Probing the M–C_NHCBond and Its Effect on the Synthesis, Structure, and Reactivity of R₂MOR(NHC) (M = Al, Ga, In) Complexes

Abstract

The studies on the reactivity of dialkylaluminum alkoxides towards N-heterocyclic carbenes (NHCs) has allowed investigation of not only the factors controlling the synthesis and properties of Me 2 AlOR(NHC) complexes. Additionally, we have focused on the effect of group 13 metals on the synthesis, structure, and reactivity of Me 2 MOR-(NHC) (M = Al, Ga, In) complexes, with regard to the strength and character of M−C NHC bonds. The reactions of simple dimethylaluminum alkoxides with NHCs lead to the monomeric Me 2 AlOR(NHC) complexes, as shown by the isolation of Me 2 AlOMe(NHC) (NHC = IMes (1a), SIMes (1b)) (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene, SIMes = 1,3-bis(2,4,6-trimethylphenyl)imidazolin-2ylidene). Despite their tendency to disproportionate, the interaction of the additional NHC molecule in Me 2 Al-(OCH 2 CH 2 OMe)(IMes)/IMes (2a/IMes) with the fifth coordinate site of aluminum has resulted in stabilization and allowed for the isolation of Me 2 Al(OCH 2 CH 2 OMe)(IMes) (2a). In contrast, the limited accessibility of the fifth coordinate site of aluminum in the case of five-coordinate [Me 2 Al(μ-OCH(Me)CO 2 Me)] 2 or four-coordinate [Me 2 Al(μ-OR)] 2 alkoxides with bulky alkoxide ligands, has affected the formation of Al−C NHC bonds and allowed only for the synthesis and isolation of stable Me 2 Al(OCPh 2 Me)(NHC) (NHC = IMes (3a), SIMes) complexes. Additionally stable aryloxide derivatives Me 2 M-(OC 6 H 4 OMe)(NHC) (NHC = IMes (4a) and SIMes (4b)) have been isolated and characterized. More ionic Al−C NHC bonds of Me 2 AlOR(NHC), in comparison with analogous Ga−C NHC and In−C NHC bonds, have been decisive for the reactivity of aluminum complexes, which includes their tendency for ligand disproportionation and activity of 1a, 4a, and 4b in the ring opening polymerization (ROP) of lactide, initiated in each case by the insertion of lactide into mainly an ionic Al−C NHC bond. The ionic character of M−C NHC bonds, decreasing in the series Al−C NHC > In−C NHC > Ga−C NHC , has been reflected by the reactivity of investigated complexes and determined by density functional theory (DFT) calculations using real-space bonding indicators (RSBIs). The structure of investigated aluminum complexes and the strength of Al−C NHC bonds have been investigated using spectroscopic methods and X-ray diffraction studies. The strength of M−C NHC bonds of investigated aluminum complexes, as well as their gallium and indium analogues, have been also determined by DFT calculations of their bond dissociation energies.

The Effect of Symmetric and Asymmetric NHCs on the Structure and Catalytic Properties of Dialkylgallium Alkoxides in the Ring-Opening Polymerization of rac-Lactide—Linking the Structure, Activity, and Stereoselectivity

“…Apart from steric hindrances of a NHCs used in this study and reflected by their V Bur parameter, the strength of Ga−C NHC bonds in the investigated Me 2 GaOR( a NHC) complexes was expected to result from the donor properties of SI(Dipp-Mes) and SI(Me-Mes), 22 as well as dominant polar covalent character of Ga− C NHC bonds. 16 Therefore, with regard to the catalytic properties of Me 2 GaOR(SI(Dipp-Mes)) and Me 2 GaOR(SI-(Me-Mes)) in the ROP of rac-LA one should expect the presence of supporting SI(Dipp-Mes) and Si(Me-Mes) ligands, which was further confirmed by the polymerization studies described below. Notably, with regard to both activity and stereoselectivity of the investigated complexes, the considerable differences between the structure of 1, 2, 4, and 5, as well as already reported Me 2 GaOR(NHC) complexes, 6a,b,7 were reflected in the solid state by the orientation of NHC respectively to Ga−C Me and Ga−O bonds, and the orientation of NHC with regard to the lone pairs located on the oxygen of alkoxide group.…”

“…However, for both 1 and 2 , small NHC tilts were indicative for the presence of strong Ga–C NHC bonds (Table ). Apart from steric hindrances of a NHCs used in this study and reflected by their V Bur parameter, the strength of Ga–C NHC bonds in the investigated Me 2 GaOR­( a NHC) complexes was expected to result from the donor properties of SI­(Dipp-Mes) and SI­(Me-Mes), as well as dominant polar covalent character of Ga–C NHC bonds . Therefore, with regard to the catalytic properties of Me 2 GaOR­(SI­(Dipp-Mes)) and Me 2 GaOR­(SI­(Me-Mes)) in the ROP of rac -LA one should expect the presence of supporting SI­(Dipp-Mes) and Si­(Me-Mes) ligands, which was further confirmed by the polymerization studies described below.…”

“…1 H and 13 C NMR spectroscopy revealed the formation of stable Me 2 GaOR­(SI­(Dipp-Mes)) ( 1 – 3 ) and Me 2 GaOR­(SI­(Me-Mes)) ( 4 , 5 ) complexes in solution. They were indicative for the presence of Ga–C NHC bonds, and most importantly, for the lack of equilibrium resulting in the dissociation of Me 2 GaOR­(NHC) to [Me 2 Ga­(μ-OR)] 2 and free NHC, the latter observed in the case of Me 2 GaOR­(6-Mes) complexes with sterically bulky N-heterocyclic carbene 1,3-bis­(2,4,6-trimethylphenyl)-3,4,5,6-tetrahydropyrimidin-1-ylidene (6-Mes), as well as Me 2 MOR­(NHC) (M = Al, In) complexes. , In the case of all investigated complexes ( 1 – 5 ), 13 C NMR was indicative for the formation of strong Ga–C NHC bonds as it revealed the considerable carbene carbon shifts, similarly to stable Me 2 GaOR­(SIMes) complexes (Table ). Therefore, with regard to further discussion on the catalytic properties of 1 – 5 in the ROP of rac -LA, both SI­(Dipp-Mes) and SI­(Me-Mes) were expected to remain bonded to gallium and act as a supporting ligand, also in the case of Me 2 Ga­(OCH­(Me)­CO 2 (PLA))­(NHC) (NHC = SI­(Dipp-Mes), SI­(Me-Mes) propagating species (see below).…”

“…We have recently shown that the strength and the character of M–C NHC bond remains crucial in order to sustain the supporting character of NHC ligand in the case of Me 2 MOR­(NHC) (M = Al, Ga, In) complexes . On the basis of the latter, we could expect the presence of a supporting NHC ligand for dialkylgallium alkoxides with NHCs, in contrast to their aluminum and indium analogues.…”

“…We have recently shown that the strength and the character of M−C NHC bond remains crucial in order to sustain the supporting character of NHC ligand in the case of Me 2 MOR-(NHC) (M = Al, Ga, In) complexes. 16 On the basis of the latter, we could expect the presence of a supporting NHC ligand for dialkylgallium alkoxides with NHCs, in contrast to their aluminum and indium analogues. In order to address the effect of an NHC supporting ligand on the activity and stereoselectivity of Me 2 GaOR(NHC), we have extended our previous studies on the synthesis, structure, and catalytic activity of this class of complexes with analogous complexes bearing asymmetric NHC ligands ( a NHCs), due to differently substituted nitrogens.…”