Aliphatic C–X (X = halogen) bond activation by transition metal complexes containing the {Pt 2S 2} core: A theoretical study of the reaction mechanism

Abstract

[L 2Pt(μ-S) 2PtL 2] (L = phosphane) complexes have been found to cleave C–X bonds in organic halides, resulting in the alkylation of the bridging sulfido ligands. Remarkably, all these reactions require a significant excess of the corresponding organic electrophile. In order to research the mechanism underlying this reactivity, the reaction between [Pt 2(μ-S) 2(dhpp) 2] (dhpp = H 2P–(CH 2) 3–PH 2) and XCH 2CH 2CH 2X (X = F, Cl, Br, I) has been studied by means of theoretical methods. The geometries of the transition states and the energy profiles are consistent with a S N2 mechanism where a sulfido bridging ligand replaces a halide anion. The energy barriers decrease in descending order within the group F > Cl > Br > I. In addition, the effect of the excess of alkyl halide has been studied by comparing the energy profiles corresponding to the reaction with a single organohalide substrate with that where a second molecule is also present. The role played by this additional XCH 2CH 2CH 2X molecule is to assist the rupture of the C–X bond by diminishing the energy barrier of the substitution process. This is achieved by establishing X⋯H interactions between the halide leaving group and the spectator organohalide molecule. In addition, the influence of the solvent in the energy barrier has been taken into account. The continuum description model used shows that consistently with the S N2 reaction mechanism polar solvents diminish the energy barrier. Notwithstanding this, the stabilizing effect of the second reactant molecule is maintained.