Probing the Effect of Six-Membered N-Heterocyclic Carbene — 6-Mes — on the Synthesis , Structure and Reactivity of Me 2 MOR ( NHC ) ( M = Ga , In ) Complexes

The investigation of the reactivity of six membered N-heterocyclic carbene 1,3-bis(2,4,6-trimethylphenyl)-3,4,5,6-tetrahydropyrimidin-1-ylidene (6-Mes) towards dialkylgallium and dialkylindium alkoxides/aryloxides has shown that both steric hindrances and donor properties of 6-Mes significantly influence the strength of M–C6-Mes bond, as well as the formation, structure and reactivity of Me2MOR(6-Mes) (M = Ga, In) complexes. While the reactions of simple dimethylgallium alkoxides with 6-Mes lead to the formation of stable monomeric Me2Ga(OCH2CH2OMe)(6-Mes) (1) and Me2GaOMe(6-Mes) complexes, the analogous Me2InOR(6-Mes) are unstable and disproportionate to methylindium alkoxides and Me3In(6-Mes) (2). The use of bulky alkoxide ligand—OCPh2Me or aryloxide ligand—OC6H4OMe allowed for the synthesis of stable Me2M(OCPh2Me)(6-Mes) (M = Ga (3) and In (4)) as well as Me2M(OC6H4OMe)(6-Mes) (M = Ga (5) and In (6)). The structures of 1–6 have been determined using both spectroscopic methods in solution and X-ray diffraction studies, which confirmed the effect of both steric hindrances and donor properties of 6-Mes on their structure and catalytic properties in the ring-opening polymerization (ROP) of rac-lactide.

Similarly to the observed effect of a metal on the corresponding M-CNHC bond, as well as the structure and ROP activity of discussed main group metal alkoxides with NHCs, we should expect a considerable effect of steric and electronic properties of NHC on both the strength and reactivity of the M-CNHC 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.In order to probe the effect of NHC on the latter, we chose 6-Mes-an N-heterocyclic carbene of stronger donor properties, along with the increased steric demand, in comparison with SIMes or IMes (Figure 1) [24,25].While the increased steric hindrances of 6-Mes result from wider N-C-N angle, this should be responsible for the hybridization at carbene carbon and the higher energy of HOMO orbital of 6-Mes occupied by a lone pair of electrons, and therefore its donating properties [26].Our studies, reported hereby, have allowed for the determination of the considerable effect of steric and electronic properties of 6-Mes, as well as the relative importance of electronic and steric properties of 6-Mes, on the synthesis, structure and stability of a series of Me2MOR(NHC) (M = Ga, In) complexes, as well as their catalytic activity in the ROP of rac-LA.

Results and Discussion
In order to observe the influence of steric and electronic properties NHC on the synthesis, structure and reactivity of Me2MOR(NHC) (M = Ga, In) complexes, we investigated the reactivity of dimethylgallium and dimethylindium alkoxides and aryloxides towards 1,3-bis(2,4,6trimethylphenyl)-3,4,5,6-tetrahydropyrimidin-1-ylidene (6-Mes).The synthesis, structure of resulting Me2MOR(6-Mes) (M = Ga, In) complexes, and their catalytic activity in the ROP of rac-LA was compared to the analogous dialkylgallium and dialkylindium derivatives stabilized with 1,3- Our studies, reported hereby, have allowed for the determination of the considerable effect of steric and electronic properties of 6-Mes, as well as the relative importance of electronic and steric properties of 6-Mes, on the synthesis, structure and stability of a series of Me 2 MOR(NHC) (M = Ga, In) complexes, as well as their catalytic activity in the ROP of rac-LA.

Reactivity of [Me 2 M(µ-OR)
] n (M= Ga, In ; OR = OCH 2 CH 2 OMe, OMe ; n = 2, 3) towards 6-Mes The reaction between dimeric gallium complex [Me 2 Ga(µ-OCH 2 CH 2 OMe)] 2 and 6-Mes (Ga:6-Mes = 1:1, Scheme 1) led to the essentially instant formation of gallium complex Me 2 Ga(OCH 2 CH 2 OMe)(6-Mes) (1).Compound 1 was isolated as an off-white solid in high yield.Unfortunately, the crystals of 1 decomposed during an X-ray diffraction experiment, which precluded the determination of its X-ray structure.Notably, the increased reactivity of 1 towards grease during X-ray experiment, in comparison with Me 2 GaOR(NHC) (NHC = SIMes, IMes), should not be surprising in the light of its structure and reactivity in solution.

