Analysis of copper 4(5)-imidazolecarboxaldehyde derivatives by ss-NMR, EPR, theoretical calculations and their advanced oxidation applications

Abstract

In this work, the synthesis and spectroscopic characterization of copper(II) complexes derived from 4(5)-imidazolecarboxaldehyde and its methyl derivatives are reported, along with their reactive oxygen species generation through hydrogen peroxide (H₂O₂) activation using DMPO for the EPR spin trapping experiments and the degradation of emerging pollutants (an azo-dye and an antibiotic). Three copper(II) complexes were synthesized: A₁ (C₁₀H₁₂Cl₂CuN₄O₂) using 4(5)-methyl-5(4)-imidazolecarboxaldehyde with copper(II) chloride (CuCl₂); A₂ (C₈H₈Cl₂CuN₄O₂) from 4(5)-imidazolecarboxaldehyde with CuCl₂; and A₄ (C₂₀H₂₄CuN₁₀O₁₀) from the methyl derivative with copper(II) nitrate. Additionally, a cobalt complex, A₃ (C₁₀H₁₂Cl₂CoN₄O₂) was synthesized for isostructural studies. The chemical structures of the metal complexes A₁, A₃ and A₄ were resolved using single-crystal X-ray crystallography, however it was not possible to isolate crystalline material for A₂. The coordination sphere of the metal ions using imidazolecarboxaldehyde molecules was described in terms of single-crystal X-ray crystallography parameters in combination with solid-state NMR, EPR and theoretical calculations. The A₁ copper complex was specifically used as a model crystalline system to predict the coordination mode of 4(5)-imidazolecarboxaldehyde when used as a ligand (A₂), particularly in cases in which non-single-crystal structures were isolated. Two-dimensional ss-NMR experiments, in which particular attention was paid to the mixing (¹H–¹H PSD) or contact times (¹H–¹³C HECTOR) were useful for the assignment of most of the NMR signals in the copper complexes. DFT calculations assisted to predict paramagnetic NMR shifts in good agreement. Only one of the evaluated complexes activated H₂O₂ and generated hydroxyl radicals (A₁). The aquatic toxicity of the copper complex that demonstrated the highest activity in decontaminant degradation was assessed through the brine shrimp toxicity test.