Schiff-base macrocycles: coordination chemistry and potential applications

Wang, Kuiyuan

Abstract

In this thesis, a number of Schiff base compounds and their complexes have been synthesized and fully characterized. Moreover, some applications including their use in catalysis, specifically the ring opening polymerization of cyclic esters, is disclosed.
Chapter 1. This chapter presents an introduction of the synthesis of macrocyclic Schiff bases together with their coordination chemistry. Applications of these Schiff base ligands and metal complexes are discussed. Topics include: photoluminescence studies; peroxidase-like mimetics; ring-opening polymerization of cyclic esters.
Chapter 2. In this chapter, the catalytic behaviour of homo binuclear versus mixed-metal analogues is discussed. In particular, homo-dinuclear Co and Zn complexes derived from the macrocycle LH2, {[2-(OH)-5-(R)-C6H2-1,3-(CH)2][CH2CH2(2-C6H4N)2]}2 (R = Me, tBu), revealed near inactivity for the ring opening polymerization (ROP) of the cyclic esters δ-valerolactone (δ-VL) and ε-caprolactone (ε-CL). By contrast, the hetero- bimetallic complexes [LCo(NCMe)(μ-Br)ZnBr]·nMeCN (n=3 or 3.25) were found to be efficient catalysts for the ROP of ε-CL and δ-VL.
Chapter 3. This chapter looks at the emission properties of some of the macrocycles and their complexes. Specifically, the emission properties of a number of solvates of the [2+2] Schiff-base macrocycles {[2-(OH)-5-(R)-C6H2-1,3-(CH)2][O(2-C6H4N)2]}2 (R = Me L1H2, tBu L2H2, Cl L3H2), formed by reacting 2,6-dicarboxy-4-R-phenol with 2,2′- oxydianiline (2-aminophenylether), (2-NH2C6H4)2O, have been investigated. The macrocycle L1-3H2 exhibited the different maximum emission wavelengths in different solvents, from max at 508 nm (in acetonitrile) to 585 nm (in dichloromethane). DFT studies on systems L1-3H2 involving solvents of different polarity (DMF versus n-hexane) indicated that the energy level gap increases with solvent polarity in line with the observed hypochromic shifts. Reaction of macrocycle L1H2 with three equivalents of ZnBr2, in the presence of Et3N, affords the complex [(ZnBr)(ZnNCMe)L1]2[ZnBr4]·2.5MeCN (9·2.5MeCN). In the case of L2H2, reaction with two equivalents of ZnBr2 affords [(ZnBr)L2H2][ZnBr3NCMe]·3MeCN (10·3MeCN), whilst in the presence of two equivalents of Et3N, work-up led to the isolation of the complex [(ZnBr)2L2]·4.5MeCN (11·4.5MeCN). The molecular structures of 9-11 are reported, together with their emission behaviour.
Chapter 4. In this chapter, the coordination chemistry of the macrocycles is extended to iron, cobalt and copper. Reaction of the [2+2] Schiff-base macrocycles {[2-(OH)-5- (R)-C6H2-1,3-(CH)2][CH2CH2(2-C6H4N)2]}2 (R = Me, LMeH2; tBu, LtBuH2) with FeBr2 afforded the complexes [FeBr(L1/L2H2)][X] (X = 0.5(FeBr3)2O, LMe·2MeCN, 12·2MeCN; LtBu, 13·0.5MeCN, X = Br, LtBu 14·3MeCN), respectively. Reaction of LtBuH2 with [KFe(OtBu)3(THF)] (formed in-situ from FeBr2 and KOtBu), following work-up, led to the isolation of the complex [Fe(LtBu)(LtBuH)]·3MeCN (15·3MeCN), whilst with [CuBr2] afforded [CuBr(LtBuH2)][Br3]·2MeCN (16·2MeCN). Attempts to form mixed Co/Ti species by reaction of [CoBrL2][CoBr3(NCMe)] with [TiCl4] resulted in [L2H4][CoBr4]·2MeCN (17·2MeCN). Use of the related oxy-bridged Schiff-base macrocycles {[2-(OH)-5-(R)-C6H2-1,3-(CH)2][O(2-C6H4N)2]}2 (R = Me, L1H2; tBu, L2H2) with CoBr2 led to the isolation of the complexes [(CoBr)2(L1)]·4C3H6O (18·4C3H6O), [Co(NCMe)2(L2H2)][CoBr4]·5MeCN (19·5MeCN), [Co(NCMe)6][CoBr3(MeCN)]2·2MeCN (20·2MeCN). For comparative structural/polymerization studies, the complexes {CoBr(NCMe)L4}2·2MeCN (21·2MeCN) and [Co(NCMe)2L4]2[CoBr3(NCMe)]2 (22), [FeBr(NCMe)L4]2·2MeCN (23·2MeCN) where L4H = 2,6-(CHO)2-4-tBu-C6H2OH, as well as the chelate-free salt [Fe(NCMe)6][FeBr3OFeBr3] (24) have been isolated and structurally characterized. The ability of these complexes to act as catalysts forthe ring opening polymerization (ROP) of ε-caprolactone (ε-CL) and δ- valerolactone (δ-VL) has been investigated, as well as the co-polymerization of γ- CL with rac-lactide (γ-LA) and vice-versa.
Chapter 5. This chapter discusses the formation of a large macrocycle and how its’ coordination chemistry with iron. The iron complex was evaluated for peroxidase- like catalytic activity. The [2+2] Schiff-base macrocycle {[2-(OH)-5-(CH3)-C6H2-1,3- (CH)2][O(2-C6H4N)2]}2 (L1H2) was reacted the with two equivalents of FeBr2 afforded either the salt complex [L1H2FeBr2]2[FeBr3OFeBr3]·7MeCN (25·7MeCN) or, in the presence of Et3N, [L1(FeBr)2]·3MeCN (26·3MeCN). The new larger [6+6] macrocycle {[2-(OH)-5-(CH3)-C6H2-1,3-(CH)2][O(2-C6H4N)2]}6 (L5H6) reacts with four equivalents of FeBr2 to afford [Fe2(L2H2)][FeBr3OFeBr3]·4MeCN (27·4MeCN). For the first time, we have evaluated such iron Schiff-base complexes for peroxidase-like catalytic activity. The electron transfer and the formation of hydroxyl radical were investigated in order to probe the mechanism of the peroxidase-like activity. Moreover, these Fe complexes were utilized for the determination of H2O2 using the colorimetric method. The linear range of H2O2 detection was 0.5-5 mM and 6-10 mM with a detection limit (LOD) of 0.05 mM. The method has good selectivity against interferents including glucose, urea, uric acid and metal ions.
Chapter 6 This chapter discuss the facile preparation, molecular structures and DFT studies of [2+2] [2+3] and [2+4] double layer macrocyclic Schiff-base compounds. The [2+3] compound has been used for the immobilization of Pd, and the morphology of the resulting composite was analyzed by PXRD and SEM. The Pd@Schiff base composite was employed as a peroxidase-like mimetic using 3,3′,5,5′-tetramethylbenzidine (TMB) as the substrate, and a colorimetric method for determining H2O2 was established.
Chapter 7 This chapter presents the experimental procedures.

Citation

Wang, K. (2021). Schiff-base macrocycles: coordination chemistry and potential applications. (Thesis). University of Hull. Retrieved from https://hull-repository.worktribe.com/output/4223398

• Thesis Type: Thesis
• Deposit Date: Sep 10, 2021
• Publicly Available Date: Feb 23, 2023
• Keywords: Chemistry
• Public URL: https://hull-repository.worktribe.com/output/4223398
• Additional Information: Department of Chemistry, The University of Hull
• Award Date: Jul 1, 2021