Density functional theory calculations carried out on a pentanuclear cluster model of Cu I -MFU-4l further support an equilibrium between O2 bound side-on and end-on to the copper sites in the framework (see Section 10 of the Supporting Information and Fig- ). In particular, the η 2 -O2 binding mode was found to be favored based on electronic energies over the η 1 -O2 binding mode (ΔE = -65 versus -46 kJ mol -1 , respectively). Additionally, the calculated O-O bond lengths for O2 bound in an η 2 and η 1 fashion are slightly longer than the O-O bond length for gaseous O2 (calculated 1.31 and 1.26 Å O2 versus 1.21 Å, respectively), consistent with electron transfer from copper(I) to O2 to form a copper(II)-superoxo (O2 •-) moiety. Taken together, the structural, spectroscopic, and computational results support the formation of Cu II -O2 •-moieties upon O2 binding in Cu I -MFU-4l. Investigation of O2, N2, Ar, and H2O Adsorption. Single-component O2, N2, and Ar isotherms were collected for Cu2.7-MFU-4l at 298 K and pressures ranging from 0-1 bar (Figures 4a). Consistent with O2 binding at the open copper sites, the material exhibits relatively steep O2 uptake at low pressures and achieves a capacity of 1.51 mmol/g at 210 mbar, the partial pressure of O2 in air. The uptake begins to level off at higher pressures, reaching a value of 2.03 mmol/g at 1 bar O2. Assuming one O2 molecule binds at every copper(I) site, the theoretical capacity (excluding adsorption at secondary sites in the material) is 2.09 mmol/g. Considering that the capacity at 1 bar represents both chemisorption and physisorption, the experimental uptake suggests that not all copper(I) sites in this framework are saturated at this pressure. Nitrogen uptake in Cu2.7-MFU-4l is more gradual at low pressures, and the material adsorbs less N2 than O2 over the entire pressure range (1.47 mmol/g at 1 bar). However, at the partial pressure of N2 in air (780 mbar), the material exhibits an N2 capacity of 1.30 mmol/g N2 that is only slightly less than the O2 capacity at 210 mbar. Finally, Cu2.7-MFU-4l adsorbs very little Ar at 298 K between 0 and 1 bar, and at 9 mbar, the partial pressure of Ar in air, the material adsorbs <0.01 mmol/g. Variable-temperature Ar, O2, and N2 adsorption isotherms were collected to determine enthalpies of adsorption in Cu2.7-MFU-4l (Figures and). Low-temperature Ar isotherms were collected at 170, 180, and 190 K, and these data could be simultaneously modeled using the single-site Langmuir-Freundlich equation (Figure ). In contrast, a dual-site Langmuir-Freundlich equation was needed to model O2 and N2 isotherms collected at 288, 298, and 308 K, consistent with primary gas binding at the copper sites and secondary interactions with the framework (Table ). Using the fits to these data and the Clausius-Clapeyron equation, we determined O2, N2, and Ar adsorption enthalpies as a function of loading (Figure ). The isosteric enthalpy of adsorption (ΔHads) at low loadings of O2 is -56.8(1) kJ/mol, higher than the values determined for N2 (-38.9(4) kJ/mol) and Ar (-10.9(1) kJ/mol). The enthalpy of Ar adsorption is consistent with weak adsorbate-framework interactions and remains essentially constant with loading, while the heats of adsorption for O2 and N2 gradually decline as the copper sites become saturated and secondary adsorption sites within the framework are occupied. The O2 and N2 adsorption enthalpies are consistent with previously reported isosteric heats of adsorption in Cu I -MFU-4l (ΔHads = -Qst = -52.6(6) and -41.6(6) kJ/mol, respectively) and indicative of strong interactions with the open copper(I) sites. This result is consistent with the DRIFTS data and electron transfer from copper(I) to O2.