The potential of metal–organic frameworks (MOFs) and covalent organic frameworks (COFs) as platforms for the development of heterogeneous single-site catalysts is reviewed thoroughly.

We present a synthetic strategy for the efficient encapsulation of a derivative of a well-defined cobaloxime proton reduction catalyst within a photoresponsive metal–organic framework (NH₂-MIL-125(Ti)). The resulting hybrid system Co@MOF is demonstrated to be a robust heterogeneous composite material. Furthermore, Co@MOF is an efficient and fully recyclable noble metal-free catalyst system for light-driven hydrogen evolution from water under visible light illumination.

The effect of a post-synthetic acid treatment on the catalytic performance of MOFs is evaluated for MIL-100(Fe), an iron-benzenetricarboxylate. The acid-treated frameworks are structurally robust as no differences have been found in XRD patterns after treatment. Porosity of the acid-treated MOFs gradually decreases, most probably as a consequence of anions remaining in the charged frameworks. Monitoring the modification of the MOFs by reactions of which the outcome depends on the acid properties of the catalyst suggests the presence of two types of active sites, with weak Brönsted acid sites in close vicinity to the Lewis acid open metal sites. This is supported by CO-chemisorption experiments which indicate a large increase of both Lewis and Brönsted acidity. In Diels–Alder reactions of oxygenated dienophiles with 1,3-cyclohexadiene, a strong increase of the activity is found for the acid-treated MOFs. This is explained by the enhanced activation of the dienophiles on the modified active sites.