Phosphorescence Induction by Host-Guest Complexation with Cyclodextrins – The Role of Regioisomerism and Affinity

We present an in-depth investigation of cyclodextrin complexes with guest compounds featuring complexation-induced room temperature phosphorescence (RTP) in aqueous solution. Very interestingly, only the complexed regioisomers bearing lateral substituents on meta-position show RTP, whereas the stronger host-guest systems with para-substituted dyes show no RTP features. The reported systems were investigated regarding their complexation behavior in water using isothermal titration calorimetry and mass spectrometry. In the case of γ-CD very strong 1 : 1 inclusion complexes (K_a up to 5.13×10⁵ M⁻¹) were unexpectedly observed. It was found that not only a strong binding to the cyclodextrin cavity is needed to restrict motion, inducing the emission, but also the conformation inside the cavity plays a pivotal role – as supported by an extensive NMR study and MD simulations.     

Synthesis, binding properties and theoretical studies of p-tert-butylhexahomotrioxacalix[3]arene tri(adamantyl)ketone with alkali, alkaline earth, transition, heavy metal and lanthanide cations

p-tert-Butylhexahomotrioxacalix[3]arene tri(adamantyl)ketone (1b) was synthesized for the first time. Compound 1b was obtained in a cone conformation in solution at room temperature, as established by NMR spectroscopy (¹H and ¹³C). The binding properties of ligand 1b for alkali, alkaline earth, transition, heavy metal and lanthanide cations have been assessed by phase transfer and proton NMR titration experiments. Molecular mechanics and ab initio techniques were also employed to complement the NMR data. The results are compared to those obtained with other closely related homooxacalixarene derivatives. Although triketone 1b is a weak extractant, it shows a strong peak selectivity for Na⁺ and also some preference for Ag⁺. Proton NMR titrations indicate the formation of 1:1 complexes between 1b and the cations studied, and also that they should be located inside the cavity defined by the phenoxy and carbonyl oxygen atoms. Although the molecular mechanics results show little correlation with the NMR data, a good agreement was obtained with the ab initio models.     

Bis(benzimidazolium)methane salts: a potential guest for dibenzo-24-crown-8 towards [2]pseudorotaxanes

We investigated the threading of bis(benzimidazolium) methane moieties with DB24C8 at 35 °C (room temperature) and found a high degree of association. The presence of threaded complexes was determined by ¹H NMR and also supported by high resolution mass spectrometry and B3LYP/6-31G**++calculations. A 2D NMR study was done to elucidate the host-guest geometry and the molecular interactions present. The variable temperature NMR experiment was done over a temperature range of 243-323 K. The threading was stable and visible even at 323 K. Acid-base equilibrium established the reversibility of the process. DFT optimized structures establish that two H-bonded links are present between O₁?O_4′ and O₃?O_2′ across the dibenzo axis of the crown.     

Exceptionally Inert Lanthanide(III) PARACEST MRI Contrast Agents Based on an 18‐Membered Macrocyclic Platform

We report a macrocyclic ligand based on a 3,6,10,13‐tetraaza‐1,8(2,6)‐dipyridinacyclotetradecaphane platform containing four hydroxyethyl pendant arms (L¹) that forms extraordinary inert complexes with Ln³⁺ ions. The [EuL¹]³⁺ complex does not undergo dissociation in 1 M HCl over a period of months at room temperature. Furthermore, high concentrations of phosphate and Zn²⁺ ions at room temperature do not provoke metal‐complex dissociation. The X‐ray crystal structures of six Ln³⁺ complexes reveal ten coordination of the ligand to the metal ions through the six nitrogen atoms of the macrocycle and the four oxygen atoms of the hydroxyethyl pendant arms. The analysis of the Yb³⁺‐ and Pr³⁺‐induced paramagnetic ¹H NMR shifts show that the solid‐state structures are retained in aqueous solution. The intensity of the ¹H NMR signal of bulk water can be modulated by saturation of the signals of the hydroxy protons of Pr³⁺, Eu³⁺, and Yb³⁺ complexes following chemical‐exchange saturation transfer (CEST). The ability of these complexes to provide large CEST effects at 25 and 37 °C and pH 7.4 was confirmed by using CEST magnetic resonance imaging experiments.     

