Highly electrophilic olefin polymerization catalysts. Quantitative reaction coordinates for fluoroarylborane/alumoxane methide abstraction and ion-pair reorganization in group 4 metallocene and 'constrained geometry' catalysts

Paul A. Deck, Colin L. Beswick, Tobin J Marks

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Abstract

Reaction enthalpies of group 4 metallocenes having the general formula L2M(CH3)2 (L = Cp, 1,2-Me2Cp, Me5Cp; L2 = Me2Si(Me4Cp)((t)BuN); M = Ti, Zr, and Hf) with the strong organo-Lewis acid B(C6F5)3 were measured using batch titration calorimetry in toluene. Methide abstraction to form the corresponding L2MCH3+CH3B(C6F5)3- contact ion pairs is highly exothermic in all cases. Exothermicity increases with increasing Cp methyl substitution: for M = Zr, ΔH = -23.1(3), -24.3(4), and -36.7(5) kcal mol-1 for L = Cp, Me2Cp, and Me5Cp, respectively for M = Hf and L = 1,2-Me2Cp, ΔH = -20.8(5) kcal mol-1. 'Constrained geometry' complexes (L2 = Me2Si(Me4Cp)((t)BuN)) exhibit similar exothermicities, with ΔH = -22.6(2), -23.9(4), and -19.3(6) kcal mol-1 for M = Ti, Zr, and Hf, respectively. In contrast, analogous reactions with methylalumoxane (M:Al = 1:50) are less exothermic, with ΔH = -10.9(3) and -8.9(4) kcal mol-1 for L = 1,2-Me2Cp and M = Zr and Hf, respectively. Under identical conditions, (1,2-Me2Cp)2M-(CH3)2 (M = Zr, Hf) complexes also undergo methide abstraction with the less Lewis-acidic triarylboranes (C6F5)2BAr (AT = 3,5-C6H3F2, Ph, and 3,5-C6H3Me2); however, conversions to the corresponding (Me2-Cp)2MCH3+ CH3B(C6F5)2Ar- ion pairs, are incomplete. Variable-temperature NMR measurements yield thermodynamic parameters for partial methide abstraction by these less Lewis-acidic boranes. For Ar = 3,5-C6H3F2, ΔH = -18.7(7) and -15.2(8) kcal mol-1 with ΔS = -42(2) and -35(3) e.u:; for Ar = Ph, ΔH = -14.8(8) and -13.3(6) kcal mol-1 with ΔS = -31(2) and -39(2) e.u.; for Ar = 3,5-C6H3Me2, ΔH = -10.8(6) and -12.7(5) with ΔS = -19(2) and -36(4) e.u., in each case for M = Zr and Hf, respectively. Dynamic NMR analyses reveal that the activation barriers for methide abstraction from the neutral metallocene dialkyls are small and relatively insensitive to the borane identity (AH = 2-6 kcal mol-1) while ion-pair separation/recombination processes are greatly facilitated by polar solvents. Ethylene polymerization activities for eight (Me2Cp)2MCH3+CH3B(C6F5)2Ar- complexes measured in toluene solution (25°C, 1 atm) follow a trend in metal (Zr > Hf)as well as a substantial trend in triarylborane (Ar = C6F5 > 3,5-C6H3F2 > Ph ~3,5-C6H3-Me2). Polymerization activities correlate roughly with MCH3+ 13C NMR chemical shifts and enthalpies of methide abstraction.

Original languageEnglish
Pages (from-to)1772-1784
Number of pages13
JournalJournal of the American Chemical Society
Volume120
Issue number8
DOIs
Publication statusPublished - Mar 4 1998

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Alkenes
Polymerization
Boranes
Olefins
Nuclear magnetic resonance
Toluene
Ions
Catalysts
Geometry
Enthalpy
Activation Analysis
Lewis Acids
Calorimetry
Chemical shift
Organometallics
Titration
Thermodynamics
Genetic Recombination
Ethylene
Substitution reactions

ASJC Scopus subject areas

  • Chemistry(all)

