KINETIC AND MECHANISTIC ASPECTS OF PROPENE OLIGOMERIZATION WITH IONIC ORGANOZIRCONIUM AND ORGANOZIRHAFNIUM COMPOUNDS - CRYSTAL-STRUCTURES OF [CP-STAR-2MME(THT)]+[BPH4]-(M = ZR, HF)

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 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(p)) and termination (k(t)). Values for (DELTA-G double-ended dagger p - DELTA-G double-ended dagger 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 Pna2(1) with a = 31.31 (1) angstrom, b = 11.844 (4) angstrom, c = 11.084 (4) angstrom, V = 4110 (2) angstrom 3, and Z = 4 for [Cp*2ZrMe(THT)]+[BPh4]- and a = 31.32 (1) angstrom, b = 11.857 (1) angstrom, c = 11.029 (1) angstrom, V = 4096 (1) angstrom 3, 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 beta-Me group in the plane between the Cp* rings are more stable than the conformations with the beta-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 beta-H transfer termination) gives inactive (meth)allyl compounds, [Cp*2M(eta(3)-C3H5)]+ and [Cp*2M(eta(3)-C4H7)]+ (M = Zr, Hf). At elevated temperatures (> 45-degrees-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.