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, Cu 2.2 Zn 2.8 Cl 1.8 (btdd) 3 (H 2 btdd = bis(1 H -1,2,3-triazolo-[4,5- b ],[4',5'- i ])dibenzo[1,4]dioxin; Cu I -MFU-4 l ) was reported to show a large H 2 adsorption enthalpy of -32 kJ/mol owing to π-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 π-backbonding interactions by tuning the pyramidal geometry of the trigonal Cu I sites. A series of isostructural frameworks, Cu 2.7 M 2.3 X 1.3 (btdd) 3 (M = Mn, Cd; X = Cl, I; Cu I M-MFU-4 l ), was synthesized through postsynthetic modification of the corresponding materials M 5 X 4 (btdd) 3 (M = Mn, Cd; X = CH 3 CO 2 , 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 Å), -27 kJ/mol for M = Mn II (0.83 Å), and -23 kJ/mol for M = Cd II (0.95 Å). Thus, Cu I Cd-MFU-4 l 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 π-backbonding to H 2 .