Probing Halogen Bonds by Scalar Couplings.

As halogen bonding is a weak, transient interaction, its description in solution is challenging. We demonstrate that scalar coupling constants (J) are modulated by halogen bonding. The binding-induced magnitude change of one-bond couplings, even up to five bonds from the interaction site, correlates to the interaction strength. We demonstrate this using the NMR data of 42 halogen-bonded complexes in dichloromethane solution and by quantum chemical calculations. Our observation puts scalar couplings into the toolbox of methods for characterization of halogen bond complexes in solution and paves the way for their applicability for other types of weak σ-hole interactions.

Halogen Bonding: An Odd Chemistry?

Halogen bonding is a flourishing field of research, but has for long been little recognized. The same goes for its scientific hero, Odd Hassel, who laid the foundations for all current developments. The crystallographic observation of halogen-oxygen interatomic distances shorter than the sum of the van der Waals radii of the involved atoms, and the interpretation of this phenomenon as a charge-transfer interaction, have been ground-breaking. Today, charge-transfer to a polarized halogen is not any longer seen as "odd", but is commonly referred to as halogen bonding, and is widely exploited in chemistry. Despite the recognition of Hassel's work with a Nobel prize in 1969, surprisingly little appreciation is given to date to the devoted scientist, who established a world-leading laboratory during one of the darkest eras of history. Herein, we wish to revive the legacy and highlight the impact of Odd Hassel's ground-breaking discoveries.

Halogen bonds of halonium ions.

Due to their electron deficiency, halonium ions act as particularly strong halogen bond donors. By accepting electrons in both lobes of their empty p-orbital, they are capable of simultaneously interacting with two Lewis bases. The interaction presumes the formation of three molecular orbitals and is accordingly typically entitled as a three-center halogen bond. In analogy to the [D-H-D]+ hydrogen bonds, which are at times entitled as short and strong bonds, the [D-X-D]+ halogen bonds of halonium ions show Bondi normalized interatomic distances of 0.6-0.7 and possess both charge transfer and electrostatic characteristics. The three-center halogen bond of halonium ions shows distinct differences in its properties from coordinative bonds of transition metals and is therefore applicable as a complementary synthon in supramolecular chemistry. The three-center halogen bond modulates the reactivity of halonium ions and is hence a useful tool for synthetic organic chemistry. Following the discussion of the nature and properties of halonium ions' halogen bonds, this tutorial review provides an overview of their current applications to stimulate future developments.

Fabrication of Porous Hydrogenation Catalysts by a Selective Laser Sintering 3D Printing Technique.

Three-dimensional selective laser sintering printing was utilized to produce porous, solid objects, in which the catalytically active component, Pd/SiO2, is attached to an easily printable supporting polypropylene framework. Physical properties of the printed objects, such as porosity, were controlled by varying the printing parameters. Structural characterization of the objects was performed by helium ion microscopy, scanning electron microscopy, and X-ray tomography, and the catalytic performance of the objects was tested in the hydrogenation of styrene, cyclohexene, and phenylacetylene. The results show that the selective laser sintering process provides an alternative and effective way to produce highly active and easily reusable heterogeneous catalysts without significantly reducing the catalytic efficiency of the active Pd/SiO2 component. The ability to control the size, porosity, mechanical properties, flow properties, physical properties, and chemical properties of the catalyst objects opens up possibilities to optimize devices for different reaction environments including batch reactions and continuous flow systems.

Surprising solvent-induced structural rearrangements in large [N···I+···N] halogen-bonded supramolecular capsules: an ion mobility-mass spectrometry study.

Coordinative halogen bonds have recently gained interest for the assembly of supramolecular capsules. Ion mobility-mass spectrometry and theoretical calculations now reveal the well-defined gas-phase structures of dimeric and hexameric [N···I+···N] halogen-bonded capsules with counterions located inside their cavities as guests. The solution reactivity of the large hexameric capsule shows the intriguing solvent-dependent equilibrium between the hexamer and an unprecedented pentameric [N···I+···N] halogen-bonded capsule, when the solvent is changed from chloroform to dichloromethane. The intrinsic flexibility of the cavitands enables this novel structure to adopt a pseudo-trigonal bipyramidal geometry with nine [N···I+···N] bonds along the edges and two pyridine binding sites uncomplexed.

