Novel oxorhenium(V) ‘3+1’ mixed ligand complexes with 3-thiapentane-1,5-dithiolate and functional mercaptobenzoyl amino acid ethyl ester Shubhamoy Chowdhury, Ali Canlier, Nobuyoshi Koshino, Yasuhisa Ikeda * Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, 2-12-1-N1-34 O-okayama, Meguro-ku, Tokyo 152-8550, Japan Received 24 April 2007; received in revised form 19 June 2007; accepted 23 June 2007 Available online 1 July 2007 Abstract Oxorhenium(V) complexes with ‘3+1’ mixed ligands, [ReO(SSS)L], where SSS is g 3 -(SCH 2 CH 2 SCH 2 CH 2 S), L = g 1 -(C 6 H 4 COOH-4- S), g 1 -(C 6 H 4 CONHCH 2 COOEt-4-S), g 1 -(C 6 H 4 CONHCH(CH 3 )COOEt-4-S), and g 1 -(C 6 H 4 CONHCH(CH 2 Ph)COOEt-4-S), have been synthesized. These L ligands and [ReO(SSS)L] complexes were characterized by IR, 1 H NMR, 13 C NMR, and MAS spectrometers. Molecular structure of [ReO(SSS){g 1 -(C 6 H 4 COOH-4-S)}] complex was determined to be a distorted square pyramidal by single crystal X-ray analytical method. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Oxorhenium(V) complexes; Molecular structure; Amino acid esters; ‘3+1’ mixed ligands; Thiolate 1. Introduction Oxorhenium(V) complexes as radiopharmaceuticals for therapy and diagnosis have been paid attention, because of their suitable radionuclear properties, i.e., 186 Re: E max = 1.1 MeV for b-emission and E max = 0.137 MeV for c-emis- sion with t 1/2 = 90.6 h, 188 Re: E max = 2.1 MeV for b-emis- sion and E max = 0.155 MeV for c-emission with t 1/2 = 17 h [1]. Generally, integrated and bifunctional approaches have been used for drug designing of Re compounds [2,3]. In such ways, ‘3+1’ mixed ligand approach has been proposed for developing novel radiopharmaceuticals containing oxo- Re(V), in which ReO 3+ core is coordinated by tridentate dithiolates (SXS, X = O, S, or NR) and unidentate thio- lates. This approach has been paid much attention [4–15], because the oxo-Re(V) complexes with ‘3+1’ mixed ligands enable the coupling of biologically relevant groups and molecules, and hence are expected to generate radiophar- maceuticals with various functions, e.g., the inhibitors of protease such as cathepsin, the receptors in the central ner- vous system (CNS), the agents for the assessment of brain perfusion, the application in nuclear medicine oncology, and so on [16–26]. However, it has been reported that the oxo-Re(V) com- plexes with ‘3+1’ mixed ligands are unstable in vitro and in vivo due to the metabolism and replacement of the unid- entate ligands by thiolated nucleophiles such as glutathione [27–29]. Hence, the oxo-Re(V) complexes with mixed ligands for overcoming such problems have been synthe- sized and characterized [30–35]. On the other hand, the introduction of the pendent ester groups (PEGs) on the unidentate thiolate coligand has been undertaken to develop new oxo-Re(V) complexes for the metabolic pathway in brain cells, because hydrolysis of PEGs by intracerebral esterases is expected to provoke trapping of oxo-Re(V) with PEGs in brain tissue due to their transformation to hydrophilic and non-diffusible metabolites [36,37]. 0020-1693/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2007.06.038 * Corresponding author. Fax: +81 3 5734 3061. E-mail address: yikeda@nr.titech.ac.jp (Y. Ikeda). www.elsevier.com/locate/ica Available online at www.sciencedirect.com Inorganica Chimica Acta 361 (2008) 145–152