Molecular and electronic structures of bis[1,4-bis(trimethylsilyl)cyclooctatetraene] sandwich complexes of titanium and zirconium

Abstract

The reaction between 2 equivalents of [Li₂{C₈H₆(SiMe₃)₂}] and [MCl₄(thf)₂](M = Ti or Zr) in tetrahydrofuran (thf ) afforded the sandwich complexes [M{C₈H₆(SiMe₃)₂}₂]. X-Ray studies showed that the zirconium compound adopts a structure in which one ring is bound in an η⁸ fashion whilst the other adopts a novel η³ co-ordination mode. This unusual bonding situation has been studied using extended-Hückel molecular-orbital calculations and photoelectron spectroscopy. In contrast, the titanium compound adopts a solid-state structure in which one ring is η⁸ bound and the other is less readily classified, being intermediate between the η³ structure found for Zr and the η⁴ structure found in [Ti(C₈H₈)₂]. Variable-temperature solution NMR studies on [M{C₈H₆(SiMe₃)₂}₂] showed that the two rings remain indistinguishable even at 193 K for M = Zr, whereas for M = Ti the two rings only become equivalent on the NMR time-scale above 328 K.