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

Photoexcitation of end-on trans-μ-1,2-peroxodicopper(II) complex [(tmpa)₂Cu^II ₂(O₂)]²⁺ (1) λ_max = 525 and 600 nm) and side-on μ-η²:η²-peroxodicopper(II) complexes [(N5)Cu^II ₂(O₂)]²⁺ (2) and [(N3)Cu^II ₂(O₂)]²⁺ (3) at ?'80 °C in acetone led to one-photon two-electron peroxide-to-dioxygen oxidation chemistry (O₂ ^2- + h→ 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^I ₂-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^I ₂/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^I ₂]²⁺ and [(N3)Cu^I ₂]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.