Synthesis of Shape‐Persistent Macrocycles with Three 1,8‐Diazaanthracene Units and Their Packing in the Single Crystal

The synthesis of four shape‐persistent macrocycles with three 1,8‐diazaanthracene units each is reported ( 2 , 3 a – 3 c ). For two of them single crystals could be obtained and the structures in the crystal be solved. The structures reveal that macrocycle 2 self‐dimerizes in the solid state; surprisingly it also forms a stable dimer in solution. The reason for this is seen in unusually efficient dispersion interactions as a consequence of the large contact areas in the dimer. All macrocycles are assessed as to their applicability in lateral polymerizations in the single crystal as well as in solution.     

Multicomponent Self‐Assembly with a Shape‐Persistent N‐Heterotriangulene Macrocycle on Au(111)

Multicomponent network formation by using a shape‐persistent macrocycle ( MC6 ) at the interface between an organic liquid and Au(111) surface is demonstrated. MC6 serves as a versatile building block that can be coadsorbed with a variety of organic molecules based on different types of noncovalent interactions at the liquid–solid interface. Scanning tunneling microscopy (STM) reveals the formation of crystalline bicomponent networks upon codeposition of MC6 with aromatic molecules, such as fullerene (C₆₀) and coronene. Tetracyanoquinodimethane, on the other hand, was found to induce disorder into the MC6 networks by adsorbing on the rim of the macrocycle. Immobilization of MC6 itself was studied in two different noncovalently assembled host networks. MC6 assumed a rather passive role as a guest and simply occupied the host cavities in one network, whereas it induced a structural transition in the other. Finally, the central cavity of MC6 was used to capture C₆₀ in a complex three‐component system. Precise immobilization of organic molecules at discrete locations within multicomponent networks, as demonstrated here, constitutes an important step towards bottom‐up fabrication of functional surface‐based nanostructures.     

Hydrogen‐Bond‐Guided Self‐Assembly of Nucleotides on a Receptor‐Array Surface

The hydrogen‐bond‐guided self‐assembly of 5′‐ribonucleotides bearing adenine(A), cytosine (C), uracil (U), or guanine (G) bases from aqueous solution on a lipid‐like surface decorated with synthetic bis(Zn^II–cyclen) (cyclen=1,4,7,10‐tetraazacyclodododecane) metal–complex receptor sites is described. The process was studied by using surface plasmon resonance spectroscopy. The data show that the mechanism of nucleotide binding to the 2D template is influenced by the chemistry of the bases and the pH value of the solution. In a neutral solution of pH 7.5, the process is cooperative and selective with respect to Watson–Crick pairs (A–U and C–G), which form stable double planes in accordance with the Chargaff rule. In a more acidic solution at pH 6.0, the interactions between complementary partners become non‐cooperative and the surface also stabilizes mismatched and wobble pairs due to the pH‐induced changes in the receptor coordination state. The results suggest that hydrogen bonding plays a key role in the self‐assembly of complementary nucleotides at the lipid‐like interface, and the cooperative character of the process stems from the ideal matching of the orientation and chemistry of all the interacting components with respect to each other in neutral solution.     

Formation of a Hydrogen‐Bonded Heptazine Framework by Self‐Assembly of Melem into a Hexagonal Channel Structure

Self‐assembly of melem C₆N₇(NH₂)₃ in hot aqueous solution leads to the formation of hydrogen‐bonded, hexagonal rosettes of melem units surrounding infinite channels with a diameter of 8.9 Å. The channels are filled with strongly disordered water molecules, which are bound to the melem network through hydrogen bonds. Single‐crystals of melem hydrate C₆N₇(NH₂)₃ ? xH₂O (x≈2.3) were obtained by hydrothermal treatment of melem at 200 °C and the crystal structure (R $\bar 3$ c, a=2879.0(4), c=664.01(13) pm, V=4766.4(13)×10⁶ pm³, Z=18) was elucidated by single‐crystal X‐ray diffraction. With respect to the structural similarity to the well‐known adduct between melamine and cyanuric acid, the composition of the obtained product was further analyzed by solid‐state NMR spectroscopy. Hydrolysis of melem to cyameluric acid during syntheses at elevated temperatures could thus be ruled out. DTA/TG studies revealed that, during heating of melem hydrate, water molecules can be removed from the channels of the structure to a large extent. The solvent‐free framework is stable up to 430 °C without transforming into the denser structure of anhydrous melem. Dehydrated melem hydrate was further characterized by solid‐state NMR spectroscopy, powder X‐ray diffraction, and sorption measurements to investigate structural changes induced by the removal of water from the channels. During dehydration, the hexagonal, layered arrangement of melem units is maintained whereas the formation of additional hydrogen bonds between melem entities requires the stacking mode of hexagonal layers to be altered. It is assumed that layers are shifted perpendicular to the direction of the channels, thereby making them inaccessible for guest molecules.     

