From single-point to three-point halogen bonding between zinc(ii) tetrathiocyanate and tetrabromomethane

Abstract

X-ray structural analysis revealed that (Bu₄N)₂[Zn(NCS)₄]·3CBr₄ co-crystals comprise a honeycomb-like network in which each thiocyanate anion is linked to the zinc(II) cation via N–Zn coordination bonds and to three tetrabromomethane molecules via S⋯Br halogen bonds. Most of the halogen bonds in this hybrid network are similar to those between the CBr₄ molecules and the separate NCS⁻ anions. However, it also contains an unusual two-point interaction involving two NCS⁻ ligands of the [Zn(NCS)₄]²⁻ complex and two bromine substituents of the CBr₄ molecule. DFT computations with ωB97XD and M062X functionals confirmed that the two-point-bonded complex represents a (local) energy minimum for the CBr₄·[Zn(NCS)₄]²⁻ dyads. These computations also revealed that complexes in which the [Zn(NCS)₄]²⁻ dianion and CBr₄ molecule are linked by three-point halogen bonds are characterized by somewhat lower energy both in the gas phase and in dichloromethane. Yet, the strengths of these two- and three-point halogen bonds are close to that between the CBr₄ molecules and NCS⁻ anions (and the experimental formation constants of CBr₄·[Zn(NCS)₄]²⁻ and CBr₄·NCS⁻ associates in dichloromethane are also similar). This indicates that each component of the multicenter interaction is significantly weaker than the one-point CBr₄·NCS⁻ halogen bond. The lack of enhancement of the strength of multi-point bonding is apparently related to the deviations of the geometries of individual halogen bonds involved in these multicenter interactions from their optimal values.