Synthesis and Structure of Neutral and Cationic Aluminum Complexes Supported by Bidentate 0,P-Phosphinophenolate Ligands and Their Reactivity with Propylene Oxide and ε-Caprolactone

The 0,P-type phosphinoplienol ligands 1a-c were round to readily react with I equiv of AIMc(3) to to afford in high yields the corresponding A1 chelate complexes {n(2-)O,P-(2-PPh2-4-R'6-R-C6H2O)}- 2a-c (R = Me R'= H, 2a; R = Ph. R'= H. 21): R 'BU, R' Me, 2c). The bis-adduct Al methyl complexes I it, {n(2-)O, P-(2-PPh2,-4-R'-6-C6-R-C6H2O)}(2)AIMe (R=Ph.(RH)-H-1=.3b; R='Bu,R'=Me,3c) formed quantitatively upon reaction also formed quantitatively upon reaction of phosphinophenols 1b, c with 0.5 equiv of AIMe(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 A1 methyl complex 3c undergoes a ligand exchange reaction in the presence of an alcohol source ('PrOH,BnOH) to generate the homoleptic tris-adduct A1 complex {n(2-)O,P-(2-PPh2-4-Me-6-'Bu-C6H3O)}(3)A1 (5c), as determined X-ray crystallographic Studies, Both the mono-and bis-adduct A1 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 A1 cationic species {n2-O,P-2-PPh2-6-Ph-C6H3O)}A1(Me) (THF)(+)(6b,c(+)) and {n(2-)O,P-(2-PPh2-4-R'-6-R-C6H3O)}(2)A1(+) (7b,c(+))(,) respectively, which were found to be highly active in propylene oxide polymerization to afford atactic polv(propyleric oxide), These cations a,lso readily initiatc the ring-opening polymerization of epsilon-caprolactone via successive ring-opening insertions of the monomer into the Al-O plienoxide bond of the phosphinophenolate chelating ligand to exclusively afford linear poly(epsilon-caprolactone) capped, at the ester end, with a (phosphino oxide)phenolate group, as deduced frorn NMR and MALDI-TOF data. In these cationic systems the PO- chelating nioiety nlay thus act as both a Supporting ligand and in initiating group for the ROP of epsilon-CL.