Nonchelated d0 zirconium-alkoxide-alkene complexes

The reaction of Cp'Zr-2((OBu)-Bu-t)Me (Cp' = C5H4Me) and [Ph3C][B(C6F5)(4)] yields the base-free complex [Cp'Zr-2((OBu)-Bu-t)][B(C6F5)(4)] (6), which exists as Cp'Zr-2((OBu)-Bu-t)(ClR)(+) halocarbon adducts in CD2Cl2 or C6D5Cl solution. Addition of alkenes to 6 in CD2Cl2 solution at low temperature gives equilibrium mixtures of Cp'(2) -Zr((OBu)-Bu-t)(alkene)(+) (12a-I), 6, and free alkene. The NMR data for 12a-I are consistent with unsymmetrical alkene bonding and polarization of the alkene C = C bond with positive charge buildup at C-int and negative charge buildup at C-term. These features arise due to the lack of d-pi(*) back-bonding. Equilibrium constants for alkene coordination to 6 in CD2Cl2 at -89 degrees C, K-eq = [12][6](-1)[alkene](-1), vary in the order: vinylferrocene (4800 M-1) >> ethylene (7.0) approximate to alpha-olefins > cis-2-butene (2.2) > trans-2-butene (< 0.1). Alkene coordination is inhibited by sterically bulky substituents on the alkene but is greatly enhanced by electron-donating groups and the beta-Si effect. Compounds 12a-I undergo two dynamic processes: reversible alkene decomplexation via associative substitution of a CD2Cl2 molecule, and rapid rotation of the alkene around the metal-(alkene centroid) axis.