Copper/α-Ketocarboxylate Chemistry With Supporting Peralkylated Diamines: Reactivity of Copper(I) Complexes and Dicopper−Oxygen Intermediates

To further understand copper-promoted oxidation reactions, the Cu(I) complexes LCuX (L = N,N′-di-tert-butyl-N,N′-dimethylethylenediamine; X = benzoylformate (BF) or p-nitro-benzoylformate) were synthesized, fully characterized by X-ray crystallography and spectroscopy in solution, and their reactivity with O2 at −80 °C examined. Oxidative decarboxylation of the α-ketocarboxylate ligand was observed, but only to a significant extent when cyclohexene, cyclooctene, or acetonitrile was present. Spectroscopic and conductivity data are consistent with mechanistic postulates involving displacement of the α-ketocarboxylate by the additives to a small extent, followed by oxygenation of the LCu(I) moiety to yield copper−oxygen species that subsequently induce decarboxylation. To test these hypotheses, spectroscopic and kinetic studies of the reactions of Bu4NBF with preformed μ-η2:η2-peroxodicopper(II) and/or bis(μ-oxo)dicopper(III) complexes supported by L or N,N,N′,N′-tetramethylpropylenediamine were performed. In an illustration of a new mode of reactivity for such dicopper−oxygen cores, decarboxylation of the added α-ketocarboxylate was observed and the intermediacy of a carboxylate-bridged μ-η2:η2-peroxodicopper(II) complex was implicated

A Structural Model of the Type 1 Copper Protein Active Site: N₂S(thiolate)S(thioether) Ligation in a Cu(II) Complex

A Structural Model of the Type 1 Copper Protein Active Site:  N2S(thiolate)S(thioether) Ligation in a Cu(II) Comple

Three-Coordinate Cu(II) Complexes: Structural Models of Trigonal-Planar Type 1 Copper Protein Active Sites

Three-Coordinate Cu(II) Complexes:  Structural Models of Trigonal-Planar Type 1 Copper Protein Active Site

Hydrogen Atom Abstraction from Hydrocarbons by a Copper(III)-Hydroxide Complex

With the aim of understanding the basis for the high rate of hydrogen atom abstraction (HAT) from dihydroanthracene (DHA) by the complex LCuOH (<b>1</b>; L = <i>N</i>,<i>N</i>′-bis­(2,6-diisopropylphenyl)-2,6-pyridinedicarboxamide), the bond dissociation enthalpy of the reaction product LCu­(H₂O) (<b>2</b>) was determined through measurement of its p<i>K</i>_a and <i>E</i>_1/2 in THF solution. In so doing, an equilibrium between <b>2</b> and LCu­(THF) was characterized by UV–vis and EPR spectroscopy and cyclic voltammetry (CV). A high p<i>K</i>_a of 18.8 ± 1.8 and a low <i>E</i>_1/2 of −0.074 V vs Fc/Fc⁺ in THF combined to yield an O–H BDE for <b>2</b> of 90 ± 3 kcal mol^–1 that is large relative to values for most transition metal oxo/hydroxo complexes. By taking advantage of the increased stability of <b>1</b> observed in 1,2-difluorobenzene (DFB) solvent, the kinetics of the reactions of <b>1</b> with a range of substrates with varying BDE values for their C–H bonds were measured. The oxidizing power of <b>1</b> was revealed through the accelerated decay of <b>1</b> in the presence of the substrates, including THF (BDE = 92 kcal mol^–1) and cyclohexane (BDE = 99 kcal mol^–1). CV experiments in THF solvent showed that <b>1</b> reacted with THF via rate-determining attack at the THF C–H­(D) bonds with a kinetic isotope effect of 10.2. Analysis of the kinetic and thermodynamic data provides new insights into the basis for the high reactivity of <b>1</b> and the possible involvement of species like <b>1</b> in oxidation catalysis

Copper/α-Ketocarboxylate Chemistry With Supporting Peralkylated Diamines: Reactivity of Copper(I) Complexes and Dicopper−Oxygen Intermediates

To further understand copper-promoted oxidation reactions, the Cu(I) complexes LCuX (L = N,N′-di-tert-butyl-N,N′-dimethylethylenediamine; X = benzoylformate (BF) or p-nitro-benzoylformate) were synthesized, fully characterized by X-ray crystallography and spectroscopy in solution, and their reactivity with O2 at −80 °C examined. Oxidative decarboxylation of the α-ketocarboxylate ligand was observed, but only to a significant extent when cyclohexene, cyclooctene, or acetonitrile was present. Spectroscopic and conductivity data are consistent with mechanistic postulates involving displacement of the α-ketocarboxylate by the additives to a small extent, followed by oxygenation of the LCu(I) moiety to yield copper−oxygen species that subsequently induce decarboxylation. To test these hypotheses, spectroscopic and kinetic studies of the reactions of Bu4NBF with preformed μ-η2:η2-peroxodicopper(II) and/or bis(μ-oxo)dicopper(III) complexes supported by L or N,N,N′,N′-tetramethylpropylenediamine were performed. In an illustration of a new mode of reactivity for such dicopper−oxygen cores, decarboxylation of the added α-ketocarboxylate was observed and the intermediacy of a carboxylate-bridged μ-η2:η2-peroxodicopper(II) complex was implicated

Involvement of a Formally Copper(III) Nitrite Complex in Proton-Coupled Electron Transfer and Nitration of Phenols

