Computational Studies of Coinage Metal Anion M- + CH3X (X = F, Cl, Br, I) Reactions in Gas Phase

We characterized the stationary points along the nucleophilic substitution (S(N)2), oxidative insertion (OI), halogen abstraction (XA), and proton transfer (PT) product channels of M- + CH3X (M = Cu, Ag, Au; X = F, Cl, Br, I) reactions using the CCSD(T)/aug-cc-pVTZ level of theory. In general, the reaction energies follow the order of PT > XA > S(N)2 > OI. The OI channel that results in oxidative insertion complex [CH3-M-X](-) is most exothermic, and can be formed through a front-side attack of M on the C-X bond via a high transition state OxTS or through a S(N)2-mediated halogen rearrangement path via a much lower transition state invTS. The order of OxTS > invTS is inverted when changing M- to Pd, a d(10) metal, because the symmetry of their HOMO orbital is different. The back-side attack S(N)2 pathway proceeds via typical Walden-inversion transition state that connects to pre- and post-reaction complexes. For X = Cl/Br/I, the invS(N)2-TS's are, in general, submerged. The shape of this M- + CH3X S(N)2 PES is flatter as compared to that of a main-group base like F- + CH3X, whose PES has a double-well shape. When X = Br/I, a linear halogen-bonded complex [CH3-X center dot center dot center dot M](-) can be formed as an intermediate upon the front-side attachment of M on the halogen atom X, and it either dissociates to CH3 + MX- through halogen abstraction or bends the C-X-M angle to continue the back-side S(N)2 path. Natural bond orbital analysis shows a polar covalent M-X bond is formed within oxidative insertion complex [CH3-M-X](-), whereas a noncovalent M-X halogen-bond interaction exists for the [CH3-X center dot center dot center dot M](-) complex. This work explores competing channels of the M- + CH3X reaction in the gas phase and the potential energy surface is useful in understanding the dynamic behavior of the title and analogous reactions.

Automated summary

We characterized the stationary points along various reaction channels of the M- + CH3X reactions and found that the reaction energies follow the order of PT > XA > S(N)2 > OI. The oxidative insertion (OI) channel is most exothermic, and can be formed through a front-side attack or a S(N)2-mediated halogen rearrangement. The order of OxTS > invTS is reversed when M- changes to Pd. The S(N)2 pathway proceeds through a typical Walden-inversion transition state. A linear halogen-bonded complex can be formed when X = Br/I. This work provides insights into the dynamic behavior of the M- + CH3X reactions.