Recognition of perrhenate and pertechnetate by a neutral macrocyclic receptor

The synthesis and anion binding properties of a neutral macrocyclic receptor bearing H-bond donor coordination sites are described. The anion binding studies by use of UV-vis and ⁹⁹Tc NMR methods revealed that the receptor can coordinate perrhenate and pertechnetate in dimethylsulfoxide and chloroform solutions with the relatively high binding constants, viz. log K _a > 4. The coordination mode of the perrhenate to the receptor was determined by a single-crystal X-ray diffraction analysis.     

Selective Encapsulation of Chloride Ions within Novel Cage Host Complexes in the Presence of Equimolar Amounts of Chloride and Bromide Ions

Four macrotricyclic cage hosts which feature four positive binding sites oriented toward the center of the intramolecular cavity are presented as promising candidates for anion receptors and they have been expected to play a important role in the selective encapsulation of the halide ion Cl⁻ or Br⁻. The complementarity between a macrotricyclic quaternary ammonium ion and Cl⁻ was achieved by fine-tuning of the four ammonium nitrogen atoms and the endocyclic methylene groups. The cage hosts [R₄N₄(C₅H₁₀)₄(C₆H₁₂)₂]⁴⁺ (abbreviated as [556]) showed perfect encapsulation of all chloride ions in acetonitrile at 0<r=([Cl⁻]_o/[[556]]_o)≤1 within the sensitivity of the ¹H NMR spectra in combination with a rather slow chemical exchange of the Cl⁻ ion in an encapsulation/decapsulation equilibrium with [556]. Further, the selective encapsulation of all the chloride ions into [556] cage occurs unambiguously at r=1 in the presence of equimolar amounts of Br⁻. The structural complementarity of the newly designed [556] host prevails over the Hofmeister-series restraints determined by differences in Gibbs free energy of halide anion solvation.     

Carboxylic acid derivatives of tetrathiafulvalene: key intermediates for the synthesis of redox-active calixarene-based anion receptors

A series of calixarene-TTF (TTF=tetrathiafulvalene) receptors incorporating amide binding units for anion recognition have been synthesized and characterized. For this purpose, two synthetically versatile new TTF carboxylic acid derivatives were prepared and characterized by X-ray diffraction, these structures demonstrating the critical role of the carboxylic function in the solid-state organization. Some of the calixarene-amide-TTF assemblies exhibit strong binding of various anions, as shown by ¹H NMR titration studies, and one receptor is able to electrochemically respond in the presence of H₂PO₄⁻, C₆H₅CO₂⁻ or CH₃CO₂⁻ anion.     

Molecular mechanics simulation studies of dienoic hydrocarbons: From alkenes to 1-Palmitoyl-2-linoleoyl-phosphatidylcholines

Although no crystal structures of mixed-chain phosphatidylcholines with unsaturated sn-2 acyl chains exist, the force field method in conjunction with the experimentally determined structure of saturated identical-chain phosphatidylcholine can be applied to simulate molecular structure for mixed-chain phosphatidylcholines. In this study, the packing models of mixed-chain 1-palmitoyl-2-linoleoyl-phosphatidylcholines in bilayers at temperatures below the gel-liquid crystalline phase transition temperature or T < T_m are simulated by using Allinger's MM3(92) force field. Our results indicate that the unsaturated sn-2 acyl chains of the mixed-chain lipid can fold into two energy-minimized topologies: the crankshaftlike and the U-shaped motifs. The folded region in the crankshiftlike sn-2 acyl chain is characterized by a sequence s⁻ Δs⁺s⁺Δs⁻, and the U-shaped chain arises from the characteristic sequence s⁻Δs⁺s⁻Δs⁺, where s^± denotes the ± skew conformation and Δ the cis carbon-carbon double bond. These modeled structures of 1-palmitoyl-2-linoleoyl-phosphatidylcholines in the bilayer at T < T_m should not be regarded as highly rigid structures, since torsion angles of carbon-carbon bonds associated with sequences s Δs⁺s⁺Δs and s Δs⁺s⁻Δ s⁺ can fluctuate somewhat without appreciably affecting the steric energy of the corresponding lipid bilayer. © 1996 by John Wiley & Sons, Inc.     

