Oxygen-Oxygen Bond Cleavage and Formation in Co(II) - American

Article Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

pubs.acs.org/JACS

Oxygen−Oxygen Bond Cleavage and Formation in Co(II)-Mediated Stoichiometric O2 Reduction via the Potential Intermediacy of a Co(IV) Oxyl Radical Lucie Nurdin,† Denis M. Spasyuk,† Laura Fairburn,† Warren E. Piers,*,† and Laurent Maron*,‡ †

Department of Chemistry, University of Calgary, 2500 University Drive NW, Calgary, Alberta T2N 1N4, Canada LPCNO, Université de Toulouse, INSA, UPS, LPCNO, 135 avenue de Rangueil, F-31077 Toulouse, France, and CNRS, LPCNO, F-31077 Toulouse, France

J. Am. Chem. Soc. Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 11/17/18. For personal use only.

‡

S Supporting Information *

ABSTRACT: In reactions of significance to alternative energy schemes, metal catalysts are needed to overcome kinetically and thermodynamically difficult processes. Often, high-oxidation-state, high-energy metal oxo intermediates are proposed as mediators in elementary steps involving O−O bond cleavage and formation, but the mechanisms of these steps are difficult to study because of the fleeting nature of these species. Here we utilized a novel dianionic pentadentate ligand system that enabled a detailed mechanistic investigation of the protonation of a cobalt(III)−cobalt(III) peroxo dimer, a known intermediate in oxygen reduction catalysis to hydrogen peroxide. It was shown that double protonation occurs rapidly and leads to a low-energy O−O bond cleavage step that generates a Co(III) aquo complex and a highly reactive Co(IV) oxyl cation. The latter was probed computationally and experimentally implicated through chemical interception and isotope labeling experiments. In the absence of competing chemical reagents, it dimerizes and eliminates dioxygen in a step highly relevant to O−O bond formation in the oxygen evolution step in water oxidation. Thus, the study demonstrates both facile O−O bond cleavage and formation in the stoichiometric reduction of O2 to H2O with 2 equiv of Co(II) and suggests a new pathway for selective reduction of O2 to water via Co(III)−O−O−Co(III) peroxo intermediates.

■

INTRODUCTION The activation and production of dioxygen, O2, as mediated by transition metal complexes, are crucial processes in oxygen reduction and water oxidation for alternative energy schemes.1,2 Critical to these reactions are O−O bond cleavage and O−O bond formation steps, whose intimate nature is often obscured in the complex multistep reaction sequences that reduce O2 to water or convert water to dihydrogen and O2 because the intermediates involved at these stages of the process are often fleeting and highly reactive.3,4 In the context of O2 reduction, cobalt(II) complexes bearing multidentate ligands have been widely studied because of the propensity of these d 7 centers to bind O 2 (usually reversibly)5−7 to yield [LCo(III)O2·]n superoxo monomers8−10 that can be reactive intermediates themselves9,11,12 or be trapped by a second [LCo(II)] to yield [LCo(III)−O− O−Co(III)L]2n peroxo dimers,13−15 in which “end-on” Pauling-type binding16 is generally the norm (Scheme 1). Another possibility involves (formal) trapping of the superoxo monomer with an [LCo(III)] center to give a superoxo dimer; these are thought to be key species in water-selective O2 reduction reactions involving binuclear Co catalysts (see below).17 The course of the reactions of Co(II) complexes © XXXX American Chemical Society

Scheme 1. Binding of Oxygen to Co(II) Complexes

with O2 is dependent on the denticity and charge of the multidentate ligand framework. More highly charged ligands (i.e., n = 0) lead to more reducing Co(II) centers and stronger O2 binding, while pentadentate ligands tend to favor the Received: July 27, 2018 Published: November 6, 2018 A

DOI: 10.1021/jacs.8b07726 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Article

Journal of the American Chemical Society

electron paramagnetic resonance (EPR) spectroscopy by its characteristic 15-line pattern due to hyperfine coupling with two spin 7/2 59Co nuclei.21,25,28,29 If it is basic enough to be protonated more rapidly than it is reduced, this triggers a twoelectron process that encourages O−O bond cleavage over one-electron reduction to the peroxo species. The resulting Co(III)OH/Co(IV)O intermediate is proposed to be highly reactive, and subsequent rapid proton-coupled electron transfer (PCET) events lead to water as the reduction product. Conversely, protonation of the peroxo derivative likely leads to the hydrogen peroxide complex shown in the intermediate box on the left side of Scheme 2. Since H2O2 is a weak ligand30−32 and the five-coordinate Co(III) centers are labile, dissociation of the hydrogen peroxide product is rapid. Thus, it is thought that in these systems, protonation of a Co(III)−O−O− Co(III) intermediate leads to H2O2. As mentioned above, however, these issues are systemdependent and, in particular, governed by the nature of the ligand set. For example, Fukuzumi et al.33 have reported that rate-limiting PCET to a Co(III)−O−O−Co(III) peroxo derivative leads selectively to water and not H2O2. In this system, a tetradentate binucleating bis(pyridyl)pyrazolate ligand in combination with capping terpyridyl ligands renders each Co(II) center in the binuclear catalyst precursor fivecoordinate and the resulting peroxo Co(III)/Co(III) complex octahedral. Furthermore, the overall charge on the complex is +3; these two factors likely discourage dissociation of H2O2 upon protonation and open new pathways that can result in O−O bond cleavage. This illustrates how new ligand systems34,35 can lead to alternate mechanistic possibilities. While the binding of O2 to Co(II) compounds to form these superoxo and peroxo compounds is relatively well understood, the intimate details concerning their protonation are more opaque. Obviously, when H2O is the final product, O−O bond cleavage must occur, and even when H2O2 is produced, the Co(II) complexes formed upon reduction of Co(III) often catalyze its disproportionation to H2O and O2,32,36 which also implies O−O bond cleavage. In some pillared dicobalt systems, spontaneous O−O bond cleavage from the Co(III)−O−O− Co(III) peroxo intermediates formed upon interaction of the Co(II)/Co(II) complexes with O2 has been proposed, albeit without much experimental justification, to form highoxidation-state Co(IV) oxo or oxyl species that immediately undergo protonation and reduction to regenerate Co(II) and water.29,37 More likely, these kinds of high-energy intermediates are generated from either the peroxo or superoxo dimers upon protonation or PCET (as shown on the right side of Scheme 2). Such reactive cobalt oxo complexes have been proposed as intermediates in both the oxygen reduction reaction29,33 and water oxidation processes38−42 but are rarely observed spectroscopically.41,43,44 Their high reactivity can be exploited in trapping experiments with Lewis acids45 or with nucleophiles or hydrogen atom sources,46 but their characterization has relied heavily on computational studies. More detailed experimental studies require well-defined systems wherein protonation of the O2-bound species can be examined to see whether these highly reactive species can be generated in a controlled way and utilized in catalytic oxidation reactions where O2 is the stoichiometric oxidant.47,48 With this in mind, we hypothesized that the the Co(II) complexes supported by a dianionic, pentadentate ligand system based on bis(pyrazolyl) diaryl borate arms mounted on a 2,6-substituted pyridyl frame that we recently reported49 may

formation of dinuclear peroxo complexes because of the stabilized d6 octahedral geometry favored in Co(III) compounds. In these compounds, O2 is formally reduced by one or two electrons and activated toward protonation or hydrogen atom acquisition, leading to the products of O2 reduction (H2O2 or H2O). Because the reductions of O2 to H2O2 and H2O are multielectron processes (two and four electrons, respectively), bimetallic Co(II) constructs have been studied extensively over the past several decades in order to encourage cooperativity15,18−20 and selectivity toward specific reduction products (Scheme 2). Indeed, so-called “Pac-man” porphyrins17,21−25 or Scheme 2. Co(II)-Mediated Oxygen Reduction

related compounds,20,26−28 in which two tetradentate macrocyclic ligand arrays are pillared by a chemical linker (represented by the dotted line in Scheme 2) that allows for a wide variance in metal−metal separation, are among the most selective catalysts for reduction of O2 to H2O. Because the Co centers in these constructs are square-planar, O2 binding leads to five-coordinate compounds that lack the ligand field stabilization of octahedral Co(III) geometries, and O2 binding is generally reversible. Strong donor solvents that coordinate the Co centers upon O2 capture can enhance O2 binding,27 in addition to the cooperative effect of having two Co centers held in proximity by the linking pillar. Studies have shown17 that the selectivity of the reaction is largely governed by the properties of the binuclear superoxo species, which is often the resting state of the system and can be detected by B


