Pore with Gate: Enhancement of the Isosteric Heat of Adsorption of Dihydrogen via Postsynthetic Cation Exchange in Metal−Organic Frameworks

Three isostructural anionic frameworks {[(Hdma)(H₃O)][In₂(L¹)₂]·4DMF·5H₂O}_∞ (NOTT-206-solv), {[H₂ppz][In₂(L²)₂]·3.5DMF·5H₂O}_∞ (NOTT-200-solv), and {[H₂ppz][In₂(L³)₂]·4DMF·5.5H₂O}_∞ (NOTT-208-solv) (dma = dimethylamine; ppz = piperazine) each featuring organic countercations that selectively block the channels and act as pore gates have been prepared. The organic cations within the as-synthesized frameworks can be replaced by Li⁺ ions to yield the corresponding Li⁺-containing frameworks {Li_1.2(H₃O)_0.8[In₂(L¹)₂]·14H₂O}_∞ (NOTT-207-solv), {Li_1.5(H₃O)_0.5[In₂(L²)₂]·11H₂O}_∞ (NOTT-201-solv), and {Li_1.4(H₃O)_0.6[In₂(L³)₂]·4acetone·11H₂O}_∞ (NOTT-209-solv) in which the pores are now unblocked. The desolvated framework materials NOTT-200a, NOTT-206a, and NOTT-208a display nonporous, hysteretic and reversible N₂ uptakes, respectively, while NOTT-206a and NOTT-200a provide a strong kinetic trap showing adsorption/desorption hysteresis with H₂. Single crystal X-ray analysis confirms that the Li⁺ ions are either tetrahedrally (in NOTT-201-solv and NOTT-209-solv) or octahedrally (in NOTT-207-solv) coordinated by carboxylate oxygen atoms and/or water molecules. This is supported by ⁷Li solid-state NMR spectroscopy. NOTT-209a, compared with NOTT-208a, shows a 31% enhancement in H₂ storage capacity coupled to a 38% increase in the isosteric heat of adsorption to 12 kJ/mol at zero coverage. Thus, by modulating the pore environment via postsynthetic cation exchange, the gas adsorption properties of the resultant MOF can be fine-tuned. This affords a methodology for the development of high capacity storage materials that may operate at more ambient temperatures.