Reactivity of [Me2M(μ-OR)]n (M= Ga, In ; OR = OCH2CH2OMe, OMe ; n = 2, 3) towards 6-Mes
The reaction between dimeric gallium complex [Me2Ga(μ-OCH2CH2OMe)]2 and 6-Mes (Ga:6-Mes = 1:1, Scheme 1) led to the essentially instant formation of gallium complex Me2Ga(OCH2CH2OMe)(6-Mes) (1).Compound 1 was isolated as an off-white solid in high yield.Unfortunately, the crystals of 1 decomposed during an X-ray diffraction experiment, which precluded the determination of its X-ray structure.Notably, the increased reactivity of 1 towards grease during X-ray experiment, in comparison with Me2GaOR(NHC) (NHC = SIMes, IMes), should not be surprising in the light of its structure and reactivity in solution.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-d8, 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 Me2Ga(OCH2CH2OMe)(SIMes) [2], was indicative of the formation of Me2Ga(OCH2CH2OMe)(6-Mes).The additional minor set of signals corresponded to [Me2Ga(OCH2CH2OMe)]2 (16%), which suggested the presence of an equilibrium, presented in Scheme 1.Such an equilibrium was further evidenced by a 2D ROESY (rotating frame Overhauser effect spectroscopy) experiment (Figure S3).Moreover, essentially the same ratio of Me2Ga(OCH2CH2OMe)(6-Mes): [Me2Ga(OCH2CH2OMe)]2, for 1, and the reaction mixture of [Me2Ga(μ-OCH2CH2OMe)]2 and 6-Mes (Ga:6-Mes = 1:1), was in line with the presence of indicated equilibrium.Most probably, the latter was also responsible for the instant reaction of 1 with CD2Cl2 resulting in the formation of [Me2Ga(μ-OCH2CH2OMe)]2 and [6-Mes-D] + , which were evidenced by NMR spectroscopy (see the Supplementary Materials).Interestingly, similar reactivity was observed for the reaction between free SIMes and methylene chloride.On the contrary, more acidic chloroform was required in order to react instantly with N-heterocyclic carbene of Me2GaOR(SIMes), while the latter remained essentially stable in CD2Cl2 during the NMR experiment [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 Me2In(OCH(Me)CO2Me)(NHC) (NHC = SIMes, IMes) [3], the one observed for 1 represents the first such equilibrium for Me2GaOR(NHC) complexes, which is in line with the weaker Ga-C6-Mes bond in comparison with Me2GaOR(NHC) (NHC = SIMes, IMes).On the contrary, the shift between free 6-Mes (Δ = 244.9ppm) and coordinated 6-Mes in the case of 1 (197.[1,2].Although the latter could indicate the presence of stronger Ga-C6-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-C6-Mes evidenced by the X-ray analysis of Me2Ga(OCPh2Me)(6-Mes) (3) (see below).While the carbene carbon signal in 13 C NMR could not be observed for Me2GaOMe(6-Mes), which was synthesized analogously to 1, the 1 H NMR was in line with the presence of the equilibrium presented in Scheme 1 (see the Supplementary Materials), and therefore, similarly to 1, indicated weaker Ga-C6-Mes bond in comparison with Ga-CNHC of Me2GaOMe(NHC) (NHC = SIMes, IMes).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.Such an equilibrium was further evidenced by a 2D ROESY (rotating frame Overhauser effect spectroscopy) experiment (Figure S3).Moreover, essentially the same ratio of Me 2 Ga(OCH 2 CH 2 OMe)(6-Mes): [Me 2 Ga(OCH 2 CH 2 OMe)] 2 , for 1, and the reaction mixture of [Me 2 Ga(µ-OCH 2 CH 2 OMe)] 2 and 6-Mes (Ga:6-Mes = 1:1), was in line with the presence of indicated equilibrium.Most probably, the latter was also responsible for the instant reaction of 1 with CD 2 Cl 2 resulting in the formation of [Me 2 Ga(µ-OCH 2 CH 2 OMe)] 2 and [6-Mes-D] + , which were evidenced by NMR spectroscopy (see the Supplementary Materials).Interestingly, similar reactivity was observed for the reaction between free SIMes and methylene chloride.On the contrary, more acidic chloroform was required in order to react instantly with N-heterocyclic carbene of Me 2 GaOR(SIMes), while the latter remained essentially stable in CD 2 Cl 2 during the NMR experiment [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  [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).While the carbene carbon signal in 13 C NMR could not be observed for Me 2 GaOMe(6-Mes), which was synthesized analogously to 1, the 1 H NMR was in line with the presence of the equilibrium presented in Scheme 1 (see the Supplementary Materials), and therefore, similarly to 1, indicated weaker Ga-C 6-Mes bond in comparison with Ga-C NHC of Me 2 GaOMe(NHC) (NHC = SIMes, IMes).