Article

Journal of the American Chemical Society allow for such a study. This “B2Pz4Py” ligand50 is a doubly negatively charged analogue of the well-known “PY5” ligand framework,51 whose Co complexes were studied in detail by

high-spin S = 3/2 configuration (μeff = 3.94μB, Evans’ method). Its structure was confirmed by X-ray crystallography (Figure S2). In THF, a 19-electron THF adduct is formed, whose structure was also confirmed by X-ray crystallography (Figure S3) and gives rise to assignable NMR spectra (Figures S4 and S5). However, the THF ligand is quite labile and can be removed by heating under vacuum. Compounds 1-THF and 1 are convenient Co(II) starting materials for reactions with O2 and other oxidizing agents. Pale-yellow solutions of 1 in THF or toluene undergo an immediate color change to blood red upon exposure to even stoichiometric (0.5 equiv) amounts of O2 (Scheme 4). The

the Long and Chang groups19,52 and the Berlinguette group53 in the contexts of proton reduction and water oxidation, respectively. The dianionic nature of the B2Pz4Py ligand results in charge neutrality for the complexes with M(II) ions, which allows for the use of organic solvents and Schlenk techniques to examine the reaction chemistry of these compounds. Here we report the treatment of a B2Pz4PyCo(II) complex with O2 and mechanistic and computational studies of the diprotonation of the resulting peroxo product. An ensemble of spectroscopic, chemical trapping, and isotope labeling studies and density functional theory (DFT) calculations suggest that a Co(IV) oxyl cation is a likely intermediate generated in this protonation reaction. This is a novel mode of reactivity for a diprotonated Co(III)−O−O−Co(III) species that is directed by the nature of the ligand system used and provides insight into both O−O bond cleavage and formation paths in Co(II)mediated transformations of O2.

Scheme 4. Reaction of 1 with O2 and Protonation of Co(III)−O−O−Co(III) Peroxo Compound 2

■

RESULTS AND DISCUSSION Synthesis, Characterization, and Reaction Chemistry of Co Complexes. Our previous reports utilizing the B2Pz4Py ligand featured a phenyl-substituted borate group;49 to enhance the solubility, the p-tolyl-substituted derivative was employed for the chemistry described here. This ligand was prepared in a manner analogous to that described for the Ph derivative, and details of its synthesis and characterization can be found in the Supporting Information and Figure S1. The pentacoordinate paramagnetic complex 1 can be prepared analytically pure in 73% yield (Scheme 3) and is essentially NMR-silent in non-coordinating apolar solvents, exhibiting a product is the diamagnetic Co(III)−O−O−Co(III) peroxo complex 2 shown in Scheme 4, which was characterized by NMR, resonance Raman (rR), and UV−vis spectroscopies and X-ray crystallography (Figure 1a). The O−O distance of 1.428(3) Å is longer than that of 1.21 Å for O2, consistent with the assignment as a formally doubly reduced peroxo complex,28,54 as are the observed 16O−16O stretch of 800 cm−1 and Co−16O stretch of 586 cm−1 in the rR spectrum. These bands shift to 757 and 554 cm−1 for the 18O−18O and Co−18O bonds in the 2-18O isotopologue, in good agreement with the calculated shifts (16/18Δcalcd = 46 cm−1, 16/18Δexptl = 43 16 18 16 18 cm−1; Co− O/Co− OΔcalcd = 28 cm−1; Co− O/Co− OΔexpt. = 32 −1 34 cm ). Binding of O2 by 1 is essentially quantitative and irreversible at temperatures below 40°C, but if 2-18O is heated to 70−80°C under an atmosphere of 16O2, exchange of bound 18 O2 with free 16O2 is observed (Figure S6). The relatively high affinity of 1 for O2 is attributable to the low potential of the Co(II/III) redox couple (−0.33 V vs Fc/Fc+, Figure S7; cf. the value of ∼0.24 V55 for [(PY5)Co]2+) and the charge neutrality of the compound. This experiment indicates that 2 is highly

Scheme 3. Synthesis of B2Pz4PyCo Compounds 1-THF, 1, and 3-Br

C


Article


Figure 1. Molecular structures and selected bond distances (in Å) of key compounds determined by X-ray crystallography. (a) Structure of peroxo complex 2. Co(1)−O(1), 1.899(2); O(1)−O(2), 1.428(3); Co(2)−O(2), 1.894(2). (b) Structure of 3-NTf2. Co(1)−O(1), 2.019(4); Co(1)− N(1), 1.929(5). (c) Structure of aquo complex 3-OH2. Co(1)−O(1), 1.979(2); O(1)···O(2), 2.730; Co(1)−N(1), 1.914(2). (d) Structure of one of the two independent molecules of 4-OPPh3; the metrical parameters of the two molecules are similar. Co(1)−O(1), 1.947(8); O(1)···O(2), 2.566; O(1)···O(3), 2.612; Co(1)−N(1), 1.956(8). In all of the structures, all of the atoms except the Cipso carbons associated with the tolyl groups on boron have been deleted for clarity; all of the hydrogens except the aquo hydrogens have also been deleted. In the structure of 4-OPPh3, the NTf2 anion is not depicted. Thermal ellipsoids are plotted at the 50% level of probability. Dark blue, Co; light blue, N; dark gray, C; light gray, H; red, O; flesh, B; light green, F; yellow, S; orange, P.

Scheme 5. Depiction of the Major Pathways Identified for the Diprotonation of 2 Leading to the Formation of 3-NTf2 and 3OH2 Along with 0.5O2

soluble bis(trifluoromethanesulfonyl)imide acid (HNTf2) at room temperature in toluene or benzene, and the products, formed quantitatively in a 1:1 ratio as determined via integration against the internal standard (Me3Si)2O, were identified as the two Co(III) mononuclear complexes 3-NTf2 and 3-OH2 (Figures S8−S13). In the former, the NTf2

stabilized but that when it does release O2, it does so via Co− O bond cleavage and not O−O bond cleavage.29,37 The high stability of 2 and its ready solubility in inert aromatic solvents allow for a detailed study of the protonation of peroxo complex 2. As depicted in Scheme 4, a clean reaction is observed when 2 is treated with 2 equiv of strong areneD


Article


Figure 2. Energy diagram of pathways A−G computed at 298 K. Enthalpies are given relative to that of compound 2 set to 0.0 kcal mol−1. Inset A shows the LUMO of complex 2-(H+)2, and inset B depicts the transition state TS1 with selected bond distances (Å). The path in green represents the lowest-energy path leading from 2 + 2H+ to the observed products 3-OH2, 3-NTf2, and 0.5O2.

