Tuning of the Copper–Thioether Bond in Tetradentate N3S(thioether) Ligands; O–O Bond Reductive Cleavage via a [CuII2(μ-1,2-peroxo)]2+/[CuIII2(μ-oxo)2]2+ Equilibrium

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}2-(O2) species L = ESE, ESP, and ESDP, which are N3S(thioether)-based ligands varied in the nature of a substituent on the S atom, along with a related N3O(ether) (EOE) ligand. Cu I and Cu complexes have been synthesized and crystallographically characterized. Copper(I) complexes are dimeric in the solid state, [{L-Cu}2](B(C6F5)4)2, however are shown by diffusion-ordered NMR spectroscopy to be mononuclear in solution. Copper(II) complexes with a general formulation [L-Cu(X)] {X = ClO4 −, n = 1, or X = H2O, n = 2} exhibit distorted square pyramidal coordination geometries and progressively weaker axial thioether ligation across the series. Oxygenation (−130 °C) of {(ESE)Cu} results in the formation of a trans-μ-1,2-peroxodicopper(II) species [{(ESE)Cu}2(μ-1,2-O2 2−)]2+ (1). Weakening the Cu−S bond via a change to the thioether donor found in ESP leads to the initial formation of [{(ESP)Cu}2(μ-1,2-O2 2−)]2+ (2) that subsequently isomerizes to a bis-μ-oxodicopper(III) complex, [{(ESP)Cu}2(μ-O )2] 2+ (2), with 2 and 2 in equilibrium (Keq = [2 ]/[2] = 2.6 at −130 °C). Formulations for these Cu/O2 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 ESDP or substitution with an ether donor in EOE leads to only a bis-μ-oxo species (3 and 4, respectively). Reactivity studies indicate that the bis-μ-oxodicopper(III) species (2, 3) and not the trans-peroxo isomers (1 and 2) are responsible for the observed ligand sulfoxidation. Our findings concerning the existence of the 2/2 equilibrium contrast with previously established ligand-Cu/O2 reactivity and possible implications are discussed.