Shape‐Controlled Synthesis and Self‐Sorting of Covalent Organic Cage Compounds

The directional bonding approach is a powerful tool to rationally control both shape and stoichiometry of three‐dimensional objects built from rigid building blocks under dynamic covalent conditions. Co‐condensation of catechol‐functionalized tribenzotriquinacene derivatives which have 90° angles between the reactive sites and diboronic acids with bite angles of 60°, 120°, and 180°, led to the efficient formation of, respectively, bipyramidal, tetrahedral, or cubic covalent organic cage compounds in a predictable manner. Investigations on the self‐sorting of ternary mixtures containing two competitive boronic acids revealed either narcissistic or social self‐sorting depending on the stability of the segregated cages relative to feasible three‐component assemblies.     

Shape‐Persistent Organic Cage Compounds by Dynamic Covalent Bond Formation

One area of supramolecular chemistry involves the synthesis of discrete three‐dimensional molecules or supramolecular aggregates through the coordination of metals. This field also concerns the chemistry of supramolecular cage compounds constructed through the use of such coordination bonds. To date, there exists a broad variety of supramolecular cage compounds; however, analogous organic cage compounds formed with only covalent bonds are relatively rare. Recent progress in this field can be attributed to important advances, not least the application of dynamic covalent chemistry. This concept makes it possible to start from readily available precursors, and in general allows the synthesis of cage compounds in fewer steps and usually higher yields.     

A Supramolecular Hydrogen‐Bonded Network as a Diffusion Barrier for Metal Adatoms

Confined in a molecular corral : A supramolecular network changes the mechanism by which underpotential deposition (UPD) of copper proceeds on a gold electrode modified by a self‐assembled monolayer (SAM). Lateral diffusion of Cu adatoms is suppressed between adjacent cells of a network/SAM hybrid structure. Instead, UPD occurs by direct deposition into the SAM filled pores of the network, where the Cu adatoms are confined.

     

Sterically‐Limited Self‐Assembly of Pt4 Macrocycles into Discrete Non‐covalent Nanotubes: Porous Supramolecular Tetramers and Hexamers

We report a template‐free strategy based on steric repulsion for the isolation of discrete columnar aggregates of macrocycles. Specifically, introduction of sterically‐demanding trityl‐derived substituents at the periphery of Pt₄ Schiff base macrocycles limits the otherwise infinite one‐dimensional columnar aggregation to discrete tetrameric and hexameric assemblies. Single crystal X‐ray diffraction studies of these compounds reveal discrete nanotubes of finite length that pack inefficiently resulting in three‐dimensional networks of interconnected void space. The discrete assemblies were studied by N₂ adsorption and show enhanced surface area when stacked. In the absence of bulky substituents the macrocycles are nonporous. This strategy for engineering discrete supramolecular macrocyclic aggregates may be generalized to other columnar assembling systems.     

Hydrogen‐Bonding‐Mediated Dynamic Covalent Synthesis of Macrocycles and Capsules: New Receptors for Aliphatic Ammonium Ions and the Formation of Pseudo[3]rotaxanes

Dynamic covalent synthesis! Intramolecular hydrogen‐bonding induces amino‐ and aldehyde‐appended aryl amides to adopt a rigid “V”‐shaped conformation. As a result, stable two‐layered capsules can be assembled quantitatively through the one‐step formation of six imine bonds. The new capsules form complexes with aliphatic diammonium ions to give unique two‐layered pseudo[3]rotaxanes (see figure).

     

Unique Hydrogen‐Bonded Complex of Hydronium and Hydroxide Ions

H₃O⁺ and OH^?, formed by the self‐ionization of two coordinating water molecules during the crystal growing of a host molecule [1,3,5‐tris(hydroxymethyl)2,4,6‐triethylbenzene ( 1 )], could be effectively stabilized by hydrogen‐bonding interactions with the preorganized hydroxy groups of three molecules of 1. The binding motifs observed in the complex ( 1 )₃?H₃O⁺?HO^? show remarkable similarity to those postulated for the hydrated hydronium and hydroxide ion complexes, which play important roles in various chemical, biological, and atmospheric processes, but their molecular structures are still not fully understood and remain a subject of intensive research.     

Encapsulation‐Induced Remarkable Stability of a Hydrogen‐Bonded Heterocapsule

Remarkably enhanced stability of the self‐assembled hydrogen‐bonded heterocapsule 1?2 by the encapsulation of 1,4‐bis(1‐propynyl)benzene 3 a was found with K_a=1.14×10⁹ M ^?1 in CDCl₃ and K_a2=1.59×10⁸ M ^?2 in CD₃OD/CDCl₃ (10 % v/v) at 298 K. The formation of 3 a @( 1?2 ) was enthalpically driven (ΔH°<0 and ΔS°<0) and there was a unique inflection point in the correlation between ΔH° versus ΔS° as a function of polar solvent content. The ab initio calculations revealed that favorable guest–capsule dispersion and electrostatic interactions between the acetylenic parts (triple bonds) of 3 a and the aromatic inner space of 1?2 , as well as less structural deformation of 1?2 upon encapsulation of 3 a , play important roles in the remarkable stability of 3 a @( 1?2 ).     

