The Nature of Halogen⋅⋅⋅Halogen Synthons: Crystallographic and Theoretical Studies

The use of polarizable [AuX2(CN)2]- (X = Br, I) building blocks toward the formation of birefringent coordination polymers

The [(n)Bu(4)N][AuX(2)(CN)(2)] (X = Br, I) salts were synthesized and structurally characterized. Both feature square-planar [AuX(2)(CN)(2)](-) anions, with trans cyano and halo ligands, which aggregate via halogen-halogen interactions. The aggregation of [AuX(2)(CN)(2)](-) units results in the parallel alignment of all of the Br-Au-Br moieties in the anions along the [110] and [110] directions. Two crystal habits of [(n)Bu(4)N][AuBr(2)(CN)(2)] were grown: with (110) and (001) as the primary faces. The birefringence in the (110) plane was found to be Δn = 0.051(4) and was <0.03 in the (001) plane. Using the [AuBr(2)(CN)(2)](-) unit, [M(phen)(2)][AuBr(2)(CN)(2)](2) (M = Fe, Ni), [Ni(terpy)(2)][AuBr(2)(CN)(2)](2), [Fe(terpy)(2)][AuBr(2)(CN)(2)][ClO(4)], and [Cu(phen)(2)(NO(3))][AuBr(2)(CN)(2)] (phen = 1,10-phenanthroline, terpy = 2,2';6',2''-terpyridine) were synthesized and structurally characterized: they formed ionic structures with coordinatively saturated metal cations and structurally aligning Br···Br interactions between the [AuBr(2)(CN)(2)](-) anions. A molecular complex, Cu(terpy)[AuBr(2)(CN)(2)](2), was prepared, as well as the coordination polymer, [Ni(en)(2)(AuBr(2)(CN)(2))][AuBr(2)(CN)(2)]·MeOH (en = ethylenediamine). The structure consists of layers of chains of Ni(en)(2)(AuBr(2)(CN)(2))(+) units and chains of unbound [AuBr(2)(CN)(2)](-) units formed via Br···Br interactions; a Δn = 0.131(3) was measured. The Δn values were related to the supramolecular structures in terms of the relative intermolecular alignment of Br-Au-Br and NC-Au-CN bonds. These measurements both demonstrate the utility of the Au-Br bonds in enhancing birefringence and show that the contribution of the M-CN units to the overall birefringence of cyanometallate coordinations polymers is non-negligible.

(E)-1,3-Bis(2,3,4,5,6-penta-fluoro-phen-yl)prop-2-en-1-one

In the title compound, C₁₅H₂F₁₀O, the two perfluorinated arene rings are tilted at an angle of 66.08 (5)° with respect to each other. The olefinic double bond adopts an E configuration and the single bond between the olefinic and carbonyl double bonds has an s-trans conformation. The carbonyl group is not in a coplanar alignment with respect to the neighbouring arene ring (0.963 Å from aryl plane) while being coplanar with regard to the olefinic double bond (0.0805 Å from olefinic bond). The crystal packing does not feature significant hydrogen-bond-type or stacking inter­actions.

4'-(2,4-Dichloro-phen-yl)-1,1'-dimethyl-piperidine-3-spiro-3'-pyrrolidine-2'-spiro-3''-indoline-4,2''-dione

In the title compound, C₂₃H₂₃Cl₂N₃O₂, the pyrroline ring adopts an envelope conformation and the piperidinone ring assumes a slightly twisted chair form. In the crystal, inversion dimers linked by pairs of N—H⋯O hydrogen bonds generate an R₂⁸ graph-set motif and a short Cl⋯Cl contact of 3.478 (1) Å occurs.

The subtle balance of weak supramolecular interactions: The hierarchy of halogen and hydrogen bonds in haloanilinium and halopyridinium salts

