Novel receptors with quinoline and amide moieties for the biologically important ions

New chromogenic anion receptors 2 and 4 utilizing quinoline and nitrophenyl groups as signaling groups were synthesized. In these receptors, amide and amine groups made strong multiple hydrogen interactions with anions. The receptors 2 and 4 bind anions with a selectivity of F⁻ > CN⁻ >  and proved to be an efficient naked-eye detector for the fluoride and cyanide ion.     

Pyrenyl-appended imidazolium receptor for selective fluorescence sensing of oxalic acid

A new fluorescent imidazolium-based cholestane receptor 4 bearing a pyrene moiety was synthesized. The binding ability of 4 toward various dicarboxylic acids was examined by UV-vis and fluorescence spectroscopy. Receptor 4 showed the highest binding constant for oxalic acid among all the tested dicarboxylic acids (K_a = 5.06 × 10⁴ M⁻¹). Oxalic acid formed a complex with 4 with a 1:2 ratio in ethanol.     

Synthesis and anion recognition properties of pyrrole-bearing acyclic receptors

A new series of acyclic anion receptors (1-4) based on methyl 5-(aminomethyl)-1H-pyrrole-2-carboxylate were designed and synthesized. The anion recognition properties of these receptors were examined by ¹H NMR spectroscopy and rationalized by density functional theoretical calculation. Receptor 1 displays the highest affinity and selectivity for anionic guests mainly due to the intramolecular hydrogen bonds and rigid molecular geometry.     

Anion recognition by a novel Fipronil-based receptor: efficient deprotonation or stable intermolecular hydrogen bonding

Strong electron-deficient heterocycles of acetyl Fipronil (F3) was designed and synthesized, its ability for anion recognition was investigated by UV and NMR analyses. This novel Fipronil-based receptor F3 shows strong binding affinity with acetate (?10⁷ M⁻¹), phosphate or fluoride ion through efficient deprotonation. In addition, its interaction with chloride anion or other weak base anions through stable intermolecular H-bonding was also reported.     

Infinite chains constructed from flexible bis(benzimidazole)-based ligands

Two neutral ligands, L¹ · 2H₂O and L² · H₂O, and seven complexes, [Cu(pmb)₂(L¹)] (1), [Cu(pmb)₂(L²)] (2), [Cu(Ac)₂(L²)] · 4H₂O (3), [Cu(4-aba)₂(L²)] (4), [Ag(4-ts)(L¹)(H₂O)] (5), [Ag₂(epes)₂(L¹)] · 2H₂O (6), [Ag(1,5-nds)_0.5(L²)] · 0.5C₂H₅OH · H₂O (7) [where L¹ = 1,1′-(1,4-butanediyl)bis(2-methylbenzimidazole); L² = 1,1′-(1,4-butanediyl)bis(2-ethylbenzimidazole), pmb = p-methoxybenzoate anion; Ac = acetate anion; 4-aba = 4-aminobenzoate anion; 4-ts = p-toluenesulfonate anion; epes = N-(2-hydroxyethyl)piperazine-N′-(2-ethanesulfonate) anion; 1,5-nds = 1,5-naphthalenedisulfonate anion], have been synthesized and characterized by elemental analysis, IR, and single-crystal X-ray diffraction. The L¹ and L² ligands in compounds 1–7 act as bridging ligands, linking metal ions into chain structures. The chains in compounds 3, 4 and 6 interlace with each other by hydrogen bonds to generate 3D supramolecular structures. In compound 5, π–π interactions between adjacent L¹ ligands hold the chains to a supramolecular layer. In compound 7, the sulfonate anions act as counterions in the framework. The thermal stabilities of 3, 6 and 7, and the luminescent properties for 5–7 in the solid states are also discussed.     

meso-Indanyl calix[4]pyrrole receptors

Two new meso-indanyl-substituted calix[4]pyrrole receptors, 2 and 3, have been synthesized. A range of calix[4]pyrrole host-neutral molecule complexes crystallise from solutions of 2 in a variety of solvents and the structures of four have been elucidated by X-ray crystallography. The F⁻ and Cl⁻ anion affinities of 2 have been measured in acetonitrile, and are significantly different from the corresponding affinities of the prototypical calix[4]pyrrole, the octamethyl-derivative, 1. ESI-FTICR-MS has been used to determine the relative F⁻ and Cl⁻ anion affinities of receptors 1 and 2 in methanol-acetonitrile solution. Deprotonation of 1 and 2 by fluoride is observed (under the conditions of the MS experiment).     

