Mechanistic Dichotomy in Proton-Coupled Electron-Transfer Reactions of Phenols with a Copper Superoxide Complex

Abstract

The kinetics and mechanism(s) of the reactions of [K(Krypt)][LCuO₂] (Krypt = 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane, L = a bis(arylcarboxamido)pyridine ligand) with 2,2,6,6-tetramethylpiperdine- N-hydroxide (TEMPOH) and the para-substituted phenols ^XArOH (X = para substituent NO₂, CF₃, Cl, H, Me, ^tBu, OMe, or NMe₂) at low temperatures were studied. The reaction with TEMPOH occurs rapidly ( k = 35.4 ± 0.3 M^-1 s^-1) by second-order kinetics to yield TEMPO^• and [LCuOOH]^- on the basis of electron paramagnetic resonance spectroscopy, the production of H₂O₂ upon treatment with protic acid, and independent preparation from reaction of [NBu₄][LCuOH] with H₂O₂ ( K_eq = 0.022 ± 0.007 for the reverse reaction). The reactions with ^XArOH also follow second-order kinetics, and analysis of the variation of the k values as a function of phenol properties (Hammett σ parameter, O-H bond dissociation free energy, p K_a, E_1/2) revealed a change in mechanism across the series, from proton transfer/electron transfer for X = NO₂, CF₃, Cl to concerted-proton/electron transfer (or hydrogen-atom transfer) for X = OMe, NMe₂ (data for X = H, Me, ^tBu are intermediate between the extremes). Thermodynamic analysis and comparisons to previous results for LCuOH, a different copper-oxygen intermediate with the same supporting ligand, and literature for other [CuO₂]⁺ complexes reveal significant differences in proton-coupled electron-transfer mechanisms that have implications for understanding oxidation catalysis by copper-containing enzymes and abiological catalysts.

Figure 1

Intermediates proposed for O₂ activation at a single copper site, emphasizing proton transfers (PT, vertical arrows), electron transfers (ET, horizontal arrows), and proton-coupled electron transfers (PCET, diagonal red arrows) that connect them. Arrows are drawn for simplicity as unidirectional, although the processes are equilibria characterized by parameters pK_a (PT), E_1/2 (ET), or BDFE (PCET), respectively, where BDFE = bond dissociation free energy. Gray boxes indicate the square schemes A and B emphasized in this work, and each intermediate is written in two forms: indicating possible formal oxidation states (blue) or overall charge (black). Figure inspired by and adapted from one published in ref .

Figure 2

Copper complexes comprising [CuOH]²⁺ and [CuO₂]⁺ cores supported by bis(arylcarboxamido)pyridine ligands. Krypt = Krypto-fix222.

Figure 3

UV−vis spectra as a function of time observed upon reaction of [K(Krypt)][LCuO₂] (red) with TEMPOH. Conditions: [Cu] = 0.7 mM, [TEMPOH]₀ = 1.4 mM, THF:MeCN 10:1, −80 °C. Inset shows linear fit in plot of k_obs versus [TEMPOH]₀ (slope = 34.9 ± 1.5, intercept = −0.016 ± 0.005, R² = 0.993).

Figure 4

Reactions of [K(Krypt)][LCuO₂] with phenols ^XArOH, with a representation of the X-ray crystal structure for the case X = H, showing all nonhydrogen atoms as 50% thermal ellipsoids (hydrogen atoms and NEt₄ counterion omitted). Selected interatomic distances (Å) and angles (deg): Cu1−O1: 1.8767(13), Cu1−N2: 1.9296(14), Cu1−N1: 2.0064(15), Cu1−N3: 2.0220(15), O1−Cu1−N2: 172.98(6), O1−Cu1−N1: 96.94(6), O1−Cu1−N3: 102.46(6) N1− Cu1−N3: 160.44(6).

