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

The O,P -type phosphinophenol ligands 1a − c were found to readily react with 1 equiv of AlMe 3 to afford in high yields the corresponding Al chelate complexes {η 2 -O,P-(2-PPh 2 -4-R′-6-R-C 6 H 2 O)}AlMe 2, 2a − c (R = Me, R′ = H, 2a; R = Ph, R′ = H, 2b; R = t Bu, R′ = Me, 2c ). The bis-adduct Al methyl complexes {η 2 -O,P-(2-PPh 2 -4-R′-6-R-C 6 H 2 O)} 2 AlMe (R = Ph, R′ = H, 3b; R = t Bu, R′ = Me, 3c ) also formed quantitatively upon reaction of phosphinophenols 1b, c with 0.5 equiv of AlMe 3 . 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 ( i PrOH, BnOH) to generate the homoleptic tris-adduct Al complex {η 2 -O,P-(2-PPh 2 -4-Me-6- t Bu-C 6 H 3 O)} 3 Al ( 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(C 6 F 5 ) 3 via a methide abstraction reaction to afford the stable and well-defined Al cationic species {η 2 -O,P-(2-PPh 2 -6-Ph-C 6 H 3 O)}Al(Me)(THF) + ( 6b, c + ) and {η 2 -O,P-(2-PPh 2 -4-R′-6-R-C 6 H 3 O)} 2 Al + ( 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.