Lanthanide Photocatalysis

光催化 镧系元素 光化学 催化作用 反应性(心理学) 激进的 激发态 化学 组合化学 无机化学 有机化学 离子 医学 替代医学 病理 物理 核物理学
作者
Yusen Qiao,Eric J. Schelter
出处
期刊:Accounts of Chemical Research [American Chemical Society]
卷期号:51 (11): 2926-2936 被引量:201
标识
DOI:10.1021/acs.accounts.8b00336
摘要

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(sp3)-C(sp3) 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 ( E1/2* ≈ -2.1 to -2.9 V versus Cp2Fe0/+) and relatively fast quenching rates ( kq ≈ 107 M-1 s-1) toward aryl halides enabled the Ce(III) guanidinate-amide complexes to participate in photocatalytic C(sp2)-C(sp2) 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 ([CeIIICl6]3-). This complex is a potent photoreductant ( E1/2* ≈ -3.45 V versus Cp2Fe0/+) and exhibits a fast quenching kinetics ( kq ≈ 109-1010 M-1 s-1) toward organohalogens. The [CeIVCl6]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 [CeIIICl6]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.
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