Pincer ligands incorporating pyrrolyl units: Versatile platforms for organometallic chemistry and catalysis

Thompson, C. Vance; Tonzetich, Zachary J.

doi:10.1016/bs.adomc.2020.04.004

Cited by 11 publications

(12 citation statements)

References 137 publications

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“…Synthesis of 8 was accomplished by addition of vinyl Grignard to [FeCl( t Bu PNP)] in thf (Scheme ). 1 H NMR spectra of 8 display several paramagnetically shifted resonances akin to other S = 1 species of ( t Bu PNP)Fe II . Notably, the tert -butyl groups of 8 occur as a single resonance near −20 ppm, which is essentially identical to that observed for 7 , further supporting the latter as a vinyl species.…”

Section: Resultsmentioning

confidence: 61%

Mechanistic Studies of Alkyne Hydroboration by a Well-Defined Iron Pincer Complex: Direct Comparison of Metal-Hydride and Metal-Boryl Reactivity

2022

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Iron-hydride and iron-boryl complexes supported by a pyrrole-based pincer ligand, tBuPNP (PNP = anion of 2,5-bis(di-tert-butylphosphinomethyl)pyrrole), were employed for a detailed mechanistic study on the hydroboration of internal alkynes. Several novel complexes were isolated and fully characterized, including iron-vinyl and iron-boryl species, which represent likely intermediates in the catalytic hydroboration pathway. In addition, the products of alkyne insertion into the Fe–B bond have been isolated and structurally characterized. Mechanistic studies of the hydroboration reaction favor a pathway involving an active iron-hydride species, [FeH( tBuPNP)], which readily inserts alkyne and undergoes subsequent reaction with hydroborane to generate product. The iron-boryl species, [Fe(BR2)( tBuPNP)] (R2 = pin or cat), was found to be chemically competent, although its use in catalysis entailed an induction period whereby the iron-hydride species was generated. Stoichiometric reactions and kinetic experiments were performed to paint a fuller picture of the mechanism of alkyne hydroboration, including pathways for catalyst deactivation and the influence of substrate bulk on catalytic efficacy.

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Section: Resultsmentioning

confidence: 61%

Mechanistic Studies of Alkyne Hydroboration by a Well-Defined Iron Pincer Complex: Direct Comparison of Metal-Hydride and Metal-Boryl Reactivity

2022

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“…Under photochemical conditions, CO can dissociate from iron, allowing ethylene to insert into the Fe−H bond and C 6 D 6 to exchange H/D with the hydride. These results add to the understanding of pyrrole‐based pincer complexes, [26] which have the potential to be developed as catalysts involving metal‐ligand cooperation [27] …”

Section: Discussionmentioning

confidence: 80%

An Iron‐Hydrogen Bond Resistant to Protonation and Oxidation

Collett

Ransohoff

Krause

et al. 2021

Zeitschrift anorg allge chemie

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A new synthetic route to cis‐(CyPNP)Fe(CO)2H (CyPNP=2,5‐bis(dicyclohexylphosphinomethyl)pyrrolyl) has been developed, involving the reaction of (CyPNP)Fe(CO)2Br with NaBH4. The Fe−H bond of this iron hydride is remarkably stable to acids (e. g., HBF4 ⋅ Et2O, HCl, and CH3CO2H) and oxidants (e. g., 1,4‐benzoquinone and galvinoxyl), only leading to the protonation of the β‐pyrrolic carbon and the oxidation of the ligand backbone, respectively. Under 369 nm UV irradiation, cis‐(CyPNP)Fe(CO)2H has been shown to react with ethylene to yield cis‐(CyPNP)Fe(CO)2Et and undergo H/D exchange with C6D6 to form cis‐(CyPNP)Fe(CO)2D and C6D5H. Both processes likely proceed via an initial CO dissociation step to yield (CyPNP)Fe(CO)H as a reactive intermediate.

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“…Our laboratory has been investigating the reactivity of cobalt complexes supported by a pyrrole-based PNP ligand, R PNP ( R PNP = anion of bis(dialkylphosphinomethyl)pyrrole). 34 These compounds have shown efficacy in several reductive bond activation reactions including C-X oxidative addition and aldehyde decarbonylation. 35 To further establish the potential of this system in reductive transformations, we turned our attention to the hydrogenation of alkenes and alkynes.…”

Section: ■ Introductionmentioning

confidence: 72%

Section: Introductionmentioning

confidence: 99%

Catalytic Hydrogenation of Alkenes and Alkynes by a Cobalt Pincer Complex: Evidence of Roles for Both Co(I) and Co(II)

2021

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The Co(I) complex, [Co(N 2 )( Cy PNP)] ( Cy PNP = anion of 2,5-bis-(dicyclohexylphosphinomethyl)pyrrole), is active toward the catalytic hydrogenation of terminal alkenes and the semi-hydrogenation of internal alkynes under 2 bar of H 2 (g) at room temperature. The products of alkyne semi-hydrogenation are a mixture of E-and Z-alkenes. By contrast, use of the related cobalt(I) precatalyst, [Co(PMe 3 )-( Cy PNP)], results in formation of exclusively Z-alkenes. A semi-stable Co(II) species, [CoH( Cy PNP)], can also be generated by treatment of degassed solutions of [Co(N 2 )( Cy PNP)] with H 2 . The Co II -hydride displays activity toward both alkene hydrogenation and isomerization, but its instability hampers implementation as a catalyst. Several species relevant to potential catalytic intermediates have been isolated and detected in solution. These compounds include alkene and alkyne adducts of Co(I) as well as a Co(III) dihydride species. Catalytic results with the compounds examined are most consistent with a process involving shuttling between Co(I) and Co(III) states. However, generation of small quantities of Co(II) during catalytic turnover appears to be responsible for the isomerization observed for alkyne semi-hydrogenation. The interplay of cobalt oxidation states within the same catalyst system is discussed in the context of mechanistic scenarios for catalytic hydrogenation.

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Pincer ligands incorporating pyrrolyl units: Versatile platforms for organometallic chemistry and catalysis

Cited by 11 publications

References 137 publications

Mechanistic Studies of Alkyne Hydroboration by a Well-Defined Iron Pincer Complex: Direct Comparison of Metal-Hydride and Metal-Boryl Reactivity

Mechanistic Studies of Alkyne Hydroboration by a Well-Defined Iron Pincer Complex: Direct Comparison of Metal-Hydride and Metal-Boryl Reactivity

An Iron‐Hydrogen Bond Resistant to Protonation and Oxidation

Catalytic Hydrogenation of Alkenes and Alkynes by a Cobalt Pincer Complex: Evidence of Roles for Both Co(I) and Co(II)

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