Nickel(II) complexes with a tetraazamacrocyclic diene ligand and its isomeric 'ane' forms: Synthesis, characterization, and antibacterial activities

Applications of macrocyclic compounds and their various metal complexes in biological fields have notably increased. Therefore, it is reasonable to synthesize novel macrocyclic compounds and assess their biological properties. In this research, several new Ni(II) complexes were prepared using hexamethyltetraaza macrocycles, and their biological activity was studied. Specifically, a fourteen-membered tetraaza macrocyclic diene ligand in its dihydroperchlorate form, Me₆[14]diene⋅2HClO₄ (L⋅2HClO₄), along with its isomers tet-a and tet-b in their saturated ‘ane’ forms, were employed for complexation with Ni(II) acetate, followed by the addition of NaClO₄. The process resulted in the formation of square planar [Ni[(L´)](ClO₄)₂ (L´ = L, tet-a, or tet-b) and octahedral [Ni(tet-b)(ClO₄)₂] complexes. Furthermore, axial addition reactions involving SCN^–, Cl^–, Br^–, and I^– with [Ni[(L´)](ClO₄)₂ led to the formation of six-coordinated octahedral complexes, including [Ni(L)(NCS)₂], [Ni(tet-a)(NCS)₂], [Ni(tet-b)(NCS)₂] [Ni(tet-b)Cl₂], [Ni(L)Br₂], [Ni(tet-b)Br₂] and [Ni(tet-b)I₂]. Additionally, axial substitution reaction on the complex [Ni(tet-b)(ClO₄)₂] with SCN^– produced a five-coordinated square pyramidal complex, [Ni(tet-b)(NCS)](ClO₄). Modern analytical and spectroscopic techniques were used to characterize these complexes. It was observed that these complexes exhibited different geometries and electrolytic properties in different organic solvents where the Ni(II) showed different coordination modes. These complexes also displayed moderate antimicrobial activity against various bacteria.