Dimeric hierarchically-assembled titanium(IV) helicates are in solvent-dependent equilibrium with the corresponding monomers. Statistically formed mixtures of such complexes bearing chiral stereocontrolling ligands and achiral diene-substituted ligands show high diastereoselectivity and reasonable enantioselectivity in the Diels–Alder reaction with maleimides if the reaction proceeds with the dimer but not with the monomer. Thus, solvent dependent switching between the monomer and dimer enables on/off switching of the enantioselectivity.

The ability of multiple CF₃-substituted arenes to act as acceptors for anions is investigated. The results of quantum-chemical calculations show that a high degree of trifluoromethyl substitution at the aromatic ring results in a positive quadrupole moment. However, depending on the polarizability of the anion and on the substitution at the arene, three different modes of interaction, namely Meisenheimer complex, side-on hydrogen bonding, or anion–π interaction, can occur. Experimentally, the side-on as well as a η²-type π-complex are observed in the crystal, whereas in solution only side-on binding is found.

Five compounds bearing electron-deficient pentafluorophenyl as well as electron-rich (salicylate or indole) aromatic moieties connected by amide or ester linkages were investigated by X-ray diffraction. In the crystals, various interactions (π–π, lone pair–π) between the different aromatic units are important structure controlling factors in addition to the stronger inter- or intramolecular hydrogen bonds induced by the amide and ester moieties. The hydrogen bonding leads to polymeric and macrocyclic assembly of the molecular building blocks.

A solid-state structural study on anion–π interaction in various N-(pentafluorobenzyl)pyridinium salts accompanied by NMR spectroscopic investigations is presented. The crystal structures of 1a–1d reveal different kinds of contacts with anions, including anion–π interactions. In particular, the solid-state structure of 1b-I₃ shows distinct evidence of anion–π interactions. Attempts to study anion–π interactions in solution were not successful, but their presence in solution could not be ruled out.

Phenanthroline-based chiral ligands L¹ and L² as well as the corresponding Eu^III and Tb^III complexes were synthesized and characterized. The coordination compounds show red and green emission, which was explored for the sensing of a series of anions such as F⁻, Cl⁻, Br⁻, I⁻, NO₃⁻, NO₂⁻, HPO₄²⁻, HSO₄⁻, CH₃COO⁻, and HCO₃⁻. Among the anions, HPO₄²⁻ exhibited a strong response in the emission property of both europium(III) and terbium(III) complexes. The complexes showed interactions with the nucleoside phosphates adenosine triphosphate (ATP), adenosine diphosphate (ADP), and adenosine monophosphate (AMP). Owing to this recognition, these complexes have been applied as staining agents in the microalgal cell Chlorella vulgaris. The stained microalgal cells were monitored through fluorescence microscopy and scanning electron microscopy. Initially, the complexes bind to the outer cell wall and then enter the cell wall through holes in which they probably bind to phospholipids. This leads to a quenching of the luminescence properties.

Compounds 1 a–f-H₂ form “monomeric” triscatecholate titanium(IV) complexes [Ti(1 a–f)₃]²⁻, which in the presence of Li cations are in equilibrium with the triple lithium-bridged “dimers” [Li₃(Ti(1 a–f)₃)₂]⁻. The equilibrium strongly depends on the donor ability of the solvent. Usually, in solvents with high donor ability, the stereochemically labile monomer is preferred, whereas in nondonor solvents, the dimer is the major species. In the latter, the stereochemistry at the complex units is “locked”. The configuration at the titanium(IV) triscatecholates is influenced by addition of chiral ammonium countercations. In this case, the induced stereochemical information at the monomer is transferred to the dimer. Alternatively, the configuration at the metal complexes can be controlled by enantiomerically pure ester side chains. Due to the different orientation of the ester groups in the monomer or dimer, opposite configurations of the triscatecholates were observed by circular dichroism (CD) spectroscopy for [Ti(1 c–e)₃]²⁻ or [Li₃(Ti(1 c–e)₃)₂]⁻. A surprising exception was found for the dimer [Li₃(Ti(1 f)₃)₂]⁻. Herein, the dimer is the dominating species in weak donor (methanol), as well as strong donor (DMSO), solvents. This is due to the bulkiness of the ester substituent destabilizing the monomer. Due to the size of the substituent in [Li₃(Ti(1 f)₃)₂]⁻ the esters have to adopt an unusual conformation in the dimer resulting in a stereocontrol of the small methyl group. Following this, opposite stereocontrol mechanisms were observed for the central metal-complex units of [Li₃(Ti(1 c–e)₃)₂]⁻ or [Li₃(Ti(1 f)₃)₂]⁻.

The binding behaviour of (thio)urea- and amide-functionalized quinoline-based anion receptors towards halide anions was investigated in detail in CDCl₃ and [D₆]DMSO solutions by using ¹H and ¹⁹F NMR spectroscopic methods. The electronic, solvent, and fluoro-substitution effects were studied. The (thio)urea- and amide-functionalized quinoline-based anion receptors showed medium to strong anion affinity and good selectivity in solution. Single crystals of key compounds were obtained and studied by X-ray diffraction spectroscopy.

