Ligand Identity-Induced Generation of Enhanced Oxidative Hydrogen Atom Transfer Reactivity for a CuII2(O2•-) Complex Driven by Formation of a CuII2(-OOH) Compound with a Strong O-H Bond

Abstract

A superoxide-bridged dicopper(II) complex, [Cu^II₂(XYLO)(O₂^•-)]²⁺ (1) (XYLO = binucleating m-xylyl derivative with a bridging phenolate ligand donor and two bis(2-{2-pyridyl}ethyl)amine arms), was generated from chemical oxidation of the peroxide-bridged dicopper(II) complex [Cu^II₂(XYLO)(O₂^2-)]⁺ (2), using ferrocenium (Fc⁺) derivatives, in 2-methyltetrahydrofuran (MeTHF) at -125 °C. Using Me₁₀Fc⁺, a 1 ⇆ 2 equilibrium was established, allowing for calculation of the reduction potential of 1 as -0.525 ± 0.01 V vs Fc^+/0. Addition of 1 equiv of strong acid to 2 afforded the hydroperoxide-bridged dicopper(II) species [Cu^II₂(XYLO)(OOH)]²⁺ (3). An acid-base equilibrium between 3 and 2 was achieved through spectral titrations using a derivatized phosphazene base. The pK_a of 3 was thus determined to be 24 ± 0.6 in MeTHF at -125 °C. Using a thermodynamic square scheme and the Bordwell relationship, the hydroperoxo complex (3) O-H bond dissociation free energy (BDFE) was calculated as 81.8 ± 1.5 (BDE = 86.8) kcal/mol. The observed oxidizing capability of [Cu^II₂(XYLO)(O₂^•-)]²⁺ (1), as demonstrated in H atom abstraction reactions with certain phenolic ArO-H and hydrocarbon C-H substrates, provides direct support for this experimentally determined O-H BDFE. A kinetic study reveals a very fast reaction of TEMPO-H with 1 in MeTHF, with k (-100 °C) = 5.6 M^-1 s^-1. Density functional theory (DFT) calculations reveal how the structure of 1 may minimize stabilization of the superoxide moiety, resulting in its enhanced reactivity. The thermodynamic insights obtained herein highlight the importance of the interplay between ligand design and the generation and properties of copper (or other metal ion) bound O₂-derived reduced species, such as pK_a, reduction potential, and BDFE; these may be relevant to the capabilities (i.e., oxidizing power) of reactive oxygen intermediates in metalloenzyme chemical system mediated oxidative processes.

Figure 1.

Experimentally derived reduction potentials of the superoxide ligand (E°′ for O₂^•−/O₂²⁻) in metal-bound superoxide, complexes.^,,,

Figure 2.

UV–vis spectra monitoring the full oxidation of [Cu^II₂(XYLO)(O₂²⁻)]⁺ (purple spectrum, λ_max = 523 nm) to [Cu^II₂(XYLO)(O₂^•−)]²⁺ (green final spectrum, λ_max = 402 nm) following addition of 1 equiv of Me₈Fc⁺ in MeTHF at −125 °C. Total reaction time is 20 min.

Figure 3.

EPR and rR spectroscopic characterization of [Cu^II₂(XYLO)(O₂^•−)]²⁺ (1). (A) Frozen MeTHF solution EPR (20 K, X-band) spectrum of 1 (black) and fitted spectrum (red). (B) rR spectra of [Cu^II₂(XYLO)(O₂^•−)]²⁺ (1) using ¹⁶O₂ (blue), ¹⁸O₂ (red), and difference spectrum (¹⁶O₂ – ¹⁸O₂, green).

Figure 4.

UV–vis spectroscopy monitoring of the incremental addition of 0.25 equiv of Me₁₀Fc⁺, from 0 to 2 equiv, to [Cu^II₂(XYLO)(O₂²⁻)]⁺ (2, purple) in MeTHF at −125 °C resulting in the generation of equilibrium mixtures shown in gray of Me₁₀Fc, [Cu^II₂(XYLO)(O₂^•−)]²⁺ (1), Me₁₀Fc⁺, and [Cu^II₂(XYLO)(O₂²⁻)]⁺ (2) (see the Experimental Section and Supporting Information for more details), which allowed the determination of the reduction potential of the 1/2 couple. The low-energy feature at 784 nm is ascribed to the decamethylferrocenium cation. (Inset) Monitoring of the absorbances at 523 (purple) and 402 (green) nm, demonstrating the decrease and increase of absorbance at the λ_;max values for [Cu^II₂(XYLO)(O₂²⁻)]⁺ (2) and [Cu^II₂(XYLO)(O₂^•−)]²⁺ (1), respectively.

Figure 5.

(A) Addition of 1 equiv of EtP₂(dma) to a −125 °C MeTHF solution of [Cu^II₂(XYLO)(OOH)]²⁺ (3) (blue, λ_max = 398 nm) resulted in full deprotonation, forming [Cu^II₂(XYLO)(O₂²⁻)]⁺ (2) (purple, λ_max = 523 nm). A spectrum was obtained every 30 s (gray spectra). (B) Incremental titration of ^tBuP₁(pyrr) into a solution of [Cu^II₂(XYLO)(OOH)]²⁺ (3) gave equilibrium mixtures (shown in gray, 15 min after each addition of ^tBuP₁(pyrr) to ensure equilibration) of ^tBuP₁(pyrr)H⁺, [Cu^II₂(XYLO)(O₂²⁻)]⁺ (λ_max = 523 nm), ^tBuP₁(pyrr), and [Cu^II₂(XYLO)(OOH)]²⁺ (λ_max = 398 nm), which allowed for the determination of the pK_a of 3 (see also Tables S3 and S4 and eq S5). (Inset) Monitoring the absorbance at 398 (blue) and 523 (purple) nm upon addition of various amounts of ^tBuP₁(pyrr), demonstrating the disappearance of 3 and concomitant formation of 2.

