Biomimetic O₂ adsorption in an iron metal-organic framework for air separation

Bio-inspired motifs for gas binding and small molecule activation can be used to design more selective adsorbents for gas separation applications. Here, we report an iron metal-organic framework, Fe-BTTri (Fe₃[(Fe₄Cl)₃(BTTri)₈]₂·18CH₃OH, H₃BTTri = 1,3,5-tris(1H-1,2,3-triazol-5-yl)benzene), that binds O₂ in a manner similar to hemoglobin and therefore results in highly selective O₂ binding. As confirmed by gas adsorption studies and Mössbauer and infrared spectroscopy data, the exposed iron sites in the framework reversibly adsorb substantial amounts of O₂ at low temperatures by converting between high-spin, square-pyramidal Fe(ii) centers in the activated material to low-spin, octahedral Fe(iii)-superoxide sites upon gas binding. This change in both oxidation state and spin state observed in Fe-BTTri leads to selective and readily reversible O₂ binding, with the highest reported O₂/N₂ selectivity for any iron-based framework.