A survey of the iron ligand-to-metal charge transfer chemistry in water

Iron, mostly as Fe2+ or Fe3+ bound in oxides, is the second most common element in the Earth's crust after aluminium. Salts of the d-block metal are cheap and commercially available and have already been used in organic chemistry in many preparative examples for the C-H functionalization of different classes of starting materials. The literature revealed that like copper or cerium, iron can undergo ligand-to-metal charge transfer processes. Different (anionic) ligands of different sizes can be added in various stoichiometries to commercial Fe(ii)/Fe(iii) salts leading to a unique orientation of the ligands around the metal centre. The resulting complexes are often coloured and can be excited by UV or visible light initiating a metal-ligand bond homolysis in which the metal part gets reduced, and the ligand loses one electron to form highly reactive radical species that can abstract hydrogen atoms or in the case of halide radicals add to double or triple bonds in a suitable substrate. In most of the literature-known procedures, the photoreaction proceeds in organic (polar protic/aprotic) solvents and the reaction medium hardly competes against the ligands (such as alcoholates or halides) for free coordination sites on the metal centre. However, in pure water, unlike other solvents (polar-protic or polar-aprotic), the iron cations get fully solvated and result in aquo-complexes due to the exclusive attachment of water molecules to a metal species, such as in sulphate, nitrate, and perchlorate salts. They possess a general stoichiometry of [M(H2O)n]z+ and have been known for a long time in inorganic and environmental chemistry showing unique and useful characteristics that will be exemplified in this review. Fully hydrated! - Iron photocatalysis tests the waters.