Intramolecular charge-transfer-induced chemiluminescent decomposition of 1,2-dioxetanes bearing a phenylmethanide anion

Dioxetanes (1) bearing a phenyl moiety substituted with a methylene or methine having an electron-withdrawing group(s) (-CH₂-Ew or -CH(X)-Ew) and dioxetane (2) bearing a 3-(1-cyanoethenyl)phenyl group were synthesized. Treatment of dioxetanes (1) with tetrabutylammonium fluoride (TBAF) caused their decomposition with accompanying emission of light with maximum wavelength at 530-758 nm. The Michael addition of a bis(methoxycarbonyl)methanide anion to dioxetane (2) produced initially an unstable dioxetane bearing a phenylmethanide anion, decomposition of which gave light with maximum wavelength at 710-740 nm. Intramolecular cyclopropanation without decomposition of the dioxetane ring took place concurrently for the Michael reaction-induced decomposition of 2 with the bis(methoxycarbonyl)chloromethanide anion.     

Novel thiophene based colorimetric and fluorescent receptor for selective recognition of fluoride ions

A colorimetric anion sensor 2,2′-(1E,1′E)-(thiophene-2,5-diylbis(methan-1-yl-1-ylidene)) was synthesized and characterized by various spectroscopic techniques. Anion binding studies were carried out using UV-visible spectrophotometric titrations and emission spectra studies, revealed that the receptor exhibits selective recognition toward F⁻over other anions. The selectivity for F⁻among the halides is attributed mainly to the hydrogen-bond interaction of the receptor with F⁻. Receptor 1 showed color change from fluorescent green to orange in the presence of tetrabutylammonium fluoride with 1:1 stoichiometry. Receptor 1 exhibits remarkably enhanced fluorescence intensity.     

A chloride selective sensor based on a calix[4]arene possessing a urea moiety

New calix[4]arene derivative 1 of 1,3-alternate conformation with a ureido moiety has been synthesized in high yield and examined for its anion recognition abilities towards anions such as fluoride, chloride, bromide, iodide, nitrate and acetate by ¹H NMR and UV-vis spectroscopy. The results show that receptor has strong binding affinity for chloride ions. A chloride ion selective electrode (ISE) was also formed which showed excellent selectivity over all the other anions tested. The limit of detection is 2.51 × 10⁻⁵ mol dm⁻³.     

The binuclear trimethyl/triethylantimony(V) bis-catecholate derivatives of four-electron reduced 4,4′-di-(3-methyl-6-tert-butyl-o-benzoquinone)

The reaction of 4,4′-di-(3-methyl-6-tert-butyl-o-benzoquinone) (Q-Q) with trimethyl- or triethylstibine proceeds as a two-electron oxidative addition of each quinone moiety and allows to prepare binuclear bis-catecholates R₃Sb(Cat-Cat)SbR₃ with a nearly quantitative yield (R = Me, 1; R = Et, 2). The products 1 and 2 were characterized by IR-, ¹H and ¹³C NMR spectroscopy. Molecular structure of triethylantimony(V) bis-catecholate 2 was confirmed by single-crystal X-ray analysis. Each Sb(V) atom adopts square pyramidal geometry with Cat moiety in basal positions, dihedral angle between Cat fragments is 74.27(5)°. The oxidation of bis-catecholates by ferrocenium tetrafluoroborate or tetrabromo-p-benzoquinone leads to loss of one trialkylantimony(V) moiety forming paramagnetic salts containing one-side decoordinated mononuclear semiquinone-catecholato [R₃Sb(Cat-SQ)]⁻ anion detected by EPR. The oxidation of 1 by air oxygen allows to prepare mononuclear quinone-catecholate Me₃Sb(Cat-Q) (3).     

Anion recognition by d-ribose-based receptors

Anion recognition properties of d-ribose-based receptors α- and β-1 were measured by ¹H NMR in CDCl₃ and MeCN-d₃. Receptor β-1 showed effective binding with anions by cooperative hydrogen bonds of cis-diol. The anomeric isomer α-1 is a less effective anion receptor which has similar cis-diol as a recognition site, indicating that the stereo configuration of the anomeric position is of significant influence on the anion recognition ability.     

Fluorescent sensing of anions using a bis-quinoxaline amidothiourea based supramolecular cleft; an example of an anion-induced deprotonation event

The quinoxaline 1, possessing a 2,6-pyridyl-based amidothiourea moiety, with the view of forming a pre-organised molecular cleft, was developed as a fluorescent anion sensor. The sensing ability of 1 was evaluated in organic solution where both the ground and the excited state of 1 was affected upon recognition of anions such as acetate [as tetrabutylammonium salt (TBAAc) solution] at the amiodothiourea moieties in MeCN. The fluorescence of 1, with λ_max at 477 nm, was, on all occasions quenched, upon anion recognition. Using TBAOH, we also show that the same anion-induced changes occurred; demonstrating that for this particular sensor, the anion-sensing takes place via a deprotonation mechanism. This anion-induced deprotonation event was further investigated by carrying out ¹H NMR titrations on 1, using both AcO⁻ and OH⁻ in DMSO-d₆.     

