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

The emergence of superior water stable Zr-based metal-organic framework UiO-66 represents a breakthrough in MOF chemistry for practical applications. However, ever since the structure was first reported in 2008 (J. Am. Chem. Soc. 2008, 130, 13850), it has often been observed that a notable concentration of defects are present in UiO-66 samples. The defect concentration can be well controlled during synthesis, leading to highly tunable 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 evidence that the missing-linker defect sites in an ambient environment are compensated with both carboxylate and water (bound through intermolecular H-bonding), which is further supported by ab initio 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 a unidentate mode. The neighboring Zr⁴⁺ site is terminated by a coordinating H₂O molecule, which helps to stabilize the COO⁻ group. This finding provides a new understanding of defect termination in UiO-66, and sheds light on the origin of its catalytic activity.