Iron Photoredox Catalysis–Past, Present, and Future

Abstract

Photoredox catalysis of organic reactions driven by iron has attracted substantial attention throughout recent years, due to potential environmental and economic benefits. In this Perspective, three major strategies were identified that have been employed to date to achieve reactivities comparable to the successful noble metal photoredox catalysis: (1) Direct replacement of a noble metal center by iron in archetypal polypyridyl complexes, resulting in a metal-centered photofunctional state. (2) In situ generation of photoactive complexes by substrate coordination where the reactions are driven via intramolecular electron transfer involving charge-transfer states, for example, through visible-light-induced homolysis. (3) Improving the excited-state lifetimes and redox potentials of the charge-transfer states of iron complexes through new ligand design. We seek to give an overview and evaluation of recent developments in this rapidly growing field and, at the same time, provide an outlook on the future of iron-based photoredox catalysis.

Ferrous and ferric complexes with cyclometalating N-heterocyclic carbene ligands: a case of dual emission revisited

“…They enable the substitution of precious metal photosensitizers and luminophores for the conversion of radiant-to-electric energy or vice versa in e.g. solar energy conversion and photoredox catalysis 5–8 or OLEDs. 9,10 Iron complexes as analogues of the archetypal ruthenium polypyridyl photosensitizers 11 have since long attracted particular interest 12–20 and the more recent progress with N-heterocyclic carbene (NHC) ligands 21–28 has eventually led to ferrous and ferric complexes with demonstrated excited-state electron transfer reactivity of their triplet metal-to-ligand charge transfer ( 3 MLCT) 29–35 and doublet ligand-to-metal charge transfer ( 2 LMCT) 36–42 states, respectively.…”

Luminescence and Excited-State Reactivity in a Heteroleptic Tricyanido Fe(III) Complex

“…Considering the ongoing climate and earth-resource crises, there is a growing emphasis on the development of cost-effective photoactive compounds that utilize more sustainable, earth-abundant metals for solar energy conversion and storage processes. − Notable photosensitizers have been achieved using first-row transition metal ions, but overall progress is slow compared with the vast library of complexes based on 4d and 5d metal ions. ,,,− During the last decades, special attention has been paid to Fe II photosensitizers where a metal-to-ligand charge transfer (MLCT) can act as a photoactive level displaying luminescence and reactivity, similarly to precious Ru II chromophores. Quite recently attention turned to Fe III compounds boosting a photoactive ligand-to-metal charge transfer (LMCT). − Early development of photofunctional d 5 complexes was devoted to hexacyanometallates such as [M III (CN) 6 ] 3– (M = Fe or Ru) where the strong ligand field splitting of the cyanide ligands generates a low-spin doublet ground state (Figure a,b). , Luminescence from a 2 LMCT excited state (λ em = 525 nm; τ = 0.5 ns) for the ruthenate analogue was detected at 77 K, while no signal was found for the ferricyanide. Despite the strong ligand field splitting provided by the cyanide ligands in the iron analogue, low lying metal-centered (MC) states induce nonradiative relaxation from the potentially emissive 2 LMCT state.…”

Multifaceted Deactivation Dynamics of Fe(II) N-Heterocyclic Carbene Photosensitizers

“…The development of Earth-abundant light-harvesting complexes has garnered growing interest for solar energy conversion and photocatalysis applications in recent years. − Iron-based complexes have received particular attention as an obvious candidate to replace the widely used Ru­(II) congener complexes. − Excitation into the metal-to-ligand charge-transfer (MLCT) states of traditional prototype Fe­(II) complexes typically results in rapid deactivation either back to the ground state (GS), or resulting in the population of a long-lived quintet metal-centered ( 5 MC) state. − Complexes of the latter type may find use as light-induced excited state spin trapping (LIESST) compounds, but unfortunately are not very promising for photocatalytic and solar cell applications . This has spurred an intense interest in investigating the ultrafast excited state dynamics of photoexcited Fe­(II) complexes as a key step to ultimately overcoming the photochemical limitations in these complexes, with particular focus on the role of triplet metal-centered ( 3 MC) states.…”