Syntheses, Structures, and Antitumor Activities of Some Transition Metal Complexes With a Novel Aroylbis( N , N ‐diethylthioureas) Derived From Vanillic Acid

A novel vanillic acid‐derived aroylthiourea ligand, 4‐acetyl‐3‐methoxybenzoyl‐ N , N ‐diethylthiourea ( HL ), was synthesized and fully characterized by elemental analysis, spectroscopic methods, and single‐crystal X‐ray diffraction. Reactions of HL with Ni(OAc) 2 ∙ 4H 2 O, CuCl 2 ∙ 2H 2 O, and (NH 4 ) 2 [PdCl 4 ] afforded the homoleptic bis ‐complexes [M(L) 2 ] (M = Ni(II) ( 1 ), Cu(II) ( 2 ) and Pd(II) ( 3 )), while reaction with Co(OAc) 2 ∙ 4H 2 O led to the Co(III) tris ‐complex [Co(L) 3 ] ( 4 ) via in situ oxidation. Heteroleptic complexes [Pd(L)(PPh 3 ) 2 ](PF 6 ) ( 5 ) and [Cu(L)(PPh 3 ) 2 ] ( 6 ) were prepared from phosphine‐containing precursors. Structural analyses revealed cis square–planar geometries for 1 , 2 , and 3 ; a facial octahedral arrangement for 4 , a square–planar configuration for 5 , and a tetrahedral geometry for 6 . In all complexes, the deprotonated ligand coordinates as a bidentate ( S , O )‐chelator. Antiproliferative evaluation against MCF‐7 and HepG2 cancer cell lines showed negligible activity for HL , 1 , and 4 , whereas 5 and 6 displayed activities comparable to cisplatin. Notably, complexes 2 and 3 exhibited significantly enhanced antiproliferative effects, with IC 50 values of 1.07 ± 0.06 and 3.57 ± 0.20 µM (MCF‐7), and 1.18 ± 0.06 and 3.17 ± 0.15 µM (HepG2), respectively. These results demonstrate the strong influence of metal coordination on biological activity and identify complexes 2 and 3 as promising candidates for further study.