equiv of HNTf2 is transferred to a solution of 1 in toluene, the amount of 2 generated indicates that 0.48(3) equiv of O2 is produced in this reaction (Figure S21). The stoichiometric reduction of O2 was demonstrated by formally closing a cycle via reduction of the products 3-OH2 and 3-NTf2 with 2 equiv of Cp2Co back to Co(II) derivative 1, which occurs essentially upon mixing of the reagents and releases 1 equiv of free H2O (Figures S22−28). Potential Mechanisms of Protonation: DFT Calculations. Establishment of the precise stoichiometry of the double protonation of 2 enables consideration of the mechanistic path by which the products 3-NTf2, 3-OH2, and 0.5O2 are generated. Plausible sequences of protonation and bond cleavages starting from peroxo compound 2 (shown in the yellow box at the top center) are depicted in Scheme 5, and most of them were probed by computational investigations using DFT with the B3PW91 functional. This methodology has proven its ability to properly account for structural and electronic data in both Co49 and Fe50 systems supported by the B2Pz4Py ligand. The details of this extensive computational study are summarized in Figure 2; Scheme 5 and Figure 2 will be referenced jointly in the discussion below. As a starting point, the optimized geometry of 2 was computed and found to be in excellent agreement with the experimental structure (Figure S29). The structure with a low-spin configuration for each Co(III) d6 center was the ground state, indicating a formally doubly reduced μ-O2. The main experimental bond lengths (in parentheses) are well-reproduced by the computational approach, e.g., the O−O distance is computed to be 1.41 Å (1.428 Å) and the two Co−O distances are 1.89 Å (1.899, 1.894 Å). The Wiberg bond index (WBI) for the O−O bond is 1.02, in line with a single bond, and the values for the Co−O

counteranion coordinates the Co(III) center through one of the sulfonyl oxygen atoms, while in the latter, the aquo ligand hydrogen bonds to the secondary-sphere anion. Notably, when only 1 equiv of acid was employed, the same products resulted, in half the amount, with 0.5 equiv of starting material 2 remaining (Figure S14). Both Co(III) cations 3-NTf2 and 3-OH2 were fully characterized, including via X-ray crystallography (Figure 1b,c) and independent synthesis from the bromide derivative 3-Br. As shown in Scheme 3, 3-Br (see the Supporting Information for details of its synthesis and characterization and Figure S15 for its X-ray structure) was prepared via oxidation of 1 with BrCPh3. Abstraction of the bromide with TMSNTf2 gave 3-NTf2 in 97% yield, and 3-OH2 was prepared via addition of water to this species. The diamagnetic nature of these d6 Co(III) products makes them easy to characterize by 1 H NMR spectroscopy, and the separately synthesized compounds were spectroscopically identical to those generated via protonation of 2. As a mixture, they give rise to sharp, separate signals in the 1H NMR spectrum, suggesting that, as expected, the aquo ligand in 3-OH2 is not labile on this time scale.56 Even the weakly coordinating NTf2 anion is rather tightly bound to the Co(III) center in 3-NTf2; the 19F NMR spectrum of 3-NTf2 indicates that the CF3 groups are inequivalent at room temperature, but heating results in coalescence of the resonances, suggesting an approximate barrier of 15.4 kcal mol−1 at 333 K for CF3 group exchange via dissociation of the anion from the Co (see Figures S17−S19). In this diprotonation sequence, stoichiometry demands that an oxygen atom must be included in order to balance the equation, and this is found in 0.5 equiv of O2; this product was identified by GC−MS and quantified by trapping with 1 (Figure S20). When the headspace of the reaction of 2 with 2 E


Article

Journal of the American Chemical Society bonds are 0.70, indicating somewhat polarized bonds. Overall, the reduction of the O2 molecule by 2 equiv of Co(II) complex 1, yielding peroxo complex 2, is computed to be enthalpically favorable by 12.4 kcal mol−1 at 298 K. Analysis of the frontier orbitals of 2 (Figure S30) gives some insight into the potential path of protonation. The doubly reduced nature of the bridging peroxy means that the HOMO in 2 is π* in character, and each oxygen develops a lone pair that is twisted orthogonally from the other; the LUMO in 2 exhibits antibonding character with respect to the Co 3d orbitals. Therefore, one can anticipate up to two protonations of either oxygen of the bridging peroxo group. With HNTf2 as the in silico proton source, it was found that monoprotonation of 2 to give 2-H+ (Scheme 5) was exceedingly favorable enthalpically (45.1 kcal mol−1; Figure 2); despite efforts, a transition state was not located for this reaction, which is not unusual for exergonic protonations, which can proceed via a “barrierless” path.57 In 2-H+, the O−O distance is only marginally elongated by 0.03 Å in comparison to that in 2, but the two Co−O bonds are significantly lengthened to 1.92 Å (Co−O−O) and 2.02 Å (Co−O(H)− O). This is in line with population of the LUMO, which displays Co−O antibonding character, and is consistent with the latter bond assuming a dative nature (cf. the distance of 1.979(2) Å for the Co−O bond in 3-OH2; Figure 1c). As shown in Scheme 5, from this monoprotonated species one might envision bond cleavage pathways (O−O, paths A or Co−O(H), path B) to occur prior to the next protonation. For paths A, both heterolytic (AHET) and homolytic (AHOMO) cleavage of the O−O bond may be considered. Path AHET leads to the mononuclear neutral Co(III) hydroxo complex 3-OH and a putative Co(V) oxo cation/Co(IV) oxyl cation labeled as I; this intermediate will be discussed in more detail below, where we discuss how this key species provides a low-energy path to the observed products 3-NTf2 and 0.5O2. Compound 3-OH can be synthesized separately (see the Supporting Information for full details and Figure S31 for the X-ray structure of 3-OH) and protonated to give the observed product 3-OH2 and thus is a potentially viable species in a mechanistic path for the double protonation of 2. However, as indicated in Figure 2, this bond cleavage is endothermic by 40 kcal mol−1, and we have not observed the presence of 3-OH in any low-temperature protonations of 2 monitored by 1H NMR spectroscopy. Similarly, the AHOMO path, which would yield the cationic Co(IV) hydroxo intermediate II and the putative Co(III) oxyl species III, is also strongly endothermic; thus, both of these paths will have prohibitively high barriers according to DFT. In path B, cleavage of the Co−O dative bond after the first protonation to give 2-H+ would yield the mononuclear Co(III) hydroperoxo complex 3-OOH58 and the product 3-NTf2. The second protonation could then occur on 3-OOH at the distal oxygen (path a) to generate water and intermediate I; alternatively, protonation at the proximal oxygen (path b) could release H2O2 and another equivalent of 3-NTf2. Computationally, the disruption of the Co−O(H) dative bond (path B) was also endothermic by more than 30 kcal mol−1, in agreement with the expectation that dissociation from the substitutionally inert d6 Co(III) center is kinetically challenging.59,60 These findings are also consistent with the observed outcome of the treatment of 2 with 1 equiv of HNTf2 discussed above, which suggests that the second protonation is much faster than any bond cleavages at this stage. Furthermore, as shown in Scheme 6, when a 1:1 mixture of

