Non-Innocent Ligands as Mediators for Visible-Light-Initiated Element-Carbon Bond Homolysis in Main Group Chemistry

Photoinitiated homolysis of element-carbon bonds is an important method for the generation of carbon-centered radicals in catalysis and organometallic or polymer chemistry. In this respect, the use of earth-abundant main group elements such as aluminum or silicon is attractive. Generally, subvalent species derived from these typically redox-inactive elements are unstable and within their high-valent configuration +III (Al) or +IV (Si) comparatively strong E─C bonds are formed. Therefore, E─C homolysis usually requires shortwave UV irradiation, which hampers their use as radical sources. Some reports in the literature show that visible-light-induced E─C homolysis is possible when a redox non-innocent ligand (NIL) is coordinated to the organometallic fragment. In a simplified view, the NILs provide chromophoric moieties, which can absorb energy in form of light and subsequently convert it to break the element-carbon bonds. The resulting main group element radicals are in turn stabilized by delocalization of the unpaired electron, effectively lowering the dissociation energy of the E─C bond. In this article, the effects of NILs as mediators for visible-light-induced E─C bond homolysis in main group chemistry are discussed on the basis of selected literature reports, and future opportunities and challenges are highlighted.