The States,Diffusion, and Concentration Distribution of Water in Radiation‐Formed PVA/PVP Hydrogels

Abstract

Hydrogels with various compositions of polyvinyl alcohol (PVA) and poly(1‐vinyl‐2‐ pyrrolidinone) (PVP) were prepared by irradiating mixtures of PVA and PVP in aqueous solutions with gamma‐rays from ⁶⁰Co sources at room temperature. The states of water in the hydrogels were characterized using DSC and NMR T₂ relaxation measurements and the kinetics of water diffusion in the hydrogels were studied by sorption experiments and NMR imaging. The DSC endothermic peaks in the temperature range ?10 to +10°C implied that there are at least two kinds of freezable water present in the matrix. The difference between the total water content and the freezable water content was referred to as bound water, which is not freezable. The weight fraction of water at which only nonfreezable water is present in a hydrogel with F_VP=0.19 has been estimated to be g_H2O/g_Polymer=0.375. From water sorption experiments, it was demonstrated that the early stage of the diffusion of water into the hydrogels was Fickian. A curve‐fit of the early‐stage experimental data to the Fickian model allowed determination of the water diffusion coefficient, which was found to lie between 1.5×10^?11 m² s^?1 and 4.5×10^?11 m² s^?1, depending on the polymer composition, the cross‐link density, and the temperature. It was also found that the energy barrier for diffusion of water molecules into PVA/PVP hydrogels was ≈24 kJ mol^?1. Additionally, the diffusion coefficients determined from NMR imaging of the volumetric swelling of the gels agreed well with the results obtained by the mass sorption method.     

Half-sandwich ruthenium complexes with Schiff base ligands bearing a hydroxyl group: Preparation,characterization and catalytic activities

Three half-sandwich ruthenium(II) complexes with hydroxyl group functionalized Schiff-base ligands [Ru(p-cymene)LCl] ( 2a-2c ) have been synthesized and characterized. All ruthenium complexes were fully characterized by ¹H and ¹³C NMR spectra, mass spectrometry and infrared spectrometry. The molecular structure of ruthenium complex 2c was confirmed by single-crystal X-ray diffraction methods. Furthermore, these half-sandwich ruthenium complexes were found to exhibit high catalytic activity for nitro compounds reduction using NaBH₄ reducing agent in the presence of cetyltrimethylammonium bromide (CTAB) in water at room temperature.     

Molecular tweezer and clip in aqueous solution: unexpected self-assembly, powerful host-guest complex formation, quantum chemical 1H NMR shift calculation

The newly prepared water-soluble naphthalene tweezer 2a and anthracene clip 4a (substituted both with lithium methanephosphonate groups in the central spacer unit) undergo an unexpected self-assembly in aqueous solution. The highly ordered intertwined structures of the self-assembled dimers [2a]2 and [4a]2 were elucidated by quantum chemical 1H NMR shift calculations. 2a and 4a form extremely stable host-guest complexes with N-methylnicotinamide 8 in methanol and water as well. According to the thermodynamic parameters determined by 1H NMR titration experiments at various temperatures the self-assembly of 2a and 4a and their strong binding to NMNA 8 observed in aqueous solution are enthalpy driven (DeltaH < 0); the enthalpic driving force is partially compensated by an unfavorable entropy (TDeltaS < 0). Self-assembly and the host-guest binding are therefore beautiful examples of the nonclassical hydrophobic effect.     

