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Negative cooperativity upon hydrogen bond-stabilized O₂ adsorption in a redox-active metal-organic framework.

ABSTRACT: The design of stable adsorbents capable of selectively capturing dioxygen with a high reversible capacity is a crucial goal in functional materials development. Drawing inspiration from biological O₂ carriers, we demonstrate that coupling metal-based electron transfer with secondary coordination sphere effects in the metal-organic framework Co₂(OH)₂(bbta) (H₂bbta = 1H,5H-benzo(1,2-d:4,5-d')bistriazole) leads to strong and reversible adsorption of O₂. In particular, moderate-strength hydrogen bonding stabilizes a cobalt(III)-superoxo species formed upon O₂ adsorption. Notably, O₂-binding in this material weakens as a function of loading, as a result of negative cooperativity arising from electronic effects within the extended framework lattice. This unprecedented behavior extends the tunable properties that can be used to design metal-organic frameworks for adsorption-based applications.

SUBMITTER: Oktawiec J

PROVIDER: S-EPMC7303157 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:Precisely locating extra-framework cations in anionic metal-organic framework compounds remains a long-standing, yet crucial, challenge for elucidating structure-performance relationships in functional materials. Single-crystal X-ray diffraction is one of the most powerful approaches for this task, but single crystals of frameworks often degrade when subjected to post-synthetic metalation or reduction. Here, we demonstrate the growth of sizable single crystals of the robust metal-organic framework Fe2(bdp)3 (bdp2- = benzene-1,4-dipyrazolate) and employ single-crystal-to-single-crystal chemical reductions to access the solvated framework materials A2Fe2(bdp)3·yTHF (A = Li+, Na+, K+). X-ray diffraction analysis of the sodium and potassium congeners reveals that the cations are located near the center of the triangular framework channels and are stabilized by weak cation-π interactions with the framework ligands. Freeze-drying with benzene enables isolation of activated single crystals of Na0.5Fe2(bdp)3 and Li2Fe2(bdp)3 and the first structural characterization of activated metal-organic frameworks wherein extra-framework alkali metal cations are also structurally located. Comparison of the solvated and activated sodium-containing structures reveals that the cation positions differ in the two materials, likely due to cation migration that occurs upon solvent removal to maximize stabilizing cation-π interactions. Hydrogen adsorption data indicate that these cation-framework interactions are sufficient to diminish the effective cationic charge, leading to little or no enhancement in gas uptake relative to Fe2(bdp)3. In contrast, Mg0.85Fe2(bdp)3 exhibits enhanced H2 affinity and capacity over the non-reduced parent material. This observation shows that increasing the charge density of the pore-residing cation serves to compensate for charge dampening effects resulting from cation-framework interactions and thereby promotes stronger cation-H2 interactions.

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Negative cooperativity upon hydrogen bond-stabilized O2 adsorption in a redox-active metal-organic framework.

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Negative cooperativity upon hydrogen bond-stabilized O₂ adsorption in a redox-active metal-organic framework.