The Origins of Dramatic Axial Ligand Effects: Closed‐Shell Mn<sup>V</sup>O Complexes Use Exchange‐Enhanced Open‐Shell States to Mediate Efficient H Abstraction Reactions

Janardanan, Deepa; Usharani, Dandamudi; Shaik, Sason

doi:10.1002/ange.201200689

Cited by 12 publications

(19 citation statements)

References 55 publications

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“…170 In a separate computational study, Shaik and coworkers proposed that the origin of the rate enhancement was due to a low-energy triplet transition state, as the most likely C–H bond cleavage pathway. 171 Higher-level NEVPT2:CAS calculations have shown that the energetically favored ground state is the singlet state for the same six-coordinate complexes, and showed that the singlet-triplet gap is much larger than predicted by the former DFT calculations. The six-coordinate complexes [Mn V (O)(X)(TBP 8 Cz)] − (X = CN − , F − ) also exhibited dramatically enhanced OAT rates relative to the non-axially ligated Mn V (O)(TBP 8 Cz) in reactions with sulfide substrates, including para -substituted thioanisole derivatives.…”

Section: Metal-oxo Porphyrinoid Complexes As Models For Biological Oxmentioning

confidence: 91%

Biomimetic Reactivity of Oxygen-Derived Manganese and Iron Porphyrinoid Complexes

2017

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Heme proteins utilize the heme cofactor, an iron porphyrin, to perform a diverse range of reactions including dioxygen binding and transport, electron transfer, and oxidation/oxygenations. These reactions share several, key metalloporphyrin intermediates, typically derived from dioxygen and its congeners such as hydrogen peroxide, and which are comprised of metal-dioxygen, metal-superoxo, metal-peroxo, and metal-oxo adducts. A wide variety of synthetic metalloporphyrinoid complexes have been synthesized to generate and stabilize these intermediates, and then employed to determine the spectroscopic features, structures, and reactivities of such species in controlled and well-defined environments. In this review, we summarize recent findings on the reactivity of these species with common porphyrinoid scaffolds employed for biomimetic studies. The proposed mechanisms of action are emphasized. The review is organized by structural type of metal-oxygen intermediate, and broken into subsections based on the metal (manganese, iron) and porphyrinoid ligand (porphyrin, corrole, corrolazine).

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Section: Metal-oxo Porphyrinoid Complexes As Models For Biological Oxmentioning

confidence: 91%

Biomimetic Reactivity of Oxygen-Derived Manganese and Iron Porphyrinoid Complexes

2017

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“…This stabilization energy is comparable to the calculated energy barrier difference between singlet and triplet pathways for this species. 311 The kinetic isotope effect (KIE) for hydrogen/deuterium HAT has been proposed to be a good mechanistic probe to probe the reactive spin state of a reaction. Theoretical calculations have shown that different spin states can display very different KIEs.…”

Section: C−h Functionalization Reactions Mediated By Oxoiron(iv) Porp...mentioning

confidence: 97%

“…This concept can be utilized to analyze the preference for the triplet state in oxoMn(V)-mediated C–H activation in Figure 32 . 311 The transition state on the S = 0 surface is an open shell singlet with no spin-exchange interactions, while for the triplet surface, three exchange interactions are present. The overall stabilization effect in the triplet state can be estimated by 3 K dd – Δ E orb ( d xy → d xz/yz ), in which Δ E orb ( d xy → d xz/yz ) is the energy needed for orbital excitation from d xy to d xz/yz .…”

Section: C–h Functionalization Reactions Mediated By Oxoiron(iv) Porpmentioning

confidence: 99%

“…This stabilization energy is comparable to the calculated energy barrier difference between singlet and triplet pathways for this species. 311 …”

Section: C–h Functionalization Reactions Mediated By Oxoiron(iv) Porpmentioning

confidence: 99%

“…A computational study by Shaik indicated that fluoride and cyanide also stabilized the triplet transition state by 14–16 kcal/mol, due to a reduction in the σ xy → π* xz/yz excitation energy and an increase in exchange coupling of unpaired d electrons in the triplet transition state. 311 However, DFT and CASSCF calculations by the Goldberg and de Visser groups on Mn V (O)(Cz)-mediated sulfoxidation of thioanisole indicated small spin–orbit coupling constants, suggesting a singlet-to-triplet spin-state crossing before the rate-limiting step is unlikely. 320 In support of this statement, only the singlet spin state can computationally reproduce a V-shaped pattern observed in a Hammett analysis with para -substituted thioanisoles, in which a negative slope was obtained for electro-donating para -substituents but a positive slope was observed for electron-withdrawing substituents.…”

Section: C–h Functionalization Reactions Mediated By Oxoiron(iv) Porpmentioning

confidence: 99%

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Oxygen Activation and Radical Transformations in Heme Proteins and Metalloporphyrins

