Halogen Bonds in Novel Polyhalogen Monoanions

Polyhalogen monoanions [X_2n+1]⁻ (X=Cl and Br; n=1, 2, 3, 4, and 5) have been systematically studied using the block-localized wave function (BLW) method, which offers a valence bond (VB) analysis. For each species, the most stable isomer can be described as a central halide anion X⁻ non-classically bonded to a number of dihalogen molecules X₂ via “halogen bonds”. VB analyses confirm the dominant role of the charge-transfer interaction between the lone pair on X⁻ and the σ-anti-bonding orbital of X₂ molecule (n→σ*) in X₃ ⁻ and higher analogues. Thus, our study demonstrates that these halogen bonds are essentially dative covalent interactions. Importantly, the charge-transfer interaction between [X_2n−1]⁻ and X₂ decreases with the increasing n, in accord with the weakening of the Lewis basicity as characterized by the corresponding HOMO energy. The reduction of the charge transfer interaction underscores the reduction of covalence in halogen bonds in [X_2n+1]⁻. This tendency highlights the anti-cooperative effect in polyhalogen monoanions. All in all, the halogen bond between X⁻ and nX₂ molecules exhibits the same trends as in X⁻ with a single X₂ molecule. In other words, halogen bonding in the larger clusters derives from the same bonding mechanism as the [X₃]⁻ anion. As such, the X⁻⋅⋅⋅X₂ halogen bond at different bond lengths forms a gauge of covalence for the entire [X_2n+1]⁻ family.