Synthesis,characterization, and insulin-enhancing studies of unsymmetrical tetradentate Schiff-base complexes of oxovanadium(IV)

A series of unsymmetrical tetradentate Schiff bases were synthesized by interaction of 2-hydroxy-1-naphthaldehyde, phenylenediamine and salicylaldehyde, or substituted salicylaldehyde in an ethanolic medium. The oxovanadium(IV) complexes and the ligands were synthesized and characterized by elemental analyses, ¹H NMR, infrared, electron paramagnetic resonance, electronic spectra, cyclic voltammetry, and room temperature magnetic susceptibility measurements. The elemental analyses for both the ligands and the metal complexes confirmed purity of the compounds as formulated. Electron paramagnetic resonance spectra of the complexes were measured as powder and in toluene/dichloromethane (9 : 1, v/v) solution at room and liquid N₂ temperatures. The g values, g _o = 1.971, g _⊥ = 1.978, and g = 1.950, are the same for all the complexes examined. The vanadium nuclear hyperfine splitting, A _o = 101–99, A _⊥ = 65–64, A _∥ = 179–177, vary slightly with substituents on the salicylaldehyde. Infrared spectra reveal strong V=O stretching bands in the range 970–988 cm^?1, typical of monomeric five-coordinate complexes. The room temperature magnetic moments of 1.6–1.8 BM for the complexes confirmed that the complexes are V(IV) complexes, with d¹ configuration. Only one quasi-reversible wave is observed for each compound and they all showed redox couples with peak-to-peak separation values (ΔE _p) ranging from 78 to 83 mV, indicating a single step one electron transfer process. Insulin-mimetic tests on C2C12 muscle cells using Biovision glucose assay showed that all the complexes significantly stimulated cell glucose utilization with negligible cytotoxicity at 0.05 µg µL^?1.     

NMR Study of Reactions between Pd,Ru, and Rh Nitrite Complexes with Sulfamic Acid

Reactions of nitrite complexes of Pd, Ru, and Rh with sulfamic acid were studied by the ^{14, 15}N, and ¹⁷O NMR method. Chemical shifts were assigned, and the predominant forms of the complexes were established. The reaction products at room temperature are cis-nitroaqua complexes. Coordination of the sulfamate ion upon storage for a long time or on heating was detected.     

Encapsulation of Trimethine Cyanine in Cucurbit[8]uril: Solution versus Solid-State Inclusion Behavior

Inclusion of polymethine cyanine dyes in the cavity of macrocyclic receptors is an effective strategy to alter their absorption and emission behavior in aqueous solution. In this paper, the effect of the host-guest interaction between cucurbit[8]uril (CB[8]) and a model trimethine indocyanine (Cy3) on dye spectral properties and aggregation in water is investigated. Solution studies, performed by a combination of spectroscopic and calorimetric techniques, indicate that the addition of CB[8] disrupts Cy3 aggregates, leading to the formation of a 1 : 1 host-guest complex with an association constant of 1.5×10⁶ M⁻¹. At concentrations suitable for NMR experiments, the slow formation of a supramolecular polymer was observed, followed by precipitation. Single crystals X-ray structure elucidation confirmed the formation of a polymer with 1 : 1 stoichiometry in the solid state.     

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⁺.     

Synthesis, characterization and properties of the polysiloxane-based episulfide resin

The polysiloxane episulfide resin (PSER) was synthesized through replacement of the oxygen atoms in 1,3,5,7-tetra-(3-glycidoxypropyl) tetramethylcyclotetrasiloxane (TGCS) with sulfur atoms using potassium thiocyanate (KSCN). It was characterized by FT-IR, ¹H NMR, MS and elemental analysis. The PSER resin was a low viscosity liquid, stable at room temperature. The polysiloxane episulfide resin was very reactive: a mixture of PSER and isophorondiamine gelated in a few seconds at room temperature. When m-phenylenediamine (m-PDA) or 2-ethyl-4-methylimidazole (2E4MZ) was used as curing agent, PSER exhibited higher reactivity compared with the parent polysiloxane epoxy resin. The reaction heat of the PSER resin was much lower in comparison with TGCS. The cured polysiloxane episulfide resin showed higher glass transition temperature and much lower water absorption, while the thermal stability was lower. It was found that methylhexahydrophthalic anhydride (MeHHPA) is not effective for curing the episulfide resin, although it is commonly used for curing epoxy resins.     