Scheme 6. Monoprotonation of Compound 2

the isotopologues 2-16O and 2-18O is treated with 1 equiv of acid, the remaining 2 contains no 16O−18O isotopologue (Figure S32), arguing strongly against the involvement of the Co(III) oxyl occurring in path AHOMO, which would be expected to rapidly and exothermically dimerize back to 2 (barrier of 8 kcal mol−1; see Figure 2) because of a high level of spin density found on the oxyl ligand (essentially 100%) in this putative species (Figure S33). This is further supported by the lack of isotope scrambling when a mixture of 2-16O and 2-18O is heated to 80 °C (Figure S34); III is not accessible from 2 either. Therefore, all of these “bond cleavage first” mechanisms were considered unlikely, and the protonation (path C) was examined next. The form of the HOMO for monoprotonated 2-H+ (Figure S35) is such that protonation at the same oxygen or the second oxygen is feasible, so each possibility was examined. However, while protonation of the same oxygen results in Co−O bond cleavage to give 3-NTf2 and, essentially, protonated 3-OOH (i.e., 3-OOH2), it is endothermic by 11.3 kcal mol−1 and is therefore not considered viable (and is not depicted in Scheme 5 or Figure 2). Conversely, protonation at the second oxygen gives the very stable complex 2-(H+)2, whose formation is 23.4 kcal mol−1 downhill from 2-H+ (Figure 2). This complex is best described as a μ-κ1-κ1-H2O2 adduct to two kinetically inert cationic d6 Co(III) centers; the O−O bond distance is 1.44 Å (cf. 1.442 Å computed for free H2O2 using the same methodology). The Co−O bond distances are 2.13 and 2.20 Å, in line with a donor−acceptor-type interaction between hydrogen peroxide and cationic cobalt(III) centers. While such hydrogen peroxide complexes are rare, recent examples have been prepared and characterized30−32 using hydrogen bonding to stabilize the weakly bound H2O2 ligand. Here the substitutional inertness of the Co(III) centers and the absence of any competing stronger donors in the system increase its stability. Nevertheless, it is highly reactive, and the LUMO of 2-(H+)2 (inset A, Figure 2) exhibits strong O−O antibonding character, so other pathways to products leading from this intermediate, depicted as the “second protonation first” paths D−F in Scheme 5, may be envisioned. Given the nature of the LUMO, direct O−O bond cleavage (path D) to yield 2 equiv of [Co(IV)OH]+ cation II was found to be enthalpically favored (by 24 kcal mol−1). This species F


Article


kcal mol−1) and yields intermediate IV, which is best described as an O2 adduct of the [B2Pz4PyCo(III)] cation. The computed O−O distance in IV is a very short 1.28 Å (close to that of 1.21 Å computed for free O2) and has a WBI of 1.75. The Co−O distances are around 1.97 Å, in line with a change to single-bond character. Another way to view IV is that it is the species that would result from two-electron oxidation of 2; thus, the LUMO of IV resembles the HOMO of 2, and because this orbital is O−O antibonding, the approximate O− O bond order in IV is 2 rather than 1 as in 2 (Figure S35). This positions IV to readily release O2 to give 3-NTf2; this was found to be exothermic by 36.3 kcal mol−1, and while not examined in detail, a scan of the potential energy surface suggests that this release occurs via two successive Co−O bond cleavages with a cationic Co(IV) superoxido intermediate.8,46 Overall, the sequence from I to IV to 3-NTf2 + O2 accounts for the other two observed products in the reaction. Experimental Evidence for Co(IV) Oxyl Cation I and Co(IV)OH Cation II. The above discussion provides a mechanistic framework that accounts for the observed products and provides a strong argument for the involvement of high-oxidation-state Co(IV) oxyl cation I as a key intermediate. We therefore sought experimental evidence to substantiate this hypothesis, and we present four lines of experiments designed to do this. First, the low-barrier equilibrium between I + 3-OH2 and II also suggests that this species may be present or that Co(IV) oxyl cation I may be implicated through generation of II by chemical oxidation of 3OH. Second, given the high level of spin density expected on the oxyl oxygen in I (Figure S33) and the O atom in II (Figure S37), we sought to chemically intercept them by protonating 2 in the presence of the hydrogen atom donor 1,4-cyclohexadiene (CHD). The cationic oxyl species might also be expected to react with oxophilic nucleophiles, so we also attempted to intercept this species with the oxygen atom acceptor triphenylphosphine (PPh3). Finally, since the hypothesized mechanism involves both O−O bond cleavage (TS1) and O−O bond formation (TS2), the diprotonation of a mixture of isotopologues 2-16O and 2-18O and analysis of the isotopic distribution in the evolved O2 should be informative. The results of these four lines of experimentation are summarized below. The redox behavior of 3-OH was probed via cyclic voltammetry, and it was found that an irreversible oxidation wave occurs at 1.12 V vs Fc/Fc+ in 1:6 THF/acetonitrile (Figure S38). Accordingly, the strong oxidant “Magic Green”68,69 was used to chemically oxidize 3-OH in benzene solution. The reaction was rapid at room temperature, and as can be seen in Scheme 7, this oxidation leads cleanly to the products 3-OH2 (partnered with the SbF6 counteranion) and 3-SbF6 (Figures S39−S41); both of these species were synthesized separately and fully characterized to confirm their identities, as they give slightly different 1H NMR spectroscopic signatures in comparison with their NTf2partnered counterparts (see the Supporting Information for details). One equivalent of the expected triarylamine byproduct was also observed. The observation of 0.23(2) equiv of O2 is significant and strongly implicates the intermediacy of I, formed via the predicted equilibrium between II and I + 3OH2 (Figure S42). Unfortunately, attempts to monitor this reaction via EPR spectroscopy gave inconclusive results. While it would not be expected that I would be observable (detection of S = 1 spin systems by EPR spectroscopy is rare70), the S =

would be expected to be highly reactive and perhaps disproportionate to give Co(III) product 3-OH2 and Co(V) oxo/Co(IV) oxyl intermediate I. Indeed, the barrier to transfer of a hydrogen atom from 3-OH2 to I to form 2 equiv of II is low enough (7.1 kcal mol−1; see Figure S36) to suggest that these three species may be in equilibrium. However, since the LUMO in 2-(H+)2 is 51.8 kcal mol−1 higher in energy than the HOMO of the system, a high barrier of 29 kcal mol−1 for this O−O bond cleavage is found by DFT. As mentioned above, dissociation of H2O2 from 2-(H+)2 (path E) is not kinetically viable, especially in the absence of any donors. However, in a catalytic situation, where water is being generated as a O2 reduction product, the equilibrium of path E may become more important. We note that, like many Co complexes, the B2Pz4PyCo system here does catalyze the decomposition of H2O2 to H2O + 0.5O2;61 little is known about the mechanism of this process,32,36 but the involvement of a species such as 2(H+)2 through path E is plausible. Thus, in the absence of excess water, paths D and E are not kinetically viable. However, a third path (F), which involves the LUMO of 2-(H+)2 by the migration of a hydrogen from one O atom to the other, provides a low-energy route to O−O bond cleavage. Here one of the Co centers serves as the reductant and thus is oxidized further in the process. The transition state for this process, which is depicted in inset B in Figure 2, is 7.6 kcal mol−1 above 2-(H+)2, and thus, this step should be rapid under the experimental conditions employed. In the transition state, the hydrogen transfer is well-advanced (O1−H, 1.32 Å; O2−H, 1.06 Å), and the O−O bond is significantly elongated to 1.73 Å. As the transfer occurs, the Co−O1 distance shortens to 1.91 Å while the Co−O2 distance increases to 2.47 Å; following the intrinsic reaction coordinate from TS1 leads directly to the product 3-OH2 and, again, the intriguing Co(V) oxo cation/Co(IV) oxyl cation I in a process that is exothermic by 28 kcal mol−1. Thus, of all the paths discussed, the sequence shown in green in Scheme 5 and Figure 2 (monoprotonation → path C → path F → path G) is clearly the kinetically and thermodynamically favored path. Intermediate I is featured in not only this sequence but the others as well and thus appears to be a key species. Highoxidation-state Co(IV) oxo/Co(III) oxyl species have been proposed as intermediates in O−O bond formation reactions important in Co-catalyzed water oxidation cycles38,40,44,62−64 and O−O bond cleavage processes in electron-rich Co(III)− O−O−Co(III) systems.46 However, Co(V) oxo/Co(IV) oxyl intermediates akin to I have also been suggested in electronrich environments.39,64,65 The computed structure of I indicates that the ground-state structure is an S = 1 triplet with significant spin density located on the oxo O (70%), suggesting that it is best described as a Co(IV) oxyl cation with substantial radical character on the oxygen (Scheme 5 inset and Figure S33). Consistent with this formulation is the very short Co−O distance of 1.67 Å, similar to the distance of 1.65 Å computed for a Co(V)O bond within a model Co oxide cluster of nuclearity 4.65 Unlike the related Co(III) oxyl species III mentioned above, oxyl cation I does not lie beyond the “oxo wall”66,67 and thus is a more viable species than III on this basis also. The high spin density on the O atom of I means that it is poised to provide a facile path for evolution of the 0.5 equiv of O2 through dimerization (path G, Scheme 5 and Figure 2). Indeed, this dimerization is highly favored enthalpically (by 36.2 kcal mol−1) and kinetically via the low-lying transition state TS2 (the activation barrier is only 2.6 G


