Oxidative Addition of the Dithiobis(formamidinium) Cation to Platinum(II) Chloro Am(m)ine Compounds: Studies on Structure, Spectroscopic Properties, Reactivity, and Cytotoxicity of a New Class of Platinum(IV) Complexes ExhibitingS-Thiourea Coordination

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Abstract
The oxidative addition of the salt [{SC(NMe2)2}2]Cl2·2H2O (1), the disulfide-like dimerized form of 1,1,3,3-tetramethylthiourea (tmtu), to Pt(II) chloro am(m)ine compounds is described. Oxidation of the [PtCl3(NH3)]- anion with 1 in methanol yields cis-[PtCl4(NH3)L] (2; L = tmtu) as the result of the trans addition of one tmtu and one chloro ligand. The same mode of oxidation is found in reactions of 1 with [PtCl(dien)]+ (dien = diethylenetriamine) and trans-[PtCl2(NH3)2]. In these cases, however, the oxidation is followed by (light-independent) cis,trans isomerizations, giving trans,mer-[PtCl2(dien)L]Cl2 (4) and fac-[PtCl3(NH3)2L]Cl·0.5MeOH (6), respectively. The single-crystal X-ray structures of 2 and trans,mer-[PtCl2(dien)L](BF4)2 (4a) have been determined. 2: monoclinic, space group P21/n, a = 6.280(1) Å, b = 13.221(3) Å, c = 16.575(2) Å, β = 96.45(1)°, Z = 4. 4a: monoclinic, space group C2/m, a = 21.093(5) Å, b = 8.9411(9) Å, c = 14.208(2) Å, β = 124.65(2)°, Z = 4. The tmtu ligands are S-bound. In 2 a pronounced trans influence of the S-donor ligand on the Pt−Cl bond (2.370(1) Å) trans to sulfur is observed. The unusual acidity of the Pt(IV) complexes exhibiting tmtu coordination trans to chloride is attributed to hydrolysis of the labilized Pt−Cltrans bond, which is supported by ion sensitive electrode measurements. An upfield shift of the 195Pt resonances is found on changing the ligand combination from NCl4S (2) to N3Cl2S (4). This order correlates with the trans influences of the ligands: tmtu > am(m)ine > chloride. The cytotoxicity of 2 and 6 in L1210 cell lines is reported and discussed in terms of a possible mechanism of action of the compounds in vivo. It is suggested that tmtu may act as a lipophilic carrier ligand and therefore enhance the cellular uptake of the new potential Pt(IV) drugs.

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