Dialkylgallium Alkoxides Stabilized with N-Heterocyclic Carbenes: Opportunities and Limitations for the Controlled and Stereoselective Polymerization of rac-Lactide

Abstract

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.

Probing the Effect of Six-Membered N-Heterocyclic Carbene—6-Mes—on the Synthesis, Structure and Reactivity of Me2MOR(NHC) (M = Ga, In) Complexes

“…In solution, both 1 H NMR, and 13 C NMR were indicative of the formation of 1. Although two sets of signals were revealed by 1 H NMR of 1 in toluene-d 8 , the main set of signals, including a singlet corresponding to Ga-Me protons (−1.06 ppm), which was significantly shifted to higher field, similarly to Me 2 Ga(OCH 2 CH 2 OMe)(SIMes) [2], was indicative of the formation of Me 2 Ga(OCH 2 CH 2 OMe)(6-Mes). The additional minor set of signals corresponded to [Me 2 Ga(OCH 2 CH 2 OMe)] 2 (16%), which suggested the presence of an equilibrium, presented in Scheme 1.…”

“…While the presence of the equilibrium presented in Scheme 1 is not surprising in the light of similar equilibrium observed in the case of Me 2 In(OCH(Me)CO 2 Me)(NHC) (NHC = SIMes, IMes) [3], the one observed for 1 represents the first such equilibrium for Me 2 GaOR(NHC) complexes, which is in line with the weaker Ga-C 6-Mes bond in comparison with Me 2 GaOR(NHC) (NHC = SIMes, IMes). On the contrary, the shift between free 6-Mes (∆ = 244.9 ppm) and coordinated 6-Mes in the case of 1 (197.1 ppm, ∆ = −47.8 ppm) was considerably larger in comparison with Me 2 MOR(NHC) (NHC = SIMes (∆ = −43.5-(−45.2) ppm), IMes (∆ = −43.1-(−43.6) ppm) [1,2]. Although the latter could indicate the presence of stronger Ga-C 6-Mes bond in solution [25], it rather reflects the stronger donor properties of 6-Mes vs SIMes/IMes in the light of both the structure of 1, as well as the weaker Ga-C 6-Mes evidenced by the X-ray analysis of Me 2 Ga(OCPh 2 Me)(6-Mes) (3) (see below).…”

“…Although the reaction of 6-Mes with [Me2In(μ-OMe)]3 led to the formation of Me2InOMe(6-Mes) complex (see the Supplementary Materials), which was more stable in comparison with Me2In(OCH2CH2OMe)(6-Mes), it seemed to disproportionate more readily in comparison with Me2InOMe(NHC) (NHC = SIMes, IMes), and as result could not be isolated in contrast to the latter [3]. While we could not estimate the strength of In-C6-Mes in the case of Me2InOR(6-Mes), due to the tendency of the latter to disproportionate , it could be analyzed for the Me3In(6-Mes) (2). The crystals of 2 suitable for X-ray analysis could be obtained either by the decomposition of Me2In(OCH2CH2OMe) or quantitative reaction between 6-Mes and Me3In.…”

“…Importantly, both the structure and reactivity of Me 2 MOR(NHC) (M = Ga, In; NHC = SIMes, IMes) were found to be strongly dependent on the M-C NHC bond. While in the case of corresponding gallium complexes, a relatively strong Ga-C NHC bond led to stable complexes [1,2], in the case of mostly unstable dialkylindium alkoxides with NHCs, a significantly weaker In-C NHC bond was observed [3]. Consequently, in the ring-opening polymerization with Me 2 GaOR(NHC), Ga-C NHC bond remained intact, resulting in the exclusive insertion of rac-LA into Ga-O alkoxide bond, similar to Zn [14][15][16], Al [17], as well as Ti and Zr [18][19][20], metal alkoxides with NHCs, which have been reported to polymerize rac-LA due to the insertion into [17], as well as Ti and Zr [18][19][20], metal alkoxides with NHCs, which have been reported to polymerize rac-LA due to the insertion into M-Oalkoxide bond.…”

“…Our initial studies revealed the adverse effect of steric hindrances of NHC on the synthesis of Me2GaOR(SIPr) as well as the formation and strength of Ga-CSIPr bond [2], which was additionally supported by the structure of Me3M(NHC) (M = Al, Ga, In) adducts [3,21,23]. However, the more detailed effect of both steric and electronic effect of NHC on the M-CNHC bond has not been addressed so far for main group metal alkoxides with NHCs, including Me2MOR(NHC) (M = Ga, In) complexes.…”