Shunichi Fukuzumi

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

Photoexcitation of end-on trans-μ-1,2-peroxodicopper(II) complex [(tmpa)2Cu(II)2(O2)](2+) (1) (λmax = 525 and 600 nm) and side-on μ-η(2):η(2)-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-) + hν → O2 + 2e(-)). Interestingly, light excitation of 2 and 3 (having side-on μ-η(2):η(2)-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+) (λmax = 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.

CCDC 1430274: Experimental Crystal Structure Determination

Related Article: Rui Cao, Claudio Saracini, Jake W. Ginsbach, Matthew T. Kieber-Emmons, Maxime A. Siegler, Edward I. Solomon, Shunichi Fukuzumi, and Kenneth D. Karlin|2016|J.Am.Chem.Soc.|138|7055|doi:10.1021/jacs.6b02404

Chapter 4:Metal Ion-Coupled and Proton-Coupled Electron Transfer in Catalytic Reduction of Dioxygen

Metal ion-coupled and proton-coupled electron-transfer processes are described for one-electron, two-electron, and four-electron reduction of dioxygen by one-electron reductants, such as ferrocene derivatives, as well as by two-electron reductants, such as NADH analogs with metal complexes. The catalytic mechanism of the four-electron reduction of dioxygen as functional models of cytochrome c oxidases has been clarified based on detailed kinetic study and the detection of intermediates.