Cooperative Effect of Monopodal Silica-Supported Niobium Complex Pairs Enhancing Catalytic Cyclic Carbonate Production

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• 作者：Valerio D鈥橢lia ; Hailin Dong ; Aaron J. Rossini ; Cory M. Widdifield ; Sai V. C. Vummaleti ; Yury Minenkov ; Albert Poater ; Edy Abou-Hamad ; J茅r茅mie D. A. Pelletier ; Luigi Cavallo ; Lyndon Emsley ; Jean-Marie Basset
• 刊名：Journal of the American Chemical Society
• 出版年：2015
• 出版时间：June 24, 2015
• 年：2015
• 卷：137
• 期：24
• 页码：7728-7739
• 全文大小：618K
• ISSN：1520-5126

文摘

Recent discoveries highlighted the activity and the intriguing mechanistic features of NbCl₅ as a molecular catalyst for the cycloaddition of CO₂ and epoxides under ambient conditions. This has inspired the preparation of novel silica-supported Nb species by reacting a molecular niobium precursor, [NbCl₅路OEt₂], with silica dehydroxylated at 700 掳C (SiO_2-700) or at 200 掳C (SiO_2-200) to generate diverse surface complexes. The product of the reaction between SiO_2-700 and [NbCl₅路OEt₂] was identified as a monopodal supported surface species, [鈮iONbCl₄路OEt₂] (1a). The reactions of SiO_2-200 with the niobium precursor, according to two different protocols, generated surface complexes 2a and 3a, presenting significant, but different, populations of the monopodal surface complex along with bipodal [(鈮iO)₂NbCl₃路OEt₂]. ⁹³Nb solid-state NMR spectra of 1a鈥?b>3a and ³¹P solid-state NMR on their PMe₃ derivatives 1b鈥?b>3b led to the unambiguous assignment of 1a as a single-site monopodal Nb species, while 2a and 3a were found to present two distinct surface-supported components, with 2a being mostly monopodal [鈮iONbCl₄路OEt₂] and 3a being mostly bipodal [(鈮iO)₂NbCl₃路OEt₂]. A double-quantum/single-quantum ³¹P NMR correlation experiment carried out on 2b supported the existence of vicinal Nb centers on the silica surface for this species. 1a鈥?b>3a were active heterogeneous catalysts for the synthesis of propylene carbonate from CO₂ and propylene oxide under mild catalytic conditions; the performance of 2a was found to significantly surpass that of 1a and 3a. With the support of a systematic DFT study carried out on model silica surfaces, the observed differences in catalytic efficiency were correlated with an unprecedented cooperative effect between two neighboring Nb centers on the surface of 2a. This is in an excellent agreement with our previous discoveries regarding the mechanism of NbCl₅-catalyzed cycloaddition in the homogeneous phase.