Kinetic and mechanistic aspects of propene oligomerization with ionic organozirconium and -hafnium compounds: crystal structures of [Cp2MMe(THT)]+[BPh4]- (M = zirconium, hafnium)

In N,N-dimethylaniline the ionic complexes [Cp{sup *}{sub 2}MMe(THT)]{sup +}[BPh{sub 4}]{sup {minus}}(M=Zr,Hf) oligomerize propene to low molecular weight oligomers. At room temperature for M = Zr a rather broad molecular weight distribution is obtained (C{sub 6} to C{sub 24}), 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 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 {beta}-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 {degrees}C are satisfactorily described by the Flory-Schulz theory. This allows the calculation of ratios of rate coefficients for propagation (k{sub p}) and termination (k{sub t}). 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 {beta}-H transfer termination) gives inactive (meth) allyl compounds, [Cp{sup *} {sub 2}M({eta}{sup 3}-C{sub 3}H{sub 5})]{sup +} and [Cp{sup *}{sub 2}M({eta}{sup 3}-C{sub 4}H{sub 7})]{sup +} (M = Zr, Hf). At elevated temperatures (>45 {degrees}C) catalytically inactive zwitterionic complexes Cp{sup *}{sub M}{sup +}-m-C{sub 6}H{sub 4}-BPh{sub 3}{sup {minus}}(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. 38 refs., 4 figs., 7 tabs.