Figure 5

(a) Hammett plot of log(k_X^DMSO/k_H^DMSO) versus ^XArOH σ_p, with X labeled. Separate linear fits are shown to data for X = NMe₂, OMe, ^tBu, and Me (slope ρ = −2.5, R² = 0.97) and for X = Me, H, Cl, CF₃, and NO₂ (slope ρ = +2.0, R² = 0.96). (b) Plot of log(k^DMSO) versus ^XArOH pK_a. Separate linear fits are shown to data for X = NMe₂, OMe, tBu, and Me (slope = +1.84, R² = 0.924) and for X = Me, H, Cl, CF₃, and NO₂ (slope = −0.22, R² = 0.998). (c) Plot of log(k^DMSO) versus ^XArOH0/+ E_1/2. Separate linear fits are shown to data for X = NMe₂, OMe, tBu, and Me (slope = −1.43, R² = 0.92) and for X = Me, H, Cl, and NO₂ (slope = +2.29, R² = 0.996).

Figure 6

UV−vis spectral traces for the reaction of [K(Krypt)]- [LCuO₂] (red) with ^XArOH (X = OMe, 20 equiv) at −60 °C, 19:1 THF:CH₃CN, over ∼30 min. Inset shows the decay trace over time following the signal at 628 (black squares) and 775 nm (red triangles). “Int.” is defined as the initially formed Cu(II) intermediate and is highlighted in the UV−vis spectrum in purple.

Figure 7

Proposed PT/ET (red) and CPET (blue) mechanisms for the reactions of [K(Krypt)][LCuO₂] with ^XArOH. The data are ambiguous for X = H, Me, ^tBu, suggesting that in these cases both pathways may be operative (not shown).

Figure 8

Complexes with [CuO₂]⁺ cores for which reactivity studies with TEMPOH (or derivatives) and phenols have been reported. DMATMPA: R = NMe₂, R₁ = R₂ = H; ^DMMTMPA: R = OMe, R₁ = R₂ = H; PVTMPA: R = NH(CO)tBu, R₁ = R₂ = H; NEOTMPA: R = R₁ = NH(neopentyl), R₂ = H). Ar = aryl, Mes = mesityl. Note: The complex ArLCuO₂ is proposed to equilibrate with an end-on (η¹) isomer (ref 39).

Figure 9

(a) log(k^DMSO) versus ^XArOH BDFE overlay plot for the reaction of [CuO₂]⁺ (red circles, −60 °C) and [CuOH]²⁺ (blue squares, −80 °C) cores with ^XArOH (X = NMe₂, OMe, Me, H, Cl, CF₃, NO₂). The red line depicts an arbitrary linear correlation for the data points for X = Me, H, Cl, CF₃, and NO₂ (R2 = 0.779, slope = 0.29). The blue line is a fit to the data for all X (R2 = 0.932, slope = −0.35). (b) log(kDMSO) versus ^XArOH σ_p overlay plot for the [CuO₂]⁺ (red circles, −60 °C) and [CuOH]²⁺ (blue squares, −80 °C) cores. The red line depicts a fit to the data for X = Me, H, Cl, CF₃, and NO₂ (R2 = 0.959, slope = 2.0). The blue line depicts a fit to the data for all X (R² = 0.806, slope = −3.2). (c) log(k^DMSO) versus ^XArOH pK_a overlay plot for the [CuO₂]⁺ (red circles, −60 °C) and [CuOH]²⁺ (blue squares, −80 °C) cores. The red and blue lines are fits to the data for all X except NMe₂ (red, R² = 0.992, slope = −0.22) or except NO₂ (blue, R² = 0.811, slope = 1.1). (d) log(k^DMSO) versus ^XArOH^0/+ E_1/2 overlay plot for the [CuO₂]⁺ (red circles, −60 °C) and [CuOH]²⁺ (blue squares, −80 °C) cores (E_1/2 values referenced to the Fc⁺/Fc redox couple). The red line is a fit for X = Me, H, Cl, and NO₂ (R² = 0.996, slope = 2.3). The blue line is a fit to the data for all X (R² = 0.798, slope = −2.5).

Figure 10

Square schemes A and B (from Figure 1), but with measured or estimated thermodynamic parameters from this work (A) or reported previously (B) indicated.

Figure 11

Plots of ΔG for the corresponding PT (red) or CPET (black) reactions with LCuOH (top, squares, adapted from ref 17) and [LCuO₂]⁻ (bottom, circles). The dashed regions highlight where there is thermodynamic potential for both PT and CPET mechanisms to occur.

Scheme 1

Reaction of [K(Krypt)][LCuO₂] with TEMPOH