Despite the adverse effect of steric hindrances of 6-Mes on the strength of Ga-C 6-Mes bond, the stronger donor properties of 6-Mes in comparison with SIMes/IMes, prompted us to investigate its effect on the synthesis and structure of Me 2 InOR(NHC) complexes.The main question was, whether the larger radius of indium, in comparison with gallium, could allow for the formation of a stronger In-C 6-Mes bond and the stabilization of Me 2 InOR(NHC), which had been previously shown by us to be unstable and prone to ligand disproportionation [3].Although the reaction between [Me 2 In(µ-OCH 2 CH 2 OMe)] 2 and 6-Mes led to the formation of Me 2 In(OCH 2 CH 2 OMe)(6-Mes), the latter complex disproportioned readily to Me 3 In(6-Mes) and Mitsubishi type complex In{Me 2 In(µ-OCH 2 CH 2 OMe)} 3 (Scheme 2) (see the Supplementary Materials), analogously to Me 2 In(OCH 2 CH 2 OMe)(NHC) (NHC = SIMes, IMes) [3].The formation of both Me 3 In(6-Mes) and In{Me 2 In(µ-OCH 2 CH 2 OMe)} 3 was evidenced by NMR spectroscopy, while, in contrast to our previous studies, In{Me 2 In(µ-OCH 2 CH 2 OMe)} 3 could be isolated as a white crystalline solid, similar to the only two other examples of Mitsubishi type indium In{Me 2 In(µ-OR)} 3 complexes [27,28].Unfortunately, we did not succeed in obtaining crystals of In{Me 2 In(µ-OCH 2 CH 2 OMe) 2 } 3 suitable for X-ray analysis, and its structure has been confirmed using 1 H and 13 C NMR spectroscopy (see the Supplementary Materials).
Despite the adverse effect of steric hindrances of 6-Mes on the strength of Ga-C6-Mes bond, the stronger donor properties of 6-Mes in comparison with SIMes/IMes, prompted us to investigate its effect on the synthesis and structure of Me2InOR(NHC) complexes.The main question was, whether the larger radius of indium, in comparison with gallium, could allow for the formation of a stronger In-C6-Mes bond and the stabilization of Me2InOR(NHC), which had been previously shown by us to be unstable and prone to ligand disproportionation [3].Although the reaction between [Me2In(μ-OCH2CH2OMe)]2 and 6-Mes led to the formation of Me2In(OCH2CH2OMe)(6-Mes), the latter complex disproportioned readily to Me3In(6-Mes) and Mitsubishi type complex In{Me2In(μ-OCH2CH2OMe)}3 (Scheme 2) (see the Supplementary Materials), analogously to Me2In(OCH2CH2OMe)(NHC) (NHC = SIMes, IMes) [3].The formation of both Me3In(6-Mes) and In{Me2In(μ-OCH2CH2OMe)}3 was evidenced by NMR spectroscopy, while, in contrast to our previous studies, In{Me2In(μ-OCH2CH2OMe)}3 could be isolated as a white crystalline solid, similar to the only two other examples of Mitsubishi type indium In{Me2In(µ-OR)}3 complexes [27,28].Unfortunately, we did not succeed in obtaining crystals of In{Me2In(μ-OCH2CH2OMe)2}3 suitable for X-ray analysis, and its structure has been confirmed using 1 H and 13 C NMR spectroscopy (see the Supplementary Materials).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)(6-Mes) or quantitative reaction between 6-Mes and Me3In.Compound 2 was synthesized and isolated in bulk using only the second method, and was used to confirm the formation of 2 in solution due to the decomposition of Me2In(OCH2CH2OMe)(6-Mes).X-ray diffraction analysis of 2 revealed, similarly to Me3In(NHC) (NHC = SIMes, IMes) monomeric structure, with the coordination sphere adopting a distorted-tetrahedral geometry (Figure 2).The In-C6-Mes bond (2.367(2) Å , 0.51 valence units (vu) [29]) in 2 turned out to be noticeably longer, and therefore weaker, than In-CSIMes (2.334(6) Å , 0.55 vu [3]; 2.316(8) Å [23]), In-CIMes (2.307(2) Å , 0.60 vu [3]; 2.304(7) Å [23], 2.292(6) Å [21]), In-CSIPr (2.342(2) Å ) [23] and In-CIPr (2.309(2) Å ) [23] in Me3In(SIMes), Me3In(IMes), Me3In(SIPr) and Me3In(IPr) adducts, respectively.It showed a more significant effect of steric hindrances of 6-Mes, in comparison with SIMes, IMes, and even SIPr and IPr, on the strength of In-C6-Mes bond.Interestingly, the latter was in agreement with smaller steric hindrances of SIMes and IMes, and in contrast with larger steric demand of SIPr and IPr, in comparison with 6-Mes, as represented by the buried volume of discussed NHCs [25].The analysis of the strength of In-C6-Mes bond of 2 in solution, in comparison with In-CNHC bonds of already characterized Me3In(NHC) adducts [3,21,23], was in sharp contrast with the solid state result.In solution, the shift between free and coordinated NHCs in 13   Although the reaction of 6-Mes with [Me 2 In(µ-OMe)] 3 led to the formation of Me 2 InOMe(6-Mes) complex (see the Supplementary Materials), which was more stable in comparison with Me 2 In(OCH 2 CH 2 OMe)(6-Mes), it seemed to disproportionate more readily in comparison with Me 2 InOMe(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-C 6-Mes in the case of Me 2 InOR(6-Mes), due to the tendency of the latter to disproportionate , it could be analyzed for the Me 3 In(6-Mes) (2).The crystals of 2 suitable for X-ray analysis could be obtained either by the decomposition of Me 2 In(OCH 2 CH 2 OMe)(6-Mes) or quantitative reaction between 6-Mes and