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 AxFe2(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 O2 uptake behavior similar to that of AxFe2(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.