Facile heterolytic bond splitting of molecular chlorine upon reactions with Lewis bases: Comparison with ICl and I2

Abstract

Formation of molecular complexes and subsequent heterolytic halogen-halogen bond splitting upon reactions of molecular Cl₂ with nitrogen-containing Lewis bases (LB) are computationally studied at M06-2X/def2-TZVPD and for selected compounds at CCSD(T)/aug-cc-pvtz//CCSD/aug-cc-pvtz levels of theory. Obtained results are compared with data for ICl and I₂ molecules. Reaction pathways indicate, that in case of Cl₂∙LB complexes the activation energies for the heterolytic Cl-Cl bond splitting are lower than the activation energies of the homolytic splitting of Cl₂ molecule into chlorine radicals. The heterolytic halogen splitting of molecular complexes of X₂∙Py with formation of [XPy₂]⁺…$X_3^{-}$ contact ion pairs in the gas phase is slightly endothermic in case of Cl₂ and I₂, but slightly exothermic in the case of ICl. Formation of {[ClPy₂]⁺…${\mathrm{Cl}}_3^{-}$}₂ dimers makes the overall process exothermic. Taking into account that polar solvents favor ionic species, generation of donor-stabilized Cl⁺ in the presence of the Lewis bases is expected to be favorable. Thus, in polar solvents the oxidation pathway via donor-stabilized Cl⁺ species is viable alternative to the homolytic Cl-Cl bond breaking.