THE SERIES OF HALOANILINIUM AND HALOPYRIDINIUM SALTS: 4-IPhNH₃Cl (1), 4-IPhNH₃Br (5), 4-IPhNH₃H₂PO₄ (6), 4-ClPhNH₃H₂PO₄ (8), 3-IPyBnCl (9), 3-IPyHCl (10) and 3-IPyH-5NIPA (3-iodopyridinium 5-nitroisophthalate, 13), where hydrogen or/and halogen bonding represents the most relevant non-covalent interactions, has been prepared and characterized by single crystal X-ray diffraction. This series was further complemented by extracting some relevant crystal structures: 4-BrPhNH₃Cl (2, CCDC ref. code TAWRAL), 4-ClPhNH₃Cl (3, CURGOL), 4-FPhNH₃Cl (4, ANLCLA), 4-BrPhNH₃H₂PO₄, (7, UGISEI), 3-BrPyHCl, (11, CIHBAX) and 3-ClPyHCl, (12, VOQMUJ) from Cambridge Structural Database for sake of comparison. Based on the X-ray data it was possible to highlight the balance between non-covalent forces acting in these systems, where the relative strength of the halogen bonding C-X...A⁻ (X = I, Br or Cl) and the ratio between the halogen and hydrogen bonds [C-X...A⁻ : D-H...A⁻] varied across the series.

Molecular symmetry and isostructural relations in crystal phases of trihalomethanes CHCl3, CHBrCl2, CHBr2Cl, and CHBr3

Bromodichloromethane (CHBrCl(2)), dibromochloromethane (CHBr(2)Cl), and their parent trihalomethanes, chloroform (CHCl(3)) and bromoform (CHBr(3)), form an intriguing series of isostructural crystal phases, the sequence of which depends on the Br/Cl substitution and thermodynamic conditions. The phase behavior of these compounds has been studied by isobaric calorimetry and isothermal compression, and the crystal structure of CHBrCl(2) has been determined at 0.10 MPa/200 K, 0.73, 1.26, 2.53 GPa (all at 295 K), and that of CHBr(2)Cl at 0.43, 1.24 GPa (all at 295 K). CHBrCl(2) frozen by isobaric cooling at 0.10 MPa crystallizes in space group P1 with Z = 2, while its high-pressure polymorph in space group Pnma (Z = 4) is stable at 295 K from its freezing pressure at 0.48 to at least 2.53 GPa. At the freezing pressure of 0.29 GPa, CHBr(2)Cl crystallizes in space group P6(3), with Z = 2, and at 1.27 GPa, it transforms to the orthorhombic structure, space group Pnma (Z = 4); CHCl(3) has the identical symmetries, but their reverse sequence was observed. A subtle isostructural phase transition has been observed at 0.10 MPa and 214.9 K in CHBr(2)Cl. The relations between isostructural phases, their symmetry, and site occupation factors of halogen atoms observed in the low-temperature and high-pressure phases of trihalomethanes (CHCl(3), CHBrCl(2), CHBr(2)Cl, and CHBr(3)) have been explained by the directional character of electrostatic interactions between the molecules. A gradual ordering of the disordered Br and Cl atoms has been achieved in the compressed crystals, where the narrower volume of the atomic sites correlates with the increased occupancy of the smaller atom (chlorine). The molecular symmetry has been shown to control the molecular aggregation in the crystalline state, consistent with the crystal site-symmetry and the balance of electrostatic matching and dispersion forces between molecules.

2,5-Dibromopyridine

In the title compound, C₅H₃Br₂N, C—H⋯N hydrogen-bonding inter­actions and Br⋯Br inter­actions [3.9418 (3) and 3.8986 (3) Å] connect the mol­ecules into planar sheets stacked perpendicular to the b axis. In addition, pyrid­yl–pyridyl inter­sheet π–π stacking inter­actions [centroid–centroid distance = 4.12 (1) Å] result in a three-dimensional network.

Bond paths as privileged exchange channels

Evidence that the bond paths of the quantum theory of atoms-in-molecules (QTAIM) signal preferred quantum-mechanical exchange channels is presented. We show how bond paths between an atom A and the atoms B in its environment appear to be determined by competition among the A-B exchange-correlation energies that always contribute to stabilize the A-B interactions. These pairwise additive stabilizations depend neither on the attractive or repulsive nature of the classical electrostatic interaction between the atoms' charge densities, nor on the change in the self energies of the atoms involved. These other terms may well cause an overall molecular-energy increase in spite of a possibly large A-B exchange-correlation stabilization. After our proposal, bond paths, both at and out of equilibrium geometries, are endowed with a specific energetic meaning that should contribute to reconcile the orthodox QTAIM interpretation with other widely accepted views, and to settle recent controversies questioning the meaning of hydrogen-hydrogen bonding and the nature of the so-called "steric interactions", the role of bond paths in endohedral complexes, and the generality of the results provided by the QTAIM. Implications for the nature of more general closed-shell interactions are also briefly discussed.