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

Abstract

Abstract: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-C 6-Mes bond, as well as the formation, structure and reactivity of Me 2 MOR(6-Mes) (M = Ga, In) complexes. While the reactions of simple dimethylgallium alkoxides with 6-Mes lead to the formation of stable monomeric Me 2 Ga(OCH 2 CH 2 OMe)(6-Mes) (1) and Me 2 GaOMe(6-Mes) complexes, the analogous Me 2 InOR(6-Mes) are unstable and disproportionate to methylindium alkoxides and Me 3 In(6-Mes) (2). The use of bulky alkoxide ligand-OCPh 2 Me or aryloxide ligand-OC 6 H 4 OMe allowed for the synthesis of stable Me 2 M(OCPh 2 Me)(6-Mes) (M = Ga (3) and In (4)) as well as Me 2 M(OC 6 H 4 OMe)(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.

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

“…The polymerization of rac -LA with Me 2 AlOMe­(IMes) ( 1a ), 3a , 4a , and 4b was performed at −20 °C similarly to previously investigated Me 2 MOR­(NHC) (M = Ga, In) complexes. , Notably, under these conditions, the insertion of rac -LA into In–C NHC of Me 2 InOR­(NHC) was observed, , while the significantly stronger Ga–C NHC bond in Me 2 GaOR­(NHC) remained intact . For the discussed dimethylaluminum alkoxide complexes with IMes, 3a showed essentially no activity in the polymerization of rac -LA at −20 °C, even after 24 h, in contrast to Me 2 In­(OCPh 2 Me)­(IMes), which polymerized rac -LA due to insertion of the latter into the In–C IMes bond.…”

“…With regard to the previously investigated Me 2 M­(OCH­(Me)­CO 2 Me)­(NHC) complexes, which mimic active species with a growing PLA chain, in the polymerization of lactide with Me 2 MOR­(NHC) (M = Ga, In; NHC = IMes, SIMes), , we approached the synthesis of analogous Me 2 Al­(( S )-OCH­(Me)­CO 2 Me)­(NHC) (Me 2 Al­(( S )- melac )­(NHC)) complexes. However, the reaction of [Me 2 Al­(μ-( S ) -melac )] 2 with IMes or SIMes resulted in oily products with complex 1 H NMR spectra (see the Supporting Information).…”

“…In recent years, main-group metal complexes with N-heterocyclic carbenes (NHCs) have become a focus of scientists, both from the point of view of fundamental studies as well as their applications in catalysis. , In this regard, NHC-stabilized main group metal alkoxides, including aluminum, gallium, indium, , magnesium, and zinc, , alkoxides, as well as Me 3 Al­(NHC) and [R 2 Al­(NHC)] + in the presence of benzyl alcohol, have been shown to catalyze ring-opening polymerization (ROP) of rac -lactide ( rac -LA), as well as other heterocyclic monomers, such as cyclohexyl oxide, trimethylene carbonate, and ε-caprolactone, in controlled and stereoselective fashion. Noteworthy, mentioned examples have shown that the M–C NHC bond strongly influences the catalytic activity of such complexes, as demonstrated, for instance, by the high activity and stereoselectivity of heteroselective Zn alkoxides with NHCs, , as well as isoselective Me 2 GaOR­(NHC) complexes, , in the polymerization of rac -LA.…”

“…With regard to the above, we have decided to investigate the effect of M–C NHC on the synthesis structure and reactivity, including activity in the ROP of lactide, of group 13 metal dialkylalkoxides and dialkylaryloxides with NHCs - Me 2 MOR­(NHC) and Me 2 MOAr­(NHC) (M = Al, Ga, In; OAr = aryloxide group) respectively. Therefore, in the first step, we have extended our studies on the synthesis, structure, and catalytic activity of Me 2 MOR­(NHC) (M = Ga, In , ) complexes in ROP, to their aluminum analogues in order to determine the strength of the Al–C NHC bond and its effect on the synthesis structure and reactivity, including both stability and activity of aluminum complexes in the ROP of lactide. Subsequently, in order to fully determine the strength and character of M–C NHC (M = Al, Ga, In) bonds, we applied density functional theory (DFT) calculations.…”

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

“…Although main group metal complexes with N-heterocyclic carbenes (NHCs) represent a still relatively new class of complexes, their catalytic properties have already become of interest in the field of catalysis, including ring-opening polymerization (ROP) of heterocyclic monomers. , With regard to the latter, the effect of NHC on the catalytic properties of main group metal complexes, such as Mg, Zn, , Al, , Ga, In, has been evidenced by a few research groups, including ours, in the ROP of lactide, as well as other heterocyclic monomers . Although in the case of listed complexes, the ring-opening polymerization could be initiated by NHCs, in the case of Zn, ,,− Ga, ,, and Al complexes, the presence of supporting NHC ligands was responsible for their activity and/or selectivity in the polymerization of lactide.…”