Department of Chemistry, Boston College, Chestnut Hill, Massachusetts 02467, United States.
J Am Chem Soc. 2021 Jan 27;143(3):1630-1640. doi: 10.1021/jacs.0c08957. Epub 2021 Jan 19.
Many enzymes utilize interactions extending beyond the primary coordination sphere to enhance catalyst activity and/or selectivity. Such interactions could improve the efficacy of synthetic catalyst systems, but the supramolecular assemblies employed by biology to incorporate second sphere interactions are challenging to replicate in synthetic catalysts. Herein, a strategy is reported for efficiently manipulating outer-sphere influence on catalyst reactivity by modulating host-guest interactions between a noncovalently encapsulated transition-metal-based catalyst guest and a metal-organic framework (MOF) host. This composite consists of a ruthenium PNP pincer complex encapsulated in the MOF UiO-66 that is used in tandem with the zirconium oxide nodes of UiO-66 and a ruthenium PNN pincer complex to hydrogenate carbon dioxide to methanol. Due to the method used to incorporate the complexes in UiO-66, structure-activity relationships could be efficiently determined using a variety of functionalized UiO-66-X hosts. These investigations uncovered the beneficial effects of the ammonium functional group (i.e., UiO-66-NH). Mechanistic experiments revealed that the ammonium functionality improved efficiency in the hydrogenation of carbon dioxide to formic acid, the first step in the cascade. Isotope effects and structure-activity relationships suggested that the primary role of the ammonium functionality is to serve as a general Brønsted acid. Importantly, the cooperative influence from the host was effective only with the functional group in close proximity to the encapsulated catalyst. Reactions carried out in the presence of molecular sieves to remove water highlighted the beneficial effects of the ammonium functional group in UiO-66-NH and resulted in a 4-fold increase in activity. As a result of the modular nature of the catalyst system, the highest reported turnover number (TON) (19 000) and turnover frequency (TOF) (9100 h) for the hydrogenation of carbon dioxide to methanol are obtained. Moreover, the reaction was readily recyclable, leading to a cumulative TON of 100 000 after 10 reaction cycles.
许多酶利用超出主配位层的相互作用来提高催化剂的活性和/或选择性。这种相互作用可以提高合成催化剂体系的功效，但生物用于引入第二配位层相互作用的超分子组装在合成催化剂中难以复制。在此，报道了一种通过调节非共价封装的过渡金属基催化剂客体与金属有机骨架（MOF）主体之间的主客体相互作用来有效操纵对催化剂反应性的外配位层影响的策略。该复合材料由封装在 MOF UiO-66 中的钌 PNP 夹复合物组成，与 UiO-66 的氧化锆节点和钌 PNN 夹复合物一起用于将二氧化碳氢化甲醇。由于用于将复合物掺入 UiO-66 的方法，可以使用各种功能化的 UiO-66-X 主体有效地确定结构-活性关系。这些研究揭示了铵官能团（即 UiO-66-NH）的有益效果。机理实验表明，铵官能团提高了二氧化碳氢化生成甲酸的效率，这是级联的第一步。同位素效应和结构-活性关系表明，铵官能团的主要作用是作为通用的布朗斯台德酸。重要的是，只有当功能基团靠近封装的催化剂时，主体的协同影响才有效。在存在分子筛以除去水的情况下进行的反应突出了 UiO-66-NH 中铵官能团的有益效果，并使活性提高了 4 倍。由于催化剂体系的模块化性质，获得了二氧化碳氢化甲醇的最高报道转化率（TON）（19000）和转化率频率（TOF）（9100 h）。此外，该反应易于回收利用，在 10 个反应循环后，累积 TON 达到 100000。
