Phenol Oxidation by a Nickel(III)–Fluoride Complex: Exploring the Influence of the Proton Accepting Ligand in PCET Oxidation

Mondal, Prasenjit; McDonald, Aidan R.

doi:10.1002/chem.202002135

Cited by 10 publications

(22 citation statements)

References 70 publications

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“…109−111 However, for a neat transfer of hydrogen atom (one-step), dependence on the redox driving force is minimal, and thus, a 0 slope is expected in the plot of (RT/F) ln k 2 versus E ox . Close to 0 slope and a relatively large KIE, on the one hand, have been reported for the phenolic O−H bond abstraction reactions of the cumylperoxyl radical (slope = −0.05) 112 and [Mn IV (ttppc)(OH)] (−0.12), 113 [Fe IV (ttppc)-(OH)] (−0.19, KIE = 2.9), 113 Ni III (Cl) (−0.12, KIE = 2.4), 48 and Ni III (F) complexes (−0.11, KIE = 18), 114 whereby a HAT reaction mechanism is implicated. On the other hand, Itoh and co-workers observed a slope of −0.72 (and KIE = 1.21−1.56) for the reaction of the (μ-η 2 :η 2 -peroxo)dicopper(II) complex and phenol derivatives, and an endergonic ET coupled with PT mechanism was documented.…”

Section: T H I S C O N T E N T Imentioning

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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Section: T H I S C O N T E N T Imentioning

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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“…This complex reacts with Ph 3 C· to give Ph 3 CN 3 (90%), indicating it is capable of efficient N 3 · to carbon radicals. Although it is known that certain Fe III complexes can carry out C–H activation without an external oxidant, − we found that 7 shows only minor thermal decomposition in the presence of excess Ph 3 CH at 50 °C, and there is no trace of the azidation product Ph 3 CN 3 .…”

Section: Resultsmentioning

confidence: 56%

Nonheme Iron(III) Azide and Iron(III) Isothiocyanate Complexes: Radical Rebound Reactivity, Selectivity, and Catalysis

et al. 2022

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The new nonheme iron complexes FeII(BNPAPh2O)(N3) (1), FeIII(BNPAPh2O)(OH)(N3) (2), FeII(BNPAPh2O)(OH) (3), FeIII(BNPAPh2O)(OH)(NCS) (4), FeII(BNPAPh2O)(NCS) (5), FeIII(BNPAPh2O)(NCS)2 (6), and FeIII(BNPAPh2O)(N3)2 (7) (BNPAPh2O = 2-(bis((6-(neopentylamino)pyridin-2-yl) methyl)amino)-1,1-diphenylethanolate) were synthesized and characterized by single crystal X-ray diffraction (XRD), as well as by 1H NMR, 57Fe Mössbauer, and ATR-IR spectroscopies. Complex 2 was reacted with a series of carbon radicals, ArX 3C· (ArX = p-X-C6H4), analogous to the proposed radical rebound step for nonheme iron hydroxylases and halogenases. The results show that for ArX 3C· (X = Cl, H, t Bu), only OH· transfer occurs to give ArX 3COH. However, when X = OMe, a mixture of alcohol (ArX 3COH) (30%) and azide (ArX 3CN3) (40%) products was obtained. These data indicate that the rebound selectivity is influenced by the electron-rich nature of the carbon radicals for the azide complex. Reaction of 2 with Ph3C· in the presence of Sc3+ or H+ reverses the selectivity, giving only the azide product. In contrast to the mixed selectivity seen for 2, the reactivity of cis-FeIII(OH)(NCS) with the X = OMe radical derivative leads only to hydroxylation. Catalytic azidation was achieved with 1 as catalyst, λ3-azidoiodane as oxidant and azide source, and Ph3CH as test substrate, giving Ph3CN3 in 84% (TON = 8). These studies show that hydroxylation is favored over azidation for nonheme iron(III) complexes, but the nature of the carbon radical can alter this selectivity. If an OH· transfer pathway can be avoided, the FeIII(N3) complexes are capable of mediating both stoichiometric and catalytic azidation.

