(65h) Effect of Capping Group Binding on Chemical Reactivity

Metal-Organic Framework UiO-66 can be engineered to have different levels of missing linker defects by using different modulators (monocarboxylic acids used as crystal growth agents) during the synthesis. It has been shown that modulators bind to open metal defect sites either in a bidentate form, with both oxygens of the monocarboxylate bound to the two Zr open metal sites, or in a unidentate form, with one oxygen bound to a Zr and a water molecule bound to the other open metal site. It is possible that different capping groups and different capping modes (bi- vs unidentate) could lead to a difference in the catalytic activity of the defective UiO-66. In this work, we use ab initio methods to calculate the binding energy differences and kinetic barriers between bi- and unidentate structures for formic acid (FA), acetic acid (AA), trifluoroacetic acid (TFA), and proline (Pr) modulators. Our calculations reveal that the modulator in unidentate binding has lower energy than bidentate binding for FA, AA, and TFA, while Pr has the opposite trend. We hypothesize that modulators having a low barrier for the transition from the bidentate to the unidentate state will have higher catalytic activity for reactions occurring at the Zr open metal site.