Syntheses, crystal structures, theoretical studies, and anticancer properties of an unsymmetrical schiff base ligand N-2-(6-methylpyridyl)-2-hydroxy-1-naphthaldimine and its Ni(II) complex

•The Schiff-base ligand, N-2-(6-methylpyridyl)-2-hydroxy-1-naphthaldimine (1) and its hitherto unknown chelate from Ni(II) salt, bis[N-2-(6-methylpyridyl)-2-oxo-1-naphthaldimnato-k3N^N^O] nickel(II) (2) have been reported.•X-ray diffraction study shows zwitter ionic keto-amine tautomer form of planar Schiff base 1 that crystallizes in orthorhombic crystal system (space group Pbca) and exists as stacked column of dimeric segments in 3D formed by C13 ̶ H13A···O1i hydrogen bonds.•While the chelate 2 crystallizes in monoclinic crystal system, (space group I2/a) and the NiN4O2 coordination kernel adopts distorted octahedral geometry around Ni+2 center.•The anticancer activity of 1 and 2 has been investigated using cytotoxicity measure (MTT assay), apoptosis assay, quantitative polymerase chain reaction (qPCR), and colony formation assays.•The Ni(II) metal complex 2, demonstrated dose-dependent cytotoxicity in vitro, killing A549 lung cancer cells via an apoptotic pathway.