The investigation of the reactivity of dialkylindium alkoxides toward N-heterocyclic carbenes (NHCs) has shown that both the character of the In−C NHC bond and alkyl and alkoxide substituents have a significant effect on the formation of R 2 InOR(NHC) complexes and the distribution of products. The reactions of simple dimethylindium alkoxides with the N-heterocyclic carbenes 1,3-bis(2,4,6trimethylphenyl)imidazolin-2-ylidene (SIMes) and 1,3-bis-(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes) lead to the monomeric complexes Me 2 InOR(NHC), as shown by the isolation of Me 2 InOMe(NHC) (NHC = IMes (3), SIMes (4)). Compounds Me 2 InOR(NHC) are unstable in solution and instantly disproportionate, which can be associated with a weaker In−C NHC bond in comparison with stable gallium analogues. As a result, Me 3 In(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 Me 2 In(OCH(Me)CO 2 Me)(NHC) complexes. The use of the bulky alkoxide ligand OCPh 2 Me allows for the synthesis of stable Me 2 In(OCPh 2 Me)(NHC) (NHC = IMes (6), SIMes (7)) from [Me 2 In(μ-OCPh 2 Me)] 2 (5). While the strongest In−C NHC bond, among the characterized Me 2 In(OR)(NHC) complexes, is crucial for the stability of 6 and 7, it is still weaker in comparison with Ga−C NHC bonds in the analogous gallium complexes Me 2 Ga(OCPh 2 Me)(NHC) (NHC = IMes (8), SIMes (9)). For [ t Bu 2 In(μ-OCH 2 CH 2 OMe)] 2 , the introduction of a bulky tertbutyl group has resulted in a lack of reactivity toward NHCs. However, the structure of t Bu 2 In(OCPh 2 Me)(IMes) has confirmed the substantial effect of bulky alkyl substituents on the strength of the In−C NHC 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, Me 2 In(OCH(Me)CO 2 Me)(NHC) complexes are highly active in the ring-opening polymerization of rac-lactide already at −20 °C, leading to isotactically enriched PLA (P m = 0.67−0.76). However, in contrast to the gallium complexes Me 2 GaOR(NHC), the noninnocent role of an NHC ligand, resulting in the formation of cyclic PLA, has been demonstrated for 6 and 7. ■ INTRODUCTIONRecently we reported Me 2 GaOR(NHC) (NHC = N-heterocyclic carbene) complexes, which constitute the first examples of well-characterized gallium, as well as group 13 metal, alkoxides with NHCs. 1 They were highly active and isoselective catalysts for the ring-opening polymerization (ROP) of lactide (LA) under mild conditions. 2,3 Moreover, the formation of isoselective Me 2 GaOR(NHC), upon coordination of the NHC to heteroselective [Me 2 GaOR] 2 , 4 allowed for the first facile stereoselectivity switch in the polymerization of rac-LA. 1,5 In addition to the gallium complexes mentioned above, a few main-group-metal complexes with NHCs have shown interesting catalytic properties in the ROP of heterocyclic monome...

The structure of a series of Me 2 GaOR(NHC) complexes with 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)) have been characterized using spectroscopic and X-ray techniques and discussed in view of their reactivity in the polymerization of rac-lactide (rac-LA). Both structure studies and density functional theory (DFT) calculations show the significant influence of NHC and OR on the structure of investigated complexes and has indicated that the Ga−C NHC bond (32.6− 39.6 kcal mol −1 ) is strong enough to form stable Me 2 GaOR-(NHC) complexes in the form of monomeric species. The reactivity of Me 2 Ga((S)-OCH(Me)CO 2 Me)(SIMes) (1) and Me 2 Ga((S)-OCH(Me)CO 2 Me)(IMes) (5) toward Lewis acids such as CO 2 and GaMe 3 has resulted in breaking of the Ga−C NHC bond with the formation of (NHC)CO 2 and Me 3 Ga(NHC) (8 and 10) and [Me 2 Ga(μ-(S)-OCH(Me)CO 2 Me)] 2 . Different results have been obtained for l,3-bis(2,6-diisopropylphenyl)-imidazolin-2-ylidene (SIPr), which coordinates more weakly to gallium, as demonstrated by the Ga−C NHC bond strength for model Me 3 GaSIMes, Me 3 GaIMes (8), and Me 3 GaSIPr (10) adducts. The reaction of SIPr with [Me 2 Ga(μ-OR)] 2 has not allowed for the breaking of Ga 2 O 2 bridges and the formation of monomeric Me 2 GaOR(SIPr) complexes, contrary to SIMes and IMes. In the case of the reaction with [Me 2 Ga(μ-(S)-OCH(Me)CO 2 Me)] 2 , the ionic compound [Me 2 Ga(OCH(Me)-CO 2 )] − [SIPrH] + (9) has been isolated. The investigated Me 2 GaOR(NHC) complexes are highly active and stereoselective in the ring-opening polymerization of rac-lactide from −20°C to room temperature, due to the insertion of rac-LA exclusively into the Ga−O alkoxide bond, leading to isotactically enriched polylactide (PLA) (P m = 0.65−0.78). It has been shown that the polymerization of lactide at low temperature is influenced by the chelate interaction of (S)-OCH(Me)CO 2 Me or (OCH(Me)C(O)) 2 OR resulting from the primary insertion of rac-LA into the Ga−O alkoxide bond, with the Ga center, which can be responsible for the low control over the molecular weight of the obtained PLA. The latter effect can be eliminated by the initial synthesis of Me 2 Ga((PLA) n OR)(NHC) with short PLA chains, which allows for controlled polymerization. Although the adverse chelate effect can be also eliminated by the polymerization of rac-LA at room temperature, the stereoselectivity of rac-LA polymerization is strongly affected by transesterification reactions. Out of investigated Me 2 GaOR(SIMes) and Me 2 GaOR(IMes) complexes, only the latter allowed for the immortal ring opening polymerization of rac-LA in the presence of i PrOH.

Chiral recognition of monomeric Me2MOR units resulting in the formation of homochiral dimeric species [Me2M(μ-OR)]2 (M = Ga, In), leads to heteroselective catalysts for the ring opening polymerization of rac-lactide (rac-LA).