in which Cl is directly coordinated to aT i 3 + center. [24] Thes ame spin Hamiltonian parameters have been used to fit W-band ELDOR-detected NMR spectra, which are reported in the Supporting Information (ELDOR = electron double resonance).…”

“…Given the superposition of the spectral features of S1 and S2, the HYSCORE spectra reflect hyperfine and nuclear quadrupole interactions associated with both Ti 3+ species. Fort he simulation of the HYSCORE spectrum, spin Hamiltonian parameters,r ecently obtained for TiCl 3 molecular complexes, [24] and the results of DFT computations (see below) were used as starting values.Anexample of areasonable simulation is shown in Figure 1c (see also the Supporting Information). Them ain feature of the experimental spectra could be reproduced by considering aC lh yperfine-interaction tensor characterized by afairly large dipolar component and relatively small Fermi contact contributions and nuclearquadrupole-interaction values (e 2 qQ/h)i nt he range 6-12 MHz.…”

Probing the coordination environment of Ti3+ions coordinated to nitrogen-containing Lewis bases

“…concentrated solution an additional broad absorption feature is clearly observed in the 3400-3800 mT region (Fig. 6 The third species (monomer 1) dominates the spectrum of the diluted solution ( Fig. Given the broad linewidth of this second signal a better resolved spectrum is obtained by recording the ESE-detected EPR spectrum, which measures the absorption signal directly (Fig.…”

Probing the Coordinative Unsaturation and Local Environment of Ti3+ Sites in an Activated High‐Yield Ziegler–Natta Catalyst

“…Fort he simulation of the HYSCORE spectrum, spin Hamiltonian parameters,r ecently obtained for TiCl 3 molecular complexes, [24] and the results of DFT computations (see below) were used as starting values.Anexample of areasonable simulation is shown in Figure 1c (see also the Supporting Information). Them ain feature of the experimental spectra could be reproduced by considering aC lh yperfine-interaction tensor characterized by afairly large dipolar component and relatively small Fermi contact contributions and nuclearquadrupole-interaction values (e 2 qQ/h)i nt he range 6-12 MHz.…”