Halogen bond symmetry: the N–X–N bond

Abstract

Halogen bonding is the attractive interaction of an electrophilic region of a halogen with a nucleophile. Its geometry, energy, and dominantly electrostatic nature resemble that of the hydrogen bond. Halogen bond strength critically depends on the electron deficiency of the halogen. Accordingly, halonium ions that bear a formal positive charge are the strongest halogen bond donors known so far. Halonium ions, similar to H + , are capable of simultaneously interacting with two Lewis bases. The structure and properties of the resulting three-center halogen bonds are reviewed herein, based on their spectroscopic and computational data, with the emphasis being placed on one representative example, the [N-XÀN] + interaction. The iodine and bromine-centered systems form symmetric three-center-four-electron halogen bonds that have comparable orbital and electrostatic contributions and a large extent of charge delocalization. Their formation yields significant stabilization. The [N-Cl-N] + bond is computationally predicted to resemble the symmetry of the three-center bond of the heavier halogens, whereas the fluorine-centered halogen bond is predicted to be asymmetric, and thus behave more similar to the analogous, asymmetric three-center [NÀHÀN] + hydrogen bond. Experimental confirmation of the latter predictions remains to be accomplished, along with studies of the possible influence of the disorder of the local environment on halogen bond symmetry, and investigations in the gaseous phase. The electronic structure and the geometry of three-center halogen bonds are of fundamental interest, and also of practical importance, for example due to the occurrence of three-centered halogen bonds in synthetic reagents.

NX⋯Y halogen bonds. Comparison with NH⋯Y H-bonds and CX⋯Y halogen bonds

“…[38][39][40] Most of this data is structural in nature and derives from crystal studies, as recently summarized by Troff et al 41 The N-X XB bond in halosuccinimides [42][43][44] shows up as a short intermolecular contact. [38][39][40] Most of this data is structural in nature and derives from crystal studies, as recently summarized by Troff et al 41 The N-X XB bond in halosuccinimides [42][43][44] shows up as a short intermolecular contact.…”

Halogen Bonding in Solution

“…Koskinen et al has recently described analogous [S-I-S] + complexes in the solid state [126]. Three-center halogen bonds were reviewed [127]. Beer et al reported the covalency of halogen bonds in a catenane-based receptor system in various solvents [128].…”

“…Such systems can be seen as the simultaneous interaction of a formally positively charged halogen with two halogen bond acceptors of comparable Lewis base strength in a linear fashion. This linear geometry is enforced by the antiparallel lobes of the empty p-orbital of the halogen, forming two "p-holes" framed by the negative belt of electron density of its filled p-orbitals (p x 2 p y 2 p z 0 ) [127]. Thus, the halogen bond donor of three-center bonds possesses analogous anisotropic electron distribution to that of two-center bonds.…”

Solvent Effects on Nitrogen Chemical Shifts

“…The jΔ 15N coord j of the complex indicates weak interaction (3 ppm) of the pyridine that is coordinating to N-fluoropyridinium (0 ppm, 235). Overall, the interaction strength order I > Br > Cl ) F is shown for the [NdXdN] + halogen bonds by both NMR and DFT [67,69,148,152], with the covalent character of the static, symmetric [N-X-N] + complexes increasing in the order Cl > Br > I.…”

“…The three-centered [N-X-N] + halogen bond comprising a positive halogen(I) and simultaneously interacting with two electron donors is regarded as a specific subclass showing especially strong interactions [148,149]. Bis(pyridine)-based [N-I-N] + and [N-Br-N] + complexes, encompassing bromine(I) or iodine(I) as electron acceptor resembling the coordination complexes of Ag(I), for example, are present as static and symmetric structures (Fig.…”