A unique high-valent copper nitrite species, LCuNO2, was accessed via the reversible one-electron oxidation of [M]­[LCuNO2] (M = NBu4+ or PPN+). The complex LCuNO2 reacts with 2,4,6-tri-tert-butylphenol via a typical proton-coupled electron transfer (PCET) to yield LCuTHF and the 2,4,6-tri-tert-butylphenoxyl radical. The reaction between LCuNO2 and 2,4-di-tert-butylphenol was more complicated. It yielded two products: the coupled bisphenol product expected from a H-atom abstraction and 2,4-di-tert-butyl-6-nitrophenol, the product of an unusual anaerobic nitration. Various mechanisms for the latter transformation were considered

Cu(I)/O₂ Chemistry Using a β-Diketiminate Supporting Ligand Derived from <i>N,N</i>-Dimethylhydrazine: A [Cu₃O₂]³⁺ Complex with Novel Reactivity

A Cu­(I) complex, LCu­(CH₃CN), was prepared and characterized, where L^– is a sterically unencumbered β-diketiminate ligand, the deprotonated version of 4-(2,2-dimethylhydrazino)­dimethylhydrazone-3-penten-2-one (LH). Analysis of FTIR spectra of the products of the reaction of LCu­(CH₃CN) with CO indicate that L^– is strongly electron donating, and support an equilibrium in solution between monomeric and dimeric forms with terminal and bridging CO ligands, respectively. Low temperature oxygenation of LCu­(CH₃CN) generated a bis­(μ-oxo)­tricopper complex with a <i>S</i> = 1 [Cu₃O₂]³⁺ core that was identified on the basis of UV–vis (λ_max (ε, M^–1 cm^–1 per Cu) = 328 (10700), 420 (1500), 590 (835) nm) and X-band electron paramagnetic resonance (EPR) spectroscopy (Δ<i>m</i>_<i>s</i> = 2 transition at 1500 G), electrospray ionization (ESI) mass spectrometry, and spectrophotometric titration (0.35(2) equiv of O₂ per copper atom), magnetic susceptibility (μ_eff = 2.8(1) BM), and H₂O₂ detection experiments (no H₂O₂ evolved upon acidification). Unlike other reported variants supported by neutral N-donor ligands, L₃Cu₃O₂ is not reduced by ferrocene, does not abstract H-atoms from phenols or 1,2-dihydroanthracene, oxidizes PPh₃ to Ph₃PO, and generates carbonate species upon exposure to CO₂. This unique reactivity for a [Cu₃O₂]³⁺ complex may be traced to the anionic charge and strong electron donating characteristics of L^–

A Structural Model of the Type 1 Copper Protein Active Site: N₂S(thiolate)S(thioether) Ligation in a Cu(II) Complex

A Structural Model of the Type 1 Copper Protein Active Site:  N2S(thiolate)S(thioether) Ligation in a Cu(II) Comple

Hydrogen Atom Abstraction from Hydrocarbons by a Copper(III)-Hydroxide Complex

With the aim of understanding the basis for the high rate of hydrogen atom abstraction (HAT) from dihydroanthracene (DHA) by the complex LCuOH (<b>1</b>; L = <i>N</i>,<i>N</i>′-bis­(2,6-diisopropylphenyl)-2,6-pyridinedicarboxamide), the bond dissociation enthalpy of the reaction product LCu­(H₂O) (<b>2</b>) was determined through measurement of its p<i>K</i>_a and <i>E</i>_1/2 in THF solution. In so doing, an equilibrium between <b>2</b> and LCu­(THF) was characterized by UV–vis and EPR spectroscopy and cyclic voltammetry (CV). A high p<i>K</i>_a of 18.8 ± 1.8 and a low <i>E</i>_1/2 of −0.074 V vs Fc/Fc⁺ in THF combined to yield an O–H BDE for <b>2</b> of 90 ± 3 kcal mol^–1 that is large relative to values for most transition metal oxo/hydroxo complexes. By taking advantage of the increased stability of <b>1</b> observed in 1,2-difluorobenzene (DFB) solvent, the kinetics of the reactions of <b>1</b> with a range of substrates with varying BDE values for their C–H bonds were measured. The oxidizing power of <b>1</b> was revealed through the accelerated decay of <b>1</b> in the presence of the substrates, including THF (BDE = 92 kcal mol^–1) and cyclohexane (BDE = 99 kcal mol^–1). CV experiments in THF solvent showed that <b>1</b> reacted with THF via rate-determining attack at the THF C–H­(D) bonds with a kinetic isotope effect of 10.2. Analysis of the kinetic and thermodynamic data provides new insights into the basis for the high reactivity of <b>1</b> and the possible involvement of species like <b>1</b> in oxidation catalysis

Mononuclear Cu–O₂ Complexes: Geometries, Spectroscopic Properties, Electronic Structures, and Reactivity

Using interwoven experimental and theoretical methods, detailed studies of several structurally defined 1:1 Cu–O2 complexes have provided important fundamental chemical information useful for understanding the nature of intermediates involved in aerobic oxidations in synthetic and enzymatic copper-mediated catalysis. In particular, these studies have shed new light on the factors that influence the mode of O2 coordination (end-on vs side-on) and the electronic structure, which can vary between Cu(II)–superoxo and Cu(III)–peroxo extremes