Tuning of the Copper–Thioether Bond in Tetradentate N₃S_(thioether) Ligands; O–O Bond Reductive Cleavage via a [Cu^II₂(μ-1,2-peroxo)]²⁺/[Cu^III₂(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-CuI}2-(O2) species L = DMMESE, DMMESP, and DMMESDP, 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. CuI and CuII complexes have been synthesized and crystallographically characterized. Copper(I) complexes are dimeric in the solid state, [{L-CuI}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-CuII(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 {(DMMESE)CuI}+ results in the formation of a trans-μ-1,2-peroxodicopper(II) species [{(DMMESE)CuII}2(μ-1,2-O22–)]2+ (1P). Weakening the Cu–S bond via a change to the thioether donor found in DMMESP leads to the initial formation of [{(DMMESP)CuII}2(μ-1,2-O22–)]2+ (2P) that subsequently isomerizes to a bis-μ-oxodicopper(III) complex, [{(DMMESP)CuIII}2(μ-O2–)2]2+ (2O), with 2P and 2O in equilibrium (Keq = [2O]/[2P] = 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 DMMESDP or substitution with an ether donor in DMMEOE leads to only a bis-μ-oxo species (3O and 4O, respectively). Reactivity studies indicate that the bis-μ-oxodicopper(III) species (2O, 3O) and not the trans-peroxo isomers (1P and 2P) are responsible for the observed ligand sulfoxidation. Our findings concerning the existence of the 2P/2O equilibrium contrast with previously established ligand-CuI/O2 reactivity and possible implications are discussed.

A N3S(thioether)-Ligated CuII-Superoxo with Enhanced Reactivity

“…6 The formulation of 2 P is confirmed by rR spectroscopy (ν O–O = 821 cm −1 , ν Cu–O = 547 cm −1 ). 7 Although the initially formed species has a short lifetime, the characteristic λ max value of 425 nm and the transformation to a more stable ( μ -1,2-peroxo)Cu II 2 , strongly suggest it is the S-ligated cupric superoxo species [( DMA N 3 S)Cu II (O 2 •− )] + ( 2 S ).…”

“…4 In an attempt to mimic the active site donors of PHM and DβM, considerable effort has been devoted toward generating thioether containing tridentate or tetradentate ligands to study their Cu I /O 2 reactivity. 5 In our own efforts we have been able to generate binuclear peroxodicopper complexes with S (thioether) ligation, 6 but there has been no report of a mononuclear Cu II (O 2 •− ) species coordinated by a S (thioether) donor. Herein, for the first time we present the spectroscopic evidence and reactivity of the new mononuclear cupric superoxo complex, ( DMA N 3 S)Cu II (O 2 •− ) ( 2 S ) (Scheme 1b) possessing thioether S-ligation that exhibits enhanced reactivity towards both O-H and C-H substrates.…”

“…4c,4h,9 Second, the thioether moiety is capped with a bulkier o -methyl benzyl substituent to slow dimerization relative to previous designs. 6d Treatment of DMA N 3 S with [Cu I (CH 3 CN) 4 ]B(C 6 F 5 ) 4 in THF, followed by pentane addition leads to the isolation of bright yellow powders with formula [( DMA N 3 S)Cu I ]B(C 6 F 5 ) 4 ( 1 ). 7 Single crystals could be grown from 2-methyltetrahydrofuran (MeTHF)/pentane at RT under Ar.…”

“…The Cu-S bond length in 1 (2.2327(5) Å) falls in the usual range (2.2 – 2.44 Å) for Cu I –S (thioether) complexes. 6 Many previously studied N 3 S (thioether) -Cu I complexes form dimers in the solid state, 6d and the incorporation of the larger o -methylbenzyl group may disfavor their formation.…”

“…Warming up the 2 S solution does not lead to sulfur oxygenation which occurs by bis(μ-oxo)Cu III 2 species. 6d …”