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-O-2 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)]ClO4 (1-ClO4), [Cu-I(L-N3S)]B(C6F5)(4) (1-B(C6F5)(4)), and [Cu-I(L-N3S')]ClO4 (2), were generated, and their redox properties, CO binding, and O-2-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)](ClO4)(2) (3) were obtained; the latter possesses axial thioether coordination. At low temperature in CH2Cl2, acetone, or 2-methyltetrahydrofuran (MeTHF), 1 reacts with O-2 and generates an adduct formulated as an end-on peroxodicopper(II) complex [{Cu-II(L-N3S)}(2)(mu-1,2-O-2(2-))](2+) (4)){lambda(max) = 530 (epsilon approximate to 9200 M-1 cm(-1)) and 605 nm (epsilon approximate to 11 800 M-1 cm(-1))}; the number and relative intensity of LMCT UV-vis bands vary from those for [{Cu-II(TMPA)}(2)(O-2(2-))](2+) {lambda(max) = 524 nm (epsilon = 11 300 M-1 cm(-1)) and 615 nm (epsilon = 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 {nu((O-O)) = 817 cm(-1) (O-16-18(2) Delta = 46 cm(-1)) and nu((Cu-O)) = 545 cm(-1) (O-16-18(2) Delta = 26 cm(-1)); these values are lower in energy than those for [{Cu-II(TMPA)}(2)(O-2(2-))](2+) {nu(Cu-O) = 561 cm(-1) and nu(O-O) = 827 cm(-1)} and can be attributed to less electron density donation from the peroxide pi* 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 A}. Following a [Cu-I(L-N3S)](+)/O-2 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 O-2-activation, and situations of biological stress involving methionine oxidation, is provided.