Highly-Selective and Reversible O₂ Binding in Cr₃(1,3,5-benzenetricarboxylate)₂

Reaction of Cr(CO)(6) with trimesic acid in DMF affords the metal-organic framework Cr(3)(BTC)(2).nDMF (BTC(3-) = 1,3,5-benzenetricarboxylate), which is isostructural to Cu(3)(BTC)(2).3H(2)O. Exchanging DMF for methanol and heating at 160 degrees C under dynamic vacuum for 48 h results in the desolvated framework Cr(3)(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 m(2)/g, respectively. At 298 K, the O(2) adsorption isotherm for Cr(3)(BTC)(2) rises steeply to a capacity of 11 wt % at 2 mbar, while the corresponding N(2) adsorption isotherm displays very little uptake, gradually rising to a capacity of 0.58 wt % at 1 bar. Accordingly, the material displays an unprecedented O(2)/N(2) selectivity factor of 22. Deoxygenation of the sample could be accomplished by heating at 50 degrees 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 O(2) adduct with partial charge transfer from the Cr(II) centers exposed on the surface of the framework. Neutron powder diffraction data confirm this mechanism of O(2) 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) A.