Structural, Spectroscopic, and Computational Characterization of O2 Binding. As noted above, Cu I -MFU-4l is known to reversibly bind O2 at room temperature, and a recent investigation of O2 binding in Cu I -MFU-4l using in situ Cu L2,3-edge near-edge x-ray absorption fine structure spectroscopy and time-dependent density functional theory (DFT) revealed that O2 adsorption is accompanied by significant electron transfer from copper(I) to O2 with partial oxidation of the copper ion. We sought to better understand the nature of O2 binding in this material using a suite of structural, spectroscopic, and computational analyses. Dosing microcrystalline Cu2.4-MFU-4l with 8 mbar of O2 at 195 K resulted in a rapid color change from off-white to pink, indicative of a change in the copper oxidation state. Analysis of powder x-ray diffraction data collected for Cu2.4-MFU-4l at 195 K before and after dosing with 8 mbar of O2 revealed a shift in the peak positions to higher 2q values with gas dosing, while dosing with higher O2 pressures of 109 and 1005 mbar did not lead to further changes in the peak positions (Figure ).