Flexible NHC-aryloxido aluminum complex and its zwitterionic imidazolium aluminate precursor in ring-opening polymerization of -caprolactone

Anionic donor-functionalized NHC (N-heterocyclic carbene) complexes of Al are rare. We report one such case here, an NHC-aryloxido AlMe 2 complex [Al(L)Me 2 ] ( 2 ), following a stepwise synthesis from the proligand [ H O-4,6- t Bu 2 -C 6 H 2 -2-CH 2 {C H (NCH&z.dbd;CHNAr)}]Br [L H 2 Br; Ar = 2,6- i Pr 2 -C 6 H 3 (Dipp)] and AlMe 3 via the zwitterionic intermediate [Al(L H )Me 2 Br] ( 1 ). The ligand's flexibility in 2 is evident from the conformational fluxionality revealed by VT- 1 H NMR spectroscopic analysis. The ∠O-Al-C ( ca. 100.5°) bite angle is also wider than the ∠O-Ti-C ( ca. 80.6°) as seen in our recently reported Ti complex [Ti(L)(NMe 2 ) 2 Br]. DFT analysis showed that the C NHC -Al bond is significantly ionic, as is the C NHC -Ti bond. Both 1 and 2 are active in the ring-opening polymerization (ROP) of -caprolactone (CL). 2 , similar to [Ti(L)(NMe 2 ) 2 Br], exhibits bifunctional MLC-type monomer activation, but only at an elevated temperature. However, the 2 /BnOH combination is catalytically active at room temperature, likely through a zwitterionic [Al(L H )Me 2 (OBn)]. The 1 /BnOH combination follows a similar mechanism but surprisingly at a faster rate. An NHC-aryloxide hybrid ligand-bound aluminum complex and its zwitterionic imidazolium precursor catalyze the ring-opening polymerization of -caprolactone through two different mechanistic pathways with and without benzyl alcohol as a cocatalyst.