Zirconium bis(indenyl) sandwich complexes with an unprecedented indenyl coordination mode and their role in the reactivity of the parent bent-metallocenes: a detailed DFT mechanistic study

The mechanisms of three closely related reactions were studied in detail by means of DFT/B3 LYP calculations with a VDZP basis set. Those reactions correspond to 1) the reductive elimination of methane from Zr(eta5-Ind)2(CH3)(H)] (Ind=C9H7-, indenyl), 2) the formation of the THF adduct, Zr(eta5-Ind)(eta6-Ind)(thf)] and 3) the interconversion between the two indenyl ligands in the Zr sandwich complex, Zr(eta5-Ind)(eta9-Ind)], which forms the link between the two former reactions. An analysis of the electronic structure of this species indicates a saturated 18-electron complex. A full understanding of the indenyl interchange process required the characterisation of several isomers of the Zr-bis(indenyl) species, corresponding to different spin states (S=0 and S=1), different coordination modes of the two indenyl ligands (eta5/eta9, eta5/eta5 and eta6/eta9), and three conformations for each isomer (syn, anti, and gauche). The fluxionality observed was found to occur in a mechanism involving bis(eta5-Ind) intermediates, and the calculated activation energy (11-14 kcal mol(-1)) compares very well with the experimental values. Two alternative mechanisms were explored for the reductive elimination of methane from the methyl/hydride complex. In the more favourable one, the initial complex, Zr(eta5-Ind)2(CH3)(H)], yields Zr(eta5-Ind)2] and methane in one crucial step, followed by a smooth transition of the Zr intermediate to the more stable eta5/eta9-species. The overall activation energy calculated (Ea=29 kcal mol(-1)) compares well with experimental values for related species. The formation of the THF adduct follows a one step mechanism from the appropriate conformer of the Zr(eta5-Ind)(eta9-Ind)] complex, producing easily (Ea=6.5 kcal mol(-1)) the known product, Zr(eta5-Ind)(eta6-Ind)(thf)], a species previously characterised by X-ray crystallography. This complex was found to be trapped in a potential well that prevents it from evolving to the 3.4 kcal mol(-1) more stable isomer, Zr(eta5-Ind)2(thf)], with both indenyl ligands in a eta5-coordination mode and a spin-triplet state (S=1).