Transition-Metal Hydride Radical Cations

Hu, Yue; Shaw, Anthony Peter Gordon; Estes, Deven P.; Norton, Jack R.

doi:10.1021/acs.chemrev.5b00532

Cited by 74 publications

(100 citation statements)

References 142 publications

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“…15d,28 The BDE M–H values measured in redox-active metal hydride species also display significant oxidation state dependence, but in this case the trend is reversed. 29 The equivalent BDE N–H values observed for P 3 Si Fe≡C–N(Me)H underscores their similar hybridizations at N(sp) and also likely reflects similar degrees of C–N bond weakening upon H atom addition to P 3 Si Fe– CNMe +/0 . Strong backbonding from Fe to C in the isocyanide P 3 Si Fe=C=NMe causes bending at N, favoring a resonance contributor with significant Fe to C π -bonding (as opposed to P 3 Si Fe–C≡NMe).…”

Section: Resultsmentioning

confidence: 77%

N–H Bond Dissociation Enthalpies and Facile H Atom Transfers for Early Intermediates of Fe–N₂ and Fe–CN Reductions

Rittle

Peters

2017

J. Am. Chem. Soc.

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Fe-mediated biological nitrogen fixation is thought to proceed via either a sequence of proton and electron transfer steps, concerted H atom transfer steps, or some combination thereof. Regardless of the specifics and whether the intimate mechanism for N2-to-NH3 conversion involves a distal pathway, an alternating pathway, or some hybrid of these limiting scenarios, Fe–NxHy intermediates are implicated that feature reactive N–H bonds. Thermodynamic knowledge of the N–H bond strengths of such species is scant, and is especially difficult to obtain for the most reactive early stage candidate intermediates (e.g., Fe–N=NH, Fe=N–NH2, Fe–NH=NH). Such knowledge is essential to considering various mechanistic hypotheses for biological (and synthetic) nitrogen fixation and to the rational design of improved synthetic N2 fixation catalysts. We recently reported several reactive complexes derived from the direct protonation of Fe–N2 and Fe–CN species at the terminal N atom (e.g., Fe=N–NH2, Fe–C≡NH, Fe≡C–NH2). These same Fe–N2 and Fe–CN systems are functionally active for N2-to-NH3 and CN-to-CH4/NH3 conversion, respectively, when subjected to protons and electrons, and hence provide an excellent opportunity for obtaining meaningful N–H bond strength data. We report here a combined synthetic, structural, and spectroscopic/analytic study to estimate the N–H bond strengths of several species of interest. We assess the reactivity profiles of species featuring reactive N–H bonds and estimate their homolytic N–H bond enthalpies via redox and acidity titrations. Very low N–H bond dissociation enthalpies (BDEN–H), ranging from 65 (e.g., Fe–C≡NH) to ≤37 kcal/mol (Fe–N=NH), are determined. The collective data presented herein provides insight into the facile reactivity profiles of early stage protonated Fe–N2 and Fe–CN species.

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Section: Resultsmentioning

confidence: 77%

N–H Bond Dissociation Enthalpies and Facile H Atom Transfers for Early Intermediates of Fe–N₂ and Fe–CN Reductions

Rittle

Peters

2017

J. Am. Chem. Soc.

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“…Such reactions would harness the controllable selectivity of metal-based hydrogen-abstracting intermediate in C–H activation step, which is hard to achieve with simple organic or inorganic radicals. In another case, carbon radicals could be generated with other radical initiation methods such as photoredox reactions, electrolysis, or other single-electron transfer (SET) processes, and later be captured by metal complexes [213–215]. The works of Kochi in 1970s have provided early examples of this type of reactions.…”

Section: Harnessing the Radical Rebound Mechanism For Novel Organic Rmentioning

confidence: 99%

Beyond ferryl-mediated hydroxylation: 40 years of the rebound mechanism and C–H activation

