Neutral Methyl and Cationic Aluminum Complexes Supported by a trans-1,2-Cyclooctanediyl-Bridged [OSSO]-Type Bis(phenolato) Ligand: Synthesis, Structures, and Use in Catalysis for Propylene Oxide Polymerization

Treatment of trans-1,2-cyclooctanediyl-bridged [OSSO]-type bis­(phenolate) 1 with Me₃Al in hexane led to the formation of methylaluminum 4 as colorless crystals. X-ray diffraction analysis of 4 revealed that the aluminum center adopts a distorted-trigonal-bipyramidal geometry which occupies the axial positions with one phenoxide oxygen and one sulfur atoms. The dealkylation reaction of 4 with 1 equiv of B­(C₆F₅)₃ in C₆D₆ afforded the novel cationic aluminum species 5²⁺ as a dimeric structure, which was isolated in the form of a [MeB­(C₆F₅)₃]⁻ salt. In the crystal structure of 5²⁺, one aluminum atom has a distorted-trigonal-bipyramidal environment, in which phenoxide and μ-bridging oxygen atoms are located at two axial positions, whereas the other aluminum atom has a distorted-square-pyramidal geometry, which occupies the axial position with a nonbridged oxygen atom. The cationic species 5²⁺ promoted the ring-opening polymerization (ROP) of propylene oxide to give an atactic polymer in 44% yield with a low molecular weight (M_n = 2500) and a very narrow molecular weight distribution (PDI = 1.04).