XB Complexes Research Articles

Is the decrease of the total electron energy density a covalence indicator in hydrogen and halogen bonds?

In this work, halogen bonding (XB) and hydrogen bonding (HB) complexes were studied with the aim of analyzing the variation of the total electronic energy density H(r b ) with the interaction strengthening. The calculations were performed at the MP2/6-311++G(2d,2p) level of approximation. To explain the nature of such interactions, the atoms in molecules theory (AIM) in conjunction with reduced variational space self-consistent field (RVS) energy decomposition analysis were carried out. Based on the local virial theorem, an equation to decompose the total electronic energy density H(r b ) in two energy densities, (-G(r b )) and 1/4∇(2)ρ(r b ), was derived. These energy densities were linked with the RVS interaction energy components. Through the connection between both decomposition schemes, it was possible to conclude that the decrease in H(r b ) with the interaction strengthening observed in the HB as well as the XB complexes, is mainly due to the increase in the attractive electrostatic part of the interaction energy and in lesser extent to the increase in its covalent character, as is commonly considered.

Complementary Selenium···Iodine Halogen Bonding and Phenyl Embraces: Cocrystals of Triphenylphosphine Selenide with Organoiodides

Se···I Halogen bonding (XB) between triphenylphosphine selenide and organoiodides: 1,2-diiodotetrafluorobenzene (1,2-F4DIB), 1,4-diiodotetrafluorobenzene (1,4-F4DIB), and tetraiodoethylene (TIE) has been used to assemble molecules into finite adducts, chains, and two-dimensional layers. Phenyl embraces involving the triphenylphosphine groups extend these moieties into additional dimensions, leading in two cases (1,4-F4DIB and TIE) to open structures, with filled or partially filled cavities. Comparison of structural parameters for 26 reported zigzag chains of embracing Ph3XY reveals that this synthon can adapt its registry to accommodate widely diverse structures. The strengths of the XB interactions were analyzed using natural bond orbital (NBO) theory, which provides an estimate of the energy of a donor–acceptor interaction (ΔEd→a) through localization of the molecular orbitals. Structures of 1:1 and 1:2 XB complexes of Ph3P═Se with 1,2-F4DIB and 1,4-F4DIB were optimized at the DFT(B97-1)/BSI level. The bond distance and angles obtained are in reasonably good agreement with the experimentally observed structures.

Halogen Bonding-Based “Catch and Release”: Reversible Solid-State Entrapment of Elemental Iodine with Monoalkylated DABCO Salts

The halogen bonding (XB) between elemental iodine (I2) and neutral 1,4-diazabicyclo[2.2.2]octane (DABCO) and its monoalkylated PF6– salts was studied by X-ray crystallographic, thermoanalytical, and computational methods. DABCO was found to form both 1:1 and 1:2 complexes with I2 showing an exceptionally strong halogen bond (ΔEcp = −73.0 kJ/mol) with extremely short N···I distance (2.37 Å) in the 1:1 complex (1a). In the more favored 1:2 complex (1b), the XB interaction was found to be slightly weaker [ΔEcp = −64.4 kJ/mol and d(N···I) = 2.42 Å] as compared to 1a. The monoalkylated DABCO salts (2PF6–7PF6) form corresponding 1:1 XB complexes with I2 {[2···I2]PF6–([7···I2]PF6} similarly to the parent free base DABCO, but both X-ray diffraction and calculated (M05-2X/def2-TZVPP) geometrical parameters indicate that the XB interactions are somewhat weaker than with DABCO itself but can nonetheless be considered as moderately strong halogen bonds. The solid -state packing of the monoalkyl DABCO complexes is greatly affected by the length of the lipophilic hydrocarbon chain as the long-tail cations show increasing amphiphilic character. However, partly as a consequence of the amphiphilic nature of parent monoalkyl DABCO PF6– salts, their I2 complexes exhibit a reversible binding of I2 into their originally nonporous crystal lattices. This was verified by thermal analysis and X-ray powder diffraction studies of 2PF6–7PF6 and their corresponding I2 complexes. By varying the length of the alkyl chain, the release temperature of I2 can be tuned from 75 °C ([4···I2]PF6) to 100 °C ([7···I2]PF6). Furthermore, these highly stable (preservable for months in normal laboratory conditions) I2 complexes can be prepared with three different routes: by mixing in solution, by mechanochemical grinding of the components, and via gas-to-solid reaction (i.e., I2 vapor to solid PF6– salts).