Synthesis and DNA cleavage properties of ternary Cu(II) complexes containing histamine and amino acids

The ternary complexes of [Cu^II(Hist)(Tyr)]⁺1 and [Cu^II(Hist)(Trp)]⁺2 have been synthesized, structurally characterized and their DNA binding and cleavage abilities probed. The intrinsic binding constants (K_b) for complexes/CT-DNA were also determined (K_b = 2.7 × 10² for complex 1 and K_b = 2.2 × 10² for complex 2). These complexes exhibit their nuclease activity on plasmid DNA, which seems to depend on the nature of the aromatic moiety. The DNA hydrolytic cleavage rate constants were also determined for complexes 1 and 2, which are 0.91 and 0.79 h⁻¹, respectively.     

Ion-pair charge transfer complexes with intense near IR absorption: Syntheses,crystal structures,electronic spectra and DFT calculations

Five ion-pair complexes, consisting of R-benzylidene-1-aminopyridinium derivatives and [Ni(mnt)₂]²⁻ (R = p-nitro (1), p-methyl (2), p-bromo (3), p-chloro (4) and m-nitro (5); mnt²⁻ = maleonitriledithiolate), were synthesized and structurally characterized. As for 1, it is interesting to observe a large deviation from square-planar coordination geometry for the Ni atom, while no deviation is observed in the other four complexes. In the solid state, UV–Vis–NIR spectra of 2–5 show similar properties with intense absorption in the 200–750 nm and moderate near IR absorption in the 750–1000 nm region, whereas 1 exhibits an intense absorption from UV/Visible to near-IR region (200–1100 nm). This unique spectral feature of 1 is attributed to its distinctive structural differences from 2 to 5, namely the strong intermolecular packing interactions between anions and cations, as well as a significant deviation from the planarity of the anion. Based on DFT and TDDFT calculations, near-IR absorbance bands in 1–5 were assigned to combined transitions of d–d, MLCT and π–π^∗ in the [Ni(mnt)₂]²⁻ anion as well as the ion-pair charge transfer (IPCT) from the anionic HOMO to the cationic LUMO. The IPCT band position in acetonitrile is independent of the substituent group feature in benzene ring of cations for 1–5, which could be interpreted that the substituent group in benzene ring only has a minor contribution to the cationic LUMO.     

Dual responsive chemosensors for anions: the combination of fluorescent PET (Photoinduced Electron Transfer) and colorimetric chemosensors in a single molecule

The design and synthesis of two novel fluorescent PET anion sensors is described, based on the principle of ‘fluorophore-spacer-(anion)receptor’. The sensors 1 and 2 employ simple diaromatic thioureas as anion receptors, and the fluorophore is a naphthalimide moiety that absorbs in the visible part of the spectrum and emits in the green. Upon recognition of anions such as F⁻ and AcO⁻ in DMSO, the fluorescence emission of 1 and 2 was ‘switched off’, with no significant changes in the UV-vis spectra. This recognition shows a 1:1 binding between the receptor and the anions. In the case of F⁻, further additions of the anion, gave rise to large changes in the UV-vis spectra, where the λ_max at 455 nm was shifted to 550 nm. These changes are thought to be due to the deprotonation of the 4-amino moiety of the naphthalimide fluorophore. This was in fact found to be the case, using simple naphthalimide derivatives such as 6. Sensors 1 and 2 can thus display dual sensing action; where at low concentrations, the fluorescence emission is quenched, and at higher concentrations the absorption spectra are modulated.     

Quaternary ammonium salt-based chromogenic and fluorescent chemosensors for fluoride ions

Chemosensors 5-7 possessing a quaternary ammonium cation (for electrostatic interactions) and an N-H group(s) (for H-bonding) as recognition sites and an anthracene-9,10-dione as both a chromogenic and fluorescent moiety exhibit absorption and emission changes with fluoride ions only. No significant response to other anions such as Cl⁻, Br⁻, I⁻, , CH₃COO⁻, , and is observed. The dual emission at λ_max 580 nm (free 5/6) and λ_max 510 and 540 nm (5/6 + F⁻) in chemosensors 5 and 6 enables ratiometric analysis of fluoride ions.     

