Ligand substituent, anion, and solvation effects on ion pair structure, thermodynamic stability, and structural mobility in "constrained geometry" olefin polymerization catalysts

An Ab initio quantum chemical investigation

G. Lanza, I. L. Fragalà, Tobin J Marks

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Abstract

Ab initio quantum chemical calculations at the MP2 level were performed on the elementary reactions and structural reorganizations involved in activation and ligand binding by the "constrained geometry" olefin polymerization catalyst series R2Si(η5-R′4C5)(R″ N)Ti(CH3)R‴ (R = H, CH3; R′ = H, CH3; R″ = CH3, t-Bu; R‴ = H, CH3, CH2CH2CH3, CH(CH3)2) in the presence of the organo-Lewis acid cocatalyst B(C6F5)3 and various solvation media. Calculated structures of the neutral precursors and resulting ion pairs are in good agreement with the experiment. Analysis of the R2Si(R′4C5)(R″N)TiR‴ + naked cations reveals the importance of α, β, and γ C-H/C-C agostic interactions in selectively stabilizing various conformations of the TiR‴ group as well as the diminished charge on Ti with the introduction of electron-donating ligand substituents. The calculated ion pair formation enthalpies for the process R2Si(η5-R′4C5)(R″ N)Ti(CH3)R‴ + B(C6F5)3 → R2Si(η5-R′4C5)(R″ N)TiR‴·H3CB(C6F5)3 are in good agreement with experiment, the magnitudes reflecting a close interplay of ligand electronic and steric characteristics which weaken the precursor Ti-CH3 bond and stabilize the cationic product. The ion pair Ti···H3CB interaction is predominantly electrostatic in character and describable by a rather flat potential energy surface for elongation, and the energetics of heterolysis are strongly influenced by the capacity of the other Ti ligands and solvation to stabilize the separated charges.

Original languageEnglish
Pages (from-to)12764-12777
Number of pages14
JournalJournal of the American Chemical Society
Volume122
Issue number51
DOIs
Publication statusPublished - Dec 27 2000

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Solvation
Alkenes
Thermodynamics
Polymerization
Olefins
Anions
Thermodynamic stability
Negative ions
Ligands
Ions
Catalysts
Geometry
Lewis Acids
Potential energy surfaces
Static Electricity
Conformations
Cations
Elongation
Electrostatics
Enthalpy

ASJC Scopus subject areas

  • Chemistry(all)

Cite this

@article{78812d9003b44e1392270a1a12218179,
title = "Ligand substituent, anion, and solvation effects on ion pair structure, thermodynamic stability, and structural mobility in {"}constrained geometry{"} olefin polymerization catalysts: An Ab initio quantum chemical investigation",
abstract = "Ab initio quantum chemical calculations at the MP2 level were performed on the elementary reactions and structural reorganizations involved in activation and ligand binding by the {"}constrained geometry{"} olefin polymerization catalyst series R2Si(η5-R′4C5)(R″ N)Ti(CH3)R‴ (R = H, CH3; R′ = H, CH3; R″ = CH3, t-Bu; R‴ = H, CH3, CH2CH2CH3, CH(CH3)2) in the presence of the organo-Lewis acid cocatalyst B(C6F5)3 and various solvation media. Calculated structures of the neutral precursors and resulting ion pairs are in good agreement with the experiment. Analysis of the R2Si(R′4C5)(R″N)TiR‴ + naked cations reveals the importance of α, β, and γ C-H/C-C agostic interactions in selectively stabilizing various conformations of the TiR‴ group as well as the diminished charge on Ti with the introduction of electron-donating ligand substituents. The calculated ion pair formation enthalpies for the process R2Si(η5-R′4C5)(R″ N)Ti(CH3)R‴ + B(C6F5)3 → R2Si(η5-R′4C5)(R″ N)TiR‴·H3CB(C6F5)3 are in good agreement with experiment, the magnitudes reflecting a close interplay of ligand electronic and steric characteristics which weaken the precursor Ti-CH3 bond and stabilize the cationic product. The ion pair Ti···H3CB interaction is predominantly electrostatic in character and describable by a rather flat potential energy surface for elongation, and the energetics of heterolysis are strongly influenced by the capacity of the other Ti ligands and solvation to stabilize the separated charges.",
author = "G. Lanza and Fragal{\`a}, {I. L.} and Marks, {Tobin J}",
year = "2000",
month = "12",
day = "27",
doi = "10.1021/ja000571r",
language = "English",
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pages = "12764--12777",
journal = "Journal of the American Chemical Society",
issn = "0002-7863",
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TY - JOUR

