Effect of In-C(NHC) Bonds on the Synthesis, Structure, and Reactivity of Dialkylindium Alkoxides: How Indium Compares to Gallium
Martyna Cybularczyk , Maciej Dranka , Janusz Zachara , Paweł Horeglad
AbstractThe 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 tertbutyl 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.
|Journal series||Organometallics, ISSN 0276-7333|
|Publication size in sheets||0.55|
|ASJC Classification||; ;|
|Score|| = 35.0, 03-02-2020, ArticleFromJournal|
= 40.0, 03-02-2020, ArticleFromJournal
|Publication indicators||= 6; = 8; : 2016 = 0.846; : 2016 = 3.862 (2) - 2016=3.492 (5)|
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