A Cationic Catechol Derivative Binds Anions in Competitive Aqueous Media

A simple dihydroxy isoquinolinium molecule (3+ ) was prepared by a modification of a literature procedure. Interestingly, during optimisation of the synthesis a small amount of the natural product pseudopalmatine was isolated, and characterised for the first time by X-ray crystallography. Compound 3+ contains a catechol motif and positive charge on the same scaffold and was found to be a potent anion receptor, binding sulfate strongly in 8 : 2 d6 -acetone:D2 O and 7 : 3 d6 -acetone:D2 O (Ka >104 and 2,100 M-1 , respectively). Unsurprisingly, chloride binding was much weaker, even in the less polar solvent mixture 9 : 1 d6 -acetone:D2 O. The sulfate binding is remarkably strong for such a simple molecule, however anion binding studies were complicated by the tendency of the molecule to react with BPh4 - or BF4 - species during anion metathesis reactions. This gave two unusual zwitterions containing tetrahedral boronate centres, which were both characterised by X-ray crystallography.

Anion recognition by silanetriol in acetonitrile

Anion recognition ability and organocatalytic activity of a silanetriol are firstly presented by comparing with those of a series of silanol derivatives.

Molecular Self-Assembly as a Tool to Construct Transmembrane Supramolecular Ion Channels

Self-assembly has become a powerful tool for building various supramolecular architectures with applications in material science, environmental science, and chemical biology. One such area is the development of artificial transmembrane ion channels that mimic naturally occurring channel-forming proteins to unveil various structural and functional aspects of these complex biological systems, hoping to replace the defective protein channels with these synthetically accessible moieties. This account describes our recent approaches to construct supramolecular ion channels using synthetic molecules and their applications in medicinal chemistry.

Using gas-phase chloride attachment for selective detection of morphine in a morphine/codeine mixture by ion mobility spectrometry

RATIONALE

Morphine and codeine are two important compounds of the opiate family that have vast applications in medicine. Several techniques have been reported for the determination of these opiates. Although ion mobility spectrometry (IMS) in positive ion mode can be applied for detection of both morphine and codeine, this technique on its own cannot detect a mixture of these two compounds because of the overlapping of their peaks.

METHODS

An IMS instrument equipped with a corona discharge ion source operating in negative ion mode was used for the detection of anionic clusters of morphine and codeine. In normal negative ion mode, NO_x ^- , CO₃ ^- , and O_n ^- act as the main reactant ions (RIs) which can deprotonate the analytes. We also used chloroform as a dopant to produce Cl^- as an alternative RI.

RESULTS

Morphine has a phenolic and an alcoholic OH group, while codeine bears only an alcoholic OH group. Because the phenolic OH group is more acidic, only morphine is deprotonated in negative ion mode in a morphine/codeine mixture. Furthermore, since morphine has two OH groups that can act as hydrogen-bond donors, it acts as an anion receptor. Hence, in the presence of chloroform where Cl^- acts as the RI, morphine traps the Cl^- anion to form a morphine-Cl^- (Mor.Cl^- ) adduct ion, while because of its structure codeine does not have this capability.

CONCLUSIONS

Using the difference in the structures of morphine and codeine, two ionization methods were proposed for selective detection of morphine. Morphine is more acidic than codeine and has greater anion-receiving capability than codeine. Hence, it can both be deprotonated and form a adduct anion with Cl^- . The Cl^- attachment method is recommended for measurements at ambient temperature.

Formation of self-assembled supramolecular polymers by anti-electrostatic anion-anion and halogen bonding interactions

We report here the formation of self-assembled supramolecular polymers in which the cooperative action of anti-electrostatic anion-anion and halogen-bonding interactions serve as a powerful driving force for the formation of large supramolecular polymers. DOSY-NMR, DLS, TEM, SEM and X-ray experiments provide evidence of the formation of supramolecular structures in solution and solid state.

