Tuning of the copper-thioether bond in tetradentate N₃S(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

Abstract

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-(O2) species L = (DMM)ESE, (DMM)ESP, and (DMM)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(II) complexes have been synthesized and crystallographically characterized. Copper(I) complexes are dimeric in the solid state, [{L-Cu(I)}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(II)(X)](n+) {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 {((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-O2(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-O2(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/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 (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)/O2 reactivity and possible implications are discussed.

Figure 1

Copper enzymes/proteins with S-ligands: (a) Peptidylglycine-α-hydroxylating monooxygenase (PHM), C–H oxygenation, (b) Azurin, electron transfer.

Chart 1

Figure 2

Representations of the dimeric Cu(I) complexes described in the crystal structures of (a) [{(^DMMESE)Cu^I}₂](B(C₆F₅)₄)₂ (1₂), (b) [{(^DMMESP)Cu^I}₂](B(C₆F₅)₄)₂ (2₂), and (c) [{(^DMMESDP)Cu^I}₂](B(C₆F₅)₄)₂ (3₂). Selected bond distances (Å) and bond angles (deg) are also listed. The H atoms and counterions were omitted for clarity.

Figure 3

Representations of the monomeric Cu(II) complexes described in the crystal structures of (a) [(^DMMESE)Cu^II(ClO₄)](ClO₄) (1a), (b) [(^DMMESP)Cu^II(H₂O)](ClO₄)₂ (2a), and (c) [(^DMMESDP)Cu^II(H₂O)](ClO₄)₂ (3a). Selected bond distances (Å) and bond angles (deg) are also listed. The H atoms (except for those of the water ligands found in 2a and 3a) and noncoordinating counterions were omitted for clarity.

Chart 2

Figure 4

(a) Structure of [{(TMPA)Cu^II}₂(μ-1,2-O₂^2–)]²⁺ obtained from X-ray crystallography and (b) low-temperature UV–vis absorption spectra of [{(TMPA)Cu^II}₂(μ-1,2-O₂^2–)]²⁺ (purple) and [{(ESE)-Cu^II}₂(μ-1,2-O₂^2–)]²⁺ (blue).^,

Figure 5

UV–vis spectra and time profile of the equilibrium between 2^P and 2^O in MeTHF at −130 °C.

Figure 6

Equilibrium between (a) a μ-1,2-trans-peroxodicopper(II) and bis-μ-oxodicopper(III) species; with 1 equiv CH₃SO₃^–, the mixture converts to a pure bis-μ-oxodicopper(III) complex, and (b) Stack and co-workers’ mixture of side-on μ-η²:η²-peroxodicopper(II) and bis-μ-oxodicopper(III) species reacts with chelating anions to give a clean side-on μ-η²:η²-peroxodicopper(II) complex.

Figure 7

Resonance Raman spectra of 2^P (a) with 647 nm excitation and 2^O (b) with 380 nm excitation in MeTHF collected at 77 K; see text.

Scheme 1

Scheme 2