Probing the M–CNHC Bond and Its Effect on the Synthesis, Structure, and Reactivity of R2MOR(NHC) (M = Al, Ga, In) Complexes

The studies on the reactivity of dialkylaluminum alkoxides towards N-heterocyclic carbenes (NHCs) has allowed investigation of not only the factors controlling the synthesis and properties of Me2AlOR(NHC) complexes. Additionally, we have focused on the effect of group 13 metals on the synthesis, structure, and reactivity of Me2MOR(NHC) (M = Al, Ga, In) complexes, with regard to the strength and character of M–CNHC bonds. The reactions of simple dimethylaluminum alkoxides with NHCs lead to the monomeric Me2AlOR(NHC) complexes, as shown by the isolation of Me2AlOMe(NHC) (NHC = IMes (1a), SIMes (1b)) (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene, SIMes = 1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene). Despite their tendency to disproportionate, the interaction of the additional NHC molecule in Me2Al(OCH2CH2OMe)(IMes)/IMes (2a/IMes) with the fifth coordinate site of aluminum has resulted in stabilization and allowed for the isolation of Me2Al(OCH2CH2OMe)(IMes) (2a). In contrast, the limited accessibility of the fifth coordinate site of aluminum in the case of five-coordinate [Me2Al(μ-OCH(Me)CO2Me)]2 or four-coordinate [Me2Al(μ-OR)]2 alkoxides with bulky alkoxide ligands, has affected the formation of Al–CNHC bonds and allowed only for the synthesis and isolation of stable Me2Al(OCPh2Me)(NHC) (NHC = IMes (3a), SIMes) complexes. Additionally stable aryloxide derivatives Me2M(OC6H4OMe)(NHC) (NHC = IMes (4a) and SIMes (4b)) have been isolated and characterized. More ionic Al–CNHC bonds of Me2AlOR(NHC), in comparison with analogous Ga–CNHC and In–CNHC bonds, have been decisive for the reactivity of aluminum complexes, which includes their tendency for ligand disproportionation and activity of 1a, 4a, and 4b in the ring opening polymerization (ROP) of lactide, initiated in each case by the insertion of lactide into mainly an ionic Al–CNHC bond. The ionic character of M–CNHC bonds, decreasing in the series Al–CNHC > In–CNHC > Ga–CNHC, has been reflected by the reactivity of investigated complexes and determined by density functional theory (DFT) calculations using real-space bonding indicators (RSBIs). The structure of investigated aluminum complexes and the strength of Al–CNHC bonds have been investigated using spectroscopic methods and X-ray diffraction studies. The strength of M–CNHC bonds of investigated aluminum complexes, as well as their gallium and indium analogues, have been also determined by DFT calculations of their bond dissociation energies.