Chiral (1,2)-Diphenylethylene-Salen Complexes of Triel Metals: Coordination Patterns and Mechanistic Considerations in the Isoselective ROP of Lactide

The synthesis of enantiomerically pure aluminium, gallium and indium complexes supported by chiral (R,R)-((ONNOHH)-O-HH) (1), (R,R)-((ONNOHMe)-O-MeH) (2), (R,R)-((ONNOtButBu)-O-tButBu) (3), (R,R)-((ONNOMeNO2)-O-MeNO2) (4), (R,R)-((ONNOHOMe)-O-HOMe) (5) and (R,R)-((ONNOClCl)-O-ClCl) (6) (1,2)-diphenylethylene-salen ligands is described. Several of these complexes have been crystallographically authenticated, which highlights a diversity of coordination patterns. Whereas all Ga complexes form [Ga-2(CH2SiMe3)(4)(ONNO)] bimetallic species (ONNO=1-3), aluminium [AlR(ONNO)] (R=Me, CH2SiMe3) and indium [In(CH2SiMe3)(ONNO)] derivatives are monometallic for ONNO=1, 2 and 4-6, and only form the bimetallic complexes [Al2R4(ONNO)] and [In-2(CH2SiMe3)(4)(ONNO)] for the most sterically crowded ligand 3. The [AlMe(ONNO)] complexes react with iPrOH to give [AlOiPr(ONNO)] complexes that are robust towards further iPrOH. The [In(CH2SiMe3)(ONNO)] congeners are inert towards excess alcohol, whereas the Ga compounds decompose easily. All these alkyl complexes, as well as the [AlOiPr(ONNO)] derivatives, catalyse the ring-opening polymerisation (ROP) of racemic lactide (rac-LA). The [AlMe(ONNO)] complexes require additional alcohol to afford controlled reactions, but [AlOiPr(ONNO)] complexes are single-component catalysts for the isoselective ROP of rac-LA, with values of P-m in the range 0.80-0.90. Experimental evidence unexpectedly shows that chain-end control leads to the isoselectivity of these aluminium catalysts; also, the more crowded the coordination sphere, the higher the isoselectivity. The bimetallic Ga complexes do not afford controlled reactions, but the binary [In(ONNO)(CH2SiMe3)/(PhCH2OH)] systems competently mediate non-stereoselective ROP; evidence is given that an activated monomer mechanism is at work. Kinetic studies show that catalytic activity decreases when electronic density and steric congestion at the metal atom increase.