A non-symmetrical pH-sensitive bistable [2]rotaxane that bears a cholesterol unit and a tetraphenylmethane group as stopper groups was designed and synthesized in 18 steps. The successful formation of the rotaxane was proven by NMR spectroscopy and MS/MS. Besides a permanent cationic alkylated triazolium unit, the axle contains a secondary amine that can act as a second pH-sensitive binding site for a crown ether. Depending on the protonation state of this amine function, the crown ether reversibly changes its position by moving between the two binding sites along the axle, as revealed by NMR spectroscopy.

Two novel supramolecular meso-helicates have been synthesized and characterized by single-crystal X-ray diffraction, variable-temperature magnetic susceptibility measurements, NMR and UV/Vis spectroscopy, mass spectrometry and density functional theory (DFT) simulations. Both compounds show a considerably high stabilization of the high-spin state of the metal centres compared with other compounds with similar types of ligands. Whereas the pyridyl complex [Fe₂L²₃] [L² = N,N′-bis(pyridin-2-ylmethylene)benzene-1,3-diamine] exhibits the beginning of a spin transition at around 350 K, the imidazolyl complex [Fe₂L¹₃] [L¹ = N,N′-bis(1H-imidazol-4-ylmethylene)benzene-1,3-diamine] still exhibits a high-spin configuration at 20 K. We applied DFT to characterize the molecular as well as solid states of both compounds. Although the low-spin state of both systems is stabilized by crystal packing, an additional stabilization of the high-spin state is induced by intramolecular interactions. This effect can be ascribed to mechanical strain in the backbone of the ligands hailing from very short CH–π interactions and being similar in effect to sterically demanding methyl groups.

Mononuclear complexes are good model systems for evaluating the effects of different ligand systems on the magnetic properties of iron(II) centres. A novel crystal structure of the title compound, [Fe(C₁₈H₂₄N₁₀)](BF₄)₂·CH₃OH, with one molecule of methanol per formula unit exhibits a strictly sixfold coordination sphere associated with a low-spin configuration at the metal centre. The incorporated methanol solvent molecule promotes extended hydrogen-bonding networks between the tetrafluoridoborate anions and the cationic units. A less constrained crystal structure regarding close contacts between the tetrafluoridoborate anions and the cationic units allows a spin transition which is inhibited in the previously published hydrate of the title compound.

Four BINOL-based bis(4-pyridyl) ligands were synthesised in enantiopure and racemic form. These ligands form metallosupramolecular [(dppp)₂M₂L₂] rhombi with cis-protected [(dppp)Pd]²⁺ and [(dppp)Pt]²⁺ ions. In principle, racemic ligands can self-assemble into three stereoisomeric rhombi. The degree of self-sorting in the self-assembly process crucially depends on the substitution pattern and the resulting bend angle of the V-shaped ligands as well as the degree of steric crowding within the assembly when racemic ligands are used. Thus, these processes either lead to homochiral assemblies in a narcissistic self-recognition manner, to heterochiral assemblies in a social self-discriminating manner, or proceed in a nonselective fashion as evidenced by NMR spectroscopy, mass spectrometry and single-crystal X-ray diffraction.

Racemic 2,2′-, 2,2′,7,7′-, and 2,2′,3,3′-substituted 9,9′-spirobifluorenes were synthesised and successfully resolved by HPLC on a Chiralpak IA stationary phase on both analytical and semipreparative scales. Their absolute configurations were determined by comparison of their specific optical rotations with literature data or by comparison of retention times with independently prepared enantiopure material. These compounds are versatile C₂-symmetric building blocks for the formation of more sophisticated cleft-like, chiral molecular architectures.

Two bis(3-pyridyl) ligands 1 and 2 based on a planar chiral [2.2]paracyclophane scaffold were synthesized in enantiomerically pure forms. These ligands act as trans-chelating ligands for square-planar-coordinated metal centres such as palladium(II) ions. Upon forming mononuclear homochiral [ML₂] complexes, they show complete chiral self-sorting in a narcissistic self-recognition manner as proven by NMR spectroscopy, mass spectrometry and single-crystal X-ray diffraction.

Invited for the cover of this issue is the group of Arne Lützen at the University of Bonn, Germany. The cover image shows two dissymmetric bis(3-pyridyl) ligands based on a planar chiral pseudo-ortho-disubstituted [2.2]paracyclophane scaffold. Upon forming a mononuclear [ML₂] complex with palladium(II) ions, these ligands act in a trans-chelating manner and undergo complete chiral self-sorting.

A new β-cyclodextrin-based receptor that showed allosteric binding behavior towards capsaicin in aqueous solution was prepared. By NMR titration and nonlinear regression, we obtained binding constants, which increased more than fivefold when an effector (Zn²⁺) was bound to a central 2,2′-bipyridine that acts as the allosteric center.

