Synthesis, structural characterization, DFT calculations and biological properties of mono- and dinuclear nickel complexes with tetradentate transformed ligands by aerobic oxidative-coupling reactions

Reaction of nickel chloride with acetylacetone bis(salicylhydrazone) (L) (prepared in situ by condensation of acetylacetone with salicylhydrazide) in methanol results in the formation of air-sensitive nickel (II) complex of symmetrical tetradentate Schiff base [Ni-II(L)] (1). Air oxidation of 1 leads to the formation of dinuclear species [Ni-II(L-L)Ni-II] (2) where the ligand (L-L)(4-) is formed by oxidative C-C coupling at the central methylene group of the acetylacetone fragment of (L)(2-). The same reaction in methanol/pyridine solution leads to oxidative C-N coupling forming a mesoionic [Ni-II(L-Py)](3). The use of Nickel acetylacetonate in methanol/pyridine or DMSO solution under aerobic conditions provide [Ni-II(L-O)(py)(2)](4), [Ni-II(L-O)(DMSO)(2)](5). In this case, the ligand (L)(2-) is partially oxidized to form (L-O) which is formed by oxidation of the central methylene group. The molecular structure of 1-5 have been determined by X-ray crystallography. The first three complexes 1, 2 and 3 have a square-planar geometry, while, 4 and 5 adopt octahedral geometry. In vitro antioxidant activities of compounds 1, 4 and 5 were evaluated against DPPH radical and hydrogen peroxide and were compared with standard antioxidant, ascorbic acid. Our results reveal that the three compounds exhibit excellent radical scavenging activities. DFT calculations have been performed on complexes 1-5 using the BP86 and B3LYP functionals to provide a complete rationalization of their structures and to describe their electronic structures elucidating the fundamental spin state and the Metal-Ligand interaction type.