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, H-2 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-2 interactions for ambient-temperature storage. Recently, Cu2.2Zn2.8Cl1.8(btdd)(3) (H(2)btdd = 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 H-2 adsorption enthalpy of -32 kJ/mol owing to pi-backbonding from Cu-I to H-2, exceeding the optimal binding strength for ambient-temperature storage (-15 to -25 kJ/mol). Toward realizing optimal H-2 binding, we sought to modulate the pi-backbonding interactions by tuning the pyramidal geometry of the trigonal Cu-I sites. A series of isostructural frameworks, Cu2.7M2.3X1.3(btdd)(3) (M = Mn, Cd; X = Cl, I; (CuM)-M-I-MFU-4l), was synthesized through postsynthetic modification of the corresponding materials M5X4(btdd)(3) (M = Mn, Cd; X = CH3CO2, I). This strategy adjusts the H-2 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 & Aring;), -27 kJ/mol for M = Mn-II (0.83 & Aring;), and -23 kJ/mol for M = Cd-II (0.95 & Aring;). Thus, (CuCd)-Cd-I-MFU-4l provides a second, more stable example of optimal H-2 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 pi-backbonding to H-2.