Heavy metal complexation by N-acyl(thio)urea-functionalized cavitands: synthesis, extraction and potentiometric studies

The synthesis and binding properties of resorcinarene-based cavitands functionalized with N-acylthiourea moieties towards different cations are described. Extraction studies with metal (Pb²⁺, Cu²⁺, Ag⁺, Hg²⁺, Cd²⁺, Eu³⁺, Fe³⁺, K⁺, Na⁺, and Ca²⁺) picrates and the incorporation in ion selective electrodes (ISEs), show that there is more than a 40% increase of the Ag⁺ extraction for N-acylthiourea ionophores (2, 3, and 8) in comparison with N-benzoyl-N′-benzylthiourea (9). Ionophore 8, which has a C₃ chain between the platform and the ionophore, extracts two times more Cu²⁺ than the more rigid one (2). Stoichiometry studies showed for ligand 2 a ligand/metal ratio of 1:1, while for model compound 9 a ratio of 1:2 was found. Potentiometric studies of electrodes revealed that cavitands 2, 3, and 8 induce a significantly different selectivity pattern compared to the cation-exchanger used, as well as model compound 9. Especially, a considerable enhancement of the selectivity towards Ag⁺ and Pb²⁺ over K⁺, Ca²⁺, and Na²⁺ ions was observed.     

Synthesis, characterization and electrochemical properties of two new calix[4]arene derivatives bearing two ferrocene imine or ferrocene amine units at the upper rim

A novel calix[4]arene derivative with two ferrocenyl Schiff-base groups at the upper rim 3 has been synthesized from 5,17-diformyl-25,27-dipropoxy-26,28-dihydroxy calix[4]arene and 4-ferrocenylaniline via condensation reaction. Reduction of 3 with sodium borohydride led to calix[4]arene derivative 4 with two amino ferrocenyl groups at the upper rim. The ferrocenyl Schiff-base calix[4]arene and its corresponding reduced amine have been purified and characterized by elemental analysis,¹H NMR, FTIR, Mass and UV-vis spectral data. Electrochemical properties of compounds 3 and 4 have been investigated. Cyclic voltammograms of 3 and 4 show reversible redox couples of ferrocene/ferrocinium at E_1/2=0.401 V and 0.346 V, respectively. Electrochemical studies show these redox active compounds electrochemically recognize trivalent lanthanides La³⁺ and Ce³⁺ and divalent Pb²⁺ and Cu²⁺cations. With ferrocenyl Schiff-base calix[4]arene 3 an anodic shift as large as 130 mV is observed on addition of one equivalent of Ce³⁺ ion. Also extraction properties of compound 4 towards some metal cations have been described. It has been observed that compound 4 has a good selectivity for metal cations Fe³⁺, Cu²⁺, Pb²⁺ and Cd²⁺ against Ni²⁺ and Co²⁺.     

Metal ion complexation in acetonitrile by upper-rim allyl-substituted, di-ionized calix[4]arenes bearing two dansyl fluorophores

The influence of Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Ag⁺, Cd²⁺, Co²⁺, Fe²⁺, Hg²⁺, Mn²⁺, Pb²⁺, Zn²⁺ and Fe³⁺ on the spectroscopic properties of two dansyl (1-dimethylaminonaphthalene-5-sulfonyl) groups linked to the lower rims of a series of three, structurally related, di-ionized calix[4]arenes is investigated by means of absorption and emission spectrophotometry. Di(tetramethylammonium) salts of the di-ionized ligands, L, L1, and L2, which differ in having no, two and four allyl groups, respectively, on the upper rim of the calix[4]arene scaffold, are utilized for the spectrofluorimetric titration experiments in MeCN. On complexation by alkaline earth metal cations, the emission spectra undergo marked red shifts and quenching of the dansyl fluorescence. These effects are weaker with alkali metal cations. Transition metal cations and Pb²⁺ interact strongly with the ligands. In particular, Fe³⁺, Hg²⁺ and Pb²⁺ cause greater than 99% quenching of the dansyl fluorescence.     

