Non-Heme Imine-Based Iron Complexes as Catalysts for Oxidative Processes

Non-heme iron complexes are emerging as powerful and versatile catalysts in several oxidative transformations. The most investigated iron complex structures are based on aminopyridine ligands, but a number of imine-based ligands have been also tested. In this review a collection of recent results obtained in oxidation catalysis with non-heme imine-based iron complexes is presented. Their catalytic performances in C-H, C-C and S oxidation are spread over a wide range of efficiency, going from very low to quite high. Such performances are discussed, whenever possible, in light of the operating reaction mechanisms and of catalyst stability. In order to facilitate the discussion, an initial survey of the most useful mechanistic tools widely applied to distinguish a metal-based oxidation from a radical-chain process is also reported. Imine-based catalysts are divided into two classes: (i) salen-Fe complexes, and (ii) imine-Fe complexes. In some cases clues for free-radical oxidation mechanisms have been reported while in other cases evidence for metal-based mechanisms has been collected. The preferred mechanistic pathway is shown to be a function of catalyst structure and features. Interestingly, some imine-based iron complexes are able to perform stereospecific oxidation reactions, demonstrating that the imine functionality can be incorporated in ligands designed for oxidation catalysis.