Lanthanide Photocatalysis

Abstract

Conspectus The use of earth-abundant, cheap, potent, and readily available lanthanide photocatalysts provides an opportunity to complement or even replace rare and precious metal photosensitizers. Moreover, lanthanide photosensitizers have been demonstrated for the generation of a variety of reactive species, including aryl radicals, alkyl radicals, and others, by single-electron-transfer (SET) and hydrogen atom transfer (HAT) pathways under mild reaction conditions. Some lanthanide photocatalysts have unprecedented reducing power from their photoexcited states, achieving the activation of challenging organic substrates that have not otherwise been activated by reported organic or transition-metal photosensitizers. In this Account, we describe our recent advances in the rational design and strategic application of lanthanide photo(redox)catalysis. Our research goals include understanding the photophysics of lanthanide luminophores and incorporating them into new photocatalysis. Among the lanthanides, we have focused on cerium because of the doublet to doublet 4f → 5d excitation and emission, which affords good conservation of energy without losses through spin-state changes, as well as a large natural abundance of that element. We have performed structural, spectroscopic, computational, and reactivity studies to demonstrate that luminescent Ce(III) guanidinate–amide complexes can mediate photocatalytic C(sp 3 )–C(sp 3 ) bond forming reactions. Taking advantage of the strong reducing power of the cerium excited states and the cerium–halogen bond forming enthalpies, we determined that the reactive, excited-state cerium metalloradical abstracts chloride anion from benzyl chloride to generate the benzyl radical. To control and predict the photocatalytic reactivities, we have also performed photophysical and photochemical studies on a series of mixed-ligand Ce(III) guanidinate–amide and guanidinate–aryloxide complexes to establish structure–property relationship for Ce(III) photocatalysts. We discovered that the emission color is directly related to ligand type and rigidity of the coordination sphere and that the photoluminescent quantum yield is correlated to variation in steric encumbrance around the cerium centers. The low excited-state reduction potentials ( E 1/2 * ≈ −2.1 to −2.9 V versus Cp 2 Fe 0/+ ) and relatively fast quenching rates ( k q ≈ 10 7 M –1 s –1 ) toward aryl halides enabled the Ce(III) guanidinate–amide complexes to participate in photocatalytic C(sp 2 )–C(sp 2 ) bond forming reactions through either inner-sphere or outer-sphere SET processes. We have also reported a simple, potent, and air-stable ultraviolet A photoreductant, the hexachlorocerate(III) anion ([Ce III Cl 6 ] 3– ). This complex is a potent photoreductant ( E 1/2 * ≈ −3.45 V versus Cp 2 Fe 0/+ ) and exhibits a fast quenching kinetics ( k q ≈ 10 9 –10 10 M –1 s –1 ) toward organohalogens. The [Ce IV Cl 6 ] 2− redox partner can also act as a potent photo-oxidant though a (presumably) long-lived chloride-to-cerium(IV) charge transfer excited state (ε = ∼6000 M –1 cm –1 ), that was used to turnover photocatalytic dechlorination and Miyuara borylation reactions. With [Ce III Cl 6 ] 3–, we achieved efficient photoinduced dehalogenation and borylation of unactivated aryl chlorides with broad substrate scope, through formally two-photon cycles where both Ce(III) and Ce(IV) act as photocatalysts. Lanthanide photoredox catalysis is now being applied in several contexts for reactions including photocatalytic dehydrogenation of amines, alkoxy-radical-mediated C–C bond cleavage and amination of alkanols, and C–H activation of alkanes. Overall, simple and potent lanthanide photocatalysis is expected to find practical applications in organic synthesis, pharmaceutical development, and small molecule activation.