Ion-pair mechanism in square planar substitution. Reactivity of cationic platinum (II) complexes with negatively charged nucleophiles in solvents of high, medium and low polarity

Abstract

The rates of displacement of dimethyl sulfoxide from the cation [Pt(phen) (CH 3) (Me 2SO)] + by a series of uncharged and negatively charged nucleophiles have been measured in a methanol/water (19:1 vol./vol.) mixture. The starting complex and the reaction products were characterized either as solids or in solution by their IR and 1H NMR spectra. The substitution reactions take place by way of a direct bimolecular attack of the ligand on the substrate. The sequence of reactivity observed is as expected on the basis of a nucleophilicity scale relevant for + 1 charged substrates ([Pt(en) (NH 3)Cl] + used as standard). The difference of reactivity between the first (t-BuNH 2) and the last (SeCN −) members of the series spans five orders of magnitude. The value measured for the nucleophilic discrimination (1.55) is the highest found so far for cationic substrates. This is a result of the easy transfer of some of the electron density brought in by the incoming ligand into the ancillary ligands. When the reaction is carried out in a series of protic and dipolar aprotic solvents, using chloride ion as nucleophile, the rate of formation of [Pt (phen) (CH 3)Cl] is dominated by the extent of solvation of Cl −, as measured by its values of the Gibbs molar energy of transfer Δ t G 0. Conductivity measurements at 25°C in dichloromethane were fitted to the Fuoss equation and the values of the dissociation constants K d for the ion pairs were calculated as follows: 2.27 × 10 −5 M for Bu 4NCl, 2.75 × 10 −5 M for Bu 4NSCN and 17.05 × 10 −5 M for [Pt(phen) (CH 3) (Me 2SO)]PF 6. The pseudo-first-order rate constants k obs for the reactions with Bu 4NCl, Bu 4NBr, Bu 4NSCN and Bu 4NI showed a curvilinear dependence on the concentration of the salt which levels off very soon (at concentrations higher than 0.005 M the kinetics are zero order in [Bu 4NX]). On addition of the inert electrolyte Bu 4NPF 6 the rates slow down and the kinetics follow the rate law k obs = kK ip[Bu 4NX]/[Bu 4NPF 6] + K ip[Bu 4NX]). These findings fit well with a reaction scheme which involves a pre-equilibrium K ip between ion pairs, followed by unimolecular substitution within the contact ion pair [Pt(phen) (CH 3) (Me 2SO)X] ip. Values of the equilibrium constants K ip for ion-pair exchange and of the internal substitution rates k were derived. The latter showed that the discrimination in reactivity between Cl −, Br −, SCN − and I − is greatly reduced with respect to aqueous solutions. The reason behind this may be desolvation of the ions coupled to the fact that a contact ion pair is already at a certain distance along the reaction coordinate in the direction of the transition state. Applications of the special salt effect and of ion pairing to synthesis are discussed.