Simple Host-Initiated Photofunctionalization of Vinyl-Appended UiO-66

Metal–organic frameworks (MOFs) serve as outstanding adsorbents for numerous separations, thanks to the high internal surface areas and tunable functionality of these materials. However, to tailor a MOF for metal ion separations, coordinating or chelating groups must be appended to the MOF scaffold. This functionalization presents a particular challenge since the coordinating groups may bind to the MOF node itself, interfering with the synthesis or structural integrity of the framework. Here, we present a novel route to install coordinating groups postsynthetically through photochemical reactions initiated by the MOF backbone. We found that a vinyl-appended derivative of 1,4-benzenedicarboxylic acid (BDC), which we term BDC-vinyl, is photoactivated by irradiation with 370 nm light. The activated BDC-vinyl reacts in a radical mechanism with substrates, including toluene, 1-propanethiol, and vinyl sulfonate, to form a covalent bond between the vinyl group of BDC-vinyl and the respective reaction partner. Crucially, this reactivity is preserved once BDC-vinyl is incorporated into a MOF as the sole linker, leading to photofunctionalization of the MOF in the absence of any external photoinitiator. We showed that BDC-vinyl could be used as the sole linker for the construction of vinyl-appended derivatives of UiO-66, CuBDC-DABCO, and MIL-53-PNO, and we carried out a series of photofunctionalization reactions on the UiO-66 analogue (termed UiO-66-vinyl). We confirmed that the products are evenly and fully functionalized through solid-state NMR, powder X-ray diffraction (PXRD), and N 2 adsorption, among other techniques. Finally, we demonstrated that the functionalization of UiO-66-vinyl with thiol or sulfonate groups modulates its capacity for rare-earth or transition metal targets, thus validating our approach to adsorbent customization. This photochemical approach to the functionalization of vinyl-appended MOFs provides chemists with a simple and mild route to custom adsorbents.