Fluorescent Charge-Assisted Halogen-Bonding Macrocyclic Halo-Imidazolium Receptors for Anion Recognition and Sensing in Aqueous Media

The synthesis and anion binding properties of a new family of fluorescent halogen bonding (XB) macrocyclic halo-imidazolium receptors are described. The receptors contain chloro-, bromo-, and iodo-imidazolium motifs incorporated into a cyclic structure using naphthalene spacer groups. The large size of the iodine atom substituents resulted in the isolation of anti and syn conformers of the iodo-imidazoliophane, whereas the chloro- and bromo-imidazoliophane analogues exhibit solution dynamic conformational behavior. The syn iodo-imidazoliophane isomer forms novel dimeric isostructural XB complexes of 2:2 stoichiometry with bromide and iodide anions in the solid state. Solution phase DOSY NMR experiments indicate iodide recognition takes place via cooperative convergent XB-iodide 1:1 stoichiometric binding in aqueous solvent mixtures. (1)H NMR and fluorescence spectroscopic titration experiments with a variety of anions in the competitive CD(3)OD/D(2)O (9:1) aqueous solvent mixture demonstrated the bromo- and syn iodo-imidazoliophane XB receptors to bind selectively iodide and bromide respectively, and sense these halide anions exclusively via a fluorescence response. The protic-, chloro-, and anti iodo-imidazoliophane receptors proved to be ineffectual anion complexants in this aqueous methanolic solvent mixture. Computational DFT and molecular dynamics simulations corroborate the experimental observations that bromo- and syn iodo-imidazoliophane XB receptors form stable cooperative convergent XB associations with bromide and iodide.

Neutral Organometallic Halogen Bond Acceptors: Halogen Bonding in Complexes of PCPPdX (X = Cl, Br, I) with Iodine (I(2)), 1,4-Diiodotetrafluorobenzene (F4DIBz), and 1,4-Diiodooctafluorobutane (F8DIBu).

The behavior of a sterically crowded neutral pincer {2,6-bis[(di-t-butylphosphino)methyl]-phenyl}palladium (PCPPd) halides, PCPPdX (X = Cl, Br or I), as XB acceptors with strong halogen bond (XB) donors, iodine (I2), 1,4-diiodotetrafluorobenzene (F4DIBz), and 1,4-diiodooctafluorobutane (F8DIBu) were studied in the solid state. The co-crystallization experiments afforded high-quality single crystals of XB complexes PCPPdCl–I2 (1a), PCPPdBr–I2 (2a), PCPPdI–I2(3a), PCPPdCl–F4DIBz (1b), PCPPdBr–F4DIBz (2b), and PCPPdBr–F8DIBu (2c). The 1:1 iodine complexes (1a, 2a, and 3a) all showed a strong halogen bonding interaction, the reduction of the sum of the van der Waals radii of halogen to iodine being 24.6 (1a), 23.9 (2a), and 19.4% (3a) with X···I–I angles of 177, 176, and 179°, respectively. While the pincer palladium chloride 1 and bromide 2 were crystallographically isomorphous and showed similar XB behavior, the palladium iodide complex, 3, exhibited markedly different properties, and unlike 1 and 2 it does not, under similar conditions, result in XB complexes with the weaker XB donors F4DIBz and F8DIBu. The results indicate that PCPPdI is not nucleophilic enough to have XB interactions with other donors than iodine. However, the weaker XB donors F4DIBz and F8DIBu form XB complexes with the chloride 1 and especially with the bromide 2. The prevalence of the halogen bonding with 2 is probably not only electronic in origin, and it seems to offer the best balance between electron poorness and steric availability. The XB interactions with F4DIBz and F8DIBu are much weaker than with iodine, the reduction of the sum of the van der Waals radii of halogen to iodine being 13.5, 12.3, and 14.6% with C–I···X angles between 163 and 179° for 1b, 2b, and 2c, respectively, and results in polymeric (···1···F4DIBz···1···F4DIBz···)n, (···2···F4DIBz···2···F4DIBz···)n, and (···2···F8DIBu···2···F8DIBu···)n one-dimensional zigzag chains in the solid state.