Host-guest complexes vs. supramolecular polymers in chalcogen bonding receptors: an experimental and theoretical study.

We describe here a comparative study between two tripodal anion receptors based on selenophene as the binding motif. The receptors use benzene or perfluorobenzene as a spacer. The presence of the electron-withdrawing ring activates the selenium atom for anion recognition inducing the formation of self-assembled supramolecular structures in the presence of chloride or bromide anions, which are bonded by the cooperative action of hydrogen and chalcogen bonding interactions. DOSY NMR and DLS experiments provided evidence for the formation of the supramolecular structures only in the presence of a perfluorobenzene based anion receptor while the analogous benzene one shows the classical anion/receptor complex without the participation of the selenium atom. The energetic and geometric features of the complexes of both receptors with the Cl and Br anions have been studied in solution. These results combined with the molecular electrostatic potential (MEP) surface plots allow us to rationalize the quite different behaviors of both receptors observed experimentally.

Anion Recognition by Neutral Chalcogen Bonding Receptors: Experimental and Theoretical Investigation.

Invited for the cover of this issue are the groups of Antonio Frontera at the Universitat de les Illes Balears and Antonio Caballero at the Universidad de Murcia. The image depicts one of the tellurophene based receptors described in this work. Read the full text of the article at 10.1002/chem.201905786.

Anion Recognition by Neutral Chalcogen Bonding Receptors: Experimental and Theoretical Investigations.

The utilization of neutral receptors for the molecular recognition of anions based on chalcogen bonding (ChB) is an undeveloped area of host-guest chemistry. In this manuscript, the synthesis of two new families of sulfur, selenium, and tellurium-based ChB binding motifs are reported. The stability of the thiophene, selenophene, and tellurophene binding motifs has enabled the determination of the association constants for ChB halide anion binding in the polar aprotic solvent THF by 1 H, 77 Se, and 125 Te NMR experiments. Two different aromatic cores are used and one or two Ch-binding motifs are incorporated with the purpose of encapsulating the anion, offering up to two concurrent chalcogen bonds. Theoretical calculations and NMR experiments reveal that, for S and Se receptors, hydrogen-bonding interactions involving the acidic H atom adjacent to the chalcogen atom are energetically favored over the ChB interaction. However, for the tellurophene binding motif, the σ-hole interaction is competitive and more favored than the hydrogen bond.

Exploiting 1,4-naphthoquinone and 3-iodo-1,4-naphthoquinone motifs as anion binding sites by hydrogen or halogen-bonding interactions.

We describe here the utilization of 1,4-naphthoquinone and 3-iodo-1,4-naphthoquinone motifs as new anion binding sites by hydrogen- or halogen-bonding interactions, respectively. These binding sites have been integrated in bidentate ester based receptors. Emission experiments reveal that both receptors selectively recognize sulfate anions, which induced a remarkable increase of a new emission band attributed to the formation of π-stacking interactions between two 1,4-naphthoquinone units. Absorption spectroscopy and mass spectrometry indicate the disruption of the ester group of the 1,4-naphthoquinone based receptor in the presence of HP2O73-, H2PO4-, F-, AcO- and C6H5CO2- and in the halogenated receptor with HP2O73-, F- and AcO- anions, while the presence of sulfate anions showed the clasical complexation behaviour. The 1H-NMR experiment showed a slow exchange process of the receptors with their sulfate complexes. The binding mode of the receptors with sulfate has been studied by DFT calculations along with the Molecular Electrostatic Potential (MEP) surface computational tool that reveals those parts of the receptors which are more suitable for interacting with anions.

Heteroditopic ferrocene-based ureas as receptors for anions and cations.

The synthesis of structurally new types of ferrocene-based ureas, in which the ferrocene moiety is simultaneously attached to two urea groups, have been prepared directly from 1,1-bis(isocyanato)ferrocene 1. Receptors 2a, 2b, and 2d show spectral and electrochemical anion-sensing capability for hydrogenphosphate and fluoride anions, in addition 2d also shows a sensing capability for acetate anion. Heteroditopic receptor 2a, bearing a pyridine binding site, does not complex Cu2+ cations but instead an intramolecular redox reaction takes place to give the oxidized form 2a+. The results obtained by using the heteroditopic receptor 2b, containing two crown ethers units, demonstrate that K+ cations can only be electrochemically detected in the presence of hydrogenphosphate anion. Whereas the new structural motif 2d, in which the two urea groups are part of the macrocycle framework, is a selective electrochemical sensor for Li+ cation in the presence of alkali metal ions. The preferred binding modes and their extent are proposed for the most representative complexes by means of DFT-based theoretical calculations.

