Harnessing the active site triad: merging hemilability, proton responsivity, and ligand-based redox-activity

Baumgardner, Douglas F.; Gilbertson, John D.

doi:10.1039/c9dt04470a

Cited by 11 publications

(12 citation statements)

References 73 publications

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“…Our work has revealed that the polypyridyl ppq ligand is the major electron acceptor during sequential redox processes of the cobalt complex and that the Co intermediate with a triply reduced ppq radical ligand promotes the hydrogen evolution reaction. Even though the noninnocent redox properties of various ligands have been demonstrated, − multiple redox forms of a polypyridine ligand bound to 3d transition metals are rarely characterized or discussed with respect to HER catalysts. , …”

Section: Introductionmentioning

confidence: 99%

Electrocatalytic Hydrogen Evolution by Cobalt Complexes with a Redox Non-Innocent Polypyridine Ligand

et al. 2021

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Novel cobalt and zinc complexes with the tetradentate ppq (8-(1″,10″-phenanthrol-2″-yl)-2-(pyrid-2′-yl)quinoline) ligand have been synthesized and fully characterized. Electrochemical measurements have shown that the formal monovalent complex [Co(ppq)(PPh 3 )] + (2) undergoes two stepwise ligand-based electroreductions in DMF, affording a [Co(ppq)-DMF] −1 species. Theoretical calculations have described the electronic structure of [Co(ppq)DMF] −1 as a low-spin Co(II) center coupling with a triple-reduced ppq radical ligand. In the presence of triethylammonium as the proton donor, the cobalt complex efficiently drives electrocatalytic hydrogen evolution with a maximum turnover frequency of thousands per second. A mechanistic investigation proposes an EECC H 2 -evolving pathway, where the second ligand-based redox process (E), generating the [Co(ppq)DMF] −1 intermediate, initiates proton reduction, and the second proton transfer process (C) is the rate-determining step. This work provides a unique example for understanding the role of redoxactive ligands in electrocatalytic H 2 evolution by transition metal sites.

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Section: Introductionmentioning

confidence: 99%

Electrocatalytic Hydrogen Evolution by Cobalt Complexes with a Redox Non-Innocent Polypyridine Ligand

et al. 2021

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“…Enzymatic systems and, in particular, metalloenzymes mediate a fascinating array of reactions via a carefully evolved secondary coordination sphere. Enzyme active sites leverage effects such as hydrogen bonding, electron transfer pathways, and electrostatic effects to precisely tune the reactivity of metallocofactors. ,,,− One example illustrating the importance of the secondary coordination sphere is in oxidase chemistry. For example, the terminal oxidant in cytochrome P450 enzymes, Compound I, consists of an Fe(IV)-oxo which is generated from O 2 via the delivery of proton and electron equivalents from cofactors and the protein superstructure.…”

Section: Introductionmentioning

confidence: 99%

Direct Aerobic Generation of a Ferric Hydroperoxo Intermediate Via a Preorganized Secondary Coordination Sphere

et al. 2021

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Enzymes exert control over the reactivity of metal centers with precise tuning of the secondary coordination sphere of active sites. One particularly elegant illustration of this principle is in the controlled delivery of proton and electron equivalents in order to activate abundant but kinetically inert oxidants such as O 2 for oxidative chemistry. Chemists have drawn inspiration from biology in designing molecular systems where the secondary coordination sphere can shuttle protons or electrons to substrates. However, a biomimetic activation of O 2 requires the transfer of both protons and electrons, and molecular systems where ancillary ligands are designed to provide both of these equivalents are comparatively rare. Here, we report the use of a dihydrazonopyrrole (DHP) ligand complexed to Fe to perform exactly such a biomimetic activation of O 2 . In the presence of O 2 , this complex directly generates a high spin Fe(III)-hydroperoxo intermediate which features a DHP • ligand radical via ligand-based transfer of an H atom. This system displays oxidative reactivity and ultimately releases hydrogen peroxide, providing insight on how secondary coordination sphere interactions influence the evolution of oxidizing intermediates in Fe-mediated aerobic oxidations.

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“…Transient coordination of a carboxylate additive (e.g., acetate, pivalate) is also a key feature of C–H activation catalysis. , The ligand switches from a κ 2 -O,O to κ 1 -O mode in order for the pendent oxygen to abstract a proton from the activated C–H bond, a mechanism that is commonly referred to as concerted-metalation deprotonation (CMD) or ambiphilic metal ligand activation (AMLA; Scheme b). , In this case, the carboxylate is ultimately a stoichiometric reagent, but there are many other examples of supporting ligands that can behave in both a structurally responsive and proton-responsive manner …”

Section: Structurally-responsive Ligands: Historic Background and Gen...mentioning

confidence: 99%

“…Recent reviews have focused on specific ligand types, such as versatile coordination pincers and hemilabile ligands with π-bonding moieties. , The current contribution showcases a variety of ligand types, that all modulate the reactivity of a metal by changing the coordination mode (Figure a). In nature, dynamic changes in the primary coordination sphere of metalloenzymes commonly influences function. − Examples include amino acid residues that dissociate to open a site for substrate binding or to assist in proton and/or electron shuttling. − For example, the function of Cytochrome c can switch from electron transfer to peroxidase due to changes bonding between iron and a methionine residue, which is one of the axial ligands (Figure b). , The mechanism that controls this switch in reactivity is complex and is still being elucidated. , However, it likely involves a weakening of the Fe–methionine interaction, coordination of a lysine residue to the axial site, and also an increased prominence of five-coordinate iron.…”

Section: Introductionmentioning

confidence: 99%

Structurally-Responsive Ligands for High-Performance Catalysts

Blacquiere

2021

ACS Catal.

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Hemilabile ligands that undergo reversible changes in denticity have long been exploited to enable catalyst activation and to increase lifetime. However, this common description does not fully reflect reality with respect to the diversity of known ligand types, modes of action, and impacts on catalysis. The term structurally responsive ligand (SRL) is employed here to encompass ligands that exhibit selftuning denticity, hapticity, or versatile coordination. This Perspective covers case study examples of SRLs in catalysis and their influence on catalyst lifetime, rate, and selectivity. The examples include leading and emerging catalysts for enabling transformations, which emphasizes both the breadth and significance of this class of ligands.

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Harnessing the active site triad: merging hemilability, proton responsivity, and ligand-based redox-activity

Abstract: Metalloenzymes catalyze important reactions by managing the proton and electron flux at the active site. In synthetic systems; hemilability, proton responsivity, and ligand-based redox-activity can be utilized as a bridge to harness this reactivity.

Cited by 11 publications

References 73 publications

Electrocatalytic Hydrogen Evolution by Cobalt Complexes with a Redox Non-Innocent Polypyridine Ligand

Electrocatalytic Hydrogen Evolution by Cobalt Complexes with a Redox Non-Innocent Polypyridine Ligand

Direct Aerobic Generation of a Ferric Hydroperoxo Intermediate Via a Preorganized Secondary Coordination Sphere

Structurally-Responsive Ligands for High-Performance Catalysts

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