Selective, High-Temperature O2 Adsorption in Chemically Reduced, Redox-Active Iron-Pyrazolate Metal–Organic Frameworks
Developing O<sub>2</sub>-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<i><sub>x</sub></i>Fe<sub>2</sub>(bdp)<sub>3</sub> (A = Na<sup>+</sup>, K<sup>+</sup>; bdp<sup>2</sup><sup>−</sup> = 1,4-benzenedipyrazolate; 0 < <i>x</i> ≤ 2), which feature coordinatively saturated iron centers, are capable of strong and selective adsorption of O<sub>2</sub> over N<sub>2</sub> 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 <sup>23</sup>Na solid-state NMR, Mössbauer, and X-ray photoelectron spectroscopies, are employed as probes of O<sub>2</sub> 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<sub>2</sub> uptake behavior to that of A<i><sub>x</sub></i>Fe<sub>2</sub>(bdp)<sub>3</sub> in an expanded-pore framework analogue and thereby gain additional insight into the O<sub>2</sub> adsorption mechanism. The chemical reduction of a robust metal–organic framework to render it capable of binding O<sub>2</sub> through such an outer-sphere electron transfer mechanism represents a promising and underexplored strategy for the design of next-generation O<sub>2</sub> adsorbents.