Complexing properties of two benzocrown-ether moieties arranged at a cyclobutane ring system

Biscrown ethers 2a-c and 3a-c arranged at a cyclobutane ring were prepared by intermolecular [2+2] photocycloaddition of vinylated benzocrown ethers. The complexing behavior of 2a-c toward alkali metal cations was evaluated by ESI-MS analysis, liquid-liquid extraction, and the comparison of complexing stability constant. An intramolecular sandwich-type 1:1 (host/guest) complexation was observed by ESI-MS analysis in the competitive system where 2a-Na⁺, 2b-K⁺, and 2c-Cs⁺ were formed selectively. In the liquid-liquid extraction, however, 2a hardly extracted any cation, while both 2b and 2c efficiently extracted larger cations such as K⁺, Rb⁺, and Cs⁺. It was found that the complexing stability constant of 2a-Na⁺ is lower than that of benzo-15-crown-5-Na⁺ though extraordinarily high values were obtained for 2b-K⁺ and 2c-Cs⁺ complexes compared with those of 18-crown-6-K⁺ and dibenzo-24-crown-8-Cs⁺ complexes, respectively. Hence, the excellent complexing ability was achieved by using the cyclobutane ring, which strongly preorganized two benzocrown-ether moieties for the larger alkali metal cations.     

Epoxy-based polymer bearing 1-naphthylamine units: highly selective fluorescent chemosensor for ferric ion

A simple epoxy-based polymer 1 bearing 1-naphthylamine units has been synthesized and its recognition behaviors toward various metal ions have been investigated in THF-water (8:2, v/v) solution. The designed polymer 1 was found to exhibit selective ON-OFF-type fluorosensing behavior toward Fe³⁺ ions over other representative metal ions such as Cu²⁺, Zn²⁺, Co²⁺, Ni²⁺, and Hg²⁺ ions.     

Fluorescent sensor of fluorene derivatives having phosphonic acid as a fluorogenic ionophore: synthesis and static quenched properties for Fe(III)

A series of novel fluorene derivatives containing –PO(OH)₂ (1, 2), –HS (3), and –N⁺ (CH₃)₃ (4) were synthesized and only phosphonic-functionalized fluorene derivatives can detect Fe³⁺ with high selectivity over other metal ions. The fluorescence quenching of 1 and 2 with titration Fe³⁺ in water–DMF solution fits the Perrin model of static quenching. Spectral analysis showed that Fe³⁺ bound with fluorene-based chemosensor through the O-atoms at the phosphonic group to form nonfluorescent complexes. The binding ratio was estimated by the Job plot and the trace levels of iron in samples were successfully monitored.     

Synthesis and characterization of ferrocenyl-acridine dyads and their multiresponse to proton and metal cations

Two new π-conjugated linked ferrocenyl-acridine dyads, (9-ethynylferrocenyl)acridine (3a) and (1-(ferrocenylethynyl)-4-ethynylbenzenyl)-acridine (3b), have been synthesized and investigated. UV-Vis spectroscopic and electrochemical studies reveal that 3a offers stronger electronic communication between terminal subunits than the extended system 3b, as shown by a stronger and lower-energy metal-to-ligand charge transfer (MLCT) transition and a more positive redox potential. Both of 3a and 3b show multiresponse to protons and selected metal ions (M = Zn²⁺, Pb²⁺, Hg²⁺, Fe³⁺, Cr³⁺), with a MLCT transition shift to the lower-energy, a redox potential shift to anode, and a luminescence increasing.     

FRET-derived ratiometric fluorescence sensor for Cu

New N-(pyrenylmethyl)naphtho-azacrown-5 (1) was synthesized as an ‘On-Off’ fluorescent chemosensor for Cu²⁺. Excited at 240 nm corresponding to the absorption of naphthalene unit (energy donor) of 1, emission at 380 nm from pyrene unit (energy acceptor) is observed, indicating that intramolecular fluorescence resonance energy transfer (FRET-On) occurs in 1. When Cu²⁺ is added to a solution of 1, however, the fluorescence of pyrene is strongly quenched (FRET-Off) whereas that of naphthalene group is revived. Such FRET ‘On-Off’ behavior of 1 is observed only in the case of Cu²⁺ binding, but not for other metal cations. The high selectivity of 1 toward Cu²⁺ can be potentially applied to a new kind of FRET-based chemosensor. The FRET On-Off behavior is supported by computational studies. The calculated molecular orbitals of HOMO and LUMOs suggest the excited-state interactions leading to FRET from naphthalene to pyrene in 1, but no electron density changes in 1·Cu²⁺ complex.     

