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

Developing O 2 -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 x Fe 2 (bdp) 3 (A = Na + , K + ; bdp 2 − = 1,4-benzenedipyrazolate; 0 < x ≤ 2), which feature coordinatively saturated iron centers, are capable of strong and selective adsorption of O 2 over N 2 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 23 Na solid-state NMR, Mössbauer, and X-ray photoelectron spectroscopies, are employed as probes of O 2 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 O 2 uptake behavior to that of A x Fe 2 (bdp) 3 in an expanded-pore framework analogue and thereby gain additional insight into the O 2 adsorption mechanism. The chemical reduction of a robust metal–organic framework to render it capable of binding O 2 through such an outer-sphere electron transfer mechanism represents a promising and underexplored strategy for the design of next-generation O 2 adsorbents.