The lower limit of metal hydride nanoconfinement is demonstrated through the coordination of a molecular hydride species to binding sites inside the pores of a metal-organic framework (MOF). Magnesium borohydride, which has a high hydrogen capacity, is incorporated into the pores of UiO-67bpy (Zr₆O₄(OH)₄(bpydc)₆ with bpydc^2- = 2,2'-bipyridine-5,5'-dicarboxylate) by solvent impregnation. The MOF retained its long-range order, and transmission electron microscopy and elemental mapping confirmed the retention of the crystal morphology and revealed a homogeneous distribution of the hydride within the MOF host. Notably, the B-, N-, and Mg-edge XAS data confirm the coordination of Mg(II) to the N atoms of the chelating bipyridine groups. In situ ¹¹B MAS NMR studies helped elucidate the reaction mechanism and revealed that complete hydrogen release from Mg(BH₄)₂ occurs as low as 200 °C. Sieverts and thermogravimetric measurements indicate an increase in the rate of hydrogen release, with the onset of hydrogen desorption as low as 120 °C, which is approximately 150 °C lower than that of the bulk material. Furthermore, density functional theory calculations support the improved dehydrogenation properties and confirm the drastically lower activation energy for B-H bond dissociation.

The highly unfavorable thermodynamics of direct aluminum hydrogenation can be overcome by stabilizing alane within a nanoporous bipyridine-functionalized covalent triazine framework (AlH₃ @CTF-bipyridine). This material and the counterpart AlH₃ @CTF-biphenyl rapidly desorb H₂ between 95 and 154 °C, with desorption complete at 250 °C. Sieverts measurements, ²⁷ Al MAS NMR and ²⁷ Al{¹ H} REDOR experiments, and computational spectroscopy reveal that AlH₃ @CTF-bipyridine dehydrogenation is reversible at 60 °C under 700 bar hydrogen, >10 times lower pressure than that required to hydrogenate bulk aluminum. DFT calculations and EPR measurements support an unconventional mechanism whereby strong AlH₃ binding to bipyridine results in single-electron transfer to form AlH₂ (AlH₃ )_n clusters. The resulting size-dependent charge redistribution alters the dehydrogenation/rehydrogenation thermochemistry, suggesting a novel strategy to enable reversibility in high-capacity metal hydrides.

Abstract: The highly unfavorable thermodynamics of direct aluminum hydrogenation can be overcome by stabilizing alane within a nanoporous bipyridine‐functionalized covalent triazine framework (AlH3@CTF‐bipyridine). This material and the counterpart AlH3@CTF‐biphenyl rapidly desorb H2 between 95 and 154 °C, with desorption complete at 250 °C. Sieverts measurements, 27Al MAS NMR and 27Al{1H} REDOR experiments, and computational spectroscopy reveal that AlH3@CTF‐bipyridine dehydrogenation is reversible at 60 °C under 700 bar hydrogen, >10 times lower pressure than that required to hydrogenate bulk aluminum. DFT calculations and EPR measurements support an unconventional mechanism whereby strong AlH3 binding to bipyridine results in single‐electron transfer to form AlH2(AlH3)n clusters. The resulting size‐dependent charge redistribution alters the dehydrogenation/rehydrogenation thermochemistry, suggesting a novel strategy to enable reversibility in high‐capacity metal hydrides.