Syntheses and characterization of monobasic tridentate Cu(ii) Schiff-base complexes for efficient oxidation of 3,5-di-tert-butylcatechol and oxidative bromination of organic substrates

Abstract

Two monomeric copper complexes [Cu^II(sal-ppzH)Cl₂] (1) and [Cu^II(hyap-ppzH)Cl₂] (2) were synthesized by reacting CuCl₂·2H₂O with the monobasic tridentate Schiff-base ligands [Hsal-ppz] (I) and [Hyap-ppz] (II) (derived by reacting 1-(2-aminoethyl) piperazine with salicylaldehyde and 2-hydroxyacetophenone) respectively. Elemental analysis, IR, UV-Vis, ¹H NMR and ¹³C NMR data confirm the structures of the ligands and those of the complexes. Both complexes are monomeric in nature in the solid state and in solution as well. Single crystal XRD data suggest a distorted square pyramidal geometry for 1 crystallized in the P space group. DFT studies established a similar molecular structure for 2. The synthesized metal complexes [Cu^II(sal-ppzH)Cl₂] (1) and [Cu^II(hyap-ppzH)Cl₂] (2) successfully catalyzed the oxidation of 3,5-di-tert-butylcatechol (3,5-DTBC) in methanol in the presence of H₂O₂ with high K_cat values of 1.182 × 10⁴ mmol h⁻¹ and 2.880 × 10⁴ mmol h⁻¹, respectively. [Cu^II(sal-ppzH)Cl₂] (1) and [Cu^II(hyap-ppzH)Cl₂] (2) were also anchored into the polymeric matrix of chloromethylated polystyrene and were analyzed by TGA, atomic absorption spectroscopy (AAS), EPR, scanning electron microscopy (SEM) as well as energy dispersive X-ray (EDX) analysis. The polymer grafted metal complexes were used as catalyst precursors in the oxidative bromination of salicylaldehyde in the presence of H₂O₂, KBr and HClO₄. Under optimized reaction conditions, both catalysts show nearly quantitative oxidative bromination of salicylaldehyde, with the order of % products formed being 5-bromosalicylaldehyde > 3,5-dibromosalicylaldehyde > 3-bromosalicylaldehyde. Plausible reactive species involved in the catalytic cycle are identified by UV-Vis spectroscopy, pH metric titration, ESI-MS, EPR and DFT studies.