Preparation and Use of Ta(CD2But)5 To Probe the Formation of (ButCD2)3Ta═CDBut. Kinetic and Mechanistic Studies of the Conversion of Pentaneopentyltantalum to the Archetypical Alkylidene Complex

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.