The crystal structures of simple triphenyl(pentafluorobenzyl)phosphonium salts provide crucial data on the influence of anion size on the molecular structure of bis(pentafluorobenzyl)phosphonium cations containing two adjacent electron-deficient moieties. Whereas the bromide anions interact by anion-π interaction in a 1:1 mode with the pentafluorobenzene unit Z-configured, the bulkier anions iodide, tetrafluoroborate, and hexafluorophosphate result in a 1:2 tweezer-like anti-configuration in which one anion interacts simultaneously with two pentafluorobenzene units. When spatial separation of the two electron-deficient rings match the size of the anion, anion-π interactions induce a conformational change from the anti-form observed for the smaller anion to the tweezer-like syn-form.

A novel quinoline-substituted crown ether has been synthesized as a receptor for ion pairs and its binding behavior was investigated in various solvents. The binding of ion pairs in [D₆]DMSO and in mixed [D₆]DMSO/CD₃CN/CDCl₃ solvents was studied and the results revealed positive cooperativity effects. As a novel solubilizing agent, the crown ether can extract halide salts into organic media, especially chloride salts. ESI MS provided evidence for the formation of salt-receptor complexes. A recycling experiment showed that the receptor can solubilize salts and can be recovered. This process can be performed as a cycle. This study validates the idea that a straightforward combination of a common anion receptor and a well-studied cation complexation moiety enables the formation of ion-pair receptors and solubilizers of salts in organic solvents.

The ligands 1-H₆ and 2-H₆ are prepared by condensation of the triangular triamines 5 and 2,3-dihydroxybenzoic acid hydrazide 7. The ligands form container-type tetrahedral coordination compounds M_x [(1 or 2)₄M′₄] (M = Li, Na, K; M′ = Ti, x = 8; M′ = Ga, x = 12). The complexes are characterized by NMR spectroscopy and ESI mass spectrometry. Despite the relatively labile acyl hydrazone unit, the complexes show high stability in water. Ligand 3-H₆ is prepared from the trisacyl hydrazide 8 and 2,3-dihydroxybenzaldehyde 9 but does not form well-defined coordination compounds with gallium(III) or titanium(IV) ions.

A series of ammonium–carboxylate and ammonium–sulfonate betaines was synthesized and studied by single-crystal X-ray diffraction analysis to investigate the weak intermolecular interactions as well as the intramolecular interactions in the solid state. None of the expected intramolecular anion–π interactions could be observed, probably because of the steric demands and the reduced nucleophilicity of the anionic part of the betaines. Nevertheless, a weak intermolecular anion–π interaction between the anionic part of the betaine and the pentafluorophenyl unit is present in the structure of 5a.

Phenanthroline-based hexadentate ligands L¹ and L² bearing two achiral semicarbazone or two chiral imine moieties as well as the respective mononuclear complexes incorporating various lanthanide ions, such as La^III, Eu^III, Tb^III, Lu^III, and Y^III metal ions, were synthesized, and the crystal structures of [ML¹Cl₃] (M=La^III, Eu^III, Tb^III, Lu^III, or Y^III) complexes were determined. Solvent or water molecules act as coligands for the rare-earth metals in addition to halide anions. The big Ln^III ion exhibits a coordination number (CN) of 10, whereas the corresponding Eu^III, Tb^III, Lu^III, and Y^III centers with smaller ionic radii show CN=9. Complexes of L², namely [ML²Cl₃] (M=Eu^III, Tb^III, Lu^III, or Y^III) ions could also be prepared. Only the complex of Eu^III showed red luminescence, whereas all the others were nonluminescent. The emission properties of the Eu derivative can be applied as a photophysical signal for sensing various anions. The addition of phosphate anions leads to a unique change in the luminescence behavior. As a case study, the quenching behavior of adenosine-5′-triphosphate (ATP) was investigated at physiological pH value in an aqueous solvent. A specificity of the sensor for ATP relative to adenosine-5′-diphosphate (ADP) and adenosine-5′-monophosphate (AMP) was found. ³¹P NMR spectroscopic studies revealed the formation of a [EuL²(ATP)] coordination species.

Ditopic ligands (1–4)-H₂ are easily obtained by coupling 8-hydroxyquinoline-2-carboxylic acid with diamines. Dinuclear helicate-type complexes [La₂(L)₃] (L = ligand) are easily obtained with alkyl-bridged ligands 2 and 3. Short hydrazine as a linkage (1) seems to lead to polymerization, whereas with 4 only [Yb₂(4)₃] could be characterized by ESIMS. Comparison of the alkyl-bridged derivatives reveals that, in the case of short spacers, alkali cations can be incorporated into the interior of the helicate, whereas this is not observed with the butyl-bridged coordination compounds.