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“…A similar type of V-shaped plot was reported for the OAT and HAT reactions of Mn V (O)(TBP 8 Cz) [TBP 8 Cz = octakis( p - tert -butylphenyl)corrolazinato 3– ], and the HAT reaction of a Mn V (NTs)(corrole) complex . A recent study showed such a mechanistic switch for the reaction of a nickel(III) fluoride complex with substituted phenols . We speculate that electron-withdrawing halide groups create a partial positive charge at the phosphorus atom through an inductive effect, which makes the (4-F-C 6 H 4 ) 3 P or (4-Cl-C 6 H 4 ) 3 P substrate electrophilic in nature, and reversal of the OAT reaction occurs with these substrates .…”

Section: Resultsmentioning

confidence: 97%

“…70 A recent study showed such a mechanistic switch for the reaction of a nickel(III) fluoride complex with substituted phenols. 71 We speculate that electron-withdrawing halide groups create a partial positive charge at the phosphorus atom through an inductive effect, which makes the (4-F-C 6 H 4 ) 3 P or (4-Cl-C 6 H 4 ) 3 P substrate electrophilic in nature, and reversal of the OAT reaction occurs with these substrates. 69 Then we examined Hammett analysis of the OAT reaction of 2-Ca 2+ with different (4-X-C 6 H 4 ) 3 P substrates.…”

Section: ■ Results and Discussionmentioning

confidence: 99%

Electrochemical Properties and Reactivity Study of [Mn^V(O)(μ-OR–Lewis Acid)] Cores

Gupta

Bera

Paul³

et al. 2021

Inorg. Chem.

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A mononuclear manganese(V) oxo complex of a bis(amidate)bis(alkoxide) ligand, (NMe4)[MnV(HMPAB)(O)] [2; H4HMPAB = 1,2-bis(2-hydroxy-2-methylpropanamido)benzene], was synthesized and structurally characterized. A Mn–Oterm distance of 1.566(4) Å was observed in the solid-state structure of 2, consistent with the MnO formulation. The reaction of redox-inactive metal ions (M n+ = Li+, Ca2+, Mg2+, Y3+, and Sc3+) with 2 resulted in the formation of 2-M n+ species, which were characterized by UV–vis, 1H NMR, cyclic voltammetry, and in situ IR spectroscopy. Theoretical calculations suggested that the alkoxide oxygen atoms of the ligand scaffold are energetically most favorable for coordinating the M n+ ions in 2. Complex 2 revealed one-electron-reduction potential at −0.01 V versus ferrocenium/ferrocene, which shifted anodically upon coordination of M n+ ions to 2, and such a shift became more prominent with stronger Lewis acids. The oxygen-atom transfer (OAT) reactivities of 2 and 2-M n+ species with triphenylphosphine were compared, which exhibited a systematic increase of the reaction rate with increasing Lewis acidity of M n+ ions, and a plot of log k 2 versus Lewis acidity of M n+ ions (ΔE) followed a linear relationship. It was observed that 2-Sc3+ was ca. 3200 times more reactive toward the OAT reaction compared to 2. Hammett analysis of 2 exhibited a V-shaped plot, indicating a change of the reaction mechanism upon going from electron-rich to electron-deficient Ar3P substrates. In contrast, 2-Ca2+ and 2-Sc3+ showed an electrophilic nature toward the OAT reaction, thus demonstrating the role of the Lewis acid in controlling the OAT mechanism. The hydrogen-atom abstraction reaction of 2 and 2-M n+ adducts with 1-benzyl-1,4-dihydronicotinamide was investigated, and it was observed that the rate of reaction did not vary considerably with the Lewis acidity of M n+ ions. On the basis of Eyring analysis of 2 and 2-M n+ adducts, we hypothesized an entropy-controlled hydrogen-atom-transfer reaction for 2-Sc3+, which is different from the reaction mechanism of 2 and 2-Ca2+.

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Phenol Oxidation by a Nickel(III)–Fluoride Complex: Exploring the Influence of the Proton Accepting Ligand in PCET Oxidation

Cited by 10 publications

References 70 publications

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

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

Nonheme Iron(III) Azide and Iron(III) Isothiocyanate Complexes: Radical Rebound Reactivity, Selectivity, and Catalysis

Electrochemical Properties and Reactivity Study of [Mn^V(O)(μ-OR–Lewis Acid)] Cores

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Phenol Oxidation by a Nickel(III)–Fluoride Complex: Exploring the Influence of the Proton Accepting Ligand in PCET Oxidation

Cited by 10 publications

References 70 publications

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

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

Nonheme Iron(III) Azide and Iron(III) Isothiocyanate Complexes: Radical Rebound Reactivity, Selectivity, and Catalysis

Electrochemical Properties and Reactivity Study of [MnV(O)(μ-OR–Lewis Acid)] Cores

Contact Info

Product

Resources

About

Electrochemical Properties and Reactivity Study of [Mn^V(O)(μ-OR–Lewis Acid)] Cores