Selective and adaptable access to N,N′-asymmetrically substituted imidazol-2-ylidene bis-NHC ligands: Pd(ii) complexes featuring wide variation in N-alkyl and aryl steric bulk

Gardiner, Michael G.; Ho, Curtis C.; Mackay, Felicity M.; McGuinness, David S.; Tucker, Matthew

doi:10.1039/c3dt32895c

Cited by 16 publications

(7 citation statements)

References 73 publications

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“…Gardiner and coworkers prepared an unsymmetricalm ethylene-linked Pdbis(NHC) complex with methyl and Mes wingtips, but this seemed to be less active in copolymerization (7 gp olymer per gram of Pd catalyst). [80] In this catalytic test,B Qw as absent from the reactionm edium and ac ontrol experiment with Pdbis(NHC) (analogous to complex 1)b earing aP F 6 À counterion did not give any product. To shed light on the mechanism of copolymerization, [81] Slaughter and Subramanium designed a methylene-bridged Pd-bis(NHC) complex with Mes wingtips together with methyl and CH 3 CN coligands.…”

Section: Copolymerizationmentioning

confidence: 74%

“…Under these optimized conditions, and by employing the complex derived from 2 a , gray poly(C 2 H 4 ‐ alt ‐CO) was obtained (810 g polymer per gram of Pd catalyst) without the formation of oligomers. Gardiner and co‐workers prepared an unsymmetrical methylene‐linked Pd–bis(NHC) complex with methyl and Mes wingtips, but this seemed to be less active in copolymerization (7 g polymer per gram of Pd catalyst) . In this catalytic test, BQ was absent from the reaction medium and a control experiment with Pd–bis(NHC) (analogous to complex 1 ) bearing a PF 6 − counterion did not give any product.…”

Section: Homogeneous Catalysismentioning

confidence: 96%

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Chelating Bis(N‐Heterocyclic Carbene) Palladium‐Catalyzed Reactions

Liu

Gou

Han

2018

Chemistry An Asian Journal

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N-Heterocyclic carbenes (NHCs) have been widely employed in the construction of various metal complexes and applied in the field of catalysis. In this review, a series of palladium complexes comprising chelating bis(NHC) donors (Pd-bis(NHC) complexes) are outlined according to their application in homogeneous catalysis. Examples of metal-organic frameworks containing Pd-bis(NHC) moieties are briefly discussed, in terms of their use in heterogeneous catalysis. The catalytic applications of Pd-bis(NHC) complexes (or moieties in heterogeneous catalysts) are mainly focused on the Mizoroki-Heck, Suzuki-Miyaura, and Sonogashira coupling reactions. Another 14 types of reactions are also summarized beyond these three types of reactions.

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Section: Copolymerizationmentioning

confidence: 74%

Section: Homogeneous Catalysismentioning

confidence: 96%

Chelating Bis(N‐Heterocyclic Carbene) Palladium‐Catalyzed Reactions

Liu

Gou

Han

2018

Chemistry An Asian Journal

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“…Different palladium(II) complexes with bulky monodentate phosphines as well as complexes of bidentate and pincer , ligands have been used, while since the discovery of stable N-heterocyclic carbenes (NHCs) by Arduengo , in 1991, there has been a growing interest in NHC ligands because of their high stability. ,− These ligands have also been found to be promising candidates for the development of catalysts for the Heck reaction. , Different groups have synthesized chelating NHC ligands that have been investigated in various catalytic reactions. − …”

Section: Introductionmentioning

confidence: 99%

Palladium Complexes with Chelating Bis-NHC Ligands in the Mizoroki–Heck Reaction—Mechanism and Electronic Effects, a DFT Study

Allolio

Straßner

2014

J. Org. Chem.

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Experimental results have shown that palladium complexes with chelating aryl- and alkyl-substituted bis-NHC ligands, including [(H3C-Im)2CH2]PdBr2, [(C6H5-Im)2CH2]PdBr2, and [(H3CO-C6H4-Im)2CH2]PdBr2 are excellent catalysts for the Mizoroki-Heck reaction. To better understand and improve the catalysts, a density functional theory (DFT) study of the Heck reaction has been performed at the B3LYP/6-31G* level of theory, complemented by M06/def2-TZVP single-point calculations. Different mechanistic pathways have been investigated and compared to available experimental results. The most likely mechanism is a cationic catalytic cycle involving the palladium oxidation states 0 and +II. We also looked at other oxidation states, but on the basis of the calculated Gibbs free energy a +II/+IV mechanism can be excluded. Aryl substitution with electron-donating groups at the para position (e.g., the methoxy group in [(H3CO-C6H4-Im)2CH2]PdBr2) was found to reduce the reaction barrier of the rate-determining step. This is in agreement with the experimental findings for the catalysts. The experimentally observed cis selectivity could also be explained by the DFT study.

