d0 metal olefin complexes.: Synthesis, structures, and dynamic properties of (C5R5)2Zr(OCMe2CH2CH2CH=CH2)+ 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 (C5R5)(2)Zr(R)(olefin)(+) intermediates in (C5R5)(2)Zr(R)(+)-catalyzed olefin polymerization, the coordination of the tethered vinyl group in (C5R5)(2)Zr(OCMe2(CH2)(n)CH=CH2)(+) species has been investigated. The reaction of (C5H5)(2)Zr(OCMe2CH2CH2CH= CH2)(Me) with B(C6F5)(3) or [Ph3C][B(C6F5)(4)] yields rhechelatrd olefin complex (C5H5)(2)Zr(OCMe2CH2CfI2CH= CH2)(+) as the MeB(C6F5)(3)(-) (12a) or B(C6F5)(4)(-) (12b) salts. In contrast, the reaction of (C5H5)(2)Zr(OCMe2CH2CH= CH2)(Me) with B(C6F5)(3) in CD2Cl2 yields the MeB(C6F5)(3)(-) adduct (C5H5)(2)Zr(+)(OCMe2CH2CH=CH2)(mu-Me)B(-)(C6F5)(3) The reaction of (C5H5)(2)Zr(OCMe2CH2CH2CH2CH=CH2(Me) with B(C6F5)(3) yields a 1.2/1 mixture (at -90 degrees C) of the chelated olefin complex (C5H5)(2)Zr(OCMe2CH2CH2CH2CH=CH2)(+) and the MeB(C6F5)(3)(-) adduct (C5H5)(2)Zr(+)(OCMe2CH2CH2CH=CH2)(mu-Me)B(-)(C6F5)(3) The reaction of rac-(EBI)-Zr(OCMe2CH2CH2CH=CH2)(Me) (EBI =ethylene-1,2-bis(1 -indenyl)) with B(C6F5)(3) or [Ph3C][B(C6F5)(4)] yields the chelated olefin complex rac-(EBI)Zr(OCMe2CH2CH2CH=CH2)(+) as the MeB(C6F5)(3)(-) 20a) or B(C6F5)(3)(-)(20a) salts, each as a 1/1 mixture of diastereomers which differ in the relative configuration of the mc-(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) Angstrom; 20a, Zr-C-term = 2.634(5),Zr-C-int = 2.819(4) Angstrom). X-ray (d(C-C)),IR ((C-C)), and NMR (H-1, C-13) data shaw 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 (C5R5)(2)Zr-(R)(alpha-olefin)(+) species. Dynamic NMR studies show that 12a,b and 20a,b undergo olefin race exchange in solution on the NMR time scale. The free energy barrier for face exchange of 20a (Delta G(FE)(double dagger) = 15.4(4) kcal/mol at 43 degrees C) is significantly greater than that for 12a (Delta G(FE)(double dagger) =10.7(5) kcal/mol at -55 degrees C). Possible origins of this difference are discussed. The face exchange of 20a is dissociative, with minimal involvement of anion, solvent, or a-complex intermediates.