Synthesis, spectral, DFT and X-ray study of a cis-MoO 2 complex with a new isothiosemicarbazone ligand REZA TAKJOO*†, JOEL T. MAGUE‡, ALIREZA AKBARI§ and MEHDI AHMADI§ †Department of Chemistry, School of Sciences, Ferdowsi University of Mashhad, Mashhad, Iran ‡Department of Chemistry, Tulane University, New Orleans, LA, USA §Department of Chemistry, Payame Noor University (PNU), Tehran, Iran (Received 2 July 2012; in final form 6 February 2013) 2,4-Dihydroxybenzaldehyde S-allylisothiosemicarbazone hydrobromide, H 2 L, reacts with dioxo- molybdenum acetylacetonate in methanol to form a stable complex of dioxomolybdenum(VI). The ligand and complex are characterized with analytical and spectroscopic techniques. Single-crystal X-ray crystallography has been also carried out for the complex, showing it has distorted octahe- dral geometry. H 2 L is a tridentate dianionic ligand bonded as an ONN donor to molybdenum. Thermogravimetric analysis of the complex shows MoO 3 as the final product above 780 °C. The results obtained from density functional theory calculations for the optimization and frequency analysis are in agreement with the experimental data. Natural bond orbital calculations show that the majority of the electron density of the donors tends to the molybdenum, since the calculated Mulliken charge for the central ion is much lesser than the formal value. Keywords: Isothiosemicarbazone; Molybdenum(VI) complex; TG; Crystal structure; DFT 1. Introduction Molybdenum is a trace element in nature and plays an important role in metabolism of plants and animals [1, 2]. As potential models for biologically active molybdenum com- pounds, Schiff base complexes of molybdenum have been used in applications related to catalytic, enzymological and oxygen transfer reactions [3]. Tridentate dibasic Schiff base complexes such as cis-MoO 2 L(D) (D = solvent) are good substrates for redox reactions because of the ability of D replacement with other solvent [4]. There are a number of crys- tal structures of molybdenum(VI) complexes of this class in all of which molybdenum coordinates to an S-alkyl-isothiosemicarbazone, two oxo ligands and solvent [5]. There is interest in new molybdenum complexes of these types due to the unique properties and potential for biological and catalytic activities. With the X-ray structures, density functional theory (DFT) computations [6] can be carried out to more fully charac- terize the complexes through calculation of optimized structures, vibrational frequencies and assessment of Natural bond orbitals (NBO’ s) [7–9]. *Corresponding author. Email: r.takjoo@um.ac.ir Journal of Coordination Chemistry , 2013 http://dx.doi.org/10.1080/00958972.2013.791922 Ó 2013 Taylor & Francis Downloaded by [Parkland College Library] at 03:38 06 May 2013