Pore Size Engineering of MOFs by Pore Edge Reaction: Tetrazine Click and Hydrogen Adsorption in Theory and Experiment

Precise control over the porosity of metal–organic frameworks (MOFs) is crucial to optimize their properties and leverage their inherent tunability. However, there are ongoing challenges in pore size engineering for each MOF platform such as preserving crystallinity and morphology and facilitating reliable theoretical predictions throughout a series of modulated structures. Among postsynthetic strategies, mainly covalent functionalization appears to simultaneously preserve structural integrity and enable accurate theoretical predictions. Here, we present a MOF platform [M2(RCOO)4(H2O)2], JUK-21(M), M = Cu or Zn, containing a tetrazine-based tetracarboxylate linker, which we covalently functionalize using the inverse electron-demand Diels–Alder reaction (iEDDA) and five dienophiles of various bulkiness, yielding a series of JUK-21(Cu)-x MOFs. In addition to experiments, the iEDDA reactivity is assessed by applying a charge distribution susceptibility analysis, including Fukui functions, hardness, and relevant donor/acceptor orbitals. Comprehensive theoretical and experimental insights into the adsorption of nitrogen and hydrogen by JUK-21(Cu)-x enable rationalization of the observed isotherms and show the isosteric heat of hydrogen adsorption as a highly sensitive parameter to validate the modification efficiency. Our findings indicate to what extent the pore size of MOFs affects the adsorption properties and highlight potential pitfalls that arise even with the precise covalent functionalization of MOFs.精准调控金属有机框架（MOFs）的孔隙率，对于优化其性能并利用其固有的可调控性至关重要。然而，针对各类金属有机框架平台开展孔径工程仍面临持续存在的挑战，例如维持结晶度与形貌，以及在一系列修饰后结构中实现可靠的理论预测。在后合成策略中，共价功能化似乎可同时保留结构完整性并实现准确的理论预测。本研究报道了一款金属有机框架平台[M₂(RCOO)₄(H₂O)₂]，即JUK-21(M)（其中M为Cu或Zn），其包含四嗪基四羧酸连接体；我们通过反电子需求狄尔斯-阿尔德反应（iEDDA）与五种不同位阻的亲双烯体对其进行共价功能化，得到一系列JUK-21(Cu)-x型金属有机框架。除实验研究外，我们还通过电荷分布敏感性分析——包括福井函数、化学硬度及相关给体/受体轨道——对iEDDA反应活性进行了评估。通过对JUK-21(Cu)-x吸附氮气与氢气的行为开展全面的理论与实验研究，我们阐明了观测到的吸附等温线，并发现氢气吸附等量热是验证修饰效率的高灵敏参数。我们的研究结果揭示了金属有机框架孔径对吸附性能的影响程度，并点明了即便通过精准共价功能化修饰金属有机框架，仍可能存在的潜在陷阱。