Analogues of Cis- and Transplatin with a Rich Solution Chemistry: cis-[PtCl2(NH3)(1-MeC-N3)] and trans-[PtI2(NH3)(1-MeC-N3)]

Mono(nucleobase) complexes of the general composition cis‐[PtCl2(NH3)L] with L=1‐methylcytosine, 1‐MeC (1 a) and L=1‐ethyl‐5‐methylcytosine, as well as trans‐[PtX2(NH3)(1‐MeC)] with X=I (5 a) and X=Br (5 b) have been isolated and were characterized by X‐ray crystallography. The Pt coordination occurs through the N3 atom of the cytosine in all cases. The diaqua complexes of compounds 1 a and 5 a, cis‐[Pt(H2O)2(NH3)(1‐MeC)]2+ and trans‐[Pt(H2O)2(NH3)(1‐MeC)]2+, display a rich chemistry in aqueous solution, which is dominated by extensive condensation reactions leading to μ‐OH‐ and μ‐(1‐MeC−‐N3,N4)‐bridged species and ready oxidation of Pt to mixed‐valence state complexes as well as diplatinum(III) compounds, one of which was characterized by X‐ray crystallography: h,t‐[{Pt(NH3)2(OH)(1‐MeC−‐N3,N4)}2](NO3)2⋅2 [NH4](NO3)⋅2 H2O. A combination of 1H NMR spectroscopy and ESI mass spectrometry was applied to identify some of the various species present in solution and the gas phase, respectively. As it turned out, mass spectrometry did not permit an unambiguous assignment of the structures of +1 cations due to the possibilities of realizing multiple bridging patterns in isomeric species, the occurrence of different tautomers, and uncertainties regarding the Pt oxidation states. Additionally, compound 1 a was found to have selective and moderate antiproliferative activity for a human cervix cancer line (SISO) compared to six other human cancer cell lines. A question of coordination: Multiple self‐condensation reactions involving water and 1‐methylcytosine (1‐MeC) ligands, as well as ready metal oxidation, dominate the aqueous‐solution chemistry of mononuclear cis‐ and trans‐[Pt(H2O)2(NH3)(1‐MeC‐N3)]2+, analogues of the diaqua species of cisplatin and its trans isomer, respectively (see scheme).