Postsynthetic Triazolate Functionalization of Ni2Cl2BTDD: Creation of Supplementary CO2 Adsorption Sites and Exceptional CO2 Selectivity

Postsynthetic modification (PSM) offers a versatile approach to tune metal-organic frameworks (MOFs) by introducing functional groups or exchanging bridging ligands while preserving open metal sites (OMSs). Nevertheless, generating additional adsorption sites via PSM remains challenging. Herein, we report a new PSM strategy that creates supplementary sorption sites within a robust MOF framework. We synthesized triazolate-exchanged Ni₂(triazolates)_xCl_2-xBTDD (BTDD = bis(1H-1,2,3-triazolo[4,5-b],[4',5'-i])dibenzo[1,4]dioxin), where bridging chlorides in the parent Ni₂Cl₂BTDD were successfully replaced with triazolate ligands. Rietveld refinement of synchrotron X-ray diffraction confirmed that triazolates bridge adjacent Ni(II) centers through 1,2-nitrogen atoms without obstructing the OMSs. This structural modification substantially enhanced CO₂ adsorption owing to newly formed binding sites and cooperative interactions between triazolate moieties and BTDDs. Among the modified frameworks, Ni_dmtz incorporating 3,5-dimethyl-1,2,4-triazolate (dmtz) showed a remarkable improvement: CO₂ uptake increased 5.2-fold at 0.15 bar and 2.6-fold at 1 bar compared to the parent framework, while also displaying enhanced stability under humid conditions. Notably, the Ideal Adsorbed Solution Theory (IAST) selectivity for CO₂/N₂ (15/85 v/v) reached 456,000, significantly surpassing most reported MOFs. Grand Canonical Monte Carlo simulations revealed the presence of a third adsorption site in Ni_dmtz, where CO₂ interacts with the methyl-substituted triazolate in addition to conventional ligand pocket sites. This study highlights a simple yet effective PSM approach to design multifunctional sorbents and provides valuable insights into the rational development of MOFs for efficient CO₂ capture.