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

• Published in: Chemistry : a European journal Vol. 11; no. 8; pp. 2505 - 2518
• Main Author: Veiros, Luis F.
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 08-04-2005; WILEY‐VCH Verlag
• Subjects: density funtional calculations; indenyl ligands; reaction mechanisms; sandwich complexes; zirconium
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.200401235

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(η5‐Ind)2(CH3)(H)] (Ind=C9H7−, indenyl), 2) the formation of the THF adduct, [Zr(η5‐Ind)(η6‐Ind)(thf)] and 3) the interconversion between the two indenyl ligands in the Zr sandwich complex, [Zr(η5‐Ind)(η9‐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 (η5/η9, η5/η5 and η6/η9), and three conformations for each isomer (syn, anti, and gauche). The fluxionality observed was found to occur in a mechanism involving bis(η5‐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(η5‐Ind)2(CH3)(H)], yields [Zr(η5‐Ind)2] and methane in one crucial step, followed by a smooth transition of the Zr intermediate to the more stable η5/η9‐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(η5‐Ind)(η9‐Ind)] complex, producing easily (Ea=6.5 kcal mol−1) the known product, [Zr(η5‐Ind)(η6‐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(η5‐Ind)2(thf)], with both indenyl ligands in a η5‐coordination mode and a spin‐triplet state (S=1).
Indenyl slippage: DFT/B3 LYP studies were performed in order to search the mechanisms of reactions involving indenyl slippage in [Zr(Ind)2] complexes. The mechanism obtained for the interconversion between the two ligands in the sandwich complex (see scheme), involves η5/η5‐intermediates and accessible activation energy (11–14 kcal mol−1).