Electronic transitions of racemic ethylene-bridged bis(indenyl)-type siloxy substituted zirconocene dichlorides. A combined UV/VIS spectroscopic and theoretical study

Mäkelä, Nora I.; Knuuttila, Hilkka; Linnolahti, Mikko; Pakkanen, Tapani A.

doi:10.1039/b004583g

Cited by 23 publications

(24 citation statements)

References 22 publications

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Section: Resultsmentioning

confidence: 99%

“…[2] This means that changes affecting the metal or ligand orbital energies will reflect in the LMCT energy. In this study UV-vis spectroscopy was used to see changes in the zirconium centre cationicity by keeping the ligand structure same in all experiments, that is, [(nBuCp) 2 Figure 1 and 2. The irradiation experiments were performed for all oxygen treated catalysts but no changes were observed in the spectra, that is, neither the absorption intensity nor its energy changed.…”

Section: Resultsmentioning

confidence: 99%

“…Simplicity of the spectrum and the number of new olefin signals at 5.0 ppm indicated not more than 3 different unsaturated main products. 2 ZrCl 2 ]/MAO/VCH system the intensity of the absorption at 300 nm, which indicates VCH interaction with cationic zirconocene, decreased ( Figure 5). This result suggests that even small amounts of O 2 interact with the cationic zirconium centres, so that the zirconocene-monomer interaction is reduced.…”

Section: Resultsmentioning

confidence: 99%

“…metallocene bond angles, electron density of the ligand or metal and orientation of the Cp based ligands. [2,3] The Cp ligand itself absorbs at higher energies in the UV range of the spectrum due to the p ! p* transitions.…”

Section: Introductionmentioning

confidence: 99%

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Effect of Light Exposure on UV‐Vis Spectrum of [(nBuCp)₂ZrCl₂] Based Catalyst

Mäkelä-Vaarne

Kallio

Reichert

et al. 2003

Macro Chemistry & Physics

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UV‐vis spectroscopic study revealed that vinylcyclohexane interaction with zirconocene catalyst is enhanced on light exposure. If oxygen is present it interacts with the catalyst, hindering vinylcyclohexane interaction and consequently lowering the catalyst activity. The catalyst activity can be recovered with light exposure if low O2 concentrations are used since in light the zirconocene‐vinylcyclohexane interaction is favoured over the zirconocene‐O2 interaction. Moreover, 1H NMR studies of the reaction products showed that the structures of the vinylcyclohexane‐based species produced with light irradiation were identical to the species produced in dark. This indicates that light exposure of the catalytic system does not change the reaction mechanism.Absorption spectrum of [(nBuCp)2ZrCl2]/MAO/VCH. (a) reaction in dark, (b) reaction with irradiation, (c) reaction with O2 treatment in dark and (d) reaction with O2 treatment with irradiation.magnified imageAbsorption spectrum of [(nBuCp)2ZrCl2]/MAO/VCH. (a) reaction in dark, (b) reaction with irradiation, (c) reaction with O2 treatment in dark and (d) reaction with O2 treatment with irradiation.

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Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

Effect of Light Exposure on UV‐Vis Spectrum of [(nBuCp)₂ZrCl₂] Based Catalyst

Mäkelä-Vaarne

Kallio

Reichert

et al. 2003

Macro Chemistry & Physics

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“…In addition to minor blocking of the metal center, such interaction would stabilize the cation resulting in higher concentration of cationic metal centers available for the incoming monomer. Possibly, the ligandϪmetal interaction in combination with the electron-donating siloxy substituent [119] results in an unusually stable active species, [117] as indicated by high catalytic activities at low MAO concentrations. [135,137,158] A third possible consequence could be the increase in the number of different active species with potentially different stereoregulating abilities, possibly resulting in broadening of the molecular weight distributions of the produced polymers.…”

Section: Cationic Methyl Intermediatesmentioning

confidence: 99%

Conformational Preferences of Racemic Ethylene-Bridged Bis(indenyl)-Type Zirconocenes: An ab initio Hartree−Fock Study

Linnolahti

Pakkanen

Leino

et al. 2001

Eur. J. Inorg. Chem.