Synthesis,characterization and metal ion complexation and extraction capabilities of calix[4]arene Schiff base compounds

The syntheses, characterization and metal ion complexation and extraction capabilities of six new calix[4]arene Schiff base compounds, 5–10, are reported. The preparation of the compounds was achieved by the condensation of 5,17-diamino-11,23-di-tert-butyl-25,27-di-n-butoxy-26,28-dihydroxycalix[4]arene with the appropriate aldehyde (5-bromosalicylaldehyde for 5, 4-anisaldehyde for 6, 4-(dimethylamino)benzaldehyde for 7, 9-anthracenecarboxaldehyde for 8, 1-pyrenecarboxaldehyde for 9, and 9-fluorenecarboxaldehyde for 10) in refluxing ethanol. The compounds were characterized by ¹H and ¹³C NMR spectroscopy, IR spectroscopy, high-resolution mass spectrometry and elemental analysis. The X-ray crystal structures of 7, 8 and 9 (as dichloromethane solvates) revealed that the calixarene molecules adopt H-bond stabilized, distorted-cone conformations and form centrosymmetric dimers in the solid state. Compounds 5–10 did not form host–guest complexes with NEt₄[(bdt)MoO₂(OSiPh₃)] (bdt^2–=benzene-1,2-dithiolate), a potential precursor for biologically relevant oxosulfido-Mo(VI/V) enzyme models; such host–guest complexes have the potential to stabilize these sought-after but highly reactive model compounds. In addition, the capabilities of 5–10 to extract selected metal ions (Ni²⁺, Co²⁺, Cu²⁺, Zn²⁺, Ag⁺, Pb²⁺, Cd²⁺ and Hg²⁺) from an aqueous into an organic phase have been assessed by picrate extraction experiments. Compound 5 displayed exceptional selectivity towards Ni²⁺, compound 7 exhibited enhanced extraction towards all of the metal ions tested and compounds 6, 9 and 10 showed very high selectivity towards Hg²⁺. On the other hand, compound 8 exhibited negligible capacity to extract any of the metal ions tested.     

Luminophore-immobilized mesoporous silica for selective Hg sensing

Novel mesoporous silica-immobilized rhodamine (MSIR) and silica particle-immobilized rhodamine (SPIR) anchored by a tren (N(CH₂CH₂NH₂)₃) were synthesized. The binding and adsorption abilities of both MSIR and SPIR for metal cations were investigated with fluorophotometry and ion chromatography, respectively. Both MSIR and SPIR show selectivity for Hg²⁺ ion over other metal cations because the Hg²⁺ ion selectively induces a ring opening of the rhodamine fluorophores. The sensitivity of the MSIR for Hg²⁺ ion is greater than that of the SPIR and the MSIR adsorbs 70% of Hg²⁺ ion while the SPIR does only 40%. The MSIR can be also easily recovered by treatment of a solution of TBA⁺OH⁻. For the application of Hg²⁺ detection in the environmental field, the MSIR-coated glass plate is also developed and exhibits an excellent function in visual and fluorescence changes with Hg²⁺ ion.     

A new Fe fluorescent chemosensor based on aggregation-induced emission

A new tetraphenylethylene (TPE)-based sensor M1 bearing double 2-methylpyridyl-2-methylthiophenylamino units linked with triazole moieties was reported. Both UV–vis and fluorescence spectroscopic studies demonstrated that M1 was highly sensitive and selective toward Fe³⁺ over other metal ions in THF/H₂O solution based on the aggregation-induced emission quenching mechanism. The lowest detection limit of M1 for Fe³⁺ is 0.7 μM. The detailed fluorescent titration study suggested that the binding stoichiometry of the M1–Fe³⁺ complex was 1:2, and the structure between M1 and the Fe³⁺ complex was confirmed by the ¹H NMR titration.     