T1 - Ligand substituent, anion, and solvation effects on ion pair structure, thermodynamic stability, and structural mobility in "constrained geometry" olefin polymerization catalysts

T2 - An Ab initio quantum chemical investigation

AU - Lanza, G.

AU - Fragalà, I. L.

AU - Marks, Tobin J

PY - 2000/12/27

Y1 - 2000/12/27

N2 - Ab initio quantum chemical calculations at the MP2 level were performed on the elementary reactions and structural reorganizations involved in activation and ligand binding by the "constrained geometry" olefin polymerization catalyst series R2Si(η5-R′4C5)(R″ N)Ti(CH3)R‴ (R = H, CH3; R′ = H, CH3; R″ = CH3, t-Bu; R‴ = H, CH3, CH2CH2CH3, CH(CH3)2) in the presence of the organo-Lewis acid cocatalyst B(C6F5)3 and various solvation media. Calculated structures of the neutral precursors and resulting ion pairs are in good agreement with the experiment. Analysis of the R2Si(R′4C5)(R″N)TiR‴ + naked cations reveals the importance of α, β, and γ C-H/C-C agostic interactions in selectively stabilizing various conformations of the TiR‴ group as well as the diminished charge on Ti with the introduction of electron-donating ligand substituents. The calculated ion pair formation enthalpies for the process R2Si(η5-R′4C5)(R″ N)Ti(CH3)R‴ + B(C6F5)3 → R2Si(η5-R′4C5)(R″ N)TiR‴·H3CB(C6F5)3 are in good agreement with experiment, the magnitudes reflecting a close interplay of ligand electronic and steric characteristics which weaken the precursor Ti-CH3 bond and stabilize the cationic product. The ion pair Ti···H3CB interaction is predominantly electrostatic in character and describable by a rather flat potential energy surface for elongation, and the energetics of heterolysis are strongly influenced by the capacity of the other Ti ligands and solvation to stabilize the separated charges.

AB - Ab initio quantum chemical calculations at the MP2 level were performed on the elementary reactions and structural reorganizations involved in activation and ligand binding by the "constrained geometry" olefin polymerization catalyst series R2Si(η5-R′4C5)(R″ N)Ti(CH3)R‴ (R = H, CH3; R′ = H, CH3; R″ = CH3, t-Bu; R‴ = H, CH3, CH2CH2CH3, CH(CH3)2) in the presence of the organo-Lewis acid cocatalyst B(C6F5)3 and various solvation media. Calculated structures of the neutral precursors and resulting ion pairs are in good agreement with the experiment. Analysis of the R2Si(R′4C5)(R″N)TiR‴ + naked cations reveals the importance of α, β, and γ C-H/C-C agostic interactions in selectively stabilizing various conformations of the TiR‴ group as well as the diminished charge on Ti with the introduction of electron-donating ligand substituents. The calculated ion pair formation enthalpies for the process R2Si(η5-R′4C5)(R″ N)Ti(CH3)R‴ + B(C6F5)3 → R2Si(η5-R′4C5)(R″ N)TiR‴·H3CB(C6F5)3 are in good agreement with experiment, the magnitudes reflecting a close interplay of ligand electronic and steric characteristics which weaken the precursor Ti-CH3 bond and stabilize the cationic product. The ion pair Ti···H3CB interaction is predominantly electrostatic in character and describable by a rather flat potential energy surface for elongation, and the energetics of heterolysis are strongly influenced by the capacity of the other Ti ligands and solvation to stabilize the separated charges.

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U2 - 10.1021/ja000571r

DO - 10.1021/ja000571r

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VL - 122

SP - 12764

EP - 12777

JO - Journal of the American Chemical Society

JF - Journal of the American Chemical Society

SN - 0002-7863

IS - 51

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