Spectroscopic characterization and reactivity studies of a copper(II) iminoxyl radical complex

Abstract

A Cu^II complex (1) of a bis-pyridine-dioxime ligand and its one-electron oxidized analog (1-ox) were thoroughly characterized by various spectroscopic techniques, including X-ray absorption spectroscopy. 1-ox was found to be a Cu^II complex of a ligand iminoxyl radical and represents the first example of such a type. Reorganization energy (λ) of 2.12 eV was determined for the 1-ox/1 couple, which is considerably higher than the type 1 protein and synthetic Cu^III/II(OH) complexes.

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The presence of redox-active ligands is often found at the active site of different metalloenzymes. The oxidative holes created by the combination of ligand and metal cantered oxidation processes help reduce a reaction's activation barrier. The formation of the iron(IV)-oxo porphyrin radical cation, for instance, occurs in the large family of cytochrome P450, which abstracts the C–H bond of substrates and performs oxygenation reactions.¹ Furthermore, the redox-active tyrosine moieties are present in photosystem II (PS II), ribonucleotide reductase, and prostaglandin H synthase.² Another example is the oxidized analogue of galactose oxidase (GO), which contains a post-translational cysteine-modified tyrosyl radical ligated Cu^II complex that oxidizes alcohol to aldehyde.³ Inspired by the active site structure of GO, various Cu^II radical species originating from catechol, phenol, ortho-aminophenol, and ortho-phenylenediamine-containing ligands have been synthesized and spectroscopically characterized.⁴ Iminoxyl radicals are a class of transient reaction intermediates involved in versatile organic transformation reactions.⁵ In addition, the generation of tyrosine iminoxyl radical species has been reported upon exposing PS II to nitric oxide.⁶ However, transition metal complexes of such a radical species are unknown. Herein, we report the spectroscopic characterization of a Cu^II-iminoxyl radical complex (Scheme 1) together with its hydrogen atom abstraction (HAA) and electron transfer (ET) reactivity studies.

Scheme 1 The reaction of 1 with one equiv. of CAN.

The Cu^II complex ([Cu^II(HL₁)(ClO₄)], 1) was prepared by reacting equimolar amounts of the ligand (H₂L₁) and Cu^II(ClO₄)₂·6H₂O in methanol. The X-ray structure of 1 is described in Fig. 1A, and metrical parameters are mentioned in Tables S1 and S2 (ESI†). A nearly perfect square pyramidal geometry around Cu^II was observed in 1 (τ₅ = 0.0093).⁷ The central Cu atom is ligated by two pyridine (d_{Cu–N(pyridine)} = 1.920(3) and 1.916(4) Å) and two imine nitrogen atoms (d_{Cu–N(imine)} = 1.912(4) and 1.929(4) Å) of the ligand. The axial position is occupied by an oxygen atom of ClO₄⁻ at a distance of 2.419(3) Å. One of the oxime groups of the ligand was further deprotonated and created a pseudo-six-membered ring around Cu in 1, which is typically observed in the Cu oxime complexes.⁸

Fig. 1 (A) The X-ray structure of 1 with 50% ellipsoid probability. (B) X-band EPR spectrum of 1 (2 mM) in methanol at 77 K. (C) CV and DPV of 1 (0.5 mM) vs. Fc⁺/Fc in methanol at 25 °C, using ⁿBu₄NClO₄ as the supporting electrolyte. (D) The change of UV-vis spectrum of 1 (0.14 mM) upon the addition of one equiv. of CAN in methanol at −40 °C.

The UV-vis spectrum of 1 was recorded in methanol, revealing the absorbance maxima at 439 nm (ε = ∼1100 M⁻¹ cm⁻¹) (Fig. S13, ESI†). The ESI-mass spectrum of 1 showed a molecular ion peak at m/z 423.07, corresponding to a composition of [Cu(HL₁)]⁺ (Fig. S15, ESI†). The X-band EPR spectrum of 1 was recorded in frozen methanol at 77 K, which is described in Fig. 1B. Simulation of the EPR spectra (Fig. S17, ESI†) revealed g_x, g_y, and g_z values of 2.045, 2.09, and 2.165, and hyperfine splitting arising from the Cu (I = 3/2) showed A^Cu_x, A^Cu_y, and A^Cu_z of 20, 30, and 210 G, respectively. Furthermore, 1 exhibited a super hyperfine structure because of the ligand donor nitrogen atoms with an average super hyperfine coupling constant (A_N) of ∼14.1 G. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) data of 1 were measured in methanol at 25 °C. Upon anodic sweep, 1 exhibited a quasi-reversible redox wave at E_1/2 of 0.48 V (ΔE = 210 mV) vs. the Fc⁺/Fc couple (Fig. 1C and Fig. S19, ESI†), which can be assigned as the ligand oxidation reaction (vide infra).

