Electron-transfer reactions of halogenated electrophiles: a different look into the nature of halogen bonding

The rates of oxidation of ferrocene derivatives by brominated molecules R-Br (CBr₃CN, CBr₄, CBr₃NO₂, CBr₃COCBr₃, CBr₃CONH₂, CBr₃F, and CBr₃H) were consistent with the predictions of the outer-sphere dissociative electron-transfer theory. The similar redox-reactions of the R-Br electrophiles with the typical halogen-bond acceptors tetramethyl-p-phenylenediamine (TMPD) or iodide were much faster than calculated using the same model. The fast redox-processes in these systems were related to the involvement of the transient halogen-bonded [R-Br, TMPD] or [R-Br, I^-] complexes in which barriers for electron transfer were lowered by the strong electronic coupling of reactants. The Mulliken-Hush treatment of the spectral and structural characteristics of the [R-Br, TMPD] or [R-Br, I^-] complexes corroborated the values of coupling elements, H_ab, of 0.2-0.5 eV implied by the kinetic data. The Natural Bond Orbital analysis of these complexes indicated a noticeable donor/acceptor charge transfer, Δq, of 0.03-0.09 ē. The H_ab and Δq values in the [R-Br, TMPD] and [R-Br, I^-] complexes (which are similar to those in the traditional charge-transfer associates) indicate significant contribution of charge-transfer (weakly-covalent) interaction to halogen bonding. The decrease of the barrier for electron transfer between the halogen-bonded reactants demonstrated in the current work points out that halogen bonding should be taken into account in the mechanistic analysis of the reactions of halogenated species.