Redox reactivity of LMCT and MLCT excited states of Earth-abundant metal complexes

Precious metal complexes, which act as excellent photoredox catalysts, have now being replaced by earth-abundant metal complexes. This review focused on redox reactivity of ligand-to-metal charge transfer (LMCT) and metal-to-ligand charge transfer (MLCT) excited states of earth-abundant metal complexes. Iron complexes with strongly σ-donating NHC-ligands (NHC = N-heterocyclic carbene) has emerged, featuring long lived LMCT excited states due to a significantly increased barrier for deactivation via metal centered states. A Fe(III)-NHC complex acts as an effective photoredox catalyst for various photocatalytic redox reactions. Although manganese(IV)-oxo complexes have no long-lived excited states (τ < < 1 ns). Once acids such as Sc(OTf)₃ and HOTf are bound to the oxo moiety of Mn(IV)-oxo complexes, the photoexcitation of acid-bound Mn(IV)-oxo complexes resulted in formation of the excited states with microseconds lifetimes, which are capable of oxidation of substrates including benzene to phenol. Photoexcited states of Mn(III), Mn(II) and Mn(I) complexes act as photoreductants to reduce substrates including O₂. On the other hand, photoexcited states of Co(IV) and Co(III) complexes act as photooxidants, whereas those of Co(II) and Co(I) complexes act as photoreductants. With regard to the excited state lifetime, [Cr(tpe)₂]³⁺ (tpe = 1,1,1-tris(pyrid-2-yl)ethane) exhibited the longest luminescence lifetime (τ = 4500 μs), acting as an effective photoredox catalyst for photocatalytic redox reactions. The LMCT state of a Cr(0) complex acts as a super photoreductant. Thus, LMCT and MLCT excited states of earth-abundant metal complexes are utilized as strong photooxidants and photoreductants, respectively.