LIGAND EFFECTS ON THE RATES OF PROTONOLYSIS AND ISOTOPIC EXCHANGE FORPLATINUM(II) ALKYLS

The protonolysis/deuterolysis of complexes L2PtRX (L = phosphorus liga nd, R = alkyl group, X = anionic ligand) has been investigated as a me chanistic probe of the reverse reaction, activation of alkanes by Pt(I I). Trans-(Et3P)(2)PtMeX (X- = triflate (OTf-), F-, NO3-) solvolyze in acidic CD3OD, forming [trans-(Et3P)(2)PtMe(CD3OD)](+) which reacts sl owly with DOTf at room temperature liberating CH3D. In dichloromethane , trans-(Et3P)(2)PtMe(OTf) reacts with HOTf at low temperatures (-70 t o -20 degrees C) to give (Et3P)(2)PtMe(H) (OTf)(2) in rapid equilibriu m with the reagents, while at higher temperatures rapid methane loss i s preceded by extensive deuterium incorporation (with DOTf) into the P t(II) methyl group. Upon treatment with acid in CD3OD, trans-(Et3P)(2) PtMeX (X = Cl, Br) also undergo H/D exchange before elimination of met hane, while trans-(Et3P)(2)PtMeI, (depe)Pt(CH3)(2) (depe=1,2-bis(dieth ylphosphine)ethane) and cis- [(MeO)(3)P] PtMeCl do not. The alpha hydr ogens of trans-(Et3P)(2)PtRCl (R = Me, Et, Bz) exchange with deuterium in CD3OD/DOTf with rates following the order Bz < Me < Et, while no e xchange is observed in the protonolysis of trans-(Et3P)(2)Pt(CH2CMe3)C l which yields (CH3)(3)CCH2D. These trends are interpreted in terms of effects on relative stabilities of key intermediates, Pt(IV) alkyl hy drides and Pt(II) alkane sigma complexes. (C) 1997 Elsevier Science S. A.