Copper active sites in biology

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Copper active sites in biology

[no abstract

A Tricopper(I) Complex Competent for O Atom Transfer, C–H Bond Activation, and Multiple O₂ Activation Steps

Oxygenation of a tricopper­(I) cyclophanate (<b>1</b>) affords reactive transients competent for C–H bond activation and O atom transfer to various substrates (including toluene, dihydroanthracene, and ethylmethylsulfide) based on ¹H NMR, gas chromatography/mass spectrometry (MS), and electrospray ionization (ESI)/MS data. Low product yields (<1%) are determined for C–H activation substrates (e.g, toluene, ethylbenzene), which we attribute to competitive ligand oxidation. The combined stopped-flow UV/visible, electron paramagnetic resonance, ESI/MS, ¹H NMR, and density functional theory (DFT) results for reaction of <b>1</b> with O₂ are consistent with transient peroxo- and di­(oxo)-bridged intermediates. DFT calculations elucidate a concerted proton-coupled electron transfer from toluene to the di­(μ-oxo) intermediate and subsequent radical rebound as the C–H activation mechanism. Our results support a multicopper oxidase-like mechanism for O₂ activation by <b>1</b>, traversing species similar to the coplanar Cu₃O₂ unit in the peroxy and native intermediates

Thermodynamics of a μ‑oxo Dicopper(II) Complex for Hydrogen Atom Abstraction

The mono-μ-hydroxo complex {[Cu­(tmpa)]₂-(μ-OH)}³⁺ (<b>1</b>) can undergo reversible deprotonation at −30 °C to yield {[Cu­(tmpa)]₂-(μ-O)}²⁺ (<b>2</b>). This species is basic with a p<i>K</i>_a of 24.3. <b>2</b> is competent for concerted proton–electron transfer from TEMPOH, but is an intrinsically poor hydrogen atom abstractor (BDFE­(OH) of 77.2 kcal/mol) based on kinetic and thermodynamic analyses. Nonetheless, DFT calculations experimentally calibrated against <b>2</b> reveal that [Cu₂O]²⁺ is likely thermodynamically viable in copper-dependent methane monoxygenase enzymes

Thermodynamics of a μ‑oxo Dicopper(II) Complex for Hydrogen Atom Abstraction

The mono-μ-hydroxo complex {[Cu­(tmpa)]₂-(μ-OH)}³⁺ (<b>1</b>) can undergo reversible deprotonation at −30 °C to yield {[Cu­(tmpa)]₂-(μ-O)}²⁺ (<b>2</b>). This species is basic with a p<i>K</i>_a of 24.3. <b>2</b> is competent for concerted proton–electron transfer from TEMPOH, but is an intrinsically poor hydrogen atom abstractor (BDFE­(OH) of 77.2 kcal/mol) based on kinetic and thermodynamic analyses. Nonetheless, DFT calculations experimentally calibrated against <b>2</b> reveal that [Cu₂O]²⁺ is likely thermodynamically viable in copper-dependent methane monoxygenase enzymes

Phenol-Induced O–O Bond Cleavage in a Low-Spin Heme–Peroxo–Copper Complex: Implications for O₂ Reduction in Heme–Copper Oxidases

This study evaluates the reaction of a biomimetic heme–peroxo–copper complex, {[(DCHIm)­(F₈)­Fe^III]–(O₂^2–)–[Cu^II(AN)]}⁺ (<b>1</b>), with a phenolic substrate, involving a net H-atom abstraction to cleave the bridging peroxo O–O bond that produces Fe^IVO, Cu^IIOH, and phenoxyl radical moieties, analogous to the chemistry carried out in heme–copper oxidases (HCOs). A 3D potential energy surface generated for this reaction reveals two possible reaction pathways: one involves nearly complete proton transfer (PT) from the phenol to the peroxo ligand before the barrier; the other involves O–O homolysis, where the phenol remains H-bonding to the peroxo O_Cu in the transition state (TS) and transfers the H⁺ after the barrier. In both mechanisms, electron transfer (ET) from phenol occurs after the PT (and after the barrier); therefore, only the interaction with the H⁺ is involved in lowering the O–O cleavage barrier. The relative barriers depend on covalency (which governs ET from Fe), and therefore vary with DFT functional. However, as these mechanisms differ by the amount of PT at the TS, kinetic isotope experiments were conducted to determine which mechanism is active. It is found that the phenolic proton exhibits a secondary kinetic isotope effect, consistent with the calculations for the H-bonded O–O homolysis mechanism. The consequences of these findings are discussed in relation to O–O cleavage in HCOs, supporting a model in which a peroxo intermediate serves as the active H⁺ acceptor, and both the H⁺ and e^– required for O–O cleavage derive from the cross-linked Tyr residue present at the active site