Me 3 In.Compound 2 was synthesized and isolated in bulk using only the second method, and was used to confirm the formation of 2 in solution due to the decomposition of Me 2 In(OCH 2 CH 2 OMe)(6-Mes).X-ray diffraction analysis of 2 revealed, similarly to Me 3 In(NHC) (NHC = SIMes, IMes) monomeric structure, with the coordination sphere adopting a distorted-tetrahedral geometry (Figure 2).The In-C 6-Mes bond (2.367(2) Å, 0.51 valence units (vu) [29]) in 2 turned out to be noticeably longer, and therefore weaker, than In-C SIMes (2.334(6) Å, 0.55 vu [3] While the relationship between the distance of essentially the same bonds and their strength should be considered a tenet in the case of interpretations of molecular structures [30], the analysis of the strength of a M-C NHC bond using the 13 C NMR spectroscopy could be influenced by other factors affecting the distribution of electron density at carbene carbon.However, it must be noticed that the shift between free and coordinated NHCs in solution, using 13 C NMR spectroscopy, reflected stronger donor properties of 6-Mes in comparison with SIMes and IMes.Notably, analogous results concerning the strength of M-C NHC bonds were obtained for alkoxide derivatives Me 2 M(OCPh 2 Me)(6-Mes) (M = Ga, In).
Inorganics 2017, 6(1), 28 5 of 16 interpretations of molecular structures [30], the analysis of the strength of a M-CNHC bond using the 13 C NMR spectroscopy could be influenced by other factors affecting the distribution of electron density at carbene carbon.However, it must be noticed that the shift between free and coordinated NHCs in solution, using 13 C NMR spectroscopy, reflected stronger donor properties of 6-Mes in comparison with SIMes and IMes.Notably, analogous results concerning the strength of M-CNHC bonds were obtained for alkoxide derivatives Me2M(OCPh2Me)(6-Mes) (M = Ga, In).4)), which is split into pitch and yaw angles [31,32]).
For complexes 3 and 4 X-ray diffraction analysis revealed the presence of four-coordinated gallium/indium species with the coordination sphere adopting a distorted-tetrahedral geometry.4)), which is split into pitch and yaw angles [31,32]).For complexes 3 and 4 X-ray diffraction analysis revealed the presence of four-coordinated gallium/indium species with the coordination sphere adopting a distorted-tetrahedral geometry.Interestingly, in contrast to essentially symmetrical gallium complex 3 (Figure 3), due to the orientation of the 6-Mes ligand with respect to In-C Me bonds, the presence of essentially asymmetrical indium complex 4 (Figure 4) was observed.Interestingly, in contrast to essentially symmetrical gallium complex 3 (Figure 3), due to the orientation of the 6-Mes ligand with respect to In-CMe bonds, the presence of essentially asymmetrical indium complex 4 (Figure 4) was observed.4)), which is split into pitch and yaw angles [31,32]).

Reactivity of [Me
While the major set of signals revealed by the 1 H NMR of 3 and 4 was in line with the formation of Me2M(OCPh2Me)(6-Mes) (M = Ga, In), the presence of minor signal could be ascribed to the [Me2Ga(μ-OCPh2Me)]2, therefore suggesting the presence of equilibrium, analogously to 1.However, due to relatively slow reaction rate between [Me2M(μ-OCPh2Me)]2 and 6-Mes (see above), the presence of minor set of signals could not be unequivocally associated with the equilibrium.The different ratios of Me2M(OCPh2Me)(6-Mes) : [Me2Ga(μ-OCPh2Me)]2 both in the reaction mixture, as well as in the spectrum of 3 or 4 were not clearly indicative of the presence of such equilibrium.In the 13 C NMR of both 3 and 4, significant shifts of carbene carbon signals between free and coordinated 6-Mes were observed.In the case of gallium complex 3, the observed shift (Δ = −47.5 ppm) was comparable with Me2Ga(OCH2CH2OMe)(6-Mes) (Δ = −47.8ppm), and slightly smaller than for Me2Ga((S)-OCH(Me)CO2Me)(6-Mes) (Δ = -48.4ppm) (see below), but was larger in comparison with Me2GaOR(NHC) (NHC = SIMes, IMes) complexes (Δmax = −45.2ppm) [2].Similarly, for 4 the observed shift (Δ = −41.7 ppm) was larger in comparison with Me2InOR(NHC) complexes (NHC = SIMes, IMes) (Δmax = −38.9ppm) [3].However, in the light of X-ray analysis of 3 and 4, which revealed weaker M-C6-Mes bonds (M = Ga, In) in comparison with M-CNHC of Me2M(OCPh2Me)(NHC) (M = Ga, In; NHC = SIMes, IMes), the shift of carbene carbon observed in 13 C NMR was indicative, similarly to 1 and 2, of the stronger donor properties of 6-Mes in comparison with SIMes or IMes, rather than stronger M-C6-Mes bonds in comparison with M-CNHC (NHC = SIMes, IMes).While the detailed discussion on the factors affecting the carbene carbon shift in 13 C NMR, and therefore the analysis of the strength of the M-CNHC bond using the 13 C NMR spectroscopy, cannot be done at the current stage, our present research in this area focuses on the effect of the character of the M-CNHC bond, rather than only its strength, on the synthesis, structure and reactivity of group 13 metal Me2MOR(NHC) complexes.4)), which is split into pitch and yaw angles [31,32]).
