The Electronic Influence of Ring Substituents and Ansa Bridges in Zirconocene Complexes as Probed by Infrared Spectroscopic, Electrochemical, and Computational Studies
The electronic influence of unbridged and ansa-bridged ring substituents on a zirconocene center has been studied by means of IR spectroscopic, electrochemical, and computational methods. With respect to IR spectroscopy, the average of the symmetric and asymmetric stretches (ν_(CO(av))) of a large series of dicarbonyl complexes (Cp^R)_2Zr(CO)_2 has been used as a probe of the electronic influence of a cyclopentadienyl ring substituent. For unbridged substituents (Me, Et, Pri, But, SiMe_3), ν_(CO(av)) on a per substituent basis correlates well with Hammett σ_(meta) parameters, thereby indicating that the influence of these substituents is via a simple inductive effect. In contrast, the reduction potentials (E°) of the corresponding dichloride complexes (Cp^R)_2ZrCl_2 do not correlate well with Hammett σ_(meta) parameters, thereby suggesting that factors other than the substituent inductive effect also influence E°. Ansa bridges with single-atom linkers, for example [Me_2C] and [Me_2Si], exert a net electron-withdrawing effect, but the effect is diminished upon increasing the length of the bridge. Indeed, with a linker comprising a three-carbon chain, the [CH_2CH_2CH_2] ansa bridge becomes electron-donating. In contrast to the electron-withdrawing effect observed for a single [Me_2Si] ansa bridge, a pair of vicinal [Me_2Si] ansa bridges exerts an electron-donating effect relative to that from the single bridge. DFT calculations demonstrate that the electron-withdrawing effect of the [Me_2C] and [Me_2Si] ansa-bridges is due to stabilization of the cyclopentadienyl ligand acceptor orbital, which subsequently enhances back-donation from the metal. The calculations also indicate that the electron-donating effect of two vicinal [Me_2Si] ansa bridges, relative to that of a single bridge, is a result of it enforcing a ligand conformation that reduces back-donation from the metal.
Additional Information© 2002 American Chemical Society. Received 15 February 2002. Published online 18 July 2002. Published in print 1 August 2002. We thank the U.S. Department of Energy, Office of Basic Energy Sciences (DE-FG02-93ER14339 to G.P. and DE-FG03-88ER13431 to J.E.B.) and the National Science Foundation (CHE-9807496 to J.E.B.) for support of this research. We thank Professor Jack Norton for use of his electrochemical equipment. Part of this work was carried out using the resources of the Oxford Super-Computing Centre and the EPSRC Columbus cluster at the Rutherford laboratory. We thank Professor Hans Brintzinger for a preprint of references 18b and 71d.
Supplemental Material - ja020236y_s.pdf