Cite this

@article{0664c232203e442fb13ef56ad4b75a46,
title = "Highly electrophilic olefin polymerization catalysts. Quantitative reaction coordinates for fluoroarylborane/alumoxane methide abstraction and ion-pair reorganization in group 4 metallocene and 'constrained geometry' catalysts",
abstract = "Reaction enthalpies of group 4 metallocenes having the general formula L2M(CH3)2 (L = Cp, 1,2-Me2Cp, Me5Cp; L2 = Me2Si(Me4Cp)((t)BuN); M = Ti, Zr, and Hf) with the strong organo-Lewis acid B(C6F5)3 were measured using batch titration calorimetry in toluene. Methide abstraction to form the corresponding L2MCH3+CH3B(C6F5)3- contact ion pairs is highly exothermic in all cases. Exothermicity increases with increasing Cp methyl substitution: for M = Zr, ΔH = -23.1(3), -24.3(4), and -36.7(5) kcal mol-1 for L = Cp, Me2Cp, and Me5Cp, respectively for M = Hf and L = 1,2-Me2Cp, ΔH = -20.8(5) kcal mol-1. 'Constrained geometry' complexes (L2 = Me2Si(Me4Cp)((t)BuN)) exhibit similar exothermicities, with ΔH = -22.6(2), -23.9(4), and -19.3(6) kcal mol-1 for M = Ti, Zr, and Hf, respectively. In contrast, analogous reactions with methylalumoxane (M:Al = 1:50) are less exothermic, with ΔH = -10.9(3) and -8.9(4) kcal mol-1 for L = 1,2-Me2Cp and M = Zr and Hf, respectively. Under identical conditions, (1,2-Me2Cp)2M-(CH3)2 (M = Zr, Hf) complexes also undergo methide abstraction with the less Lewis-acidic triarylboranes (C6F5)2BAr (AT = 3,5-C6H3F2, Ph, and 3,5-C6H3Me2); however, conversions to the corresponding (Me2-Cp)2MCH3+ CH3B(C6F5)2Ar- ion pairs, are incomplete. Variable-temperature NMR measurements yield thermodynamic parameters for partial methide abstraction by these less Lewis-acidic boranes. For Ar = 3,5-C6H3F2, ΔH = -18.7(7) and -15.2(8) kcal mol-1 with ΔS = -42(2) and -35(3) e.u:; for Ar = Ph, ΔH = -14.8(8) and -13.3(6) kcal mol-1 with ΔS = -31(2) and -39(2) e.u.; for Ar = 3,5-C6H3Me2, ΔH = -10.8(6) and -12.7(5) with ΔS = -19(2) and -36(4) e.u., in each case for M = Zr and Hf, respectively. Dynamic NMR analyses reveal that the activation barriers for methide abstraction from the neutral metallocene dialkyls are small and relatively insensitive to the borane identity (AH = 2-6 kcal mol-1) while ion-pair separation/recombination processes are greatly facilitated by polar solvents. Ethylene polymerization activities for eight (Me2Cp)2MCH3+CH3B(C6F5)2Ar- complexes measured in toluene solution (25°C, 1 atm) follow a trend in metal (Zr > Hf)as well as a substantial trend in triarylborane (Ar = C6F5 > 3,5-C6H3F2 > Ph ~3,5-C6H3-Me2). Polymerization activities correlate roughly with MCH3+ 13C NMR chemical shifts and enthalpies of methide abstraction.",
author = "Deck, {Paul A.} and Beswick, {Colin L.} and Marks, {Tobin J}",
year = "1998",
month = "3",
day = "4",
doi = "10.1021/ja972912f",
language = "English",
volume = "120",
pages = "1772--1784",
journal = "Journal of the American Chemical Society",
issn = "0002-7863",
publisher = "American Chemical Society",
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TY - JOUR

T1 - Highly electrophilic olefin polymerization catalysts. Quantitative reaction coordinates for fluoroarylborane/alumoxane methide abstraction and ion-pair reorganization in group 4 metallocene and 'constrained geometry' catalysts

AU - Deck, Paul A.

AU - Beswick, Colin L.