Self-Complementary Dimers of Oxalamide-Functionalized Resorcinarene Tetrabenzoxazines.

Self-complementarity is a useful concept in supramolecular chemistry, molecular biology and polymeric systems. Two resorcinarene tetrabenzoxazines decorated with four oxalamide groups were synthesized and characterized. The oxalamide groups possessed self-complementary hydrogen bonding sites between the carbonyls and amide groups. The self-complementary nature of the oxalamide groups resulted in self-included dimeric assemblies. The hydrogen bonding interactions within the tetrabenzoxazines gave rise to the formation of dimers, which were confirmed by single-crystal X-ray diffractions analysis and supported by NMR spectroscopy and mass spectrometry. The self-included dimers were connected by numerous and strong intermolecular N-H⋅⋅⋅O and C-H⋅⋅⋅O hydrogen bonds supplemented with C-H⋅⋅⋅π interactions, forming one-dimensional polymers, which were then further linked into three-dimensional networks.

Tetrameric and Dimeric [N⋠⋠⋠I+ ⋠⋠⋠N] Halogen-Bonded Supramolecular Cages.

Tripodal N-donor ligands are used to form halogen-bonded assemblies via structurally analogous Ag+ -complexes. Selective formation of discrete tetrameric I6 L4 and dimeric I3 L2 halonium cages, wherein multiple [N⋅⋅⋅I+ ⋅⋅⋅N] halogen bonds are used in concert, can be achieved by using sterically rigidified cationic tris(1-methyl-1-azonia-4-azabicyclo[2.2.2]octane)-mesitylene ligand, L1(PF6 )3 , and flexible ligand 1,3,5-tris(imidazole-1-ylmethyl)-2,4,6-trimethylbenzene, L2, respectively. The iodonium cages, I6 L14 (PF6 )18 and I3 L22 (PF6 )3 , were obtained through the [N⋅⋅⋅Ag+ ⋅⋅⋅N]→ [N⋅⋅⋅I+ ⋅⋅⋅N] cation exchange reaction between the corresponding Ag6 L14 (PF6 )18 and Ag3 L22 (PF6 )3 coordination cages, prepared as intermediates, and I2 . The synthesized metallo- and halonium cages were studied in solution by NMR, in gas phase by ESI-MS and in the solid-state by single crystal X-ray diffraction.

[N⋠⋠⋠I+ ⋠⋠⋠N] Halogen-Bonded Dimeric Capsules from Tetrakis(3-pyridyl)ethylene Cavitands.

Two [N⋅⋅⋅I+ ⋅⋅⋅N] halogen-bonded dimeric capsules using tetrakis(3-pyridyl)ethylene cavitands with different lower rim alkyl chains are synthesized and analyzed in solution and the gas phase. These first examples of symmetrical dimeric capsules making use of the iodonium ion (I+ ) as the main connecting module are characterized by 1 H NMR spectroscopy, diffusion ordered NMR spectroscopy (DOSY), electrospray ionization mass spectrometry (ESI-MS), and ion mobility-mass spectrometry (TW-IMS) experiments. The synthesis and effective halogen-bonded dimerization proceeds through analogous dimeric capsules with [N⋅⋅⋅Ag+ ⋅⋅⋅N] binding motifs as the intermediates as evidenced by the X-ray structures of (CH2 Cl2 )2 @[3 a2 ⋅Ag4 ⋅(H2 O)2 ⋅OTs4 ] and (CH2 Cl2 )2 @[3 a2 ⋅Ag4 ⋅(H2 O)4 ⋅OTs4 ], two structurally different capsules.