15 research outputs found
Modulating the Reduction Potential of Mononuclear Cobalt(II) Complexes via Selective Deprotonation of Tris[(2-benzimidazolyl)methyl]amine
A Tricopper(I) Complex Competent for O Atom Transfer, C–H Bond Activation, and Multiple O<sub>2</sub> Activation Steps
Oxygenation
of a tricopperÂ(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 <sup>1</sup>H 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, <sup>1</sup>H NMR, and density functional theory
(DFT) results for reaction of <b>1</b> with O<sub>2</sub> are
consistent with transient peroxo- and diÂ(oxo)-bridged intermediates.
DFT calculations elucidate a concerted proton-coupled electron transfer
from toluene to the diÂ(μ-oxo) intermediate and subsequent radical
rebound as the C–H activation mechanism. Our results support
a multicopper oxidase-like mechanism for O<sub>2</sub> activation
by <b>1</b>, traversing species similar to the coplanar Cu<sub>3</sub>O<sub>2</sub> unit in the peroxy and native intermediates
Thermodynamics of a μ‑oxo Dicopper(II) Complex for Hydrogen Atom Abstraction
The
mono-μ-hydroxo complex {[CuÂ(tmpa)]<sub>2</sub>-(μ-OH)}<sup>3+</sup> (<b>1</b>) can undergo reversible deprotonation at
−30 °C to yield {[CuÂ(tmpa)]<sub>2</sub>-(μ-O)}<sup>2+</sup> (<b>2</b>). This species is basic with a p<i>K</i><sub>a</sub> of 24.3. <b>2</b> is competent for concerted
proton–electron transfer from TEMPOH, but is an intrinsically
poor hydrogen atom abstractor (BDFEÂ(OH) of 77.2 kcal/mol) based on
kinetic and thermodynamic analyses. Nonetheless, DFT calculations
experimentally calibrated against <b>2</b> reveal that [Cu<sub>2</sub>O]<sup>2+</sup> 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 {[CuÂ(tmpa)]<sub>2</sub>-(μ-OH)}<sup>3+</sup> (<b>1</b>) can undergo reversible deprotonation at
−30 °C to yield {[CuÂ(tmpa)]<sub>2</sub>-(μ-O)}<sup>2+</sup> (<b>2</b>). This species is basic with a p<i>K</i><sub>a</sub> of 24.3. <b>2</b> is competent for concerted
proton–electron transfer from TEMPOH, but is an intrinsically
poor hydrogen atom abstractor (BDFEÂ(OH) of 77.2 kcal/mol) based on
kinetic and thermodynamic analyses. Nonetheless, DFT calculations
experimentally calibrated against <b>2</b> reveal that [Cu<sub>2</sub>O]<sup>2+</sup> 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 O<sub>2</sub> Reduction in Heme–Copper Oxidases
This
study evaluates the reaction of a biomimetic heme–peroxo–copper
complex, {[(DCHIm)Â(F<sub>8</sub>)ÂFe<sup>III</sup>]–(O<sub>2</sub><sup>2–</sup>)–[Cu<sup>II</sup>(AN)]}<sup>+</sup> (<b>1</b>), with a phenolic substrate, involving a net H-atom abstraction
to cleave the bridging peroxo O–O bond that produces Fe<sup>IV</sup>O, Cu<sup>II</sup>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<sub>Cu</sub> in the transition
state (TS) and transfers the H<sup>+</sup> 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<sup>+</sup> 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<sup>+</sup> acceptor, and both
the H<sup>+</sup> and e<sup>–</sup> 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)ÂCuÂ(II)]<sub>2</sub>-(μ-OH)<sub>2</sub>}Â(OTf)<sub>2</sub> (L = Me<sub>2</sub>TMPA = bisÂ((6-methyl-2-pyridyl)Âmethyl)Â(2-pyridylmethyl)Âamine)
in which water oxidation proceeds with high Faradaic efficiency (>90%)
and moderate rates (33 s<sup>–1</sup> at ∼1 V overpotential,
pH 12.5). A large kinetic isotope effect (<i>k</i><sub>H</sub>/<i>k</i><sub>D</sub> = 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)ÂCuÂ(II)Â(OH)]}<sup>+</sup> adduct (<i>K</i><sub>eq</sub> = 0.0041). Calculations that reproduce the experimental
findings reveal that oxidation of either the mononuclear or dinuclear
species results in a common dinuclear intermediate, {[LCuÂ(III)]<sub>2</sub>-(μ-O)<sub>2</sub>}<sup>2+</sup>, which avoids formation
of terminal CuÂ(IV)î—»O/CuÂ(III)–O<sup>•</sup> intermediates.
Calculations further reveal that both intermolecular water nucleophilic
attack and redox isomerization of {[LCuÂ(III)]<sub>2</sub>-(μ-O)<sub>2</sub>}<sup>2+</sup> 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)ÂCuÂ(II)]<sub>2</sub>-(μ-OH)<sub>2</sub>}Â(OTf)<sub>2</sub> (L = Me<sub>2</sub>TMPA = bisÂ((6-methyl-2-pyridyl)Âmethyl)Â(2-pyridylmethyl)Âamine)
in which water oxidation proceeds with high Faradaic efficiency (>90%)
and moderate rates (33 s<sup>–1</sup> at ∼1 V overpotential,
pH 12.5). A large kinetic isotope effect (<i>k</i><sub>H</sub>/<i>k</i><sub>D</sub> = 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)ÂCuÂ(II)Â(OH)]}<sup>+</sup> adduct (<i>K</i><sub>eq</sub> = 0.0041). Calculations that reproduce the experimental
findings reveal that oxidation of either the mononuclear or dinuclear
species results in a common dinuclear intermediate, {[LCuÂ(III)]<sub>2</sub>-(μ-O)<sub>2</sub>}<sup>2+</sup>, which avoids formation
of terminal CuÂ(IV)î—»O/CuÂ(III)–O<sup>•</sup> intermediates.
Calculations further reveal that both intermolecular water nucleophilic
attack and redox isomerization of {[LCuÂ(III)]<sub>2</sub>-(μ-O)<sub>2</sub>}<sup>2+</sup> 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