Selective, High-Temperature O2 Adsorption in Chemically Reduced, Redox-Active Iron-Pyrazolate Metal–Organic Frameworks

Developing O2-selective adsorbents that can produce high-purity oxygen from air remains a significant challenge. Here, we show that the chemically reduced metal–organic framework AxFe2(BDP)3 (A = Na+, K+; BDP2 = 1,4-benzenedipyrazolate; 0 < x ≤ 2), which features coordinatively-saturated iron centers, is capable of strong and selective adsorption of O2 over N2 at ambient (25 °C) or even elevated (200 °C) temperature. Through a combination of gas adsorption measurements, single-crystal X-ray diffraction, and numerous spectroscopic probes, including 23Na solid-state NMR and X-ray photoelectron spectroscopy, we demonstrate that selective O2 uptake likely occurs as a result of outer-sphere electron transfer from the framework to form superoxide species, which are subsequently stabilized by intercalated alkali metal cations that reside in the one-dimensional triangular pores of the framework. The chemical reduction of a robust metal–organic framework to render it capable of binding O2 through an outer-sphere electron transfer mechanism thus represents a promising and underexplored strategy for the design of next-generation O2 adsorbents.