Dalton Transactions Dynamic Article Links Cite this: Dalton Trans., 2012, 41, 658 www.rsc.org/dalton PAPER Crystal structure, electronic properties and cytotoxic activity of palladium chloride complexes with monosubstituted pyridines† Agnieszka Krogul,* a Jakub Cedrowski, a Katarzyna Wiktorska, b Wojciech P. Ozimi´ nski, b,c Jadwiga Skupi´ nska a and Grzegorz Litwinienko* a Received 26th July 2011, Accepted 28th September 2011 DOI: 10.1039/c1dt11412c Palladium(II) complexes attract great attention due to their remarkable catalytic and biological activity. In the present study X-ray characterization, UV-Vis and Time-Dependent Density Functional Theory (TD-DFT) calculations for six PdCl 2 (XPy) 2 complexes (where: Py = pyridine; X = H, CH 3 or Cl) were applied in order to investigate substituent effects on their crystal structures and electronic properties and to combine the results with their catalytic and cytotoxic activity. The structures of complexes PdCl 2 (3-MePy) 2 , PdCl 2 (4-MePy) 2 and PdCl 2 (2-ClPy) 2 , have been described for the first time and we compared our results with available data for the whole series of six complexes. All compounds exhibit a square planar coordination geometry in which the palladium ion coordinates two nitrogen atoms of pyridine ligands and two chlorine atoms in trans positions. For complexes with ortho substituted XPy ligands a cis disposition of substituents takes place, whereas for other ligands: 3-MePy and 3-ClPy – the substituents are in trans positions. For XPy the energies of p–p* and n–p* transitions depend on the position and nature of the X substituent in the XPy ring. After complex formation a hipsochromic shift (24–34 nm) of p–p* and a bathochromic shift of n–p* bands are observed. The UV-Vis spectra of PdCl 2 (XPy) 2 confirm that square planar coordination geometry of complexes I–VI and two dp–p* transitions are expected. With the help of the TD-DFT calculations we proved that dp–p* transitions in solutions of PdCl 2 (XPy) 2 complexes result from MLCT (metal-to-ligand charge transfer) with contribution from chlorine atoms to palladium. We also studied substituent effects on cytotoxic properties of Pd(II) complexes against the human breast cancer cell line MCF7, the human prostate cancer cell line PC3, and the human T-cell lymphoblast-like cell line CCRF. The studied complexes were the most active against the CCRF cell line and less or even no cytotoxic effect was observed for PC3 cells. Complexes with MePy ligands showed increased cytotoxic activity compared to unsubstituted pyridine ligands. Introduction Complexes of palladium are considered as promising novel materi- als of wide applicability. Palladium square planar complexes with pyridine ligands are relatively ease to prepare and have remarkable catalytic activity, 1–6 thus, complexes of palladium(II) with amines a University of Warsaw, Pasteura 1, 02-093, Warsaw, Poland. E-mail: litwin@ chem.uw.edu.pl, akrogul@chem.uw.edu.pl; Fax: (48) 22 8222380; Tel: (48) 22 8220211 ext. 378 b National Medicines Institute, Chelmska 30/34, 00-725, Warsaw, Poland c Institute of Nuclear Chemistry and Technology, Dorodna 16, 03-195, Warsaw, Poland † Electronic supplementary information (ESI) available: Elemental analy- sis of complexes I, II and VI. Structures of complexes III–V. Selected bond distances and angles for complexes III–V. UV-Vis spectra of free XPy ligands and PdCl 2 (XPy) 2 complexes. Frontier Orbitals involved in dp–p* transitions in complexes I and VI. Visualisation of dp–p* transitions in complexes I and VI. Theoretical UV-Vis spectra of complexes I, III and VI. CCDC reference numbers 837204–837206. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c1dt11412c are efficient catalysts for industrially important processes, i.e. carbonylations, 2,3 polymerizations 4 and other organic reactions like, for example, the Heck reaction, Suzuki–Miyaura cross- coupling, Sonogashira coupling, Negishi coupling, Stille cross coupling, etc. 5,6 Moreover, palladium(II) complexes are supposed to be potential anticancer agents and this expectation flows from a close resemblance of Pd complexes to Pt complexes. 7 The most recognized anticancer agent is cis-diamminedichloroplatinum(II), a compound colloquially named cisplatin. Unfortunately, thera- peutic application of cisplatin has been limited by its serious side effects like, for example, nephrotoxicity, ototoxicity, neurotoxicity, allergy. An additional reason which has limited the applicability of Pt complexes is that some kinds of tumor cells (i.e. ovarian or small cell lung cancers) have shown a resistance after initial treatment with cisplatin. 8,9 This kind of chemoresistance has been also observed by Reedijk et al. 10 for other Pt complexes (being simple analogs of cisplatin). Recently, some attention has been focused on compounds with bulky planar ligands which are believed to be kinetically and thermodynamically more stable than simple 658 | Dalton Trans., 2012, 41, 658–666 This journal is © The Royal Society of Chemistry 2012 Downloaded by University of Warsaw on 13 December 2011 Published on 08 November 2011 on http://pubs.rsc.org | doi:10.1039/C1DT11412C View Online / Journal Homepage / Table of Contents for this issue