Highly-Selective and Reversible O-2 Binding in Cr-3(1,3,5-benzenetricarboxylate)(2)

Reaction of Cr(CO)6 with trimesic acid in DMF affords the metal−organic framework Cr3(BTC)2·nDMF (BTC3− = 1,3,5-benzenetricarboxylate), which is isostructural to Cu3(BTC)2·3H2O. Exchanging DMF for methanol and heating at 160 °C under dynamic vacuum for 48 h results in the desolvated framework Cr3(BTC)2. Nitrogen gas adsorption measurements performed at 77 K revealed a type I isotherm, indicating BET and Langmuir surface areas of 1810 and 2040 m2/g, respectively. At 298 K, the O2 adsorption isotherm for Cr3(BTC)2 rises steeply to a capacity of 11 wt % at 2 mbar, while the corresponding N2 adsorption isotherm displays very little uptake, gradually rising to a capacity of 0.58 wt % at 1 bar. Accordingly, the material displays an unprecedented O2/N2 selectivity factor of 22. Deoxygenation of the sample could be accomplished by heating at 50 °C under vacuum for 48 h, leading to a gradually diminishing uptake capacity over the course of 15 consecutive adsorption/desorption cycles. Infrared and X-ray absorption spectra suggest formation of an O2 adduct with partial charge transfer from the CrII centers exposed on the surface of the framework. Neutron powder diffraction data confirm this mechanism of O2 binding and indicate a lengthening of the Cr−Cr distance within the paddle-wheel units of the framework from 2.06(2) to 2.8(1) Å.