Synthesis and structures of mono- and di-nuclear aluminium and zinc complexes bearing α-diimine and related ligands, and their use in the ring opening polymerization of cyclic esters

Abstract

A series of organoaluminium imino-amido complexes of the type {[ArNC(Me₂)C(Me)NAr]AlMe₂} (Ar = 2,6-iPr₂C₆H₃ (1), Ar = 2,6-Et₂C₆H₃ (2); Ar = 2,6-Me₂C₆H₃ (3) have been prepared via reaction of AlR₃ and the respective α-diimine. Similar reaction of the bis(α-diimine) [ArNC(Me)C(Me)N-]₂ (Ar = 2,6-iPr₂C₆H₃) with AlMe₃ afforded the bimetallic complex [ArN−C(Me)₂C(Me)NAlMe₂]₂ (4), whilst reaction of the acetyl-imino compound [OC(Me)C(Me)NAr] (Ar = 2,6-Et₂C₆H₃) with AlMe₃ afforded the bimetallic complex {[OCMe₂CH(Me)NAr]AlMe₂}₂ (5). In related organozinc chemistry, we have isolated {[ArNC(Me)(Et)C(Me)NAr]ZnEt} (Ar = 2,6-iPr₂C₆H₃, 6) and the trinuclear complex {[ArNC(Me)COCHCO(Me)C(Me)NAr][OCH(Me)C(Me)NAr](ZnEt)₃} (Ar = 2,6-iPr₂C₆H₃, 7) from reactions of ZnEt₂ with ArNC(Me)C(Me)NAr or [OC(Me)C(Me)NAr], respectively. Reaction of the bis(α-diimine), L^{iPr–N₂–ArCH₂Ar–N₂}, derived from 4,4′-methylenebis(2,6-diisopropylaniline), with ZnCl₂ affords [L^{iPr–N₂–ArCH₂Ar–N₂}(ZnCl₂)₂] (8). The molecular structures of complexes 1–8 are reported. Preliminary results of the ability of 1–8, along with the previously reported metal–metal bonded complex {[ArNC(Me)C(Me)NAr]Al(THF)}₂ (9), to act as catalysts for the ring opening polymerization (ROP) of the cyclic esters ε-caprolactone (ε-CL), δ-valerolactone (δ-VL) and rac-lactide (r-LA) are presented. For ε-CL and δ-VL, best results were obtained using the metal–metal bonded complex 9. For r-LA, the Al-based systems exhibited moderate activity affording only liquid oligomers, whilst the Zn-based systems performed better affording at 80 °C isotactic PLA with M_nca. 10 kDa with conversions of up to 66%. The co-polymerization of ε-CL with δ-VL was also examined, and differing preferences were noted for monomer incorporation.