Aluminium complexes of bidentate N,O- and N,N-ligands derived from oxidative functionalization of amido phosphines: synthesis, structure and reactivity

Abstract

A series of amido phosphinoxide and amido phosphinimine ligands that are electronic variations of monoanionic N,O- and N,N-ketiminates have been prepared and employed to examine the coordination chemistry of aluminium. Oxidation of the previously established N-(2-diphenylphosphinophenyl)-2,6-dialkylaniline in the presence of H₂O₂ or organic azides RN₃ (R = 2,6-C₆H₃ⁱPr₂, SiMe₃) led to phosphinoxides (H[NO] 1a–b) and phosphinimines (H[NN] 1c–d), respectively. Alkane elimination reactions of these protio-ligand precursors with trialkylaluminium in toluene or pentane solutions afforded cleanly the corresponding organoaluminium complexes, including dimethyl 2a–d, diethyl 3a–d and diisobutyl derivatives 4a–b and 4d. Solution NMR studies revealed Cs symmetry for these organoaluminium species, in which the α-hydrogen atoms are all diastereotopic. The correlation between the steric congestion of these molecules and the degree of resolution of the multiplet signals corresponding to the diastereotopic α-hydrogen atoms observed by the ¹H NMR spectroscopy is of particular interest. Dichloroaluminium complexes 5c–d were prepared in high yields by protonolysis of MeAlCl₂ with 1c–d. Single-crystal X-ray diffraction analyses of 2c–d, 3a, 3d, 4a, and 4d elucidated a mononuclear, distorted tetrahedral core for all of these aluminium species. Interestingly, complexes 2c–d are active initiators for catalytic ring-opening oligomerization of ε-caprolactone, whereas 2a–b are rather inactive, highlighting the significance of the steric hindrance imposed by the amido phosphinimine ligands, as compared to that imposed by the phosphinoxide counterparts.