Cation‐Directed Dimeric versus Tetrameric Assemblies of Lanthanide‐Stabilized Dilacunary Keggin Tungstogermanates

Reaction of mid‐ to late lanthanide ions with GeO₂ and Na₂WO₄ in NaOAc buffer results in a library of [Ln₂(GeW₁₀O₃₈)]^6? clusters ( Ln₂ ), which consist of dilacunary Keggin fragments stabilized by the insertion of 4f atoms in the vacant sites and show the ability to undergo cation‐directed self‐assembly processes. In the presence of Na⁺, two β‐ Ln₂ subunits assemble by means of Ln‐O(WO₅)‐Ln bridges to form the chiral [Ln₄(H₂O)₆(β‐GeW₁₀O₃₈)₂]^12? dimeric anions (ββ‐ Ln₄ , Ln=Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu). When Cs⁺ is present, two Ln₄ ‐like dimers further assemble into the [{Ln₄(H₂O)₅(GeW₁₀O₃₈)₂}₂]^24? species ( Ln₈ , Ln=Ho, Er, Tm, Yb, Lu). Two types of tetramers coexist in the solid state: One shows a full ββ‐ Ln₈ architecture, whereas the other one is a mixed αβ‐ Ln₈ assembly in which each β‐subunit is linked to its corresponding α‐ Ln₂ derivative. Regardless of differences in isomeric forms and the relative arrangement of Ln₂ subunits, all anions display virtually identical {Ln₄} cores as a common structural feature. A combination of ESI mass spectrometry and ¹⁸³W NMR spectroscopy experiments indicates that Ln₈ tetramers fragment into Ln₄ dimers upon dissolution, which undergo partial dissociation into Ln₂ monomers and slow dimer/monomer equilibration. This is most likely followed by β‐to‐α isomerization of Ln₂ clusters with consequent reassembly, as indicated by isolation of three additional αα‐ Ln₄ derivatives. Magnetic and photoluminescence properties in the Na ‐ββ‐ Ln₄ series are also discussed.     

Crown‐Shaped Tungstogermanates as Solvent‐Controlled Dual Systems in the Formation of Vesicle‐Like Assemblies

Reaction of early lanthanides, GeO₂, and Na₂WO₄ in a NaOAc buffer results in large crown‐shaped polyoxometalates based on [Ln₂GeW₁₀O₃₈]^6? subunits. By using Ni²⁺ as a crystallizing agent, [Na?Ln₁₂Ge₆W₆₀O₂₂₈(H₂O)₂₄]^35? ( Na?Ln₁₂ ) hexamers formed by alternating β(1,5)/β(1,8) subunits were obtained for Ln=Pr, Nd. The addition of K⁺ led to a similar anion for Ln=Sm, namely, [K?Sm₁₂Ge₆W₆₀O₂₂₈(H₂O)₂₂]^35? ( K?Sm₁₂ ) and [K?K₇Ln₂₄Ge₁₂W₁₂₀O₄₄₄(OH)₁₂(H₂O)₆₄]^52? ( K?Ln₂₄ ) dodecamers that consist of a central core identical to K?Sm₁₂ decorated with six external γ(3,4) subunits for Ln=Pr, Nd. These anions dissociate in water into hexameric cores and monomeric entities, as shown by ESI mass spectrometry. The former self‐assemble into spherical, hollow, and single‐layered blackberry‐type structures with radii of approximately 75 nm, as monitored by laser light scattering (LLS) and TEM techniques. Analogous studies performed for K?Nd₂₄ in water/acetone mixtures show that the dodecamers remain stable and form in turn their own type of blackberries with sizes that increase from approximately 20 to 50 nm with increasing acetone content. This control over both the composition and size of the vesicle‐like assemblies is achieved for the first time by modifying the architecture of the species that undergoes supramolecular association through the solvent polarity.     

Gel Sculpture: Moldable,Load‐Bearing and Self‐Healing Non‐Polymeric Supramolecular Gel Derived from a Simple Organic Salt

An easy access to a library of simple organic salts derived from tert‐butoxycarbonyl (Boc)‐protected L ‐amino acids and two secondary amines (dicyclohexyl‐ and dibenzyl amine) are synthesized following a supramolecular synthon rationale to generate a new series of low molecular weight gelators (LMWGs). Out of the 12 salts that we prepared, the nitrobenzene gel of dicyclohexylammonium Boc‐glycinate ( GLY.1 ) displayed remarkable load‐bearing, moldable and self‐healing properties. These remarkable properties displayed by GLY.1 and the inability to display such properties by its dibenzylammonium counterpart ( GLY.2 ) were explained using microscopic and rheological data. Single crystal structures of eight salts displayed the presence of a 1D hydrogen‐bonded network (HBN) that is believed to be important in gelation. Powder X‐ray diffraction in combination with the single crystal X‐ray structure of GLY.1 clearly established the presence of a 1D hydrogen‐bonded network in the xerogel of the nitrobenzene gel of GLY.1 . The fact that such remarkable properties arising from an easily accessible (salt formation) small molecule are due to supramolecular (non‐covalent) interactions is quite intriguing and such easily synthesizable materials may be useful in stress‐bearing and other applications.     