A highly selective turn-on fluorescent chemosensor for copper(II) ion

A new pyrene derivative (1) containing a diaminomaleonitrile moiety exhibits high selectivity for Cu²⁺ detection. Significant fluorescence enhancement was observed with chemosensor 1 in the presence of Cu²⁺. However, the metal ions Ag⁺, Ca²⁺, Cd²⁺, Co²⁺, Fe²⁺, Fe³⁺, Hg²⁺, Mg²⁺, Mn²⁺, Ni²⁺, Pb²⁺, and Zn²⁺ produced only minor changes in fluorescence values for the system. The apparent association constant (K_a) of Cu²⁺ binding in chemosensor 1 was found to be 5.55×10³ M⁻¹. The maximum fluorescence enhancement caused by Cu²⁺ binding in chemosensor 1 was observed over the pH range 5-7.5.     

Synthesis, characterization, and evaluation of rhodamine based sensors for nerve gas mimics

Six new rhodamine-B based compounds were synthesized (1-6) and used as fluorescent turn-ON sensors for diethyl chlorophosphate (DCP) in aqueous media at pH 7.0. Compound 1 and 3 gave high fluorescent enhancement with DCP compared to the other compounds. Very high selectivity and sensitivity were observed as these compounds did not show significant fluorescent enhancement with dimethyl methylphosphonate (DMMP), HCl, and metal ions, such as Na⁺, K⁺, Ca²⁺, Cr³⁺, Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Hg²⁺, Pb²⁺. Depending on the way the sensor is presented, results are instantaneous or observed over some minutes.     

Selective photoswitching of a dyad with diarylethene and spiropyran units

A dyad bearing diarylethene and spiropyran units were synthesized. Ultraviolet light, visible light, H⁺, and Fe³⁺ inputs induce the multiple interconversion among the colorless diarylethene with spiropyran form (3), the colored closed form of diarylethene with spiropyran form (4), ME (5), MEH (6, 7) and MEH·Fe³⁺ (8). The efficient energy transfer from the anthracene emission to MEH·Fe³⁺ or ME·Fe³⁺ form was achieved. Using multi-mode photo switching in a dyad 3, logic gates may be built.     

Synthesis and crystal structure characterization of iron(II), cobalt(II) and nickel(II) complexes with 1,3-bis(2-pyridylmethylthio)propane

Three new mononuclear complexes of nitrogen–sulfur donor sets, formulated as [Fe^II(L)Cl₂] (1), [Co^II(L)Cl₂] (2) and [Ni^II(L)Cl₂] (3) where L = 1,3-bis(2-pyridylmethylthio)propane, were synthesized and isolated in their pure form. All the complexes were characterized by physicochemical and spectroscopic methods. The solid state structures of complexes 1 and 3 have been established by single crystal X-ray crystallography. The structural analysis evidences isomorphous crystals with the metal ion in a distorted octahedral geometry that comprises NSSN ligand donors with trans located pyridine rings and chlorides in cis positions. In dimethylformamide solution, the complexes were found to exhibit Fe^II/Fe^III, Co^II/Co^III and Ni^II/Ni^III quasi-reversible redox couples in cyclic voltammograms with E_1/2 values (versus Ag/AgCl at 298 K) of +0.295, +0.795 and +0.745 V for 1, 2 and 3, respectively.     

Synthesis, NMR characterization and ion binding properties of 1,3-bridged p-tert-butyldihomooxacalix[4]crown-6 bearing pyridyl pendant groups

1,3-Di(2-pyridylmethoxy)-p-tert-butyldihomooxacalix[4]arene-crown-6 (2) was synthesized for the first time. 2 was isolated in a cone conformation in solution at room temperature, as established by NMR spectroscopy (¹H, ¹³C and NOESY). Complete assignment of both proton and carbon NMR spectra was achieved by a combination of COSY, HSQC and HMBC experiments. The binding properties of ligand 2 towards alkali, alkaline earth, transition and heavy metal cations have been assessed by phase transfer and proton NMR titration experiments. The results are compared to those obtained with other dihomooxacalix[4]arene-crowns-6 and closely-related calix[4]arene-crown derivatives. 2 shows a preference for the soft heavy metal cations (except for Cd²⁺), with a very strong affinity for Ag⁺. Some transition metal cations are also well extracted. 2 forms 1:1 complexes with K⁺, Ca²⁺ and Ag⁺, and ¹H NMR titrations indicate that they should be encapsulated into the cavity defined by the crown ether unit and by the two pyridyl pendant arms. A 1:2 (ML₂) complex is formed with Zn²⁺ and two species, probably 1:1 and 1:2 complexes, are obtained with Pb²⁺.     

