Versatile coordination behavior of N,N-di(alkyl/aryl)-N 0 -benzoylthiourea ligands: Synthesis, crystal structure and cytotoxicity of palladium(II) complexes N. Selvakumaran a , Seik Weng Ng b , Edward R.T. Tiekink b , R. Karvembu a,⇑ a Department of Chemistry, National Institute of Technology, Tiruchirappalli 620015, India b Department of Chemistry, University of Malaya, 50603 Kuala Lumpur, Malaysia article info Article history: Received 8 March 2011 Received in revised form 10 June 2011 Accepted 16 June 2011 Available online 23 June 2011 Keywords: Palladium(II) Thiourea Synthesis X-ray structures Anti-cancer activity abstract The synthesis and characterization of Pd(II) complexes with the general formula cis-[Pd(L-O,S) 2 ] (HL = N,N- diethyl-N 0 -benzoylthiourea, N,N-diisobutyl-N 0 -benzoylthiourea or N,N-dibenzyl-N 0 -benzoylthiourea) and trans-[PdCl 2 (HL-S) 2 ] (HL = N,N-diphenyl-N 0 -benzoylthiourea, N,N-di-n-butyl-N 0 -benzoylthiourea or N,N- diisopropyl-N 0 -benzoylthiourea) are reported. These complexes were formed from the reaction between PdCl 2 and N,N-di(alky/aryl)-N 0 -benzoylthiourea in acetonitrile with the formulation dependent on the nature of HL. The new Pd(II) complexes have been characterized by analytical and spectral (FT-IR, UV–Vis, 1 H NMR and 13 C NMR, Mass) techniques. The molecular structures of two of the complexes (1 and 5) have been conformed by X-ray crystallography. Complex 1 shows cytotoxicity against human breast cancer cells. Ó 2011 Elsevier B.V. All rights reserved. 1. Introduction We are currently interested in the coordination chemistry of N,N- di(alkyl/aryl)-N 0 -benzoylthiourea ligands (Fig. 1) in view of their interesting and versatile coordination behavior towards transition metals [1,2]. Three different coordination modes have been found so far for this type of ligand in their mononuclear transition metal complexes. They are monobasic bidentate (O,S) [3], neutral mono- dentate (S) [4] and neutral bidentate (O,N) [5]. Among these, mono- basic bidentate (O,S) coordination is very common and leads to stable Pd(II) [6,7], Cu(II) [8], Pt(II) [9,10], Co(III) [10], Rh(III) [11], Ni(II) [10,12] and Co(II) [12] complexes. On the other hand, coordi- nation through S only is rare and observed in few transition metal complexes. When present, the monodentate coordination was ex- plained on the basis of intramolecular hydrogen bond formation be- tween the carbonyl O atom and the thiourea NH group (Fig. 2) [13]. This is true in their Pt(II) [14], Pd(II) [13], Cu(I) [15] and Cd(II) [16] complexes. There are very few reports in which N,N-di(alkyl/aryl)- N 0 -benzoylthiourea ligands that is without the thiourea NH moiety, where coordinated to the metal is through S only [17,18]. The reac- tion between Pd(II) salts and N,N-di(alkyl/aryl)-N 0 -benzoylthiourea ligands (HL) have so far yielded only complexes of the type cis- [Pd(L-O,S) 2 ] [6–8]. Interestingly, we were able to obtain Pd(II) complexes of the type cis-[Pd(L-O,S) 2 ] or trans-[Pd(HL-S) 2 ] when reaction was carried out between PdCl 2 and N,N-di(alkyl/aryl)-N 0 - benzoylthiourea ligand. N,N-di(ethyl/isobutyl/benzyl)-N 0 -ben- zoylthiourea ligands gave cis-[Pd(L-O,S) 2 ] and N,N-di(phenyl/n-bu- tyl/isopropyl)-N 0 -benzoylthiourea ligands formed trans-[PdCl 2 (HL- S) 2 ] on reaction with PdCl 2 in acetonitrile. It was already realized that N-substituents can influence the coordination mode of ligand; for example, coordination of dimethyl substituted ferrocene car- bonyl thiourea (HL 0 ) to Cu(II) ion differed substantially from that of the corresponding diethyl substituted analog (HL 00 ). The former yielded a bis-sulfur bridged Cu(II) dimer, [{CuL 0 (HL 0 )Cl} 2 ] [17] while the latter only coordinated in a bidentate manner to yield a single mononuclear Cu(II) complex [CuL 00 2 ] [19]. N,N-Di(alkyl/aryl)-N 0 -benzoylthiourea ligands were effectively utilized for liquid–liquid extraction of the platinum group metals from hydrochloric acid solutions [20–23]. The determination of traces of the platinum group metals by means high performance thin layer chromatography (HPTLC) after complexation with suit- ably modified fluorescent N-aroylthiourea has also been of interest [24–27]. Recently N,N-diethyl-N 0 -benzoylthiourea has been used for the selective online preconcentration and highly selective trace determination of Pd(II) by means of graphite furnace atomic absorption spectroscopy [28]. Transition metal complexes with thiourea derivatives are also known to exhibit a wide range of biological activities such as antivi- ral, antibacterial, antifungal, anticancer, antitubercular, antithyroi- dal, insecticidal, antimalarial, etc. [29–33]. N-Substituents not only influenced the coordination behavior of ligands but also the chemi- cal and physical properties of its complexes. Indeed cytotoxicity 0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2011.06.031 ⇑ Corresponding author. Tel.: +91 4312503636; fax: +91 431 2500133. E-mail address: kar@nitt.edu (R. Karvembu). Inorganica Chimica Acta 376 (2011) 278–284 Contents lists available at ScienceDirect Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica