The great strides of iron photosensitizers for contemporary organic photoredox catalysis: On our way to the holy grail?

The scarcity of 2nd and 3rd row transition metals such as ruthenium, iridium, rhodium and osmium, coupled with a global desire for sustainable chemistry has led to the resurgence of research centered on earth-abundant metal-based photosensitizers. Amongst these, iron photosensitizers have often been considered a 'holy grail' because of iron's high abundance in the earth's crust, low toxicity, and low environmental impact. In the last years especially, two different concepts to use these photosensitizers for photoredox catalysis were developed and an overview of both approaches will be presented herein. The first one is related to the inner sphere reactivity of in situ generated photosensitizers that allows the activation of several ligand classes for radical-based transformations using simple iron salts. The second one covers the diffusional excited-state reactivity for recently developed structurally well-defined photosensitizers. For both approaches the applications are structured according to the (proposed) photoactivation step in the light-driven applications and discussed based on the versatile examples recently reported in this highly emerging research field.