Synthesis,Conformational Interconversion,and Photophysics of Tethered Porphyrin–Fullerene Dyads with Parachute Topology

The synthesis of a porphyrin–fullerene dyad with “parachute” topology is reported. To determine whether the dyad is “flexing” at room temperature, low‐temperature NMR experiments were used. Computational modeling has shown the low‐energy conformation of the dyad to be nonsymmetric. Although, ¹H NMR spectroscopy at room temperature is consistent with a molecule with C_2v symmetry, the spectrum changes on lowering the temperature consistent with “windshield wiper”‐like motion, in which the porphyrin moiety rotates from one side of the C₆₀ sphere to the other. Nanosecond and picosecond fluorescence lifetime experiments show two components contribute to the fluorescence decay, also consistent with the presence of more than one conformer.     

Synthesis,spectroscopy, and biological activity of heterobimetallic complexes containing Sn(IV) and Pd(II) with 4-(hydroxymethyl)piperidine-1-carbodithioic acid

4-(Hydroxymethyl)piperidine-1-carbodithioic acid has been synthesized by reacting 4-piperidinemethanol with carbon disulfide at room temperature. The synthesized ligand was treated with organotin(IV) chlorides to yield organotin complexes. These complexes by further treatment with palladium chloride afford heterobimetallic complexes containing Sn(IV) and Pd(II). The synthesized complexes along with ligand has been characterized by elemental analysis, FTIR, NMR (¹H, ¹³C) spectroscopy. The IR data confirmed the complexation through the sulfur and oxygen donor sites of the ligand. NMR data revealed the tetrahedral geometry in the solution. Biological activity was checked against selected bacterial and fungal strains. All complexes along with the ligand were found to be biologically active with few exceptions.     

Conformational equilibria in 7-membered ring metal chelate complexes

Bis(diphosphine)metal and (diphosphine)(diene)metal (M = Rh, Ir) cationic complexes containing 7-membered chelate rings have been studied by low temperature ³¹P and ¹H NMR spectroscopy. For cyclooctadiene-1,4-bis(diphenylphosphino)butane- and -DIOP-rhodium, and cyclooctadiene-1,4-bis(diphenylphosphino)butane-iridium complexes, boat and chair conformations may be distinguished at low temperature. ¹H NMR spectra of these and analogous complexes suggest that both boat and chair conformers are significantly populated at room temperature. Bis(diphosphine) complexes of rhodium and iridium show very complex dynamic behaviour.     

Synthesis, spectroscopic, and thermal studies of some Hg(II) and Cd(II) coordination compounds of N,N-bis[(E)-3-(phenylprop)-2-enylidene]propanediamine

Some new coordination compounds of cadmium(II) and mercury(II) with N,N-bis[(E)-3-(phenylprop)-2-enylidene]propanediamine (L) as a new bidentate Schiff base ligand with general formula MLX₂ (X = Cl^?, Br^?, I^?, SCN^?, and N₃ ^?) have been prepared. They were characterized by elemental analysis, FT-infrared (FT-IR) and Ultraviolet–Visible spectra, ¹H- and ¹³C-NMR spectra. The reasonable shifts of FT-IR and NMR spectral signals of the complexes with respect to the free ligand confirm well coordination of ligand and anions(X-) in inner sphere coordination mode. The thermal behavior of the complexes from room temperature to 800 °C shows weight loss by decomposition of the anions and ligand segments in the subsequent steps. The results showed that cadmium complexes have no water molecules (neither as lattice nor as coordinated water) and are decomposed in two temperature steps except about cadmium thiocyanate complex that is decomposed in three steps. Final residual contents of cadmium complexes are suggested to be cadmium oxide or sulfide. Mercury complexes were decomposed in three to four temperature steps. Mercury bromide and azide complexes leave out a little amount of mercury oxide in final, while mercury chloride, iodide, and thiocyanate complexes were found to be completely decomposed without any residual matter.     