Simple pentafluorobenzyl-substituted ammonium and pyridinium salts with different anions can be easily obtained by treatment of the parent amine or pyridine with the respective pentafluorobenzyl halide. Hexafluorophosphate is introduced as the anion by salt metathesis. In the case of the ammonium salt 4, water co-crystallisation seems to suppress effective anion–π interactions of bromide with the electron-deficient aromatic system, whereas with salts 5 and 6 such interactions are observed despite the presence of water. However, due to asymmetric hydrogen-bonding interactions with ammonium side chains, the anion of 5 is located close to the rim of the pentafluorophenyl group (η¹ interaction). In 6 the CH–anion hydrogen bonding is more symmetric and fixes the anion on top of the ring (η⁶). A similar structure-controlling effect is observed in case of the 1,4-diazabicyclo[2.2.2]octane derivatives 7. Here the position of the anion (Cl, Br, I) is shifted according to the length of the weak CH–halide interaction. The hexafluorophosphate 7 d reveals that this “non-coordinating” anion can be located on top of an aromatic π system. In the methyl-substituted pyridinium salts 9 and 10 different locations of the bromide anions with respect to the π system are observed. This is due to different conformations of the mono- versus disubstituted pyridine, which leads to different directions of the weak, but structurally important, H_MeBr bonds.

Based on chiral, enantiomerically pure 7-[(S)-phenylethylurea]-8-hydroxyquinoline (1-H), trinuclear helicate-type complexes 2–5 are formed with divalent transition-metal cations. X-ray structural analyses reveal the connection of two monomeric complex units [M(1)₃]⁻ (M=Zn, Mn, Co, Ni) by a central metal ion to form a “dimer”. Due to the enantiopurity of the ligand, the complexes are obtained as pure enantiomers, resulting in pronounced circular dichroism (CD) spectra. Single-ion effects and intra- and intermolecular coupling are observed with dominating ferromagnetic coupling in the case of the cobalt(II) and nickel(II) and dominating antiferromagnetic coupling in the case of the manganese(II) complex.

The directionality of interaction of electron-deficient π systems with spherical anions (e.g,. halides) can be controlled by secondary effects like NH or CH hydrogen bonding. In this study a series of pentafluorophenyl-substituted salts with polyhalide anions is investigated. The compounds are obtained by aerobic oxidation of the corresponding halide upon crystallization. Solid-state structures reveal that in bromide 2, directing NH–anion interactions position the bromide ion in an η¹-type fashion over but not in the center of the aromatic ring. The same directing forces are effective in corresponding tribromide salt 3. In the crystal, the bromide ion is paneled by four electron-deficient aromatic ring systems. In addition, compounds 4 and 6, which have triiodide and the rare tetraiodide dianion as anions, are described. Computational studies reveal that the latter is highly unstable. In the present case it is stabilized by the crystal lattice, for example, by interaction with electron-deficient π systems.

Indole-based anion receptors with an carboxamide unit in 2- and an urea in 7-position were prepared and found to bind halides (as well as acetate and nitrate) in chloroform solutions at room temperature. Investigations of the binding behaviour show that the receptor is selective for chloride. Surprisingly, the truncated receptor 3 without the 2-carbamoyl substituent shows the highest affinity for Cl^–. Thorough ¹H, ¹³C and ¹⁵N NMR investigations indicate different binding modes for acetate, nitrate and halides to the receptor 2. The observation of a major conformational change of this receptor during the binding of the halide ions leads to an understanding of the relative binding affinities of 3 > 4 > 2 for chloride. The results of the NMR study are supported by ab initio calculations. In addition, ESI FTICR MS competition experiments of the indole 2 and the quinoline 1 reveal the “self-aggregation” of the receptors and show that halides have a higher affinity to 2 than to 1.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Homonuclear helicates with rare-earth-metal(III) ions or heteronuclear derivatives with rare-earth-metal and aluminium or zinc centres are obtained in alkali-metal-templated self-assembly processes from isobutenylidene-bridged homoditopic bis(2-carbamido-8-hydroxyquinoline)-derived ligands 1H₂ and 2H₂ or heteroditopic (8-hydroxyquinoline)(2-carbamido-8-hydroxyquinoline)-derived ligands 3H₂ and 4H₂. Diamagnetic coordination compounds possess a high stability in organic solvents such as CDCl₃, [D₄]MeOH or [D₆]DMSO and can be well characterised by ¹H NMR spectroscopy by using methylene protons and the protons of the vinylic units of the ligand as stereochemical or symmetry probes, respectively. Some of the homonuclear complexes could be crystallised and were characterised by using X-ray diffraction studies. The complexes adopt a triple-stranded helical structure with a central templating cation encapsulated in their interior. An unusual orientation of the double bond of one spacer towards this cation is observed. The homo- and heterodinuclear helicates with ytterbium(III), neodymium(III) or erbium(III) of ligands 2 and 4 were of special interest owing to their near-infrared (NIR) emitting properties, which were investigated depending on the lanthanide and on the encapsulated alkali-metal cation.