2016

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Since our initial report in 1976, the oxygen rebound mechanism has become the consensus mechanistic feature for an expanding variety of enzymatic C–H functionalization reactions and small molecule biomimetic catalysts. For both the biotransformations and models, an initial hydrogen atom abstraction from the substrate (R–H) by high-valent iron-oxo species (Fen=O) generates a substrate radical and a reduced iron hydroxide, [Fen−1–OH ·R]. This caged radical pair then evolves on a complicated energy landscape through a number of reaction pathways, such as oxygen rebound to form R–OH, rebound to a non-oxygen atom affording R–X, electron transfer of the incipient radical to yield a carbocation, R+, desaturation to form olefins, and radical cage escape. These various flavors of the rebound process, often in competition with each other, give rise to the wide range of C–H functionalization reactions performed by iron-containing oxygenases. In this review, we first recount the history of radical rebound mechanisms, their general features, and key intermediates involved. We will discuss in detail the factors that affect the behavior of the initial caged radical pair and the lifetimes of the incipient substrate radicals. Several representative examples of enzymatic C–H transformations are selected to illustrate how the behaviors of the radical pair [Fen−1–OH ·R] determine the eventual reaction outcome. Finally, we discuss the powerful potential of “radical rebound” processes as a general paradigm for developing novel C–H functionalization reactions with synthetic, biomimetic catalysts. We envision that new chemistry will continue to arise by bridging enzymatic “radical rebound” with synthetic organic chemistry.

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“…41,42 We have calculated the BDE C–H ’s for both endo - and exo -Cp*Co(η 4 -C 5 Me 5 H) + as 31 kcal/mol (Figure 3B; Table 2), indicating that they should be among the strongest PCET reagents accessible in this catalyst cocktail. Indeed, they would be among the strongest PCET reagents known.…”

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confidence: 98%

Catalytic N₂-to-NH₃ Conversion by Fe at Lower Driving Force: A Proposed Role for Metallocene-Mediated PCET

et al. 2017

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We have recently reported on several Fe catalysts for N2-to-NH3 conversion that operate at low temperature (−78 °C) and atmospheric pressure while relying on a very strong reductant (KC8) and acid ([H(OEt2)2][BArF4]). Here we show that our original catalyst system, P3BFe, achieves both significantly improved efficiency for NH3 formation (up to 72% for e– delivery) and a comparatively high turnover number for a synthetic molecular Fe catalyst (84 equiv of NH3 per Fe site), when employing a significantly weaker combination of reductant (Cp*2Co) and acid ([Ph2NH2][OTf] or [PhNH3][OTf]). Relative to the previously reported catalysis, freeze-quench Mössbauer spectroscopy under turnover conditions suggests a change in the rate of key elementary steps; formation of a previously characterized off-path borohydrido–hydrido resting state is also suppressed. Theoretical and experimental studies are presented that highlight the possibility of protonated metallocenes as discrete PCET reagents under the present (and related) catalytic conditions, offering a plausible rationale for the increased efficiency at reduced driving force of this Fe catalyst system.

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Transition-Metal Hydride Radical Cations

Cited by 74 publications

References 142 publications

N–H Bond Dissociation Enthalpies and Facile H Atom Transfers for Early Intermediates of Fe–N₂ and Fe–CN Reductions

N–H Bond Dissociation Enthalpies and Facile H Atom Transfers for Early Intermediates of Fe–N₂ and Fe–CN Reductions

Beyond ferryl-mediated hydroxylation: 40 years of the rebound mechanism and C–H activation

Catalytic N₂-to-NH₃ Conversion by Fe at Lower Driving Force: A Proposed Role for Metallocene-Mediated PCET

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Transition-Metal Hydride Radical Cations

Cited by 74 publications

References 142 publications

N–H Bond Dissociation Enthalpies and Facile H Atom Transfers for Early Intermediates of Fe–N2 and Fe–CN Reductions

N–H Bond Dissociation Enthalpies and Facile H Atom Transfers for Early Intermediates of Fe–N2 and Fe–CN Reductions

Beyond ferryl-mediated hydroxylation: 40 years of the rebound mechanism and C–H activation

Catalytic N2-to-NH3 Conversion by Fe at Lower Driving Force: A Proposed Role for Metallocene-Mediated PCET

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Product

Resources

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N–H Bond Dissociation Enthalpies and Facile H Atom Transfers for Early Intermediates of Fe–N₂ and Fe–CN Reductions

N–H Bond Dissociation Enthalpies and Facile H Atom Transfers for Early Intermediates of Fe–N₂ and Fe–CN Reductions

Catalytic N₂-to-NH₃ Conversion by Fe at Lower Driving Force: A Proposed Role for Metallocene-Mediated PCET