Well-Defined Ti Surface Sites in Ziegler-Natta Pre-Catalysts from ^47/49Ti Solid-State Nuclear Magnetic Resonance...

Treatment of Ziegler-Natta (ZN) catalysts with BCl₃ improves their activity by increasing the number of active sites. Here we show how ^47/49Ti solid-state nuclear magnetic resonance (NMR) spectroscopy enables us to understand the electronic structure of the Ti surface sites present in such treated ZN pre-catalysts, prior to activation with alkyl aluminum. High-field (21.1 T) and low-temperature (∼100 K) NMR augmented by DFT modeling on the pre-catalyst and corresponding molecular analogues enables the detection of ^47/49Ti NMR signatures and a molecular level understanding of the electronic structure of Ti surface sites. The associated Ti surface sites exhibit ⁴⁹Ti NMR signatures (δ_iso, exp = -170 ppm; C_Q, exp = 9.3 MHz; κ = 0.05) corresponding to well-defined fully chlorinated hexacoordinated Ti sites adsorbed on a distorted surface of the MgCl₂ support, formed upon post-treatment with BCl₃ and removal of the alkoxo ligands, paralleling the increased polymerization activity.