Synthesis and structures of novel 1-methylcytosinato-bridged (ethylenediamine)platinum(ii) and platinum(iii) dinuclear complexes

Abstract

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)₂Pt(en)](NO₃)₂ (1). The species during the reactions prior to formation of 1 have been investigated by using ¹⁹⁵Pt 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) Å in 1 is well comparable with the corresponding value found in the cis-diammineplatinum(II) analog (2.981(2) Å). 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)₂Pt(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/aquaht-1-methylcytosinato bridged (en)Pt(III) dimer ht-[H₂O(en)Pt(MetCyt-H)₂Pt(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) Å in 6.