Synthesis of the nickel(II) complexes bearing tetradentate thiosemicarbazone through Michael addition of n-alcohols. Experimental, theoretical characterization and antioxidant properties

New thiosemicarbazone derivative, 3-benzylidene-2,4-pentanedione-S-methyl-thiosemicarbazone hydrogen iodide (TSC), was synthesized and characterized by elemental analysis, IR, 1 H NMR and single crystal X-ray diffraction. The novel nickel(II) complexes, Ni1-4, were synthesized from the TSC and salicylaldehyde by template effect of nickel(II) ion. The reactions were accomplished by a proton provided from the addition of alcohols which are methyl, ethyl, propyl and allyl for complexes Ni1, 2, 3 and 4, respectively. Spectroscopic data indicated that the formation of the complexes occurred through Michael addition of the alcohols to the 2,4-pentanedione moiety of the TSC. Distorted square planar structures of complexes Ni1 and Ni2 were confirmed by single crystal X-ray diffraction. In addition, the detailed computations were performed by using the theory DFT for experimentally obtained structures of the TSC, Ni1 and Ni2. The antioxidant property of the TSC and nickel(II) complexes along with the standard ascorbic acid was studied by using DPPH radical scavenging and Cupric Reducing Antioxidant Capacity (CUPRAC) assays.