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

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 alpha-diimine. Similar reaction of the bis(alpha-diimine) [ArN=C(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 [O=C(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 {[ArN=C(Me)COCHCO(Me)C(Me)=NAr][OCH(Me)C(Me)=NAr](ZnEt)3} (Ar = 2,6-iPr2C6H3, 7) from reactions of ZnEt2 with ArN=C(Me)C(Me)=NAr or [O=C(Me)C(Me)=NAr], respectively. Reaction of the bis(alpha-diimine), LiPr-N2-ArCH2Ar-N2, derived from 4,4/-methylenebis(2,6-diisopropylaniline), with ZnCl2 affords [LiPr-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 {[ArN=C(Me)C(Me)=NAr]Al(THF)}2 (9), to act as catalysts for the ring opening polymerization (ROP) of the cyclic esters epsilon-caprolactone (epsilon-Cl), delta-valerolactone (delta-VL) and rac-lactide (r-LA) are presented. For epsilon-CL and delta-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 oC isotactic PLA with Mn ca. 10 KDa with conversions of up to 66%. The co-polymerization of epsilon-CL with delta-VL was also examined, and differing preferences were noted for monomer incorporation.