Metal-organic frameworks (MOFs) hold great potential in catalysis as they promise to marry the advantages of homogeneous and heterogeneous catalysts. Their high metal concentrations and nearly fully exposed active sites, make them close to ideal singlesite catalysts sought after in heterogeneous catalysis. A challenge is controlling the metal's coordination environment, which in MOFs is often limited by the geometries of available secondary building units. Zr-MOFs, composed of hexameric Zr-oxo cluster Zr 6 O 4 (OH) 4 (RCOO) 12 (where RCOO represents carboxylic acid ligands), exemplify this concept, with clusters functioning as the smallest possible ZrO 2 catalyst. Their reactivity is not only shaped by the design principle of reticular chemistry but to a great extent by defects. Keeping the analogy to bulk ZrO 2 catalysts, the typical {Zr 6 } (= hexameric Zr oxo clusters) metal node resembles the local structure of cubic zirconia (c-ZrO 2 ). However, lower symmetry phases, like monoclinic zirconia (m-ZrO 2 ) are desired for catalysis. Zr cations in m-ZrO 2 are 7-coordinated with two different Zr-O bond lengths, in contrast to the high-symmetry ZrO 8 sites in c-ZrO 2 . This creates coordinatively unsaturated Zr (Zr cus ) sites in m-ZrO 2 , and their concentration can be enhanced by inducing oxygen vacancies (O-vacancies), improving catalytic performances. In a perfect Zr-MOF crystal, every metal site is 8-coordinated, just like in c-ZrO 2 . To reduce their valency, defects have to be induced.