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1:1 and 1:2 cocrystals of 4, 4-bipyridine and N- bromosuccinimide: the complexes with a very short N∙∙∙Br halogen bond

Halogen bonds that form between polarisable and electron-deficient halogen atoms (Br, I) and electron-rich Lewis bases have been utilised as alternatives to hydrogen bonds in the planned construction of cocrystals, salts, liquid crystals and ionic inclusion frameworks [1-3]. Typical halogen bonds are similar to hydrogen bonds both in terms of length and directionality. Herein we report the halogen bonded 1:1 and 1:2 cocrystals of N-bromosuccinimide (nbs) with a halogen bond acceptor 4, 4-bipyridine (bpy) prepared by liquid-assisted grinding (LAG) as well as by conventional solution-based method. We have demonstrated how nbs, a commercially available compound commonly used as a brominating agent in organic synthesis, can be used as halogen bond donor [4]. The N–Br covalent bond of nbs is polarised by electron withdrawing amide moiety and in the reaction with bpy leads to cocrystals with an interesting (CO)2N–Br∙∙∙N halogen bond synthon. The Br∙∙∙N distance of 2.586 Å in (nbs)(bpy) is 23.9 % less then the sum of the van der Waals (VDW) radii of nitrogen (1.55 Å) and bromine (1.85 Å). Furthermore, crystal structure determination of (nbs)2(bpy) reveals two pairs of symmetry equivalent (CO)2N–Br∙∙∙N halogen bonds with contact distances of 2, 358 and 2, 373 Å which are 30.6 and 30.2 % less than the sum of the VDW radii, respectively. Molecules of both cocrystals show an interesting packing in crystal, where the halogen bonded complexes are connected into a 3D network via C–H∙∙∙O hydrogen bonds.

Halogen bonds; crystal structure; cocrystals

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