Gate Opening without Volume Change Triggers Cooperative Gas Interactions, Underpins an Isotherm Step in Metal–Organic Frameworks

Abstract

Three halogenated metal–organic frameworks (MOFs) reported recently exhibited a second step in their CO2 gas adsorption isotherms. The emergence of halogen-bonding interactions beyond a threshold gas pressure between the framework halogen and the CO2 guest was conjectured to be the underlying reason for the additional step in the isotherm. Our investigation employing periodic density functional theory calculations did not show significant interactions between the halogen and CO2 molecules. Further, using a combination of DFT-based ab initio molecular dynamics and grand canonical Monte Carlo simulations, we find that the increased separation of framework nitrate pairs facing each other across the pore channel enables the accommodation of an additional CO2 molecule which is further stabilized by cooperative interactionsan observation that facilely explains the second isotherm step. The increased separation between the nitrate groups can occur without any lattice expansion, consistent with experiments. The results point to a structural feature to achieve this isotherm step in MOFs that neither possess large pores nor exhibit large-scale structural changes such as breathing.

Interplay of halogen bonding and solvation in protein–ligand binding

“…It is likely that a more accurate description of the halogen atom is needed, taking into account that the electron density is modified by surrounding atoms, breaking the symmetry so that the negative ring splits into regions of more or less negative charge. 48 , 49 , 50 We also performed free-energy perturbation calculations of the relative binding affinities of the five ligands and showed that the standard parameters reproduced the experimental binding affinities excellently (within 1.8 kJ/mol), whereas the EP parameters gave much worse results ( cf . Table S3 ).…”

Noncovalent interaction guided selectivity of haloaromatic isomers in a flexible porous coordination polymer

“…55 This geometry-based criterion for halogen bonding identication is a necessary condition but not a sufficient one, especially for the case of guests conned within MOF/ PCP pores. 56 The electronic structure, however, denitively reveals the presence of such interactions. Hence, to elucidate this specic interaction (blue-dashed line in Fig.…”

“…Hence, to elucidate this specific interaction (blue-dashed line in Fig. 4a), we have calculated maximally localized Wannier functions (MLWFs), and their corresponding centers – maximally localised Wannier function centers (MLWFCs), 56–58 MLWFs (violet surfaces) and MLWFCs (yellow spheres) of CO1 (Fig. 4g–h) show that the π-electrons of the double bond face the C–Cl axis.…”

Porous Metal‐Organic Cages Based on Rigid Bicyclo[2.2.2]oct‐7‐ene Type Ligands: Synthesis, Structure, and Gas Uptake Properties

“…Given the differences in the functionalisation of the ligands, we speculate that there may be enhanced interactions between CO 2 molecules and the bromo‐functionalised ligand of 3 a in comparison to the cages 1 a and 2 a . Halogen bonding interactions with CO 2 have been proposed as leading to pseudo‐gate opening behaviour in MOFs, [20] but the role of those interactions is contested [21] . The lack of structural information for MOPs in their activated state is common in the field, and represents a challenge for understanding their structure‐property relationships, particularly with gas sorption behaviour.…”

“…Given the differences in the functionalisation of the ligands, we speculate that there may be enhanced interactions between CO 2 molecules and the bromo-functionalised ligand of 3 a in comparison to the cages 1 a and 2 a. Halogen bonding interactions with CO 2 have been proposed as leading to Chemistry-A European Journal pseudo-gate opening behaviour in MOFs, [20] but the role of those interactions is contested. [21] The lack of structural information for MOPs in their activated state is common in the field, and represents a challenge for understanding their structure-property relationships, particularly with gas sorption behaviour. The post-sorption analysis of 1 a, 2 a, and 3 a using IR spectroscopy and PXRD showed that the materials retain the packing associated with the MeOH solvate phases 1-MeOH, 2-MeOH, and 3-MeOH, suggesting that the materials do not decompose under activation (Figures S33-S38), although there is an appreciable loss in crystallinity.…”