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 chemically reduced metal–organic framework materials of the type A_xFe₂(bdp)₃ (A = Na⁺, K⁺; bdp²⁻ = 1,4-benzenedipyrazolate; 0 < x ≤ 2), which feature coordinatively saturated iron centers, are capable of strong and selective adsorption of O₂ over N₂ at ambient (25 °C) or even elevated (200 °C) temperature. A combination of gas adsorption analysis, single-crystal X-ray diffraction, magnetic susceptibility measurements, and a range of spectroscopic methods, including ²³Na solid-state NMR, Mössbauer, and X-ray photoelectron spectroscopies, are employed as probes of O₂ uptake. Significantly, the results support a selective adsorption mechanism involving 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 structure. We further demonstrate similar O₂ uptake behavior to that of A_xFe₂(bdp)₃ in an expanded-pore framework analogue and thereby gain additional insight into the O₂ adsorption mechanism. The chemical reduction of a robust metal–organic framework to render it capable of binding O₂ through such an outer-sphere electron transfer mechanism represents a promising and underexplored strategy for the design of next-generation O₂ adsorbents.