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

Halogen-Bonded Organic Frameworks (XOFs) Based on N⋅⋅⋅Br+⋅⋅⋅N Bonds: Robust Organic Networks Constructed by Fragile Bonds

Organic frameworks face a trade-off between the framework stability and the bond dynamics, which necessitates the development of innovative linkages that can generate stable frameworks without hindering efficient synthesis. Although iodine(I)-based halogen-bonded organic frameworks (XOFs) have been developed, constructing XOFs based on bromine(I) is desirable yet challenging due to the high sensitivity of bromine(I) species. In this work, we present the inaugural construction of stable bromine(I)-bridged two-dimensional (2D) halogen-bonded organic frameworks, XOF(Br)-TPy-BF4/OTf, based on sensitive [N⋅⋅⋅Br⋅⋅⋅N]+ halogen bonds. The formation of XOF(Br)-TPy-BF4/OTf was monitored by 1H NMR, XPS, IR, SEM, TEM, HR-TEM, SEAD. Their framework structures were established by the results from PXRD, theoretical simulations and SAXS. More importantly, XOF(Br) displayed excellent chemical and thermal stabilities. They exhibited stable two-dimensional framework structures in various organic solvents and aqueous media, even over a wide pH range (pH 3-12), while the corresponding model compounds BrPy2BF4/OTf decomposed quickly even in the presence of minimal water. Furthermore, the influence of the counterions were investigated by replacing BF4 with OTf, which improved the stability of XOF(Br). This characteristic enabled XOF(Br) to serve as an efficient oxidizing reagent in aqueous environments, in contrast with the sensitivity of BrPy2BF4/OTf, which performed well only in organic media. This study not only deepens our fundamental understanding of organic frameworks but also opens new avenues for the development and application of multifunctional XOFs.

Structural studies of supramolecular complexes and assemblies by ion mobility mass spectrometry

Recent advances in instrumentation and development of computational strategies for ion mobility mass spectrometry (IM-MS) studies have contributed to an extensive growth in the application of this analytical technique to comprehensive structural description of supramolecular systems. Apart from the benefits of IM-MS for interrogation of intrinsic properties of noncovalent aggregates in the experimental gas-phase environment, its merits for the description of native structural aspects, under the premises of having maintained the noncovalent interactions innate upon the ionization process, have attracted even more attention and gained increasing interest in the scientific community. Thus, various types of supramolecular complexes and assemblies relevant for biological, medical, material, and environmental sciences have been characterized so far by IM-MS supported by computational chemistry. This review covers the state-of-the-art in this field and discusses experimental methods and accompanying computational approaches for assessing the reliable three-dimensional structural elucidation of supramolecular complexes and assemblies by IM-MS.

Halogen Bonds of Halogen(I) Ions─Where Are We and Where to Go?

Halenium ions, X+, are particularly strong halogen-bond donors that interact with two Lewis bases simultaneously to form linear [D···X···D]+-type halonium complexes. Their three-center, four-electron halogen bond is both fundamentally interesting and technologically valuable as it tames the reactivity of halogen(I) ions, opening up new horizons in a variety of fields including synthetic organic and supramolecular chemistry. Understanding this bonding situation enables the development of improved halogen(I) transfer reactions and of advanced functional materials. Following a decade of investigations of basic principles, the range of applications is now rapidly widening. In this Perspective, we assess the status of the field and identify its key advances and the main bottlenecks. Clearing common misunderstandings that may hinder future progress, we aim to inspire and direct future research efforts.