Light-Modulated Self-Blockage of a Urea Binding Site in a Stiff-Stilbene Based Anion Receptor

Anion binding to a receptor based on stiff-stilbene, which is equipped with a urea hydrogen bond donating group and a phosphate or phosphinate hydrogen bond accepting group, can be controlled by light. In one photoaddressable state (E isomer) the urea binding site is available for binding, while in the other (Z isomer) it is blocked because of an intramolecular interaction with its hydrogen bond accepting motif. This intramolecular interaction is supported by DFT calculations and ¹H NMR titrations reveal a significantly lower anion binding strength for the state in which anion binding is blocked. Furthermore, the molecular switching process has been studied in detail by UV/Vis and NMR spectroscopy. The presented approach opens up new opportunities toward the development of photoresponsive anion receptors.     

Rate constants for the reactions of C2H5O,i‐C3H7O,and n‐C3H7O with NO and O2 as a function of temperature

The rate constants of the reactions of ethoxy (C₂H₅O), i‐propoxy (i‐C₃H₇O) and n‐propoxy (n‐C₃H₇O) radicals with O₂ and NO have been measured as a function of temperature. Radicals have been generated by laser photolysis from the appropriate alkyl nitrite and have been detected by laser‐induced fluorescence. The following Arrhenius expressions have been determined: (R₁) C₂H₅O + O₂ → products k₁ = (2.4 ± 0.9) × 10⁻¹⁴ exp(−2.7 ± 1.0 kJmol⁻¹/RT) cm³ s⁻¹ 295K < T < 354K p = 100 Torr (R₂) i‐C₃H₇O + O₂ → products k₂ = (1.6 ± 0.2) × 10⁻¹⁴ exp(−2.2 ± 0.2 kJmol⁻¹/RT) cm³ s⁻¹ 288K < T < 364K p = 50–200 Torr (R₃) n‐C₃H₇O + O₂ → products k₃ = (2.5 ± 0.5) × 10⁻¹⁴ exp(−2.0 ± 0.5 kJmol⁻¹/RT) cm³ s⁻¹ 289K < T < 381K p = 30–100 Torr (R₄) C₂H₅O + NO → products k₄ = (2.0 ± 0.7) × 10⁻¹¹ exp(0.6 ± 0.4 kJmol⁻¹/RT) cm³ s⁻¹ 286K < T < 388K p = 30–500 Torr (R₅) i‐C₃H₇O + NO → products k₅ = (8.9 ± 0.2) × 10⁻¹² exp(3.3 ± 0.5 kJmol⁻¹/RT) cm³ s⁻¹ 286K < T < 389K p = 30–500 Torr (R₆) n‐C₃H₇O + NO → products k₆ = (1.2 ± 0.2) × 10⁻¹¹ exp(2.9 ± 0.4 kJmol⁻¹/RT) cm³s⁻¹ 289K < T < 380K p = 30–100 Torr All reactions have been found independent of total pressure between 30 and 500 Torr within the experimental error. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 860–866, 1999     

Facile and Reliable Emission-Based Nanomolar Anion Sensing by Luminescent Iridium Receptors Featuring Chelating Halogen-Bonding Sites

An anion sensor is presented that combines a bidentate hydrogen- (HB) or halogen-bonding (XB) site with a luminescent monocationic Ir fragment for strong binding of common anions (K_a up to 6×10⁴ m ⁻¹) with diagnostic emission changes. A new emission-based protocol for fast and reliable detection was derived on the basis of correction for systematic but unspecific background effects. Such a simple correction routine circumvents the hitherto practical limitations of systematic emission-based analysis of anion binding with validated open-source software (BindFit). The anticipated order of K_a values was obeyed according to size and basicity of the anions (Cl>Br=OAc) as well as the donor atom of the receptor (XB: 6×10⁴ m ⁻¹ > HB: 5×10³ m ⁻¹), and led to submicromolar limits of detection within minutes. The results were further validated by advanced NMR techniques, and corroborated by X-ray crystallographic data and DFT analysis, which reproduced the structural and electronic features in excellent agreement. The results suggest that corrected emission-based sensing may become a complementary, reliable, and fast tool to promote the use of XB in various application fields, due to the simple and fast optical determination at high dilution.     

