Trummer David, Nobile Anna Giorgia, Payard Pierre-Adrien, Ashuiev Anton, Kakiuchi Yuya, Klose Daniel, Jeschke Gunnar, Copéret Christophe
ETH Zürich Department of Chemistry and Applied Biosciences Vladimir-Prelog-Weg 2 Zürich CH-8093 Switzerland
Chem Sci. 2022 Sep 15;13(37):11091-11098. doi: 10.1039/d2sc04235e. eCollection 2022 Sep 28.
The Union Carbide (UC) ethylene polymerization catalysts, based on chromocene dispersed on silica, show distinct features from the Phillips catalysts, but share the same heated debate regarding the structure of their active sites. Based on a combination of IR, EPR spectroscopies, labeling experiments, and DFT modeling, we identified monomeric surface-supported Cr(iii) hydrides, ([triple bond, length as m-dash]SiO)Cr(Cp)-H, as the active sites of the UC catalyst. These sites are formed in the presence of grafted and adsorbed chromocene as well as residual surface OH groups, only possible at high Cr loading, and involve a C-H activation of the Cp ring. These Cr-hydrides initiate polymerization, yielding Cr(iii) alkyl species that insert ethylene through a Cossee-Arlman-type mechanism, as evidenced by spectroscopic studies. These insights inspired the design of a well-defined analog, CpCr(CH(SiMe)) grafted on partially dehydroxylated silica, that shows similar spectroscopic and polymer structure to the UC catalyst, further supporting the proposed active site structure.
联合碳化物公司（UC）基于负载在二氧化硅上的二茂铬的乙烯聚合催化剂，与菲利普斯催化剂具有不同的特征，但在其活性位点结构方面也存在同样激烈的争论。基于红外光谱、电子顺磁共振光谱、标记实验和密度泛函理论建模的综合研究，我们确定单体表面负载的氢化铬（III），即（[三键，长度为m破折号]SiO）Cr（Cp）-H，为UC催化剂的活性位点。这些位点是在接枝和吸附的二茂铬以及残留的表面羟基存在的情况下形成的，只有在高铬负载量时才可能出现，并且涉及Cp环的C-H活化。这些氢化铬引发聚合反应，生成通过科塞-阿尔曼型机理插入乙烯的铬（III）烷基物种，光谱研究证明了这一点。这些见解启发了一种明确的类似物的设计，即接枝在部分脱羟基二氧化硅上的CpCr（CH（SiMe）），它显示出与UC催化剂相似的光谱和聚合物结构，进一步支持了所提出的活性位点结构。