Two enantiomerically pure 9,9′-spirobifluorene-based bis(pyridine) ligands 1 and 2 were prepared to study their self-assembly behavior upon coordination to cis-protected palladium(II) ions. Whereas the sterically more demanding ligand, 2, gave rise to the expected dinuclear metallosupramolecular M₂L₂ rhombi, the sterically less demanding ligand, 1, acts as a template to give rise to a homochiral metallosupramolecular M₄L₄ catenane.

Coordination-driven self-assembly is one of the most powerful strategies to prepare nanometer-sized discrete (supra)molecular assemblies. Herein, we report on the use of two constitutionally isomeric BINOL-based bis(pyridine) ligands for this purpose. Upon coordination to Pd^II ions these self-assemble into enantiomerically pure endo- and exo-functionalized hexa- and dodecanuclear metallosupramolecular spheres with a chiral skeleton depending on the substitution pattern of the BINOL core. These aggregates were characterized by NMR, MS, DLS, TEM, and EELS as well as ECD. Furthermore, experimental ECD data could be compared to those obtained from theoretical simulations using a simplified Tamm–Dancoff approximation to time-dependent DFT to rationalize the extraordinary high molar circular dichroisms. Despite the rotational freedom around the central aryl–aryl bond of these ligands, the self-assembly process happens completely selective in a “narcissistic” self-recognition manner.

Ein enantiomerenreiner Bis(3-pyridyl)-Ligand 1 auf der Basis eines BINOL-Gerüsts bildet mit tetravalenten Pd^II-Ionen einen homochiralen [Pd₄(1)₈]-Komplex. Zwei Tetrafluoroborationen dienen dabei als Template für die Bildung dieses Aggegates und werden in zwei periphere Kavitäten eingeschlossen. Die dadurch resultierende Struktur repräsentiert ein neues Strukturmotiv für diese Sorte von metallosupramolekularen Assemblaten, in dem die vier Palladiumionen in einer verzerrt tetraedrischen Anordnung zu finden sind. Dies zwingt den Liganden 1, zwei verschiedene Konformationen in dem Aggregat einzunehmen. Beide Phänomene sind einzigartig und führen überdies zur Bildung einer dreidimensionalen Struktur, die noch über eine weitere, chirale und hydrophile zentrale Kavität verfügt.

An enantiomerically pure BINOL-based bis(3-pyridyl) ligand 1 assembles into a homochiral [Pd₄(1)₈] complex upon coordination to tetravalent Pd^II ions. The formation of this aggregate is templated by two tetrafluoroborate counterions that are encapsulated in two peripheral cavities. The resulting structure is a new structural motif for this kind of metallosupramolecular assemblies that arranges the palladium ions in a distorted tetrahedral fashion and forces ligand 1 to adopt two different conformations. Both phenomena are unique and cause an overall three-dimensional structure that has another confined, chiral, and hydrophilic central cavity.

Die koordinationsgetriebene Selbstorganisation ist eine der leistungsfähigsten Strategien zum Aufbau nanometergroßer diskreter (supra-)molekularer Aggregate. Hier berichten wir über die Verwendung von zwei konstitutionsisomeren Bis(pyridin)-Liganden auf der Basis von BINOL zu diesem Zweck. Je nach Substitutionsmuster des BINOL-Gerüsts bilden sich bei der Koordination an Pd^II-Ionen enantiomerenreine, endo- oder exo-funktionalisierte sechs- oder zwölfkernige metallosupramolekulare Polyeder mit einem chiralen Gerüst. Die Aggregate wurden mittels NMR, MS, DLS, TEM und EELS sowie ECD charakterisiert. Die außergewöhnlich hohen molaren Circulardichroismen der Komplexe konnten durch den Vergleich experimenteller ECD-Daten mit simulierten Daten (vereinfachte Tamm-Dancoff-Näherung zur zeitabhängigen DFT) nachvollzogen werden. Hervorzuheben ist ferner, dass die Selbstorganisationsprozesse vollständig selektiv im Sinne einer “narzisstischen” Selbsterkennung ablaufen, obwohl die Liganden ein hohes Maß an Rotationsfreiheit um die zentrale Aryl-Aryl-Bindung aufweisen.

Racemic 4,12-difunctionalized [2.2]paracyclophanes were synthesized and successfully resolved by (recycling) HPLC on a stationary CHIRALPAK IA phase at a semipreparative scale. Their absolute configurations were determined by X-ray crystal structure analysis and/or by comparison of their specific optical rotations with literature data. These are valuable functionalized C₂-symmetric building blocks for the formation of more sophisticated V-shaped, chiral molecular architectures, as demonstrated by some exploratory transformations.

