Toward a reverse hierarchy of halogen bonding between bromine and iodine

Abstract

We compare here the halogen bond characteristics of bimolecular adducts involving either N-bromo- or N-iodosaccharin as strong halogen bond donors, with 4-picoline as a common XB acceptor. In the NBSac·Pic system, the bromine atom of NBSac is displaced toward the picoline, almost at a median position between the two nitrogen atoms, N and N', with NBr and BrN' distances at 2.073(6) and 2.098(6) Å respectively. This extreme situation contrasts with the analogous iodine derivative, NISac·Pic, where the N-I and IN' distances amount to 2.223(4) and 2.301(4) Å respectively. Periodic DFT calculations, and molecular calculations of adducts (PBEPBE-D2 aug-cc-pVTZ) either at the experimental frozen geometry or with optimization of the halogen position, indicate a more important degree of covalency (i.e. shared-shell character) in the adduct formed with the bromine atom. A stronger charge transfer to the picoline is also found for the bromine (+0.27 |e|) than for the iodine (+0.18 |e|) system. This inversion of halogen bond strength between I and Br finds its origin in the strong covalent character of the interaction in these adducts, in line with the strength of covalent N-Br and N-I bonds. Detailed characterization of the critical points (CPs) of the L(r) = -∇ρ(r) function along bonding directions has permitted the adducts to be distinguished and they can be respectively described as "neutral" NISac/Pic and "intermediate" NSac/Br/Pic, the latter with Br being close to formal equivalent NBr and BrN' interactions but still more associated to the XB donor than to the picoline, as indicated by the topological and energetic properties of the ρ(r) function at the bond critical points (BCPs).

Quantum Crystallography: Current Developments and Future Perspectives

“…In the widely used QTAIM theory, [193] this kind of analysish as been deeplyd eveloped for 1ðr)t oc haracterize many aspects that this functione xhibits in atoms,m olecules and their interactions. [212,213] Thus, in the polyiodide chains of the crystal structure of tyrosinium polyiodide hydrate (Figure 10 b), iodines are distinguished from iodides by the type of CPs found in their interactions with surrounding atoms. Hence, the study of the topological critical points (CP) of r 2 1 r ðÞhas indicated that charge concentration (CC) and charge depletion (CD) sites found in the valence shell of atoms are drivingg eometric preferences of molecules in the solid state.…”

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

“…[8b] FourmiguØ et al have reported an almost symmetric XB between N-bromosaccharin and 4-methylpyridine with ac onsiderable charge transfer (0.27 e) to the acceptor. [9] On the other hand, Ř ezµč and de la Lande have determined that the charge transfer component, although present in XB-s,contributes only up to 10 %t othe overall energy even in strong XBs. [10] As the question of the charge transfer in XB has primarily been addressed from the computational point of view,w e have opted for experimental determination of charge density of asystem containing astrong, potentially partially covalent XB.Only ahandful of experimental charge density studies of halogen bonding have been published to date.…”

Disentanglement of orthogonal hydrogen and halogen bonds via natural orbital for chemical valence: A charge displacement analysis

“…, respectively, whereas one of the pillar of halogen bonding is that heavier halogens give stronger XBs, with numerous experimental and theoretical confirmations [2] and only one counterexample. [54] Obviously, ΔE int depends on all the interactions, therefore in this case the disentanglement of XB results to be particularly interesting.…”

“…From the analysis of the homologues NCS and NIS, something unexpected emerges. The total interaction energy, with respect to relaxed fragments (Δ E tot ), is −26.2, −27.3, and −15.8 for [ SeU ] … [NIS], [ SeU ] … [NBS], and [ SeU ] … [NCS], respectively, whereas one of the pillar of halogen bonding is that heavier halogens give stronger XBs, with numerous experimental and theoretical confirmations and only one counter‐example . Obviously, Δ E int depends on all the interactions, therefore in this case the disentanglement of XB results to be particularly interesting.…”