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The ab initio Hartree−Fock method has been utilized to rationalize the conformational preferences of racemic ethylene-bridged bis(indenyl)-type zirconocenes in dichloride and cationic methyl forms. This group of catalysts is characterized by conformational isomerism due to fluxionality of the ethylene bridge resulting in two distinct orientations of the indenyl ligands, indenyl-forward (Π) and indenyl-backward (Y). A systematic investigation of the influence of methyl and tri-

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Weakly Coordinating Cations as Alternatives to Weakly Coordinating Anions

et al. 2009

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The ongoing pursuit of weakly coordinating anions [1] (WCAs) has allowed the synthesis, isolation, and catalytic utilization of highly electrophilic transition-metal [2] and main-group [3] À . [4,5] While a truly noncoordinating anion has not been identified, there is little doubt that this class of anion has been a wellspring of both fundamental insights and industrial applications. [6] During our studies of sterically expanded single-site catalysts for olefin polymerization, [7] we observed consistent trends indicating that increased steric bulk around the electrophilic transition metal yielded increased catalytic activity, for both metallocene catalysts ( Figure 1) and constrained-geometry catalysts (CGCs).[8] These observations are reminiscent of the increased activities observed with increasingly non-coordinating anions, but they must have a different root cause, as the same methylaluminoxane (MAO)-derived counteranion was common among these polymerizations.We postulated that the increased activity was a result of steric shielding effected by the expanded ancillary ligands; this steric sheath diminished the ability of the encased electrophilic metal center to accept the larger counteranion, but it still allowed access to the relatively small monomers. Figure 2 illustrates how steric bulk might retard the accepting ability of such electrophiles, resulting in what should be weakly coordinating cations (WCCs).[9] We then sought additional evidence to test this supposition and turned our attention to UV/Vis spectroscopy. [10] Upon activation with MAO (5000 equiv), zirconocene [{Ph 2 C(C 5 H 4 )(C 13 H 8 )}ZrCl 2 ] (1-Flu) undergoes an immediate red shift of its visible absorption maximum l max from 499 to 557 nm (DE = À5.97 kcal mol À1 = À25.0 kJ mol À1 ). Zirconocene [{Ph 2 C(C 5 H 4 )(C 29 H 36 )}ZrCl 2 ] (1-Oct) also undergoes a rapid red shift, but of significantly greater magnitude: 521 to 622 nm (DE = À8.91 kcal mol À1 = À37.3 kJ mol À1 ). For these species, l max corresponds to a ligand-to-metal charge transfer (LMCT), [10a, 11] and the change in l max upon MAO activation is a direct measure of the resultant electron density at the zirconium center, because the LMCT is correspondingly more facile for more electron-deficient metals. Thus, the zirconocenium ion pair generated from 1-Oct features a more electron-deficient metal center, and a plausible explanation for this electron deficiency is a greater average cation-anion separation caused by steric shielding (Figure 2).Over the course of 3 h, the l max of activated 1-Oct relaxes back to 590 nm, while the l max of activated 1-Flu remains unchanged at 557 nm. Figure 3 plots this dynamic process and

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Electronic transitions of racemic ethylene-bridged bis(indenyl)-type siloxy substituted zirconocene dichlorides. A combined UV/VIS spectroscopic and theoretical study

Cited by 23 publications

References 22 publications

Effect of Light Exposure on UV‐Vis Spectrum of [(nBuCp)₂ZrCl₂] Based Catalyst

Effect of Light Exposure on UV‐Vis Spectrum of [(nBuCp)₂ZrCl₂] Based Catalyst

Conformational Preferences of Racemic Ethylene-Bridged Bis(indenyl)-Type Zirconocenes: An ab initio Hartree−Fock Study

Weakly Coordinating Cations as Alternatives to Weakly Coordinating Anions

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Electronic transitions of racemic ethylene-bridged bis(indenyl)-type siloxy substituted zirconocene dichlorides. A combined UV/VIS spectroscopic and theoretical study

Cited by 23 publications

References 22 publications

Effect of Light Exposure on UV‐Vis Spectrum of [(nBuCp)2ZrCl2] Based Catalyst

Effect of Light Exposure on UV‐Vis Spectrum of [(nBuCp)2ZrCl2] Based Catalyst

Conformational Preferences of Racemic Ethylene-Bridged Bis(indenyl)-Type Zirconocenes: An ab initio Hartree−Fock Study

Weakly Coordinating Cations as Alternatives to Weakly Coordinating Anions

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Effect of Light Exposure on UV‐Vis Spectrum of [(nBuCp)₂ZrCl₂] Based Catalyst

Effect of Light Exposure on UV‐Vis Spectrum of [(nBuCp)₂ZrCl₂] Based Catalyst