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

Abstract

Anionic donor-functionalized NHC (N-heterocyclic carbene) complexes of Al are rare. We report one such case here, an NHC-aryloxido AlMe₂ complex [Al(L)Me₂] (2), following a stepwise synthesis from the proligand [HO-4,6-^tBu₂-C₆H₂-2-CH₂{CH(NCHCHNAr)}]Br [LH₂Br; Ar = 2,6-ⁱPr₂-C₆H₃ (Dipp)] and AlMe₃via the zwitterionic intermediate [Al(LH)Me₂Br] (1). The ligand's flexibility in 2 is evident from the conformational fluxionality revealed by VT-¹H 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)(NMe₂)₂Br]. 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 ε-caprolactone (CL). 2, similar to [Ti(L)(NMe₂)₂Br], 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(LH)Me₂(OBn)]. The 1/BnOH combination follows a similar mechanism but surprisingly at a faster rate.