Tuning the Redox Activity of Metal-Organic Frameworks for Enhanced, Selective O₂ Binding: Design Rules and Ambient...

Metal-organic frameworks (MOFs) with coordinatively unsaturated metal sites are appealing as adsorbent materials due to their tunable functionality and ability to selectively bind small molecules. Through the use of computational screening methods based on periodic density functional theory, we investigate O₂ and N₂ adsorption at the coordinatively unsaturated metal sites of several MOF families. A variety of design handles are identified that can be used to modify the redox activity of the metal centers, including changing the functionalization of the linkers (replacing oxido donors with sulfido donors), anion exchange of bridging ligands (considering μ-Br^-, μ-Cl^-, μ-F^-, μ-SH^-, or μ-OH^- groups), and altering the formal oxidation state of the metal. As a result, we show that it is possible to tune the O₂ affinity at the open metal sites of MOFs for applications involving the strong and/or selective binding of O₂. In contrast with O₂ adsorption, N₂ adsorption at open metal sites is predicted to be relatively weak across the MOF dataset, with the exception of MOFs containing synthetically elusive V²⁺ open metal sites. As one example from the screening study, we predicted that exchanging the μ-Cl^- ligands of M₂Cl₂(BBTA) (H₂BBTA = 1H,5H-benzo(1,2-d:4,5-d')bistriazole) with μ-OH^- groups would significantly enhance the strength of O₂ adsorption at the open metal sites without a corresponding increase in the N₂ affinity. Experimental investigation of Co₂Cl₂(BBTA) and Co₂(OH)₂(BBTA) confirms that the former exhibits weak physisorption of both N₂ and O₂, whereas the latter is capable of chemisorbing O₂ at room temperature in a highly selective manner. The O₂ chemisorption behavior is attributed to the greater electron-donating character of the μ-OH^- ligands and the presence of H-bonding interactions between the μ-OH^- bridging ligands and the reduced O₂ adsorbate.