Encapsulation in Charged Droplets Generates Distorted Host-Guest Complexes

The ability of various hydrogen-bonded resorcinarene-based capsules to bind α,ω-alkylbisDABCOnium (DnD) guests of different lengths was investigated in solution and in the gas-phase. While no host-guest interactions were detected in solution, encapsulation could be achieved in the charged droplets formed during electrospray ionisation (ESI). This included guests, which are far too long in their most stable conformation to fit inside the cavity of the capsules. A combination of three mass spectrometric techniques, namely, collision-induced dissociation, hydrogen/deuterium exchange, and ion-mobility mass spectrometry, together with computational modelling allow us to determine the binding mode of the DnD guests inside the cavity of the capsules. Significant distortions of the guest into horseshoe-like arrangements are required to optimise cation-π interactions with the host, which also adopt distorted geometries with partially open hydrogen-bonding seams when binding longer guests. Such quasi "spring-loaded" capsules can form in the charged droplets during the ESI process as there is no competition between guest encapsulation and ion pair formation with the counterions that preclude encapsulation in solution. The encapsulation complexes are sufficiently stable in the gas-phase - even when strained - because non-covalent interactions significantly strengthen in the absence of solvent.

Structure and Bonding of Halonium Compounds

The geometrical parameters and the bonding in [D···X···D]⁺ halonium compounds, where D is a Lewis base with N as the donor atom and X is Cl, Br, or I, have been investigated through a combined structural and computational study. Cambridge Structural Database (CSD) searches have revealed linear and symmetrical [D···X···D]⁺ frameworks with neutral donors. By means of density functional theory (DFT), molecular electrostatic potential (MEP), and energy decomposition analyses (EDA) calculations, we have studied the effect of various halogen atoms (X) on the [D···X···D]⁺ framework, the effect of different nitrogen-donor groups (D) attached to an iodonium cation (X = I), and the influence of the electron density alteration on the [D···I···D]⁺ halonium bond by variation of the R substituents at the N-donor upon the symmetry, strength, and nature of the interaction. The physical origin of the interaction arises from a subtle interplay between electrostatic and orbital contributions (σ-hole bond). Interaction energies as high as 45 kcal/mol suggest that halonium bonds can be exploited for the development of novel halonium transfer agents, in asymmetric halofunctionalization or as building blocks in supramolecular chemistry.

[N···I···N]+ Type Halogen-Bonding-Driven Supramolecular Helical Polymers with Modulated Chirality

The [N···I···N]⁺ type halogen bond has been utilized to synthesize supramolecular architectures, while the applications in constructing helical motifs and modulating supramolecular chirality have been unexplored so far. In this work, the [N···I···N]⁺ halogen bond was introduced to drive the formation of supramolecular helical polymers via a Ag(I) coordination intermediate, showing tunable supramolecular chirality. Pyridine segments were conjugated to the asymmetric ferrocene skeleton, which show "open" and "closed" geometry depending on the sp² N positions. Coordination with Ag(I) generated one-dimensional (1D) double helices and 2D helicates featured the [Ag(O)···I···Ag(O)]⁺ bond, which further stacked into 3D porous frameworks with chiral channels and adjustable pore sizes. Ionic exchange afforded 1D supramolecular helical polymers in solution phases driven by the [N···I···N]⁺ type halogen bonds, which was evidenced by the experimental results and density functional theory calculation. Fc2 exclusively demonstrated tunable supramolecular chirality in the formation of coordinated and halogen bonded polymers. In addition, solvent change would further inverse the helicity of halogen bonded supramolecular helical polymers depending on the rotation of the ferrocenyl core whose "closed" and "open" states were accompanied by the breakage of intramolecular hydrogen bonds. This work introduces a [N···I···N]⁺ type ionic halogen bond to prepare supramolecular helical polymers, providing multiple protocols in regulating helicity by ion exchange and solvent environments.

Impact of Host Flexibility on Selectivity in a Supramolecular Host-Catalyzed Enantioselective aza-Darzens Reaction

A highly enantioselective aza-Darzens reaction (up to 99% ee) catalyzed by an enantiopure supramolecular host has been discovered. To understand the role of host structure on reaction outcome, nine new gallium(III)-based enantiopure supramolecular assemblies were prepared via substitution of the external chiral amide. Despite the distal nature of the substitution in these catalysts, changes in enantioselectivity (61 to 90% ee) in the aziridine product were observed. The enantioselectivities were correlated to the flexibility of the supramolecular host scaffold as measured by the kinetics of exchange of a model cationic guest. This correlation led to the development of a best-in-class catalyst by substituting the gallium(III)-based host with one based on indium(III), which generated the most flexible and selective catalyst.

