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Halogen Bond Asymmetry in Solution

Artikel i vetenskaplig tidskrift
Författare Sofia Lindblad
Krenare Mehmeti
Alberte X. Veiga
Bijan Nekoueishahraki
Jürgen Gräfenstein
Mate Erdelyi
Publicerad i Journal of the American Chemical Society
Volym 140
Sidor 13503-13513
ISSN 0002-7863
Publiceringsår 2018
Publicerad vid Svenskt NMR-centrum vid Göteborgs universitet
Institutionen för kemi och molekylärbiologi
Sidor 13503-13513
Språk en
Länkar doi.org/10.1021/jacs.8b09467
Ämneskategorier Kemi

Sammanfattning

© 2018 American Chemical Society. 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]+ r← [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.

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