Ligand effects on the rates of protonolysis and isotopic exchange for platinum(II) alkyls

The protonolysis/deuterolysis of complexes L2PtRX (L = phosphorus ligand, R = alkyl group, X = anionic ligand) has been investigated as a mechanistic probe of the reverse reaction, activation of alkanes by Pt(II). Trans-(Et3P)(2)PtMeX (X- = triflate (OTf-), F-, NO3-) solvolyze in acidic CD3OD, forming [trans-(Et3P)(2)PtMe(CD3OD)](+) which reacts slowly with DOTf at room temperature liberating CH3D. In dichloromethane, trans-(Et3P)(2)PtMe(OTf) reacts with HOTf at low temperatures (-70 to -20 degrees C) to give (Et3P)(2)PtMe(H) (OTf)(2) in rapid equilibrium with the reagents, while at higher temperatures rapid methane loss is preceded by extensive deuterium incorporation (with DOTf) into the Pt(II) methyl group. Upon treatment with acid in CD3OD, trans-(Et3P)(2)PtMeX (X = Cl, Br) also undergo H/D exchange before elimination of methane, while trans-(Et3P)(2)PtMeI, (depe)Pt(CH3)(2) (depe=1,2-bis(diethylphosphine)ethane) and cis- [(MeO)(3)P] PtMeCl do not. The alpha hydrogens 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 exchange is observed in the protonolysis of trans-(Et3P)(2)Pt(CH2CMe3)Cl which yields (CH3)(3)CCH2D. These trends are interpreted in terms of effects on relative stabilities of key intermediates, Pt(IV) alkyl hydrides and Pt(II) alkane sigma complexes. (C) 1997 Elsevier Science S.A.