Phosphido-diphosphine pincer aluminum complexes as catalysts for ring opening polymerization of cyclic esters

New aluminum alkyl complexes, supported by o-phenylene-derived phosphido diphosphine pro-ligands [Ph-PPP]-H and [iPr-PPP]-H ([Ph-PPP]-H5bis(2-diphenylphosphinophenyl) phosphine; [iPr-PPP]-H5bis(2-diisopropylphosphinophenyl) phosphine) are reported. Compounds [Ph-PPP]AlMe2 (1), [iPr-PPP]AlMe2 (2), and [Ph-PPP]AliBu2 (3) have been synthesized by reaction of the pro-ligand with the appropriate trialkyl aluminum precursor and have been characterized by 1H, 13C and 31P NMR spectroscopy. The solution NMR data and theoretical calculations suggest for all complexes trigonal bipyramidal structures with C2v symmetry in which the phosphido diphosphine ligand acts as a j3 coordinated ligand. All complexes promote the ring-opening polymerization of e-caprolactone, L- and rac-lactide. Polyesters with controlled molecular parameters (Mn, end groups) and low polydispersities are obtained. Upon addition of isopropanol, efficient binary catalytic systems for the immortal ring-opening polymerization of the cyclic esters are produced. Preliminary investigations show the ability of these complexes to promote copolymerization of L-lactide and E-caprolactone to achieve copolymers whose microstructures are depending on the structure of the catalyst.