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)

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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.

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