Supramolecular networks by imine halogen bonding

Halogen bonding of neutral donors using imine groups of porous organic cage compounds as acceptors leads to the formation of halogen-bonded frameworks. We report the use of two different imine cages, in combination with three electron-poor halogen bond donors. Four resulting solid-state structures elucidated by single-crystal X-ray analysis are presented and analysed for the first time by plane-wave DFT calculations and QTAIM-analyses of the entire unit cells, demonstrating the formation of halogen bonds within the networks. The supramolecular frameworks can be obtained either from solution or mechanochemically by liquid-assisted grinding.

Cagearenes: synthesis, characterization, and application for programmed vapor release

Here, we announce the establishment of a new family of organic molecular cages, named cagearenes, by taking advantage of a versatile strategy. These cagearenes were prepared via the Friedel–Crafts reaction by condensing two equivalents of a precursor bearing three 1,4-dimethoxybenzene groups and three equivalents of formaldehyde. Two cages, namely cagearene-1 and cagearene-2, are obtained and well characterized. The cagearene-1 solid exhibits the ability to adsorb benzene vapour from an equimolar benzene/cyclohexane mixture with a purity of 91.1%. Then, the adsorbed benzene molecules can be released from the cage at a relatively lower temperature, namely 70 °C, as a consequence of which, cyclohexane with a high purity was left within the cage solid. Heating the cage solid further at 130 °C led to the production of cyclohexane with a purity up to 98.7%. As inferred from the single crystal structures and theoretical calculations, the ability of the cage in programmed release of benzene and cyclohexane results from the different binding modes of these two guests.

Two organic cages, cagearene-1 and cagearene-2, are prepared. The cagearene-1 solid selectively absorbs benzene vapor from a benzene/cyclohexane mixture and is used to achieve temperature-controlled programmed vapor release.

Hierarchical study of mono- and multicharged imidazolium encapsulation in p-sulfonatocalix[4]arene molecular capsules

Four ternary multi-component crystal structures comprising water-soluble p-sulfonatocalix[4]arene anion as the cavitand in the presence of different types of positively charged guest molecules comprising 1-butylimidazolium, 3,3'-(1,4-phenylenebis(methylene))bis(1-methylimidazolium), 3,3',3''-(benzene-1,3,5-triyltris(methylene))tris(1-methylimidazolium), or 3,3',3'',3'''-(benzene-1,2,4,5-tetrayltetrakis(methylene))tetrakis(1-methylimidazolium) and...

Dimeric iodine(i) and silver(i) cages from tripodal N-donor ligands via the [N–Ag–N]+ to [N–I–N]+ cation exchange reaction

Complexation of tripodal ligands with silver(I) salts generated M3L2 cage complexes that encapsulated anions within their cavities. Subsequent [N–Ag–N]+ to [N–I–N]+ cation exchange with I2 resulted in the corresponding halogen-bonded iodine(I) cages.

Macrocyclic complexes based on [N⋯I⋯N]+ halogen bonds

New 1-2 nm macrocyclic iodine(I) complexes prepared VIA a simple ligand exchange reaction manifest rigid 0.5-1 nm cavities that bind the hexafluorophosphate anion in the gas phase. The size of the cavities and the electrostatic interactions with the iodine(I) cations influence the anion binding properties of these macrocyclic complexes.

Are bis(pyridine)iodine(I) complexes applicable for asymmetric halogenation?

