Note Silylated mixed-ligand rhenium complexes with the [PNS/S] donor atom set Torsten Kniess a,b, *, Celia Fernandes a , Isabel Santos a , Werner Kraus c , Hartmut Spies b a Departamento de Quimica, ITN, Estrada Nacional 10, 2686-953, Sacavem Codex, Portugal b Institute of Bioinorganic & Radiopharmaceutical Chemistry, FZ-Rossendorf, PF 510119, D-01314 Dresden, Germany c Federal Institute of Materials Research and Testing, Berlin, Richard-Willsta ¨tter-Str. 11, D-12489 Berlin, Germany Received 8 August 2002; accepted 18 November 2002 Abstract New oxorhenium complexes with 2-(diphenylphosphanyl)-N -(2-thioethyl)benzamide (H 2 PNS) and trimethyl-, triethyl- and triphenyl-hydroxyl silylated monodentate thiols are reported. These new complexes have been prepared by reacting [N n Bu 4 ][R- e(O)Cl 4 ] with the tridentate H 2 PNS and the corresponding silylated thiol at room temperature. The characterization of the complexes involved elemental analysis, 31 P and 1 H NMR spectroscopies and X-ray crystallographic analysis for the triethyl-silylated Re complex. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Crystal structure; Rhenium oxo mixed-ligands complexes 1. Introduction Our research group has been evaluating the possibility of using heterofunctionalized phosphines for preparing Re and 99m Tc complexes with biological interest [1  /6]. We found that 2-(diphenylphosphanyl)-N -(2-hydro- xyethyl)benzamide (H 2 PNO), N -(2-aminoethyl)-2-di- phosphanl)-benzamide (H 2 PNN) or 2-(diphenyl- phosphanyl)-N -(2-thioethyl)benzamide (H 2 PNS) react with different rhenium and technetium precursors lead- ing to stable Re(V) or Re(I) complexes [1 /6]. For M(V), several mixed-ligand oxocomplexes with [(PNO/S)(S)] donor atom sets have been synthesized, among them arylpiperazine (a receptor ligand for the 5-HT 1A ser- otonergic receptors) bearing species derived from the most stable oxocompounds have also been synthesized [3,6]. For increasing the lipophilicity of Re and Tc com- pounds and to facilitate their transportation in the organism some efforts were made on the synthesis of hydroxy silylated mixed-ligand rhenium  /oxocomplexes [7,8]. It is expected that by silylation of Re and 99m Tc compounds their bio-availability will be improved, especially in terms of crossing the blood brain barrier and on the uptake in brain tissues. Successful synthesis of 99m Tc silylated ‘3 /1’ complexes with the tridentate 3- thiapentane-1,5-dithiol (HSSSH) was demonstrated re- cently [9]. However, stability problems observed for this class of complexes [10] led us to focus on the synthesis and characterization of analogous compounds stabilized with the tridentate 2-(diphenylphosphanyl)-N -(2- thioethyl)benzamide ligand (H 2 PNS), which has shown to form 3 /1 oxocompounds stable towards glutathione exchange [6]. We report on the synthesis and characterisation of new oxorhenium complexes stabilized by the tridentate 2-(diphenylphosphanyl)-N -(2-thioethyl)benzamide and by several monodentate silylated co-ligands: [Re(O)(k 3 - PNS)(S-(CH 2 ) n -OSiR 3 )] with n /2,3 and R /CH 3 , C 2 H 5 and C 6 H 5 . 2. Experimental All chemicals were of reagent grade and used without further purification. [ n Bu 4 N][Re(O)Cl 4 ] and 2-(diphe- * Corresponding author. Fax:  /49-351-2602-915. E-mail address: kniess@fz-rossendorf.de (T. Kniess). Inorganica Chimica Acta 348 (2003) 237  /241 www.elsevier.com/locate/ica 0020-1693/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0020-1693(02)01486-X