AU - Marks, Tobin J

PY - 1998/3/4

Y1 - 1998/3/4

N2 - Reaction enthalpies of group 4 metallocenes having the general formula L2M(CH3)2 (L = Cp, 1,2-Me2Cp, Me5Cp; L2 = Me2Si(Me4Cp)((t)BuN); M = Ti, Zr, and Hf) with the strong organo-Lewis acid B(C6F5)3 were measured using batch titration calorimetry in toluene. Methide abstraction to form the corresponding L2MCH3+CH3B(C6F5)3- contact ion pairs is highly exothermic in all cases. Exothermicity increases with increasing Cp methyl substitution: for M = Zr, ΔH = -23.1(3), -24.3(4), and -36.7(5) kcal mol-1 for L = Cp, Me2Cp, and Me5Cp, respectively for M = Hf and L = 1,2-Me2Cp, ΔH = -20.8(5) kcal mol-1. 'Constrained geometry' complexes (L2 = Me2Si(Me4Cp)((t)BuN)) exhibit similar exothermicities, with ΔH = -22.6(2), -23.9(4), and -19.3(6) kcal mol-1 for M = Ti, Zr, and Hf, respectively. In contrast, analogous reactions with methylalumoxane (M:Al = 1:50) are less exothermic, with ΔH = -10.9(3) and -8.9(4) kcal mol-1 for L = 1,2-Me2Cp and M = Zr and Hf, respectively. Under identical conditions, (1,2-Me2Cp)2M-(CH3)2 (M = Zr, Hf) complexes also undergo methide abstraction with the less Lewis-acidic triarylboranes (C6F5)2BAr (AT = 3,5-C6H3F2, Ph, and 3,5-C6H3Me2); however, conversions to the corresponding (Me2-Cp)2MCH3+ CH3B(C6F5)2Ar- ion pairs, are incomplete. Variable-temperature NMR measurements yield thermodynamic parameters for partial methide abstraction by these less Lewis-acidic boranes. For Ar = 3,5-C6H3F2, ΔH = -18.7(7) and -15.2(8) kcal mol-1 with ΔS = -42(2) and -35(3) e.u:; for Ar = Ph, ΔH = -14.8(8) and -13.3(6) kcal mol-1 with ΔS = -31(2) and -39(2) e.u.; for Ar = 3,5-C6H3Me2, ΔH = -10.8(6) and -12.7(5) with ΔS = -19(2) and -36(4) e.u., in each case for M = Zr and Hf, respectively. Dynamic NMR analyses reveal that the activation barriers for methide abstraction from the neutral metallocene dialkyls are small and relatively insensitive to the borane identity (AH = 2-6 kcal mol-1) while ion-pair separation/recombination processes are greatly facilitated by polar solvents. Ethylene polymerization activities for eight (Me2Cp)2MCH3+CH3B(C6F5)2Ar- complexes measured in toluene solution (25°C, 1 atm) follow a trend in metal (Zr > Hf)as well as a substantial trend in triarylborane (Ar = C6F5 > 3,5-C6H3F2 > Ph ~3,5-C6H3-Me2). Polymerization activities correlate roughly with MCH3+ 13C NMR chemical shifts and enthalpies of methide abstraction.

AB - Reaction enthalpies of group 4 metallocenes having the general formula L2M(CH3)2 (L = Cp, 1,2-Me2Cp, Me5Cp; L2 = Me2Si(Me4Cp)((t)BuN); M = Ti, Zr, and Hf) with the strong organo-Lewis acid B(C6F5)3 were measured using batch titration calorimetry in toluene. Methide abstraction to form the corresponding L2MCH3+CH3B(C6F5)3- contact ion pairs is highly exothermic in all cases. Exothermicity increases with increasing Cp methyl substitution: for M = Zr, ΔH = -23.1(3), -24.3(4), and -36.7(5) kcal mol-1 for L = Cp, Me2Cp, and Me5Cp, respectively for M = Hf and L = 1,2-Me2Cp, ΔH = -20.8(5) kcal mol-1. 'Constrained geometry' complexes (L2 = Me2Si(Me4Cp)((t)BuN)) exhibit similar exothermicities, with ΔH = -22.6(2), -23.9(4), and -19.3(6) kcal mol-1 for M = Ti, Zr, and Hf, respectively. In contrast, analogous reactions with methylalumoxane (M:Al = 1:50) are less exothermic, with ΔH = -10.9(3) and -8.9(4) kcal mol-1 for L = 1,2-Me2Cp and M = Zr and Hf, respectively. Under identical conditions, (1,2-Me2Cp)2M-(CH3)2 (M = Zr, Hf) complexes also undergo methide abstraction with the less Lewis-acidic triarylboranes (C6F5)2BAr (AT = 3,5-C6H3F2, Ph, and 3,5-C6H3Me2); however, conversions to the corresponding (Me2-Cp)2MCH3+ CH3B(C6F5)2Ar- ion pairs, are incomplete. Variable-temperature NMR measurements yield thermodynamic parameters for partial methide abstraction by these less Lewis-acidic boranes. For Ar = 3,5-C6H3F2, ΔH = -18.7(7) and -15.2(8) kcal mol-1 with ΔS = -42(2) and -35(3) e.u:; for Ar = Ph, ΔH = -14.8(8) and -13.3(6) kcal mol-1 with ΔS = -31(2) and -39(2) e.u.; for Ar = 3,5-C6H3Me2, ΔH = -10.8(6) and -12.7(5) with ΔS = -19(2) and -36(4) e.u., in each case for M = Zr and Hf, respectively. Dynamic NMR analyses reveal that the activation barriers for methide abstraction from the neutral metallocene dialkyls are small and relatively insensitive to the borane identity (AH = 2-6 kcal mol-1) while ion-pair separation/recombination processes are greatly facilitated by polar solvents. Ethylene polymerization activities for eight (Me2Cp)2MCH3+CH3B(C6F5)2Ar- complexes measured in toluene solution (25°C, 1 atm) follow a trend in metal (Zr > Hf)as well as a substantial trend in triarylborane (Ar = C6F5 > 3,5-C6H3F2 > Ph ~3,5-C6H3-Me2). Polymerization activities correlate roughly with MCH3+ 13C NMR chemical shifts and enthalpies of methide abstraction.

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