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Substituent Effects on the [N–I–N]+ Halogen Bond

Journal article
Authors Anna-Carin Carlsson
Krenare Mehmeti
Martin Uhrbom
Alavi Karim
Michele Bedin
Rakesh Puttreddy
Roland Kleinmaier
Alexei A Nevorov
Bijan Nekoueishahraki
Jürgen Gräfenstein
Kari Rissanen
Mate Erdelyi
Published in Journal of the American Chemical Society
Volume 138
Issue 31
Pages 9853–9863
ISSN 0002-7863
Publication year 2016
Published at Swedish NMR Centre at Göteborg University
Department of Chemistry and Molecular Biology
Pages 9853–9863
Language en
Links dx.doi.org/10.1021/jacs.6b03842
https://gup.ub.gu.se/file/201536
Subject categories Chemical Sciences

Abstract

We have investigated the influence of electron density on the three-center [N–I–N]+ halogen bond. A series of [bis(pyridine)iodine]+ and [1,2-bis((pyridine-2-ylethynyl)benzene)iodine]+ BF4– complexes substituted with electron withdrawing and donating functionalities in the para-position of their pyridine nitrogen were synthesized and studied by spectroscopic and computational methods. The systematic change of electron density of the pyridine nitrogens upon alteration of the para-substituent (NO2, CF3, H, F, Me, OMe, NMe2) was confirmed by 15N NMR and by computation of the natural atomic population and the π electron population of the nitrogen atoms. Formation of the [N–I–N]+ halogen bond resulted in >100 ppm 15N NMR coordination shifts. Substituent effects on the 15N NMR chemical shift are governed by the π population rather than the total electron population at the nitrogens. Isotopic perturbation of equilibrium NMR studies along with computation on the DFT level indicate that all studied systems possess static, symmetric [N–I–N]+ halogen bonds, independent of their electron density. This was further confirmed by single crystal X-ray diffraction data of 4-substituted [bis(pyridine)iodine]+ complexes. An increased electron density of the halogen bond acceptor stabilizes the [N···I···N]+ bond, whereas electron deficiency reduces the stability of the complexes, as demonstrated by UV-kinetics and computation. In contrast, the N–I bond length is virtually unaffected by changes of the electron density. The understanding of electronic effects on the [N–X–N]+ halogen bond is expected to provide a useful handle for the modulation of the reactivity of [bis(pyridine)halogen]+-type synthetic reagents.

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