π covalency in the halogen bond

Abstract

Halogen bonds are a highly directional class of intermolecular interactions widely employed in chemistry and chemical biology. This linear interaction is commonly viewed to be analogous to the hydrogen bond because hydrogen bonding models also intuitively describe the σsymmetric component of halogen bonding. The possibility of π-covalency in a halogen bond is not contemplated in any known models. Here we present evidence of π-covalency being operative in halogen bonds formed between chloride and halogenated triphenylamine-based radical cations. We reach this conclusion through computational analysis of chlorine K-edge X-ray absorption spectra recorded on these halogen bonded pairs. In light of this result, we contend that halogen bonding is better described by analogy to metal coordination bonds rather than hydrogen bonds. Our revised description of the halogen bond suggests that these interactions could be employed to influence the electronic properties of conjugated molecules in unique ways.

Nature of the Hydrogen Bond Enhanced Halogen Bond

“…Moreover, the EDA data indicate that the halogen bond in these complexes is not merely electrostatic but exhibits a high degree of covalency, as can be seen from the high contribution of the ∆E orb term (ranging from 42 to 47% to the total ∆E int term). A similar result was found in other halogen bonds [20,24,25], and in particular, in systems involving carbenes [26] and more recently, also carbones [27]. Interestingly, the electrostatic interactions as well as the orbital interactions (albeit to a lesser extent) steadily increase with the number of adjacent hydrogen bonds in the system (i.e., none < one < two).…”

(15‐crown‐5)BiI3 as a Building Block for Halogen Bonded Supramolecular Aggregates

“…In conclusion, our results show that crown ether complexes like (15-crown-5)BiI 3 (1) can act as halogen bond acceptors for typical halogen bond donors like TIE, allowing for the isolation of supramolecular aggregates like (15-crown-5)BiI 3 • 0.5TIE (2). We are now in the process of trying to translate our findings to the on-surface chemistry of exfoliated BiI 3 crystals.…”

“…Halogen bonding has emerged as an important interaction in supramolecular chemistry, [1] and continues to be of great current interest regarding its theoretical description [2] and in fields of application from catalysis [3] to material science [4] and biochemistry. [5] Metal halide complexes can also be used in the construction of supramolecular aggregates involving halogen bonds, serendipitously or by design when halogenated solvents like halomethanes are used, [6,7] in many polyhalide compounds [8][9][10][11][12][13][14][15] or through addition of typical halogen bond donors like tetraiodoethylene (TIE) or 1,4diiodotetrafluorobenzene.…”

“…[25] A search in the Cambridge Crystallographic Database [26] reveals that up to now the use of chalcogen-based ligands like crown ethers has been the only way to enforce the trigonal pyramidal shape of BiI 3 in the gas phase [27] within a neutral coordination complex. [28,29] Here, we present the synthesis, crystal structure and thermal, vibrational and optical properties of (15-crown-5)BiI 3 (1) in comparison with the halogen bonded adduct (15-crown-5)BiI 3 • 0.5TIE (2).…”

Catalytic Activation via π –Backbonding in Halogen Bonds?

“…Notably, π-type bonding in XB interactions was recently observed both computationally 28 and experimentally. 29 Exploring the relevance and importance of such contributions in halogen bonding is only just beginning. Simplified view of different modes of activation in XB-assisted catalysis.…”