Synthesis and Structure of Neutral and Cationic Aluminum Complexes Supported by Bidentate <i>O</i>,<i>P</i>-Phosphinophenolate Ligands and Their Reactivity with Propylene Oxide and ε-Caprolactone

Abstract

The O,P-type phosphinophenol ligands 1a−c were found to readily react with 1 equiv of AlMe3 to afford in high yields the corresponding Al chelate complexes {η2-O,P-(2-PPh2-4-R′-6-R-C6H2O)}AlMe2, 2a−c (R = Me, R′ = H, 2a; R = Ph, R′ = H, 2b; R = tBu, R′ = Me, 2c). The bis-adduct Al methyl complexes {η2-O,P-(2-PPh2-4-R′-6-R-C6H2O)}2AlMe (R = Ph, R′ = H, 3b; R = tBu, R′ = Me, 3c) also formed quantitatively upon reaction of phosphinophenols 1b,c with 0.5 equiv of AlMe3. Both the mono- and bis-adduct Al methyl species 2a−c and 3b,c are stable monomeric species whether in solution or in the solid state and remain stable in coordinating solvents such as thf. In contrast, the bis-adduct Al methyl complex 3c undergoes a ligand exchange reaction in the presence of an alcohol source (iPrOH, BnOH) to generate the homoleptic tris-adduct Al complex {η2-O,P-(2-PPh2-4-Me-6-tBu-C6H3O)}3Al (5c), as determined from X-ray crystallographic studies. Both the mono- and bis-adduct Al methyl species 2b,c and 3b,c react fast with B(C6F5)3 via a methide abstraction reaction to afford the stable and well-defined Al cationic species {η2-O,P-(2-PPh2-6-Ph-C6H3O)}Al(Me)(THF)+ (6b,c+) and {η2-O,P-(2-PPh2-4-R′-6-R-C6H3O)}2Al+ (7b,c+), respectively, which were found to be highly active in propylene oxide polymerization to afford atactic poly(propylene oxide). These cations also readily initiate the ring-opening polymerization of ε-caprolactone via successive ring-opening insertions of the monomer into the Al−O phenoxide bond of the phosphinophenolate chelating ligand to exclusively afford linear poly(ε-caprolactone) capped, at the ester end, with a (phosphino oxide)phenolate group, as deduced from NMR and MALDI-TOF data. In these cationic systems, the PO− chelating moiety may thus act as both a supporting ligand and an initiating group for the ROP of ε-CL.