Positron annihilation studies of bio-related N 2 S 2 -tetradentate ligands and their zinc complexes S.Y. Shaban a,b , K.R. Mahmoud c , T. Sharshar c,d,n a Chemistry Department, Faculty of Science, Kafrelsheikh University, Kafr El Sheikh, Egypt b Institut f¨ ur Anorganische Chemie der Unversit¨ at Erlanger-N¨ urnberg, Egerlander Strasse 1, D-91058 Erlanger, Germany c Physics Department, Faculty of Science, Kafrelsheikh University, Kafr El Sheikh, Egypt d Physics Department, Faculty of Science, Taif University, Taif, 888 Hawiya, Saudi Arabia article info Article history: Received 2 November 2010 Accepted 2 September 2012 Available online 10 September 2012 Keywords: Amine-thiolate ligands Zinc thiolate complexes Positron annihilation lifetime spectroscopy abstract In this study, a series of three N 2 S 2 -tetradentate ligands and their zinc complexes were investigated by positron annihilation lifetime spectroscopy. The measurements were performed at room temperature. The analysis of lifetime spectra of all samples yielded four lifetime components, except in one sample that yielded three components. The results showed that the formation probability and lifetime of ortho- positronium in this series are dependent on the structure. & 2012 Elsevier Ltd. All rights reserved. 1. Introduction Zinc complexes with sulfur amine-ligand cores are of increasing interest as structural and spectroscopic models for metal binding sites in a number of metallothioneins and metalloregulatory pro- teins (Dance et al., 1990; Bochmann et al., 1991; Santos et al., 1991; Zhang et al., 1991, 2003; Mann and Ozin, 1996; Shaban et al., 2007). Ligands containing NS moieties are more frequently used in these model compound studies (Nekola and Rehder, 2002; Shaban et al., 2011; Sellmann et al., 2001, 2003b, 1995; Sellmann and Sutter, 2003). The pentadentate, dianionic ligand 2,6-bis(2 0 -mercaptophe- nylthio)dimethylpyridine, NS 4 2 , which contains two thioether (S) and two thiophenolate (S  ) functions, reacts with [V 2 (m-Cl) 3 (thf) 6 ] 2 [ZnCl 6 ] to form the zinc complex [Zn(NS 4 )] CH 2 Cl 2 . The relevance of this compound as a model for Zn–S and Zn  S  binding in biomolecules is addressed (Nekola and Rehder, 2002). Sellmann et al. have found that the N 2 H 2 S 2  H 2 and N 2 Me 2 S 2  H 2 ligands can bind dinitrogen for the first time and act as models for nitrogenases (Sellmann et al., 2001, 2003b; Sellmann and Sutter, 2003). It was found that these ligands can stabilize high iron and ruthenium oxidation states (Sellmann et al., 1995). A recent work showed that N 2 H 2 S 2 –H 2 and N 2 Me 2 S 2 –H 2 ligand systems can control the coordination number and geometry as well as the availability and functionality of labile coordination sites in zinc complexes (Shaban et al., 2011). It is of interest to use other techniques such as positron annihilation lifetime (PAL) spectroscopy to characterize this type of sulfur amine-ligand as well as their zinc complexes. Although, a PAL is one of characterizing techniques, the PAL measurements of metal complexes are still very scarce especially for zinc com- plexes containing N 2 S 2 -ligands. In PAL spectroscopy, the energetic positrons are implanted in molecular solids and their probability of their formation (I) and lifetimes (t) are measured. These parameters provide information about the physical and chemical properties of such solids (Schrader and Jean, 1988). Positrons can form positronium (Ps) in molecular solids (Schrader and Jean, 1988). Two Ps varieties can be formed: p  Ps (singlet) and o-Ps (triplet), depending on the relative spins of the positron and the electron. Therefore, if Ps formation occurs, the lifetime spectra consist of three or more exponential components standing for p-Ps (t 1 ¼ 120 ps), free and bound positrons (t 2 ¼ 150–500 ps) and o-Ps (t 3 ,etc. 4900 ps), with the associated formation probabilities, I 1 , I 2 , I 3 , etc., respectively. Normally one work with the parameter of the longer lived variety, o-Ps (see section 2.2). The parameters of PAL measurements were correlated with chemical structure of transition metal compounds by Kajcsos et al. (1982). They found very complex relationships between the annihilation parameters and the nature of ligands and geometries of the complexes. They found evidences for the formation of o-Ps in zinc complexes [Zn{S 2 CN(CH 2 C 6 H 5 )} 2 ] and [Zn(pyridine) 2 ]Cl 2 . Several studies showed that a careful choice of transition metals and ligands could lead to complexes where the formation of Ps could be observed (Marques-Netto et al., 1985). The main aim of this study is, therefore, to apply the PAL technique to investigate the electronic structure of N 2 S 2 Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/radphyschem Radiation Physics and Chemistry 0969-806X/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.radphyschem.2012.09.001 n Corresponding author at: Physics Department, Faculty of Science, Taif University, Taif, 888 Hawiya, Saudi Arabia. Tel.: þ966 2 7274299; fax: þ966 2 7255529. E-mail address: tahersharshar@hotmail.com (T. Sharshar). Radiation Physics and Chemistry 82 (2013) 12–15