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

Treatment of Ziegler-Natta (ZN) catalysts with BCl3 improves their activity by increasing the number of active sites. Here we show how Ti-47/49 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 (similar to 100 K) NMR augmented by DFT modeling on the pre-catalyst and corresponding molecular analogues enables the detection of Ti-47/49 NMR signatures and a molecular level understanding of the electronic structure of Ti surface sites. The associated Ti surface sites exhibit Ti-49 NMR signatures (delta(iso, exp) = -170 ppm; C-Q,C- exp = 9.3 MHz; kappa = 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.