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“…Halomethylation of imidazolylpyridine 2 using an excess of chloroiodomethane did not work out as well as was reported for other arylimidazole precursors in the literature . Even at a 50-fold excess of chloroiodomethanein comparison to an 8–12-fold excess in the literature–distinct amounts of NCCN ligand precursor P3 were obtained, which could be separated from 5 by stepwise precipitation of the raw product.…”

Section: Resultsmentioning

confidence: 79%

“…At first glance, the synthesis of a ligand precursor with an external pyridine unit may seem even more complicated, as only one linearly linked triazolium fragment has been reported to date and modification of this precursor hardly appears possible . Nevertheless, a combination of two recently developed strategies to access nonsymmetrically substituted diimidazolium salts was found to be successful. ,, Following the initial work of Nozaki et al., who targeted asymmetrically substituted monoimidazolium salts, the group of Gardiner was able to access N,N′-asymmetrically substituted di-NHC ligands by isolating halo-methylated imidazolium salts . Instead of the arylimidazole precursors used in their work, we reacted 2-imidazolylpyridine with chloroiodomethane.…”

Section: Resultsmentioning

confidence: 99%

NHC Versus Pyridine: How “Teeth” Change the Redox Behavior of Iron(II) Complexes

et al. 2015

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A series of octahedral iron(II) complexes with tetradentate NHC/pyridine hybrid ligands containing up to three pyridyl units was designed to study the influence of NHC and pyridine donors on the electronic structure of the metal center. Structural analysis of the iron complexes by NMR spectroscopy and single-crystal X-ray diffraction reveals different coordination modes of the ligand depending on the linkage of the different donor moieties. The oxidation potentials of all complexes correlate linearly with the number of NHC moieties coordinated to iron, as shown by cyclic voltammetry. The influence, although minor, of structural properties on the oxidation potential and (in one case) the influence of the oxidation state of the coordination geometry of the hybrid ligand are also demonstrated. ■ INTRODUCTIONN-heterocyclic carbene (NHC) ligands have become a multifunctional tool in organometallic chemistry and homogeneous catalysis, as steric and electronic properties can be easily influenced by modification of the substituents. 1−6 During the past decade, iron NHC complexes have especially received increasing attention. 7−9 Examples of remarkable reactivity were obtained on the basis of the use of chelating, polydentate NHC ligands, 10−17 including oxidation catalysis with NHC/pyridine hybrid ligands. 18,19 Recently, it could be shown that replacing acetonitrile ligands of the catalytically active iron(II) NCCN complex has a strong influence on the electronic structure of the metal center. 20 However, the literature lacks comprehensive studies on the influence of NHC/pyridine hybrid ligands with different NHC/pyridyl ratios on the oxidation potential of the metal. Such manipulations would allow fine-tuning of the metal center without blocking labile binding sites.Several alkylene-bridged tetradentate NHC/pyridine hybrid ligands are known (Figure 1). Ligands of the general types A (NCCN) and B and C (both CNNC) as well as their arylated derivatives 21−27 count for open-chain structures, while E fits into the class of macrocyclic ligands. 28 Three types of alkylenebridged tetra-NHC ligands (CCCC) have been described so far: open-chain (D), 29 macrocyclic (F), 30−32 and mixed aryl-or heteroatom-bridged ligands. 33−38 The coordination chemistry of the aforementioned acyclic type A di-NHC ligand derivatives has been well investigated. Complexes are known for group 10 metals, 21,22,25,26,39−41 coinage metals, 22,42 cobalt, 41 ruthenium, 43 and iron. 41,44 Only one example of class B is known; it is a highly rigid ligand and does not permit mononuclear, chelated metal complexes, with only a silver complex being reported. 23 The more flexible skeletal structure of type C offers chelating abilities, 24,27,45 although the reported palladium complex is strongly distorted with N−Pd−N and N−Pd−C angles of 80°in comparison to C−Pd−C angles of 120°. 24 Acyclic tetra-NHC ligands (D) are highly flexible, and complexes are known for coinage metals, group 10 metals, and iron. 29,46 For macrocyclic ligands of type E and F a broad r...

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Selective and adaptable access to N,N′-asymmetrically substituted imidazol-2-ylidene bis-NHC ligands: Pd(ii) complexes featuring wide variation in N-alkyl and aryl steric bulk

Cited by 16 publications

References 73 publications

Chelating Bis(N‐Heterocyclic Carbene) Palladium‐Catalyzed Reactions

Chelating Bis(N‐Heterocyclic Carbene) Palladium‐Catalyzed Reactions

Palladium Complexes with Chelating Bis-NHC Ligands in the Mizoroki–Heck Reaction—Mechanism and Electronic Effects, a DFT Study

NHC Versus Pyridine: How “Teeth” Change the Redox Behavior of Iron(II) Complexes

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