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 (v_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, Bu^t, SiMe₃), v_CO(av) on a per substituent basis correlates well with Hammett δ_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)₂ZrCl₂ do not correlate well with Hammett δmeta parameters, thereby suggesting that factors other than the substituent inductive effect also influence E°. Ansa bridges with single-atom linkers, for example [Me₂C] and [Me₂Si], 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 [CH₂CH₂CH₂] ansa bridge becomes electron-donating. In contrast to the electron-withdrawing effect observed for a single [Me₂Si] 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 [Me₂C] and [Me₂Si] 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 [Me₂Si] 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.