Synthesis,characterization, semi-empirical study,and biological activities of organotin(IV) complexes with cyclohexylcarbamodithioic acid as biological active ligand

Cyclohexylcarbamodithioic acid has been synthesized by the reaction of cyclohexylamine with carbon disulfide at room temperature. Its complexes have been synthesized by the reaction of cyclohexylcarbamodithioic acid with organotin(IV) chlorides in 1?:?1/1?:?2 molar ratio. The ligand and complexes have been characterized by elemental analysis, infrared (IR), and multinuclear (¹H, ¹³C, and ¹¹⁹Sn) NMR spectroscopy. Elemental data show good agreement between calculated and found values of carbon, hydrogen, nitrogen, and sulfur. IR data show that the ligand is bidentate and complexes exhibit a five-coordinate geometry in the solid state, which is also confirmed by semi-empirical studies. NMR data show that the complexes exhibit tetrahedral geometry in solution state. The ligand and its complexes were screened for their in vitro mutagenic, antimicrobial, MIC, antioxidant activities, and cytotoxicity. Biological screening data demonstrate that complexes show significant activity against various bacterial and fungal strains and are good antioxidants. The cytotoxicity data show positive lethality for complexes as compared to ligand and can play a very significant role in drug development.     

Synthesis,characterization, semi-empirical study and biological activities of homobimetallic complexes of tranexamic acid with organotin(IV)

The Schiff base has been synthesized by reacting tranexamic acid with indol-3-carboxyaldehyde in the first step and then with carbon disulfide at room temperature in the second step. The homobimetallic complexes have been synthesized by reaction of Schiff base with R₂SnCl₂ and R₃SnCl in 1?:?2?M ratio under stirring, where R?=?methyl, n-butyl and phenyl. The ligand and complexes have been characterized by elemental analysis, FT-IR, multinuclear NMR (¹H, ¹³C and ¹¹⁹Sn) and semi-empirical study. IR data reveal the bidentate nature of the ligand. Five- or six-coordinate geometry was confirmed in solution by NMR spectroscopy. The homobimetallic complexes and ligand were tested in vitro against some pathogenic bacteria and fungi to assess their antimicrobial properties. The complexes show biological activities with few exceptions.     

A spectroscopic investigation of incompletely condensed polyhedral oligomeric silsesquioxanes (POSS-mono-ol, POSS-diol and POSS-triol): Hydrogen-bonded interaction and host-guest complex

¹H NMR dilution experiment and FTIR were used to investigate the hydrogen bonded interaction in three different types of incompletely condensed silsesquioxanes (POSS-mono-ol, POSS-diol and POSS-triol). For POSS-triol, there existed a dynamic equilibrium between single molecule and hydrogen-bonded dimer, and the dimerization constants (K_dim) of POSS-triol in different solvents were determined by ¹H NMR dilution experiment. In addition, based on hydroxy group which acted as hydrogen bond donors, the possibility of three POSS silanols as anion receptors to form host-guest complexes was also explored in this paper.     

In and F MAS NMR study of (NH4)3InF6 phases

This study presents for the first time an NMR spectroscopic characterization of the room and high temperature phases of (NH₄)₃InF₆ using ¹⁹F and ¹¹⁵In as probe nuclei. The reversible phase transition to the cubic phase at 353 K was followed by MAS NMR in situ. Static NMR experiments of the room temperature phase and MAS NMR experiments of the high temperature phase allowed the determination of the NMR parameters of both nuclei. Finally, the scalar In-F coupling, rarely observed in solid state NMR, is evidenced in both room and high temperature phases of (NH₄)₃InF₆, and measured in the high temperature phase.