Design and Synthesis of a Water-Soluble Schiff Base Copper Complex and its Crystal Structure as an Efficient Catalyst for “Click”-Enabled Construction of 1,4-Disubstituted 1,2,3-Triazoles and a DFT study

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.