Flubendazole Pd(II) complexes: structural studies, cytotoxicity, and quantum chemical calculations

[Pd(FLU)₂X₂]·yH₂O·zCH₃OH (FLU = flubendazole; X = Cl (1), y = 0, z = 0; X = Br (2), NO₃ (3), y = 2, z = 0; X = SCN (4), y = 2, z = 3) were synthesized as potential anticancer complexes, and their structures were elucidated using elemental analysis, TG/DTA, IR, ¹H NMR, UV–vis., and conductivity measurements. FLU interacts with Pd(II) ions as a neutral unidentate ligand via the pyridine-type nitrogen of the benzimidazole ring. Geometry optimization, molecular electrostatic potential maps and natural bond orbital analysis were performed by DFT/B3LYP method. FLU, in comparison to its complexes, was screened for its antibacterial and cytotoxic activity. Complexes 1–4 possess strong anticancer activity with IC₅₀ values (4.13–3.68 μg ml⁻¹) compared with 3.57 μg ml⁻¹ reported for cis-platin. The cytotoxicity was shown to be affected by the nature of the anion, where the sequence is 3 > 2 > 4 > 1 in case of MCF7 cell line. Structural-activity relationships suggested that E_HOMO, energy gap and dipole moment were the most significant descriptors for the correlation with the antitumor activity.