Multidentate halogen bond acceptors: from fluorides to iodides. Anticooperativity in halogen-bonded clusters

A survey of the Cambridge Structural Database together with a statistical analysis of its results has been conducted in order to investigate the frequency of occurrence of halide anions as halogen bond acceptors of different denticity. The results show that for the majority of studied motifs (C-I)_<i>n</i>⋯Y^- (Y = F, Cl, Br and I) the number of halogen bond donors <i>n</i> ≤ 4, and for the fluoride anion there are no motifs containing more than four ligands in the crystal structure. To assess the impact of denticity of halides as acceptors on interaction energy and its nature, computational studies have been performed for model halogen-bonded complexes (F₃C-I)_<i>n</i>⋯Y^- (Y = F, Cl, Br, and I) with different number of halogen bond donors. A canonical energy decomposition analysis (canonical EDA) reveals the trends in interaction energy and its components when increasing the number of halogen-bonded ligands. Moreover, the canonical EDA allows to establish the factors making the adduct (F₃C-I)₅⋯F^-, containing the structural motif which does not occur in the crystal structure, different from analogous complexes. Additionally, the non-additivity of interactions in the model systems and its source have been studied.