Defect Termination in the UiO-66 Family of Metal–Organic Frameworks: The Role of Water and Modulator

Abstract

The defect concentration in the prototypical metal-organic framework UiO-66 can be well controlled during synthesis, leading to precisely tunable physicochemical properties for this structure. However, there has been a long-standing debate regarding the nature of the compensating species present at the defective sites. Here, we present unambiguous spectroscopic evidence that the missing-linker defect sites in an ambient environment are compensated with both carboxylate and water (bound through intermolecular hydrogen bonding), which is further supported by <i>ab initio</i> calculations. In contrast to the prevailing assumption that the monocarboxylate groups (COO^-) of the modulators form bidentate bonding with two Zr⁴⁺ sites, COO^- is found to coordinate to an open Zr⁴⁺ site in an unidentate mode. The neighboring Zr⁴⁺ site is terminated by a coordinating H₂O molecule, which helps to stabilize the COO^- group. This finding not only provides a new understanding of defect termination in UiO-66, but also sheds light on the origin of its catalytic activity.