Article

Journal of the American Chemical Society Scheme 7. Oxidation of 3-OH with Magic Green

Scheme 8. Chemical Interception of I/II with 1,4Cyclohexadiene as a Hydrogen Atom Donor

1

/2 cation II might be detectable, and the significant amount of spin density on cobalt in II (Figure S37) should manifest itself in hyperfine coupling to the 59Co nucleus.45 However, the oxidation of 3-OH by Magic Green is slow at low temperatures, and while a weak signal was observed, the presence of the triarylamine radical oxidizing agent made the spectra obtained uninterpretable and certainly not assignable. We then examined ways to chemically intercept these reactive paramagnetic species. Compound 2 is unreactive toward CHD under ambient conditions (Scheme 8). Protonation in the presence of 2−800 equiv of CHD gave an increased ratio of 3-OH2 over 3-NTf2 of up to 75:25, and benzene was also produced in the expected quantity on the basis of additional 3-OH2 (Figures S43−S48). Both I and II would be able to accept a hydrogen atom, the ultimate products being 3-OH2 and benzene. DFT computations also indicated that the reaction of Co(IV) oxyl cation I with CHD proceeds with a small barrier of 4.0 kcal mol−1, although this is approximately double the barrier for dimerization of I. Thus, while competitive with dimerization, the complete reaction of I/II with CHD was not observed at achievable concentrations of CHD. Nonetheless, these experiments implicate the intermediacy of I and II. Because of the incomplete chemical modification of I/II using CHD, we sought to intercept these intermediates with an oxygen atom acceptor, PPh371 (Scheme 9). Again, no reaction was observed between 2 and PPh3; protonation in the presence of 2−32 equiv of phosphine lead to nearly quantitative production of OPPh3 at high equivalencies (Figures S49−S52). The same results were obtained when separately prepared [HPPh3][NTf2] was employed. Because the product OPPh3 interacts dynamically with both 3-OH2 and 3-NTf2, the speciation of the Co(III) products was affected but could be assigned through separate control experiments. Accordingly, the 31P{1H} spectrum of the product mixture (Figure S50) presents three distinct peaks: a singlet at −5.6 ppm corresponding to unreacted PPh3, a broad resonance at 28.7

Scheme 9. Chemical Interception of I/II with PPh3 as an Oxygen Atom Acceptor

ppm, and a sharp singlet at 44.8 ppm. The reaction between 3NTf2 and OPPh3 shows that the latter signal is due to the adduct 3-OPPh 3 (Figures S53−S58), in which OPPh 3 displaces the NTf2 anion at room temperature. Lower temperatures favor 3-NTf2 and liberated OPPh3 (Figures S59 and S60). The broader downfield signal is due to triphenylphosphine oxide hydrogen-bonded to the aquo ligand in 3-OH2 to give 4 as depicted in Scheme 9; this was unraveled through experiments in which isolated 3-OH2 was treated with OPPh3 and variable-temperature 1H and 31P NMR spectra were recorded (Figures S61 and S62). Crystals grown from a H


Article


apparatus shown in Figure S70, and the resulting sample of 2-nO was isolated and analyzed in a similar fashion. As can be seen from the spectrum at the bottom of Figure 3, a broad band centered at 780 cm−1, the expected position for 2-16/18O,72 was observed. Close inspection of this band shows that it has two shoulders on either side, and a deconvolution treatment of the data suggests that the expected bands for 2-16O and 2-18O are possibly present (Figure S71) but unresolved. Unfortunately, because of the high propensity of peroxo complex 2 to burn at a laser power above 30 mW, a more resolved rR spectrum could not be obtained. Nonetheless, the experiment clearly shows that the formation of 2-16/18O is observed, as expected in the proposed mechanism involving intermediate I; any II present in the absence of a chemical modifier also evolves oxygen by disproportionating into I and 3-OH2 through the equilibrium discussed above.

1:1 mixture of 3-OH2 and OPPh3 gave the bis(OPPh3) adduct 4-OPPh3 (Figure 1d), but complex 4 is the major species in solution, as indicated through integration of the 1H NMR spectrum, and exhibits a broad signal around 28 ppm in the 31P NMR spectrum (Figures S63 and S64). These results collectively confirm that OPPh3 is produced in these reactions in up to 98% NMR yield (Figures S51 and S52). When a mixture of isotopologues of 2 were protonated in the presence of PPh3, low-temperature 31P NMR spectroscopy showed a 1:1 mixture of 3-16OPPh3 and 3-18OPPh3 (Figure S65); high-resolution mass spectrometry also confirmed this finding (Figures S66 and S67). Finally, an experiment in which 2 was protonated with 1 equiv of [HPPh3][NTf2] at low temperature showed via 1H and 31P NMR spectroscopy that OPPh3 only begins to appear concurrently with the reemergence of resonances for 2 (Figures S68 and S69). This implies that oxygen atom transfer to PPh3 does not occur from the protonated compounds 2-H+ and 2-(H+)2. Together, these results show that the oxygen atom in the phosphine oxide arises from peroxy compound 2 and that this transfer occurs through intermediate I or II, which is captured by PPh3 before I undergoes dimerization to release O2. A final experiment implicating I involved the protonation of a 1:1 mixture of 2-16O and 2-18O and an analysis of the isotopic composition of the evolved 0.5 equiv of nO2 (Figure 3). As mentioned above, the two pure isotopologues give distinct isotopically shifted bands in the rR spectra, shown as the top trace in Figure 3. The O2 arising from the protonation reaction was harvested by Co(II) complex 1 using the

■

CONCLUSIONS This study reports the reactivity of a neutral Co(II) complex of a dianionic pentadentate ligand with O2. The resulting peroxo complex 2 is highly stable toward release of O2 and can further react with a strong acid (HNTf2) to afford two fully characterized products, 3-NTf2 and 3-OH2, along with 0.5 equiv of O2. The mechanism of this protonation reaction was probed in detail both experimentally and computationally. The most favorable reaction path demonstrates both facile O−O bond cleavage (TS1, 7.6 kcal mol−1) and O−O bond formation (TS2, 2.3 kcal mol−1) steps as part of this process. While protonation of Co(III)−O−O−Co(III) complexes is typically thought to lead to production of H2O2,17 the dianionic nature of our ligand design and the kinetically inert nature of octahedral Co(III) centers allows for an alternate path involving O−O bond cleavage leading to H2O in our system (Scheme 10). Thus, protonation of 2 in the absence of Scheme 10. Diprotonation of Co(III)−O−O−Co(III) Complexes

water reveals the proposed involvement of the highly reactive cationic Co(IV) oxyl intermediate I, the result of O−O bond cleavage upon protonation of 2, as the most likely actor in this reaction. Intermediate I dimerizes rapidly under these conditions to release O2 in an example of O−O bond formation39,63 through coupling of two highly reactive metal oxyl species. The favorability of this unusual path of O2 reduction to water is dictated by the nature of the ligand system (pentacoordinate, dianionic), and although the chemistry reported here is stoichiometric, this path may be important in some catalytic systems.29

