Thermally induced structural transformations of a series of palladium(II) complexes with N-heteroaromatic bidentate hydrazone ligands Nebojša Begovi  c a, b , Vladimir A. Blagojevi  c a , Sanja B. Ostoji  c b , Darko M. Mici  c b , Nenad Filipovi  c c , Katarina Andjelkovi  c d , Dragica M. Mini  c a, * a Faculty of Physical Chemistry, University of Belgrade, Serbia b Institute of General and Physical Chemistry, Belgrade, Serbia c Faculty of Agriculture, University of Belgrade, Serbia d Faculty of Chemistry, University of Belgrade, Serbia A R T I C L E I N F O Article history: Received 17 May 2014 Received in revised form 8 July 2014 Accepted 3 August 2014 Available online xxx Keywords: Palladium complex Thermal degradation DFT calculations Reaction mechanism Chelate ligand A B S T R A C T Thermal stability of a series of palladium(II) complexes with N-heteroaromatic bidentate hydrazone ligands was investigated using a combination of experimental measurements and DFT calculations. All complexes exhibit a reversible second-order transition around 333 K, which can be attributed to structural reorganization of the ligand molecules. Thermal degradation begins in 570–610 K temperature region, with an endothermic peak, followed by exothermic peaks in DSC. TG measurements show a well-defined mass loss corresponding to the initial degradation, while subsequent processes are poorly separated. DFT calculations suggest that the initial degradation step occurs with release of Cl, which then reacts with remaining part of the complex molecule in an exothermic process. This leads to decomposition of the ligand molecule into four fragments corresponding to ethyl chloride, carbon dioxide, methyl amine, and the fragment with the aromatic group. Mass spectrum suggests that creation of these fragments most likely corresponds to the initial degradation, after which some of these coordinate to Pd center, whose coordination sphere is left incomplete by release of Cl. TG measurement to 1123 K indicates that the final degradation product at this temperature is palladium. ã 2014 Elsevier B.V. All rights reserved. 1. Introduction Synthesis and properties of palladium(II) complexes has been a focus of some interest for some time [1–3]. They have been recognized as effective catalysts [4–7], as well as potential metal based anticancer drugs [8–10], since they exhibit the same geometry and electronic configuration as platinum. However, the ligand-exchange kinetics of platinum(II) and palladium(II) derivatives is quite different: hydrolysis of palladium(II) com- pounds is nearly 10 5 times faster, leading to very reactive species that are unable to reach pharmacological targets [11,12]. It was shown that stabilization of palladium(II) complexes by prevention of possible cis–trans isomerisation can be achieved by using chelate ligands in complex synthesis which possess similar or even better activity than platinum-approved drugs [13]. In light of this potential application, their thermal stability has been considered as an important issue [14–17], although, to the best of our knowledge, there has been no investigation to the impact of ligand peripheral substitution on thermal stability. Farran and House studied effect of the basicity of the ligands on the enthalpies and the activation energies of thermal decomposition of palladium(II) chloride complexes with pyridine and pyridine derivatives using TG and DSC [18]. During thermal degradation, complexes of type PdL 2 Cl 2 lost two ligands in a single step, resulting in PdCl 2 , with no relationship between the enthalpy values of decomposition and other properties of the complexes. With only a few exceptions, calculated E a values increased, while the procedural onset temperatures (Te) of decomposition decreased with increase in basicity of the ligands. This was due to trans influence of one ligand in the square planar complexes, causing the first ligand to be released easier than the second. Gomez-Vaamonde et al. investigated thermal stability of trans-bis (2-amino pyridine)dichloro palladium(II) complex, where palladi- um(II) is coordinated to two monodentate ligands [19]. The process * Corresponding author. E-mail address: dminic@ffh.bg.ac.rs (D.M. Mini c). http://dx.doi.org/10.1016/j.tca.2014.08.005 0040-6031/ ã 2014 Elsevier B.V. All rights reserved. Thermochimica Acta 592 (2014) 23–30 Contents lists available at ScienceDirect Thermochimica Acta journa l home page : www.e lsevier.com/loca te/tca