A 1,1′-binaphthyl-based bis(pyridine) ligand (1) was prepared in racemic and enantiomerically pure form to study the formation of [Pd₂(1)₄] complexes upon coordination to palladium(II) ions with regard to the degree of chiral self-sorting. The self-assembly process proceeds in a highly selective narcissistic self-recognition manner to give only homochiral supramolecular M₂L₄ cages, which were characterized by ESI-MS, NMR, and electronic circular dichroism (ECD) spectroscopy, as well as by single-crystal XRD analysis.

Cooperative effects in the binding of two or more substrates to different binding sites of a receptor that are a result of a conformational change caused by the binding of the first substrate—also referred to as the effector—are called allosteric effects. In biological systems, allosteric regulation is a widely used mechanism to control the function of proteins and enzymes in cellular metabolism. Inspired by this a lot of efforts have been made in supramolecular chemistry to implement this concept into artificial systems to control functions as molecular recognition, signal amplification, or even reactivity and catalysis. This review gives an up-to-date overview over the different approaches that have been reported ever since the first examples from the late 1970s/early 1980s. It covers both homo- and heterotropic examples and is divided according to the nature of the effector—cationic, anionic, or neutral—effectors and systems that use combinations of those.

A series of ten palladiumbis(pyridine) complexes, as well as their corresponding platinum complexes, have been synthesized. The pyridine ligands in each series carried different σ-donor and/or π-acceptor/donor substituents at the para-position of their pyridine rings. These complexes were analysed by NMR spectroscopy, X-ray crystallography, (tandem) MS, and isothermal titration calorimetry (ITC) to validate whether these methods allowed us to obtain a concise and systematic picture of the relative and absolute thermodynamic stabilities of the complexes, as determined by the electronic effects of the substituents. Interestingly, the NMR spectroscopic data hardly correlated with the expected substituent effects but the heteronuclear platinumphosphorus coupling constants did. Crystallographic data were found to be blurred by packing effects. Instead, tandem MS and ITC data were in line with each other and followed the expected trends.

Seven racemic derivatives of Tröger’s base—the 1,7-dibromo-substituted derivative 3, the 2,8-dibromo-substituted derivative 4, the 2,8-diiodo-substituted derivative 5, the 3,9-diiodo-substituted derivative 6, the 4,10-dibromo-substituted derivative 7, its singly debrominated analogue 8, and the 2,8-diamino-substituted derivative 9 in its Fmoc-protected form—were synthesized and successfully resolved by (recycling) HPLC on a stationary Whelk-O1 phase at a semipreparative scale. These are valuable functionalized C₂-symmetric building blocks for further applications. Their absolute configurations were determined by X-ray crystal structure analysis and/or by comparison of their quantum chemically calculated circular dichroism and UV/Vis spectra with the experimental obtained spectra.

Five racemic dissymmetric bis(pyridyl) ligands based on 2,8- or 3,9-difunctionalised Tröger’s base derivatives have been synthesised. Only those derived from a 2,8-difunctionalised scaffold were found to undergo selective self-assembly to discrete self-assembled dinuclear metallosupramolecular aggregates of rhomboid shape upon coordination to cis-protected Pd²⁺ or Pt²⁺ ions, as evidenced by ESI mass spectrometry, NMR spectroscopy and single-crystal X-ray diffraction. Interestingly, these processes were found to be highly diastereoselective leading to the formation of C_2v-symmetric heterochiral assemblies in a self-discriminating manner.

The synthesis and self-assembly behaviour of a series of enantiomerically pure bis(chelating) ligands is reported. The ligands differ in the spacer unit between a BINOL core and two bipyridyl groups as the chelating entities and were found to undergo completely diastereoselective self-assembly to dinuclear double-stranded helicates with silver(I) salts, as demonstrated by NMR and CD spectroscopy and ESI mass spectrometry. Upon coordination to iron(II) or zinc(II) ions, however, a dramatic loss in the diastereoselectivity of the self-assembly of dinuclear triple-stranded helicates was observed as a result of increasing spacer length. In the case of zinc(II), the self-assembly processes were even found to be nonselective with regard to the composition of the helicates.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Based on a first example of an allosteric hemicarcerand (1) we prepared four new 2,2′-bipyridines that carry resorcinarene moieties in a highly convergent manner. Upon coordination to suitable transition metal ions or their complexes these compounds undergo conformational changes in a way that they switch between “open” and “closed” forms (2, 3, and 4) or vice versa (5), thus, bringing together or separating the two functional moieties on the central bipyridine. Among the transition metal complexes that act as effectors for the conformational switching, [Re(CO)₅Cl] and monomeric copper(I) complexes of sterically hindered 2,9-arylated 1,10-phenanthrolines proved to be very effective. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Based on the rigid and V-shaped structure of Tröger's base, several dissymmetrical bis(bipyridine) ligands with both rigid and flexible spacer units have been synthesized, and their self-assembly to double- and triple-stranded helicates upon coordination to late transition metal ions has been investigated by NMR spectroscopic and ESI mass spectrometricmeans.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)