Synthesis and Self‐Assembly of Cyclic 2,7‐Anthrylene Ethynylene 1,3‐Phenylene Ethynylene Trimer with a Planar Conformation

Cyclic arylene ethynylene hexamer 1 , composed of alternating 2,7‐anthrylene ethynylene units and meta‐phenylene ethynylene units, was synthesized. It shows C₃ symmetry and possesses a flat and rigid conformation with a large equilateral triangle‐like cavity. Macrocycle 1 self‐associates through π–π stacking interactions between the anthracene‐containing macrocyclic aromatic cores with indefinite‐association constant K_E=6980 m ^?1 in CDCl₃ at 303 K. Macrocycle 1 also self‐assembles into π‐stacked nanofibers in the drop‐cast film.     

Evolution of a Constitutional Dynamic Library Driven by Self‐Organisation of a Helically Folded Molecular Strand

Conversion of macrocyclic imine entities into helical strands was achieved through three‐ and four‐component exchange reactions within constitutionally dynamic libraries. The generation of sequences of the intrinsic helicity codon, based on the hydrazone–pyrimidine fragment obtained by condensation of pyrimidine dialdehyde A with pyrimidine bis‐hydrazine B , shifted the equilibrium between all the possible macrocycles and strands towards the full expression (>98%) of helical product [ A / B ]. Furthermore, it was shown that chain folding accelerated the dynamic exchange reactions among the library members. Lastly, in four‐component experiments (involving A , B , E and either C or D ), even though the macrocyclic entities ([ A / C ], [ B / E ]; [ A / D ], [ B / E ]) were the kinetically preferred products, over time dialdehyde A relinquished its initial diamine partners C or D to opt for bis‐hydrazine B , which allowed the preferential formation of the helically folded strand. The present results indicate that self‐organisation pressure was able to drive the dynamic system towards the selective generation of the strand undergoing helical folding.     

Salt‐Induced Adaptation of a Dynamic Combinatorial Library of Pseudopeptidic Macrocycles: Unraveling the Electrostatic Effects in Mixed Aqueous Media

Dynamic combinatorial libraries are powerful systems for studying adaptive behaviors and relationships, as models of more complex molecular networks. With this aim, we set up a chemically diverse dynamic library of pseudopeptidic macrocycles containing amino‐acid side chains with differently charged residues (negative, positive, and neutral). The responsive ability of this complex library upon the increase of the ionic strength has been thoroughly studied. The families of the macrocyclic members concentrating charges of the same sign showed a large increase in its proportion as the ionic strength increases, whereas those with residues of opposite charges showed the reverse behavior. This observation suggested an electrostatic shielding effect of the salt within the library of macrocycles. The top‐down deconvolution of the library allowed us to obtain the fundamental thermodynamic information connecting the library members (exchange equilibrium constants), as well as to parameterize the adaptation to the external stimulus. We also visualized the physicochemical driving forces for the process by structural analysis using NMR spectroscopy and molecular modeling. This knowledge permitted the full understanding of the whole dynamic library and also the de novo design of dynamic chemical systems with tailored co‐adaptive relationships, containing competing or cooperating species. This study highlights the utility of dynamic combinatorial libraries in the emerging field of systems chemistry.     

Molecular Spoked Wheels: Synthesis and Self‐Assembly Studies on Rigid Nanoscale 2D Objects

We present the efficient synthesis of a new molecular spoked‐wheel structure ( MSW‐3 ). Two derivatives with diameters of approximately 4 nm have been prepared. By highlighting the importance of pseudo‐high‐dilution conditions during cyclization, we were able to access the compounds on a several hundred milligram scale. In addition to the standard characterization (NMR spectroscopy, MS), we describe a detailed investigation of the optical properties of the fluorescent MSWs by comparison with appropriate model chromophores. Furthermore, a comprehensive study of the structure in solution by means of light‐ and X‐ray scattering experiments has been conducted. Scanning tunneling microscopy (STM) revealed the two‐dimensional organization of the molecules on highly oriented pyrolytic graphite and emphasized the spoked‐wheel structure. The diameter of these molecules measured by small‐angle X‐ray scattering is in very good agreement with that obtained from STM and matches the results of molecular modeling. This confirms the rigidifying effect of the spokes, which results in highly shape‐persistent nanometer‐sized oblate organic compounds.