Effect of In–CNHC Bonds on the Synthesis, Structure, and Reactivity of Dialkylindium Alkoxides: How Indium Compares to Gallium

The investigation of the reactivity of dialkylindium alkoxides toward N-heterocyclic carbenes (NHCs) has shown that both the character of the In–CNHC bond and alkyl and alkoxide substituents have a significant effect on the formation of R2InOR­(NHC) complexes and the distribution of products. The reactions of simple dimethylindium alkoxides with the N-heterocyclic carbenes 1,3-bis­(2,4,6-trimethylphenyl)­imidazolin-2-ylidene (SIMes) and 1,3-bis­(2,4,6-trimethylphenyl)­imidazol-2-ylidene (IMes) lead to the monomeric complexes Me2InOR­(NHC), as shown by the isolation of Me2InOMe­(NHC) (NHC = IMes (3), SIMes (4)). Compounds Me2InOR­(NHC) are unstable in solution and instantly disproportionate, which can be associated with a weaker In–CNHC bond in comparison with stable gallium analogues. As a result, Me3In­(NHC) (NHC = IMes (1), SIMes (2)) adducts, as well as Mitsubishi-type methylindium alkoxides, are formed. The exchange of a simple alkoxy group with chelating (S)-methyl lactate (S-melac) has resulted in the more stable Me2In­(OCH­(Me)­CO2Me)­(NHC) complexes. The use of the bulky alkoxide ligand OCPh2Me allows for the synthesis of stable Me2In­(OCPh2Me)­(NHC) (NHC = IMes (6), SIMes (7)) from [Me2In­(μ-OCPh2Me)]2 (5). While the strongest In–CNHC bond, among the characterized Me2In­(OR)­(NHC) complexes, is crucial for the stability of 6 and 7, it is still weaker in comparison with Ga–CNHC bonds in the analogous gallium complexes Me2Ga­(OCPh2Me)­(NHC) (NHC = IMes (8), SIMes (9)). For [tBu2In­(μ-OCH2CH2OMe)]2, the introduction of a bulky tert-butyl group has resulted in a lack of reactivity toward NHCs. However, the structure of tBu2In­(OCPh2Me)­(IMes) has confirmed the substantial effect of bulky alkyl substituents on the strength of the In–CNHC bond. The structures of 1, 2, 4–6, and 8 have been determined using both spectroscopic methods in solution and X-ray diffraction studies. Similarly to their gallium analogues, Me2In­(OCH­(Me)­CO2Me)­(NHC) complexes are highly active in the ring-opening polymerization of rac-lactide already at −20 °C, leading to isotactically enriched PLA (Pm = 0.67–0.76). However, in contrast to the gallium complexes Me2GaOR­(NHC), the noninnocent role of an NHC ligand, resulting in the formation of cyclic PLA, has been demonstrated for 6 and 7.