Reversible and Selective O₂ Chemisorption in a Porous Metal–Organic Host Material

The metal–organic host material [{Co^III₂(bpbp)(O₂)}₂bdc](PF₆)₄ (1·2O₂; bpbp^– = 2,6-bis(N,N-bis(2-pyridylmethyl)aminomethyl)-4-tert-butylphenolato; bdc^2– = 1,4-benzenedicarboxylato) displays reversible chemisorptive desorption and resorption of dioxygen through conversion to the deoxygenated Co(II) form [{Co^II₂(bpbp)}₂bdc](PF₆)₄ (1). Single crystal X-ray diffraction analysis indicates that the host lattice 1·2O₂, achieved through desorption of included water guests from the as-synthesized phase 1·2O₂·3H₂O, consists of an ionic lattice containing discrete tetranuclear complexes, between which lie void regions that allow the migration of dioxygen and other guests. Powder X-ray diffraction analyses indicate that the host material retains crystallinity through the dioxygen desorption/chemisorption processes. Dioxygen chemisorption measurements on 1 show near-stoichiometric uptake of dioxygen at 5 mbar and 25 °C, and this capacity is largely retained at temperatures above 100 °C. Gas adsorption isotherms of major atmospheric gases on both 1 and 1·2O₂ indicate the potential suitability of this material for air separation, with a O₂/N₂ selectivity factor of 38 at 1 atm. Comparison of oxygen binding in solution and in the solid state indicates a dramatic increase in binding affinity to the complex when it is incorporated in a porous solid.