Abstract
Manipulating the structure of alkylgallium alkoxides with asymmetric N-heterocyclic carbenes (NHCs) has led to the formation of essentially “on” and “off” states of catalytic gallium centers in the ring-opening polymerization (ROP) of rac-lactide (rac-LA), and has allowed to rationalize the effect of the NHC on the activity and stereoselectivity of Me2GaOR(NHC) complexes. The reactions of dimethylgallium alkoxides with asymmetric NHCs, due to differently substituted nitrogens, resulted in the formation of Me2GaOR(SI(Dipp-Mes)) (OR = OMe (1), OCH2CH2OMe (2), OCH(Me)CO2Me (3)) and Me2GaOR(SI(Me-Mes)) (OR = OMe (4), OCPh2Me (5)) complexes (SI(Dipp-Mes) = 1-(diisopropylphenyl)-3-(mesityl)-imidazolin-2-ylidene, SI(Me-Mes) = 1-(methyl)-3-(mesityl)-imidazolin-2-ylidene). The X-ray analysis of 1, 2, 4, and 5 revealed a strong effect of NHCs on their structure, manifested by the orientation of NHC with respect to Ga–CMe and Ga–O bonds, and OR alkoxide group. The latter, represented by a dihedral O–Ga–CNHC–N angle, have allowed to define the cavity required for the interaction of lactide with gallium and alkoxide groups, and subsequent insertion of lactide into the Ga–Oalkoxide bond. Despite small differences between the cavities of 1 and Me2GaOMe(SIMes) (SIMes = 1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene), the structures of NHCs were responsible for their isoselectivities, of Pm-max = 0.83 and Pm-max = 0.78, respectively, in the ROP of rac-LA at −20 °C. The much smaller cavity of 4 led to a minor activity and lower isoselectivity (Pm-max = 0.56 at −20 °C). Further decrease of the activity of Me(O,CNHC)GaOR (6), possessing both small cavity and, in contrast to 4, restrained rotation of Ga–CNHC bond of the chelate ligand, has supported the effect of NHC on the structure and activity of Me2GaOR(NHC) complexes. DFT calculations of the energy profile of the consecutive ring-opening of two lactide molecules into Ga–Oalkoxide bonds of 1, 4, and Me2GaOMe(SIMes) have confirmed the effect of NHC on their stereoselectivity.
Highlights
IntroductionMain 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,3,4 Al,4d,5 Ga, In,7 has been evidenced by a few research groups, including ours, in the ROP of lactide, as well as other heterocyclic monomers. in the case of listed complexes, the ring-opening polymerization could be initiated by NHCs, in the case of Zn,3,4a,c−f Ga,6a,b,d and Al5a complexes, the presence of supporting NHC ligands was responsible for their activity and/or selectivity in the polymerization of lactide.In these cases, the NHC remained intact toward the monomer, while the formation of M−CNHC bond resulted in the considerable modification of the structure and, as a result, catalytic properties
In order to address the effect of an N-heterocyclic carbenes (NHCs) supporting ligand on the activity and stereoselectivity of Me2GaOR(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, due to differently substituted nitrogens
In the course of our studies on Me2GaOR(aNHC), we isolated a series of Me2GaOR(SI(Dipp-Mes) (SI(Dipp-Mes) = 1-(diisopropylphenyl)-3-(mesityl)-imidazolin-2-ylidene) and Me2GaOR(SI(Me-Mes)) (SI(Me-Mes) = 1-(methyl)-3-(mesityl)-imidazolin-2-ylidene) complexes (Scheme 1), which were crucial in order to relate the structure and catalytic properties of
Summary
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,3,4 Al,4d,5 Ga, In,7 has been evidenced by a few research groups, including ours, in the ROP of lactide, as well as other heterocyclic monomers. in the case of listed complexes, the ring-opening polymerization could be initiated by NHCs, in the case of Zn,3,4a,c−f Ga,6a,b,d and Al5a complexes, the presence of supporting NHC ligands was responsible for their activity and/or selectivity in the polymerization of lactide.In these cases, the NHC remained intact toward the monomer, while the formation of M−CNHC bond resulted in the considerable modification of the structure and, as a result, catalytic properties. 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.. The explanation of the role of supporting NHC ligands on the activity and stereoselectivity of the discussed class of catalysts should be of essence in order to tailor their catalytic properties.9 It should be, of importance for the rational design of new main group metal complexes with supporting NHCs in the ROP of lactide, with regard to switchable catalysts, essential for the synthesis of PLA The explanation of the role of supporting NHC ligands on the activity and stereoselectivity of the discussed class of catalysts should be of essence in order to tailor their catalytic properties. It should be, of importance for the rational design of new main group metal complexes with supporting NHCs in the ROP of lactide, with regard to switchable catalysts, essential for the synthesis of PLA
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