Metal-stabilized rare tautomers of nucleobases .5. Iminooxo tautomer of cytosine coordinated to Pt(II) with metal and nucleobase in syn and anti orientations

A Pt-II complex containing N4 bound neutral 1-methylcytosine (1-MeC), trans-[Pt(NH3)(2)(1-MeC-N4)(2)](ClO4)(2) (5), has been prepared and characterized by X-ray analysis. The complex contains the rare iminooxo tautomer form of the cytosine nucleobase. Pt-II binding is through the exocyclic N4 position of the nucleobases, with Pt and the N3 positions in a syn orientation. As a consequence, the proton at N3 is pointing toward the heavy metal, thereby allowing an agostic Pt ... HN interaction. Formation of 5 is achieved via oxidation of the linkage isomer trans-[Pt(NH3)(2)(1-MeCN3)(2)](2+) (1) to a Pt-IV species (2), followed by metal migration to N4, and subsequent reduction to Pt-II. This process is a text-book example for a redox-assisted metal migration at a heterocyclic ligand. The existence of various rotamers of 5 in aqueous solution is evident from H-1 NMR spectroscopy. The possible role of these rotamers of the metalated rare tautomer, and in particular of those having Pt and the N3 position in an anti arrangement, with regard to base mispairing is discussed.