The addition of one equiv. of ceric ammonium nitrate (CAN) to 1 in methanol at −40 °C resulted in the formation of a new species (1-ox) having λ_max at 543 nm (ε = 8860 M⁻¹ cm⁻¹, Fig. 1D), which can be assigned to the ligand-to-metal charge-transfer (LMCT) transition.

A similar LMCT band has been reported for the oxidized Cu^II complexes of imine-oxime and dimethyl glyoxime ligands.⁹ The species decomposed slowly upon warming the reaction solution at 25 °C. Furthermore, no signal was observed for 1-ox in the X-band EPR spectrum in frozen acetonitrile at 77 K, and the species revealed a magnetic moment of 2.8μ_B (Fig. S23, ESI†) in methanol-d₄ at −40 °C, suggesting the presence of an S = 1 state in 1-ox. This observation indicates that the oxidation of 1 results in forming a ligand-oxidized Cu^II species (1-ox, supported by X-ray absorption near-edge structure (XANES) studies, vide infra). The S = 1 state of 1-ox may result from the ferromagnetic coupling between the unpaired electron at Cu^II and an unpaired electron at the ligand backbone. It's important to note that monoradical Cu^II species have been stabilized using ligand scaffolds having anionic iminobenzosemiquinone,¹⁰ tripyrrindione,¹¹ and redox-active [NNN] pincer backbones.¹² In the mentioned examples, anionic ligands were employed, which get oxidized easily before the Cu-centered ET reaction, thus making the ligand radical containing Cu^II species relatively stable.

Furthermore, to get additional information about species 1-ox, we prepared a deuterated analogue (the aniline moiety of the ligand is enriched with deuterium, described in the ESI†) of the Cu^II complex (1-d₅) and the one-electron oxidized product (1-ox-d₅). We then compared the IR and Raman spectra of 1-ox and 1-ox-d₅ (Fig. S24 and S25, ESI†). Nonetheless, we failed to observe any shift in the stretching vibration in the 1000–1600 cm⁻¹ region. This experiment suggests that the origin of the ligand oxidation in 1-ox is not the C₆H₅NR₂ scaffold of the ligand but rather the oxime moiety, which results in the formation of a Cu^II complex containing an iminoxyl radical (Scheme 1).

The Cu complexes were further investigated by XANES and extended X-ray absorption fine structure (EXAFS) spectroscopy (Fig. 2 and Table S3, ESI†). Cu K-edge XANES is generally characterized by a 1s → (4p + shakedown) transition¹³ along the rising maximum edge between the pre-edge and principal maximum, assigned as the 1s → 4p transition with concurrent ligand to metal charge transfer (LMCT), as illustrated in Fig. 2A.¹⁴

Fig. 2 (A) Normalized Cu K-edge XANES spectra of 1 and 1-ox were recorded at 20 K; bottom inset: experimental and top inset: calculated zoomed-in view of the pre-edge regions of 1 (black) and 1-ox (red). (B) Fourier transforms (FT) of k³-weighted Cu EXAFS of 1 and 1-ox. Inset: k³[χ(k)]-weighted traces as a function of k, the photoelectron wavevector (solid lines) and fitting (dashed lines) of 1 (black) and ligand oxidized 1-ox (red). Experimental spectra were calculated for k values of 1.5–14 Å⁻¹.

Complex 1 displays the principal maximum or white line attributed to the first sharp rise in the X-ray absorption spectrum at 8998.79 eV (Fig. 2A). 1 further displays the 1s → (4p + shakedown) peak at 8990.77 eV and conventional pre-edge 1s → 3d Cu transitions at 8979.4 eV (Table S5 and Fig. S26, S27, ESI†), which are typical for Cu^II complexes.^13c,14

The EXAFS spectrum of 1 is shown in Fig. 2B. A prominent peak (peak I) is observed in the EXAFS spectra (Fig. 2B), corresponding to the averaged Cu–N bond distances. The EXAFS fits for the first coordination sphere, and the entire spectrum are shown in Table S3 (ESI†), Fig. 2B inset, which resolves 4 Cu–N bond distances at 1.94 Å and a Cu–O distance of 2.35 Å (Fit 3 and Table S3, Fig. S29, ESI†) for 1.

