Dialkylgallium Alkoxides Stabilized with <i>N</i>‑Heterocyclic Carbenes: Opportunities and Limitations for the Controlled and Stereoselective Polymerization of <i>rac</i>-Lactide

Abstract

The structure of a series of Me₂GaOR­(NHC) complexes with <i>N</i>-heterocyclic carbenes (1,3-bis­(2,4,6-trimethylphenyl)­imidazolin-2-ylidene (SIMes) and 1,3-bis­(2,4,6-trimethylphenyl)­imidazol-2-ylidene (IMes)) have been characterized using spectroscopic and X-ray techniques and discussed in view of their reactivity in the polymerization of <i>rac</i>-lactide (<i>rac</i>-LA). Both structure studies and density functional theory (DFT) calculations show the significant influence of NHC and OR on the structure of investigated complexes and has indicated that the Ga–C_NHC bond (32.6–39.6 kcal mol^–1) is strong enough to form stable Me₂GaOR­(NHC) complexes in the form of monomeric species. The reactivity of Me₂Ga­((<i>S</i>)-OCH­(Me)­CO₂Me)­(SIMes) (<b>1</b>) and Me₂Ga­((<i>S</i>)-OCH­(Me)­CO₂Me)­(IMes) (<b>5</b>) toward Lewis acids such as CO₂ and GaMe₃ has resulted in breaking of the Ga–C_NHC bond with the formation of (NHC)­CO₂ and Me₃Ga­(NHC) (<b>8</b> and <b>10</b>) and [Me₂Ga­(μ-(<i>S</i>)-OCH­(Me)­CO₂Me)]₂. Different results have been obtained for l,3-bis­(2,6-diisopropylphenyl)-imidazolin-2-ylidene (SIPr), which coordinates more weakly to gallium, as demonstrated by the Ga–C_NHC bond strength for model Me₃GaSIMes, Me₃GaIMes (<b>8</b>), and Me₃GaSIPr (<b>10</b>) adducts. The reaction of SIPr with [Me₂Ga­(μ-OR)]₂ has not allowed for the breaking of Ga₂O₂ bridges and the formation of monomeric Me₂GaOR­(SIPr) complexes, contrary to SIMes and IMes. In the case of the reaction with [Me₂Ga­(μ-(<i>S</i>)-OCH­(Me)­CO₂Me)]₂, the ionic compound [Me₂Ga­(OCH­(Me)­CO₂)]⁻[SIPrH]⁺ (<b>9</b>) has been isolated. The investigated Me₂GaOR­(NHC) complexes are highly active and stereoselective in the ring-opening polymerization of <i>rac</i>-lactide from −20 °C to room temperature, due to the insertion of <i>rac</i>-LA exclusively into the Ga–O_alkoxide bond, leading to isotactically enriched polylactide (PLA) (<i>P</i>_m = 0.65–0.78). It has been shown that the polymerization of lactide at low temperature is influenced by the chelate interaction of (<i>S</i>)-OCH­(Me)­CO₂Me or (OCH­(Me)­C­(O))₂OR resulting from the primary insertion of <i>rac</i>-LA into the Ga–O_alkoxide bond, with the Ga center, which can be responsible for the low control over the molecular weight of the obtained PLA. The latter effect can be eliminated by the initial synthesis of Me₂Ga­((PLA)_<i>n</i>OR)­(NHC) with short PLA chains, which allows for controlled polymerization. Although the adverse chelate effect can be also eliminated by the polymerization of <i>rac</i>-LA at room temperature, the stereoselectivity of <i>rac</i>-LA polymerization is strongly affected by transesterification reactions. Out of investigated Me₂GaOR­(SIMes) and Me₂GaOR­(IMes) complexes, only the latter allowed for the immortal ring opening polymerization of <i>rac</i>-LA in the presence of ^<i>i</i>PrOH