2017

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As a result of the adaptation of life to an aerobic environment, nature has evolved a panoply of metalloproteins for oxidative metabolism and protection against reactive oxygen species. Despite the diverse structures and functions of these proteins, they share common mechanistic grounds. An open-shell transition metal like iron or copper is employed to interact with O2 and its derived intermediates such as hydrogen peroxide to afford a variety of metal–oxygen intermediates. These reactive intermediates, including metal-superoxo, -(hydro)peroxo, and high-valent metal–oxo species, are the basis for the various biological functions of O2-utilizing metalloproteins. Collectively, these processes are called oxygen activation. Much of our understanding of the reactivity of these reactive intermediates has come from the study of heme-containing proteins and related metalloporphyrin compounds. These studies not only have deepened our understanding of various functions of heme proteins, such as O2 storage and transport, degradation of reactive oxygen species, redox signaling, and biological oxygenation, etc., but also have driven the development of bioinorganic chemistry and biomimetic catalysis. In this review, we survey the range of O2 activation processes mediated by heme proteins and model compounds with a focus on recent progress in the characterization and reactivity of important iron–oxygen intermediates. Representative reactions initiated by these reactive intermediates as well as some context from prior decades will also be presented. We will discuss the fundamental mechanistic features of these transformations and delineate the underlying structural and electronic factors that contribute to the spectrum of reactivities that has been observed in nature as well as those that have been invented using these paradigms. Given the recent developments in biocatalysis for non-natural chemistries and the renaissance of radical chemistry in organic synthesis, we envision that new enzymatic and synthetic transformations will emerge based on the radical processes mediated by metalloproteins and their synthetic analogs.

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Spin‐Regulated Electron Transfer and Exchange‐Enhanced Reactivity in Fe₄S₄‐Mediated Redox Reaction of the Dph2 Enzyme During the Biosynthesis of Diphthamide

2021

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The [4Fe-4S]-dependent radical S-adenosylmethionine (SAM) proteins is one of large families of redox enzymes that are able to carry apanoply of challenging transformations. Despite the extensive studies of structure-function relationships of radical SAM (RS) enzymes,t he electronic statedependent reactivity of the [4Fe-4S] cluster in these enzymes remains elusive.U sing combined MD simulations and QM/ MM calculations,wedeciphered the electronic state-dependent reactivity of the [4Fe-4S] cluster in Dph2, ak ey enzyme involved in the biosynthesis of diphthamide.O ur calculations show that the reductive cleavage of the S À C (g) bond is highly dependent on the electronic structure of [4Fe-4S].Interestingly, the six electronic states can be classified into alow-energy and ahigh-energy groups,whichare correlated with the net spin of Fe4atom ligated to SAM. Due to the driving force of Fe4ÀC (g) bonding,the net spin on the Fe4moiety dictate the shift of the opposite spin electron from the Fe1-Fe2-Fe3 blockt oS AM. Such spin-regulated electron transfer results in the exchangeenhanced reactivity in the lower-energy group compared with those in the higher-energy group.T his reactivity principle provides fundamental mechanistic insights into reactivities of [4Fe-4S] cluster in RS enzymes.

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The Origins of Dramatic Axial Ligand Effects: Closed‐Shell Mn^VO Complexes Use Exchange‐Enhanced Open‐Shell States to Mediate Efficient H Abstraction Reactions

Cited by 12 publications

References 55 publications

Biomimetic Reactivity of Oxygen-Derived Manganese and Iron Porphyrinoid Complexes

Biomimetic Reactivity of Oxygen-Derived Manganese and Iron Porphyrinoid Complexes

Oxygen Activation and Radical Transformations in Heme Proteins and Metalloporphyrins

Spin‐Regulated Electron Transfer and Exchange‐Enhanced Reactivity in Fe₄S₄‐Mediated Redox Reaction of the Dph2 Enzyme During the Biosynthesis of Diphthamide

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The Origins of Dramatic Axial Ligand Effects: Closed‐Shell MnVO Complexes Use Exchange‐Enhanced Open‐Shell States to Mediate Efficient H Abstraction Reactions

Cited by 12 publications

References 55 publications

Biomimetic Reactivity of Oxygen-Derived Manganese and Iron Porphyrinoid Complexes

Biomimetic Reactivity of Oxygen-Derived Manganese and Iron Porphyrinoid Complexes

Oxygen Activation and Radical Transformations in Heme Proteins and Metalloporphyrins

Spin‐Regulated Electron Transfer and Exchange‐Enhanced Reactivity in Fe4S4‐Mediated Redox Reaction of the Dph2 Enzyme During the Biosynthesis of Diphthamide

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The Origins of Dramatic Axial Ligand Effects: Closed‐Shell Mn^VO Complexes Use Exchange‐Enhanced Open‐Shell States to Mediate Efficient H Abstraction Reactions

Spin‐Regulated Electron Transfer and Exchange‐Enhanced Reactivity in Fe₄S₄‐Mediated Redox Reaction of the Dph2 Enzyme During the Biosynthesis of Diphthamide