How big is a Cp? Novel cycloheptatrienyl zirconium complexes with tri-, tetra- and pentasubstituted cyclopentadienyl ligands.

Heiko Bauer,Sebastian Schäfer,Matthias Tamm,Yu Sun,Andreas Glöckner,Helmut Sitzmann,Alain C Tagne Kuate,Marc D Walter,Matthias Freytag

doi:10.1039/c4dt01581a

Abstract

The new bulky cyclopentadienyl anions 1,2,4-tri(cyclopentyl)cyclopentadienide and 2,3-diisopropyl-1,4-dimethyl-5-trimethylsilyl-cyclopentadienide were prepared. These and the already known 1,2,4-tri(cyclohexyl)-, 1,2,4-tri(isopropyl)-, 2,3-diisopropyl-1,4-dimethyl-, 1,3,4-triisopropyl-2,5-dimethyl-, pentaphenyl-, and p-butylphenyl-tetraphenyl-cyclopentadienide as well as tert-butylindenide were coordinated to the cycloheptatrienylzirconium fragment [(CHT)ZrCl(tmeda)]. The nine zirconium complexes of the [(CHT)Zr(Cp)] type were characterized by elemental analysis and NMR spectroscopy. For five of the sandwich complexes X-ray crystal structure determination could be carried out; structures of the four others were obtained by DFT calculations. The data serve as a basis for cone angle measurements of cyclopentadienyl ligands to evaluate the steric demand of these ligands.

Highlights

In the late forties of the last century the parent cyclopentadienyl ligand was seized by freshly reduced iron from cyclopentadiene vapors at 300 °C1 to form a complex later called ferrocene and initiating a rush for cyclopentadienyl complexes of other metals as well.[2,3,4] About 20 years later, when titanium tetrachloride was reacted with butane isomers and other olefins at 300 °C in an autoclave, unexpected pentamethylcyclopentadienyltitanium trichloride[5] was assembled which stimulated experiments with this prototype of polysubstituted cyclopentadienyl ligands.[6]
While the number of cyclopentadienyl complexes known in the literature is in the same order of magnitude as the number of complexes with tertiary phosphine ligands,[11] the attention received by one article on the steric bulk of phosphine ligands[12] by far exceeds the combined impact of a number of review articles on the quantification of the steric bulk of cyclopentadienyl ligands
The steric bulk of the cyclopentadienyl ligands in titanocene derivatives was concluded to affect both the angular opening between the two Cp planes corresponding to the access of reactants to the metal center and the angular opening in the plane bisecting the two Cp planes, which limits the angle between the Ti–C( polymer) σ bond and the direction of the olefin approach.[16]

Summary

Introduction

In the late forties of the last century the parent cyclopentadienyl ligand was seized by freshly reduced iron from cyclopentadiene vapors at 300 °C1 to form a complex later called ferrocene and initiating a rush for cyclopentadienyl complexes of other metals as well.[2,3,4] About 20 years later, when titanium tetrachloride was reacted with butane isomers and other olefins at 300 °C in an autoclave, unexpected pentamethylcyclopentadienyltitanium trichloride[5] was assembled which stimulated experiments with this prototype of polysubstituted cyclopentadienyl ligands.[6]. A discussion of proton and 13C NMR spectra of complex 6 can be found in the ESI.† A structure model had to be calculated by DFT methods (Fig. 6 and Table 2) which shows a slightly bent sandwich structure with an interplanar angle of 8.6°.

Results

Conclusion