Halogen Bond Asymmetry in Solution

Abstract

Halogen bonding is the noncovalent interaction of halogen atoms in which they act as electron acceptors. Whereas three-center hydrogen bond complexes, [D···H···D]+ where D is an electron donor, exist in solution as rapidly equilibrating asymmetric species, the analogous halogen bonds, [D···X···D]+, have been observed so far only to adopt static and symmetric geometries. Herein, we investigate whether halogen bond asymmetry, i.e., a [D–X···D]+ bond geometry, in which one of the D–X bonds is shorter and stronger, could be induced by modulation of electronic or steric factors. We have also attempted to convert a static three-center halogen bond complex into a mixture of rapidly exchanging asymmetric isomers, [D···X–D]+ ⇄ [D–X···D]+, corresponding to the preferred form of the analogous hydrogen bonded complexes. Using 15N NMR, IPE NMR, and DFT, we prove that a static, asymmetric geometry, [D–X···D]+, is obtained upon desymmetrization of the electron density of a complex. We demonstrate computationally that conversion into a dynamic mixture of asymmetric geometries, [D···X–D]+ ⇄ [D–X···D]+, is achievable upon increasing the donor–donor distance. However, due to the high energetic gain upon formation of the three-center-four-electron halogen bond, the assessed complex strongly prefers to form a dimer with two static and symmetric three-center halogen bonds over a dynamic and asymmetric halogen bonded form. Our observations indicate a vastly different preference in the secondary bonding of H+ and X+. Understanding the consequences of electronic and steric influences on the strength and geometry of the three-center halogen bond provides useful knowledge on chemical bonding and for the development of improved halonium transfer agents.

Halogen Bonding Helicates Encompassing Iodonium Cations

“…The silver(I) complexes 7 and 8 were formed as precursors to 3 and 4 , following a previously described pathway (Scheme ) . The formation of the silver(I) and iodonium complexes in CD 2 Cl 2 were associated with large 15 N NMR coordination shifts, approximately 60 ppm and 90 ppm, respectively, consistent with previous reports …”

Phosphine Oxides as Spectroscopic Halogen Bond Descriptors: IR and NMR Correlations with Interatomic Distances and Complexation Energy

“…[24][25][26][27][28][29][30][31][32], in organocatalysis [33][34][35][36][37][38] and in design of pharmaceuticals (here, halogen bonds are considered primarily as a type of hydrophobic functional groups, increasing the lipophilic properties of molecules and allowing them to pass through cell membranes) [39,40]. Besides, the halogen bond has been a subject of numerous theoretical works [41][42][43][44][45][46][47].…”

Strong N−X⋅⋅⋅O−N Halogen Bonds: A Comprehensive Study on N‐Halosaccharin Pyridine N‐Oxide Complexes

“…The difference in δ 15 N coord chemical shifts for − O−N + groups, as a qualitative assessment, reflects the strong N−I⋅⋅⋅ − O−N + and weak N−Br⋅⋅⋅ − O−N + interactions. The δ 15 N coord chemical shifts of the N−X⋅⋅⋅ − O−N + XBs range from 2.3 to 22.3 ppm and are 45 to 5 times smaller than the δ 15 N coord values (≥100 ppm) found in pyridine‐based [N−X−N] + complexes, and one to three times larger than those observed for weak C−X⋅⋅⋅N (X=Br, I; δ 15 N coord range, 2.0 to 8.0 ppm) XB interactions . The significantly larger δ 15 N coord values for [N−X−N] + are due to the direct participation of the N‐atoms in the XB coordination and the effective positive charge delocalization over a 3c‐4e [N−X−N] + halogen‐bonded system .…”

“…The results by Erdélyi et al. provided useful information on the strength and geometry of the [N−X−N] + XBs . In [bis(pyridine)iodine] + complexes, computational studies revealed that the substituents at the para ‐position to the pyridine N‐atom influence the XB stabilization energies, reflected by their [N−I + ] bond lengths .…”

“…In our previous study, we have shown that the strong and tuneable N−X⋅⋅⋅ − O−N + halogen bonds are formed by using two heteronuclear aromatic components, namely N‐haloimides as donors and pyridine N ‐oxides (PyNOs) as acceptors . Unlike the [N−X−N] + synthesis, this approach does not require dry solvent conditions, either for solution NMR studies or crystallization experiments. Solution studies of the 1:1 N‐haloimide/PyNO complexes showed that high binding constants manifested shorter X⋅⋅⋅O halogen bond distances with R XB values <0.70 …”