Isotopically labeled zirconocene methyl neopentyl complexes of the formula (CpR n) 2Zr-(CH 3)(CH 2C(CH 3) 2CD 3) are obtained via reaction of LiCH 2C(CH 3) 2CD 3 with (CpR n) 2Zr(CH 3)-(Cl). Addition of B(C 6F 5) 3 to (CpR n) 2Zr(CH 3)(CH 2C(CH 3) 2CD 3) results in β-methyl elimination, forming the ion-paired species [(CpR n) 2Zr(CH 3)] [CH 3B(C 6F 5) 3] along with isotopologs of isobutene. The relative amounts of d 3- and d 0-isobutene afford the isotope effect for β-methyl elimination. For Cp 2Zr(CH 3)(CH 2C(CH 3) 2CD 3) (Cp = (n 5-C 5H 5)), a kinetic deuterium isotope effect of 1.40(2) has been measured at 23°C. Comparable deuterium kinetic isotope effects have been observed for four other zirconocene methyl neopentyl compounds: [Cp*(C 5Me 4H)-Zr] (Cp* = (n 5-C 5Me 5)), [Cp* 2Zr], [rac-(EBI)Zr] (EBI - ethylenebis(indenyl)), and [(THP)Zr] (THP = 1,2-(SiMe 2) 2(n 5-3,5-C 5H(CHMe 2) 2)(n 5-C 5H 3)). The direction and magnitude of these effects are consistent with γ-agostic assistance in the transition state for β-methyl elimination, the microscopic reverse of α-agostic assistance in the transition state for olefin insertion into the Zr-methyl bond.
All Science Journal Classification (ASJC) codes
- Physical and Theoretical Chemistry
- Organic Chemistry
- Inorganic Chemistry