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 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.