Quantum Chemical Modeling of Hydrogen Binding in Metal--Organic Frameworks: Validation, Insight, Predictions and Challenges.

A detailed chemical understanding of \ce{H2} interactions with binding sites in the nanoporous crystalline structure of metal--organic frameworks (MOFs) can lay a sound basis for the design of new sorbent materials. Computational quantum chemical calculations can aid in this quest. To set the stage, we review general thermodynamic considerations that control the usable storage capacity of a sorbent. We then discuss cluster modeling of \ce{H2} ligation at MOF binding sites using state-of-the-art density functional theory (DFT) calculations, and how the binding can be understood using energy decomposition analysis (EDA). Employing these tools, we illustrate the connections between the character of the MOF binding site and the associated adsorption thermodynamics using four experimentally characterized MOFs, highlighting the role of open metal sites (OMSs) in accessing binding strengths relevant to room temperature storage. The sorbents are MOF-5, with no open metal sites, \ce{Ni2}(\textit{m}-dobdc), containing Lewis acidic Ni(II) sites, Cu(I)-MFU-4\textit{l}, containing $\pi$ basic Cu(I) sites and V\textsubscript{2}Cl\textsubscript{2.8}(btdd), also containing $\pi$-basic V(II) sites. We next explore the potential for binding multiple \ce{H2} molecules at a single metal site, with thermodynamics useful for storage at ambient temperature; a materials design goal which has not yet been experimentally demonstrated. Computations on Ca$^{2+}$ or Mg$^{2+}$ bound to catecholate or Ca$^{2+}$ bound to porphyrin show the potential for binding up to 4 \ce{H2}; there is precedent for the inclusion of both catecholate and porphyrin motifs in MOFs. Turning to transition metals, we discuss the prediction that two \ce{H2} molecules can bind at V(II)-MFU-4\textit{l}, a material that has been synthesized with solvent coordinated to the V(II) site. Additional calculations demonstrate binding three equivalents of hydrogen per OMS in Sc(I) or Ti(I)-exchanged MFU-4\textit{l}. Overall, the results suggest promising prospects for experimentally realizing higher capacity hydrogen storage MOFs, if nontrivial synthetic and desolvation challenges can be overcome. Coupled with the unbounded chemical diversity of MOFs, there is ample scope for additional exploration and discovery.