Cp* Iridium Complexes Give Catalytic Alkane Hydroxylation with Retention of Stereochemistry

Zhou, Meng; Schley, Nathan D.; Crabtree, Robert H.

doi:10.1021/ja1058247

Cited by 110 publications

(87 citation statements)

References 15 publications

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“…Since then a wide number of molecular iridium precursors have been reported, with half sandwich iridium compounds showing the highest activities ,. Although the exact nature of the active species is still a matter of debate, it has been shown that the Cp*Ir III complexes are precursors which undergo oxidative activation with loss of the Cp* ligand, either chemically, or electrochemically, before entering catalysis . A crucial feature of the most effective members of that family is an oxidatively robust chelate ligand that remains bound to the iridium to prevent decomposition into IrO x and modulates the active site ,.…”

Section: Introductionmentioning

confidence: 99%

Electrochemical and Kinetic Insights into Molecular Water Oxidation Catalysts Derived from Cp*Ir(pyridine‐alkoxide) Complexes

2018

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We report the solution‐phase electrochemistry of seven half‐sandwich iridium(III) complexes with varying pyridine‐alkoxide ligands to quantify electronic ligand effects that translate to their activity in catalytic water oxidation. Our results unify some previously reported electrochemical data of Cp*Ir complexes by showing how the solution speciation determines the electrochemical response: cationic complexes show over 1 V higher redox potentials that their neutral forms in a distinct demonstration of charge accumulation effects relevant to water oxidation. Building on previous work that analysed the activation behaviour of our pyalk‐ligated Cp*Ir complexes 1–7, we assess their catalytic oxygen evolution activity with sodium periodate (NaIO4) and ceric ammonium nitrate (CAN) in water and aqueous tBuOH solution. Mechanistic studies including H/D kinetic isotope effects and reaction progress kinetic analysis (RPKA) of oxygen evolution point to a dimer‐monomer equilibrium of the IrIV resting state preceding a proton‐coupled electron transfer (PCET) in the turnover‐limiting step (TLS). Finally, true electrochemically driven water oxidation is demonstrated for all catalysts, revealing surprising trends in activity that do not correlate with those obtained using chemical oxidants.

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Section: Introductionmentioning

confidence: 99%

Electrochemical and Kinetic Insights into Molecular Water Oxidation Catalysts Derived from Cp*Ir(pyridine‐alkoxide) Complexes

2018

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“…11,12,20–22 Both Cp* (Cp* = pentamethylcyclopentadienyl) and Cp (Cp = cyclopentadienyl) half-sandwich Ir III complexes are viable precatalysts, not only for water oxidation but also selective CH oxidations. 23,24 We have used NaIO 4 as a primary oxidant in this chemistry because of its ease of handling, good solubility, and lack of absorptions in the visible and near-UV. The disadvantage is that we cannot be certain if the reaction catalyzed in this case is true water oxidation rather than periodate dismutation: 2 IO 4 − → 2 IO 3 − + O 2 .…”

Section: Introductionmentioning

confidence: 99%

Probing the Viability of Oxo-Coupling Pathways in Iridium-Catalyzed Oxygen Evolution

et al. 2013

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A series of Cp*IrIII dimers have been synthesized to elucidate the mechanistic viability of radical oxo-coupling pathways in iridium-catalyzed O2 evolution. The oxidative stability of the precursors toward nanoparticle formation and their oxygen evolution activity have been investigated and compared to suitable monomeric analogues. We found that precursors bearing monodentate NHC ligands degraded to form nanoparticles (NPs), and accordingly their O2 evolution rates were not significantly influenced by their nuclearity or distance between the two metals in the dimeric precursors. A doubly chelating bis-pyridine–pyrazolide ligand provided an oxidation-resistant ligand framework that allowed a more meaningful comparison of catalytic performance of dimers with their corresponding monomers. With sodium periodate (NaIO4) as the oxidant, the dimers provided significantly lower O2 evolution rates per [Ir] than the monomer, suggesting a negative interaction instead of cooperativity in the catalytic cycle. Electrochemical analysis of the dimers further substantiates the notion that no radical oxyl-coupling pathways are accessible. We thus conclude that the alternative path, nucleophilic attack of water on high-valent Ir-oxo species, may be the preferred mechanistic pathway of water oxidation with these catalysts, and bimolecular oxo-coupling is not a valid mechanistic alternative as in the related ruthenium chemistry, at least in the present system.

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“…The translation of lab-scale aqueous procedures ('in water' 'on water' or 'by water' 81 ) to the industrial level can be seen in the same 'green' light, courtesy in part to the design of water-soluble reagents: note, for example, the application of copper sulfonatosalen complexes for amination of aryl halides in water 82 and copper 2-pyridylaldoxime complexes for the the aqueous conversion of aryl halides to phenols 83 . Looking more abstractly at the current highlights, mention might be given to developments in alkane oxidation: note the angular hydroxylation of bicyclic hydrocarbons with Ce(IV) and a catalytic amount of Ir(III) 84 , and the aerobic oxidation of cycloalkanes with robust copper-containing ceramic material (Cu@SiCN), in which the copper content can be simply varied to adjust selectivity 85 . An interesting terminal functionalization of alkanes in one pot is also at hand via iridium-catalyzed dehydrogenation to the terminal alkene and subsequent regiospecific hydrozirconation prior to electrophilic substitution 86 .…”

Section: Further Trends and Developments In Synthetic Organic Chemistmentioning

confidence: 99%

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2011

Theilheimer's Synthetic Methods of Organic Chemistry

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Cp* Iridium Complexes Give Catalytic Alkane Hydroxylation with Retention of Stereochemistry

Cited by 110 publications

References 15 publications

Electrochemical and Kinetic Insights into Molecular Water Oxidation Catalysts Derived from Cp*Ir(pyridine‐alkoxide) Complexes

Electrochemical and Kinetic Insights into Molecular Water Oxidation Catalysts Derived from Cp*Ir(pyridine‐alkoxide) Complexes

Probing the Viability of Oxo-Coupling Pathways in Iridium-Catalyzed Oxygen Evolution

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