Highly robust and thermally stable MFU-4-type frameworks caught our attention to design welldefined strong adsorption sites for H2. Within the MFU-4 family, the MFU-4l (Zn5Cl4(btdd)3) (H2btdd = bis(1H-1,2,3-triazolo [4,5-b],[4′,5′-i])dibenzo [1,4]dioxin) framework is particularly noteworthy due to its longer linker and corresponding larger pore size, which facilitates postsynthetic modifications by allowing easier diffusion of molecules through the pores. MFU-4l was originally synthesized through a solvothermal reaction of zinc chloride with the H2btdd linker. This process yields a cubic framework with a large lattice parameter of approximately 30 Å (Figure ) and a secondary building unit (SBU) comprised of a Kuratowski-type cluster with composition Zn5Cl4(triazolate)6 (Figure ). The Tdsymmetric SBU features two types of Zn II ions: a central ion having octahedral coordination to six organic linkers and four peripheral Zn II ions. Each of these peripheral ions has pseudo-tetrahedral coordination with three triazolate ligands and a chloride ion, although the latter can be substituted by various anionic ligands, including hydroxide, azide, and formate. The peripheral Zn II ions in the MFU-4l framework can be substituted by other M I or M II metals through the post-synthetic ion-exchange reaction. Additionally, the substituted M II atom can be chemically reduced, yielding coordinatively unsaturated M I sites. This approach has been experimentally documented exclusively for Cu II to date. This synthetic flexibility represents a powerful tuning tool for MFU-4-type MOFs, as exemplified with the recent discoveries of Li I Zn II -MFU-4l and Cu I Zn II -MFU-4l. However, relative to the ideal enthalpy range, Li I Zn II -MFU-4l shows too weak H2 binding (isosteric enthalpy of adsorption, Qst = -3 to -5 kJ/mol). In turn, the Cu I Zn II -MFU-4l exhibits an excessively strong binding of hydrogen with Qst = -32 kJ/mol; thus, the majority of H2 is adsorbed below 5 bar and cannot contribute to the usable capacity. Another example is a recent computational study of the MFU-4l frameworks with vanadium (II) open metal sites, which was predicted to allow multiple hydrogen molecules to ligate per V II site, increasing the overall hydrogen usable capacity. Nevertheless, the practical utility of V II Zn II -MFU-4l is significantly limited due to the high sensitivity of vanadium(II)-containing frameworks to oxygen. Among various H2 binding mechanisms observed in MOFs with unsaturated open metal sites, πbackdonation has been demonstrated as the predominant binding mechanism for Cu I Zn II -MFU-4l and V II Zn II -MFU-4l. This involves the dxz and dyz orbitals of the peripheral transition metal ion donating electron density to the σ* anti-bonding orbital of the H2 molecule. Consequently, computational studies indicate the H-H bond distance increases upon coordination with the metal, with a general trend indicating that stronger M-H2 interactions lead to greater activation and, thus, elongation of the H-H bond distance.