Halogen Bonds in Novel Polyhalogen Monoanions

Polyhalogen monoanions [X2n+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 X-2 via halogen bonds. VB analyses confirm the dominant role of the charge-transfer interaction between the lone pair on X- and the s-anti-bonding orbital of X-2 molecule (n -> sigma*) in X-3(-) and higher analogues. Thus, our study demonstrates that these halogen bonds are essentially dative covalent interactions. Importantly, the charge-transfer interaction between [X2n-1](-) and X-2 decreases with the in-creasing 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 [X2n+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 X-2 molecule. In other words, halogen bonding in the larger clusters derives from the same bonding mechanism as the [X-3](-) anion. As such, the X-center dot center dot center dot X-2 halogen bond at different bond lengths forms a gauge of covalence for the entire [X2n+1](-) family.