Ambiphilicity of a mononuclear cobalt(III) superoxo complex

Abstract

Addition of HOTf to a mixture of Co^III(BDPP)(O₂˙) (1, H₂BDPP = 2,6-bis((2-(S)-diphenylhydroxylmethyl-1-pyrrolidinyl)methyl)pyridine) and Cp*₂Fe produced H₂O₂ in high yield implying formation of Co^III(BDPP)(OOH) (3), and reaction of Sc(OTf)₃ with the same mixture gave a peroxo-bridged Co^III/Sc^III5. These findings demonstrate the ambiphilic property of Co^III-superoxo 1.

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Metal-superoxo species are often believed to be the first intermediate following dioxygen (O₂) association in the catalytic cycle of O₂ activating metalloenzymes.¹ Despite intensive work in the past, the chemistry of metal-superoxo complexes remains largely unexplored, and hence attracts significant attention from chemists and biochemists.²Inter alia, metal-superoxo intermediates can react with NO˙ or organic radicals to furnish metal-peroxynitrite³ and -alkylperoxo⁴ complexes via radical coupling. Furthermore, they exhibit considerable electrophilicity as indicated by their capability of performing hydrogen atom abstraction (HAA)⁵ from weak C–H and O–H bonds and oxygen atom transfer⁶ to triphenylphosphine or thiol anisoles. On the other hand, they can initiate deformylation processes when treated with 2-phenylpropionaldehyde, thereby revealing their nucleophilic character.^7,8 Besides the aforementioned well know activities, in a given elementary transformation metal-superoxo intermediates may function not only as an electrophile but also as a nucleophile. In fact, ambiphilicity of metal-superoxo species has been postulated in a series of theoretical and experimentally investigations including O₂ activation catalyzed by α-ketoglutarate dependent dioxygenases,⁹ and by Cu, Fe and Co model complexes.¹⁰ Only recently has such ambiphilic property been experimentally confirmed.⁸ In our continuing efforts devoted to investigating reactivity of metal-superoxo intermediates, some of us succeeded in preparing a range of homologous Fe^III-, Co^III- and Mn^III-superoxo species by reacting O₂ with the corresponding divalent precursors.¹¹ It has been shown that these trivalent metal-superoxo complexes can convert into the metal-hydroperoxo complexes via HAA. In particular, the reaction of Mn^III(BDP^BrP)(O₂˙) (H₂BDP^BrP = 2,6-bis((2-(S)-di(4-bromo)-phenylhydroxylmethyl-1-pyrrolidinyl)methyl)pyridine) with trifluoroacetic acid (TFA) and Sc(OTf)₃ yields rare examples of Mn^IV-hydroperoxo complexes, Mn^IV(BDP^BrP)(OOH), and [Mn^IV(μ-OO)Sc(OTf)_n]⁽³⁻ⁿ⁾⁺ as evidenced by the combined spectroscopic and computational studies (Scheme 1).⁸ Obviously, these proton- and metal-coupled electron transfer processes provide the first experimental support for the proposed ambiphilicity of metal-superoxo species. In this regard, more examples are desired to fully understand how the ambiphilic property of metal-superoxo species affects their chemical reactivity. To this end, we examined the reaction of a Co^III-superoxo complex, Co^III(BDPP)(O₂˙) (1, H₂BDPP = 2,6-bis((2-(S)-diphenylhydroxylmethyl-1-pyrrolidinyl)methyl)pyridine) with TFA and Sc(OTf)₃ together with external electron donors.

Scheme 1 Ambiphilicity of metal-superoxo complexes.

Treating 1 with HOTf in THF at −90 °C gave a gray-green solution attributed to intermediate 2 having two weak absorption bands at 470 and 640 nm, which reached maxima when 1 equiv. of HOTf was added (the inset of Fig. 1). The existence of an isosbestic point at 590 suggested that no intermediate was formed in the course of conversion of 1 to 2 (Fig. 1). Conversely, complex 1 can be retrieved from deprotonation of 2 by 1 equiv. of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) with a yield of 80% with respect to 1 (Fig. S1, ESI†). Moreover, complex 2 can be obtained from one-electron oxidation of the hydroperoxo complex Co^III(BDPP)(OOH) (3). Adding equimolar of tris(4-bromophenyl)ammoniumyl hexachloroantimonate, which is often referred to as magic blue, to a THF solution of 3 at −90 °C resulted in a gray-green solution, whose absorption spectrum displayed the same signature features as those found for 2 (Fig. S2, ESI†).

