†KAUST Catalysis Center, King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi Arabia.
‡Centre de RMN à Très Hauts Champs (CNRS/ENS-Lyon/UCB Lyon 1), Université de Lyon, 5 rue de la Doua, 69100 Villeurbanne, France.
J Am Chem Soc. 2015 Jun 24;137(24):7728-39. doi: 10.1021/jacs.5b02872. Epub 2015 Jun 15.
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·OEt2], with silica dehydroxylated at 700 °C (SiO(2-700)) or at 200 °C (SiO(2-200)) to generate diverse surface complexes. The product of the reaction between SiO(2-700) and [NbCl5·OEt2] was identified as a monopodal supported surface species, [≡SiONbCl4·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 [(≡SiO)2NbCl3·OEt2]. (93)Nb solid-state NMR spectra of 1a-3a and (31)P solid-state NMR on their PMe3 derivatives 1b-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 [≡SiONbCl4·OEt2] and 3a being mostly bipodal [(≡SiO)2NbCl3·OEt2]. A double-quantum/single-quantum (31)P NMR correlation experiment carried out on 2b supported the existence of vicinal Nb centers on the silica surface for this species. 1a-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.
最近的发现强调了 NbCl5 作为分子催化剂在环境条件下催化二氧化碳和环氧化物环加成的活性和有趣的机械特征。这启发了通过将分子铌前体[NbCl5·OEt2]与在 700°C（SiO2-700）或 200°C（SiO2-200）下脱羟的二氧化硅反应来制备新型硅负载 Nb 物种。SiO2-700）与[NbCl5·OEt2]之间的反应产物被鉴定为单齿支撑表面物种[≡SiONbCl4·OEt2]（1a）。根据两种不同的方案，SiO2-200）与铌前体的反应生成表面配合物 2a 和 3a，具有显著但不同的单齿表面配合物种群以及双齿[(≡SiO)2NbCl3·OEt2]。1a-3a 的（93）Nb 固态 NMR 谱和其 PMe3 衍生物 1b-3b 的（31）P 固态 NMR 谱导致 1a 被明确分配为单齿单齿 Nb 物种，而 2a 和 3a 被发现具有两种不同的表面支撑成分，2a 主要是单齿[≡SiONbCl4·OEt2]，而 3a 主要是双齿[(≡SiO)2NbCl3·OEt2]。在 2b 上进行的双量子/单量子（31）P NMR 相关实验支持该物种在二氧化硅表面上存在相邻 Nb 中心。1a-3a 是在温和催化条件下从二氧化碳和环氧丙烷合成碳酸丙烯酯的非均相催化剂；发现 2a 的性能明显优于 1a 和 3a。在对模型二氧化硅表面进行系统 DFT 研究的支持下，观察到的催化效率差异与 2a 表面上两个相邻 Nb 中心之间前所未有的协同效应相关。这与我们之前关于均相相中 NbCl5 催化环加成反应机制的发现非常吻合。
