Synthesis and Structural Characterization of Various N,O,N-Chelated Aluminum and Gallium Complexes for the Efficient ROP of Cyclic Esters and Carbonates: How Do Aluminum and Gallium Derivatives Compare ?

The novel Al and Ga coordination compounds 4, 5, 7, and 8 of the type (NON)AlX (NON2– = {RNC6H4}2O2–; 4, R = Cy, X = Me; 5, R = C5H9, X = OCH2Ph; 7, R = C6F5, X = Me) and (NON)GaX (NON2– = {R2NHC6H4}2O2–; 8, R = C6F5, X = Me) have been synthesized via, in the case of 4, 7, and 8, a methane elimination reaction between the corresponding protio ligands (NON)H2 and MMe3 precursors (M = Al, Ga). The Al alkoxide derivative 5 was prepared via an alcoholysis reaction between κ3-N,O,N-{(C5H9)NC6H4}2O}2AlNMe2 and PhCH2OH. The tetracoordinate Al–THF adduct κ2-N,N′-{(C5H9)NC6H4}2O}2Al(Me)(THF) (6) was prepared via a methane elimination reaction between the protio ligand TfNONH2 (Tf = CF3SO2, 1c) and AlMe3 in a CH2Cl2/THF solvent mixture. As determined from X-ray studies, complexes 4, 7, and 8 are monomeric in the solid state and feature a central tetracoordinate metal center effectively κ3-N,O,N′ chelated and adopting a trigonal-monopyramidal geometry. The presence of Ga···F(o-C6F5) contacts in the solid-state structure of compound 8 are likely to reflect the Lewis acidity of the Ga center. The Al alkoxide derivative [κ3-N,O,N-{(C5H9)NC6H4}2O}2AlOCH2Ph]2 (5) was isolated as a dimer containing two five-coordinate (trigonal-pyramidal) Al centers and retains its dimeric structure in solution, while the Al–THF adduct 6 crystallizes in a monomeric form with an Al center in a tetrahedral geometry. The catalytic performances of the Al and Ga species 4, 5, 7, and 8 as ROP initiators of rac-lactide (rac-LA), ε-caprolactone (ε-CL), and trimethylene carbonate (TMC) were estimated, and most of them efficiently mediate the controlled and immortal ROP of these three monomers in the presence of BnOH, such an alcohol acting as an effective chain transfer agent. Of particular interest, the Ga amido species κ3-N,O,N-{(C5H9)NC6H4}2O}2Ga-NMe2 (3) outperforms its Al counterpart κ3-N,O,N-{(C5H9)NC6H4}2O}2AlNMe2 (2) in the ROP of rac-LA, whether in terms of control or activity, to afford isotactically enriched PLA (Pm = 0.7). In contrast, all the Al derivatives are more efficient catalysts for the polymerization of ε-CL or TMC than the Ga analogues. For the ROP of TMC initiated by the Al and Ga complexes 2–5, an increased Lewis acidity of the metal center is clearly beneficial to both the activity and the ROP control. Notably, the C6F5N Ga species 8 was found to be inactive in the ROP of rac-LA, ε-CL, and TMC, which may be related to both electronic (C–F···Ga interactions) and steric factors.