Figure 3. Scrambling of the nO label in the protonation of a 1:1 mixture of 2-16O and 2-18O as determined by resonance Raman spectroscopy: (top) rR spectra of 2-16O (green trace) and 2-18O (purple trace); (bottom) rR spectrum of the 2-nO isotopologues (black trace) obtained by reaction of evolved nO2 with 1, overlaid on the difference spectrum (2-18O) − (2-16O) (green trace). The rR spectra were obtained with 532 nm laser excitation at room temperature. I


Article


■

peroxido dicobalt species formed reversibly from CoII and O2. Chem. Commun. 2017, 53, 11782−11785. (9) Anson, C. W.; Ghosh, S.; Hammes-Schiffer, S.; Stahl, S. S. Co(salophen)-Catalyzed Aerobic Oxidation of p-Hydroquinone: Mechanism and Implications for Aerobic Oxidation Catalysis. J. Am. Chem. Soc. 2016, 138, 4186−4193. (10) Wang, C.-C.; Chang, H.-C.; Lai, Y.-C.; Fang, H.; Li, C.-C.; Hsu, H.-K.; Li, Z.-Y.; Lin, T.-S.; Kuo, T.-S.; Neese, F.; Ye, S.; Chiang, Y.W.; Tsai, M.-L.; Liaw, W.-F.; Lee, W.-Z. A Structurally Characterized Nonheme Cobalt−Hydroperoxo Complex Derived from Its Superoxo Intermediate via Hydrogen Atom Abstraction. J. Am. Chem. Soc. 2016, 138, 14186−14189. (11) Wang, Y.-H.; Pegis, M. L.; Mayer, J. M.; Stahl, S. S. Molecular Cobalt Catalysts for O2 Reduction: Low-Overpotential Production of H2O2 and Comparison with Iron-Based Catalysts. J. Am. Chem. Soc. 2017, 139, 16458−16461. (12) Wang, Y.-H.; Goldsmith, Z. K.; Schneider, P. E.; Anson, C. W.; Gerken, J. B.; Ghosh, S.; Hammes-Schiffer, S.; Stahl, S. S. Kinetic and Mechanistic Characterization of Low-Overpotential, H2O2-Selective Reduction of O2 Catalyzed by N2O2-Ligated Cobalt Complexes. J. Am. Chem. Soc. 2018, 140, 10890−10899. (13) Werner, A.; Mylius, A. Beitrag zur Konstitution anorganischer Verbindungen. XII. Mitteilung. Ü ber Oxykobaltiake und Anhydrooxykobaltiake. Z. Anorg. Chem. 1898, 16, 245−267. (14) Schmidt, S.; Heinemann, F. W.; Grohmann, A. A Structurally Characterised Pair of Dicobalt(III) Peroxo/Superoxo Complexes with C2-Symmetrical Tetrapodal Pentadentate Amine Ligands, and Some Reactivity en Route. Eur. J. Inorg. Chem. 2000, 2000, 1657−1667. (15) Rigsby, M. L.; Mandal, S.; Nam, W.; Spencer, L. C.; Llobet, A.; Stahl, S. S. Cobalt analogs of Ru-based water oxidation catalysts: overcoming thermodynamic instability and kinetic lability to achieve electrocatalytic O2 evolution. Chem. Sci. 2012, 3, 3058−3062. (16) Pauling, L. Nature of the Iron−Oxygen Bond in Oxyhæmoglobin. Nature 1964, 203, 182−183. (17) Rosenthal, J.; Nocera, D. G. Role of Proton-Coupled Electron Transfer in O−O Bond Activation. Acc. Chem. Res. 2007, 40, 543− 553. (18) Gavrilova, A. L.; Qin, C. J.; Sommer, R. D.; Rheingold, A. L.; Bosnich, B. Bimetallic reactivity. One-site addition two-metal oxidation reaction of dioxygen with a bimetallic dicobalt(II) complex bearing five- and six-coordinate sites. J. Am. Chem. Soc. 2002, 124, 1714−1722. (19) Di Giovanni, C.; Gimbert-Suriñach, C.; Nippe, M.; BenetBuchholz, J.; Long, J. R.; Sala, X.; Llobet, A. Dinuclear Cobalt Complexes with a Decadentate Ligand Scaffold: Hydrogen Evolution and Oxygen Reduction Catalysis. Chem. - Eur. J. 2016, 22, 361−369. (20) Volpe, M.; Hartnett, H.; Leeland, J. W.; Wills, K.; Ogunshun, M.; Duncombe, B. J.; Wilson, C.; Blake, A. J.; McMaster, J.; Love, J. B. Binuclear Cobalt Complexes of Schiff-Base Calixpyrroles and Their Roles in the Catalytic Reduction of Dioxygen. Inorg. Chem. 2009, 48, 5195−5207. (21) Chang, C. J.; Deng, Y.; Shi, C.; Chang, C. K.; Anson, F. C.; Nocera, D. G. Electrocatalytic four-electron reduction of oxygen to water by a highly flexible cofacial cobalt bisporphyrin. Chem. Commun. 2000, 1355−1356. (22) Chang, C. J.; Loh, Z.-H.; Shi, C.; Anson, F. C.; Nocera, D. G. Targeted Proton Delivery in the Catalyzed Reduction of Oxygen to Water by Bimetallic Pacman Porphyrins. J. Am. Chem. Soc. 2004, 126, 10013−10020. (23) Collman, J. P.; Wagenknecht, P. S.; Hutchison, J. E. Molecular Catalysts for Multielectron Redox Reactions of Small Molecules: The “Cofacial Metallodiporphyrin” Approach. Angew. Chem., Int. Ed. Engl. 1994, 33, 1537−1554. (24) Chang, C. K.; Liu, H. Y.; Abdalmuhdi, I. Electroreduction of oxygen by pillared cobalt(II) cofacial diporphyrin catalysts. J. Am. Chem. Soc. 1984, 106, 2725−2726. (25) Le Mest, Y.; Inisan, C.; Laouénan, A.; L’Her, M.; Talarmin, J.; El Khalifa, M.; Saillard, J.-Y. Reactivity toward Dioxygen of Dicobalt Face-to-Face Diporphyrins in Aprotic Media. Experimental and

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.8b07726. Experimental and characterization details for all new compounds, including spectroscopic data and X-ray crystallographic data (PDF) X-ray crystallographic data for 1-THF (CIF) X-ray crystallographic data for 1 (CIF) X-ray crystallographic data for 2 (CIF) X-ray crystallographic data for 3-Br (CIF) X-ray crystallographic data for 3-NTf2 (CIF) X-ray crystallographic data for 3-OH2 (CIF) X-ray crystallographic data for 4-OPPh3 (CIF) X-ray crystallographic data for B2Pz4Py (CIF)

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Warren E. Piers: 0000-0003-2278-1269 Laurent Maron: 0000-0003-2653-8557 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS W.E.P. thanks the Canada Research Chair Secretariat for a Tier I CRC (2013−2020). Funding for the experimental work described was provided by the Natural Sciences and Engineering Research Council of Canada in the form of a Discovery Grant to W.E.P. Funding for the computational work described was provided by a French CNRS PICS project. L.N. thanks Alberta Innovates Technology Futures and the Vanier Canada Graduate Scholarships for support. The authors thank Prof. Paul Hayes, Prof. René Boeré, and Nathan Hill (University of Lethbridge) for help with acquiring EPR data and Prof. Michael Neidig (University of Rochester) for useful discussions regarding EPR data interpretation.

