Dynamic processes in platinum(II)–adenosine complexes. Preparation, NMR spectroscopic characterisation and crystal structure of isomeric PtII(dien)–adenosine complexes

Abstract

The distribution of aquated Pt^II(dien) between the adenosine N¹ and N⁷ sites has been found to be affected in aqueous solution not only by lowering the pH, but also by equilibration of the mixture of Pt^II and nucleoside at high temperature. In the former case Pt^II predominantly binds to the N⁷ site (1), whereas the latter procedure significantly increases the amount of the N¹-bound complex (2) at the expense of the N⁷ isomer. Both complexes exhibit two ¹⁹⁵Pt resonances at ambient temperature, but ¹H and ¹³C NMR spectra recorded in D₂O revealed only signals with expected multiplicities for the adenine moiety. However, in [²H₇]dmf the aromatic protons and the dien NH proton resonances were split into two components at –50 °C and the C⁶–NH₂ protons were no longer equivalent in both complexes. This suggests restricted rotation about the platinum–nucleoside bonds and/or conformational changes of the dien ligand. Also rotation of the exocyclic amino group about the C⁶–N⁶ bond appears to be restricted at low temperatures. Both complexes were crystallised as perchlorate salts and their crystal structures determined. Based on X-ray data, further disturbance in the platinum(II) co-ordination sphere may be induced by close proximity of the exocyclic amino group of the nucleobase and Pt^II.