Copper(I) complex O₂-reactivity with a N₃S thioether ligand: A copper-dioxygen adduct including sulfur ligation, ligand oxygenation, and comparisons with all nitrogen ligand analogues

In order to contribute to an understanding of the effects of thioether sulfur ligation in copper-O₂ reactivity, the tetradentate ligands L^N3S (2-ethylthio-N,N-bis(pyridin-2-yl)methylethanamine) and L ^N3S′ (2-ethylthio-N,N-bis(pyridin-2-yl)ethylethanamine) have been synthesized. Corresponding copper(I) complexes, [Cu^I(L ^N3S)]CIO₄ (1-CIO₄), [Cu^I-(L ^N3S)]B(C₆F₅)₄ (1-B(C ₆F₅)4), and [Cu^I(L^N3S′)] CIO₄ (2), were generated, and their redox properties, CO binding, and O₂-reactivity were compared to the situation with analogous compounds having all nitrogen donor ligands, [Cu^I-(TMPA)(MeCN)] ⁺ and [Cu^I(PMAP)]⁺ (TMPA = tris(2- pyridylmethyl)amine; PMAP = bis[2-(2-pyridyl)ethyl]-(2-pyridyl)-methylamine). X-ray structures of 1-B(C₆F₅)₄, a dimer, and copper(II) complex [Cu^II(L^N3S)(MeOH)](CIO ₄)₂ (3) were obtained; the latter possesses axial thioether coordination. At low temperature in CH₂CI₂, acetone, or 2-methyltetrahydrofuran (MeTHF), 1 reacts with O₂ and generates an adduct formulated as an end-on peroxodicopper-(II) complex [{CU^II(L^N3S)}₂(μ-1,2-O₂ ^2-)]²⁺ (4)){λ_max = 530 (ε ≈ 9200 M^-1 cm^-1) and 605 nm (ε ≈ 11 800 M^-1 cm^-1)}; the number and relative intensity of LMCT UV-vis bands vary from those for [{Cu^II(TMPA)}₂(O₂ ^2-)]²⁺ {λ_max = 524 nm (ε = 11 300 M^-1 cm^-1)} and 615 nm (ε = 5800 M^-1 cm^-1)} and are ascribed to electronic structure variation due to coordination geometry changes with the L^N3S ligand. Resonance Raman spectroscopy confirms the end-on peroxo-formulation {ν_(O-O) = 817 cm^-1 (^16-18O₂ Δ = 46 cm^-1) and ν_(Cu-O) = 545 cm^-1 (^16-18O₂ Δ = 26 cm^-1); these values are lower in energy than those for [{Cu^II(TMPA)}₂(O₂^2-)]²⁺ {μ(Cu-O) = 561 cm^-1 and Δ(O-O) = 827 cm^-1} and can be attributed to less electron density donation from the peroxide π* orbitals to the Cu(II) ion. Complex 4 is the first copper-dioxygen adduct with thioether ligation; direct evidence comes from EXAFS spectroscopy {Cu K-edge; Cu-S = 2.4 Å}. Following a [Cu^I(L ^N3S)]⁺/O₂ reaction and warming, the L ^N3S thioether ligand is oxidized to the sulfoxide in a reaction modeling copper monooxygenase activity. By contrast, 2 is unreactive toward dioxygen probably due to its significantly increased Cu^II/Cu ^I redox potential, an effect of ligand chelate ring size (in comparison to 1). Discussion of the relevance of the chemistry to copper enzyme O₂-activation, and situations of biological stress involving methionine oxidation, is provided.