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 H(2)O(2) or organic azides RN(3) (R = 2,6-C(6)H(3)(i)Pr(2), SiMe(3)) 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 (1)H NMR spectroscopy is of particular interest. Dichloroaluminium complexes 5c-d were prepared in high yields by protonolysis of MeAlCl(2) 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.

Synthesis, Structure, and Ring‐Opening Polymerization Catalysis of Zinc Complexes Containing Amido Phosphinimine Ligands

“…For instance, the two o-alkyl groups in each N-aryl moiety are all chemically equivalent, so are the two phosphorus-bound phenyl rings. Consistent with the more oxidized nature of the phosphinimine functionality, the 31 (Table 1) are all comparatively downfield from those of H [1] (ca. -20 ppm).…”

“…The formation rates of 3-4 appear to increase in the order a Ͻ b Ͻ c, consistent with that expected from a steric standpoint. Prior to the formation of the zinc alkyls 3-4, no intermediate was detected by 31 …”

“…[31] Interestingly, these three-coordinate zinc alkyls do not bind strong coordinating solvents such as THF or diethyl ether in spite of their coordinatively unsaturated nature, which highlights the sufficient steric crowdedness imposed by these amido phosphinimine ligands. The preparation of three-coordinate zinc complexes that are free of coordinating solvents is of current interest.…”

“…Following established protocols, [31] the protio ligand precursors H [2] were readily prepared by Staudinger reactions [44] of N-(2-diphenylphosphanylphenyl)-2,6-dialkylaniline (H [1]) [36,43] with appropriate organoazides [45][46][47] in toluene or DME solutions at 80°C (Scheme 1). Piers et al recently described a distinct strategy to prepare H[2a] [48] and H[2b].…”

“…Of particular note are zinc complexes of tris(pyrazolyl)hydroborates [11] and β-diketiminates, [12,13] which have shown remarkable catalytic activities in a controlled manner. We are currently exploring the reaction chemistry with metal complexes of amido phosphinimine ligands, [31] which may be regarded as an electronic modification of β-diketiminates. The amido phosphinimine ligands are oxidative derivatives of amido phosphanes, [32][33][34][35][36][37][38][39][40][41] as either solvent-free complexes or solvated adducts depending on the reaction solvents employed (toluene, OEt2, or THF).…”