A Redox-Active Mesoporous Cobalt-Pyrazolate Framework for Reversible O₂ Sorption

Expanding pyrazolate metal-organic frameworks (MOFs) beyond microporous architectures is a formidable synthetic challenge, as the strong and directional M-N bonds impose strict geometric constraints that hinder the integration of mesoporosity and active sites. Such limitations have restricted the structural diversity of pyrazolate MOFs compared with their carboxylate analogues, despite the former offering superior chemical stability and fantastic performance in gas storage, separation, and catalysis. Here we present mesoporous BUT-45 as the first example of csq-type pyrazolate MOF, which was constructed from the low-symmetry (C_s) tetra-pyrazolate ligand 1,3,6,8-tetra(1H-pyrazolate-4-yl)-9H-carbazole (CTP^4-) and 8-connected Co₆ clusters (D_2h). Interestingly, the presence of rich active Co sites enables instantaneous O₂ chemisorption at ambient temperature in BUT-45, and good framework stability allows this process to be fully reversible. Single-crystal x-ray diffraction and in situ spectroscopy analyses provide structural insights into the as-synthesized, O₂-loaded, and regenerated phases, revealing the mechanism of Co-O₂ adduct formation and demonstrating complete reversibility via hydrazine hydrate reduction. This work highlights how reticular chemistry can map target nets from carboxylate to pyrazolate, while offering desired properties and direct visualization of redox chemistry in MOFs.