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

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, Cu2.2Zn2.8Cl1.8(btdd)3 (H2btdd = bis(1H-1,2,3-triazolo-[4,5-b],[4',5'-i])dibenzo[1,4]dioxin; CuI-MFU-4l) was reported to show a large H2 adsorption enthalpy of -32 kJ/mol owing to pi-backbonding from CuI 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 pi-backbonding interactions by tuning the pyramidal geometry of the trigonal CuI sites. A series of isostructural frameworks, Cu2.7M2.3X1.3(btdd)3 (M = Mn, Cd; X = Cl, I; CuIM-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 CuI sites according to the ionic radius of the central metal ion of the pentanuclear cluster node, leading to -33 kJ/mol for M = ZnII (0.74 A), -27 kJ/mol for M = MnII (0.83 A), and -23 kJ/mol for M = CdII (0.95 A). Thus, CuICd-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 CuI sites, weakening the pi-backbonding to H2.