Dynamic NMR study of a 1,3-diyne bridged dinuclear bis(cyclopentadienyl)titanium complex: understanding the C?C single bond metathesis reaction

Abstract

The coordination of 1,3-diynes as bridging ligands between two metallocene units is in many cases associated with the cleavage of their central C—C single bond. The present study was conducted with the complex [Ti(η5-C5H5)2]2 (µ-(1,3-η),(2,4-η′)-Me3Si-CC-CC-CMe3) (1a) which at first sight does not exhibit such a bond cleavage. 1H and 13C NMR studies revealed a dynamic behaviour leading to an equivalence of all four cyclopentadienyl ligands (which the solid-state structure is lacking). The most likely explanation for this invokes an equilibrium between 1a and an isomer with a cleaved central C—C bond and two bridging σ, π-alkynyl ligands that may interchange their binding function between the metal centres. This interpretation is supported by a theoretical analysis from the literature. The energetic differences between such isomeric complexes are small, so experimental conditions may shift the assumed equilibrium to either side. Examination of the 1H NMR spectra gives , which is in accord with theoretical predictions. The discussed equilibrium is an essential feature in the C—C single bond metathesis reaction of 1,3-diynes which is accomplished by complexation with two titanocene fragments and dissociation of the primarily formed dinuclear complexes, e.g. 1a. Copyright © 2000 John Wiley & Sons, Ltd.