Concerted proton-electron transfer oxidation of phenols and hydrocarbons by a high-valent nickel complex

Fisher, Katherine; Feuer, Margalit L; Lant, Hannah M. C.; Mercado, Brandon Q.; Crabtree, Robert H.; Brudvig, Gary W.

doi:10.1039/c9sc05565g

Cited by 16 publications

(28 citation statements)

References 48 publications

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“…Conversely, no apparent correlation exists with the p K a of organic substrates (Figure S4), ruling out a p K a driven mechanism. Instead, the linear correlation implicates a concerted H-atom abstraction pathway, and this observation is consistent with other metal complexes that react via a CPET mechanism. − Additionally, large differences between the p K a values of 4 and C–H substrates (Δp K a > 30) and between the E ° values of 2 (Co III/IV couple) and C–H substrates (Δ E ° > 0.6 V) suggest that discrete proton or electron transfer is not thermodynamically accessible, disfavoring stepwise proton transfer-electron transfer (PTET) and electron transfer-proton transfer (ETPT) pathways. Taken together, these data strongly suggest that H-atom abstraction by 2 occurs by a CPET mechanism.…”

Section: Results and Discussionsupporting

confidence: 74%

Concerted Proton–Electron Transfer Reactivity at a Multimetallic Co₄O₄ Cubane Cluster

et al. 2020

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High-valent oxocobalt(IV) species have been invoked as key intermediates in oxidative catalysis, but investigations into the chemistry of proton-coupled redox reactions of such species have been limited. Herein, the reactivity of an established water oxidation catalyst, [Co 4 O 4 (OAc) 4 (py) 4 ][PF 6 ], toward H-atom abstraction reactions is described. Mechanistic analyses and density functional theory (DFT) calculations support a concerted proton−electron transfer (CPET) pathway in which the high energy intermediates formed in stepwise pathways are bypassed. Natural bond orbital (NBO) calculations point to cooperative donor−acceptor σ interactions at the transition state, whereby the H-atom of the substrate is transferred to an orbital delocalized over a Co 3 (μ 3 -O) fragment. The mechanistic insights provide design principles for the development of catalytic C−H activation processes mediated by a multimetallic oxo metal cluster.

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Section: Results and Discussionsupporting

confidence: 74%

Concerted Proton–Electron Transfer Reactivity at a Multimetallic Co₄O₄ Cubane Cluster

et al. 2020

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“…Notably, a number of high-valent non-oxo–metal species of late transition metals have been recently described as strong oxidants that activate C–H bonds. Representative examples include a series of Ni(III) and Cu(III) complexes developed by the groups of Tolman, − McDonald, − Company and Brudvig . In this work, we obtained a high-valent Co IV –dinitrate complex (Co IV –(ONO 2 ) 2 , 2 ) by one-electron oxidation of its Co(III) precursor.…”

Section: Introductionmentioning

confidence: 99%

“…Representative examples include a series of Ni(III) and Cu(III) complexes developed by the groups of Tolman, 10−12 McDonald, 13−15 Company 16 and Brudvig. 17 In this work, we obtained a high-valent Co IV − dinitrate complex (Co IV −(ONO 2 ) 2 , 2) by one-electron oxidation of its Co(III) precursor. 2 is characterized structurally by X-ray crystallography, and its electronic structure has been probed using combined spectroscopic and computational approaches.…”

Section: ■ Introductionmentioning

confidence: 99%

Crystal Structure and C–H Bond-Cleaving Reactivity of a Mononuclear Co^IV–Dinitrate Complex

et al. 2020

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High-valent FeIVO intermediates with a terminal metal–oxo moiety are key oxidants in many enzymatic and synthetic C–H bond oxidation reactions. While generating stable metal–oxo species for late transition metals remains synthetically challenging, notably, a number of high-valent non-oxo–metal species of late transition metals have been recently described as strong oxidants that activate C–H bonds. In this work, we obtained an unprecedented mononuclear CoIV–dinitrate complex (2) upon one-electron oxidation of its Co(III) precursor supported by a tridentate dianionic N3 ligand. 2 was structurally characterized by X-ray crystallography, showing a square pyramidal geometry with two coordinated nitrate anions. Furthermore, characterization of 2 using combined spectroscopic and computational methods revealed that 2 is a low-spin (S = 1/2) Co(IV) species with the unpaired electron located on the cobalt d z2 orbital, which is well positioned for substrate oxidations. Indeed, while having a high thermal stability, 2 is able to cleave sp3 C–H bonds up to 87 kcal/mol to afford rate constants and kinetic isotope effects (KIEs) of 2–6 that are comparable to other high-valent metal oxidants. The ability to oxidize strong C–H bonds has yet to be observed for CoIV–O and CoIIIO species previously reported. Therefore, 2 represents the first high-valent Co(IV) species that is both structurally characterized by X-ray crystallography and capable of activating strong C–H bonds.

