d⁰ Metal Olefin Complexes. Synthesis, Structures, and Dynamic Properties of (C₅R₅)₂Zr(OCMe₂CH₂CH₂CHCH₂)⁺ Complexes:  Models for the Elusive (C₅R₅)₂Zr(R)(Olefin)⁺ Intermediates in Metallocene-Based Olefin Polymerization Catalysis

To model the Zr−olefin interaction in the as-yet unobserved (C 5 R 5 ) 2 Zr(R)(olefin) + intermediates in (C 5 R 5 ) 2 Zr(R) + -catalyzed olefin polymerization, the coordination of the tethered vinyl group in (C 5 R 5 ) 2 Zr(OCMe 2 (CH 2 ) n CH CH 2 ) + species has been investigated. The reaction of (C 5 H 5 ) 2 Zr(OCMe 2 CH 2 CH 2 CH CH 2 )(Me) with B(C 6 F 5 ) 3 or [Ph 3 C][B(C 6 F 5 ) 4 ] yields the chelated olefin complex (C 5 H 5 ) 2 Zr(OCMe 2 CH 2 CH 2 CH CH 2 ) + as the MeB(C 6 F 5 ) 3 - ( 12a ) or B(C 6 F 5 ) 4 - ( 12b ) salts. In contrast, the reaction of (C 5 H 5 ) 2 Zr(OCMe 2 CH 2 CH CH 2 )(Me) with B(C 6 F 5 ) 3 in CD 2 Cl 2 yields the MeB(C 6 F 5 ) 3 - adduct (C 5 H 5 ) 2 Zr (+) (OCMe 2 CH 2 CH CH 2 )(μ-Me)B ( - ) (C 6 F 5 ) 3 . The reaction of (C 5 H 5 ) 2 Zr(OCMe 2 CH 2 CH 2 CH 2 CH CH 2 )(Me) with B(C 6 F 5 ) 3 yields a 1.2/1 mixture (at −90 °C) of the chelated olefin complex (C 5 H 5 ) 2 Zr(OCMe 2 CH 2 CH 2 CH 2 CH CH 2 ) + and the MeB(C 6 F 5 ) 3 - adduct (C 5 H 5 ) 2 Zr (+) (OCMe 2 CH 2 CH 2 CH CH 2 )(μ-Me)B ( - ) (C 6 F 5 ) 3 ‘. The reaction of rac -(EBI)Zr(OCMe 2 CH 2 CH 2 CH CH 2 )(Me) (EBI = ethylene-1,2-bis(1-indenyl)) with B(C 6 F 5 ) 3 or [Ph 3 C][B(C 6 F 5 ) 4 ] yields the chelated olefin complex rac -(EBI)Zr(OCMe 2 CH 2 CH 2 CH CH 2 ) + as the MeB(C 6 F 5 ) 3 - ( 20a ) or B(C 6 F 5 ) 4 - ( 20b ) salts, each as a 1/1 mixture of diastereomers which differ in the relative configuration of the rac -(EBI)Zr unit and the internal carbon of the coordinated olefin. X-ray diffraction analyses of 12a and the S,S,R / R,R,S isomer of 20a, and NMR data for 12a, b and 20a, b establish that the Zr−olefin bonding in these species is unsymmetrical and consists of a weak Zr−C term interaction and minimal Zr−C int interaction ( 12a, Zr−C term = 2.68(2), Zr−C int = 2.89(2) Å; 20a, Zr−C term = 2.634(5), Zr−C int = 2.819(4) Å). X-ray ( d C C ), IR (ν C C ), and NMR ( 1 H, 13 C) data show that the Zr−olefin interaction does not significantly perturb the structure of the coordinated olefin but does polarize the C C bond such that positive charge buildup occurs at C int . Similar unsymmetrical bonding and polarization effects may contribute to the high insertion reactivity of (C 5 R 5 ) 2 Zr(R)(α-olefin) + species. Dynamic NMR studies show that 12a, b and 20a, b undergo olefin face exchange in solution on the NMR time scale. The free energy barrier for face exchange of 20a (Δ G ⧧ FE = 15.4(4) kcal/mol at 43 °C) is significantly greater than that for 12a (Δ G ⧧ FE = 10.7(5) kcal/mol at −55 °C). Possible origins of this difference are discussed. The face exchange of 20a is dissociative, with minimal involvement of anion, solvent, or σ-complex intermediates.