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
Journal of the American Chemical Society
by Claudio Saracini, Kei Ohkubo, Tomoyoshi Suenobu, Gerald J. Meyer, Kenneth D. Karlin and Shunichi Fukuzumi
DOI: 10.1021/jacs.5b10177
http://pubs.acs.org/doi/pdf/10.1021/jacs.5b10177
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DOI: 10.1021/jacs.5b10177
ABSTRACT: Photoexcitation of end-on trans-μ-1,2-
peroxodicopper(II) complex [(tmpa)2CuII2(O2)]2+ (1) (λmax= 525 and 600 nm) and side-on μ -η2:η2-peroxodicopper(II)
complexes [(N5)CuII2(O2)]2+ (2) and [(N3)CuII2(O2)]2+ (3) at −80 °C in acetone led to one-photon two-electron
peroxide-to-dioxygen oxidation chemistry (O22− + 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: [CuII(O2•−)CuI]2+ (λmax = 685−740 nm). For 1, this intermediate underwent further fast intramolecular electron transfer to yield an “O2-caged” dicopper(I) adduct, CuI2− 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 [CuII(O2•−)CuI]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−CuII/I redox potentials; (N5)CuI and (N3)CuI complexes are far more stable than (tmpa)CuI species. The fast CuI2/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)CuI2]2+ and [(N3)CuI2]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.
peroxodicopper(II) complex [(tmpa)2CuII2(O2)]2+ (1) (λmax= 525 and 600 nm) and side-on μ -η2:η2-peroxodicopper(II)
complexes [(N5)CuII2(O2)]2+ (2) and [(N3)CuII2(O2)]2+ (3) at −80 °C in acetone led to one-photon two-electron
peroxide-to-dioxygen oxidation chemistry (O22− + 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: [CuII(O2•−)CuI]2+ (λmax = 685−740 nm). For 1, this intermediate underwent further fast intramolecular electron transfer to yield an “O2-caged” dicopper(I) adduct, CuI2− 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 [CuII(O2•−)CuI]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−CuII/I redox potentials; (N5)CuI and (N3)CuI complexes are far more stable than (tmpa)CuI species. The fast CuI2/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)CuI2]2+ and [(N3)CuI2]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.
http://pubs.acs.org/doi/pdf/10.1021/jacs.5b10177
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