Carbon monoxide insertion into zirconium– and hafnium–carbon σ-bonds: synthesis, and structural and thermodynamic data for alkyl- and aryl-η-acylbis(η-cyclopentadienyl)derivatives of zirconium(IV) and hafnium(IV)

Abstract

Carbonylation of toluene solutions of bis(η-cyclopentadientyl)diphenyl-zirconium(IV) and -hafnium(IV), [M(cp)₂(Ph)₂](cp =η-C₅H₅; M = Zr or Hf), with CO at atmospheric pressure gave acyl derivatives [M(cp)₂(COPh)(Ph)][ν(CO)= 1 500 cm^–1]. The same results were obtained for [M(cp)₂R₂](M = Zr or Hf; R = CH₂Ph or Me), but in these cases the carbonyl insertion was found to be reversible. The isolation of [M(cp)₂(COR)R][ν(CO)= 1 530–1 550 cm^–] was possible only for zirconium, while under the same conditions equilibrium constants and thermodynamic data were obtained for CO insertion into the Hf–Me (ΔH^⊖=–12.7 ± 1.6 kcal mol^–1; ΔS^⊖=–33.0 ± 5.6 cal K^–1 mol^–1) and Hf–CH₂Ph (ΔH^⊖=–11.5 ± 0.2 kcal mol^–1; ΔS^⊖=–28.7 ± 0.6 cal K^–1 mol^–1) bonds. All the acyls show a low CO stretching frequency, which indicates an η-acyl bonded group, as was confirmed by X-ray structural data.