The reaction of PtCl 62− with aromatic compounds to afford anionic σ-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)

Abstract

The kinetics of the thermal reaction of the PtCl 6 2− ion with various aromatic compounds in CF 3COOH/H 2O or CH 3COOH to afford σ-aryl complexes of platmum(IV) have been studied at temperatures of 60–100 °C. The reaction is first order both in PtCl 6 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 σ-aryl complexes decreases with increasing concentration of LiCl added to the reaction mixture. Additions of AgNO 3, Na 2PtCl 4, Hg(OCOCH 3) 2, NaOCOCH 3, BF 3 · OEt 2 and SeO 2 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 (C 6H 5X) decrease in the following sequence of X: OH > OCH 3 > CH 3 > C 2H 5 > OC 6H 5 > CH(CH 3) 2 > H > C 6H 5 > F > COCH 3 > COOH > Cl > NO 2. The logarithms of the relative rates correlate with Hammett's σ and Brown's σ + constants (ϱ = −3.0 and ϱ + = −1.5). The kinetic hydrogen isotope effect of the reaction is small ( ∼ 3 for benzene and ∼ 2.3 for toluene). The following mechanism of the reaction is proposed. The first stage is the dissociation of PtCl 6 2− followed by coordination with arene to form a π-complex. The π-complex then transforms into a Wheland-type complex, which gives a σ-aryl complex of platmum(IV) after proton elimination. The reaction of PtCl 6 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 > OC 2H 5 > OCH 3 > OC 6H 5 > CH 3 (ϱ + = −1.5). The isotope effect for the reaction with toluene ( k H/ k D ∼ 2) was determined. No para-meta isomerisation was observed in the photoinduced reaction at room temperature. The ESR spectra of frozen solutions of PtCl 6 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, [ArH] + [Pt IIICl 5 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 PtCl 6 2− with Ar 2Hg in aqueous acetone to afford σ-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 Ar 2Hg appears to involve an electron-transfer stage.