A Combination of Factors: Tuning the Affinity of Europium Receptors for Phosphate in Water

Although recognition of hard anions by hard metal ions is primarily achieved via direct coordination, electrostatic and hydrogen-bonding interactions also play essential roles in tuning the affinity of such supramolecular receptors for their target. In the case of Eu^III hydroxypyridinone-based complexes, the addition of a single charged group (-NH₃⁺, -CO₂^-, or -SO₃^-) or neutral hydrogen-bonding moiety (-OH) peripheral to the open coordination site substantially affects the affinity of the metal receptor for phosphate in water at neutral pH. A single primary ammonium increases the first association constant for phosphate in neutral water by 2 orders of magnitude over its neutral analogue. The addition of a peripheral alcohol group also increases the affinity of the receptor but to a lesser degree (21-fold). On the other hand, negatively charged complexes bearing either a carboxylate or sulfate moiety have negligible affinity for phosphate. Interestingly, the peripheral group also influences the stoichiometry of the lanthanide receptor for phosphate. While the complex bearing a -NH₃⁺ group binds phosphate in a 1:2 ratio, those with -OH and H (control) both form 1:3 complexes. Although the positively charged Eu^III-Lys-HOPO has the highest K_a1 for phosphate, a greater increase in luminescence intensity (36-fold) is observed with the neutral Eu^III-Ser-HOPO complex. Notably, whereas the affinity of the Eu^III complexes for phosphate is substantially influenced by the presence of a single charged group or hydrogen-bond donor, their selectivity for phosphate over competing anions remains unaffected by the addition of the peripheral groups.

Anion coordination chemistry using O-H groups

This review covers significant advances in the use of O-H groups in anion coordination chemistry. The review focuses on the use of these groups in synthetic anion receptors, as well as more recent developments in transport, self-assembly and catalysis.

Easily-prepared Hydroxy-containing Receptors Recognize Anions in Aqueous Media

Despite their ready availability, O-H groups have received relatively little attention as anion recognition motifs. Here, we report two simple hydroxy-containing anion receptors that are prepared in two facile steps followed by anion exchange, without the need for chromatographic purification at any stage. These receptors contain a pyridinium bis(amide) motif as well as hydroxyphenyl groups, and bind mono- and divalent anions in 9:1 CD₃ CN:D₂ O, showing a selectivity preference for sulfate. Notably, a "model" receptor that does not contain hydroxy groups shows only very weak sulfate binding in this competitive solvent mixture. In the solid state, X-ray crystallographic studies show that the receptors tend to form extended assemblies with anions; however, ¹ H and DOSY NMR studies as well as molecular dynamics simulations show that only 1:1 complexes are present in solution. Molecular dynamics simulations suggest that one of the receptors suffers from competing intramolecular hydrogen bonding, while another binds partially-hydrated anions, with the receptor's O-H groups forming hydrogen bonds to water molecules within the anion's coordination sphere.

Synthesis of per(5-N-carboxamide-5-dehydroxylmethyl)-β-cyclodextrins and their selective recognition ability utilizing multiple hydrogen bonds

An amide cyclodextrin with anion recognition ability exhibits unique binding mode in which unsymmetrically arranged functional groups play distinctive roles.

Efficient stabilisation of a dihydrogenphosphate tetramer and a dihydrogenpyrophosphate dimer by a cyclic pseudopeptide containing 1,4-disubstituted 1,2,3-triazole moieties

A cyclic pseudooctapeptide 2 is described containing 1,4-disubstituted 1,2,3-triazole moieties. This compound features eight converging hydrogen bond donors along the ring, namely four amide NH and four triazole CH groups, which enable 2 to engage in interactions with anions. While fully deprotonated sulfate anions exhibit only moderate affinity for 2, protonated anions such as dihydrogenpyrophosphate and dihydrogenphosphate anions are strongly bound. Complexation of the phosphate-derived anions involves sandwiching of a dihydrogenpyrophosphate dimer or a dihydrogenphosphate tetramer between two pseudopeptide rings. X-ray crystallography provided structural information, while 1H NMR spectroscopy, mass spectrometry, and isothermal titration calorimetry demonstrated that these complexes are stable in solution (2.5 vol% water/DMSO) and can even be transferred without decomposition into the gas phase. The observed high thermodynamic stabilities are attributed to the mutual reinforcement of the interactions between the individual complex components, namely, hydrogen-bonding between the anions, multiple hydrogen bonding interactions between the anion aggregates and the triazole CH and NH hydrogen bond donors of 2, and potential dispersive interactions between the closely arranged pseudopeptide rings. Pseudopeptide 2 thus represents a promising lead for the construction of phosphate receptors, whose binding selectivity makes use of the unique ability of certain anions to assemble into higher aggregates.

