A new complex, [Ni(Tren)(Barb)(H2O)](Barb)(Barb-H)·2H2O (NiC) {Tren = tris(2-aminoethyl)amine and Barb = 5,5-diethlybarbiturate}, has been synthesized and characterized. The nickel(II) complex was grafted to g-C3N4, forming NiC@g-C3N4. The obtained NiC complex and its hybrid material NiC@g-C3N4 were characterized by FT-IR, TGA-DSC, UV–vis, SEM-EDX, and XRD. Single crystal X-ray crystallography shows that NiC exhibits NiN5O octahedral geometry. The results also show that the complex molecules are connected via N–H⋅⋅⋅⋅O hydrogen bonds. The complex NiC and NiC@g-C3N4 show oxidative stress effects on in vivo experiments. They also showed an increase in tissue damage markers that are represented by increase in tissue levels of malondialdehyde (MDA) in brain, liver, kidney and heart. Both materials showed inhibitory activity for examined antioxidants such as glutathione-S-transferase (GST), superoxide dismutase (SOD), glutathione (GSH) and glutathione reductase (GSH-R). The destructive effects were clearer for the NiC@g-C3N4 composite in liver, brain and kidney, respectively. The grafting of NiC complex in g-C3N4 is responsible for improving the catalytic activity of the compound, due to the synergistic effect of both components and facile recovery, as well recycling of the catalyst. The immobilization of the synthesized nickel(I) complex [Ni(Tren)(Barb)(H2O)](Barb)(Barb-H)•2H2O (NiC) to the graphitic carbon nitride (g-C3N4) as electron rich surface, forming the hybrid material NiC@g-C3N4, which provides both the benefits i.e improved catalytic activity owing to the synergistic effect of both components and facile recovery, recycling of the catalyst. [ABSTRACT FROM AUTHOR]