Novel Neutral and Cationic Aluminium Alkyl Complexes Supported by Potentially Tridentate O,N,L‐Type Aminophenolate Ligands and Their Use in Propylene Oxide Polymerization

Abstract

AbstractPotentially tridentate O,N,L‐type aminophenol proligands of the type 2‐{CH2N(C4H8L)}‐6‐R‐C6H3OH (1a: R = Ph, L = O; 1b: R = Ph, L = NMe; 1c: R = tBu, L = O; 1d: R = tBu, L′ = NMe) and 6‐{CH2L}‐2‐CPh3‐4‐Me‐C6H2OH (1e: R = CPh3, L = O; 1f: R = CPh3, L′ = NMe) readily react with AlMe3 through an alkane elimination reaction to afford the corresponding aminophenolate aluminium dimethyl complexes η2‐N,O‐[2‐{CH2N(C4H8L)}‐6‐R‐C6H3O]AlMe2 (2a: R = Ph, L = O; 2b: R = Ph, L = NMe; 2c: R = tBu, L = O; 2d: R = tBu, L′ = NMe) and η2‐N,O‐[6‐{CH2(C4H8L)}‐2‐CPh3‐4‐Me‐C6H2O]AlMe2 (2e: R = CPh3, L = O; 2f: R = CPh3, L′ = NMe), respectively, as determined by X‐ray crystallography in the case of compounds 2b–e. These neutral Al dimethyl complexes all feature a (η2‐O,N)Al chelate, whether in solution or in the solid state, and complexes 2a, 2b, 2d and 2f may be readily ionized by B(C6F5)3 to yield Al cations of the type (η3‐O,N, L)AlMe+ (3a, 3b+, 3d+ and 3f+) as dissociated MeB(C6F5)3– salts in solution. The stability of these Al cations appears to be greatly dependent on the steric crowding around the Al centre. Despite ring strain associated with the coordination of the extra L ligand, the solution behaviour of such Al cations are consistent with the effective coordination of the extra L ligand to the Al metal centre under the studied conditions. Some of these cations were found to be highly active in propylene oxide (PO) polymerization under mild conditions to yield atactic PPO with a moderate polydispersity. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)