Tuning of the Copper–Thioether Bond in Tetradentate N(3)S((thioether)) Ligands; O–O Bond Reductive Cleavage via a [Cu(II)(2)(μ-1,2-peroxo)](2+)/[Cu(III)(2)(μ-oxo)(2)](2+) Equilibrium

[Image: see text] Current interest in copper/dioxygen reactivity includes the influence of thioether sulfur ligation, as it concerns the formation, structures, and properties of derived copper-dioxygen complexes. Here, we report on the chemistry of {L-Cu(I)}(2)-(O(2)) species L = (DMM)ESE, (DMM)ESP, and (DMM)ESDP, which are N(3)S((thioether))-based ligands varied in the nature of a substituent on the S atom, along with a related N(3)O((ether)) (EOE) ligand. Cu(I) and Cu(II) complexes have been synthesized and crystallographically characterized. Copper(I) complexes are dimeric in the solid state, [{L-Cu(I)}(2)](B(C(6)F(5))(4))(2), however are shown by diffusion-ordered NMR spectroscopy to be mononuclear in solution. Copper(II) complexes with a general formulation [L-Cu(II)(X)](n+) {X = ClO(4)(–), n = 1, or X = H(2)O, n = 2} exhibit distorted square pyramidal coordination geometries and progressively weaker axial thioether ligation across the series. Oxygenation (−130 °C) of {((DMM)ESE)Cu(I)}(+) results in the formation of a trans-μ-1,2-peroxodicopper(II) species [{((DMM)ESE)Cu(II)}(2)(μ-1,2-O(2)(2–))](2+) (1(P)). Weakening the Cu–S bond via a change to the thioether donor found in (DMM)ESP leads to the initial formation of [{((DMM)ESP)Cu(II)}(2)(μ-1,2-O(2)(2–))](2+) (2(P)) that subsequently isomerizes to a bis-μ-oxodicopper(III) complex, [{((DMM)ESP)Cu(III)}(2)(μ-O(2–))(2)](2+) (2(O)), with 2(P) and 2(O) in equilibrium (K(eq) = [2(O)]/[2(P)] = 2.6 at −130 °C). Formulations for these Cu/O(2) adducts were confirmed by resonance Raman (rR) spectroscopy. This solution mixture is sensitive to the addition of methylsulfonate, which shifts the equilibrium toward the bis-μ-oxo isomer. Further weakening of the Cu–S bond in (DMM)ESDP or substitution with an ether donor in (DMM)EOE leads to only a bis-μ-oxo species (3(O) and 4(O), respectively). Reactivity studies indicate that the bis-μ-oxodicopper(III) species (2(O), 3(O)) and not the trans-peroxo isomers (1(P) and 2(P)) are responsible for the observed ligand sulfoxidation. Our findings concerning the existence of the 2(P)/2(O) equilibrium contrast with previously established ligand-Cu(I)/O(2) reactivity and possible implications are discussed.