Syntheses and X-ray structural studies of the novel complexes [Ni(en) 2 (3-pyt) 2 ] and [Cu(en) 2 ](3-pyt) 2 based on 5-(3-pyridyl)-1,3,4-oxadiazole-2-thione Mamata Singh a , R.J. Butcher b , N.K. Singh a, * a Department of Chemistry, Banaras Hindu University, Varanasi 221 005, India b Department of Chemistry, Howard University, 525 College Street NW, Washington, DC 20059, USA article info Article history: Received 6 June 2008 Accepted 26 June 2008 Available online 4 August 2008 Keywords: 5-(3-Pyridyl)-1,3,4-oxadiazole-2-thione Mixed-ligand complexes Ethylenediamine complexes Supramolecular architecture X-ray abstract Two novel mononuclear mixed-ligand complexes [Ni(en) 2 (3-pyt) 2 ](1) and [Cu(en) 2 ](3-pyt) 2 (2), derived from potassium [N 0 -(pyridine-3-carbonyl)-hydrazinecarbodithioate [K + (H 2 L)  ] and containing en as a co- ligand, have been synthesized. The [K + (H 2 L)  ] undergoes cyclization in the presence of ethylenediamine (en) and is converted to 5-(3-pyridyl)-1,3,4-oxadiazole-2-thione (3-pyt)  . [Ni(en) 2 (3-pyt) 2 ] and [Cu(en) 2 ](3-pyt) 2 have been characterized with the aid of elemental analyses, IR, UV–Vis, magnetic sus- ceptibility and single crystal X-ray studies. The complexes 1 and 2 crystallize in the orthorhombic and monoclinic systems with space groups Pca2(1) and C2/c, respectively. The single crystal X-ray diffraction studies of both complexes indicate that (3-pyt)  adopts a thione form in 1 but is present as a thiolato form in 2. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Several methods have been reported in the literature for the synthesis of 1,3,4-oxadiazoles [1–3]. Oxadiazole derivatives are an important group of heterocyclic compounds [4,5] due to which their chemistry and uses have been highlighted in numerous re- ports [6–8]. They have been used in a wide variety of biological applications [9]. The heterocyclic thiones represent an important type of compound in the field of coordination chemistry because of their potential multifunctional donor sites, viz either exocyclic sulfur or endocyclic nitrogen [10]. The cyclization of 3-acyldithioc- arbazate esters, N-aroyldithiocarbazates and their salts to 1,3,4- oxadiazoles in the presence of a base are reported in the literature [11,12]. Earlier we have reported the metal assisted cyclization of N-benzoyldithiocarbazates to 5-phenyl-1,3,4-oxadiazole-2-thiol in the presence of excess en [13]. This has prompted us to synthe- size the complexes of 1,3,4-oxadiazole-2-thiones in order to ascer- tain the cyclization of other dithiocarbazates. Since the pyridyl oxadiazoles are promising bridging ligands for the synthesis of coordination polymers [14], several authors have already used cyc- lized pyridyl 1,3,4-oxadiazoles for the synthesis of metal com- plexes [14–16]. There are a few reports on the metal complexes of 5-phenyl-1,3,4-oxadiazole-2-thione [13,17], 5-(4-pyridyl)- 1,3,4-oxadiazole-2-thione [14,15] and 5-(2-pyridyl)-1,3,4-oxadiaz- ole-2-thione [16]. To the best of our knowledge we are reporting for the first time metal complexes based on 5-(3-pyridyl)-1,3,4- oxadiazole-2-thione and their crystallographic studies. 2. Experimental 2.1. Materials and methods Commercial reagents were used without further purification and all experiments were carried out in the open atmosphere. Nic- otinamide (Sigma Aldrich), CS 2 (SD Fine Chemicals, India) and KOH (Qualigens) were used as received. All the solvents were purchased from Merck Chemicals, India, and used after purification. Nicotinic acid hydrazide [18] and [Ni(en) 2 (NCS) 2 ] were prepared from the reported methods [19]. 2.2. Synthesis of potassium [N 0 -(pyridine-3-carbonyl)- hydrazinecarbodithioate [K + (H 2 L)  ] This compound was synthesized by the reaction of CS 2 (4 ml, 53 mmol) with a solution of nicotinic acid hydrazide (7 g, 53 mmol) in MeOH (35 ml) in the presence of KOH (2.86 g, 51 mmol). The solution was stirred continuously for about 2 h at room temperature and the precipitated product was filtered off and washed with diethyl ether. Yield: 80%. M.p.: 252 °C. Anal. Calc. for C 7 H 6 N 3 OS 2 K (251.36): C, 33.45; H, 2.41; N, 16.72; S, 25.51. Found: C, 33.50; H, 2.32; N, 16.78; S, 25.50%. IR data (m cm 1 , KBr): m(NH) 3218m, 3148m; m(C@O)1665s; [b(NH) + m(CN)] (thioamide I) 1470s; [m(CN)+b(NH)] (thioamide II) 1329s; m(N–N) 1087m; m(C@S) 989s; pyridine ring 0277-5387/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2008.06.038 * Corresponding author. Tel.: +91 542 2318529; fax: +91 542 2368127. E-mail address: singhnk_bhu@yahoo.com (N.K. Singh). Polyhedron 27 (2008) 3151–3159 Contents lists available at ScienceDirect Polyhedron journal homepage: www.elsevier.com/locate/poly