Enantiopure halogenated molecules are of tremendous importance as synthetic intermediates in the construction of pharmaceuticals, fragrances, flavours, natural products, pesticides, and functional materials. Enantioselective halofunctionalizations remain poorly understood and generally applicable procedures are lacking. The applicability of chiral trans-chelating bis(pyridine)iodine(i) complexes in the development of substrate independent, catalytic enantioselective halofunctionalization has been explored herein. Six novel chiral bidentate pyridine donor ligands have been designed, routes for their synthesis developed and their [N–I–N]⁺-type halogen bond complexes studied by ¹⁵N NMR and DFT. The chiral complexes encompassing a halogen bond stabilized iodenium ion are shown to be capable of efficient iodenium transfer to alkenes; however, without enantioselectivity. The lack of stereoselectivity is shown to originate from the availability of multiple ligand conformations of comparable energies and an insufficient steric influence by the chiral ligand. Substrate preorganization by the chiral catalyst appears a necessity for enantioselective halofunctionalization.

The enantioselectivity of the iodine(i) transfer process from chiral bis(pyridine)iodine(i) complexes to alkenes is explored.

The Influence of Secondary Interactions on the [N-I-N]+ Halogen Bond

[Bis(pyridine)iodine(I)]⁺ complexes offer controlled access to halonium ions under mild conditions. The reactivity of such stabilized halonium ions is primarily determined by their three-center, four-electron [N-I-N]⁺ halogen bond. We studied the importance of chelation, strain, steric hindrance and electrostatic interaction for the structure and reactivity of halogen bonded halonium ions by acquiring their ¹⁵ N NMR coordination shifts and measuring their iodenium release rates, and interpreted the data with the support of DFT computations. A bidentate ligand stabilizes the [N-I-N]⁺ halogen bond, decreasing the halenium transfer rate. Strain weakens the bond and accordingly increases the release rate. Remote modifications in the backbone do not influence the stability as long as the effect is entirely steric. Incorporating an electron-rich moiety close by the [N-I-N]⁺ motif increases the iodenium release rate. The analysis of the iodine(I) transfer mechanism highlights the impact of secondary interactions, and may provide a handle on the induction of stereoselectivity in electrophilic halogenations.

Elastic Collision Based Dynamic Partitioning Scheme for Hybrid Simulations

In hybrid simulations, such as the QM/MM approach, the system is partitioned into regions that are treated at different levels of theory. The key question then becomes how to evaluate the interactions between particles on opposite sides of the boundary. One approach is to place the boundary in such a way that particles near the boundary on both sides are of the same type, thus simplifying the evaluation of the interactions. If mobile particles are present, such as solvent molecules, and particles are allowed to cross the boundary, the conservation of energy and atomic forces is problematic unless the computational effort is increased significantly. By preventing particles from crossing the boundary but allowing the boundary to be flexible, an accurate estimate of average thermodynamic properties is obtained in principle as illustrated by the flexible inner region ensemble separator (FIRES) method [C. Rowley and B. Roux, J. Chem. Theory Comput. 2012, 8, 3526]. In FIRES, a harmonic restraint is applied to particles near the boundary. Therefore, it can occur that particle cross the boundary to some extent resulting in anomalies in the particle density. Here, a constraint approach is presented where particles instantaneously scatter from the boundary. This scattering-adapted FIRES (SAFIRES) implementation makes use of a variable-time-step propagation algorithm where the time step is scaled automatically to identify the moment a collision should occur. If the length of the time step is kept constant, this propagator reduces to a regular Langevin dynamics algorithm, and to the velocity Verlet algorithm for conservative dynamics if the friction coefficient is set to zero. Correct average ensemble statistics are obtained as demonstrated in simulations where, for testing purposes, the particles in the two regions are treated at the same level of theory, namely, a homogeneous Lennard-Jones (LJ) liquid and liquid water based on the TIP4P potential function. In order to illustrate this approach in solid-liquid interface simulations, a LJ liquid in contact with the surface of a crystal is also simulated. The simulations using SAFIRES are shown to reproduce the unconstrained reference simulations without significant deviations in the particle density and the dynamics are shown to conserve energy when coupling to the heat bath is turned off.

