Synthesis and structural characterization of NHC-stabilized Al(III) and Ga(III) alkyl cations and use in the ring-opening polymerization of lactide

•First examples of NHC group 13 metal cationic alkyls were prepared and structurally characterized.•NHC Al(III) and Ga(III) cationic alkyls are robust and strong Lewis acids.•Such species polymerize rac-lactide in a controlled manner and good activity.

Group 13 metal (Al, Ga) alkyls supported by N-heterocyclic carbenes (NHCs), (NHC)MMe3 (NHC = IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene, IDipp = 1,3-di-diisopropylphenylimidazol-2-ylidene; M = Al, Ga), were successfully ionized via a methide abstraction route with B(C6F5)3 in the presence of an external Lewis base (THF, Et2O) to afford the corresponding cations of the type (NHC)MMe2(L)+ (L labile) as CH3B(C6F5)3- salts. Such cations, which are the first examples of NHC-stabilized Al and Ga cationic alkyls, are stable in solution (CH2Cl2) and the molecular structure of cation (IMes)AlMe2(Et2O)+ was determined by X-ray crystallographic diffraction studies. All cationic compounds were also tested as initiators of the ring-opening polymerization (ROP) of rac-lactide (rac-LA). The Al cation (IMes)AlMe2(THF)+ effectively polymerizes rac-LA at room temperature in the presence of BnOH to produce chain-length-controlled and narrow disperse polylactide (TOF = 160 h−1).

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