Stefan Matthias Huber

A modular synthesis to a tetracationic halogen bonding foldamer is presented, which outperforms classical bidentate halogen bond donors in halide-abstraction.

While halogen bonding has been employed for the construction of organic frameworks, no metal‐organic frameworks (MOFs) with additional halogen bonding functionality are known to date. Here, we present a suitable ligand for such systems, which combines a polyiodinated and ‐fluorinated terphenyl core with two carboxyl groups for metal coordination. Depending on the crystallization conditions, the ligand assembles with bivalent zinc cations into three distinct crystalline systems, either: (i) a metal‐containing hydrogen‐bonded organic framework; (ii) a MOF; or (iii) one‐dimensional metal‐organic chains with intermolecular halogen bonding and hydrogen bonding between adjacent chains. The 1D chains (iii) are obtained when using carbon disulfide as the crystallization medium, underscoring the potential of uncommon solvents in the design of structurally diverse crystalline materials formed from different supramolecular interactions. This study showcases the role of noncovalent interactions and halogen chemistry in directing crystal packing and highlights the challenges associated with the preparation of porous halogen bonding MOFs.

ZUSAMMENFASSUNG: Die Einbindung ‐hybridisierter Kohlenstoffzentren in planare ‐Kohlenstoffnanostrukturen hat sich in der Lösungschemie als wirkungsvolle Strategie zur Erzeugung gewünschter elektronischer und struktureller Eigenschaften etabliert. Während effiziente Kupplungsstrategien für ‐Systeme in der Synthese auf Oberflächen bereits existieren, wurde die Bildung von ‐‐Bindungen auf Oberflächen bislang wenig beachtet. In diesem Artikel wird das oberflächenvermittelte Analogon der Friedel‐Crafts‐Alkylierung auf einer Metalloberfläche präsentiert. Dabei fungiert die Metalloberfläche als Katalysator für eine intramolekulare ‐‐Kupplung, die auf einem eigens konzipierten fluorierten aromatischen Edukt basiert. Die Wechselwirkung zwischen Edukt und Katalysator stabilisiert das Zwischenprodukt zur direkten Abbildung auf Einzelmolekülebene im Realraum. Basierend auf hochauflösender Rastertunnelmikroskopie und Röntgenphotoelektronenspektroskopie wird in Kombination mit ab‐initio‐Rechnungen jeder Elementarschritt des Reaktionswegs aufgeklärt. Die Ergebnisse erweitern das Repertoire der Synthese auf Oberflächen durch einen innovativen Zugang zur kontrollierten Integration von ‐Zentren in hybride ‐‐Kohlenstoffnanostrukturen.

The incorporation of ‐hybridized carbon centers into planar carbon nanostructures is a powerful strategy in solution chemistry for tuning their electronic and structural properties. While on‐surface synthesis has demonstrated efficient coupling strategies for systems, ─ bond formation on surfaces remains largely unexplored. Here, we report the on‐surface analogue of Friedel–Crafts alkylation on a metal surface: an intramolecular ─ coupling reaction based on a specifically designed fluorinated aromatic reactant, with the metal surface itself acting as catalyst. Our reactant‐catalyst design stabilizes the key intermediate; allowing for direct real‐space imaging at the single‐molecule level. By integrating high‐resolution scanning tunneling microscopy, x‐ray photoelectron spectroscopy, and first‐principles computations, we elucidate each elementary step of the on‐surface Friedel–Crafts alkylation pathway. These results open a new route toward the controlled integration of centers into hybrid ─ carbon nanostructures, expanding the synthetic toolbox of on‐surface chemistry.

