Using Postsynthetic X-Type Ligand Exchange to Enhance CO2 Adsorption in Metal–Organic Frameworks with Kuratowski-Type Building Units

Postsynthetic modification methods have emerged as indispensable tools for tuning the properties and reactivity of metal–organic frameworks (MOFs). In particular, postsynthetic X-type ligand exchange (PXLE) at metal building units has gained increasing attention as a means of immobilizing guest species, modulating the reactivity of framework metal ions, and introducing new functional groups. The reaction of a Zn–OH functionalized analogue of CFA-1 (1-OH, Zn(ZnOH)4(bibta)3, where bibta2– = 5,5′-bibenzotriazolate) with organic substrates containing mildly acidic E–H groups (E = C, O, N) results in the formation of Zn–E species and water as a byproduct. This Brønsted acid–base PXLE reaction is compatible with substrates with pKa(DMSO) values as high as 30 and offers a rapid and convenient means of introducing new functional groups at Kuratwoski-type metal nodes. Gas adsorption and diffuse reflectance infrared Fourier transform spectroscopy experiments reveal that the anilide-exchanged MOFs 1-NHPh0.9 and 1-NHPh2.5 exhibit enhanced low-pressure CO2 adsorption compared to 1-OH as a result of a Zn–NHPh + CO2 ⇌ Zn–O2CNHPh chemisorption mechanism. The MFU-4l analogue 2-NHPh ([Zn5(OH)2.1(NHPh)1.9(btdd)3], where btdd2– = bis(1,2,3-triazolo)dibenzodioxin), shows a similar improvement in CO2 adsorption in comparison to the parent MOF containing only Zn–OH groups.