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Issue 5, 2020
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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

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Abstract

A series of organoaluminium imino-amido complexes of the type {[ArNC(Me2)C(Me)[double bond, length as m-dash]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) [ArN[double bond, length as m-dash]C(Me)C(Me)[double bond, length as m-dash]N-]2 (Ar = 2,6-iPr2C6H3) with AlMe3 afforded the bimetallic complex [ArN−C(Me)2C(Me)[double bond, length as m-dash]NAlMe2]2 (4), whilst reaction of the acetyl-imino compound [O[double bond, length as m-dash]C(Me)C(Me)[double bond, length as m-dash]NAr] (Ar = 2,6-Et2C6H3) with AlMe3 afforded the bimetallic complex {[OCMe2CH(Me)[double bond, length as m-dash]NAr]AlMe2}2 (5). In related organozinc chemistry, we have isolated {[ArNC(Me)(Et)C(Me)[double bond, length as m-dash]NAr]ZnEt} (Ar = 2,6-iPr2C6H3, 6) and the trinuclear complex {[ArN[double bond, length as m-dash]C(Me)COCHCO(Me)C(Me)[double bond, length as m-dash]NAr][OCH(Me)C(Me)[double bond, length as m-dash]NAr](ZnEt)3} (Ar = 2,6-iPr2C6H3, 7) from reactions of ZnEt2 with ArN[double bond, length as m-dash]C(Me)C(Me)[double bond, length as m-dash]NAr or [O[double bond, length as m-dash]C(Me)C(Me)[double bond, length as m-dash]NAr], respectively. Reaction of the bis(α-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[double bond, length as m-dash]C(Me)C(Me)[double bond, length as m-dash]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 Mnca. 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.

Graphical abstract: 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

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Supplementary files

Article information


Submitted
08 Nov 2019
Accepted
28 Dec 2019
First published
30 Dec 2019

Dalton Trans., 2020,49, 1456-1472
Article type
Paper

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

L. Xiao, Y. Zhao, S. Qiao, Z. Sun, O. Santoro and C. Redshaw, Dalton Trans., 2020, 49, 1456
DOI: 10.1039/C9DT04332B

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