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. 2016 Jan 13;138(1):356-68.
doi: 10.1021/jacs.5b10985. Epub 2015 Dec 22.

Perturbing the Copper(III)-Hydroxide Unit through Ligand Structural Variation (VSports手机版)

Debanjan Dhar  1 , Gereon M Yee  1 , Andrew D Spaeth  1 , David W Boyce  1 , Hongtu Zhang  1 , Büsra Dereli  1 , Christopher J Cramer  1 , William B Tolman  1
Affiliations

V体育2025版 - Perturbing the Copper(III)-Hydroxide Unit through Ligand Structural Variation

Debanjan Dhar et al. J Am Chem Soc. .

Abstract

Two new ligand sets, (pipMe)LH2 and (NO2)LH2 ((pipMe)L = N,N'-bis(2,6-diisopropylphenyl)-1-methylpiperidine-2,6-dicarboxamide, (NO2)L = N,N'-bis(2,6-diisopropyl-4-nitrophenyl)pyridine-2,6-dicarboxamide), are reported which are designed to perturb the overall electronics of the copper(III)-hydroxide core and the resulting effects on the thermodynamics and kinetics of its hydrogen-atom abstraction (HAT) reactions VSports手机版. Bond dissociation energies (BDEs) for the O-H bonds of the corresponding Cu(II)-OH2 complexes were measured that reveal that changes in the redox potential for the Cu(III)/Cu(II) couple are only partially offset by opposite changes in the pKa, leading to modest differences in BDE among the three compounds. The effects of these changes were further probed by evaluating the rates of HAT by the corresponding Cu(III)-hydroxide complexes from substrates with C-H bonds of variable strength. These studies revealed an overarching linear trend in the relationship between the log k (where k is the second-order rate constant) and the ΔH of reaction. Additional subtleties in measured rates arise, however, that are associated with variations in hydrogen-atom abstraction barrier heights and tunneling efficiencies over the temperature range from -80 to -20 °C, as inferred from measured kinetic isotope effects and corresponding electronic-structure-based transition-state theory calculations. .

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Conflict of interest statement

Notes

The authors declare no competing financial interest.

Figures

Figure 1
Figure 1
(top) Hydrogen-atom transfer (HAT) by previously reported copper(III)–hydroxide complex, and (bottom) variants of the copper(III)–hydroxide complex studied herein.
Figure 2
Figure 2
Square scheme with associated thermodynamic parameters.
Figure 3
Figure 3
Representations of the X-ray crystal structures of (a) the anionic portion of (Et4N)[NO2LCu(OH)] ((Et4N)[3−]), (b) the Na-bridged anions in (Na)[pipHLCu(OH)] ((Na)[4a−]), and (c) the Na-bridged anions in (Na)[pipMeLCu(OH)] ((Na)[4b−]), (d) NO2LCu(OH2) THF (5), (e) NO2LCu(THF) (6), and (f) NO2LCu(CH3CN)·CH3CN (7). All non-hydrogen atoms are shown as 50% thermal ellipsoids and the hydrogen atoms attached to N(2) and O(3) atoms as spheres.
Figure 4
Figure 4
X-band EPR spectra of THF frozen solutions of the indicated complexes (30K).
Figure 5
Figure 5
Cyclic voltammagrams of compounds 1a− (red), 3− (black), and 4b− (blue). Conditions: 0.2 M Bu4NPF6 in 1,2-difluorobenzene, 20°C, 50 mV s−1.
Figure 6
Figure 6
UV–vis spectra after chemical oxidation of 1a− (to form LCu(OH), 1a, black), 3− (to form NO2LCu(OH), 3, red), and 4b− (to form pipMeLCu(OH), 4b, blue) in DFB at −25 °C.
Figure 7
Figure 7
TD-DFT difference plots (yellow lobes = loss of electron density, pink lobes = gain of elecron density) of the LMCT band taken at the highest oscillator strength for (a) LCu(OH) (1a), (b) NO2LCu-(OH) (3), and (c) pipMeLCu(OH) (4b). Gray, white, blue, red, and green atoms represent C, H, N, O, and Cu, respectively.
Figure 8
Figure 8
Eyring plots of ln(k/T) vs 1/T with linear least-squares fits to the data for reactions of LCu(OH) (1a) (black circles), NO2LCu(OH) (3) (red diamonds), and pipMeLCu(OH) (4b) (blue squares) with DHA or DHA-d4 in CH2Cl2. (a) Overlay of plots for reactions of the complexes with DHA. (b) Plots for the reactions of LCuOH (1a) with DHA (H, solid line) or DHA-d4 (D, dashed line). (c) Plots for the reactions of NO2LCu(OH) (3) and pipMeLCu(OH) (4b) with DHA (H, solid lines) or DHA-d4 (D, dashed lines). Indicated linear fits used to determine activation parameters (Table 4) have R2 > 0.99; standard deviations in the k values are shown in Table S9.
Figure 9
Figure 9
Plots of ln(KIE) vs 1/T constructed from the activation parameters (Table 4) for LCu(OH) (1a, black dashed line), NO2LCu-(OH) (3, blue solid line), and pipMeLCu(OH) (4b, red dotted line).
Figure 10
Figure 10
Plot of log(k) vs ΔH (equivalent to the ΔBDE between the aqua complexes 1a, 5, and 8 and the C–H bonds of the substrates) for reactions of 1a (black), 4b (red), and 3 (blue) with the substrates DHA (filled circles), cyclohexene (open circles), diphenylmethane (filled squares), THF (open squares), toluene (filled stars), and cyclohexane (open stars) at −25 °C in DFB. The indicated linear fit has a slope of −0.265 (R2 = 0.895).
Figure 11
Figure 11
(a) Optimized HAT TS structure for LCu(OH) (1a). Dotted lines indicate the forming and breaking OH and CH bonds. Atoms are copper (green), nitrogen (blue), oxygen (red), carbon (gray), and hydrogen (white) (b) with calculated interatomic distances indicated (given in Å).
Figure 12
Figure 12
Plots of Az from fits to the EPR spectra of Cu(II) complexes (black, R2 > 0.99) and DFT-calculated Cu partial charges for the oxidized Cu(III) complexes (blue, R2 = 0.94) versus E1/2 values for the CuIII/II couples for the systems supported by the indicated ligands, with arbitrary linear fits.
Figure 13
Figure 13
Plot of the E1/2 for the CuIII/II couple and the pKa for the Cu(II) aqua complex for the systems supported by the indicated ligands. The line is a fit to the data with a slope of −0.094 V/pKa (R2 = 0.98).
Scheme 1
Scheme 1
Synthesis of Ligand Precursors
Scheme 2
Scheme 2
Complexes Studied in This Work
Scheme 3
Scheme 3
Equilibria Involved in Determining pKa of pipMeLCu(OH2) (8) (eqs 2–4) and O–H BDE Values (eq 5)
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