Introducing Nonstructural Ligands to Zirconia-like Metal–Organic Framework Nodes To Tune the Activity of Node-Supported Nickel Catalysts for Ethylene Hydrogenation

Previous work has shown that introduction of hexafluoroacetylacetone (Facac) units as nonstructural ligands for the zirconia-like nodes of the eight-connected metal–organic framework (MOF), NU-1000, greatly alters the selectivity of node-supported oxy-nickel clusters for ethylene dimerization vs oligomerization. Here we explore a related concept: tuning of support/catalyst interactions, and therefore, catalyst activity, via parallel installation of organic modifiers on the support itself. As modifiers we focused on para-substituted benzoates (R-BA–; R = −NH2, −OCH3, −CH3, −H, −F, and −NO2) where the substituents were chosen to present similar steric demand, but varying electron-donating or electron-withdrawing properties. R-benzoate-engendered shifts in the node-based aqua O–H stretching frequency for NU-1000, as measured by DRIFTS (diffuse-reflectance infrared Fourier-transform spectroscopy), together with systematic shifts in Ni 2p peak energies, as measured by X-ray photoelectron spectroscopy, show that the electronic properties of the support can be modulated. The vibrational and electronic peak shifts correlate with the putative electron-withdrawing vs electron-donating strength of the para-substituted benzoate modifiers. Subsequent installation of node-supported, oxy-Ni­(II) clusters for ethylene hydrogenation yield a compelling correlation between log (catalyst turnover frequency) and the electron donating or withdrawing character of the substituent of the benzoate units. Single crystal X-ray diffraction measurements reveal that each organic modifier makes use of only one of two available carboxylate oxygens to accomplish grafting. The remaining oxygen atom is, in principle, well positioned to coordinate directly to an installed Ni­(II) ion. We postulate that the unanticipated direct coordination of the catalyst by the node-modifier (rather than indirect modifier-based tuning of support­(node)/catalyst electronic interactions) is the primary source of the observed systematic tuning of hydrogenation activity. We suggest, however, that regardless of mechanism for communication with active-sites of MOF-supported catalysts, intentional elaboration of nodes via grafted, nonstructural organic species could prove to be a valuable general strategy for fine-tuning supported-catalyst activity and/or selectivity.

已有研究表明，向八连接金属有机框架（metal–organic framework, MOF）NU-1000的类氧化锆节点引入六氟乙酰丙酮（hexafluoroacetylacetone, Facac）单元作为非结构配体，可显著改变节点负载氧合镍团簇催化乙烯二聚相较于齐聚反应的选择性。本研究探索了一项相关策略：通过在载体（节点）自身同步接枝有机改性剂，调控载体与催化剂的相互作用，进而调节催化剂活性。本次研究选取的改性剂为对位取代苯甲酸盐（R-BA–；取代基R分别为-NH₂、-OCH₃、-CH₃、-H、-F及-NO₂），所选取代基的空间位阻需求相近，但电子给体与电子受体性质存在差异。通过漫反射傅里叶变换红外光谱（diffuse-reflectance infrared Fourier-transform spectroscopy, DRIFTS）测得，NU-1000节点结合的羟基O-H伸缩振动频率因苯甲酸盐改性剂发生偏移；结合X射线光电子能谱（X-ray photoelectron spectroscopy, XPS）测得的Ni 2p峰能系统偏移，证实载体的电子性质可被调控。振动光谱与电子能谱的峰偏移与对位取代苯甲酸盐改性剂的推定电子给体/受体强度呈现相关性。后续在节点负载氧合Ni(II)团簇用于催化乙烯加氢后，催化剂的对数转化频率（log(turnover frequency)）与苯甲酸盐单元取代基的给/吸电子特性之间存在显著相关性。单晶X射线衍射测试结果显示，每种有机改性剂仅利用两个可用羧酸根氧原子中的一个完成接枝。剩余的氧原子理论上处于可直接与负载的Ni(II)离子配位的理想位置。我们推测，节点改性剂与催化剂发生未预期的直接配位（而非通过改性剂间接调控载体（节点）与催化剂的电子相互作用），是观测到加氢活性被系统调控的主要原因。不过我们认为，无论与MOF负载催化剂活性位点的沟通机制如何，通过接枝非结构有机物种对节点进行精准修饰，都有望成为调控负载催化剂活性与/或选择性的通用有效策略。