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

摘要

A series of organoaluminium imino-amido complexes of the type {[ArNC(Me-2)C(Me)-NAr]AlMe2} (Ar = 2,6-iPr(2)C(6)H(3) (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-iPr(2)C(6)H(3)) with AlMe3 afforded the bimetallic complex [ArN-C(Me)(2)C(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-iPr(2)C(6)H(3), 6) and the trinuclear complex {[ArN-C(Me)COCHCO(Me)C(Me)-NAr][OCH(Me)C(Me)-NAr](ZnEt)(3)} (Ar = 2,6-iPr(2)C(6)H(3), 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), L-2(iPr-N)-ArCH2Ar-N-2, derived from 4,4 '-methylenebis(2,6-diisopropylaniline), with ZnCl2 affords [L-2(iPr-N)-ArCH2Ar-N-2(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 degrees C isotactic PLA with M(n)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.