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

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. 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.