The XANES spectra of 1-ox are further illustrated in Fig. 2A. Complex 1-ox revealed the pre-edge energy transition at 8979.4 eV, which is identical to the transition obtained for 1 (Table S5 and Fig. S27 and S28, ESI†), suggesting that the oxidation of 1 causes the removal of the electron from the ligand backbone rather than from Cu^II together with a small energy shift of 0.54 eV from 8990.17 to 8990.71 eV compared to 1. However, the 1s → 3d transitions of Cu^II and Cu^III complexes have been well-known to appear at ∼8979 ± 0.5 eV^13c and ∼8981 ± 0.5 eV,¹⁵ respectively, with a shift of ∼1.5 to 2 eV between these two oxidation states.¹⁶ By contrast, in the event of a ligand-based oxidation process, the shift in the pre-edge energy has been known to be negligible or shifted by less than 1 eV.^15,17 Thus, comparing the shift of the 1s → 3d transition energy clearly establishes the ligand-derived oxidation process for 1. The experimental pre-edge and rising edge features were further compared to several calculated models (Fig. S27B, S29 and Table S5, ESI†), and the best agreement was found for the ligand oxidized 1-ox with a bound solvent molecule having an S = 1 state (Fig. 2A inset and Fig. S29, ESI†). Both the calculated pre-edge and energy shift of 0.52 eV in the rising edge region between 1 and 1-ox with a bound solvent molecule agree well with the experimental XANES spectra, which display a comparable energy shift of 0.54 eV as elaborated above (Fig. S27B, ESI†). By contrast, a calculated four-coordinated ligand oxidized 1-ox model illustrates a decreased shift in the rising edge region compared to 1 with a ClO₄⁻ ligand due to the lesser energy needed to eject a core 1s electron from a four vs. five-coordinated ligand oxidized complex (Fig. S27B, ESI†).¹⁸ The theoretically calculated different Cu oxidized models of 1, such as [Cu^III(HL₁)]²⁺, [Cu^III(HL₁)(CH₃CN)]²⁺, and [Cu^III(HL₁)(ClO₄)]⁺, further showed a shift of ∼1.5 eV in contrast to experimental pre-edge trends (Fig. S29, ESI†).

The structure of 1-ox was further evidenced by their EXAFS spectra and fits (Fig. 2B and Tables S3, S4, Fig. S29, ESI†). The complex 1-ox shows slightly elongated Cu–N bond distances of 1.96 Å in agreement with 1-ox with a bound solvent molecule (Table S7, ESI†) and similar Cu–C/O single, Cu–C multiple scattering bond distances of 2.86 Å and 3.09 Å as its parent complex 1 (Fit 6 vs. 3 in Table S3, Fig. S28, ESI†). The increased Cu–N bond distance may be attributed to the accumulation of positive charge on the ligand. The experimental EXAFS bond distances for the 1-ox were additionally in very good agreement with the DFT-optimized distances (Tables S4 and S5, ESI†). The DFT optimized structure of [Cu^II(L₁˙)(ClO₄)]⁺, having a ClO₄⁻ molecule at the axial position, showed an elongated Cu–O distance of 2.30 Å (Table S6, ESI†), which contradicts the best-fitted metrical parameters of 1-ox obtained from EXAFS analysis (Fit 6, Table S3, ESI†). Thus, the DFT calculations, TD-DFT XANES simulations, and EXAFS studies altogether imply the formation of a solvent-coordinated Cu^II iminoxyl radical complex. Electron density difference calculations between 1 and 1-ox further show most of the spin density distributions to be around the iminoxyl radical (Fig. 4A, Table S8, ESI†).

Thus, the spectroscopic characterization of 1-ox, together with theoretical calculations, clearly established the formation of a ligand-oxidized Cu(II) complex. While the generation of such radical species has been speculated in Cu-catalyzed water oxidation reactions,^8b it has never been characterized before.

Metalloradical species, such as Cu^II(phenoxyl radical) and Cu^II(anilino radical) complexes, exhibit HAA reactivities.¹⁹ Thus, to examine the ability of 1-ox toward HAA, we evaluated the reaction of 1-ox with TEMPOH (BDE = 67 kcal mol⁻¹) at −40 °C, which resulted in the immediate decay of the intermediate (Fig. 3A). The reaction was found to be very fast, and the estimation of the accurate rate constant of the reaction was not possible. Subsequently, we evaluated the final reaction solution by UV-vis spectroscopy, which showed features identical to 1. Furthermore, the final reaction solution's X-band EPR spectrum exhibited spectral features identical to the starting Cu^II complex (1) (Fig. S30, ESI†). We tentatively suggest that the reaction follows a hydrogen atom transfer (HAT) pathway (Fig. 4B) to initially generate a protonated Cu^II species (1^H), which subsequently releases a proton to generate 1. Next, we studied the ET reaction of 1-ox with different ferrocene derivatives having different E_ox values (Fig. S31, ESI†), and the determined second-order electron-transfer rate constant (k_et) values are described in Table S6 (ESI†). Initially, we examined the reaction of 1-ox with ferrocene in methanol at −60 °C, which resulted in the rapid decay of the absorption at 543 nm (Fig. 3B). A k_et of 1.3 × 10³ M⁻¹ s⁻¹ was estimated from the plot of 1/[1-ox] vs. time (s). Likewise, k_et values of 2.5 × 10³, 96, 24, 20.7, and 11 M⁻¹ s⁻¹ were determined using ethylferrocene, bromoferrocene, methyl ferroceylethynyl ketone, (methoxycarbonyl) ferrocene, and acetylferrocene, respectively, at −60 °C (Fig. S32–S37, ESI†). The k_et values decreased with increasing E_ox of the ferrocene derivatives. We subsequently utilized these k_et values of the outer sphere ET reactions to determine the reorganization energy (λ) of ET using eqn (1).²⁰

