Strong Halogen Bonds Research Articles

On the role of halogen bond in the halophilic reaction: A theoretical study

Halophilic reactions have been modeled theoretically by employing CBr 4, Cl 3CCN, Cl 3CCOCl, CCl 4 and Cl 3CF as substrates and Cl − as a nucleophile. It was found that the formation of the strong halogen bond is a necessary but not sufficient condition for the occurring of the halophilic reaction: the strong red-shifting halogen bond facilitates the reaction whereas the strong blue-shifting halogen bond retards it. The theoretical results are in good agreement with the experimental results.

Hierarchical halogen bonding induces polymorphism

Co-crystals of 1-iodo-3,5-dinitrobenzene and 1,4-diazabicyclo[2.2.2]octane manifest either two strong or one strong and one weak intermolecular 2 : 1 halogen bond (XB) motifs in polymorphic structures I and II, respectively, whereas weaker XB-donor 4,4-bipyridine just forms 1 : 1 structure III with one strong halogen bond.

Metal Fluorides Form Strong Hydrogen Bonds and Halogen Bonds: Measuring Interaction Enthalpies and Entropies in Solution

The organometallic compound trans-(tetrafluoropyrid-2-yl)bis(triethylphosphine)-fluoronickel(II) (NiF) is shown to serve as a strong hydrogen bond and halogen bond acceptor in solution via intermolecular interactions with the fluoride ligand. The nature of the interactions has been confirmed by multinuclear NMR spectroscopy. Experimental binding constants, enthalpies, and entropies of interaction with hydrogen-bond-donor indole and halogen-bond-donor iodopentafluorobenzene have been determined by 19F NMR titration. In toluene-d8 solution indole forms a 1:1 and 2:1 complex with NiF (K1 = 57.9(3), K2 = 0.58(4)). Interaction enthalpies and entropies are -23.4(2) kJ mol-1 and -44.5(8) J mol-1 K-1, respectively, for the 1:1 complex; -14.8(8) kJ mol-1 and -53(3) J mol-1 K-1, respectively, for the 2:1 complex. In toluene-d8 solution iodopentafluorobenzene forms only a 1:1 complex (K1 = 3.41(9)) with enthalpy and entropy of interaction of -16(1) kJ mol-1 and -42(4) J mol-1 K-1, respectively. A marked solvent effect was observed for the halogen bond interaction. NMR titrations in heptane solution indicated formation of both 1:1 and 2:1 complexes of iodopentafluorobenzene with NiF (K1 = 21.8(2), K2 = 0.22(4)). Interaction enthalpies and entropies are -26(1) kJ mol-1 and -63(4) J mol-1 K-1, respectively, for the 1:1 complex; -21(1) kJ mol-1 and -83(5) J mol-1 K-1, respectively, for the 2:1 complex. There is a paucity of such experimental energetic data particularly for halogen bonds despite substantial structural data. These measurements demonstrate that halogen bonds are competitive with hydrogen bonds as intermolecular interactions and provide a suitable benchmark for theoretical calculations and quantitative input into design efforts in supramolecular chemistry and crystal engineering.

A Stepwise Mechanism for the Mechanochemical Synthesis of Halogen-Bonded Cocrystal Architectures

A molecular-level mechanism is proposed for two cases of mechanochemical cocrystallization via halogen bonds. The proposed stepwise mechanism is based on the observation and structural characterization of intermediates that appear in early stages of the reaction. The mechanism arises from the competition of strong and weak intermolecular halogen bonds of the N...I and S...I type and involves the initial formation of finite molecular assemblies, held together via N...I bonds that subsequently polymerize into infinite chains by cross-linking through S...I bonds. This mechanism has been observed in the synthesis of linear as well as zigzag halogen-bonded chain architectures.

Ab Initio Investigation of the Complexes between Bromobenzene and Several Electron Donors: Some Insights into the Magnitude and Nature of Halogen Bonding Interactions

Halogen bonding, a specific intermolecular noncovalent interaction, plays crucial roles in fields as diverse as molecular recognition, crystal engineering, and biological systems. This paper presents an ab initio investigation of a series of dimeric complexes formed between bromobenzene and several electron donors. Such small model systems are selected to mimic halogen bonding interactions found within crystal structures as well as within biological molecules. In all cases, the intermolecular distances are shown to be equal to or below sums of van der Waals radii of the atoms involved. Halogen bonding energies, calculated at the MP2/aug-cc-pVDZ level, span over a wide range, from -1.52 to -15.53 kcal/mol. The interactions become comparable to, or even prevail over, classical hydrogen bonding. For charge-assisted halogen bonds, calculations have shown that the strength decreases in the order OH- > F- > HCO2- > Cl- > Br-, while for neutral systems, their relative strengths attenuate in the order H2CS > H2CO > NH3 > H2S > H2O. These results agree with those of the quantum theory of atoms in molecules (QTAIM) since bond critical points (BCPs) are identified for these halogen bonds. The QTAIM analysis also suggests that strong halogen bonds are more covalent in nature, while weak ones are mostly electrostatic interactions. The electron densities at the BCPs are recommended as a good measure of the halogen bond strength. Finally, natural bond orbital (NBO) analysis has been applied to gain more insights into the origin of halogen bonding interactions.

Structure systematics in A3Mo 2X9, X = Cl, Br, I, from Rietveld refinement of X-ray powder data

The structures of the compounds A 3Mo 2Cl 9, A = K, NH 4, Rb, Cs, Me 4N; A 3Mo 2Br 9, A = K, Rb, Cs, Me 4N; Cs 3Mo 2I 9, and Cs 3W 2Cl 9 have been refined by Rietveld analysis of powder X-ray diffraction data. The results have been used to analyze the effect of change of the A cation on the geometry of the ( M 2 X 9) 3− complex anion, which comprises two octahedra sharing a common face. The observed increase in the M- M separation with increasing size of the A cation is interpreted in terms of a balance between attractive M- M bonding forces, which draw the metals together, and strong covalent M-terminal halogen bonds which, together with elongation of ( M 2 X 9) 3− due to packing effects, cause a net lengthening of M- M. In the series A 3Mo 2Cl 9, for example, the MoMo separation increases from 2.524(8) to 2.778(8) Å when the A cation changes from K to Me 4N.