Ferrocenoyl-amino acids: redox response towards di- and trivalent metal ions

The interaction of six ferrocene-amino acid conjugates (Fc-CO-Gly-OH (2a), Fc-CO-Asp-OH (3a), Fc-CO-Glu-OH (4a), 1,1′-Fc(CO-Gly-OH) (2b), 1,1′-Fc(CO-Asp-OH) (3b), and 1,1′-Fc(CO-Glu-OH) (4b)) with Mg²⁺, Ca²⁺, Zn²⁺, La³⁺, and Tb³⁺ was investigated in aqueous solutions using cyclic voltammetry (CV) and differential pulse voltammetry (DPV). Addition of metal ions to solutions caused, in some cases, large changes in the half-wave potential, E_1/2. Our electrochemical results show that the Gly systems, 2a and 2b, show a preference for binding Mg²⁺, whereas the Asp and Glu conjugates prefer binding Ln³⁺.     

Spectroscopic,structure and DFT studies of copper(II) and palladium(II) complexes of pyridine-2-carboxaldehyde-2-pyridylhydrazone: A chromogenic agent for palladium(II)

A heterocyclic hydrazone ligand, pyridine-2-carboxaldehyde-2-pyridylhydrazone, HL, 1, was investigated as a new chromogenic agent for selective detection of Pd²⁺. The ligand HL, 1, undergoes 1:1 complexation with Pd²⁺ and Cu²⁺ to form complexes [Pd(L)Cl], 1a and [Cu(HL)Cl₂], 1b respectively. The complex 1a gives a characteristic absorption peak at 536 nm with distinct reddish-pink coloration. The change in color can easily be distinguished from other metal complexes by the naked eye. No obvious interference was observed in the presence of other metal ions (Na⁺, K⁺, Mg²⁺, Ca²⁺, Al³⁺, Mn²⁺, Fe³⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Sn²⁺, Hg²⁺, Pb²⁺). The association constants, K_ass (UV–Vis), were found to be 5.52 ± 0.004 × 10⁴ for 1a and 4.94 ± 0.006 × 10⁴ for 1b at 298 K. On excitation at 295 nm, the ligand HL, 1 strongly emits at 372 nm due to an intraligand ¹(π–π^∗) transition. Upon complexation the emission peaks are blue shifted (λ_ex 295 nm, λ_em 358 nm for 1a and λ_ex 295 nm, λ_em 367 nm for 1b) along with a quenching (F/F₀ 0.32 for 1a and 0.88 for 1b) in the emission intensity. DFT and TDDFT calculations were highly consistent with the spectroscopic behavior of the ligand and complexes. The molecular structure of the complex 1b has been determined by single crystal X-ray diffraction studies.     

Synthesis and metal ion complexation of spin labeled 18-crown-6 ethers containing an acridone or an acridine fluorophore unit

Double (spin and fluorescence) labeled pyrroline derivatives of crown ethers containing an acridone or an acridine fluorophore unit (1 and 2) and their diamagnetic analogues (3 and 4) were synthesized. Their fluorescent behavior as well as their complexation properties toward selected metal ions (Na⁺, K⁺, Mg²⁺, Ca²⁺, Zn²⁺) were examined.     

On the different coordination of Ni, Zn and Cd cations in their model Schiff base complexes - Single crystal X-ray and solid state NMR studies

Three model metal complexes: Ni(NCS)L, Zn₂(NCS)₂L₂ and Cd₂(NCS)₂L₂, consisting of the SCN⁻ anion(s) and L = 2-[(2-dimethylaminoethylimino)-methyl]-phenolate, have been studied by X-ray diffraction and solid state NMR spectroscopy. The metal cations in these complexes have different coordination modes: Ni²⁺ is almost square-planar, the Zn²⁺ cation in Zn₂(NCS)₂L₂ is pentacoordinated, whereas Cd²⁺ is penta- and hexacoordinated in [Cd₂(NCS)₂L₂]. The different coordination of the metal cations influences the chemical shifts of the metal cations and also the nitrogen atoms. These chemical shifts can be correlated with the M-N and M-O bond lengths (M = Ni²⁺, Zn²⁺, Cd²⁺).