Formation and Geometrical Control of Polygon‐Like Metal‐Coordination Assemblies

Polygon‐like [2+2]‐ and [3+3]‐type metal complexes were prepared from dipyrrin dimers connected by acute‐angled spacers. The electrical conduction depends strongly on the packing alignment of the compounds, revealing the presence of effective hopping pathways for holes with relatively high mobility up to 0.11 cm² V^?1 s^?1 along the aligned axis of [3+3]‐type metal‐bridged assemblies. These observations correlated with the geometrical control of the π‐conjugated metal complexes in the cyclic structures, which enables their ordered arrangement in the assemblies.     

Non‐Covalent Interactions in Supramolecular Assemblies Investigated with Electron Spectroscopies

A new view: A picture of the different non‐covalent interactions relevant for the self‐assembly of organic layers and their spectroscopic fingerprints is provided (see figure). In particular, state‐of‐the‐art spectroscopic measurements are performed for supramolecular assemblies, comparing the electronic structure of single‐component layers with that of binary organic layers.

     

When Self‐Assembly Fails: Stepwise Metal‐Directed Synthesis of [2]Catenanes

On the attempted synthesis of a series of homo‐ and heterotrimetallic [2]catenanes by the self‐assembly of a 2‐(pyridin‐4‐ylmethyl)‐2,7‐diazapyrenium ligand, (ethylenediamine)palladium(II) or platinum(II) nitrate, and a dioxoaryl bis(N‐monoalkyl‐4,4′‐bipyridinium) salt as building blocks, both the one‐pot direct self‐assembly of the components and the so called “magic ring” approach fail to produce the expected trinuclear [2]catenanes under thermodynamically driven conditions. However, one of the target supramolecules is obtained by following a stepwise protocol, consisting of the threading of a dinuclear Pt^II metallacycle and the dioxoaryl bis(N‐monoalkyl‐4,4′‐bipyridinium) axle, followed by kinetically controlled Pt^II‐directed cyclization of the corresponding pseudorotaxane.     

One Ligand in Dual Roles: Self‐Assembly of a Bis‐Rhomboidal‐Shaped,Three‐Dimensional Molecular Wheel

A facile high yield, self‐assembly process that leads to a terpyridine‐based, three‐dimensional, bis‐rhomboidal‐shaped, molecular wheel is reported. The desired coordination‐driven supramolecular wheel involves eight structurally distorted tristerpyridine (tpy) ligands possessing a 60° angle between the adjacent tpy units and twelve Zn²⁺ ions. The tpy ligand plays dual roles in the self‐assembly process: two are staggered at 180° to create the internal hub, while six produce the external rim. The wheel can be readily generated by mixing the tpy ligand and Zn²⁺ in a stoichiometric ratio of 2:3; full characterization is provided by ESI‐MS, NMR spectroscopy, and TEM imaging.     

Transformation of a Cp*–Iridium(III) Precatalyst for Water Oxidation when Exposed to Oxidative Stress

The reaction of [Cp*Ir(bzpy)NO₃] ( 1 ; bzpy=2‐benzoylpyridine, Cp*=pentamethylcyclopentadienyl anion), a competent water‐oxidation catalyst, with several oxidants (H₂O₂, NaIO₄, cerium ammonium nitrate (CAN)) was studied to intercept and characterize possible intermediates of the oxidative transformation. NMR spectroscopy and ESI‐MS techniques provided evidence for the formation of many species that all had the intact Ir–bzpy moiety and a gradually more oxidized Cp* ligand. Initially, an oxygen atom is trapped in between two carbon atoms of Cp* and iridium, which gives an oxygen–Ir coordinated epoxide, whereas the remaining three carbon atoms of Cp* are involved in a η³ interaction with iridium ( 2 a ). Formal addition of H₂O to 2 a or H₂O₂ to 1 leads to 2 b , in which a double MeCOH functionalization of Cp* is present with one MeCOH engaged in an interaction with iridium. The structure of 2 b was unambiguously determined in the solid state and in solution by X‐ray single‐crystal diffractometry and advanced NMR spectroscopic techniques, respectively. Further oxidation led to the opening of Cp* and transformation of the diol into a diketone with one carbonyl coordinated at the metal ( 2 c ). A η³ interaction between the three non‐oxygenated carbons of “ex‐Cp*” and iridium is also present in both 2 b and 2 c . Isolated 2 b and mixtures of 2 a – c species were tested in water‐oxidation catalysis by using CAN as sacrificial oxidant. They showed substantially the same activity than 1 (turnover frequency values ranged from 9 to 14 min^?1).     

