Recyclable, immobilized transition-metal photocatalysts

Abstract

The use of transition-metal complexes asphotocatalysts have allowed the performance of diverse organic transformations in an outstanding manner, characterized not only by high yields, TOF,and selectivity values, but also by modulating and providing access to novel molecular structures that, without them, would be difficult if not impossible. However, one of the biggest concerns regarding theuse of these photocatalysts relies on the difficulties associated with their isolation from reaction media and reutilization once the chemical process ends. The above, besides contaminating reaction products and requiring out tedious purification processes, prompts the inevitable loss of the catalyst, directly affectingits recyclability. In addition, this situation results innegative outcomes from an economic and environ-mental perspective, since transition-metal complexesare usually expensive materials, and their unsuccess-ful recovery could result in leakage into the environ-ment. These drawbacks served as inspiration for the elaboration of the present review focused on the development of novel strategies developed during the last decade for the successful recovery of these species. The strategies summarized herein, whether for homogeneous or heterogeneous systems, are based on the use of thermotropic solvents, changes in the hydrophilic balance of the catalyst, the employ-ment of polymers, copolymers, porous macromolecular structures, and inorganic nanostructures as supportof these entities. Moreover, the use of organized and confining media, such as micelles and gels in this context, is also discussed. We hope that this review will motivate the search for new strategies to develop novel catalytic systems, understanding that high performance is based not only on yields but also on recyclability, sustainability, and responsibility to the environment.