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

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

Claudio Saracini†, Kei Ohkubo‡§, Tomoyoshi Suenobu‡, Gerald J. Meyer†, Kenneth D. Karlin*†, and Shunichi Fukuzumi*‡§∥

† Department of Chemistry, The Johns Hopkins University, Baltimore, Maryland 21218, United States

‡ Department of Material and Life Science, Graduate School of Engineering, Osaka University, ALCA and SENTAN, Japan Science and Technology Agency (JST), Suita, Osaka 565-0871, Japan

§ Department of Chemistry and Nano Science, Ewha Womans University, Seoul 120-750, Korea

∥ Faculty of Science and Engineering, Meijo University, ALCA and SENTAN, Japan Science and Technology Agency (JST), Nagoya, Aichi 468-0073, Japan

J. Am. Chem. Soc., 2015, 137 (50), pp 15865–15874

DOI: 10.1021/jacs.5b10177

Publication Date (Web): December 11, 2015

*karlin@jhu.edu, *fukuzumi@chem.eng.osaka-u.ac.jp

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 1indicated 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.