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

Article abstract of DOI:10.1021/om9003362

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 (^tPrOH, 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(C ₆F₅)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 e-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(e-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.