Group 13 metal (Al, Ga, In) alkyls supported by N -heterocyclic carbenes for use in lactide ring-opening polymerization catalysis

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

“…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].…”

“…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-C 6-Mes bond of 2 in solution, in comparison with In-C NHC bonds of already characterized Me 3 In(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 [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.…”

“…On the other hand, rac-LA could insert, even at low temperature, into In-CNHC bond of Me2GaOR(NHC), considerably weaker in comparison with Ga-CNHC [3]. Analogously, the insertion of lactide into the M-CNHC bond of Mg alkoxides with alkoxide ligands possessing NHC termini [16], Me3Al(NHC) adducts [21], or yttrium and titanium complexes [22], have been reported so far.…”

“…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.…”

Sterically Bulky NHC Adducts of GaMe3 and InMe3 for H2 Activation and Lactide Polymerization

“…The NMR data for 3 and 4 only display sharp resonances As determined through X-ray crystallographic studies (XRD), the solid-state molecular structure of 2 confirmed the effective coordination of IPr* to InMe 3 , resulting in a four-coordinate In(III) adopting a distorted tetrahedral geometry and coplanar to the NHC heterocyclic ring (Figure 2). The In-C carbene bond distance (2.330(5) Å) in 2 is a bit longer than those in (IMes)InMe 3 and (IPr)InMe 3 (2.292(6) and 2.309(2) Å, respectively; IMes = 1,3-bis(2,4,6-trimethylphenyl)-1,3-dihydro-imidazol-2-ylidene; IPr = 1,3-bis{2,6-bis(1-methylethyl)phenyl})-1,3-dihydro-imidazol-2-ylidene), likely reflecting greater steric hindrance between the NHC and InMe 3 in 2 [20,21]. However, the shortest contacts between IPr* and InMe 3 (H···H = 2.257 Å and C···H = 2.747 Å) remain close to the sum of the van der Waals (vdW) radii of the corresponding atoms, in line with no severe steric congestion in 2 and in agreement with the observed stability of adduct (IPr*)InMe 3 in solution [22].…”