Neutral and cationic aluminium complexes containing a chiral (OSSO)-type bis(phenolato) ligand: synthesis, structures and polymerization activity

Reaction of trimethylaluminium with the linked bis(phenol) trans-1,4-dithiocyclohexanediyl-2,2'-bis(4,6-di-tert-butylphenol) (cydtbpH(2), 1a) led to the chiral methyl aluminium complex [Al(cydtbp) Me] (2a) as both racemate and resolved enantiomers, which were characterized by multinuclear NMR spectroscopy and elemental analysis. Rac-{trans-1,2-dithiocyclohexanediyl-2,2'-bis(6-tert-butyl-4-methylphenol)} (cytbmpH(2), rac-1b) similarly gave rac-2b. Single-crystal X-ray diffraction of (S, S)-2a and rac-2b showed a strongly distorted trigonal bipyramidal geometry with significantly differing Al-S distances. Reaction of rac-2b with (-)-borneol gave the alkoxy complex 5 as a mixture of two diastereomers. Methyl abstraction from the neutral metal complex rac-2a using B(C6F5)(3) gave the cationic complex [rac-Al(cydtbp)(THF)(2)](+)[MeB(C6F5)(3)](-)(rac-6.(THF)(2)) that according to X-ray crystallography adopts an octahedral coordination geometry. Upon reacting trimethylaluminium with a bis(phenol) with a longer link, 3,4-trans-butanediyl-1,6-dithiahexanediyl)-2,2'-bis(6-tertbutyl-4-methylphenol), (cmtbmpH(2), 3, a dinuclear compound 4 was obtained that contains two square pyramidal aluminium centers with two fly-over bis(phenolate) ligands. The neutral methyl aluminium complexes are active in the polymerization of methyl methacrylate (MMA) and in the ring-polymerization of rac-lactide (LA), producing PMMAs and PLAs in a controlled fashion. The cationic aluminium complexes were active in the cationic polymerization of isoprene and benzofuran.