Carbonyl Hypoiodites as Extremely Strong Halogen Bond Donors

Neutral halogen-bonded O-I-N complexes were prepared from in situ formed carbonyl hypoiodites and aromatic organic bases. The carbonyl hypoiodites have a strongly polarized iodine atom with larger σ-holes than any known uncharged halogen bond donor. Modulating the Lewis basicity of the selected pyridine derivatives and carboxylates leads to halogen-bonded complexes where the classical O-I⋅⋅⋅N halogen bond transforms more into a halogen-bonded COO- ⋅⋅⋅I-N+ ion-pair (salt) with an asymmetric O-I-N moiety. X-ray analyses, NMR studies, and calculations reveal the halogen bonding geometries of the carbonyl hypoiodite-based O-I-N complexes, confirming that in the solid-state the iodine atom is much closer to the N-atom of the pyridine derivatives than its original position at the carboxylate O-atom.

Iodonium complexes of the tertiary amines quinuclidine and 1-ethylpiperidine

Iodonium complexes incorporating tertiary amines have been synthesised to study and explore why such species comprised of alkyl amines are relatively rare. The complexes were characterised in solution (1H and 15N NMR spectroscopy) and the solid state (SCXRD), and analysed computationally.

Reaction Monitoring and Structural Characterisation of Coordination Driven Self-Assembled Systems by Ion Mobility-Mass Spectrometry

Nature creates exquisite molecular assemblies, required for the molecular-level functions of life, via self-assembly. Understanding and harnessing these complex processes presents an immense opportunity for the design and fabrication of advanced functional materials. However, the significant industrial potential of self-assembly to fabricate highly functional materials is hampered by a lack of knowledge of critical reaction intermediates, mechanisms, and kinetics. As we move beyond the covalent synthetic regime, into the domain of non-covalent interactions occupied by self-assembly, harnessing and embracing complexity is a must, and non-targeted analyses of dynamic systems are becoming increasingly important. Coordination driven self-assembly is an important subtype of self-assembly that presents several wicked analytical challenges. These challenges are "wicked" due the very complexity desired confounding the analysis of products, intermediates, and pathways, therefore limiting reaction optimisation, tuning, and ultimately, utility. Ion Mobility-Mass Spectrometry solves many of the most challenging analytical problems in separating and analysing the structure of both simple and complex species formed via coordination driven self-assembly. Thus, due to the emerging importance of ion mobility mass spectrometry as an analytical technique tackling complex systems, this review highlights exciting recent applications. These include equilibrium monitoring, structural and dynamic analysis of previously analytically inaccessible complex interlinked structures and the process of self-sorting. The vast and largely untapped potential of ion mobility mass spectrometry to coordination driven self-assembly is yet to be fully realised. Therefore, we also propose where current analytical approaches can be built upon to allow for greater insight into the complexity and structural dynamics involved in self-assembly.