While the major set of signals revealed by the 1 H NMR of 3 and 4 was in line with the formation of Me 2 M(OCPh 2 Me)(6-Mes) (M = Ga, In), the presence of minor signal could be ascribed to the [Me 2 Ga(µ-OCPh 2 Me)] 2 , therefore suggesting the presence of equilibrium, analogously to 1.However, due to relatively slow reaction rate between [Me 2 M(µ-OCPh 2 Me)] 2 and 6-Mes (see above), the presence of minor set of signals could not be unequivocally associated with the equilibrium.The different ratios of Me 2 M(OCPh 2 Me)(6-Mes):[Me 2 Ga(µ-OCPh 2 Me)] 2 both in the reaction mixture, as well as in the spectrum of 3 or 4 were not clearly indicative of the presence of such equilibrium.In the 13 C NMR of both 3 and 4, significant shifts of carbene carbon signals between free and coordinated 6-Mes were observed.In the case of gallium complex 3, the observed shift (∆ = 47.5 ppm) was comparable with Me 2 Ga(OCH 2 CH 2 OMe)(6-Mes) (∆ = −47.8ppm), and slightly smaller than for Me 2 Ga((S)-OCH(Me)CO 2 Me)(6-Mes) (∆ = −48.4ppm) (see below), but was larger in comparison with Me 2 GaOR(NHC) (NHC = SIMes, IMes) complexes (∆ max = −45.2ppm) [2].Similarly, for 4 the observed shift (∆ = −41.7 ppm) was larger in comparison with Me 2 InOR(NHC) complexes (NHC = SIMes, IMes) (∆ max = −38.9ppm) [3].However, in the light of X-ray analysis of 3 and 4, which revealed weaker M-C 6-Mes bonds (M = Ga, In) in comparison with M-C NHC of Me 2 M(OCPh 2 Me)(NHC) (M = Ga, In; NHC = SIMes, IMes), the shift of carbene carbon observed in 13 C NMR was indicative, similarly to 1 and 2, of the stronger donor properties of 6-Mes in comparison with SIMes or IMes, rather than stronger M-C 6-Mes bonds in comparison with M-C NHC (NHC = SIMes, IMes).While the detailed discussion on the factors affecting the carbene carbon shift in 13 C NMR, and therefore the analysis of the strength of the M-C NHC bond using the 13 C NMR spectroscopy, cannot be done at the current stage, our present research in this area focuses on the effect of the character of the M-C NHC bond, rather than only its strength, on the synthesis, structure and reactivity of group 13 metal Me 2 MOR(NHC) complexes.

Reactivity of [Me 2 M(µ-OC 6 H 4 OMe)] 2 (M = Ga, In) towards 6-Mes
Although the investigation of the synthesis and structure of Me 2 M(OC 6 H 4 OMe)(6-Mes) (M = Ga (5), In ( 6)) (Scheme 4) was important prior to the studies on their catalytic activity in the ring-opening polymerization (ROP) of rac-lactide (rac-LA) (see below), they were also interesting with regard to our studies on Me 2 Ga(O,C NHC ) complexes possessing chelate alkoxide and aryloxide ligands with NHC functionality [33].The latter showed that aryloxide (O Ar ,C NHC ) ligands, in comparison with alkoxide chelate (O,C NHC ) ligands, resulted in stronger Ga-C NHC bond, which could be crucial for the synthesis and structure of 5 and 6 in comparison with their alkoxide analogues Me 2 MOR(6-Mes) (M = Ga, In).The reactions of 6-Mes with [Me 2 M(µ-OC 6 H 4 OMe)] 2 (M:6-Mes = 1:1, M = Ga, In) led to the instant formation of 5 and 6, which were isolated in high yields as white crystalline solids.However, the crystals suitable for X-ray analysis could not be isolated.Therefore, the structure of 5 and 6 was determined in solution using NMR spectroscopy.

