Assigning H chemical shifts in paramagnetic mono- and bimetallic surface sites using DFT: a case study on the Union Carbide polymerization catalyst.

The Union Carbide (UC) ethylene polymerization catalyst, based on silica-supported chromocene, is one of the first industrial catalysts prepared by surface organometallic chemistry, though the structure of the surface sites remains elusive. Recently, our group reported that monomeric and dimeric Cr(ii) sites, as well as Cr(iii) hydride sites, are present and that their proportion varies as a function of the Cr loading. While H chemical shifts extracted from solid-state H NMR spectra should be diagnostic of the structure of such surface sites, unpaired electrons centered on Cr atoms induce large paramagnetic H shifts that complicate their NMR analysis. Here, we implement a cost-efficient DFT methodology to calculate H chemical shifts for antiferromagnetically coupled metal dimeric sites using a Boltzmann-averaged Fermi contact term over the population of the different spin states. This method allowed us to assign the H chemical shifts observed for the industrial-like UC catalyst. The presence of monomeric and dimeric Cr(ii) sites, as well as a dimeric Cr(iii)-hydride sites, was confirmed and their structure was clarified.