Copper(II) complexes of a heterotopic N- heterocyclic carbene ligand: Preparation and catalytic application

Lin, Shih-Chieh Aaron; Liu, Yi-Hung; Peng, Shie‐Ming; Liu, Shiuh‐Tzung

doi:10.1016/j.jorganchem.2018.01.053

“…N -Heterocyclic carbenes (NHCs) are renowned for their efficacy as ligands in homogeneous metal catalysis for many reactions within contemporary organic synthesis. − Over the past decade, NHCs have also emerged as versatile molecular modifiers for the metal surfaces of heterogeneous catalysts. − The broad application of NHC ligands in catalysis is largely attributed to their adaptable steric and electronic parameters, which allow for effective modulation of catalytic activity. − Moreover, their strong affinity for transition metals ensures the stability and performance of M/NHC catalytic systems. ,− However, metal complexes paired with NHC ligands (M/NHC) can undergo transformations that entail cleaving the M–C(NHC) bond, resulting in NHC-detached metal species. − These bond cleavage reactions can be reversible or irreversible and frequently contribute to the progressive deactivation of many M/NHC catalytic systems. ,, Notably, the breakage of M-NHC bonds can lead to the emergence of new NHC-disconnected metal species and often to the formation of novel metal nanoparticles . These new entities can exhibit distinct catalytic activities and, in some reactions, function as the primary active centers. , Such metal–NHC bond cleavage is common across various metals and NHC ligands and can significantly influence the activity, selectivity, stability, and catalysis mechanism of M/NHC systems …”

Section: Introductionmentioning

confidence: 99%

Quantitative Determination of Active Species Transforming the R-NHC Coupling Process under Catalytic Conditions

Patil,

Burykina,

Eremin

et al. 2024

Inorg. Chem.

0

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Palladium complexes with N-heterocyclic carbenes (Pd/NHC) serve as prominent precatalysts in numerous Pd-catalyzed organic reactions. While the evolution of Pd/NHC complexes, which involves the cleavage of the Pd− C(NHC) bond via reductive elimination and dissociation, is acknowledged to influence the catalysis mechanism and the performance of the catalytic systems, conventional analytic techniques [such as NMR, IR, UV−vis, gas chromatography−mass spectrometry (GC−MS), and high-performance liquid chromatography (HPLC)] frequently fail to quantitatively monitor the transformations of Pd/NHC complexes at catalyst concentrations typical of real-world conditions (below approximately 1 mol %). In this study, for the first time, we show the viability of using electrospray ionization mass spectrometry (ESI-MS). This approach was combined with the use of selectively deuterated H-NHC, Ph-NHC, and O-NHC coupling products as internal standards, allowing for an in-depth quantitative analysis of the evolution of Pd/NHC catalysts within actual catalytic systems. The reliability of this approach was affirmed by aligning the ESI-MS results with the NMR spectroscopy data obtained at greater Pd/NHC precatalyst concentrations (2−5 mol %) in the Mizoroki−Heck, Sonogashira, and alkyne transfer hydrogenation reactions. The efficacy of the ESI-MS methodology was further demonstrated through its application in the Mizoroki−Heck reaction at Pd/NHC loadings of 5, 0.5, 0.05, and 0.005 mol %. In this work, for the first time, we present a methodology for the quantitative characterization of pivotal catalyst transformation processes commonly observed in M/NHC systems.

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An Adaptable N‐Heterocyclic Carbene Macrocycle Hosting Copper in Three Oxidation States

Liu

¹

,

Resch

²

,

Klawitter

³

et al. 2020

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A neutral hybrid macrocycle with two trans‐positioned N‐heterocyclic carbenes (NHCs) and two pyridine donors hosts copper in three oxidation states (+I–+III) in a series of structurally characterized complexes (1–3). Redox interconversion of [LCu]+/2+/3+ is electrochemically (quasi)reversible and occurs at moderate potentials (E1/2=−0.45 V and +0.82 V (vs. Fc/Fc+)). A linear CNHC‐Cu‐CNHC arrangement and hemilability of the two pyridine donors allows the ligand to adapt to the different stereoelectronic and coordination requirements of CuI versus CuII/CuIII. Analytical methods such as NMR, UV/Vis, IR, electron paramagnetic resonance, and Cu Kβ high‐energy‐resolution fluorescence detection X‐ray absorption spectroscopies, as well as DFT calculations, give insight into the geometric and electronic structures of the complexes. The XAS signatures of 1–3 are textbook examples for CuI, CuII, and CuIII species. Facile 2‐electron interconversion combined with the exposure of two basic pyridine N sites in the reduced CuI form suggest that [LCu]+/2+/3+ may operate in catalysis via coupled 2 e−/2 H+ transfer.

show abstract

An Adaptable N‐Heterocyclic Carbene Macrocycle Hosting Copper in Three Oxidation States

Liu

¹

,

Resch

²

,

Klawitter

³

et al. 2020

View full text Add to dashboard Cite

A neutral hybrid macrocycle with two trans‐positioned N‐heterocyclic carbenes (NHCs) and two pyridine donors hosts copper in three oxidation states (+I–+III) in a series of structurally characterized complexes (1–3). Redox interconversion of [LCu]+/2+/3+ is electrochemically (quasi)reversible and occurs at moderate potentials (E1/2=−0.45 V and +0.82 V (vs. Fc/Fc+)). A linear CNHC‐Cu‐CNHC arrangement and hemilability of the two pyridine donors allows the ligand to adapt to the different stereoelectronic and coordination requirements of CuI versus CuII/CuIII. Analytical methods such as NMR, UV/Vis, IR, electron paramagnetic resonance, and Cu Kβ high‐energy‐resolution fluorescence detection X‐ray absorption spectroscopies, as well as DFT calculations, give insight into the geometric and electronic structures of the complexes. The XAS signatures of 1–3 are textbook examples for CuI, CuII, and CuIII species. Facile 2‐electron interconversion combined with the exposure of two basic pyridine N sites in the reduced CuI form suggest that [LCu]+/2+/3+ may operate in catalysis via coupled 2 e−/2 H+ transfer.

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Copper(II) complexes of a heterotopic N- heterocyclic carbene ligand: Preparation and catalytic application

Cited by 10 publications

References 19 publications

Quantitative Determination of Active Species Transforming the R-NHC Coupling Process under Catalytic Conditions

Quantitative Determination of Active Species Transforming the R-NHC Coupling Process under Catalytic Conditions

An Adaptable N‐Heterocyclic Carbene Macrocycle Hosting Copper in Three Oxidation States

An Adaptable N‐Heterocyclic Carbene Macrocycle Hosting Copper in Three Oxidation States

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