Selective nitrogen adsorption via backbonding in a metal-organic framework with exposed vanadium sites. # These authors contributed equally to this work Industrial processes prominently feature π-acidic gases, and an adsorbent capable of selectively interacting with these molecules could enable a number of important chemical separations 1-4 . In nature, enzymes, and correspondingly their synthetic analogues, use accessible, reducing metal centers to bind and even activate weakly π-acidic species such as N 2 through backbonding interactions 5-7 , and incorporation of similar moieties into a porous material should give rise to a new mechanism of adsorption for these gaseous substrates 8 .However, synthetic challenges have prevented realization of such a material. Here, we report a metal-organic framework featuring exposed vanadium(II) centers with an electronic configuration and 3d-orbital energies conducive to the back-donation of electron density to weak π-acids, thereby enabling highly selective adsorption. This new adsorption mechanism, together with the presence of a high concentration of available adsorption sites, results in record N 2 capacities and selectivities for the removal of N 2 from mixtures with CH 4 , while further enabling the separation of olefins from paraffins at elevated temperatures.Ultimately, incorporating such π-basic metal centers into tunable porous materials offers a new handle for capturing and activating key molecular species within next-generation adsorbents.The implementation of adsorbent-based technology stands as a promising route toward mitigating the high energy and emission costs associated with current industrial chemical The synthesis of V 2 Cl 2.8 (

The widespread implementation of H2 as a fuel is currently hindered by the high pressures or cryogenic temperatures required to achieve reasonable storage densities. In contrast, the realization of materials that strongly and reversibly adsorb hydrogen at ambient temperatures and moderate pressures could transform the transportation sector and expand adoption of fuel cells in other applications. To date, however, no adsorbent has been identified that exhibits a binding enthalpy within the optimal range of −15 to −25 kJ/mol for ambient-temperature hydrogen storage. Here, we report the hydrogen adsorption properties of the metal-organic framework (MOF) V2Cl2.8(btdd) (H2btdd, bis(1H-1,2,3-triazolo[4,5b],[4′,5′-i])dibenzo [1,4]dioxin), which features exposed vanadium(II) sites capable of backbonding with weak π acids. Significantly, gas adsorption data reveal that this material binds H2 with an enthalpy of −21 kJ/mol. This binding energy enables usable hydrogen capacities that exceed that of compressed storage under the same operating conditions. The Kubas-type vanadium(II)-dihydrogen complexation is characterized by a combination of techniques. From powder neutron diffraction data, a V-D2(centroid) distance of 1.966(8) Å is obtained, the shortest yet reported for a MOF. Using in situ infrared spectroscopy, the vibration of the vanadium-bound H2 is identified, and it displays a red shift of 242 cm −1 relative to free H2. Electronic structure calculations show that a main contribution to bonding stems from the interaction between the vanadium dπ and H2 σ* orbital. Ultimately, the pursuit of MOFs containing high densities of weakly π-basic metal sites may enable storage capacities under ambient conditions that far surpass those accessible with compressed gas storage.

This Perspective summarizes progress in the development of O2-selective metal–organic frameworks for adsorptive air separations and identifies key metrics and design considerations toward optimizing material performance for practical applications.