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

Abstract

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).

Indenyl Ruthenium Complexes with an Unusual η3 Coordination Mode

“…Complexes with η 1 coordinated indenyl ligands, predominantly with the late transition metals, have also been reported . Recently, Chirik and co-workers have identified and characterized a series of [Zr(η 5 -ind)­(η 9 -ind)] complexes . However, despite extensive research of indenyl complexes, a rather unusual η 3 coordination mode with bonding through the junction carbon and the adjacent carbons of the five- and six-membered rings of the indenyl ligand has not been reported.…”

Indenyl effect in dissociative reactions. Nucleophilic substitution in iron carbonyl complexes: a case study

“…In order to stabilize the unsaturated intermediate, [Fe(Ind)(CO)I], and decrease the reaction barrier, the coordination of the benzene ring of the indenyl ligand to the metal was probed. This is reminiscent of the indenyl coordination in zirconium sandwich complexes, [Zr(Ind) 2 ], 17, 18 and was originally proposed by Mawby et al, 5,6 but all attempts were unsuccessful. Moreover, possible solvent (n-octane) participation in a dissociative path, or even substitution of one CO ligand by one molecule of octane in a concerted step, were also investigated.…”

Mechanisms of reactions in solution

“…There has been a comparative DFT study of reductive elimination of CH 4 from [Zr-(Z 5 -Ind) 2 (CH 3 )(H)], to yield [Zr-(Z 5 -Ind)(Z 6 -Ind)(THF)], formation of the THF adduct and switching between the two indenyl species yielding [Zr-(Z 5 -Ind)(Z 9 -Ind)], a sandwich complex. 213 The favoured mechanism for the reductive elimination involves loss of CH 4 in a single step, followed by formation of the Z 5 /Z 9 species which is then available to react with THF.…”