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

Abstract

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.