THE REACTION OF PtCl62- WITH AROMATIC COMPOUNDS TO AFFORD ANIONIC ?-ARYL COMPLEXES OF PLATINUM(IV). VIII. KINETICS AND MECHANISMS OF THERMAL, PHOTOCHEMICAL AND γ-INDUCED REACTIONS WITH ARENES AND ARYLMERCURY COMPOUNDS (ELECTROPHILIC SUBSTITUTION INVOLVING ELECTRON TRANSFER)

Article abstract of DOI:10.1016/0022-328X(84)80609-9

The kinetics of the thermal reaction of the PtCl6^2- ion with various aromatic compounds in CF3COOH/H2O or CH3COOH to afford ?-aryl complexes of platinum(IV) have been studied at temperatures of 60-100 deg C.The reaction is first order both in PtCl6^2- and arene.The process of formation of platinated toluene (as well as some other complexes) is accompanied with its para-meta isomerisation.The rate of formation of the ?-aryl complexes decreases with increasing concentration of LiCl added to the reaction mixture.Additions of AgNO3, Na2PtCl4, Hg(OCOCH3)2, NaOCOCH3, BF3*OEt2 and SeO2 accelerate the reaction.The activation energies of the formation and the para-meta isomerisation are ca. 100 kJ mol^-1.The relative rates of the reaction with different aromatic compounds (C6H5X) decrease in the following sequence of X: OH > OCH3 > CH3 > C2H5 > OC6H5 > CH(CH3)2 > H > C6H5 > F > COCH3 > COOH > Cl > NO2.The logarithms of the relative rates correlate with Hammett's ? and Brown's ?⁺ constants (ρ = -3.0 and ρ⁺ = -1.5).The kinetic hydrogen isotope effect of the reaction is small (ca. 3 for benzene and ca. 2.3 for toluene).The following mechanism of the reaction is proposed.The first stage is the dissociation of PtCl6^2- followed by coordination with arene to form a ?-complex.The ?-complex then transforms into a Wheland-type complex, which gives a ?-aryl complex of platinum(IV) after proton elimination.The reaction of PtCl6^2- with arenes may be carried out at room temperature if it is induced by light or γ-irradiation.The relative rates of the photoinduced reaction decrease in the following sequence: OH > OC2H5 > OCH3 > OC6H5 > CH3 (ρ⁺ = -1.5).The isotope effect for the reaction with toluene (k_H/k_D ca. 2) was determined.No para-meta isomerisation was observed in the photoinduced reaction at room temperature.The ESR spectra of frozen solutions of PtCl6^2- and arenes irradiated at 77 K exhibited signals due to platinum(III) complexes and organic radicals.The proposed mechanism involves electron transfer from an arene to a platinum(IV) complex to give an intermediate ion-radical pair, radical kation IIICl5^2->.The latter may then be transformed into a Wheland-type complex.Such a mechanism may be termed the S_E2e.t. mechanism.The thermal reaction of PtCl6^2- with Ar2Hg in aqueous acetone to afford ?-aryl complexes of platinum(IV) is first order in the platinum(IV) complex and zero order in the aryl mercurial.The rate of the reaction decreases upon addition of LiCl.The mechanism of the reaction with Ar2Hg appears to involve an electron-transfer stage.