Preparation and Use of Ta(CD2But)5 To Probe the Formation of (ButCD2)3TaCDBut. Kinetic and Mechanistic Studies of the Conversion of Pentaneopentyltantalum to the Archetypical Alkylidene Complex
journal contributionposted on 17.06.2009, 00:00 by Julia K. C. Abbott, Liting Li, Zi-Ling Xue
Pentaneopentyltantalum, Ta(CH2But)5 (1), was directly observed earlier in the formation of the archetypical alkylidene complex (ButCH2)3TaCHBut (2) from the reaction of either (ButCH2)3TaCl2 (3) with 2 equiv of ButCH2Li or (ButCH2)4TaCl (4) with 1 equiv of ButCH2Li. Ta(CH2But)5 (1) was, however, short-lived, and its 1H NMR resonances were mixed with those of (ButCH2)3TaCHBut (2), ButCH2Li, (ButCH2)3TaCl2 (3), (ButCH2)4TaCl (4), and CMe4 in a fairly narrow region. In the current work, deuterium-labeled Ta(CD2But)5 (1-d10) has been prepared from the reactions of (ButCD2)3TaCl2 (3-d6) with 2 equiv of ButCD2Li as well as (ButCD2)4TaCl (4-d8) with 1 equiv of ButCD2Li. Due to a kinetic isotope effect, Ta(CD2But)5 (1-d10) has a much longer life than 1. In addition, there are fewer peaks in the 1H NMR spectra of Ta(CD2But)5 (1-d10). 2H NMR spectroscopy can also be used to characterize 1-d10. These properties provide an opportunity to identify and study 1-d10 in detail. Kinetic studies of the Ta(CD2But)5 (1-d10) → (ButCD2)3TaCDBut (2-d7) and Ta(CH2But)5 (1) → (ButCH2)3TaCHBut (2) conversions yield a kinetic isotope effect (KIE) = 14.1(0.8) at 273 K. In addition, kinetic studies of the 1-d10 → 2-d7 conversion at 273−298 K give ΔH⧧D = 21.1(1.5) kcal/mol and ΔS⧧D = −4(6) eu for the α-deuterium abstraction reaction.