Steric Control of the Coordination Mode of the Salicylaldehyde Thiosemicarbazone Ligand. Syntheses, Structures, and Redox Properties of Ruthenium and Osmium Complexes Falguni Basuli, 1a Shie-Ming Peng, 1b and Samaresh Bhattacharya* ,1a Inorganic Chemistry Section, Department of Chemistry, Jadavpur University, Calcutta 700032, India, and Department of Chemistry, National Taiwan University, Taipei, Taiwan, ROC ReceiVed May 1, 1997 Introduction The chemistry of transition metal complexes of thiosemicar- bazones has been receiving considerable attention largely because of their pharmacological properties. 2 Thiosemicarba- zones usually bind to a metal ion, either in the neutral thione form (1) or in the anionic thiolate form (2), as bidentate N,S- donor ligands forming five-membered chelate rings. 2,3 How- ever, incorporation of a third donor site (D) into these thiosemicarbazone ligands, linked to the carbonylic carbon via one or two intervening atoms, normally results in D,N,S tricoordination (3). 2,4,5 In this note we report the chemistry of two ruthenium and osmium complexes of the same ligand, Viz. salicylaldehyde thiosemicarbazone (Hsaltsc, where H stands for the dissociable proton). Though free Hsaltsc exists in the thione form (4), 6 it is known to coordinate as a dianionic tridentate O,N,S donor. 5 Reaction of Hsaltsc with [M(PPh 3 ) 3 X 2 ] (M ) Ru, Os; X ) Cl, Br) afforded complexes of the type [M(PPh 3 ) 2 - (saltsc) 2 ] where the salicylaldehyde thiosemicarbazone ligand is coordinated, in spite of having the phenolic oxygen as the potential third donor site, as a bidentate N,S-donor ligand, forming a four-membered chelate ring (5). The steric bulk of the coligand PPh 3 appears to be the driving force for this rather unexpected coordination mode of the salicylaldehyde thiosemi- carbazone ligand. The syntheses, characterization, and cyclic voltammetric properties of these two [M(PPh 3 ) 2 (saltsc) 2 ] com- plexes are described here. Experimental Section Materials. [Ru(PPh3)3Cl2] and [Os(PPh3)3Br2] were synthesized according to reported procedures. 7 Salicylaldehyde thiosemicarbazone (Hsaltsc) was prepared by reacting equimolar amounts of salicylalde- hyde and thiosemicarbazide in hot ethanol. Purification of dichlo- romethane and preparation of tetraethylammonium perchlorate (TEAP) for electrochemical work were performed as reported in the literature. 8 Preparation of [Ru(PPh3)2(saltsc)2]. To a solution of Hsaltsc (42 mg, 0.22 mmol) in ethanol (40 mL) was added [Ru(PPh 3)3Cl2] (100 mg, 0.10 mmol) followed by NEt 3 (0.22 mg, 0.22 mmol). The resulting mixture was stirred for 30 min at ambient temperature. The yellow precipitate of [Ru(PPh 3)2(saltsc)2] was collected by filtration, washed thoroughly with ethanol, and dried in air. Recrystallization of the crude product from 1:1 dichloromethane-hexane solution gave [Ru(PPh 3)2- (saltsc) 2] as a golden yellow crystalline solid. Yield: 72%. Anal. Calcd for C 52H46N6O2P2S2Ru: C, 61.60; H, 4.54; N, 8.29. Found: C, 61.54; H, 4.59; N, 8.26. Preparation of [Os(PPh 3)2(saltsc)2]. This was prepared by fol- lowing the above procedure (except that stirring was continued for 2 h at 60 °C) using [Os(PPh 3)3Br2] instead of [Ru(PPh3)3Cl2]. Yield: 67%. Anal. Calcd for C 52H46N6O2P2S2Os: C, 56.62; H, 4.17; N, 7.62. Found: C, 56.54; H, 4.21; N, 7.58. Physical Measurements. Microanalyses (C, H, N) were performed using a Perkin-Elmer 240C elemental analyzer. IR spectra were obtained on a Perkin-Elmer 783 spectrometer with samples prepared as KBr pellets. Electronic spectra were recorded on a Simadzu UV- 1601 spectrophotometer. Magnetic susceptibilities were measured using a PAR 155 vibrating-sample magnetometer. 1 H NMR spectra were obtained on a Bruker AC-200 NMR spectrometer using TMS as the internal standard. Electrochemical measurements were made using a PAR model 273 potentiostat. 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