Activity of Me2Ga(OCPh2Me)(6-Mes) (3 and 4), Me2M(OC6H4OMe)(6-Mes) (5 and 6) (M = Ga, In) and Me2Ga(OCH2CH2OMe)(6-Mes) (1) in the ROP of rac-lactide
In order to investigate the effect of 6-Mes on the catalytic properties of Me2MOR(NHC), we examined the activity of 1, 3, 4, 5 and 6 complexes in the ring-opening polymerization (ROP) of rac-LA.With regard to the latter, we focused on the effect of M-C6-Mes bond on the catalytic properties of selected complexes.While the M-C6-Mes bond affects the structure of all selected complexes, it should be expected to influence their reactivity, as well the structure and reactivity of Me2MO(PLA)(6-Mes), where O(PLA) represents growing polylactide chain, and therefore the microstructure of resulting polylactide (PLA).While our previous results showed that both gallium and indium Me2MOR(NHC) complexes were highly active already at −20 °C [1][2][3], in order to compare our results with the latter, we investigated the ROP of rac-LA with 1, 3, 4, 5 and 6 at identical conditions.Gallium complexes: alkoxide derivative Me2M(OCPh2Me)(6-Mes) (3) and aryloxide derivative Me2Ga(OC6H4OMe)(6-Mes) (5), were inactive in the polymerization of rac-LA at −20 °C.While in the case of complexes 3 and 5 the bulky alkoxide or aryloxide group did not allow for the insertion of rac-LA into Ga-O bond, most importantly, the strong Ga-C6-Mes bond precluded, the initiation of rac-LA polymerization by Nheterocyclic carbene, similarily to Me2Ga(OCPh2Me)(NHC) (NHC = SIMes, IMes) [3].Contrary to the latter, a much weaker In-CNHC bond in Me2In(OCPh2Me)(NHC) (NHC = SIMes, IMes) resulted in the initiation of rac-LA by N-heterocyclic carbenes [3], and similar reactivity was observed in the case of Me2In(OCPh2Me)(6-Mes) (4) and Me2In(OC6H4OMe)(6-Mes) (6), which led to the formation of cyclic PLA (see the Supplementary Materials).However, the observed reactivity of 6-Mes of indium complexes 4 and 6 towards lactide, as well as the lack of activity of gallium derivatives 3 and 5, may also indicate the considerable effect of a character of the M-CNHC bond on the reactivity of investigated complexes.As we were unable to isolate any stable alkoxide Me2InOR(6-Mes) complex, we focused on the activity of 1 in the ROP of rac-LA.Due to the reactivity of 1 towards CH2Cl2, which led to the almost quantitative and instant formation of [Me2Ga(μ-OCH2CH2OMe)]2 [9,35], no activity of 1 in the ROP of rac-LA in CH2Cl2, at −20 °C was observed.On the other hand, the polymerization of rac-LA with 1 (25:1) in toluene led to essentially full conversion after 12 hours (−20 °C) or 10 minutes (room temperature).For the PLA obtained at both temperatures the MALDI-TOF analysis revealed the presence of an end group of 76 Da, which was in agreement with the insertion of rac-LA into Ga-OCH2CH2OMe bond (see the Supplementary Materials).Additionally, for PLA obtained at both −20  For the PLA obtained at both temperatures the MALDI-TOF analysis revealed the presence of an end group of 76 Da, which was in agreement with the insertion of rac-LA into Ga-OCH 2 CH 2 OMe bond (see the Supplementary Materials).Additionally, for PLA obtained at both −20 • C and r.t. the extensive intermolecular transesterification was evidenced, while minor distributions referring to cyclic PLA could be assigned to the presence of an intermolecular transesterification or additional initiation of rac-LA ROP by 6-Mes.Although the initiation of the lactide polymerization by 6-Mes of 1, especially at −20 • C, was unlikely in the light of the lack of activity of 3 and 5, it could be assumed for Me 2 GaO(PLA)(6-Mes) propagating species.However, as we were not able to synthesize Me 2 Ga((S)-melac)(6-Mes) (see above), which mimics propagating species Me 2 GaO(PLA)(6-Mes), we synthesized the latter in the reaction between 1 and 10 equiv of rac-LA both at −20 • C and r.t..Although the 1 H NMR spectra after 36 h, both at −20 • C and r.t. were essentially the same, as the spectrum registered after mixing of 1 and 10 equiv of rac-LA at room temperature, they were inconclusive for the presence of Me 2 GaO(PLA)(6-Mes) with 6-Mes coordinated to gallium.In this case, the presence of complex Ga-Me signals in 1 H NMR in the range between −0.31 and 0.02 was in contrast to Me 2 Ga((S)-melac)(SIMes) (−0.80, −0.82 ppm) or Me 2 Ga(OCH(Me)C(O)) 2 O(CH 2 ) 2 OMe(IMes) (−0.79, −0.87 ppm) [2], and could not strongly support the presence of Me 2 GaO(PLA)(6-Mes).Moreover, the lack of signal corresponding to coordinated/uncoordinated carbene carbon in 13 C NMR was also inconclusive for the formation of the latter.Therefore, we could not unequivocally conclude whether the initiation of the ROP of rac-LA by 6-Mes was possible, or the presence of discussed transesterification reactions simply resulted from the catalytic properties of Me 2 GaO(PLA)(6-Mes) propagating species.However, irrespective of the origin, the presence of transesterification in the case of the ROP of rac-LA with 1 at -20 • C, which is in contrast with the essentially no transesterification observed for the ROP of rac-LA with Me 2 GaOR(NHC) (NHC = SIMes, IMes) [1,2], led to a considerable decrease of isoselectivity (P m (probability of meso linkages in PLA) = 0.66-0.69 at −20 • C, P m = 0.63-0.66 at r.t.) in comparison with the ROP of rac-LA with Me 2 GaOR(NHC) (P m = 0.76-0.78at −20 • C, P m = 0.65-0.68 at r.t.).Importantly, the ROP of rac-LA with 1 showed the considerable effect of the structure of NHC on the catalytic properties of Me 2 GaOR(NHC), which should be of importance and is of our current interest as Me 2 GaOR(NHC) are still rare examples of highly active metal alkoxides for the isoselective ROP of rac-LA [5][6][7][8][9][10][11][12][13].
