Geometric Tuning of Coordinatively Unsaturated Copper(I) Sites in Metal-Organic Frameworks for Ambient-Temperature...

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, H₂ 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-H₂ interactions for ambient-temperature storage. Recently, Cu_2.2Zn_2.8Cl_1.8(btdd)₃ (H₂btdd = bis(1H-1,2,3-triazolo-[4,5-b],[4',5'-i])dibenzo[1,4]dioxin; Cu^I-MFU-4l) was reported to show a large H₂ adsorption enthalpy of -32 kJ/mol owing to π-backbonding from Cu^I to H₂, exceeding the optimal binding strength for ambient-temperature storage (-15 to -25 kJ/mol). Toward realizing optimal H₂ binding, we sought to modulate the π-backbonding interactions by tuning the pyramidal geometry of the trigonal Cu^I sites. A series of isostructural frameworks, Cu_2.7M_2.3X_1.3(btdd)₃ (M = Mn, Cd; X = Cl, I; Cu^IM-MFU-4l), was synthesized through postsynthetic modification of the corresponding materials M₅X₄(btdd)₃ (M = Mn, Cd; X = CH₃CO₂, I). This strategy adjusts the H₂ adsorption enthalpy at the Cu^I sites according to the ionic radius of the central metal ion of the pentanuclear cluster node, leading to -33 kJ/mol for M = Zn^II (0.74 Å), -27 kJ/mol for M = Mn^II (0.83 Å), and -23 kJ/mol for M = Cd^II (0.95 Å). Thus, Cu^ICd-MFU-4l provides a second, more stable example of optimal H₂ 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^I sites, weakening the π-backbonding to H₂.