Stabilisation of the iminooxo tautomer of 1-methylcytosine in PtII complexes: Role of the ancillary ligands

Reaction of cis-[L2Pt(μ-OH)]2(NO3)2 (L = PPh3) with 1-methylthymine (1-MeTy), in DMF, leads to the formation of the mononuclear neutral adduct cis-L2Pt{1-MeTy(–H)}(ONO2) (1) whose structure in the solid state has been obtained by single crystal X-ray diffraction. The deprotonated nucleobase is bounded at the N(3) site, with the pyrimidinic ring almost perpendicular (78.0(1)°) to the metal coordination plane. The fourth ligand is a monodentate nitrate group. Addition of 1 equiv. of 1-methylcytosine (1-MeCy) causes the immediate replacement of the nitrato ligand to form the cationic complex cis-[L2Pt{1-MeTy(–H)}(1-MeCy,N3)]NO3 (2) in which both the nucleobases are N(3)-platinated. In CD2Cl2 at −40 °C 2 exists as a mixture of two conformers (2:1 molar ratio) arising from the different orientation of the nucleobases with respect to the metal coordination plane. In solution of DMSO, DMF or chlorinated solvents, 2 slowly converts into the isomer cis-[L2Pt{1-MeTy(–H)}(1-MeCy,N4)]NO3 (3), containing the tautomeric form of the cytosine stabilised through the coordination at the N(4) atom, as a mixture of conformers whose relative abundance is dependent on the solvent and the temperature. In contrast, the analogous complex of 2 containing the phosphine PMe3, cis-[(PMe3)2Pt{1-MeTy(–H)}(1-MeCy,N3)]NO3 (4), also isolated as pure compound, in DMSO solution slowly rearranges leading to the elimination of the neutral 1-MeTy, with the formation of the dinuclear cytosinate complex cis-[(PMe3)2Pt{1-MeCy(–H),N3N4}]2(NO3)2, previously characterised by us.