Abstract
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(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 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-4l), 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-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 π-backbonding to H(2).