{Phenoxy-imine}aluminum versus -indium Complexes for the Immortal ROP of Lactide: Different Stereocontrol, Different Mechanisms

Abstract

A series of dialkylaluminum and -indium {ONR}MR′2 complexes (M = Al, R′ = Me; M = In, R′ = Me, CH2SiMe3) stabilized by a phenoxy-imine {ONR}− ligand platform, with variable R-imino substituents and functionalized by a bulky o-SiPh3 in the phenoxy moiety, has been prepared and structurally characterized in solution and in the solid state. {ONR}AlMe2 complexes reacted with alcohols, in particular with alkyl (S)-H-lactate, to generate the corresponding {ONR}Al(OR)2 compounds. On the other hand, the indium complexes {ONR}InR′2 proved largely inert toward alcohols. When they were combined with an alcohol (iPrOH, BnOH), the {ONR}AlMe2 complexes promoted the living (immortal) ring-opening polymerization ((i)ROP) of racemic lactide (rac-LA) with a good control over the molecular weights and various microstructures, dependent on the R-imino substituent. Complexes having benzyl-type imino substituents enabled the achievement of significant isotacticity (Pm up to 0.80), following grossly the bulkiness of the aryl moiety. The analogous {ONR}InR′2 proved similarly active for the (i)ROP of rac-LA in presence of an external alcohol, but the polymerizations were less controlled and none of the complexes induced stereoselectivity, except one (3a,Pm = 0.70). Kinetic studies revealed different rate laws, with an apparent zero-order dependence on monomer for the aluminum system 1m/iPrOH and a first-order dependence on monomer for the analogous indium system 3m/iPrOH. On the basis of the stoichiometric reactivity of model compounds, two different operative ROP mechanisms are suggested, depending on the nature of the metal center: Al-based complexes proceed through coordination–insertion, while In-based complexes are proposed to operate through an activated monomer mechanism.