Fig. 1 UV-vis spectral changes of the reaction of 1 (1.0 mM, blue trace) with 1 equiv. of HOTf in THF at −90 °C. Inset: Titration curve of 1 with HOTf. X-axis: the equivalents of HOTf, Y-axis: the conversion ratio of the absorption peak at 640 nm.

To identify the exact nature of the resulting species 2, we have undertaken detailed spectroscopic characterization and DFT calculations. The EPR measurement of 2 exhibited a spectrum similar to that of 1 (A_Co = 18 G) except for a slightly larger ⁵⁹Co hyperfine coupling constant (A_Co = 24 G) seen in Fig. 2, thus indicating that 2 still consists of a Co^III center coupled with a radical ligand yielding an overall doublet ground state. The radical ligand thus would be a hydroperoxyl radical or a superoxo having a strong hydrogen bonding interaction with the protonated BDPP²⁻ ligand (Scheme 1) as suggested by the crystal structure of 3.^11b However, the EPR spectrum of the product generated by reacting 1 with deuterated triflate acid (DOTf) is almost identical to that of 2 without discernable line broadening, which essentially rules out the possibility of the radical ligand being a hydroperoxyl radical (Fig. 2C). Repeated attempts to obtain the O–O vibrational frequencies of 2 from resonance Raman measurements did not accomplish, largely because 2 has only weak chromophores in the usual UV-vis region (Fig. 1). Consequently, the intensity of the O–O stretching signal is too low to be readily detected.

Fig. 2 X-band EPR spectra of (A) 1 (g₁ = 2.098, A_Co = 18 G); (B) 1 with 1 equiv. of HOTf (g₁ = 2.066, A_Co = 24 G); (C) 1 with 1 equiv. of DOTf (g₁ = 2.064, A_Co = 24 G) and (D) 1 with 1 equiv. of Sc(OTf)₃ (g₁ = 2.066, A_Co = 24 G). Measurement condition: T = 77 K, f_mw = 9.6 GHz.

DFT calculations also suggested the O donor of the BDPP²⁻ ligand to be the favored protonation site of 1, consistent with experiment. Even when the starting geometry contained a OOH ligand in which the distal H atom forms a hydrogen bond with the BDPP²⁻ ligand, the geometry optimizations invariably shifted the H atom back to the O atom of BDPP²⁻ and eventually converged to A (Fig. 3). We also tested the initial geometry without the hydrogen bond by tilting the H atom upward. The computations indeed yielded a Co^III center bound to a hydroperoxyl radical ligand (C), but C lies 23.2 kcal mol⁻¹ higher in energy above A (Fig. 3). Moreover, formation of a hydrogen bond between the superoxo motif and the proton of the OH group of the protonated BDPP²⁻ ligand stabilized A by 10.4 kcal mol⁻¹ relative to B (Fig. 3). Thus, A is best deemed as the most appropriate model for 2.

Fig. 3 Computed models for 2.

Alternatively, to transform 1 into the corresponding peroxo product, we then added 1 equiv. of decamethylferrocene (Cp*₂Fe) or sodium naphthalenide (NaC₁₀H₈) to THF solutions of 1 at −90 °C, but UV-vis measurements suggested that no reactions occurred (Fig. S3 and S4, ESI†). Taken together, neither proton nor electron donors alone can realize the superoxo-to-peroxo conversion for 1.

Interestingly, upon treating a mixture containing equimolar 1 and Cp*₂Fe with 1 equiv. of HOTf, the color of the reaction solution gradually changed from gray-green to dark green then orange; meanwhile, characteristic features of decamethylferrocenium (Cp*₂Fe⁺) emerged suggesting that Co^III-superoxo 1 was reduced in the presence of both HOTf and Cp*₂Fe (Fig. 4A). During this process, we did not observe the formation of Co^III-hydroperoxo 3. Instead, the reaction produced 19% of H₂O₂ with respect to 1, as determined by iodometric titration (Fig. S5, ESI†). When 2 equiv. of HOTf was added, 42% of H₂O₂ was furnished (Fig. S6, ESI†). Thus, we reasoned that the aforementioned reaction indeed generates 3; however, once formed, 3 further reacted with HOTf to produce H₂O₂. On the other hand, treating 2 with 1 equiv. of Cp*₂Fe (Fig. 4B) also generated 23% of H₂O₂ (Fig. S7, ESI†). Therefore, all experimental findings revealed that transformation of 1 to 3 proceeds via concerted proton coupled electron transfer, which clearly demonstrated the ambiphilicity of 1.

