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 π-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 π-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 Å), −27 kJ/mol for
M = MnII (0.83 Å), and −23 kJ/mol for M = CdII (0.95 Å). 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 π-backbonding to H2.