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

The distribution of aquated Pt-II(dien) between the adenosine N-1 and N-7 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 N-7 Site (1), whereas the latter procedure significantly increases the amount of the N-1-bound complex (2) at the expense of the N-7 isomer. Both complexes exhibit two Pt-195 resonances at ambient temperature, but H-1 and C-13 NMR spectra recorded in D2O revealed only signals with expected multiplicities for the adenine moiety. However, in [H-2(7)]dmf the aromatic protons and the dien NH proton resonances were split into two components at -50 degrees C and the C-6-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 C-6-N-6 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.