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“…Therefore, spectroscopic interrogation and reactivity studies of high-valent Ni complexes have been of great interest to chemists given their strong biological relevance, application in catalysis, and energy-related applications. A few high-valent mononuclear Ni complexes have been characterized, and the reactivity of such species has been investigated in recent years by Ray, Company, , McDonald, − and others. , The X-ray crystal structure of a few organometallic high-valent Ni complexes supported by electron-rich carbanion/carbene ligands has further been isolated. ,− However, the effect of the secondary coordination sphere on the electronic properties and reactivity of high-valent Ni complexes has not been investigated. Thus, in this study, we set out to synthesize a Ni(III) complex of a bisamidate-bisalkoxide N 2 O 2 donor set of ligand so that the alkoxide arms of the ligand can coordinate Ni as well as LA.…”

Section: Introductionmentioning

confidence: 99%

Effect of Redox-Inactive Metal Ion–Nickel(III) Interactions on the Redox Properties and Proton-Coupled Electron Transfer Reactivity

et al. 2022

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Mononuclear nickel(II) and nickel(III) complexes of a bisamidate-bisalkoxide ligand, (NMe4)2[NiII(HMPAB)] (1) and (NMe4)[NiIII(HMPAB)] (2), respectively, have been synthesized and characterized by various spectroscopic techniques including X-ray crystallography. The reaction of redox-inactive metal ions (M n+ = Ca2+, Mg2+, Zn2+, Y3+, and Sc3+) with 2 resulted in 2-M n+ adducts, which was assessed by an array of spectroscopic techniques including X-ray absorption spectroscopy (XAS), electron paramagnetic resonance (EPR), and reactivity studies. The X-ray structure of Ca2+ coordinated to Ni(III) complexes, 2-Ca2+T, was determined and exhibited an average Ni–Ca distance of 3.1253 Å, close to the metal ions’ covalent radius. XAS analysis of 2-Ca2+ and 2-Y3+ in solution further revealed an additional coordination to Ca and Y in the 2-M n+ adducts with shortened Ni–M distances of 2.15 and 2.11 Å, respectively, implying direct bonding interactions between Ni and Lewis acids (LAs). Such a short interatomic distance between Ni(III) and M is unprecedented and was not observed before. EPR analysis of 2 and 2-M n+ species, moreover, displayed rhombic signals with g av > 2.12 for all complexes, supporting the +III oxidation state of Ni. The NiIII/NiII redox potential of 2 and 2-M n+ species was determined, and a plot of E 1/2 of 2-M n+ versus pK a of [M(H2O) n ] m+ exhibited a linear relationship, implying that the NiIII/NiII potential of 2 can be tuned with different redox-inactive metal ions. Reactivity studies of 2 and 2-M n+ with different 4-X-2,6-ditert-butylphenol (4-X-DTBP) and other phenol derivatives were performed, and based on kinetic studies, we propose the involvement of a proton-coupled electron transfer (PCET) pathway. Analysis of the reaction products after the reaction of 2 with 4-OMe-DTBP showed the formation of a Ni(II) complex (1a) where one of the alkoxide arms of the ligand is protonated. A pK a value of 24.2 was estimated for 1a. The reaction of 2-M n+ species was examined with 4-OMe-DTBP, and it was observed that the k 2 values of 2-M n+ species increase by increasing the Lewis acidity of redox-inactive metal ions. However, the obtained k 2 values for 2-M n+ species are much lower compared to the k 2 value for 2. Such a variation of PCET reactivity between 2 and 2-M n+ species may be attributed to the interactions between Ni(III) and LAs. Our findings show the significance of the secondary coordination sphere effect on the PCET reactivity of Ni(III) complexes and furnish important insights into the reaction mechanism involving high-valent nickel species, which are frequently invoked as key intermediates in Ni-mediated enzymatic reactions, solar-fuel catalysis, and biomimetic/synthetic transformation reactions.

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Concerted proton-electron transfer oxidation of phenols and hydrocarbons by a high-valent nickel complex

Abstract: A high-valent nickel(III) compound performs fast concerted proton–electron transfer on O–H and C–H bonds. Thermodynamic analysis suggests that the oxidizing power of the compound and the formation of a strong ligand O–H bond lead to high reactivity.

Cited by 16 publications

References 48 publications

Concerted Proton–Electron Transfer Reactivity at a Multimetallic Co₄O₄ Cubane Cluster

Concerted Proton–Electron Transfer Reactivity at a Multimetallic Co₄O₄ Cubane Cluster

Crystal Structure and C–H Bond-Cleaving Reactivity of a Mononuclear Co^IV–Dinitrate Complex

Effect of Redox-Inactive Metal Ion–Nickel(III) Interactions on the Redox Properties and Proton-Coupled Electron Transfer Reactivity

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Concerted proton-electron transfer oxidation of phenols and hydrocarbons by a high-valent nickel complex

Abstract: A high-valent nickel(III) compound performs fast concerted proton–electron transfer on O–H and C–H bonds. Thermodynamic analysis suggests that the oxidizing power of the compound and the formation of a strong ligand O–H bond lead to high reactivity.

Cited by 16 publications

References 48 publications

Concerted Proton–Electron Transfer Reactivity at a Multimetallic Co4O4 Cubane Cluster

Concerted Proton–Electron Transfer Reactivity at a Multimetallic Co4O4 Cubane Cluster

Crystal Structure and C–H Bond-Cleaving Reactivity of a Mononuclear CoIV–Dinitrate Complex

Effect of Redox-Inactive Metal Ion–Nickel(III) Interactions on the Redox Properties and Proton-Coupled Electron Transfer Reactivity

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Concerted Proton–Electron Transfer Reactivity at a Multimetallic Co₄O₄ Cubane Cluster

Concerted Proton–Electron Transfer Reactivity at a Multimetallic Co₄O₄ Cubane Cluster

Crystal Structure and C–H Bond-Cleaving Reactivity of a Mononuclear Co^IV–Dinitrate Complex