article · Journal of Molecular Structure
Researchers have developed a water-soluble Schiff base copper(II) complex designed to catalyse azide-alkyne cycloaddition reactions, widely known as click chemistry. Synthesised from 2-hydroxy-1-naphthaldehyde, 2-((2-aminoethyl)amino)ethanol, and copper sulphate, the crystalline complex was fully characterised using techniques including single-crystal X-ray diffraction. In a water-ethanol solvent mixture alongside sodium ascorbate, the catalyst facilitated the synthesis of 1,4-disubstituted 1,2,3-triazoles in high isolated yields. The catalytic system demonstrated operational simplicity and maintained its activity across five consecutive reuse cycles without significant loss of performance. Analytical and computational studies, including mass spectrometry and density functional theory, indicate that the complex acts as a precursor to an active copper(I) species in solution. The calculations confirmed a thermodynamically favourable reaction pathway with defined activation barriers for triazole formation.
Click chemistry is a foundational technique used to connect molecular building blocks efficiently, but many copper catalysts require toxic organic solvents or degrade rapidly. Demonstrating an effective, recyclable copper catalyst that works in an environmentally benign water-ethanol medium provides a cleaner, more practical method for assembling complex chemical architectures like triazoles without generating excessive chemical waste.
The complex serves as a recyclable catalyst for chemical synthesis, which could benefit synthetic chemists and manufacturers seeking greener, water-compatible click chemistry processes. Because the work is limited to laboratory-scale synthesis, characterisation, and reaction modelling, the technology is at an early research stage and would require further scale-up testing before industrial adoption in fine chemicals or pharmaceutical manufacturing.
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The preparation of a Schiff base-Cu(II) complex designed as a water-soluble copper-catalyzed azide–alkyne cycloaddition (CuAAC) catalyst to form new 1,4-disubstituted 1,2,3-triazoles is described. The Schiff base itself was formed in situ from 2-hydroxy-1-naphthaldehyde and 2-((2-aminoethyl)amino)ethanol which was then treated with copper sulphate. The crystalline product formed was characterized by FT-IR, EDX, SEM, and single-crystal X-ray diffraction. The catalytic performance of the complex with sodium ascorbate in a homogeneous water–ethanol mixed solvent system was evaluated in several CuAAC or “click” reactions with representative aldehydes. The 1,2,3-triazoles were formed in high isolated yields and the catalyst could be recovered and reused for up to five consecutive cycles without significant deterioration of its catalytic activity. The operational simplicity highlights the efficiency of this catalytic system. It is hypothesized that the crystalline product whose X-ray structure was determined, is a precursor to the actual Schiff base-Cu(I) catalyst which is formed in the aqueous medium. ESI-MS and DFT analysis support the hypothesis envisioned. DFT calculations were performed for the formation of 5a , selected as a convenient representative model for all of the triazole products produced, since they only differ in their aryl ring substituents. The optimized transition state for 5a was confirmed by the presence of a single imaginary frequency, and IRC analysis connected the reactant- and product-side regions. Frequency analysis confirmed the product-side structure as a true minimum. Relative to the reactant-side minimum, the calculated electronic and Gibbs activation barriers were 70.36 and 82.17 kJ mol⁻¹, respectively. The product-side minimum was substantially lower in energy than the reactants, with ΔE = -314.01 kJ mol⁻¹ and ΔG = -279.09 kJ mol⁻¹, indicating the thermodynamically favorable formation of 5a . Together, ESI-TOF mass spectrometry and DFT calculations support the proposed Cu(I)-mediated formation of 1,2,3-triazoles.
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DOI: 10.1016/j.molstruc.2026.147466
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