Rapid Reversible Oxidative Addition of Group 14−Halide Bonds to Platinum(II):  Rates, Equilibria, and Bond Energies

Abstract

The reversible oxidative addition reactions of methyl(halogeno)tin and methyl(halogeno)germanium compounds to electron-rich platinum(II) complexes of the type [PtMe 2 (diimine)] have been studied. Complete kinetic and thermodynamic parameters have been obtained by VT 1 H NMR for the reversible oxidative addition of Me 3 EX (E = Sn, X = Cl, Br, I) to [PtMe 2 (bpy- t bu 2 )] (bpy- t bu 2 = 4,4‘-di- tert - butyl-2,2‘-bipyridyl) and related compounds, while partial data have been obtained for the reductive elimination of Me 2 SnCl 2 from [PtClMe 2 (Me 2 SnCl)(bpy- t bu 2 )] and for the oxidative addition of Me 3 GeCl to [PtMe 2 (bpy- t bu 2 )]. UV−visible spectroscopic studies have also yielded equilibrium constants and Δ G ° for the reversible oxidative addition reactions of Me 3 SnX (X = Cl, Br, I) to [PtMe 2 (diimine)]. Thermodynamic studies quantitatively establish the halogen effect on the oxidative addition reactions studied according to the favorability series I > Br > Cl. Kinetic studies clearly point to an S N 2 mechanism for the reactions studied, and this is further supported by the observation of the cationic complex [PtMe 2 (Me 3 Sn)(bpy- t bu 2 ){OC(CD 3 ) 2 }] + at low temperature in acetone- d 6 . Extremely large second-order rate constants are observed for the oxidative addition of Sn−X bonds to dimethylplatinum(II) complexes, some being greater than 10 8 M - 1 s - 1, and it is established that rates follow the series Sn > Ge > Si > C and I > Br > Cl. Estimates have been made of the Pt−MMe 3 bond dissociation energies for [PtXMe 2 (MMe 3 )(bpy- t bu 2 )], X = halide, and these are 233, 182, and 172 kJ mol - 1 for M = Si, Ge, and Sn, respectively; the values are useful in rationalizing the chemistry of the Pt−M bonds.