Preparation and Use of Ta(CD₂Bu^t)₅ To Probe the Formation of (Bu^tCD₂)₃Ta═CDBu^t. Kinetic and Mechanistic Studies of the Conversion of Pentaneopentyltantalum to the Archetypical Alkylidene Complex

Abstract

Pentaneopentyltantalum, Ta(CH(2)Bu(t))(5) (1), was directly observed earlier in the formation of the archetypical alkylidene complex (Bu(t)CH(2))(3)Ta=CHBu(t) (2) from the reaction of either (Bu(t)CH(2))(3)TaCl(2) (3) with 2 equiv of Bu(t)CH(2)Li or (Bu(t)CH(2))(4)TaCl (4) with 1 equiv of Bu(t)CH(2)Li. Ta(CH(2)Bu(t))(5) (1) was, however, short-lived, and its (1)H NMR resonances were mixed with those of (Bu(t)CH(2))(3)Ta=CHBu(t) (2), Bu(t)CH(2)Li, (Bu(t)CH(2))(3)TaCl(2) (3), (Bu(t)CH(2))(4)TaCl (4), and CMe(4) in a fairly narrow region. In the current work, deuterium-labeled Ta(CD(2)Bu(t))(5) (1-d(10)) has been prepared from the reactions of (Bu(t)CD(2))(3)TaCl(2) (3-d(6)) with 2 equiv of Bu(t)CD(2)Li as well as (Bu(t)CD(2))(4)TaCl (4-d(8)) with 1 equiv of Bu(t)CD(2)Li. Due to a kinetic isotope effect, Ta(CD(2)Bu(t))(5) (1-d(10)) has a much longer life than 1. In addition, there are fewer peaks in the (1)H NMR spectra of Ta(CD(2)Bu(t))(5) (1-d(10)). (2)H NMR spectroscopy can also be used to characterize 1-d(10). These properties provide an opportunity to identify and study 1-d(10) in detail. Kinetic studies of the Ta(CD(2)Bu(t))(5) (1-d(10)) --> (Bu(t)CD(2))(3)Ta=CDBu(t) (2-d(7)) and Ta(CH(2)Bu(t))(5) (1) --> (Bu(t)CH(2))(3)Ta=CHBu(t) (2) conversions yield a kinetic isotope effect (KIE) = 14.1(0.8) at 273 K. In addition, kinetic studies of the 1-d(10) --> 2-d(7) conversion at 273-298 K give DeltaH(double dagger)(D) = 21.1(1.5) kcal/mol and DeltaS(double dagger)(D) = -4(6) eu for the alpha-deuterium abstraction reaction.