Active metal template synthesis of a neutral indolocarbazole-containing [2]rotaxane host system for selective oxoanion recognition

A CuAAC active metal template approach is used to prepare a new neutral indolocarbazole-containing [2]rotaxane anion host system which exhibits a rare interlocked host selectivity for oxoanions over halides.

Hydrogen bonding-induced conformational change in a crystalline sugar derivative

We report crystallographic evidence of the change of a regular chair conformation to a skew boat conformation in a partially protected sugar derivative.

Phosphate binding with a thiophene-based azamacrocycle in water

Structural characterization of the phosphate complex with a thiophene-based macrocycle suggests that two dihydrogen phosphates in a dimeric form are encapsulated in the cavity via several hydrogen bonds from NH···O and CH···O interactions. In the lattice framework, the two dimers are linearly hydrogen-bonded to form a tetramer. ¹H NMR titrations suggest that the host forms a 1:1 complex with phosphate, showing an association constant of 120 M^-1 in D₂O at pH = 5.5. The host guest complexation was further confirmed by ESI-MS in a gas phase.

Head-to-tail intermolecular hydrogen bonding of OH and NH groups with fluoride

To explore the anion-recognition ability of the phenolic hydroxyl group and the amino hydrogen, we synthesized three different acridinedione (ADD) based anion receptors, 1, 2 and 3, having OH, NH, and combination of OH and NH groups, respectively. Absorption, emission and (1)H NMR spectral studies revealed that receptor 1, having only a phenolic OH group, shows selective deprotonation of the hydroxyl proton towards F(-), which results in an "ON-OFF"-type signal in the fluorescence spectral studies. Receptor 2, which only has an amino hydrogen, also shows deprotonation of the amino hydrogen with F(-), whereas receptor 3 (having both OH and NH groups) shows head-to-tail intermolecular hydrogen bonding of OH and NH groups with F(-) prior to deprotonation. The observation of hydrogen bonding of the OH and NH groups in a combined solution of 1 and 2 with F(-) in a head-to-tail hetero-intermolecular fashion, and the absence of head-to-head and tail-to-tail intermolecular hydrogen bonding in 1 and 2 with F(-), prove that the difference in the acidity of the OH and NH protons leads to the formation of an intermolecular hydrogen-bonding complex with F(-) prior to deprotonation. The presence of this hydrogen-bonding complex was confirmed by absorption spectroscopy, 3D emission contour studies, and (1)H NMR titration.

Spectroscopic properties of the receptors based on 2- and 3-linked tri(indolyl)methanes for anion binding

The anion binding properties of two title tri(indolyl)methane compounds have been evaluated by using UV-vis absorption and fluorescence spectroscopy. Different from 3-linked receptor which exhibited faint even no obvious spectral response to anions, 2-linked receptor can bind F(-), AcO(-) and H(2)PO(4)(-), with the bathochromic shifts of the UV absorption and fluorescence quenching. This selectivity will be attributed to suitable steric array of NH binding sites of 2-linked receptor, which helps to form multiple hydrogen bonding interactions with anions.

Fluorescent detection of phosphate anion by a highly selective chemosensor in water

A macrocyclic dinuclear copper complex, [Cu(2) (II)(1)Br(4)]·2H(2)O has been synthesized and characterized by X-ray crystallography, in which each metal ion is pentacoordinated in a square pyramidal environment and the macrocycle is folded to form a boat-shaped empty cavity. As studied by an indicator displacement assay, the dinuclear complex shows strong selectivity for phosphate over sulfate, nitrate, perchlorate and halides in water at physiological pH.

Fluorescent carbazolylurea anion receptors

A series of fluorescent carbazolylurea base anion receptors have been synthesised that show a high affinity for oxo-anions (particularly bicarbonate and acetate). The fluorescence of dicarbazolylurea (1) is quenched upon addition of benzoate anions in DMSO-0.5% water.