Using Postsynthetic X-Type Ligand Exchange to Enhance CO<sub>2</sub> Adsorption in Metal–Organic Frameworks with Kuratowski-Type Building Units
Caitlin E. Bien, Zhongzheng Cai, Casey R. Wade
Abstract
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.