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 LN3S (2-ethylthio-N,N-bis(pyridin-2-yl)methylethanamine) and LN3S‘(2-ethylthio-N,N-bis(pyridin-2-yl)ethylethanamine) have been synthesized. Corresponding copper(I) complexes, [CuI(LN3S)]ClO4 (1-ClO4), [CuI(LN3S)]B(C6F5)4 (1-B(C6F5)4), and [CuI(LN3S‘)]ClO4 (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, [CuI(TMPA)(MeCN)]+ and [CuI(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 [CuII(LN3S)(MeOH)](ClO4)2 (3) were obtained; the latter possesses axial thioether coordination. At low temperature in CH2Cl2, acetone, or 2-methyltetrahydrofuran (MeTHF), 1 reacts with O2 and generates an adduct formulated as an end-on peroxodicopper(II) complex [{CuII(LN3S)}2(μ-1,2-O22-)]2+ (4)){λmax = 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 [{CuII(TMPA)}2(O22-)]2+ {λmax = 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 LN3S 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 [{CuII(TMPA)}2(O22-)]2+ {ν(Cu−O) = 561 cm-1 and ν(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 [CuI(LN3S)]+/O2 reaction and warming, the LN3S 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 CuII/CuI 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.