Unusually Stable Vinyl Cations

Müller, Thomas; Meyer, Rita; Lennartz, Dirk; Siehl, Hans‐Ullrich

doi:10.1002/1521-3773(20000901)39:17<3074::aid-anie3074>3.0.co;2-t

Cited by 82 publications

(14 citation statements)

References 27 publications

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“…A similar temperaturedependent Al···N interaction has been employed in alkylamine-functionalized zirconocene/MAO to modulate the molecular-weight distribution of polyethylene. [24] Clearly, MAO, in the form of [Me-MAO] À , acts as an in situ ansabridge and plays the unprecedented role of endowing aspecific class I unbridged-metallocene precatalysts with isospecificity, although cocatalysts are also known to affect the stereoselectivity of unbridged-metallocene catalysts to some extent. [25][26][27] In conclusion, we have devised a novel synthetic strategy for the synthesis of isospecific, unbridged-metallocene catalytic systems for propylene polymerization.…”

mentioning

confidence: 99%

Endowing Aspecific, Unbridged‐Metallocene Propylene‐Polymerization Catalysts with Isospecificity: The Unprecedented Role of MAO

Kim

Lee

et al. 2006

Angew Chem Int Ed

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The relationship between the nature of catalyst systems and the resulting polymer has been well established in single-site olefin-polymerization systems and now provides the opportunity to tailor the polymers properties. [1][2][3][4] In particular, chiral ansa-metallocene catalysts, which follow an enantiomorphic site-control mechanism, have been intensively investigated to obtain stereochemical control of propylene polymerization by varying the ligand structure. [5][6][7] Unbridgedmetallocene-based systems, on the other hand, have received little attention owing to their aspecific nature [8,9] in spite of their ability to produce iso-rich [10,11] or isotactic-atactic block polypropylene [12,13] when containing rotationally hindered ligands. As unbridged metallocenes are far easier to synthesize than ansa-metallocenes, we have been investigating isospecific, unbridged-metallocene catalytic systems that can be generated in situ during the activation step. To this end, we have designed "class I" unbridged metallocenes, a new class analogous to the known aspecific, unbridged metallocenes of "class II". [8,9] The Lewis basic sites E in class I complexes are found to interact with [Me-MAO] À to generate rigid, rac-like cationic active species, thereby endowing aspecific, unbridged-metallocene precatalysts with isospecificity. Herein, we report a novel example of a sterically unhindered, unbridged zirconocene system that is able to produce highly isotactic polypropylene through the unprecedented role of methyl aluminum oxane (MAO). The amine-functionalized, unbridged zirconocenes [{1-(pMe 2 NC 6 H 4 )-3,4-Me 2 C 5 H 2 } 2 ZrX 2 ] [(AP) 2 ZrX 2 ; X = Cl (2), X = Me (3)] were obtained from newly synthesized ligand 1 as outlined in Scheme 1. The molecular structure of 2 has C 2 symmetry in the solid state (Figure 1). The polymerization of propylene with 2/MAO ([Al]/[Zr] = 1000) was performed at various temperatures (T p = 0, 25, 50, and 70 8C; Table 1, entries 1-4, respectively). The 2/MAO system shows lower catalytic activity but produces higher molecular weight polypropylenes than the well-known isospecific catalyst rac-[Et(Ind) 2 ZrCl 2 ] (EBIZr)/MAO (entry 9) under identical reaction conditions. The differential scanning calorimetry (DSC) and gel permeation chromatography (GPC) traces indicate that all the crude polypropylenes from the 2/MAO system show multiple melting transitions (T m ) and broad molecular-weight distributions (M w /M n ) of between 4.5 and 11 (Figure 2).The crude polypropylenes were fractionated by stepwise solvent extraction [14] into three portions for further analysis, 1) diethyl ether soluble, 2) diethyl ether insoluble and nheptane soluble, and 3) diethyl ether insoluble and n-heptane insoluble. The mmmm methyl pentad values in Table 2 suggest that these portions correspond to atactic-like, mod- [*] Dr.