Using Diffusion NMR To Characterize Guanosine Self‐Association: Insights into Structure and Mechanism

This paper presents results from a series of pulsed field gradient (PFG) NMR studies on lipophilic guanosine nucleosides that undergo cation‐templated assembly in organic solvents. The use of PFG‐NMR to measure diffusion coefficients for the different aggregates allowed us to observe the influences of cation, solvent and anion on the self‐assembly process. Three case studies are presented. In the first study, diffusion NMR confirmed formation of a hexadecameric G‐quadruplex [G 1 ]₁₆ ? 4 K⁺ ? 4 pic^? in CD₃CN. Furthermore, hexadecamer formation from 5′‐TBDMS‐2′,3′‐isopropylidene G 1 and K⁺ picrate was shown to be a cooperative process in CD₃CN. In the second study, diffusion NMR studies on 5′‐(3,5‐bis(methoxy)benzoyl)‐2′,3′‐isopropylidene G 4 showed that hierarchical self‐association of G₈‐octamers is controlled by the K⁺ cation. Evidence for formation of both discrete G₈‐octamers and G₁₆‐hexadecamers in CD₂Cl₂ was obtained. The position of this octamer–hexadecamer equilibrium was shown to depend on the K⁺ concentration. In the third case, diffusion NMR was used to determine the size of a guanosine self‐assembly where NMR signal integration was ambiguous. Thus, both diffusion NMR and ESI‐MS show that 5′‐O‐acetyl‐2′,3′‐O‐isopropylidene G 7 and Na⁺ picrate form a doubly charged octamer [G 7 ]₈ ? 2 Na⁺ ? 2 pic^? 9 in CD₂Cl₂. The anion's role in stabilizing this particular complex is discussed. In all three cases the information gained from the diffusion NMR technique enabled us to better understand the self‐assembly processes, especially regarding the roles of cation, anion and solvent.     

Supramolecular Synthons in Designing Low Molecular Mass Gelling Agents: L‐Amino Acid Methyl Ester Cinnamate Salts and their Anti‐Solvent‐Induced Instant Gelation

Easy access to a class of chiral gelators has been achieved by exploiting primary ammonium monocarboxylate ( PAM ), a supramolecular synthon. A combinatorial library comprising of 16 salts, derived from 5 l ‐amino acid methyl esters and 4 cinnamic acid derivatives, has been prepared and scanned for gelation. Remarkably, 14 out of 16 salts prepared (87.5 % of the salts) show moderate to good gelation abilities with various solvents, including commercial fuels, such as petrol. Anti‐solvent induced instant gelation at room temperature has been achieved in all the gelator salts, indicating that the gelation process is indeed an aborted crystallization phenomenon. Rheology, optical and scanning electron microscopy, small angle neutron scattering, and X‐ray powder diffraction have been used to characterize the gels. A structure‐property correlation has been attempted, based on these data, in addition to the single‐crystal structures of 5 gelator salts. Analysis of the FT‐IR and ¹H NMR spectroscopy data reveals that some of these salts can be used as supramolecular containers for the slow release of certain pest sex pheromones. The present study clearly demonstrates the merit of crystal engineering and the supramolecular synthon approach in designing new materials with multiple properties.     

Self‐Assembly and Structure of Cobalt(II) Complexes Using Diaminodiamide Ligands

The self‐assembly of Co(II) with two diaminodiamide ligands, 4,7‐diazadecanediamide and 4,8‐diazaundecanediamide, gave two different crystals, [(C₈H₁₈N₄O₂)Co(OH)₂Co(C₈H₁₈N₄O₂)]Cl₂ ( 1 ) [Co(C₉H₂₀N₄O₂)(Cl)(H₂O)]·Cl·2H₂O ( 2 ). Structures of 1 and 2 were characterized by single‐crystal X‐ray diffraction analysis. Structural data for 1 shows a novel type of binuclear complex with distorted octahederal coordination geometry around the Co atoms through the hydroxo bridges. By using inter‐connector N‐H···N hydrogen bonding interactions as building forces, each cationic moiety [(C₈H₁₈N₄O₂)Co(OH)₂Co(C₈H₁₈N₄O₂)]²⁺ is linked to neighboring ones, producing a charged hydrogen‐bonded 1D chain‐like structure. The chains are further connected into a 2D layer in a (4,4)‐topology via N‐H···Cl_free hydrogen‐bonding interactions. Structural data for 2 indicate that the cobalt atom adopts a six‐coordinated N₂O₄ environment, giving a distorted octahedral geometry, where two N‐ and two O‐donor sets of ligand located at equatorial positions and one water and one chloride occupied at axial positions. Through NH···Cl‐Co and OH···Cl‐Co contacts, each cationic moiety [Co(C₉H₂₀N₄O₂)(Cl)(H₂O)]⁺ in 2 is linked to neighboring ones, producing a charged hydrogen‐bonded 1D chainlike structure. Thus, the crystal‐engineering approach has proved successful in the solid‐state packing due to steric strain effect of the diaminodiamide ligand.     

