Redox reactions of platinum-(II) and -(IV) complexes: influence of the σ-donor ability of non-labile 2,2′-bipyridyl and substituted 1,10-phenanthroline ligands on the rates of reaction

Abstract

The reduction of [PtCl₄(N–N)] by [NBu₄]I and the oxidation of [PtCl₂(N–N)] by [AuCl₄]^– in the presence of chloride ions have been kinetically investigated in acetonitrile [N–N = 1,10-phenanthroline (phen), 5Me-, 5NO₂-, 5,6Me₂-, and 3,4,7,8Me₄-phen, or 2,2′-bipyridyl(bipy)]. The rate law for the reduction of [PtCl₄(N–N)] has the form: Rare =k₁[PtCl₄(N–N)]+k₂[PtCl₄(N–N)][I^–]. The first-order rate term is ascribed to a dissociative Pt–Cl rate-determining bond rupture, followed by a fast redox step. An inner-sphere redox mechanism is thought to be responsible for the second-order rate term. The dissociative path is only slightly affected by changes in N–N, whereas the second-order term is strongly influenced by the σ-donor ability of the uncharged ligands, the relation log k₂=–0.62 PK_a+ 2.05 being found at 25 °C. The rate law for the oxidation of [PtCl₂,(N–N)] by [AuCl₄]^– has the form: Rate =k₃[PtCl₂(N–N)][AuCl₄^–][Cl^–]. The rates are not sensitive to changes in N–N, indicating that the energetics of the interactions occurring in the activated complexare on the whole nearly constant throughout the set of ligands emproved. Atentative explanation of this behaviour takes into consideration the opposite effects caused by changing N–N on the formation free energy of the precursor inner-sphere intermediate, and on the free energy required to form the transition state for electron transfer from such an intermediate, both effects contributing to the free energy of activation of the redox reaction.