k_et = Z exp[−(λ/4)(1 + ΔG_et/λ)²/k_BT] (1)

−ΔG_et = −e(E_ox − E_red) (2)

Fig. 3 (A) Change of UV-vis spectrum of 1-ox (0.14 mM) upon addition of one equiv. of TEMPO-H in methanol at −40 °C. Inset: Decay of 1-ox at 543 nm. (B) A plot of 1/[1-ox] vs. time (s) for the reaction of 1-ox with ferrocene. (C) The X-band EPR spectrum of the reaction solution was obtained upon the addition of one equiv. of ferrocene to a methanol solution of 1-ox (1 mM). The sample was collected immediately after the addition of Fc to 1-ox. (D) A plot of the logarithm of the ET rate constant (k_et) of the reaction of 1-ox with different ferrocene derivatives vs. driving force (−ΔG_et) of the ET reaction. Substrates: acetylferrocene (a), methoxycarbonyl ferrocene (b), methyl ferrocenylethynyl ketone (c), bromoferrocene (d), ferrocene (e), and ethylferrocene (f).

Fig. 4 (A) Electron density difference between 1 and 1-ox. (B) Reactivity studies of 1-ox.

The value of Z is the collision frequency, which is usually taken as 1 × 10¹¹ M⁻¹ s⁻¹.^20bk_B and T are the Boltzmann constant and absolute temperature, respectively. The driving force (−ΔG_et) of the outer sphere ET reactions was calculated from the one-electron reduction potential (E_redvs. Fc⁺/Fc) of 1-ox and the one-electron oxidation potential (E_oxvs. Fc⁺/Fc) of the ferrocene derivatives using eqn (2). The outer sphere ET rate constants of 1-ox with different ferrocene derivatives fit well in the Marcus equation of outer sphere ET (eqn (1)) and revealed a λ value of 2.12 eV (Fig. 3D), which is significantly large. According to the Marcus theory of ET, the λ represents the energy required to structurally reorganize a molecule and the surrounding solvent sphere upon ET. Thus, a high λ value is associated with a significant structure change upon ET, which includes a change of coordination number around the metal ion and a large structural distortion. The large λ of 1-ox/1, therefore, is consistent with the large ΔE value obtained in the CV analysis and accounts for the ClO₄⁻ dissociation/solvent coordination event upon oxidation of 1. A combined EXAFS analysis and DFT studies supports the structural change associated with the transformation of 1 to 1-ox. Nonetheless, λ > 2.0 eV has been reported in several iron-oxo complexes.²¹ Furthermore, the λ value for the 1-ox/1 couple is considerably larger than the Cu^III/II(OH) and other reported Cu^III/Cu^II redox couples.²²

In conclusion, we report the first example of a Cu^II-iminoxyl radical complex (1-ox) and its thorough spectroscopic characterization. The species undergoes a HAT reaction and reveals high reorganization energy.

S. P. acknowledges the Science and Engineering Research Board (SERB, CRG/2022/005842) for funding. The authors thank the CRF at IIT Delhi for EPR and NMR measurements. X-ray data of 1 was measured in an IOE-funded single-crystal X-ray diffractometer at the Department of Chemistry, IIT Delhi. S. K. acknowledges the CSIR for a doctoral research fellowship. A. D. and M. B. thank IIT Delhi for a doctoral fellowship. D. M. acknowledges funding from the Ramon y Cajal grant RYC2020-029863-I through the Instituto de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones Cientificas (CSIC-ICMM), PIE grant from CSIC-ICMM (20226AT001), and the Spanish Ministerio de Ciencia, Innovación y Universidades grants (PID2019-111086RA-I00, TED2021-1327 57B-I00, PID2022-143013OB-I00, CNS2023-145046). L. V. acknowledges the Communidad de Madrid grant (PIPF-2022/ECO-25801) for a predoctoral fellowship.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2333487. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4cc02922d

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