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

Xiao, Lin; Zhao, Yanxia; Qiao, Sijie; Sun, Ziyue; Santoro, Orlando; Redshaw, Carl

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), 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 {[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 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.

Citation

Xiao, L., Zhao, Y., Qiao, S., Sun, Z., Santoro, O., & Redshaw, C. (2020). 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. Dalton Transactions : an international journal of inorganic chemistry, 49(5), 1456-1472. https://doi.org/10.1039/c9dt04332b

• Journal Article Type: Article
• Acceptance Date: Dec 28, 2019
• Online Publication Date: Dec 30, 2019
• Publication Date: Feb 7, 2020
• Deposit Date: Jan 2, 2020
• Publicly Available Date: Dec 31, 2020
• Journal: Dalton Transactions
• Print ISSN: 1477-9226
• Electronic ISSN: 1477-9234
• Publisher: Royal Society of Chemistry
• Peer Reviewed: Peer Reviewed
• Volume: 49
• Issue: 5
• Pages: 1456-1472
• DOI: https://doi.org/10.1039/c9dt04332b
• Keywords: Inorganic Chemistry
• Public URL: https://hull-repository.worktribe.com/output/3340444
• Publisher URL: https://pubs.rsc.org/en/content/articlelanding/2020/DT/C9DT04332B#!divAbstract
• Additional Information: : This document is Similarity Check deposited; : Supplementary Information; : Single-blind; : Received 8 November 2019; Accepted 28 December 2019; Accepted Manuscript published 30 December 2019