■

REFERENCES

(1) Pegis, M. L.; Wise, C. F.; Martin, D. J.; Mayer, J. M. Oxygen Reduction by Homogeneous Molecular Catalysts and Electrocatalysts. Chem. Rev. 2018, 118, 2340−2391. (2) Lewis, N. S.; Nocera, D. G. Powering the planet: Chemical challenges in solar energy utilization. Proc. Natl. Acad. Sci. U. S. A. 2006, 103, 15729−15735. (3) Ray, K.; Heims, F.; Pfaff, F. F. Terminal Oxo and Imido Transition-Metal Complexes of Groups 9−11. Eur. J. Inorg. Chem. 2013, 2013, 3784−3807. (4) Fukuzumi, S.; Lee, Y.-M.; Nam, W. Mechanisms of TwoElectron versus Four-Electron Reduction of Dioxygen Catalyzed by Earth-Abundant Metal Complexes. ChemCatChem 2018, 10, 9−28. (5) Ramprasad, D.; Gilicinski, A. G.; Markley, T. J.; Pez, G. P. Cobalt(II) Polypyridine Complexes and Their Reversible Reactivity with Dioxygen. Inorg. Chem. 1994, 33, 2841−2847. (6) Jones, R. D.; Summerville, D. A.; Basolo, F. Synthetic oxygen carriers related to biological systems. Chem. Rev. 1979, 79, 139−179. (7) Huang, X.; Groves, J. T. Oxygen Activation and Radical Transformations in Heme Proteins and Metalloporphyrins. Chem. Rev. 2018, 118, 2491−2553. (8) Corona, T.; Padamati, S. K.; Acuna-Pares, F.; Duboc, C.; Browne, W. R.; Company, A. Trapping of superoxido cobalt and J


Article

Journal of the American Chemical Society Theoretical Aspects. Possible Mechanistic Implication in the Reduction of Dioxygen. J. Am. Chem. Soc. 1997, 119, 6095−6106. (26) Askarizadeh, E.; Yaghoob, S. B.; Boghaei, D. M.; Slawin, A. M.; Love, J. B. Tailoring dicobalt Pacman complexes of Schiff-base calixpyrroles towards dioxygen reduction catalysis. Chem. Commun. 2010, 46, 710−2. (27) Devoille, A. M. J.; Love, J. B. Double-pillared cobalt Pacman complexes: synthesis, structures and oxygen reduction catalysis. Dalton Trans. 2012, 41, 65−72. (28) Givaja, G.; Volpe, M.; Edwards, M. A.; Blake, A. J.; Wilson, C.; Schroder, M.; Love, J. B. Dioxygen reduction at dicobalt complexes of a Schiff base calixpyrrole ligand. Angew. Chem., Int. Ed. 2007, 46, 584−6. (29) Fukuzumi, S.; Okamoto, K.; Gros, C. P.; Guilard, R. Mechanism of Four-Electron Reduction of Dioxygen to Water by Ferrocene Derivatives in the Presence of Perchloric Acid in Benzonitrile, Catalyzed by Cofacial Dicobalt Porphyrins. J. Am. Chem. Soc. 2004, 126, 10441−10449. (30) Wallen, C. M.; Bacsa, J.; Scarborough, C. C. Hydrogen Peroxide Complex of Zinc. J. Am. Chem. Soc. 2015, 137, 14606− 14609. (31) Wallen, C. M.; Bacsa, J.; Scarborough, C. C. Coordination of Hydrogen Peroxide with Late-Transition-Metal Sulfonamido Complexes. Inorg. Chem. 2018, 57, 4841−4848. (32) Wallen, C. M.; Palatinus, L.; Bacsa, J.; Scarborough, C. C. Hydrogen Peroxide Coordination to Cobalt(II) Facilitated by Second-Sphere Hydrogen Bonding. Angew. Chem., Int. Ed. 2016, 55, 11902−11906. (33) Fukuzumi, S.; Mandal, S.; Mase, K.; Ohkubo, K.; Park, H.; Benet-Buchholz, J.; Nam, W.; Llobet, A. Catalytic four-electron reduction of O2 via rate-determining proton-coupled electron transfer to a dinuclear cobalt-μ-1,2-peroxo complex. J. Am. Chem. Soc. 2012, 134, 9906−9909. (34) Wada, T.; Maki, H.; Imamoto, T.; Yuki, H.; Miyazato, Y. Fourelectron reduction of dioxygen catalysed by dinuclear cobalt complexes bridged by bis(terpyridyl)anthracene. Chem. Commun. 2013, 49, 4394−4396. (35) Monte-Pérez, I.; Kundu, S.; Chandra, A.; Craigo, K. E.; Chernev, P.; Kuhlmann, U.; Dau, H.; Hildebrandt, P.; Greco, C.; Van Stappen, C.; Lehnert, N.; Ray, K. Temperature Dependence of the Catalytic Two- versus Four-Electron Reduction of Dioxygen by a Hexanuclear Cobalt Complex. J. Am. Chem. Soc. 2017, 139, 15033− 15042. (36) Waldmeier, P.; Prijs, B.; Sigel, H. Metal Ions and Hydrogen Peroxide. XXV. On the Kinetics of the Catalase-Like Activity of the Cobalt(II) Complex of 4,4′,4″,4‴-Tetrasulfophthalocyanine. Z. Naturforsch., B: J. Chem. Sci. 1972, 27, 95−100. (37) Kadish, K. M.; Frémond, L.; Shen, J.; Chen, P.; Ohkubo, K.; Fukuzumi, S.; El Ojaimi, M.; Gros, C. P.; Barbe, J.-M.; Guilard, R. Catalytic Activity of Biscobalt Porphyrin-Corrole Dyads Toward the Reduction of Dioxygen. Inorg. Chem. 2009, 48, 2571−2582. (38) Crandell, D. W.; Ghosh, S.; Berlinguette, C. P.; Baik, M.-H. How a [CoIV−O]2+ Fragment Oxidizes Water: Involvement of a Biradicaloid [CoII−(·O·)]2+ Species in Forming the O−O Bond. ChemSusChem 2015, 8, 844−852. (39) Nguyen, A. I.; Ziegler, M. S.; Oña-Burgos, P.; SturzbecherHohne, M.; Kim, W.; Bellone, D. E.; Tilley, T. D. Mechanistic Investigations of Water Oxidation by a Molecular Cobalt Oxide Analogue: Evidence for a Highly Oxidized Intermediate and Exclusive Terminal Oxo Participation. J. Am. Chem. Soc. 2015, 137, 12865− 12872. (40) Du, H. Y.; Chen, S. C.; Su, X. J.; Jiao, L.; Zhang, M. T. RedoxActive Ligand Assisted Multielectron Catalysis: A Case of Co(III) Complex as Water Oxidation Catalyst. J. Am. Chem. Soc. 2018, 140, 1557−1565. (41) Wang, B.; Lee, Y.-M.; Tcho, W.-Y.; Tussupbayev, S.; Kim, S.T.; Kim, Y.; Seo, M. S.; Cho, K.-B.; Dede, Y.; Keegan, B. C.; Ogura, T.; Kim, S. H.; Ohta, T.; Baik, M.-H.; Ray, K.; Shearer, J.; Nam, W.

