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–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.
Highlights
Over recent years, we have reported Me2 MOR(NHC) (M = Ga(μ-(S)-melac)]2 with SIPr (Ga) [1,2], In [3]; NHC = N-heterocyclic carbene) complexes, which constitute the first examples of gallium and indium, as well as group13 metal alkoxides, with NHCs [4]
Ga–CNHC bond led to stable complexes [1,2], in the case of mostly unstable dialkylindium alkoxides with NHCs, a significantly weaker In–CNHC bond was observed [3]
In order to investigate the effect of 6-Mes on the catalytic properties of Me2 MOR(NHC), In order to investigate the effect of 6-Mes on the catalytic properties of Me2MOR(NHC), we we examined the activity of 1, 3, 4, 5 and 6 complexes in the ring-opening polymerization (ROP) of examined the activity of 1, 3, 4, 5 and 6 complexes in the ring-opening polymerization (ROP) of racrac-LA
Summary
13 metal alkoxides, with NHCs [4] They represent isoselective catalysts for the ring-opening polymerization (ROP) of rac-lactide (rac-LA), which are highly active at low temperatures. They still constitute rare examples of isoselective catalysts operating at mild conditions, both among group 13 metal complexes [5,6,7,8,9], and beyond [10,11,12,13]. In the ring-opening polymerization with Me2 GaOR(NHC), Ga–CNHC bond remained intact, resulting in the exclusive insertion of rac-LA into Ga–Oalkoxide bond, similar to Zn [14,15,16], Al [17], as well as Ti and Zr [18,19,20], metal alkoxides with NHCs, which have been reported to polymerize rac-LA due to the insertion into
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