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)xCl2-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.