Role of the ancillary ligands on the stabilization of the imino-oxo tautomer of 1-methylcytosine in PtII complexes

Abstract

The mixed nucleobases complexes cis-[L₂Pt{1-MeTy(-H)}(1-MeCy,N³)]NO₃ (L = PPh₃, 1a; PMePh₂, 1b), containing the N(3)-deprotonated 1-methylthymine (1-MeTy(-H)) and the neutral 1-methylcytosine (1-MeCy) have been prepared and characterised. The compounds were obtained by reacting the hydroxo complexes cis-[L₂Pt(μ-OH)]₂(NO₃)₂ with 1-methylthymine (1-MeTy), followed by the addition of 1 equivalent of 1-MeCy. In solution of DMSO, DMF or chlorinated solvents, 1a converts quantitatively into the isomer cis-[L₂Pt{1-MeTy(-H)}(1-MeCy,N⁴)]NO₃ (2a) containing the tautomeric form of the cytosine stabilized through the coordination at the N(4) atom, as shown by single-crystal X-ray analysis. The structural determination of 2a shows the presence in the unit cell of two crystallographic independent complexes having similar conformation, with a different orientation of the two nucleobases (head–head and head–tail) according to the presence of both isomers in solution. Complex 1b, having the less hindered PMePh₂ ligands, in DMSO solution, contains the tautomeric forms of the cytosine in equilibrium and the migration of the metal from the N(3) to N(4) site occurs only to a minor extent.