Kinetics and mechanism of complexation of trans-[PtCl(NH3)2(H2O)]+ with inosine and 1-methylinosine in aqueous solution at different pH values

Abstract

The kinetics of complexation of trans-[PtCl(NH₃)₂(H₂O)]⁺ with inosine and its 1-methyl derivative has been studied at 298.2 K in aqueous solution (pH 2.8–8.4) by employing HPLC as an analytical tool. Under these conditions trans-[PtCl(NH₃)₂(H₂O)]⁺ behaves like a monofunctional platinum(II) species. Throughout the pH range studied the complex formation can be explained by substitution of the aqua ligand by the incoming nucleoside, the OH group appearing to be inert toward substitution. 1-Methylinosine forms only a N⁷-bound 1 : 1 complex. This binding mode is favoured also with inosine when pH < 6, whereas above this pH N¹ becomes an additional binding site which facilitates the formation of a N¹,N⁷-bonded diplatinum species, too. Rate parameters obtained for the formation of the inosine 1 : 1 complexes show that the N⁷ site is preferred over the N¹ site, whereas in the binding of the second platinum(II) unit to the different 1 : 1 complexes the N¹ site is slightly more favourable than the N⁷ site. Co-ordination of trans-[PtCl(NH₃)₂(H₂O)]⁺ to the inosine N⁷ site lowers the basicity of N¹H site by about 1.3 log units.