Halogenotrinitromethanes: A Combined Study in the Crystalline and Gaseous Phase and Using Quantum Chemical Methods

The halogenotrinitromethanes FC(NO₂)₃ ( 1 ), BrC(NO₂)₃ ( 2 ), and IC(NO₂)₃ ( 3 ) were synthesized and fully characterized. The molecular structures of 1 – 3 were determined in the crystalline state by X‐ray diffraction, and gas‐phase structures of 1 and 2 were determined by electron diffraction. The Hal?C bond lengths in F?, Cl?, and Br?C(NO₂)₃ in the crystalline state are similar to those in the gas phase. The obtained experimental data are interpreted in terms of Natural Bond Orbitals (NBO), Atoms in Molecules (AIM), and Interacting Quantum Atoms (IQA) theories. All halogenotrinitromethanes show various intra‐ and intermolecular non‐bonded interactions. Intramolecular N ??? O and Hal ??? O (Hal=F ( 1 ), Br ( 2 ), I ( 3 )) interactions, both competitors in terms of the orientation of the nitro groups by rotation about the C?N bonds, lead to a propeller‐type twisting of these groups favoring the mentioned interactions. The origin of the unusually short Hal?C bonds is discussed in detail. The results of this study are compared to the molecular structure of ClC(NO₂)₃ and the respective interactions therein.     

A Supramolecular Sorting Hat: Stereocontrol in Metal–Ligand Self‐Assembly by Complementary Hydrogen Bonding

A combination of self‐complementary hydrogen bonding and metal–ligand interactions allows stereocontrol in the self‐assembly of prochiral ligand scaffolds. A unique, non‐tetrahedral M₄L₆ structure is observed upon multicomponent self‐assembly of 2,7‐diaminofluorenol with 2‐formylpyridine and Fe(ClO₄)₂. The stereochemical outcome of the assembly is controlled by self‐complementary hydrogen bonding between both individual ligands and a suitably sized counterion as template. This hydrogen‐bonding‐mediated stereoselective metal–ligand assembly allows the controlled formation of nonsymmetric discrete cage structures from previously unexploited ligand scaffolds.     

Synthesis and Supramolecular Self‐Assembly of Coil‐Rod‐Coil Molecules: The Relationship between Self‐Assembled Nanostructures and Molecular Structures

Herein, the relationship between the supramolecularly self‐assembled nanostructures and the chemical structures of coil‐rod‐coil molecules is discussed. A series of nonamphiphilic coil‐rod‐coil molecules with different alkyl chains, central mesogenic groups, and chemical linkers were designed and synthesized. The solvent‐mediated supramolecular self‐assembling of these coil‐rod‐coil molecules resulted in rolled‐up nanotubes, nanofibers, submicron sized belts, needle‐like microcrystals, and amorphous structures. The self‐assembling behaviors of these coil‐rod‐coil molecules have been systematically investigated to reveal the relationship between the supramolecularly self‐assembled nanostructures and their chemical structures. With respect to the formation of rolled‐up nanotubes by self‐assembly of coil‐rod‐coil molecules, we have systematically investigated the following three influencing structural factors: 1) the alkyl chain length; 2) the central mesogenic group; (3) the linker type. These studies disclosed the key structural features of coil‐rod‐coil molecules for the formation of rolled‐up nanotubes.     

Self‐Assembly of a Low‐Symmetry Monodendron Containing Two Asymmetrically Linked Molecular Wedges

Herein, we describe the synthesis of a low‐symmetry monodendron, 3,4‐bis(dodecyloxy)‐5‐[3,4,5‐tris(dodecyloxy)benzyloxy]benzoic acid, following a simple route which starts from gallic acid ethyl ester and does not require any protecting groups. The self‐assembled structures formed by the compound in 3D and 2D were investigated by synchrotron X‐ray scattering and scanning force microscopy (SFM). In 3D, the compound forms a stable crystalline phase with an orthorhombic lattice in which the alkyl chains connected to different benzene rings form crystalline and amorphous domains. Upon cooling from the isotropic melt the compound exhibits a monotropic smectic mesophase. In 100‐nm‐thick films on a neutral substrate the structure loses its biaxiality, adopting a hexagonal columnar structure with the columns oriented parallel to the substrate. By contrast, in ultrathin films on graphite the SFM likely reveals two crystal orientations, which can develop due to the epitaxial adsorption on the substrate of the alkyl chains pertinent to different benzene rings.