Synthesis of 2. A stirred solution of Me 3 In (116 mg, 0.73 mmol) in toluene (5 mL) was cooled to −20 • C, and a toluene solution (2 mL) of 6-Mes (232 mg, 0.73 mmol) was added dropwise.After addition the reaction mixture was warmed to room temperature and stirred for 2 h.Solvent and volatile residues were then removed under vacuum to give a white solid.The solid was subsequently recrystallized from a toluene/hexane solution (2/6 mL) at −20  (96 mg, 0.32 mmol) in toluene (6 mL) was added 2 mL of a toluene solution of 6-Mes (103 mg, 0.32 mmol) at room temperature, and the resulting solution was stirred for 24 h.Then, toluene was removed under vacuum to give a light yellow solid.The solid was subsequently crystallized from toluene (2 mL) after addition of hexane (6 mL) at −20 )] 2 (118 mg, 0.53 mmol) in toluene (6 mL) was added 2 mL of a toluene solution of 6-Mes (171 mg, 0.53 mmol) at room temperature, and the resulting solution was stirred for 2 h.Then toluene was removed under vacuum to give a pale white solid.The solid was subsequently crystallized from toluene (1.5 mL) after addition of hexane (5 mL) at −20

X-Ray Structure Determination
Single crystals of 2, 3, 4 and (6-Mes)=CH 2 were grown from toluene/hexane solutions.Suitable single crystals were selected under a polarizing microscope and glued to a glass capillary.The diffraction data were collected on an Oxford Diffraction Gemini A ultra diffractometer at room temperature.Data collection and reduction were performed using the CrysAlis PRO software developed by Rigaku Oxford Diffraction [39].The structures were solved with the ShelXT structure solution program using Intrinsic Phasing and refined with the ShelXL refinement package using Least Squares minimization [40].These programs were invoked from within Olex2 suite which was also used for the production of Figures 2-5

Conclusions
In summary, we have investigated the influence of steric and electronic properties of 1,3-bis(2,4,6-trimethylphenyl)-3,4,5,6-tetrahydropyrimidin-1-ylidene (6-Mes) on the synthesis, structure and reactivity of Me 2 MOR(NHC).Particularly, we have focused on the M-C 6-Mes bond in Me 2 MOR(6-Mes), how it is influenced by the structure of 6-Mes in comparison with other NHCs, and how it can influence the structure of Me 2 MOR(6-Mes) as well as their catalytic activity in the ring-opening polymerization (ROP) of racemic lactide (rac-LA).The considerable effect of steric hindrances of 6-Mes was reflected both by significantly longer, and therefore weaker M-C 6-Mes in the solid state in comparison to M-C SIMes and M-C IMes , which was revealed by X-ray analysis and the presence of equilibrium between Me 2 MOR(NHC) and [Me 2 M(µ-OR)] 2 /NHC in solution.The stronger donor properties of 6-Mes, in comparison with other NHCs, were revealed in solution by significant shift of carbene carbon in 13  With regard to the catalytic activity of Me 2 MOR(NHC), the effect of Ga-C 6-Mes and a resulting structure of Me 2 GaOR(NHC) influenced mostly stereoselectivity, as well as the extent of transesterification reactions in the polymerization of rac-LA, which demonstrated the crucial role of NHC on the catalytic properties of Me 2 GaOR(NHC).The mechanism of the ROP of rac-LA was in line with the insertion of rac-LA into Ga-O alkoxide bond, although the insertion into Ga-C 6-Mes could not be excluded.As a result of even weaker In-C 6-Mes bond in comparison to other In-C NHCs in Me 2 InOR(NHC) the insertion of rac-LA into In-C 6-Mes occurred and was not surprising in the light of the polymerization of rac-LA with previously described by us Me 2 InOR(NHC) (NHC = SIMes, IMes).Importantly, the reported studies, concerning both the structure and catalytic activity of Me 2 MOR(6-Mes), revealed the effect of the character of M-C NHC , rather than only its strength, which is the subject of our current research.

