A nearly flat -∆Hads curve was observed up to approximately 80% of the expected uptake for one H2 per Cu I based on the formula unit of each Cu I M-MFU-4l framework. The H2 loading when -∆Hads starts to decrease (1.2, 1.9, and 1.4 mmol/g for Cu I Zn-MFU-4l, Cu I Mn-MFU-4l, and Cu I Cd-MFU-4l, respectively) is reminiscent of H2 uptake in each Cu I M-MFU-4l framework at 1 mbar and 77 K (1.3, 1.9, and 1.7 mmol/g for Cu I Zn-MFU-4l, Cu I Mn-MFU-4l, and Cu I Cd-MFU-4l, respectively). These Clausius-Clapeyron analyses also confirm that the H2 binding enthalpy follows the order Cu I Zn-MFU-4l > Cu I Mn-MFU-4l > Cu I Cd-MFU-4l, as shown in the 298-K isotherm data (Figure ). This order follows the trend in size of the central metal ion within each pentanuclear cluster, with larger metals giving lower binding enthalpies. Notably, ∆Hads for Cu I Cd-MFU-4l (-22.7 kJ/mol) is in the range of the optimal binding enthalpy for ambient temperature H2 storage (-15 to -25 kJ/mol). Thus, the material represents just the second example of a MOF that exhibits the optimal H2 binding enthalpy. We also note that, although the Cu I -containing framework NU-2100 was recently reported to show a high initial ∆Hads of -32 kJ/mol, the strong H2 binding in this material is limited to only ~1% of Cu I in the framework (0.025 mmol/g) and the nature of the sites responsible has not been elucidated. In contrast, the quantity of strong adsorption sites in the Cu I M-MFU-4l series approaches the stoichiometric value for one H2 per Cu I , thereby allowing an examination of structure-property effects with well-defined Cu I sites. The difference IR spectra for the three Cu I M-MFU-4l frameworks exhibit two peaks for H2 (Figure ). One is a broad asymmetric peak most intense at 3250, 3300, and 3380 cm -1 for Cu I Zn-MFU-4l, Cu I Mn-MFU-4l, and Cu I Cd-MFU-4l, respectively (Table ). The other peak is sharp and lower in intensity, and is centered around 4120 cm -1 for all Cu I M-MFU-4l frameworks. Given that the H-H stretching frequency, ν(H-H), of free H2 arises at 4161 cm -1 , we assign the broad peaks (for which the extent of peak shift depends on the central metal identity) to the stretching mode of H2 strongly bound to Cu I sites, whereas we assign the peaks at ~4120 cm -1 to H2 weakly bound at secondary, non-metal adsorption sites. The broadening of an H-H stretching signal is commonly observed for molecular H2 complexes and has also been seen previously for Cu I Zn-MFU-4l, supporting our assignment of the signal for H2 bound to Cu I sites. Such peak broadening is characteristic of a chemically activated H2 molecule, which results in a considerably broad, asymmetric probability distribution for the internuclear distance. The trend in the shifts of ν(H-H) confirms that the H-H bond is less activated when bound to a Cu I site residing at a pentanuclear secondary building unit with a larger central metal.