Photoredox catalysts based on earth-abundant metal complexes

Over the last decade, visible light photoredox catalysis has exploded into the consciousness of the synthetic chemist. The principal photocatalysts used are based on rare and toxic ruthenium(ii) and iridium(iii) complexes. This critical review focusses on Earth-abundant metal complexes as potential replacement photocatalysts and summarizes the use of photoactive Cu(i), Zn(ii), Ni(0), V(v), Zr(iv), W(0), W(vi), Mo(0), Cr(iii), Co(iii) and Fe(ii) complexes in photoredox reactions. The optoelectronic properties of these complexes and relevant structurally related analogs, not yet used for photoredox catalysis, are discussed in combination with the reaction scope reported for each photocatalyst. Prospects for the future of photocatalyst design are considered.