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 [PtMe2(diimine)] have been studied. Complete kinetic and thermodynamic parameters have been obtained by VT 1H NMR for the reversible oxidative addition of Me3EX (E = Sn, X = Cl, Br, I) to [PtMe2(bpy-tbu2)] (bpy-tbu2 = 4,4‘-di-tert-butyl-2,2‘-bipyridyl) and related compounds, while partial data have been obtained for the reductive elimination of Me2SnCl2 from [PtClMe2(Me2SnCl)(bpy-tbu2)] and for the oxidative addition of Me3GeCl to [PtMe2(bpy-tbu2)]. UV−visible spectroscopic studies have also yielded equilibrium constants and ΔG° for the reversible oxidative addition reactions of Me3SnX (X = Cl, Br, I) to [PtMe2(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 SN2 mechanism for the reactions studied, and this is further supported by the observation of the cationic complex [PtMe2(Me3Sn)(bpy-tbu2){OC(CD3)2}]+ at low temperature in acetone-d6. Extremely large second-order rate constants are observed for the oxidative addition of Sn−X bonds to dimethylplatinum(II) complexes, some being greater than 108 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−MMe3 bond dissociation energies for [PtXMe2(MMe3)(bpy-tbu2)], 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.