Synthesis, Structural Characterization, Redox, and Antibacterial Studies of 12‐Membered Tetraaza Macrocyclic Cu(II), Ni(II), Co(II), Zn(II), and VO(IV) Complexes Derived from 1,2‐(Diimino‐4′‐antipyrinyl)‐1,2‐diphenylethane and o‐Phenylenediamine

Abstract

A tetraaza macrocyclic Schiff base (L), synthesized from 1,2‐(diimino‐4′‐antipyrinyl)‐1,2‐diphenylethane and o‐phenylenediamine, acts as a tetradentate ligand and forms solid cationic complexes with Cu(II), Ni(II), Co(II) and VO(IV) salts in ethanol. All the synthesized compounds were characterized by microanalytical data, magnetic susceptibility measurements, IR, UV‐Vis., 1H NMR, 13C NMR, ESR and mass spectral techniques. The IR and UV‐Vis. spectral data and magnetic susceptibilities of the complexes suggest that the [CuL]Cl2, [NiL]Cl2, [CoL]Cl2 and [ZnL]Cl2 complexes exhibit square‐planar geometry whereas the [CuL(Y)2]Cl2 complexes show octahedral geometry, where Y = pyridine (py), imidazole (im) or triphenylphosphine (PPh3). The [VOL]SO4 complex has a square‐pyramidal geometry. The electrolytic behaviour of the chelates was assessed from their molar conductance data. The monomeric nature of the chelates was confirmed from their magnetic susceptibility values. Cyclic voltammogram of the [CuL]Cl2 complex in MeCN reveals that the unusual oxidation states such as Cu(III), Cu(I) and Cu(0) are stabilized by the ligand systems. The [CuL(py)2]Cl2 and [CuL(im)2]Cl2 complexes strongly enhance the copper(0) → copper(II) oxidation while the [CuL(PPh3)2]Cl2 complex completely suppresses the Cu(II)/Cu(III) couple and copper(0) → copper(II) oxidation. The cyclic voltammogram of the Ni(II), Co(II) and VO(IV) complexes were also recorded in MeCN solution and their salient features are reported. The ESR spectra of the copper and vanadyl complexes in DMSO solution at 300 and 77 K were recorded and their M.O. coefficients were calculated. The investigated compounds and uncomplexed metal salts were tested against bacteria like Staphylococcus aureus, Bacillus subtilis, Klebsiella pneumoniae, Salmonella typhi, Pseudomonas aeruginosa and Shigella flexneri. The metal salts have higher activity than the free ligand and the metal chelates.