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 <i>trans</i>-μ-1,2-peroxodicopper­(II) complex [(tmpa)₂­Cu^II₂­(O₂)]²⁺ (<b>1</b>) (λ_max = 525 and 600 nm) and side-on μ-η²:η²-peroxodicopper­(II) complexes [(N5)­Cu^II₂­(O₂)]²⁺ (<b>2</b>) and [(N3)­Cu^II₂­(O₂)]²⁺ (<b>3</b>) at −80 °C in acetone led to one-photon two-electron peroxide-to-dioxygen oxidation chemistry (O₂^2– + <i><i>h</i></i>ν → O₂ + 2e^–). Interestingly, light excitation of <b>2</b> and <b>3</b> (having side-on μ-η²:η²-peroxo ligation) led to release of dioxygen, while photoexcitation of <b>1</b> (having an end-on <i>trans-</i>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 <b>1</b>, 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 <b>1</b> occurred. Femtosecond laser excitation of <b>2</b> and <b>3</b> 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 <b>3</b>. Such remarkable differences in reaction pathways likely result from the well-known ligand-derived stability of <b>2</b> and <b>3</b> vs <b>1</b> 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 <b>2</b> and <b>3</b>, 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