Ambiphilicity of a mononuclear cobalt( III ) superoxo complex

Addition of HOTf to a mixture of Co III (BDPP)(O 2 (cid:2) ) (1, H 2 BDPP = 2,6-bis((2-( S )-diphenylhydroxylmethyl-1-pyrrolidinyl)methyl)pyridine) and Cp* 2 Fe produced H 2 O 2 in high yield implying formation of Co III (BDP-P)(OOH) (3), and reaction of Sc(OTf) 3 with the same mixture gave a peroxo-bridged Co III /Sc III 5. These findings demonstrate the ambiphilic property of Co III -superoxo 1.

Metal-superoxo species are often believed to be the first intermediate following dioxygen (O 2 ) association in the catalytic cycle of O 2 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. 2 Inter alia, metal-superoxo intermediates can react with NO or organic radicals to furnish metal-peroxynitrite 3 and -alkylperoxo 4 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 transfer 6 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 O 2 activation catalyzed by a-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 metalsuperoxo intermediates, some of us succeeded in preparing a range of homologous Fe III -, Co III -and Mn III -superoxo species by reacting O 2 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 Br P) (O 2 ) (H 2 BDP Br P = 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 Br P)(OOH), and [Mn IV (m-OO) Sc(OTf) n ] (3Àn)+ 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)(O 2 ) (1, H 2 BDPP = 2,6-bis((2-(S)-diphenylhydroxylmethyl-1-pyrrolidinyl)methyl)pyridine) with TFA and Sc(OTf) 3 together with external electron donors.
Treating 1 with HOTf in THF at À90 1C 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, 8diazabicyclo[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 (BDP-P)(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 À90 1C resulted in a gray-green solution, whose absorption spectrum displayed the same signature features as those found for 2 ( Fig. S2, ESI †).
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 59 Co 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 BDPP 2À 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.
DFT calculations also suggested the O donor of the BDPP 2À 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 BDPP 2À ligand, the geometry optimizations invariably shifted the H atom back to the O atom of BDPP 2À 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 À1 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 BDPP 2À ligand stabilized A by 10.4 kcal mol À1 relative to B (Fig. 3). Thus, A is best deemed as the most appropriate model for 2.
Alternatively, to transform 1 into the corresponding peroxo product, we then added 1 equiv. of decamethylferrocene (Cp* 2 Fe) or sodium naphthalenide (NaC 10 H 8 ) to THF solutions of 1 at À90 1C, 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* 2 Fe 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* 2 Fe + ) emerged suggesting that Co III -superoxo 1 was reduced in the presence of both HOTf and Cp* 2 Fe (Fig. 4A).
During this process, we did not observe the formation of Co IIIhydroperoxo 3. Instead, the reaction produced 19% of H 2 O 2 with respect to 1, as determined by iodometric titration (Fig. S5, ESI †). When 2 equiv. of HOTf was added, 42% of H 2 O 2 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 H 2 O 2 . On the other hand, treating 2 with 1 equiv. of Cp* 2 Fe (Fig. 4B) also generated 23% of H 2 O 2 (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.
In comparison with the similar reaction found for Mn III (BDP Br P) (O 2 ) (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 BDPP 2À 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 O 2 À motif and the protonated BDPP 2À ligand and a superoxo-bridged binuclear Co III /Sc III 4, 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* 2 Fe produced 42% of H 2 O 2 suggesting the formation of Co III -hydroperoxo 3, and the reaction of Sc(OTf) 3 with 1 in the presence of Cp* 2 Fe gave a peroxo-bridged binuclear Co III /Sc III 5. 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 oneelectron 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 O 2 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

Conflicts of interest
There are no conflicts to declare.