Synthesis and structures of novel 1-methylcytosinato-bridged (ethylenediamine)platinum(II) and platinum(III) dinuclear complexes

The reaction of [Pt(en)(H2O)(2)](2+) with 1 equivalent of 1-methylcytosine at an initial pH of 6 ultimately led to the 1-methylcytosinato-bridged (ethylenediamine) platinum(II) dimer, [(en)Pt(MetCyt-H)(2)Pt(en)](NO3)(2) (1). The species during the reactions prior to formation of 1 have been investigated by using Pt-195 NMR spectroscopy. From the reaction mixture the [Pt(en)(MetCyt)(2)](NO3)(2) complex (3) could be isolated and structurally characterized. A rational synthesis of 1 was also achieved by dimerization of [Pt(en)(MetCyt)(H2O)](NO3)(2). According to X-ray structure analysis, the Pt-Pt distance of 2.9816(3) angstrom in 1 is well comparable with the corresponding value found in the cis-diammineplatinum(II) analog (2.981(2) angstrom). Oxidation of 1 with K2S2O8 led to the symmetrically sulfate-capped diplatinum(III) complex with two ethylenediamine ligands equatorially bound to the two Pt atoms ht-[(SO4)(en) Pt(MeCyt-H)(2)Pt(en)(SO4)] (5), as established by X-ray crystallography. When the oxidation was carried out in the presence of HClO4, the formation of the hydroxido/aqua ht-1-methylcytosinato bridged (en) Pt(III) dimer ht-[H2O(en) Pt(MetCytH)(2)Pt(en) OH](ClO4)(3) (6) was observed, as supported by elemental and X-ray structure analysis. In the structure of 6, two ethylenediamine ligands are differently bound to the two Pt centers: one of them equatorially while the second one caps the opposite axial site of the diplatinum(III) core. The Pt-Pt distances in the platinum(III) dimers are 2.5982(4) in 5 and 2.5569(3) angstrom in 6.