Volume 58, Issue 44 pp. 15702-15706
Communication

A Crystallographic Charge Density Study of the Partial Covalent Nature of Strong N⋅⋅⋅Br Halogen Bonds

Mihael Eraković

Mihael Eraković

Department of Physical Chemistry, Rudjer Bošković Institute, Bijenička 54, HR-10000 Zagreb, Croatia

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Prof. Dominik Cinčić

Prof. Dominik Cinčić

Department of Chemistry, Faculty of Science, University of Zagreb, Horvatovac 102a, HR-10000 Zagreb, Croatia

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Dr. Krešimir Molčanov

Corresponding Author

Dr. Krešimir Molčanov

Department of Physical Chemistry, Rudjer Bošković Institute, Bijenička 54, HR-10000 Zagreb, Croatia

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Prof. Vladimir Stilinović

Corresponding Author

Prof. Vladimir Stilinović

Department of Chemistry, Faculty of Science, University of Zagreb, Horvatovac 102a, HR-10000 Zagreb, Croatia

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First published: 23 August 2019
Citations: 55

In memory of Professor Drago Grdenić (1919–2018)

Graphical Abstract

Crystallographic charge density analysis of a strong N−Br⋅⋅⋅N halogen bond in a N-bromosuccinimide and 3,5-dimethylpyridine co-crystal (NBS-lut) reveals a partially covalent character. Comparisons with pure crystalline NBS and the covalent bond in a bis(3-methylpyridine)bromonium cation suggest that there is a continuum of interactions between the intermolecular “non-bonding” halogen bond and a three centre–two electron covalent bond.

Abstract

The covalent nature of strong N−Br⋅⋅⋅N halogen bonds in a cocrystal (2) of N-bromosuccinimide (NBS) with 3,5-dimethylpyridine (lut) was determined from X-ray charge density studies and compared to a weak N−Br⋅⋅⋅O halogen bond in pure crystalline NBS (1) and a covalent bond in bis(3-methylpyridine)bromonium cation (in its perchlorate salt (3). In 2, the donor N−Br bond is elongated by 0.0954 Å, while the Br⋅⋅⋅acceptor distance of 2.3194(4) is 1.08 Å shorter than the sum of the van der Waals radii. A maximum electron density of 0.38 e Å−3 along the Br⋅⋅⋅N halogen bond indicates a considerable covalent contribution to the total interaction. This value is intermediate to 0.067 e Å−3 for the Br⋅⋅⋅O contact in 1, and approximately 0.7 e Å−3 in both N−Br bonds of the bromonium cation in 3. A calculation of the natural bond order charges of the contact atoms, and the σ*(N1−Br) population of NBS as a function of distance between NBS and lut, have shown that charge transfer becomes significant at a Br⋅⋅⋅N distance below about 3 Å.

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