Synthesis, structure and molecular dynamics of η2-iminoacyl compounds [Cp(ArO)Zr(η2-ButNCCH2Ph)(CH2Ph)] and [Cp(ArO)Zr(η2-ButNCCH2Ph)2]

Abstract

The reaction of the substrate [CpZr(CH₂Ph)₃] with the ligands 2,3,5,6-tetraphenyl- 1, 2-phenyl-4,6-di-tert-butyl- 2 or 2-(1-naphthyl)-4,6-di-tert-butyl-phenol 3 leads to the corresponding compounds [Cp(ArO)Zr(CH₂Ph)₂] 4–6. Solution studies show that naphthyl rotation is slow on the NMR timescale at ambient temperatures within 6 leading to non-equivalent benzyl groups with diastereotopic methylene protons. The solid-state structure of 5 shows both benzyl ligands to be η¹-bound. Compounds 4–6 react with tert-butylisocyanide in hydrocarbon solvents to initially produce the mono(iminoacyl) derivatives [Cp(ArO)Zr(η²-Bu^tNCCH₂Ph)(CH₂Ph)] 7–9 followed by the bis(iminoacyl) products [Cp(ArO)Zr(η²-Bu^tNCCH₂Ph)₂] 10–12. In the solution ¹³C NMR spectra of the iminoacyl derivatives the Zr–CN carbon atom resonates at δ 242–244 ppm (7–9) and δ 229–234 ppm (10–12) consistent with the η²-C,N binding. This was confirmed in the solid state by X-ray diffraction studies of 8, 11 and 12. At ambient temperatures only one set of iminoacyl resonances are observed for 10 and 11, indicating that both iminoacyl rotation and aryloxide rotation are facile. At lower temperatures the iminoacyl ligands in 11 become non-equivalent in the ¹H NMR spectrum consistent with restricted rotation about the Zr–O–Ar bond. The iminoacyl ligands in the 2-(1-naphthyl) derivative 12 are non-equivalent in solution NMR spectra from −75 to +85 °C. The solution fluxionality of these molecules as determined by NMR studies is discussed in detail.