<p>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 O<sub>2</sub> and N<sub>2</sub> 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<sup>-</sup>, μ-Cl<sup>-</sup>, μ-F<sup>-</sup>, μ-SH<sup>-</sup>,
or μ-OH<sup>-</sup> groups), and altering the formal oxidation state of the
metal. As a result, we show that it is possible to tune the O<sub>2</sub>
affinity at the open metal sites of MOFs for applications involving the strong
and/or selective binding of O<sub>2</sub>. In contrast with O<sub>2</sub>
adsorption, N<sub>2</sub> adsorption at open metal sites is predicted to be
relatively weak across the MOF dataset, with the exception of MOFs containing
synthetically elusive V<sup>2+</sup> open metal sites. As one example from the
screening study, we predict that exchanging the μ-Cl<sup>-</sup> ligands of M<sub>2</sub>Cl<sub>2</sub>(BBTA)
(H<sub>2</sub>BBTA = 1<i>H</i>,5<i>H</i>-benzo(1,2-d:4,5-d′)bistriazole) with
μ-OH<sup>-</sup> groups would significantly enhance the strength of O<sub>2</sub>
adsorption at the open metal sites without a corresponding increase in the N<sub>2</sub>
affinity. Experimental investigation of Co<sub>2</sub>Cl<sub>2</sub>(BBTA) and
Co<sub>2</sub>(OH)<sub>2</sub>(BBTA) confirms that the former exhibits only weak
physisorption, whereas the latter is capable of chemisorbing O<sub>2</sub> at
room temperature. The chemisorption behavior is attributed to the greater
electron-donating character of the μ-OH<sup>-</sup><sub> </sub>ligands and the
presence of H-bonding interactions between the μ-OH<sup>-</sup> bridging
ligands and the O<sub>2</sub> adsorbate.</p>
Metal-Organic Frameworks for Highly Selective Separations