We report the synthesis, structural characterization, and optoelectronic properties of a highly electron‐deficient bi(cyclopropylidene)‐framework ( CN8CP2 ). The developed one‐pot synthesis gives access to the dianionic species via thermally induced homocoupling of an iodinated precursor. The controlled oxidation yields the radical anion, whereas the neutral molecule is accessible only as an electrochemically generated in situ species. Single‐crystal X‐ray diffraction studies of the dianion reveal molecular layers separated by counterions, thereby enabling fluorescence in the solid state. The structure of the radical anion reveals a highly ordered arrangement of π‐stacked molecules. Optical spectroscopy and quantum chemical calculations indicate that the vibronic fine structure is governed by the vibrational modes of the cyclopropane core. The analysis of the electronic structures confirms extensive spin delocalization for the radical anion and a pronounced σ‐aromatic character. The exceptionally low energy levels of the acceptor orbitals are determined as −5.66 eV for the radical anion and −6.18 eV for the neutral species. Consequently, charge transfer to the neutral molecule or the radical anion results in the formation of the closed‐shell dianion, which circumvents instabilities that are associated with open‐shell species formed for conventional electron acceptors. Thus, CN8CP2 appears as one of the strongest small‐molecule organic acceptors for advanced organic electronic materials.

Even though halogen bonding is routinely used in crystal engineering and beyond, motifs offering multipoint interactions are still rare. This applies in particular to self‐complementary systems which incorporate both halogen bond donating and accepting moieties. In this study, we present the first isolated homodimers based on a zwitterionic halogen bond donor, in which an iodolium core acts as Lewis acidic unit and a sulfonate group acts as Lewis base. Initial experiments with a carboxylate variant featured too strong coordination, leading to poor solubility. The optimized sulfonate systems, in contrast, resulted in strongly bound dimers with halogen bond distances of up to 2.73 Å. We anticipate this motif to form the basis for the further development of halogen‐bonding‐based linkers.

We recently introduced multidentate halogen‐bonding receptors based on C‐iodinated ortho ‐Carborane moieties. Following up on our results in solution, which indicated a second binding event for sub‐stochiometric concentrations of halide guests, we herein present a systematic co‐crystallization study with halides to further evaluate the halogen‐bonding capacities of these halogen‐bonding catalysts. The obtained solid‐state structures include two rare examples of octahedral coordination via halogen bonding of a halide ion by a pair of tridentate receptors.

A novel 1,2-bis(dicyanomethylene)-cyclopropanide-substituted anionic chalcogen bond donor displays “anti-electrostatic” interactions in the solid state.

The coordination of allene dicarbonyl compounds to cyclic iodonium salts is presented as a new biaxial halogen bonding recognition motif. 1H¬ NMR titrations and isothermal titration calorimetry (ITC) of such adducts were performed, resulting in a binding constant as high as 6.9·10^5 M-1 for the complex of dibenzo[b,d]iodolium with an allene diamide. The biaxial binding was confirmed via the comparison of the non-functionalized iodolium with variants in which the halogen bonding axis are blocked by methyl groups, and it was further corroborated by theoretical calculations and a crystal structure analysis. Furthermore, we could experimentally demonstrate the stabilizing effect of the halogen bond donor donor on an allene diamide. In order to differentiate (and eventually separate) the enantiomers of the allene, chiral iodolium diamide derivatives (with one or two chiral side-arms) were synthesized. 1H NMR experiments with these halogen bond donors strongly suggest chiral recognition of allenes.

The first study of the high-pressure behavior of organic halogen bond donors in solution is presented: HP-NMR titrations were performed in various solvents, primarily with a neutral tridentate halogen bond donor and an orthoamide or halides as Lewis bases. While the orthoamide showed stronger binding upon increased pressure in CDCl₃, CD₂Cl₂, acetone-d₆, and THF-d₈, the halides unexpectedly featured decreased association constants for the first three solvents. For comparison, bromide was also titrated against dicationic halogen and chalcogen bond donors as well as a thiourea as a representative hydrogen bond donor, with the latter showing weaker binding at elevated pressure. Qualitative agreement with this data was achieved via thermodynamic modeling using PC-SAFT. These findings stress the important role of the solvent in halogen-bonded complexes involving charged species and are expected to provide the basis for a rational application of high pressure in applications of halogen bonding in solution.