Kinetic and mechanistic aspects of propene oligomerization with ionic organozirconium and -hafnium compounds: crystal structures of [Cp*2MMe(THT)]+[BPh4]- (M = zirconium, hafnium)

In N,N-dimethylaniline the ionic complexes [Cp*2MMe(THT)]+[BPh4]- (M = Zr, Hf) oligomerize propene to low molecular weight oligomers. At room temperature for M = Zr a rather broad molecular weight distribution is obtained (C6 to C24), whereas for M = Hf only one dimer (4-methyl-1-pentene) and one trimer (4,6-dimethyl-1-heptene) are formed. With an increase in reaction temperature the product composition shifts to lower molecular weights, but the specific formation of head-to-tail oligomers is retained. The oligomers are formed by β-Me transfer from the growing oligopropene alkyl chain to the metal center. The molecular weight distributions of the oligomers produced at temperature between 5 and 45 °C are satisfactorily described by the Flory-Schulz theory. This allows the calculation of ratios of rate coefficients for propagation (kp) and termination (kt). Values for (ΔG‡p - ΔG‡t)298K were calculated as -1.9 (3) and -1.4 (4) kcal·mol-1 for [Cp*2ZrMe(THT)]+[BPh4]- and [Cp*2HfMe(THT)]+[BPh4]-, respectively. Both complexes crystallize in the space group Pna21 with a = 31.31 (1) Å, b = 11.844 (4) Å, c = 11.084 (4) Å, V = 4110 (2) Å3, and Z = 4 for [Cp*2ZrMe(THT)]+[BPh4]- and a = 31.32 (1) Å, b = 11.857 (1) Å, c = 11.029 (1) Å, V = 4096 (1) Å3, and Z = 4 for [Cp*2HfMe(THT)]+[BPh4]-. A molecular modeling study based on the molecular structures using the ALCHEMY software package suggests that the conformations with the β-Me group in the plane between the Cp* rings are more stable than the conformations with the β-Me group pointing toward one of the Cp* ligands. Inactivation of the catalysts is caused by two different mechanisms. At room temperature allylic C-H activation of monomer and isobutene (formed by a minor β-H transfer termination) gives inactive (meth)allyl compounds, [Cp*2M(η3-C3H5)]+ and [Cp*2M(η3-C4H7)]+ (M = Zr, Hf). At elevated temperatures (>45 °C) catalytically inactive zwitterionic complexes Cp*2M+-m-C6H4-BPh3- (M = Zr, Hf) are formed through aromatic C-H activation. Reactivation of the inactive (meth)allyl complexes can be achieved by addition of hydrogen to the oligomerization mixtures.