Ion-pair mechanism in square planar substitution. Reactivity of cationic platinum(II) complexes

The rates of displacement of dimethyl sulfoxide from the cation [Pt(phen)(CH3)(Me(2)SO)](+) 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 H-1 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)(NH3)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)(CH3)Cl] is dominated by the extent of solvation of Cl-, as measured by its values of the Gibbs molar energy of transfer Delta(t)G(0). Conductivity measurements at 25 degrees 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 x 10(-5) M for Bu(4)NCl, 2.75 X 10(-5) M for Bu(4)NSCN and 17.05 X 10(-5) M for [Pt(phen)(CH3)(Me(2)SO)]PF6. The pseudo-first-order rate constants k(obs) for the reactions with Bu(4)NCl, Bu(4)NBr, Bu(4)NSCN and Bu(4)NI 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(4)NX]). On addition of the inert electrolyte Bu(4)NPF(6) the rates slow down and the kinetics follow the rate law k(obs) = kK(ip)[Bu(4)NX]/([Bu(4)NPF(6)] + K-ip[Bu(4)NX]). 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)(CH3)(Me(2)SO)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.