properties of the MX-MFU-4l and Cu I M-MFU-4l frameworks, we collected low-pressure H2 adsorption isotherms at 77 K (Figure ). All parent MX-MFU-4l frameworks show similar adsorption profiles at 77 K, with no steep uptake below 1 mbar indicating the absence of strong H2 adsorption sites. In contrast, all Cu I M-MFU-4l frameworks exhibit steep H2 adsorption profiles below 1 mbar at 77 K. Notably, H2 uptake at 77 K and 1 mbar in Cu I M-MFU-4l reaches approximately 80% of the expected value for one H2 per Cu I site of the frameworks in each Cu I M-MFU-4l (80%, 85%, and 87% for Cu I Zn-MFU-4l, Cu I Mn-MFU-4l, and Cu I Cd-MFU-4l, respectively), suggesting that the initial steep H2 uptake results from H2 binding at the open Cu I sites. The slightly lower H2 uptake than theoretically expected implies the presence of inactive Cu for H2, possibly present at defect sites. Since the uptake below 1 mbar at 77 K was too steep to compare the H2 binding strengths of the frameworks, H2 adsorption isotherms were also collected at 298 K (Figure ). The initial slope of the isotherms (Henry's law constant, Table ) indicates that the H2 binding strength ranks in the order Cu I Zn-MFU-4l > Cu I Mn-MFU-4l > Cu I Cd-MFU-4l.