Towards NHC stabilized alkylgallium alkoxide/aryloxide cations – The advances, the limitations and the challenges

Abstract

Here we describe and discuss an approach to the synthesis of alkylgallium alkoxide and aryloxide cations stabilized with N-heterocyclic carbenes (NHCs). The reaction of methyl abstracting agent B(C6F5)3 with Me2Ga(OAr,CNHC) complexes stabilized by aryloxide ligands with N-heterocyclic carbene functionalities (1 and 2) has led to the formation of cationic complexes [MeGa(OAr,CNHC)]22+ (32+ and 42+). [3][MeB(C6F5)3]2 and [4][MeB(C6F5)3]2, which are the first examples of alkylgallium aryloxide cationic complexes, have been isolated and characterized by both spectroscopic and X-ray techniques. In contrast, the reaction of alkoxide derivative Me2Ga(O,CNHC) (5) with B(C6F5)3 and [Ph3C][B(C6F5)4] has led to the formation of adducts Me2Ga(O,CNHC)·B(C6F5)3 (6) and [Me2Ga(O,CNHC)·Ph3C]+. Although, the reactions of B(C6F5)3 with Me2Ga(OC6H4OMe)(SIMes) (7) or Me2GaOMe(SIMes) (8) (SIMes = 1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene) have resulted in the formation of cationic species [MeGaOR(SIMes)]+, as evidenced by spectroscopic techniques, the side reactions resulting in the formation of (SIMes)B(C6F5)3 and boron alkoxide species have also been observed. Cationic complexes [3][MeB(C6F5)3]2 and [4][MeB(C6F5)3]2 are poor catalysts for the ring-opening polymerization of cyclic esters and are essentially not active in the low pressure polymerization of ethylene.