Mechanism of reductive elimination of ethane from some halogenotrimethylbis(tertiary phosphine)platinum(IV) complexes

Abstract

Specifically deuteriated complexes have been prepared by stereospecific oxidative addition of CD₃I to cis-[PtMe₂(L)₂](L = PMe₂Ph or AsMe₂Ph) and of MeX (X = Cl, Br, or I) to cis-[Pt(CD₃)₂(PMe₂Ph)₂]. These complexes undergo intramolecular scrambling of Me and CD₃ groups in solution at room temperature (L = AsMe₂Ph) or above (L = PMe₂Ph), and decompose by intramolecular reductive elimination of ethane. Kinetic studies of the decomposition of fac-[PtX(Me)₃L₂](I: X = Cl or Br, L = PMe₂Ph; X = I, L = PMe₃, PMe₂Ph, or PMePh₂) indicate that ethane is eliminated from a five-co-ordinate intermediate formed by dissociation of a phosphine ligand, but when L₂= 1,2-bis(diphenylphosphino)ethane (dppe) ethane is eliminated without prior phosphine dissociation. In his case the activation energy (69 kJ mol^–1) for ethane elimination is considerably lower than the estimated methyl–platinum bond energy (144 kJ mol^–1) for (I; X = I, L = PMe₂Ph) obtained from a study of the ethane elimination by differential-scanning calorimetry. Methyl groups are eliminated from the platinum(IV) complexes more readily than [²H₃]methyl groups, with a mean secondary kinetic-isotope effect k(H)/k(D) of 1·07. The results are interpreted in terms of concerted reductive elimination, with the transition state resembling an ethane complex of platinum(II).