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.