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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