Supplementary Materials:
The following are available online at www.mdpi.com/2304-6740/6/1/28/s1.Figures S1-S58 (including mainly NMR spectra of gallium and indium complexes, NMR data of PLA obtained with selected indium and gallium complexes, NMR spectra of experiments of 1 with 10 eq rac-LA, MALDI-TOF of PLA obtained with 1 and weak interactions within crystal structures of 3 and 4), cif and cif-checked files.
C NMR, indicated stronger In-C6-Mes bond (δ = 207.5 ppm, Δ = −37.4ppm) in comparison with In-CSIMes (Δ = −35.2ppm) and In-CIMes (Δ = −35.7 ppm) of Me3In(SIMes) and Me3In(IMes) respectively.While the relationship between the distance of essentially the same bonds and their strength should be considered a tenet in the case of

2. 5 .
Activity of Me 2 Ga(OCPh 2 Me)(6-Mes) (3 and 4), Me 2 M(OC 6 H 4 OMe)(6-Mes) (5 and 6) (M = Ga, In) and 2 Ga(OCH 2 CH 2 OMe)(6-Mes) (1) in the ROP of rac-Lactide In order to investigate the effect of 6-Mes on the catalytic properties of Me 2 MOR(NHC), we examined the activity of 1, 3, 4, 5 and 6 complexes in the ring-opening polymerization (ROP) of rac-LA.With regard to the latter, we focused on the effect of M-C 6-Mes bond on the catalytic properties of selected complexes.While the M-C 6-Mes bond affects the structure of all selected complexes, it should be expected to influence their reactivity, as well the structure and reactivity of Me 2 MO(PLA)(6-Mes), where O(PLA) represents growing polylactide chain, and therefore the microstructure of resulting polylactide (PLA).While our previous results showed that both gallium and indium Me 2 MOR(NHC) complexes were highly active already at −20 • C [1-3], in order to compare our results with the latter, we investigated the ROP of rac-LA with 1, 3, 4, 5 and 6 at identical conditions.Gallium complexes: alkoxide derivative Me 2 M(OCPh 2 Me)(6-Mes) (3) and aryloxide derivative Me 2 Ga(OC 6 H 4 OMe)(6-Mes) (5), were inactive in the polymerization of rac-LA at −20 • C. While in the case of complexes 3 and 5 the bulky alkoxide or aryloxide group did not allow for the insertion of rac-LA into Ga-O bond, most importantly, the strong Ga-C 6-Mes bond precluded, the initiation of rac-LA polymerization by N-heterocyclic carbene, similarily to Me 2 Ga(OCPh 2 Me)(NHC) (NHC = SIMes, IMes) [3].Contrary to the latter, a much weaker In-C NHC bond in Me 2 In(OCPh 2 Me)(NHC) (NHC = SIMes, IMes) resulted in the initiation of rac-LA by N-heterocyclic carbenes [3], and similar reactivity was observed in the case of Me 2 In(OCPh 2 Me)(6-Mes) (4) and Me 2 In(OC 6 H 4 OMe)(6-Mes) (6), which led to the formation of cyclic PLA (see the Supplementary Materials).However, the observed reactivity of 6-Mes of indium complexes 4 and 6 towards lactide, as well as the lack of activity of gallium derivatives 3 and 5, may also indicate the considerable effect of a character of the M-C NHC bond on the reactivity of investigated complexes.As we were unable to isolate any stable alkoxide Me 2 InOR(6-Mes) complex, we focused on the activity of 1 in the ROP of rac-LA.Due to the reactivity of 1 towards CH 2 Cl 2 , which led to the almost quantitative and instant formation of [Me 2 Ga(µ-OCH 2 CH 2 OMe)] 2 [9,35], no activity of 1 in the ROP of rac-LA in CH 2 Cl 2 , at −20 • C was observed.On the other hand, the polymerization of rac-LA with 1 (25:1) in toluene led to essentially full conversion after 12 h (−20 • C) or 10 min (room temperature).
C NMR upon coordination of 6-Mes to Ga and In of Me 2 MOR(6-Mes) (M = Ga, In) and Me 3 In(6-Mes).However, in light of other results, including X-ray analysis, they could not indicate the presence of a stronger M-C 6-Mes bond for investigated complexes in comparison with M-C SIMes , M-C IMes , and M-C SIPr of their Me 2 MOR(NHC) (M = Ga, In; NHC = SIMes, IMes) and Me 3 In(NHC) (NHC = SIMes, IMes, SIPr) analogues.The formation of In-C 6-Mes bond in the case of dimethylindiumalkoxides was not sufficient in order to stabilize Me 2 InOR(NHC) complexes.Finally, the investigation of the reactivity of Me 2 M(OCH(Me)CO 2 Me) towards 6-Mes, with regard to the structure of propagating Me 2 M(OPLA)(6-Mes) species in the ROP of rac-LA, allowed to clarify the mechanism of the reaction of Me 2 InOR(NHC) towards sterically hindered NHCs.