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confidence: 99%

Endowing Aspecific, Unbridged‐Metallocene Propylene‐Polymerization Catalysts with Isospecificity: The Unprecedented Role of MAO

Kim

Lee

et al. 2006

Angew Chem Int Ed

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“…(5tf) Figure 9 shows the molecular structure of vinyl cation 20 in the salt 20 e [CB u H 6 Br 6 ]. As previously predicted by computations (59,60) and deduced from 13 C NMR chemical shift parameters (10,35), the vinyl cation is linear around the positively charged carbon atom C + (a(C^-C + -C q ) = 178.8°), which suggests a sp-hybridization for C + . As a consequence the C^=C + double bond is unusually short (r(C p =C + ) = 125.5 pm) and approaches the bond length for a normal OC triple bond.…”

Section: R = I-butylmentioning

confidence: 57%

“…This interpretation of the experimental data implies that the contribution of /?-SiC hyperconjugation to the overall thermodynamic stability of vinyl cations is not constant, but is determined by the electron demand at the electron deficient dicoordinated carbon atom. Alkyl Substituted Vinyl Cations (35,36) Stabilization of the vinyl cations 8-10 by aryl substituents is important but even substituents without strong resonance effects provide adequate stabilization to allow the synthesis of persistent vinyl cations at ambient temperature. This is demonstrated by the synthesis and isolation of the salts of alkyl-substituted vinyl cations 19 and 20, and their characteristic NMR parameters are summarized in Table 1.…”

Section: σ-Delocalization Versus ^-Conjugation In Vinyl Cationsmentioning

confidence: 99%

Unusually Stable Vinyl Cations

Müller

Margraf

Syha

et al. 2007

ACS Symposium Series

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Recent results on the chemistry of persistent vinyl cations are summarized, β, β-Disilyl-substituted vinyl cations were synthesized by intramolecular addition of transient silylium ions to alkynes. The vinyl cations are stable at ambient temperature and were isolated in the form of their tetrakispentafluorophenylborate and hexabromocarboranate salts. The vinyl cations were characterized by IR and NMR spectroscopy and by X-ray crystallography. The experimental results for the α-alkyl-and α-aryl-substituted vinyl cations confirm their Y-shape structures, consisting of a linear dicoordinated, formally positively charged α-carbon atom and a trigonal planar coordinated β-carbon atom. In addition, the spectroscopic data clearly indicate the consequences of, β-silyl hyperconjugation in these vinyl cations. Scope and limitations of the synthetic approach to vinyl cations via addition of silylium ions to C≡C triple bonds are discussed.

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“…The catalyst seeds were the result of oxidizing a giant metallorganic cluster surrounded by ligandterminated, icosahedral FeMo cages. Our synthesis of the metallorganic cluster reproduced previous reports (10,11). The clusters were diluted in ethanol (30 mg/L), and then 0.5 mL of solution was spincast onto 4" p ++ Si/SiO 2 wafers to give a dilute coating of catalyst particles.…”

Section: A Carbon Nanotube Synthesis and Device Fabricationmentioning

confidence: 99%

Electrode Characteristics of Individual, MnO₂ Coated Carbon Nanotubes

Corso¹,

Pérez²,

Collins³

2012

ECS Trans.

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We investigate interfacial charge transfer between a manganese oxide (MnO 2 ) pseudocapacitor material and graphitic carbon supports in the limit where the graphitic carbon is defect free. We achieve this limit experimentally by fabricating model electrodes comprising MnO 2 deposited on single, pristine, isolated singlewalled carbon nanotubes. Li ion cyclic voltammetry of the composites gives a specific capacitance in accord with MnO 2 storage capacities, but with kinetics limited by the poor electron transfer properties of defect free carbon. By fitting the data to an equivalent circuit model, we determine the charge transfer resistivity of MnO 2 -carbon interfaces to be 9 x 10 7 :-cm when defects are absent, a limiting value for high power cathodes.

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Unusually Stable Vinyl Cations

Cited by 82 publications

References 27 publications

Endowing Aspecific, Unbridged‐Metallocene Propylene‐Polymerization Catalysts with Isospecificity: The Unprecedented Role of MAO

Endowing Aspecific, Unbridged‐Metallocene Propylene‐Polymerization Catalysts with Isospecificity: The Unprecedented Role of MAO

Unusually Stable Vinyl Cations

Electrode Characteristics of Individual, MnO₂ Coated Carbon Nanotubes

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Unusually Stable Vinyl Cations

Cited by 82 publications

References 27 publications

Endowing Aspecific, Unbridged‐Metallocene Propylene‐Polymerization Catalysts with Isospecificity: The Unprecedented Role of MAO

Endowing Aspecific, Unbridged‐Metallocene Propylene‐Polymerization Catalysts with Isospecificity: The Unprecedented Role of MAO

Unusually Stable Vinyl Cations

Electrode Characteristics of Individual, MnO2 Coated Carbon Nanotubes

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Electrode Characteristics of Individual, MnO₂ Coated Carbon Nanotubes