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

Abstract

In this work, syntheses of Schiff-base ligand, N-2-(6-methylpyridyl)-2-hydroxy-1-naphthaldimine (1) and its hitherto unknown chelate with a Ni(II) salt, bis[N-2-(6-methylpyridyl)-2-oxo-1-naphthaldimnato-k3N^N^O] nickel(II) (2) have been reported and characterized by spectral techniques (IR, 1H NMR, Mass). Solid state structures and non-covalent interactions persisting in 1 and 2 are studied by Density Functional Theory (DFT) optimizations and Hirshfeld Surface (HS) analysis. X-ray diffraction (XRD) study shows zwitter ionic keto-amine tautomer form of planar Schiff base 1 that exists as dimer formed by C13 ̶ H13A···O1i [(i) -x+1, -y, -z+1] hydrogen bonds and distorted octahedral geometry around Ni+2 center in chelate 2, where overall crystal structure stability may be attributed to weak C ̶ H···π, π···π stacking, van der Waals interactions, and C ̶ H···O type intermolecular hydrogen bonds. The HS study and 2D Finger Print (FP) plots corroborate well with XRD data and show prominent O···H/H···O spikes (2.2 Å < de+di < 2.3 Å) and C···H/H···C (2.5 Å < de+di < 2.6 Å) spikes that arise from C ̶ H···O type H-bonds and C ̶ H···π interactions respectively, along with significant C···C interactions (de+di ∼ 3.3 Å) due to π···π stacking (2). The anticancer activity has been investigated by using cytotoxicity measure (MTT assay), apoptosis assay, quantitative polymerase chain reaction (qPCR), and colony formation assays. The Ni(II) metal complex demonstrates dose-dependent cytotoxicity in vitro, killing A549 lung cancer cells via an apoptotic pathway.