Electrocatalytic Water Oxidation by a Homogeneous Copper Catalyst Disfavors Single-Site Mechanisms

Deployment of solar fuels derived from water requires robust oxygen-evolving catalysts made from earth abundant materials. Copper has recently received much attention in this regard. Mechanistic parallels between Cu and single-site Ru/Ir/Mn water oxidation catalysts, including intermediacy of terminal Cu oxo/oxyl species, are prevalent in the literature; however, intermediacy of late transition metal oxo species would be remarkable given the high d-electron count would fill antibonding orbitals, making these species high in energy. This may suggest alternate pathways are at work in copper-based water oxidation. This report characterizes a dinuclear copper water oxidation catalyst, {[(L)­Cu­(II)]₂-(μ-OH)₂}­(OTf)₂ (L = Me₂TMPA = bis­((6-methyl-2-pyridyl)­methyl)­(2-pyridylmethyl)­amine) in which water oxidation proceeds with high Faradaic efficiency (>90%) and moderate rates (33 s^–1 at ∼1 V overpotential, pH 12.5). A large kinetic isotope effect (<i>k</i>_H/<i>k</i>_D = 20) suggests proton coupled electron transfer in the initial oxidation as the rate-determining step. This species partially dissociates in aqueous solution at pH 12.5 to generate a mononuclear {[(L)­Cu­(II)­(OH)]}⁺ adduct (<i>K</i>_eq = 0.0041). Calculations that reproduce the experimental findings reveal that oxidation of either the mononuclear or dinuclear species results in a common dinuclear intermediate, {[LCu­(III)]₂-(μ-O)₂}²⁺, which avoids formation of terminal Cu­(IV)O/Cu­(III)–O^• intermediates. Calculations further reveal that both intermolecular water nucleophilic attack and redox isomerization of {[LCu­(III)]₂-(μ-O)₂}²⁺ are energetically accessible pathways for O–O bond formation. The consequences of these findings are discussed in relation to differences in water oxidation pathways between Cu catalysts and catalysts based on Ru, Ir, and Mn

Electrocatalytic Water Oxidation by a Homogeneous Copper Catalyst Disfavors Single-Site Mechanisms

Deployment of solar fuels derived from water requires robust oxygen-evolving catalysts made from earth abundant materials. Copper has recently received much attention in this regard. Mechanistic parallels between Cu and single-site Ru/Ir/Mn water oxidation catalysts, including intermediacy of terminal Cu oxo/oxyl species, are prevalent in the literature; however, intermediacy of late transition metal oxo species would be remarkable given the high d-electron count would fill antibonding orbitals, making these species high in energy. This may suggest alternate pathways are at work in copper-based water oxidation. This report characterizes a dinuclear copper water oxidation catalyst, {[(L)­Cu­(II)]₂-(μ-OH)₂}­(OTf)₂ (L = Me₂TMPA = bis­((6-methyl-2-pyridyl)­methyl)­(2-pyridylmethyl)­amine) in which water oxidation proceeds with high Faradaic efficiency (>90%) and moderate rates (33 s^–1 at ∼1 V overpotential, pH 12.5). A large kinetic isotope effect (<i>k</i>_H/<i>k</i>_D = 20) suggests proton coupled electron transfer in the initial oxidation as the rate-determining step. This species partially dissociates in aqueous solution at pH 12.5 to generate a mononuclear {[(L)­Cu­(II)­(OH)]}⁺ adduct (<i>K</i>_eq = 0.0041). Calculations that reproduce the experimental findings reveal that oxidation of either the mononuclear or dinuclear species results in a common dinuclear intermediate, {[LCu­(III)]₂-(μ-O)₂}²⁺, which avoids formation of terminal Cu­(IV)O/Cu­(III)–O^• intermediates. Calculations further reveal that both intermolecular water nucleophilic attack and redox isomerization of {[LCu­(III)]₂-(μ-O)₂}²⁺ are energetically accessible pathways for O–O bond formation. The consequences of these findings are discussed in relation to differences in water oxidation pathways between Cu catalysts and catalysts based on Ru, Ir, and Mn