New Cu(II), Co(II) and Ni(II) azo-Schiff base complexes: Synthesis, characterization, catalytic oxidation of alkenes and DFT study

Abstract

Three new complexes with general formula of ML (M = Cu (1), Co (2), Ni (3)) containing an azo-Schiff base ligand (H2L) derived from 2,3-butanediamine and 4-(benzeneazo) salicylaldehyde were synthesized by template method. Characterization of the ligand and complexes were accomplished with FT-IR, UV–Vis, and 1H NMR. The catalytic activity of the complexes (1–3) were tested for the oxidation of various alkenes (cyclooctene, cyclohexene, styrene, α-methyl styrene, and norbornene) applying tert-butyl hydroperoxide (TBHP) as an oxidizing agent, and it was found that they were acceptable catalysts. Under the optimized reaction conditions, CuL complex displayed 94% conversion for the oxidation of cyclooctene, and CoL and NiL complexes exhibited 90 and 85% conversions for oxidizing α-methyl styrene, respectively. Based on our density functional computations, diffuse functions are compulsory in the basis set for geometry optimization of these systems. Therefore, the most stable structures and the vibrational frequencies were calculated at the M06-2X/6–311++G(d,p) level. By establishing the correlation between observed and calculated frequencies, the assignment of the vibrational modes was performed. Based on natural charge analysis (NAO), the back electron transfer from ML to the TBHP breaks the OO bond and facilitates the formation of tert-butoxyl radicals.