Figure 6.

UV–vis spectra monitoring of the reaction of [Cu^II₂(XYLO)(O₂^•−)]²⁺ (green spectrum) with 30 equiv of p-OMe-2,6-DTBP in MeTHF at −80 °C to give [Cu^II₂(XYLO)(OOH)]²⁺ (blue final spectrum, λ_max = 398 and 612 nm). Spectra were taken every 5 min (gray spectra, total reaction time is 3 h). (Inset) EPR spectra (20 K) of [Cu^II₂(XYLO)(O₂^•−)]²⁺ (1) (green spectrum, broad g = 2 peak) and its reaction with 30 equiv of p-OMe-2,6-DTBP to give [Cu^II₂(XYLO)(OOH)]²⁺ (3) (X-band EPR silent) and the phenoxyl radical (blue spectrum, sharp g = 2 peak). The yield calculated based on the absorption of [Cu^II₂(XYLO)(OOH)]²⁺ at 398 nm is 77%. The yield calculated based on spin quantification of the p-OMe-2,4,-di-tert-butylphenoxyl radical is 71%. See Figure S8 for a time trace of this reaction.

Figure 7.

UV–vis spectra monitoring the reaction of [Cu^II₂(XYLO)-(O₂^•−)]²⁺ (green spectrum) with 500 equiv of diphenylmethane in MeTHF at −80 °C to give [Cu^II₂(XYLO)(OOH)]²⁺ (blue final spectrum). Spectra were taken every 10 min (gray spectra, total reaction time was 8 h). Yield calculated based on the absorption of [Cu^II₂(XYLO)(OOH)]²⁺ at 398 nm is 80%. See Figure S16 for a time trace of this reaction.

Figure 8.

UV–vis spectra monitoring the reaction of [Cu^II₂XYLO)-(O₂^•−)]²⁺ at 402 nm (green spectrum) with 20 equiv of TEMPO–H in MeTHF at −100 °C to yield [Cu^II₂(XYLO)(OOH)]²⁺ at 398 nm (blue final spectrum, total reaction time was 15 min). (Inset) Plot of k_obs (s⁻¹) vs varying concentrations of TEMPO–H (M) to obtain the second-order rate constant, k₂ = 5.6 ± 0.4 M⁻¹ s⁻¹. Yield calculated based on the absorption of [Cu^II₂(XYLO)(OOH)]²⁺ at 398 nm is 96%. See the Supporting information for a larger version of the inset (Figure S11), which also includes the origin (0, 0).

Figure 9.

Comparison of the peroxide-binding modes in (A) [Cu^II₂(XYLO)(O₂²⁻)]⁺ and (B) [Cu^II₂(pyrazolate)(O₂²⁻)]⁺.

Figure 10.

Lowest unoccupied molecular orbitals for [Cu^II₂(XYLO)-(O₂^•−)]²⁺ (A) and [Cu^II₂(pyrazolate)(O₂^•−)]²⁺ (B), with the structure and unpaired spins (Cu = green, O₂^•− = red) for each complex shown on the left. Antiferromagnetic coupling between unpaired electrons is denoted below the structures on the left. For each complex, the singly occupied superoxo π* orbital (backbonding acceptor) is indicated in red. The percent of (formally) occupied orbital character mixed into the unoccupied orbitals is given for Cu atoms and chelating ligand (contributions from the bridging phenolate(Ph)/pyrazolate(Pyr) and the remaining ligand (N_L) are given separately); orbitals active in backbonding are bolded.

Chart 1.

Strong, EtP₂(dma) (Left, Conjugate Acid pK_a(THF) = 28.1), and Weaker, ^tBuP₁(pyrr) (Right, Conjugate Acid pK_a(THF) = 22.8), Phosphazene Base Derivatives Capable of Deprotonating [Cu^II₂(XYLO)(OOH)]²⁺ (3) To Give the Peroxo Complex [Cu^II₂(XYLO)(O₂²⁻)]⁺ (2)

Scheme 1.

(A) Four-Electron, Four-Proton Reduction of Dioxygen to Water; (B) O₂ Addition at a Mixed-Valent Dicopper Site Giving a Bridging Superoxo Complex, Which Can Then Be Externally Reduced with or without a Proton Present to Give a Binuclear Hydroperoxo or Peroxo Complex, Respectively

Scheme 2.

Previously Deduced Catalytic Mechanism for the 2H⁺/2e⁻ Reduction of O₂ to H₂O₂ by [Cu^II₂(XYLO)(OH)]²⁺ in the Presence of Acid (Trifluoroacetic Acid (TFA)) and Reductant (Decamethylferrocene)^{a a} All species depicted have been characterized in separate studies presently (vide infra) or previously.

Scheme 3.

Thermodynamic Square Scheme Relevant to the Interconversion of Superoxo, Peroxo, and Hydroperoxo XYLO Dicopper(II) Complexes and the Present Determination of the O–H Bond Dissociation Free Energy of [Cu^II₂(XYLO)(OOH)]²⁺ in MeTHF at −125 °C

Scheme 4.

[Cu^II₂(XYLO)(O₂^•−)]²⁺ Can Abstract an H-Atom from Organic Solvent Soluble Derivatives of Ascorbic Acid (Green) and from Ascorbate (Blue)

Scheme 5.

Comparison of the Thermodynamic Parameters Determined for the Phenolate-Bridged (XYLO, Reported Herein) and the Pyrazolate-Bridged Dicopper(II) Complexes Reported ^, by Meyer and Co-workers^{a a} Note that reduction potentials shown are vs Fc^+/0.