Synthesis and reactivity of a mononuclear non-haem cobalt(IV)−oxo complex. Nat. Commun. 2017, 8, 14839. (42) Wang, D.; Groves, J. T. Efficient water oxidation catalyzed by homogeneous cationic cobalt porphyrins with critical roles for the buffer base. Proc. Natl. Acad. Sci. U. S. A. 2013, 110, 15579−15584. (43) McAlpin, J. G.; Surendranath, Y.; Dincă, M.; Stich, T. A.; Stoian, S. A.; Casey, W. H.; Nocera, D. G.; Britt, R. D. EPR Evidence for Co(IV) Species Produced during Water Oxidation at Neutral pH. J. Am. Chem. Soc. 2010, 132, 6882−6883. (44) Zhang, M.; de Respinis, M.; Frei, H. Time-resolved observations of water oxidation intermediates on a cobalt oxide nanoparticle catalyst. Nat. Chem. 2014, 6, 362−367. (45) Hong, S.; Pfaff, F. F.; Kwon, E.; Wang, Y.; Seo, M.-S.; Bill, E.; Ray, K.; Nam, W. Spectroscopic Capture and Reactivity of a Low-Spin Cobalt(IV)−Oxo Complex Stabilized by Binding Redox-Inactive Metal Ions. Angew. Chem., Int. Ed. 2014, 53, 10403−10407. (46) Nguyen, A. I.; Hadt, R. G.; Solomon, E. I.; Tilley, T. D. Efficient C−H bond activations via O2 cleavage by a dianionic cobalt(II) complex. Chem. Sci. 2014, 5, 2874−2878. (47) Stahl, S. S. Palladium Oxidase Catalysis: Selective Oxidation of Organic Chemicals by Direct Dioxygen-Coupled Turnover. Angew. Chem., Int. Ed. 2004, 43, 3400−3420. (48) Anson, C. W.; Stahl, S. S. Cooperative Electrocatalytic O2 Reduction Involving Co(salophen) with p-Hydroquinone as an Electron-Proton Transfer Mediator. J. Am. Chem. Soc. 2017, 139, 18472−18475. (49) Nurdin, L.; Spasyuk, D. M.; Piers, W. E.; Maron, L. Reactions of Neutral Cobalt(II) Complexes of a Dianionic Tetrapodal Pentadentate Ligand: Cobalt(III) Amides from Imido Radicals. Inorg. Chem. 2017, 56, 4157−4168. (50) Spasyuk, D. M.; Carpenter, S. H.; Kefalidis, C. E.; Piers, W. E.; Neidig, M. L.; Maron, L. Facile hydrogen atom transfer to iron(III) imido radical complexes supported by a dianionic pentadentate ligand. Chem. Sci. 2016, 7, 5939−5944. (51) Jonas, R. T.; Stack, T. D. P. C−H Bond Activation by a Ferric Methoxide Complex: A Model for the Rate-Determining Step in the Mechanism of Lipoxygenase. J. Am. Chem. Soc. 1997, 119, 8566− 8567. (52) Zee, D. Z.; Chantarojsiri, T.; Long, J. R.; Chang, C. J. Metal− Polypyridyl Catalysts for Electro- and Photochemical Reduction of Water to Hydrogen. Acc. Chem. Res. 2015, 48, 2027−2036. (53) Wasylenko, D. J.; Palmer, R. D.; Berlinguette, C. P. Homogeneous water oxidation catalysts containing a single metal site. Chem. Commun. 2013, 49, 218−227. (54) Avdeef, A.; Schaefer, W. P. Reversible oxygen carriers. Synthesis and structure of μ-dioxygen-bis[N,N′-ethylenebis(salicylideniminato)piperidinecobalt] acetonate piperidinate (C5H11NCoSalen)2(O2)· 0.67(CH3)2CO·0.33C5H11N7. Inorg. Chem. 1976, 15, 1432−1440. (55) Sun, Y.; Bigi, J. P.; Piro, N. A.; Tang, M. L.; Long, J. R.; Chang, C. J. Molecular Cobalt Pentapyridine Catalysts for Generating Hydrogen from Water. J. Am. Chem. Soc. 2011, 133, 9212−9215. (56) Mechanisms of Inorganic Reactions; Kleinberg, J., Murmann, R. K., Fraser, R. T. M., Bauman, J., Eds.; Advances in Chemistry Series, Vol. 49; American Chemical Society: 1965; Chapter 2, p 31−54. (57) Riplinger, C.; Sampson, M. D.; Ritzmann, A. M.; Kubiak, C. P.; Carter, E. A. Mechanistic Contrasts between Manganese and Rhenium Bipyridine Electrocatalysts for the Reduction of Carbon Dioxide. J. Am. Chem. Soc. 2014, 136, 16285−16298. (58) Shin, B.; Sutherlin, K. D.; Ohta, T.; Ogura, T.; Solomon, E. I.; Cho, J. Reactivity of a Cobalt(III)−Hydroperoxo Complex in Electrophilic Reactions. Inorg. Chem. 2016, 55, 12391−12399. (59) Wiesendanger, H. U. D.; Jones, W. H.; Garner, C. S. Exchange of Ammonia between Metal−Ammine Cations and Solvent in Liquid Ammonia. J. Chem. Phys. 1957, 27, 668−672. (60) Basolo, F.; Pearson, R. G. Mechanisms of Inorganic Reactions; John Wiley and Sons: London, 1958. (61) Stauber, J. M.; Bloch, E. D.; Vogiatzis, K. D.; Zheng, S.-L.; Hadt, R. G.; Hayes, D.; Chen, L. X.; Gagliardi, L.; Nocera, D. G.; Cummins, C. C. Pushing Single-Oxygen-Atom-Bridged Bimetallic K


Article

Journal of the American Chemical Society Systems to the Right: A Cryptand-Encapsulated Co−O−Co Unit. J. Am. Chem. Soc. 2015, 137, 15354−15357. (62) Surendranath, Y.; Kanan, M. W.; Nocera, D. G. Mechanistic Studies of the Oxygen Evolution Reaction by a Cobalt-Phosphate Catalyst at Neutral pH. J. Am. Chem. Soc. 2010, 132, 16501−16509. (63) Mattioli, G.; Giannozzi, P.; Amore Bonapasta, A.; Guidoni, L. Reaction Pathways for Oxygen Evolution Promoted by Cobalt Catalyst. J. Am. Chem. Soc. 2013, 135, 15353−15363. (64) Das, D.; Pattanayak, S.; Singh, K. K.; Garai, B.; Sen Gupta, S. Electrocatalytic water oxidation by a molecular cobalt complex through a high valent cobalt oxo intermediate. Chem. Commun. 2016, 52, 11787−11790. (65) Li, X.; Siegbahn, P. E. M. Water Oxidation Mechanism for Synthetic Co−Oxides with Small Nuclearity. J. Am. Chem. Soc. 2013, 135, 13804−13813. (66) Winkler, J. R.; Gray, H. B. Electronic Structures of Oxo−Metal Ions. Struct. Bonding 2011, 142, 17−28. (67) Gray, H. B.; Winkler, J. R. Living with Oxygen. Acc. Chem. Res. 2018, 51, 1850−1857. (68) Murata, Y.; Cheng, F.; Kitagawa, T.; Komatsu, K. Generation of fullerenyl cation (EtO)2P+(OH)CH2−C60+ from RC60−H and from RC60−C60R (R = CH2P(O)(OEt)2). J. Am. Chem. Soc. 2004, 126, 8874−8875. (69) Connelly, N. G.; Geiger, W. E. Chemical redox agents for organometallic chemistry. Chem. Rev. 1996, 96, 877−910. (70) Hendrich, M. P.; Debrunner, P. G. Integer-spin electron paramagnetic resonance of iron proteins. Biophys. J. 1989, 56, 489− 506. (71) Kindermann, N.; Dechert, S.; Demeshko, S.; Meyer, F. ProtonInduced, Reversible Interconversion of a μ-1,2-Peroxo and a μ-1,1Hydroperoxo Dicopper(II) Complex. J. Am. Chem. Soc. 2015, 137, 8002−8005. (72) Klotz, I. M.; Kurtz, D. M. Binuclear oxygen carriers: hemerythrin. Acc. Chem. Res. 1984, 17, 16−22.

L


Oxygen-Oxygen Bond Cleavage and Formation in Co(II) - American

Recommend Documents