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

Reacting 1-phenyl-2-(diphenylphosphino)ethanone (P(1)similar toOH), 1-tert-butyl-2-(diphenylphosphino)ethanone (P(2)similar toOH) and 1-phenyl-2-(diphenylphosphino)propanone (P(3)similar toOH) with ReOCl3(PPh3)(2) and Re(NPh)Cl-3(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(Psimilar toO)(2) compounds obtained in toluene adopt the 'twisted' cis-(P, P) octahedral structure. With Re(NPh)Cl-3(PPh3)(2), P(1)similar toOH and P(2)similar toOH give the corresponding Re(NPh)Cl(Psimilar toO)(2) compounds and the trans-(P,P)-trans-(Cl,Cl) isomer of monosubstituted Re(NPh)Cl-2(PPh3)(Psimilar toO) as the major products, together with small amounts of 'twisted' trans-(P,P) Re(NPh)Cl(Psimilar toO)(2). The monosubstituted complex is the only species isolated with P(3)similar toOH. 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)(Psimilar toO)(2) are isolated, resulting from stereoselective substitution of the halide in the 'twisted' cis-(P,P) octahedral complexes. The reactions with Re(NPh)Cl-3(PPh3)(2) are less selective and give rise to mixtures of complexes. The trans-ethoxo-phenylimido compound Re(NPh)(OEt)(P(1)similar toO)(2) is obtained in good yield. A crystallographic study reveals an unexpected 'equatorial' trans-(P,P) structure with the ethoxo ligand trans to the Re=NPh bond. H-1 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.