Measurement of Barriers for Alkene Dissociation and for Inversion at Zirconium in a d0Zirconium−Alkyl−Alkene Complex

The β-allyl zirconacyclobutane complex Cp* 2 Zr[CH 2 CH(CH 2 CH CH 2 )CH 2 ] ( 7 ) reacted rapidly with B(C 6 F 5 ) 3 in CD 2 Cl 2 at −78 °C to form the zwitterionic d 0 zirconium(IV) chelate complex Cp* 2 Zr[η 1,η 2 -CH 2 CH[CH 2 B(C 6 F 5 ) 3 ]CH 2 CH CH 2 ] ( 2a and 2b ). Low-temperature 1 H, 13 C, TOCSY1D, and NOESY1D NMR spectroscopy of 2 established the bonding of the tethered alkene to the d 0 metal center. A dynamic NMR study of the interconversion of 2a and 2b allowed measurement of the alkene dissociation energy (Δ G ⧧ = 10.5 ( 2a to 2b ) and 10.3 ( 2b to 2a ) kcal mol - 1 ), but the complex decomposed before the barrier for site epimerization at the zirconium center could be determined. Reaction of 7 with [(C 6 H 5 ) 2 (CH 3 )NH][B(C 6 F 5 ) 4 ] led to the formation of two isomeric d 0 zirconium(IV)−alkyl−alkene chelates Cp* 2 Zr[η 1,η 2 -CH 2 CH(CH 3 )CH 2 CH CH 2 ][B(C 6 F 5 ) 4 ] ( 8a and 8b ). This more thermally stable zirconium−alkyl−alkene complex allowed the measurement of barriers associated with decomplexation of the alkene (Δ G ⧧ =10.7 and 11.1 kcal mol - 1 ) and site epimerization at the zirconium center (Δ G ⧧ =14.4 kcal mol - 1 ) by line shape analysis of variable-temperature 1 H and 13 C NMR spectra.