Materials Synthesis and Characterization. The framework Cu I -MFU-4l was initially synthesized following the literature protocol. In brief, Zn5Cl4(btdd)3 (MFU-4l) was treated with excess CuCl2 in N,N-dimethylacetamide under an inert atmosphere at 60 °C to give Cu II Cl-MFU-4l (Cu II 2.2Zn2.8Cl4(btdd)3 based on inductively coupled plasma optical emission spectroscopy, ICP-OES). Subsequent anion exchange with lithium formate and thermolysis at 180 °C afforded Cu I -MFU-4l (Figure ). When prepared using this route, Cu I -MFU-4l has been reported to contain a mixture of copper(I) and copper(II) ions. The copper(I) sites in this material are known to strongly bind H2, and therefore as a means of estimating the number of these sites in Cu I -MFU-4l, we collected H2 adsorption isotherms at 77 K and pressures ranging from 0 to 1.2 bar (Figure ). The material exhibits steep H2 uptake at low pressures and achieves a capacity of 1.26 mmol/g at 1 mbar, followed by more gradual uptake at higher pressures indicative of H2 physisorption. If all of the copper sites in the material were trigonal pyramidal copper(I), and assuming a 1:1 stoichiometry for H2 binding, we would expect a low-pressure uptake of approximately 1.9 mmol/g (based on the copper site stoichiometry determined for the Cu II -MFU-4l precursor from ICP-OES). From the measured uptake of 1.26 mmol/g at 1 mbar, we then estimate ~68% of the copper ions are exposed copper(I) sites. While estimates of copper(I) loading achieved in this way are qualitative, we propose that the H2 uptake at 1 mbar may be a useful metric for more consistently evaluating and reporting copper(I) loading in Cu I -MFU-4l.