Alternating copolymerization of propylene oxide and carbon dioxide with highlv efficient and selective (salen)Co(III) catalysts: Effect of ligand and cocatalyst variation

Synthetic routes to a series of new (salen)CoX (salen = N,N'-bis(salicylidene)-1,2-diaminoalkane; X = Br or pentafluorobenzoate (OBzF(5))) species are described. Several of these complexes are active for the copolymerization of propylene oxide (PO) and CO2, yielding regioregular poly(propylene carbonate) (PPC) without the generation of propylene carbonate byproduct. Variation of the salen ligand, as well as the inclusion of organic-based ionic or Lewis basic cocatalysts, has dramatic effects on the resultant (salen) CoX catalytic activity. Highly active (RR)-(salen-1)CoOBzF(5) (salen-1 N,N'-bis(3,5di-tert-butylsalicylidene)-1,2-diaminocyclohexane) catalysts with [Ph4P]Cl or [PPN]Y ([PPN] = bis(triphenylphosphine)iminium; Y = Cl or OBzF(5)) cocatalysts exhibited turnover frequencies up to 720 h(-1) for rac-PO/CO2 copolymerization, yielding PPC with greater than 90% head-to-tail connectivity. Additionally, the (RR)-(salen-1)CoOBzF(5)/ [PPN]Cl catalyst system demonstrated a k(rel) of 9.7 for the enchainment of (S)- over (R)PO when the copolymerization was carried out at low temperatures. (c) 2006 Wiley Periodicals, Inc.