Charge-Assisted Halogen Bonds in Halogen-Substituted Pyridinium Salts: Experimental Electron Density

Favorable electrostatic interactions dominate the packing of three halogensubstituted pyridinium salts with chloride or tetrachloridometallate counteranions. The electron density in these solids was determined based on high resolution X-ray diffraction experiments. Residues carrying opposite charges subtend short interhalogen contacts X···X in which the anionic chlorides act as electron donors and the cationic halogen substituents of the pyridinium cations act as acceptors. The X···X interactions coexist and in part compete with classical N−H···Cl hydrogen bonds. Properties derived from the experimental charge density such as the position of the bond critical point, electron density, Laplacian and energy densities help to establish the relevance of the competing interactions. Multipole-derived charge densities allow classification of short contacts more reliably than exclusively geometry-based analyses and simple promolecule densities