Laser-Induced Dynamics of Peroxodicopper(II) Complexes Vary with the Ligand Architecture. One-Photon Two-Electron O2 Ejection and Formation of Mixed-Valent CuICuII–Superoxide Intermediates

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• 作者：Claudio Saracini ; Kei Ohkubo ; Tomoyoshi Suenobu ; Gerald J. Meyer ; Kenneth D. Karlin ; Shunichi Fukuzumi
• 刊名：Journal of the American Chemical Society
• 出版年：2015
• 出版时间：December 23, 2015
• 年：2015
• 卷：137
• 期：50
• 页码：15865-15874
• 全文大小：635K
• ISSN：1520-5126

文摘

Photoexcitation of end-on trans-μ-1,2-peroxodicopper(II) complex [(tmpa)₂Cu^II₂(O₂)]²⁺ (1) (λ_max = 525 and 600 nm) and side-on μ-η²:η²-peroxodicopper(II) complexes [(N5)Cu^II₂(O₂)]²⁺ (2) and [(N3)Cu^II₂(O₂)]²⁺ (3) at ?80 °C in acetone led to one-photon two-electron peroxide-to-dioxygen oxidation chemistry (O₂^2– + hν → O₂ + 2e^–). Interestingly, light excitation of 2 and 3 (having side-on μ-η²:η²-peroxo ligation) led to release of dioxygen, while photoexcitation of 1 (having an end-on trans-1,2-peroxo geometry) did not, even though spectroscopic studies revealed that both reactions proceeded through previously unknown mixed-valent superoxide species: [Cu^II(O₂^?–)Cu^I]²⁺ (λ_max = 685–740 nm). For 1, this intermediate underwent further fast intramolecular electron transfer to yield an “O₂-caged” dicopper(I) adduct, Cu^I₂–O₂, and a barrierless stepwise back electron transfer to regenerate 1 occurred. Femtosecond laser excitation of 2 and 3 under the same conditions still led to [Cu^II(O₂^?–)Cu^I]²⁺ intermediates that, instead, underwent O₂ release with a quantum yield of 0.14 ± 0.1 for 3. Such remarkable differences in reaction pathways likely result from the well-known ligand-derived stability of 2 and 3 vs 1 indicated by ligand–Cu^II/I redox potentials; (N5)Cu^I and (N3)Cu^I complexes are far more stable than (tmpa)Cu^I species. The fast Cu^I₂/O₂ rebinding kinetics was also measured after photoexcitation of 2 and 3, with the results closely tracking those known for the dicopper proteins hemocyanin and tyrosinase, for which the synthetic dicopper(I) precursors [(N5)Cu^I₂]²⁺ and [(N3)Cu^I₂]²⁺ and their dioxygen adducts serve as models. The biological relevance of the present findings is discussed, including the potential impact on the solar water splitting process.