Lu Zhiyong, Liu Jian, Zhang Xuan, Liao Yijun, Wang Rui, Zhang Kun, Lyu Jiafei, Farha Omar K, Hupp Joseph T
College of Mechanics and Materials, Hohai University, Nanjing 210098, P. R. China.
Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States.
J Am Chem Soc. 2020 Dec 16;142(50):21110-21121. doi: 10.1021/jacs.0c09782. Epub 2020 Dec 2.
High-stability, zirconium-based metal-organic frameworks are attractive as heterogeneous catalysts and as model supports for uniform arrays of subsequently constructed heterogeneous catalysts-for example, MOF-node-grafted metal-oxy and metal-sulfur clusters. For hexa-Zr(IV)-MOFs characterized by nodes that are less than 12-connected, sites not used for linkers are ideally occupied by reactive and displaceable OH/HO pairs. The desired pairs are ideal for grafting the aforementioned catalytic clusters, while aqua-ligand lability renders them effective for exposing highly Lewis-acidic Zr(IV) sites (catalytic sites) to candidate reactants. New single-crystal X-ray studies of an eight-connected Zr-MOF, , reveal that conventional activation fully removes modulator ligands, but replaces them with three node-blocking formate ligands (from solvent decomposition) and only one OH/HO pair, not four-a largely overlooked complication that now appears to be general for Zr-MOFs. Here we describe an alternative activation protocol that effectively removes modulators, avoids formate, and installs the full complement of terminal OH/HO pairs. It does so via an unusual isolatable intermediate featuring eight aqua ligands and four non-ligated chlorides-again as supported by single-crystal X-ray data. We find that complete replacement of node-blocking modulators/formate with the originally envisioned OH/OH pairs has striking consequences; here we touch upon just three. First, elimination of unrecognized formate renders aqua ligands much more thermally labile, enabling open Zr(IV) sites to be obtained at lower temperature. Second, in the absence of formate, which otherwise links and locks pairs of node Zr(IV) ions, reversible removal of aqua ligands engenders reversible contraction of MOF meso- and micropores, as evidenced by X-ray diffraction. Third, formate replacement with OH/OH pairs renders ca.10× more active for catalytic hydrolytic degradation of a representative simulant of G-type chemical warfare agents.
高稳定性的锆基金属有机框架作为多相催化剂以及后续构建的多相催化剂均匀阵列的模型载体具有吸引力，例如MOF节点接枝的金属氧和金属硫簇。对于以连接数小于12的节点为特征的六锆（IV）-MOF，未用于连接体的位点理想地被反应性和可取代的OH/HO对占据。所需的这些对对于接枝上述催化簇是理想的，而水配体的不稳定性使其能够有效地将高度路易斯酸性的Zr（IV）位点（催化位点）暴露于候选反应物。对一种八连接Zr-MOF的新的单晶X射线研究表明，传统活化完全去除了调节剂配体，但用三个阻塞节点的甲酸盐配体（来自溶剂分解）和仅一个OH/HO对取代了它们，而不是四个——这是一个很大程度上被忽视的复杂情况，现在看来对Zr-MOF来说是普遍存在的。在这里，我们描述了一种替代活化方案，该方案有效地去除了调节剂，避免了甲酸盐，并安装了完整的末端OH/HO对。它通过一种不寻常的可分离中间体来实现，该中间体具有八个水配体和四个未配位的氯——同样由单晶X射线数据支持。我们发现，用最初设想的OH/OH对完全取代阻塞节点的调节剂/甲酸盐会产生显著的后果；这里我们只提及三点。首先，可以消除未识别的甲酸盐，使水配体在热稳定性上大大降低，从而能够在较低温度下获得开放的Zr（IV）位点。其次，在没有甲酸盐的情况下（否则甲酸盐会连接并锁定节点Zr（IV）离子对），水配体的可逆去除会导致MOF介孔和微孔的可逆收缩，这由X射线衍射证明。第三，用OH/OH对取代甲酸盐使 对G型化学战剂代表性模拟物的催化水解降解活性提高约10倍。
