On the Heterogeneous Nature of Cisplatin-1-Methyluracil Complexes: Coexistence of Different Aggregation Modes and Partial Loss of NH3 Ligands as Likely Explanation

The conversion of the 1 : 1-complex of Cisplatin with 1-methyluracil (1MeUH), cis-[Pt(NH₃)₂(1MeU-N3)Cl] ( 1 a ) to the aqua species cis-[Pt(NH₃)₂(1MeU-N3)(OH₂)]⁺ ( 1 b ), achieved by reaction of 1 a with AgNO₃ in water, affords a mixture of compounds, the composition of which strongly depends on sample history. The complexity stems from variations in condensation patterns and partial loss of NH₃ ligands. In dilute aqueous solution, 1 a , and dinuclear compounds cis-[(NH₃)₂(1MeU-N3)Pt(μ-OH)Pt(1MeU-N3)(NH₃)₂]⁺( 3 ) as well as head-tail cis-[Pt₂(NH₃)₄(μ-1MeU-N3,O4)₂]²⁺ ( 4 ) represent the major components. In addition, there are numerous other species present in minor quantities, which differ in metal nuclearity, stoichiometry, stereoisomerism, and Pt oxidation state, as revealed by a combination of ¹H NMR and ESI-MS spectroscopy. Their composition appears not to be the consequence of a unique and repeating coordination pattern of the 1MeU ligand in oligomers but rather the coexistence of distinctly different condensation patterns, which include μ-OH, μ-1MeU, and μ-NH₂ bridging and combinations thereof. Consequently, the products obtained should, in total, be defined as a heterogeneous mixture rather than a mixture of oligomers of different sizes. In addition, a N₂ complex, [Pt(NH₃)(1MeU)(N₂)]⁺ appears to be formed in gas phase during the ESI-MS experiment. In the presence of Na⁺ ions, multimers n of 1 a with n=2, 3, 4 are formed that represent analogues of non-metalated uracil quartets found in tetrastranded RNA.