Ferrocene-based ureas as multisignaling receptors for anions.

The synthesis of structurally new types of ferrocene-based ureas, in which the ferrocene moiety is simultaneously attached to two urea groups, have been prepared directly from 1,1'-bis(isocyanato)ferrocene 1. Homoditopic receptors 2a-e show spectral and electrochemical anion-sensing action: they display a selective downfield shift of the urea protons and a cathodic shift of the ferrocene/ferrocinium redox couple with hydrogen phosphate and fluoride anions. In addition, receptor 2d based on an unprecedent tetraaza[9]ferrocenophane architecture, shows spectral, electrochemical, and selective fluorescent responses to fluoride anion.

Ferrocene-thiophene dyads with azadiene spacers: electrochemical, electronic and cation sensing properties.

The synthesis, electrochemical, electronic and cation sensing properties of ferrocene-thiophene ligands, linked by 2-aza-1,3-butadiene bridges, whose characteristics have been systematically varied by introducing the ferrocene moiety at the 1- or 4-position of the aza bridge, are presented. Spectroelectrochemical studies revealed the presence of low-energy bands in the partially oxidized forms, which indicate the existence of intramolecular electron-transfer between the iron center and the organic bridges. Some of the reported ligands have also shown to be efficient chemosensors for metal ions. Compounds 5a and 5b function as highly selective chemosensors for Mg2+ ions, while not showing any response to Ca2+ or alkali metal ions, whereas ligand 3b shows a selective sensing response to the soft Cd2+ metal ions over Hg2+ cations.

Preparation, structure, and anion sensing properties of 1,n-diaza[n]ferrocenophanes.

The synthesis of structurally new types of azaferrocenophanes is reported in this note: 4, which comprises a 1,2-fused azaheterocycle to a 1,3-diaza[3]ferrocenophane framework; 1,3-diaza[3]ferrocenophanes 5 and 7, which can be considered as a 1,1'-ferrocenylene N,N'-guanidine or urea, respectively; and 1,3,6,8-tetraza[8]ferrocenophanes 9 and 10, bearing two ureido moieties in the ansa-bridge. These compounds were prepared directly from 1,1'-bis(isocyanato)ferrocene 1 and thoroughly characterized by spectroscopic means. Tetraza[8]ferrocenophanes 9 and 10 show spectral and electrochemical anion-sensing action: they display a selective downfield shift of the urea protons and a cathodic shift of the ferrocene/ferrocinium redox couple with dihydrogenphosphate and fluoride anions. The crystal structure of compound 4 has been determined by single-crystal X-ray methods.

Multifunctional linear triferrocene derivatives linked by oxidizable bridges: optical, electronic, and cation sensing properties.

[structure: see text]. A novel linear triferrocene derivative has been prepared and proven to be of special interest in the study of intramolecular electron transfer and as a specific optical and electrosensor for Mg(2+) cation.

A ferrocene-based heteroditopic ligand for electrochemical sensing of cations and anions.

A ferrocene-based heteroditopic receptor containing urea and crown ether units shows electrochemical responses to dihydrogenphosphate and fluoride anions. K+ cations can only be detected in the presence of dihydrogenphosphate.

Selective fluorescence sensing of Li+ in an aqueous environment by a ferrocene-anthracene-linked dyad.

A new 1-(9-anthryl)-4-ferrocenyl-2-aza-1,3-butadiene showing selective fluorescence enhancement upon binding to lithium cations over other alkaline cations in aqueous environment has been conveniently synthesized and characterized. [structure: see text]

A new fluoride selective electrochemical and fluorescent chemosensor based on a ferrocene-naphthalene dyad.

A new difunctionalized receptor based on an aza-ferrocenophane structure shows electrochemical and fluorescent responses to fluoride anion.

Modulation of the metal recognition properties of a new type of azaferrocenophane-based chemosensors: co-ordination chemistry towards Mg2+, Ca2+, Zn2+ and Ni2+.

The structure-redox chemistry relationship of a new type of azaferrocenophane-based chemosensors, 3 and 4, in the presence of protons and several kinds of metal ions, has been studied. Electrochemical studies, carried out in CH2Cl2, in the presence of increasing amounts of Mg2+, Ca2+, Zn2+ and Ni2+ showed that the wave corresponding to the Fc/Fc+ couple of the uncomplexed ligands is gradually replaced by a new reversible wave at more positive potentials corresponding to the Fc/Fc+ couple of the complexed ligands. The maximum shift of the ferrocene oxidation wave was found for 4b in the presence of Mg2+, whereas for 3f a selective sensing response for Mg2+ in the presence of hydrated Ca2+ cations was observed, with a concomitant highly visual output response consisting of a deep purple colour.