Palladium and half sandwich ruthenium(II) complexes of selenated and tellurated benzotriazoles: Synthesis, structural aspects and catalytic applications

1-(Phenylselenomethyl)-1H-benzotriazole (L¹) and 1-(4-methoxyphenyltelluromethyl)-1H-benzotriazole (L²) have been synthesized by reacting 1-(chloromethyl)-1H-benzotriazole with in situ generated nucleophiles PhSe⁻ and ArTe⁻, respectively. The complexes of L¹ and L² with Pd(II) and Ru(II)(η⁶-p-cymene) have been synthesized. Proton, carbon-13, Se-77 and/or Te-125 NMR spectra authenticate both the ligands and their complexes. The single crystal structures of L¹, L² and [RuCl(η⁶-p-cymene)(L)][PF₆] (L = L¹: 3, L = L²: 4) have been solved. The Ru-Se and Ru-Te bond lengths have been found 2.4801(11) and 2.6183(10) Å, respectively. The palladium complexes, [PdCl₂(L)] (L = L¹: 1, L = L²: 2) have been explored for Heck and Suzuki-Miyaura C-C coupling reactions. The TON values are upto 95,000. The Ru-complexes have been found promising for catalytic oxidation of alcohols (TON ∼ 7.8-9.4 × 10⁴). The complexes of telluroether ligands are as efficient catalysts as those of selenoether ones and in fact better for catalytic oxidation.     

A novel strapped porphyrin receptor for molecular recognition

A novel strapped porphyrin receptor Zn1, in which two electron-rich bis(p-phenylene)-34-crown ether-10 units are incorporated, has been designed and synthesized from the newly developed intermediate 7 for investigating new chemistry of molecular recognition. ¹H NMR and UV-Vis studies revealed that Zn1 displays relatively weak binding abilities to neutral electron deficient naphthalene-1,8,4,5-tetracarboxydiimide (NDI) derivatives 13 (no simple complexing stoichiometry was observed), 19 (K_a=48(±5) M⁻¹) and 30 (K_a=46(±5) M⁻¹) in chloroform-d, strong binding ability to pyridine derivative 25, (K_a=1.5(±0.12)×10³ M⁻¹) in chloroform, moderately strong binding ability to tetracationic compound 35·4PF₆ (K_a=475(±50) M⁻¹) in acetone-d₆, and very strong binding affinity to compound 22 (K_a=6.5(±0.7)×10⁵ M⁻¹), which consists of one pyridine and two NDI units, in chloroform. Remarkable cooperative effect of the intermolecular metal-ligand coordination and donor-acceptor interactions in complex Zn1·22 was observed by comparing the complexing behaviors between Zn1 and the appropriately designed guests. Complex Zn1·22 possesses an unique three-dimensional tri-site binding feature. For comparison, the complexing affinity of 1 toward compounds 13, 19, and 30 in chloroform-d and 35·4PF₆ in acetone-d₆ has also been investigated and the binding patterns in different complexes were explored. The results demonstrate that strapped porphyrin derivatives are ideal precursors for constructing new generation of three-dimensional multi-site artificial receptors for molecular recognition and host-guest chemistry.     

New water-soluble iminecalix[4]arene with a deep hydrophobic cavity

We report the synthesis of a new water-soluble iminecalix[4]arene host 4c with a deep hydrophobic cavity. The negatively charged four carboxylate functions on the top of the cavity play a major role in the recognition of charged molecular species. The ¹H NMR titration experiments revealed that host 4c binds with cationic (10-12) and neutral guests (6-9) in water with high binding constants in the order of 10⁴-10⁵ M⁻¹. Cationic guest 9 showed highest binding constant of 2.81 × 10⁵ M⁻¹ with host 4c amongst all tested guests. Selectivity over anionic guests (13-17) is established by the presence of negative charges at the top of the deep hydrophobic cavity, as guests 15 and 17 were not recognized by host 4c. Neutral pyridine derivatives with hydrophobic chains at para positions showed high binding constants of 6.02 × 10⁴-2.23 × 10⁵ M⁻¹. The data obtained for the recognition of the guests by host 4c revealed that the ionic as well as the hydrophobic-hydrophobic interactions are crucial in the molecular recognition in aqueous medium.     

A “sodium trap” based on benzo-15-crown-5 with an exocyclic N-(thiophosphoryl)thiourea moiety

For N-(thio)phosphorylthioureas of the common formula RC(S)NHP(X)(OiPr)₂HL^I (R = N-(4′-aminobenzo-15-crown-5), X = S), HL^II (R = N-(4′-aminobenzo-15-crown-5), X = O), HL^III (R = PhNH, X = S), HL^IV (R = PhNH, X = O), and (N,N′-bis-[C(S)NHP(S)(OiPr)₂]₂-1,10-diaza-18-crown-6) H₂L^V, salts LiL^I,III,IV, NaL^I–IV, KL^I–IVM₂L^V (M = Li⁺, Na⁺, K⁺), Ba(L^I,III,IV)₂, and BaL^V have been synthesized and investigated. Compounds NaL^I,II quantitatively drop out as a deposit in ethanol medium, allowing the separation of Na⁺ and K⁺ cations. This effect is not displayed for the other compounds. The crystal structures of HL^III and the solvate of the composition [K(Me₂CO)L^III] have been investigated by X-ray crystallography.