Nobile Anna Giorgia, Trummer David, Berkson Zachariah J, Wörle Michael, Copéret Christophe, Payard Pierre-Adrien
ETH Zürich Department of Chemistry and Applied Biosciences Vladimir-Prelog-Weg 2 CH-8093 Zürich Switzerland
Université de Lyon, Université Claude Bernard Lyon I, CNRS, INSA, CPE, UMR 5246, ICBMS Rue Victor Grignard F-69622 Villeurbanne Cedex France.
Chem Sci. 2023 Feb 2;14(9):2361-2368. doi: 10.1039/d2sc06827c. eCollection 2023 Mar 1.
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.
联合碳化物公司（UC）基于二氧化硅负载的二茂铬的乙烯聚合催化剂，是最早通过表面有机金属化学制备的工业催化剂之一，尽管表面位点的结构仍然难以捉摸。最近，我们小组报告说，存在单体和二聚体Cr（ii）位点以及Cr（iii）氢化物位点，并且它们的比例随Cr负载量而变化。虽然从固态氢核磁共振谱中提取的氢化学位移应该能够诊断此类表面位点的结构，但以Cr原子为中心的未成对电子会引起较大的顺磁氢位移，使它们的核磁共振分析变得复杂。在这里，我们采用一种经济高效的密度泛函理论（DFT）方法，通过对不同自旋态的布居数使用玻尔兹曼平均费米接触项来计算反铁磁耦合金属二聚体位点的氢化学位移。这种方法使我们能够确定在类似工业UC催化剂中观察到的氢化学位移。单体和二聚体Cr（ii）位点以及二聚体Cr（iii）-氢化物位点的存在得到了证实，并且它们的结构也得到了阐明。
