Synthesis and characterisation of oxo- and phenylimido-rhenium(v) complexes containing bidentate phosphinoenolato ligands

Abstract

Reacting 1-phenyl-2-(diphenylphosphino)ethanone (P₁∼OH), 1-tert-butyl-2-(diphenylphosphino)ethanone (P₂∼OH) and 1-phenyl-2-(diphenylphosphino)propanone (P₃∼OH) with ReOCl₃(PPh₃)₂ and Re(NPh)Cl₃(PPh₃)₂ in toluene and ethanol in the presence of NEt₃ yields complexes in which these ligands bind as monoanionic enolato chelating agents. Bromo and iodo analogues are prepared similarly. The reactions are solvent dependent. The disubstituted ReOCl(P∼O)₂ compounds obtained in toluene adopt the ‘twisted’ cis-(P,P) octahedral structure. With Re(NPh)Cl₃(PPh₃)₂, P₁∼OH and P₂∼OH give the corresponding Re(NPh)Cl(P∼O)₂ compounds and the trans-(P,P)-trans-(Cl,Cl) isomer of monosubstituted Re(NPh)Cl₂(PPh₃)(P∼O) as the major products, together with small amounts of ‘twisted’ trans-(P,P) Re(NPh)Cl(P∼O)₂. The monosubstituted complex is the only species isolated with P₃∼OH. The major structural difference between the two systems is the small stability gap between the cis- and the trans-(P,P) isomers of the imido complexes. In ethanol, unexpected cis-ethoxo-oxo complexes ReO(OEt)(P∼O)₂ are isolated, resulting from stereoselective substitution of the halide in the ‘twisted’ cis-(P,P) octahedral complexes. The reactions with Re(NPh)Cl₃(PPh₃)₂ are less selective and give rise to mixtures of complexes. The trans-ethoxo-phenylimido compound Re(NPh)(OEt)(P₁∼O)₂ is obtained in good yield. A crystallographic study reveals an unexpected ‘equatorial’ trans-(P,P) structure with the ethoxo ligand trans to the ReNPh bond. ¹H NMR indicates that the ethoxo ligand acts as an electron reservoir in these complexes, since its protons are strongly deshielded in the cis-oxo-ethoxo species and strongly shielded in the trans-phenylimido-ethoxo complex.