Gas-Phase Structural Analysis of Supramolecular Assemblies

Ion mobility spectrometry and gas-phase IR action spectroscopy are two structure-sensitive mass-spectrometric methods becoming more popular recently. While ion mobility spectrometry provides collision cross sections as a size and shape dependent parameter of an ion of interest, gas-phase spectroscopy identifies functional groups and is capable of distinguishing different isomers. Both methods have recently found application for the investigation of supramolecular assemblies. We here highlight several aspects.Starting with the characterization of switching states in azobenzene photoswitches as well as redox-switchable lasso-type pseudorotaxanes, structures of isomers can be distinguished and mechanistic details analyzed. Ion mobility mass spectrometry in combination with gas-phase H/D-exchange reactions unravels subtle structural details as described for the chiral recognition of crown ether amino acid complexes. Gas-phase IR spectroscopy allows identification of details of the binding patterns in dimeric amino acid clusters as well as the serine octamer. This research can be extended into the analysis of peptide assemblies that are of medical relevance, for example, in Alzheimer's disease, and into a general hydrophobicity scale for natural as well as synthetic amino acids. The development of ultracold gas-phase spectroscopy that for example makes use of ions trapped in liquid helium droplets provides access to very well resolved spectra. The combination of ion mobility separation of ions with subsequent spectroscopic analysis even permits separation of different isomers and studying them separately with respect to their structure. This represents a great advantage of these gas-phase methods over solution experiments, in which the supramolecular complexes under study typically equilibrate and thus prevent a separate investigation of different isomers. At the end of this overview, we will discuss larger and more complex supramolecules, among them giant halogen-bonded cages and complex intertwined topologies such as molecular knots and Solomon links.

Dimeric capsules self-assembled through halogen and chalcogen bonding

Supramolecular capsules are nanoscale containers useful for the study of molecular behavior in confined spaces. They offer practical applications in catalysis, molecular transport, drug delivery, and materials science. Self-assembly has proven to be an effective approach in constructing supramolecular capsules and a variety of well-known noncovalent interactions including hydrogen bonding, metal coordination and ionic interactions have been deployed. Bowl-shaped cavitand structures built up from resorcinarenes have particular advantages for the construction of supramolecular capsules, and this highlight discusses the unconventional self-assembly of molecular capsules held together by halogen and chalcogen bonds.

Halogen Bond of Halonium Ions: Benchmarking DFT Methods for the Description of NMR Chemical Shifts

Because of their anisotropic electron distribution and electron deficiency, halonium ions are unusually strong halogen-bond donors that form strong and directional three-center, four-electron halogen bonds. These halogen bonds have received considerable attention owing to their applicability in supramolecular and synthetic chemistry and have been intensely studied using spectroscopic and crystallographic techniques over the past decade. Their computational treatment faces different challenges to those of conventional weak and neutral halogen bonds. Literature studies have used a variety of wave functions and DFT functionals for prediction of their geometries and NMR chemical shifts, however, without any systematic evaluation of the accuracy of these methods being available. In order to provide guidance for future studies, we present the assessment of the accuracy of 12 common DFT functionals along with the Hartree-Fock (HF) and the second-order Møller-Plesset perturbation theory (MP2) methods, selected from an initial set of 36 prescreened functionals, for the prediction of 1H, 13C, and 15N NMR chemical shifts of [N-X-N]+ halogen-bond complexes, where X = F, Cl, Br, and I. Using a benchmark set of 14 complexes, providing 170 high-quality experimental chemical shifts, we show that the choice of the DFT functional is more important than that of the basis set. The M06 functional in combination with the aug-cc-pVTZ basis set is demonstrated to provide the overall most accurate NMR chemical shifts, whereas LC-ωPBE, ωB97X-D, LC-TPSS, CAM-B3LYP, and B3LYP to show acceptable performance. Our results are expected to provide a guideline to facilitate future developments and applications of the [N-X-N]+ halogen bond.

O-I-O halogen bond of halonium ions

The reactivity of halonium ions is conveniently modulated by three-center, four-electron halogen bonds. Such stabilized halonium complexes are valuable reagents for oxidations and halofunctionalization reactions. We report the first example of the stabilization of a halenium ion in a three-center, four-electron halogen bond with two oxygen ligands. The influence of electron density and solvent on the stability of the complexes is assessed. O-I-O halogen bond complexes are applicable as synthetic reagents and as supramolecular synthons.

