Thermodynamic and kinetic study of the interaction between the Pt(ii) centres in [Pt2(N,N,N′,N′-tetrakis(2-pyridylmethyl)diamine)(H2O)2]4+. Influence of the bridging ligand

Abstract

A series of binuclear Pt(II) complexes were synthesized to investigate the influence of four different bridging diamines, viz. benzene-1,3-diamine, benzene-1,4-diamine, 1,3-propanediamine and 1,4-butanediamine, on the reactivity of the platinum centres in complexes of the type [Pt₂(N,N,N′,N′-tetrakis(2-pyridylmethyl)diamine)(H₂O)₂]⁴⁺ in comparison to the corresponding mononuclear complex, [Pt(bis(2-pyridylmethyl)amine)(H₂O)]²⁺. The pK_a values for both deprotonation steps were determined, and the kinetics of the reaction with chloride was studied under pseudo-first order conditions as a function of chloride concentration and temperature. The results indicate that the electrophilicity of the binuclear complexes is higher than that of the mononuclear complex, which results in lower pK_a values, higher reaction rates and a decrease in ΔH^‡. In addition, the reactivity of one Pt(II) centre shows a clear dependence on the nature of the other Pt(II) centre, leading to different thermodynamic and kinetic properties for the first and second reaction steps. The interaction between the two Pt(II) centres mainly depends on the Pt–Pt distance and not on the aromatic or aliphatic nature of the bridging diamine group. Temperature dependent ¹H NMR-studies were performed on the [Pt₂(N,N,N′,N′-tetrakis(2-pyridylmethyl)-1,3-propanediamine)Cl₂]²⁺ complex, and the results indicate that a non-symmetric, folded species of this complex dominates at low temperatures. This behaviour may account for the unusual behaviour of the corresponding diaqua complex.