Tight-Contact Ion Pairs Involving Pt(II) Dithiooxamide Complexes: the Acid-BaseReactions between Hydrohalogenated Ion-Paired Complexes and Pyridine

The equilibrium constants relative to HCl exchange between Pt(II)-containing tight contact ion pairs (TCIP) and pyridine have been investigated in chloroform solution at 298 K. The general formulas of the metal species are: {[Pt(H2-R2-dithiooxamide)2]2+, 2Cl-} (a-type compounds; R=methyl (1a), ethyl (2a), n-propyl (3a), iso-propyl (4a), n-butyl (5a), cyclohexyl (6a), benzyl (7a), β-phenyl-ethyl (8a), allyl (9a)) and {[(H-R2-dithiooxamidate)Pt(H2-R2-dithiooxamide)]+, Cl-} (b-type compounds; R has the same meanings as before, given rise to 1b-9b species; moreover, the mixed R compound 10b, containing R=benzyl on a DTO (dthiooxamidate/dithiooxamide) ligand and R = ethyl on the other DTO ligand, has also been investigated). Moreover, the parent species [Pt(H-R2-dithiooxamidate)2] (c-type compounds; 1c-10c) have also been prepared. Out of 29 compounds reported in the paper, 19 compounds are here reported for the first time, and their synthesis and characterization data are also given. Compounds of a-type exhibit two successive equilibrium constants, which are related to successive HCl transfer from the TCIP to pyridine. By comparing the equilibrium constants Kc of the various b-type species, we have been able to (i) obtain information on the relative stability of the TCIP and, by taking advantage of the two equilibrium constants Kc1 and Kc2 found for each a-type species, (ii) gain knowledge on the electronic interaction between the two basic sites of the Pt(II) bis-dithiooxamide complexes, mediated by the metal center. Linear relationships are found between the pKc of the compounds and the σ-Taft value (Σσ*) of the amine substituents of the DTO ligands. Interestingly, the slope of such linear correlations is much steeper for pKc2 than for pKc1, indicating that the electronic interaction between the basic sites increases with the electron donating ability of the R substituent. A parallel is proposed between the splitting of HCl transfer equilibrium constants in a-type TCIP and oxidation potential splitting in dinuclear, bridge-linked metal complexes.