Yabuuchi Yuto, Furukawa Hiroyasu, Carsch Kurtis M, Klein Ryan A, Tkachenko Nikolay V, Huang Adrian J, Cheng Yongqiang, Taddei Keith M, Novak Eric, Brown Craig M, Head-Gordon Martin, Long Jeffrey R
Department of Chemistry, University of California, Berkeley, California 94720, United States.
Institute for Decarbonization Materials, University of California, Berkeley, California 94720, United States.
J Am Chem Soc. 2024 Aug 14;146(32):22759-22776. doi: 10.1021/jacs.4c08039. Epub 2024 Aug 2.
Porous solids can accommodate and release molecular hydrogen readily, making them attractive for minimizing the energy requirements for hydrogen storage relative to physical storage systems. However, H adsorption enthalpies in such materials are generally weak (-3 to -7 kJ/mol), lowering capacities at ambient temperature. Metal-organic frameworks with well-defined structures and synthetic modularity could allow for tuning adsorbent-H interactions for ambient-temperature storage. Recently, CuZnCl(btdd) (Hbtdd = bis(1-1,2,3-triazolo-[4,5-],[4',5'-])dibenzo[1,4]dioxin; Cu-MFU-4) was reported to show a large H adsorption enthalpy of -32 kJ/mol owing to π-backbonding from Cu to H, exceeding the optimal binding strength for ambient-temperature storage (-15 to -25 kJ/mol). Toward realizing optimal H binding, we sought to modulate the π-backbonding interactions by tuning the pyramidal geometry of the trigonal Cu sites. A series of isostructural frameworks, CuMX(btdd) (M = Mn, Cd; X = Cl, I; CuM-MFU-4), was synthesized through postsynthetic modification of the corresponding materials MX(btdd) (M = Mn, Cd; X = CHCO, I). This strategy adjusts the H adsorption enthalpy at the Cu sites according to the ionic radius of the central metal ion of the pentanuclear cluster node, leading to -33 kJ/mol for M = Zn (0.74 Å), -27 kJ/mol for M = Mn (0.83 Å), and -23 kJ/mol for M = Cd (0.95 Å). Thus, CuCd-MFU-4 provides a second, more stable example of optimal H binding energy for ambient-temperature storage among reported metal-organic frameworks. Structural, computational, and spectroscopic studies indicate that a larger central metal planarizes trigonal Cu sites, weakening the π-backbonding to H.
多孔固体能够轻松地容纳和释放分子氢，相对于物理存储系统而言，这使得它们在降低储氢能量需求方面具有吸引力。然而，这类材料中的氢吸附焓通常较弱（-3至-7 kJ/mol），导致在环境温度下的储氢容量较低。具有明确结构和合成模块性的金属有机框架能够实现对吸附剂与氢相互作用的调控，以用于环境温度下的储氢。最近，据报道CuZnCl(btdd)（Hbtdd = 双(1-1,2,3-三唑并-[4,5-],[4',5'-])二苯并[1,4]二恶英；Cu-MFU-4）由于铜与氢之间的π反馈键作用，显示出-32 kJ/mol的大氢吸附焓，超过了环境温度储氢的最佳结合强度（-15至-25 kJ/mol）。为了实现最佳的氢结合，我们试图通过调整三角铜位点的金字塔形几何结构来调节π反馈键相互作用。通过对相应材料MX(btdd)（M = Mn, Cd；X = CHCO, I）进行后合成修饰，合成了一系列同构框架CuMX(btdd)（M = Mn, Cd；X = Cl, I；CuM-MFU-4）。该策略根据五核簇节点中心金属离子的离子半径来调整铜位点的氢吸附焓，对于M = Zn（0.74 Å）为-33 kJ/mol，对于M = Mn（0.83 Å）为-27 kJ/mol，对于M = Cd（0.95 Å）为-23 kJ/mol。因此，CuCd-MFU-4在已报道的金属有机框架中提供了第二个更稳定的环境温度储氢最佳氢结合能的例子。结构、计算和光谱研究表明，更大的中心金属使三角铜位点平面化，减弱了与氢的π反馈键作用。
