Alkyne and Alkene Complexes of a d0 Zirconocene Aryl Cation

The generation and properties of nonchelated Zr−aryl−alkyne and Zr−aryl−alkene complexes that are stabilized by the presence of β-Si-substituents in the alkyne and alkene ligands and fluorination of the aryl ligand are described. Reaction of [Cp'2Zr(OtBu)(ClCD2Cl)][B(C6F5)4] (1, Cp' = C5H4Me) with alkyne and alkene substrates (L) generates Cp'2Zr(OtBu)(L)+ adducts (L = HC⋮CCH2SiMe3 (2); H2CCHCH2SiMe3 (3); HC⋮CMe (4); H2CCHCH2CMe3 (5)). Equilibrium constants for substrate binding (Keq = [Zr-L][1]-1[L]-1; CD2Cl2, −89 °C) are much larger for the β-Si-substituted compounds 2 (1.0(2) × 105 M-1) and 3 (1.7(4) × 103 M-1) than for hydrocarbon analogues 4 (3.6(7) × 102 M-1) and 5 (1.9(1) M-1), which is ascribed to β-Si stabilization of the partial positive charge on Cint of the bound substrate. [Cp2Zr(C6F5)][B(C6F5)4] (7, Cp = C5H5) was generated by the reaction of Cp2Zr(C6F5)Me with [Ph3C][B(C6F5)4] in C6D5Cl. Reaction of 7 with alkyne and alkene substrates (L) generates Cp2Zr(C6F5)(L)+ adducts (L = HC⋮CCH2SiMe3 (8); H2CCHCH2SiMe3 (10)). No insertion of the substrate into the Zr−C6F5 bond is observed in 8 (at −38 °C) or 10 (up to 22 °C). The allyltrimethylsilane ligand in 10 undergoes nondissociative alkene face exchange ("alkene flipping", i.e., exchange of the Cp2Zr(C6F5)+ unit between the two alkene enantiofaces without alkene dissociation), with a first-order rate constant kflip = 23(1) s-1 (C6D5Cl, −38 °C). 10 also undergoes slower reversible decomplexation of the alkene (kdissoc = 5.0(8) s-1; C6D5Cl, −38 °C).