Fig. 4 UV-vis spectral changes of addition of (A) 1 equiv. of HOTf into a solution of 1 (1.0 mM) in the presence of 1 equiv. of Cp*₂Fe and (B) 1 equiv. of Cp*₂Fe into a solution of 2 (1.0 mM) in THF at −90 °C.

Reaction of 1 with Sc(OTf)₃ in THF at −90 °C generated a product, which showed nearly identical absorption and EPR spectra to those of 2 (Fig. S8 (ESI†) and Fig. 2D). On the basis of these observations, we tentatively suggest that the reaction furnished a Co^III-superoxo⋯Sc^III species, [Co(BDPP)(O₂˙)⋯Sc(OTf)_n]⁽³⁻ⁿ⁾⁺ (4). Furthermore, addition of Sc(OTf)₃ to the mixture of 1 and Cp*₂Fe in THF at −90 °C afforded a Co^III-peroxo-Sc^III complex, [Co(BDPP)(μ-OO)Sc(OTf)_n]⁽²⁻ⁿ⁾⁺ (5), as depicted in Scheme 1 (Fig. S9, ESI†). Addition of 2 equiv. of HOTf to the THF solution of 5 produced 76% H₂O₂ as quantified by iodometric titration experiments (Fig. S10, ESI†). Formation of H₂O₂ thus strongly supports the identity of peroxo-bridged binuclear 5. These findings further reinforce the ambiphilic property of 1.

In comparison with the similar reaction found for Mn^III(BDP^BrP)(O₂˙) (Scheme 1), the difference can be readily attributed to the much higher oxidation potential of Co^III to Co^IV than that of Mn^III to Mn^IV, which can ultimately be rooted back to the distinct effective nuclear charge of low spin Co^III compared to high spin Mn^III centers. Therefore, formation of an otherwise hydroperoxo O–H bond does not provide a sufficient driving force to trigger an electron transfer from the Co^III center to the superoxo ligand. Consequently, the superoxo motif is not electron rich enough to accommodate the incoming proton from HOTf and protonation of the supporting BDPP²⁻ ligand is preferred. Therefore, to effect surperoxo-to-peroxo conversion for 1, an external electron source has to be provided in addition to Brønsted or Lewis acids.

In conclusion, treatment of Co^III-superoxo 1 with HOTf and Sc(OTf)₃ afforded the ligand-protonated Co^III-superoxo 2 with a hydrogen bond formed between the O₂˙⁻ motif and the protonated BDPP²⁻ ligand and a superoxo-bridged binuclear Co^III/Sc^III4, and Co^III-superoxo 1 can be regenerated from deprotonation of 2 by DBU. However, addition of 2 equiv. of HOTf into the reaction mixture of 1 and Cp*₂Fe produced 42% of H₂O₂ suggesting the formation of Co^III-hydroperoxo 3, and the reaction of Sc(OTf)₃ with 1 in the presence of Cp*₂Fe gave a peroxo-bridged binuclear Co^III/Sc^III5. These findings provided strong experimental support for the ambiphilic property of Co^III-superoxo 1. Interestingly, the ligand-protonated Co^III-superoxo 2 can be prepared from one-electron oxidation of Co^III-hydroperoxo 3. The unveiled results underline the critical property of ambiphilicity for metal-superoxo species and direct us to design further investigation strategies towards better understanding O₂ activation processes carried out by metalloenzymes and related catalysts.

We are grateful for the financial supports from the Ministry of Science and Technology of Taiwan (MOST 108-2113-M-003-009-MY3 to W.-Z. L.) and the Max-Planck Society. W.-Z. L. and S. Y. also acknowledge the MOST-DAAD Project-Based Personnel Exchange Program (MOST 107-2911-I-003-502 and DAAD 57320810). Open Access funding provided by the Max Planck Society.

Conflicts of interest

There are no conflicts to declare.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0cc05337f

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