AlMe3 and ZnMe2 Adducts of a Titanium Imido Methyl Cation: A Combined Crystallographic, Spectroscopic, and DFT Study
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- 作者：Paul D. Bolton ; Eric Clot ; Andrew R. Cowley ; Philip Mountford
- 刊名：Journal of the American Chemical Society
- 出版年：2006
- 出版时间：November 22, 2006
- 年：2006
- 卷：128
- 期：46
- 页码：15005 - 15018
- 全文大小：284K
- 年卷期：v.128,no.46(November 22, 2006)
- ISSN：1520-5126
文摘
A combined experimental and DFT study of the reactions of the titanium imido methyl cation[Ti(NtBu)(Me3[9]aneN3)Me]+ (4+) with AlMe3 and ZnMe2 is described. Reaction of 4+ with AlMe3 gave [Ti(NtBu)(Me3[9]aneN3)(
-Me)2AlMe2]+ (7+), the first structurally characterized AlMe3 adduct of a transition metalalkyl cation and a model for the presumed resting state in MAO-activated olefin polymerizations. Reactionof 4+ with ZnMe2 also gave a methyl-bridged heterobinuclear species, namely [Ti(-NtBu)(Me3[9]aneN3)(-Me)2ZnMe]+ (8+), the first directly observed ZnMe2 adduct of a transition metal alkyl cation. At roomtemperature, all three metal-bound methyls of 8+ underwent rapid exchange with those of free ZnMe2,whereas at 233 K only the terminal Zn-Me group exchanged significantly. Addition of AlMe3 to 8+quantitatively formed 7+ and ZnMe2. Reaction of 4+ with Cp2ZrMe2 gave [Ti(NtBu){Me2(-CH2)[9]aneN3}(-CH2)ZrCp2]+ (10+) via a highly selective double C-H bond activation reaction in which both alkyl groupsof Cp2ZrMe2 were lost. DFT calculations on models of 7+ confirmed the approximately square-basedpyramidal geometries for the bridging methyl groups. Calculations on 8+ found that the formation of theTi(-Me)2Zn moiety is assisted by an Nimide
Zn dative bond. DFT calculations for the sterically lessencumbered methyl cation [Ti(NMe)(H3[9]aneN3)Me]+ found strong thermodynamic preferences for adductsfeaturing NimideM (M = Al or Zn) interactions. This offers insight into recently observed structure-productivity trends in MAO-activated imido-based polymerization catalysts. Calculations on the metalloceniumadducts [Cp2Ti(-Me)2AlMe2]+ and [Cp2Ti(-Me)2ZnMe]+ are described, each showing 
-agostic interactionsfor the bridging methyl groups. For these systems and the imido ones, the coordination of AlMe3 to thecorresponding monomethyl cation is ca. 30 kJ mol-1 more favorable than for ZnMe2.
-Me)2AlMe2]+ (7+), the first structurally characterized AlMe3 adduct of a transition metalalkyl cation and a model for the presumed resting state in MAO-activated olefin polymerizations. Reactionof 4+ with ZnMe2 also gave a methyl-bridged heterobinuclear species, namely [Ti(-NtBu)(Me3[9]aneN3)(-Me)2ZnMe]+ (8+), the first directly observed ZnMe2 adduct of a transition metal alkyl cation. At roomtemperature, all three metal-bound methyls of 8+ underwent rapid exchange with those of free ZnMe2,whereas at 233 K only the terminal Zn-Me group exchanged significantly. Addition of AlMe3 to 8+quantitatively formed 7+ and ZnMe2. Reaction of 4+ with Cp2ZrMe2 gave [Ti(NtBu){Me2(-CH2)[9]aneN3}(-CH2)ZrCp2]+ (10+) via a highly selective double C-H bond activation reaction in which both alkyl groupsof Cp2ZrMe2 were lost. DFT calculations on models of 7+ confirmed the approximately square-basedpyramidal geometries for the bridging methyl groups. Calculations on 8+ found that the formation of theTi(-Me)2Zn moiety is assisted by an NimideZn dative bond. DFT calculations for the sterically lessencumbered methyl cation [Ti(NMe)(H3[9]aneN3)Me]+ found strong thermodynamic preferences for adductsfeaturing NimideM (M = Al or Zn) interactions. This offers insight into recently observed structure-productivity trends in MAO-activated imido-based polymerization catalysts. Calculations on the metalloceniumadducts [Cp2Ti(-Me)2AlMe2]+ and [Cp2Ti(-Me)2ZnMe]+ are described, each showing -agostic interactionsfor the bridging methyl groups. For these systems and the imido ones, the coordination of AlMe3 to thecorresponding monomethyl cation is ca. 30 kJ mol-1 more favorable than for ZnMe2.