Synthesis, Structures, and Dynamic Features of d0 Zirconocene–Allyl Complexes

Abstract

The reaction of [Cp2ZrMe][MeB(C6F5)3] (1) with 2,4-dimethyl-1-pentene and 2,4-dimethyl-1-heptene, respectively, in C6D5Cl at 25 °C results in irreversible formation of the cationic Cp2Zr+–allyl complexes [Cp2Zr(η3-CH2C(CH2R)CH2)]+ (2a, 3a) and [Cp2Zr(η3-CH2C(Me)CHR)]+ (2b, 3b) (2a,b, R = CH(CH3)2; 3a,b, R = CH(CH3)CH2CH2CH3) and release of methane. The Cp2Zr+–allyl complexes were characterized with regard to their structures and rearrangement dynamics of their allyl ligands by NMR spectroscopy. Variable-temperature 1H NMR experiments show that the allyl ligands of complexes 2a,b and 3a,b are fluxional. The fluxional behavior in these complexes is mainly due to a mechanism that involves η3 to η1 isomerization, rotation of the allyl carbon–carbon π unit about the carbon–carbon σ bond, and reversion to the η3-allyl coordination mode, when both the allyl syn/anti hydrogen exchange and apparent Cp ligand exchange occur. The rotation of the C═C unit about the allyl carbon–carbon σ bond also results in a reversal of the η3-allyl coordination face relative to the Cp2Zr+ moiety. A second mechanism which may account for the apparent Cp ligand exchange in the Cp2Zr+–allyl complexes under investigation consists of rotation of the η3-coordinated allyl ligand about the metal–allyl bond. The free energy of activation for the exchange processes, as estimated from the coalescence temperature of the two Cp ligands, is between 54 and 60 kJ/mol. Aside from the intramolecular allyl exchange processes described above, this study also shows that the η1- and η3-coordinated allyl forms of a particular Zr–allyl complex coexist in solution and that the equilibrium composition of these species is temperature dependent. The Cp2Zr+–allyl complexes described in this paper serve as models for similar cationic Cp2Zr+–allyl intermediates implicated in zirconocene-catalyzed alkene polymerization reactions.