Neutral NiII, PdII and PtII ONS-pincer complexes of 5-acetylbarbituric-4N-dimethylthiosemicarbazone: synthesis, characterization and properties

Abstract

Octahedral 1 : 1 Ni^II and square-planar 1 : 1 Pd^II and Pt^II complexes of formulae [Ni(HAcb4DM)(AcO)(H₂O)₂]·H₂O (1), [Pd(HAcb4DM)Cl]·5H₂O (2) and [Pt(HAcb4DM)Cl]·3H₂O (3), where H₂Acb4DM = 5-acetylbarbituric-4N-dimethylthiosemicarbazone (H₂ denoting its two dissociable protons, one enolic and one thiolic), have been synthesized and characterized by elemental analysis and by ¹H and ¹³C NMR, UV-vis, and IR spectroscopy. Crystallisation of compounds 1–3 from DMSO afforded complexes of formulae [Ni(HAcb4DM)₂]·2H₂O (1a), [Pd(Acb4DM)(DMSO)]·DMSO (2a) and [Pt(Acb4DM)(DMSO)]·DMSO (3a), the molecular and crystal structures of which were determined by X-ray diffractometry. The thiosemicarbazone in 1a coordinates to the metal ions in an ONS-tridentate manner in the O-enolate/S-thione form, but in complexes 2a and 3a the thiosemicarbazone binds Pd^II or Pt^II as an ONS-pincer ligand in the O-enolate/S-thiolate form. The ¹⁹⁵Pt NMR spectrum of 3 shows a signal at −2950 ppm along with two new signals at −3348 and −2731 ppm, indicating the presence of solvolysis products. The catalytic activity of complex 2a has been explored in aryl–aryl Suzuki cross-coupling reactions. H₂Acb4DM and complexes 2 and 3 were screened for in vitro cytotoxicity against a human tumour cell line (HeLa-229), with the clinically employed drug cisplatin as a reference.