Guest Binding Drives Host Redistribution in Libraries of CoII 4 L4 Cages

Two CoII 4 L4 tetrahedral cages prepared from similar building blocks showed contrasting host-guest properties. One cage did not bind guests, whereas the second encapsulated a series of anions, due to electronic and geometric effects. When the building blocks of both cages were present during self-assembly, a library of five CoII LA x LB 4-x cages was formed in a statistical ratio in the absence of guests. Upon incorporation of anions able to interact preferentially with some library members, the products obtained were redistributed in favor of the best anion binders. To quantify the magnitudes of these templation effects, ESI-MS was used to gauge the effect of each template upon library redistribution.

A porous fluorinated organic [4+4] imine cage showing CO2 and H2 adsorption

We report the first POC, containing perfluorinated aromatic panels forming quickly and in high purity, despite low preorganization encoded in the starting materials.

Anion templated crystal engineering of halogen bonding tripodal tris(halopyridinium) compounds

Crystal engineering of halogen bonding tripodal receptors is found to be highly dependent on solvent and choice of anion.

Asymmetric [N–I–N]+ halonium complexes

The first examples of unrestrained asymmetric silver(i) and halonium complexes have been prepared and characterised.

Halogen bonds of halonium ions

Halonium ions are particularly strong halogen bond donors, and are accordingly valuable tools for a variety of fields, such as supramolecular and synthetic organic chemistry.

Fluorinated azobenzenes as supramolecular halogen-bonding building blocks

ortho-Fluoroazobenzenes are a remarkable example of bistable photoswitches, addressable by visible light. Symmetrical, highly fluorinated azobenzenes bearing an iodine substituent in para-position were shown to be suitable supramolecular building blocks both in solution and in the solid state in combination with neutral halogen bonding acceptors, such as lutidines. Therefore, we investigate the photochemistry of a series of azobenzene photoswitches. Upon introduction of iodoethynyl groups, the halogen bonding donor properties are significantly strengthened in solution. However, the bathochromic shift of the π→π* band leads to a partial overlap with the n→π* band, making it slightly more difficult to address. The introduction of iodine substituents is furthermore accompanied with a diminishing thermal half-life. A series of three azobenzenes with different halogen bonding donor properties are discussed in relation to their changing photophysical properties, rationalized by DFT calculations.

Halogen Bonding Helicates Encompassing Iodonium Cations

The first halonium-ion-based helices were designed and synthesized using oligo-aryl/pyridylene-ethynylene backbones that fold around reactive iodonium ions. Halogen bonding interactions stabilize the iodonium ions within the helices. Remarkably, the distance between two iodonium ions within a helix is shorter than the sum of their van der Waals radii. The helical conformations were characterized by X-ray crystallography in the solid state, by NMR spectroscopy in solution and corroborated by DFT calculations. The helical complexes possess potential synthetic utility, as demonstrated by their ability to induce iodocyclization of 4-penten-1-ol.

Recommendations for reporting ion mobility Mass Spectrometry measurements

Here we present a guide to ion mobility mass spectrometry experiments, which covers both linear and nonlinear methods: what is measured, how the measurements are done, and how to report the results, including the uncertainties of mobility and collision cross section values. The guide aims to clarify some possibly confusing concepts, and the reporting recommendations should help researchers, authors and reviewers to contribute comprehensive reports, so that the ion mobility data can be reused more confidently. Starting from the concept of the definition of the measurand, we emphasize that (i) mobility values (K0 ) depend intrinsically on ion structure, the nature of the bath gas, temperature, and E/N; (ii) ion mobility does not measure molecular surfaces directly, but collision cross section (CCS) values are derived from mobility values using a physical model; (iii) methods relying on calibration are empirical (and thus may provide method-dependent results) only if the gas nature, temperature or E/N cannot match those of the primary method. Our analysis highlights the urgency of a community effort toward establishing primary standards and reference materials for ion mobility, and provides recommendations to do so. © 2019 The Authors. Mass Spectrometry Reviews Published by Wiley Periodicals, Inc.

[2+2] Halogen-bonded boxes employing azobenzenes

Herein, we report the synthesis and crystal structures of three [2+2] supramolecular boxes assembled by halogen bonding.