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

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]⁺… <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msubsup><mml:mi>X</mml:mi> <mml:mn>3</mml:mn> <mml:mo>-</mml:mo></mml:msubsup> </mml:mrow> </mml:math> 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]⁺… <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msubsup><mml:mi>Cl</mml:mi> <mml:mn>3</mml:mn> <mml:mo>-</mml:mo></mml:msubsup> </mml:mrow> </mml:math> }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.