Dynamic Inversion of Stereoselective Phosphate Binding to a Bisurea Receptor Controlled by Light and Heat

A chiral bisurea anion receptor, derived from a first‐generation molecular motor, can undergo photochemical and thermal isomerization operating as a reconfigurable system. The two possible cis configurations in the isomerization cycle are opposite in helicity, as is shown by CD spectroscopy. ¹H NMR titrations demonstrate that the P and M helical cis isomers hold opposite enantioselectivity in the binding of binol phosphate, while anion complexation by the intermediate trans isomer is not selective. The difference in the binding affinity of the enantiomers was rationalized by DFT calculations, revealing very distinct binding modes. Thus, the enantiopreferred substrate binding in this receptor can be inverted in a dynamic fashion using light and heat.     

Square-wave voltammetry of 4,4′-diisopropyl-2,2′-bithiazoline and its complex with copper(II)

(4S)-4′-diisopropyl-2,2′-bithiazoline (DPT) is an electroactive organic chiral compound giving two reduction responses in square-wave voltammograms at potentials about −0.2 and −0.4 V by forming a complex with mercury which deposits at the electrode surface. By the addition of copper(II) ion to the solution of DPT a third peak appears between them at about −0.3 V, which corresponds to the reduction of adsorbed Cu-DPT complex. Optimal pH for the investigation of those redox processes was found to be 2.8. By square-wave voltammetric measurements it was interpreted that these redox reactions were quasireversible with immobilized reactants. By plotting i_p/f vs. frequency a quasireversible maximum was obtained, and the apparent standard reaction rate constants were calculated: log (k_s)DPT=(0.91 ± 0.9) and 1 < k_s < 65_S⁻¹, log (k_s)_CuDPT= (0.35 ± 0.9) and 0.3 < k_s < 18 S⁻¹ in 0.55 M NaCl.     

A new tripodal anion receptor with selective binding for H2PO4 and F ions

The anions binding properties of the pyrrole-based tripodal anion receptor 1 were studied by X-ray crystallography, ¹H NMR, and ESI-MS. It revealed that this new tripodal receptor has a preference for binding H₂PO₄⁻ and F⁻ ions.     

Boosting Electroreduction of CO2 over Cationic Covalent Organic Frameworks: Hydrogen Bonding Effects of Halogen Ions

We present the first example of charged imidazolium functionalized porphyrin-based covalent organic framework (Co-iBFBim-COF-X) for electrocatalytic CO₂ reduction reaction, where the free anions (e.g., F⁻, Cl⁻, Br⁻, and I⁻) of imidazolium ions nearby the active Co sites can stabilize the key intermediate *COOH and inhibit hydrogen evolution reaction. Thus, Co-iBFBim-COF-X exhibits higher activity than the neutral Co-BFBim-COF, following the trend of F⁻<Cl⁻<Br⁻<I⁻. Particularly, the Co-iBFBim-COF-I⁻ showed nearly 100 % CO₂ selectivity at a low full-cell voltage of 2.3 V, and achieved a high CO₂ partial current density of 52 mA cm⁻² with a turnover frequency of 3018 h⁻¹ at 2.4 V in the anion membrane electrode assembly, which is 3.57 times larger than that of neutral Co-BFBim-COF. This work provides new insight into the importance of free anions in the stabilization of intermediates and decreasing the local binding energy of H₂O with active moiety to enhance CO₂ reduction reaction.     

Atmospheric Chemistry of Camphor

Rate constants have been measured at 296 ± 2 K for the gas‐phase reactions of camphor with OH radicals, NO₃ radicals, and O₃. Using relative rate methods, the rate constants for the OH radical and NO₃ radical reactions were (4.6 ± 1.2) × 10⁻¹² cm³ molecule⁻¹ s⁻¹ and <3 × 10⁻¹⁶ cm³ molecule⁻¹ s⁻¹, respectively, where the indicated error in the OH radical reaction rate constant includes the estimated overall uncertainty in the rate constant for the reference compound. An upper limit to the rate constant for the O₃ reaction of <7 × 10⁻²⁰ cm³ molecule⁻¹ s⁻¹ was also determined. The dominant tropospheric loss process for camphor is calculated to be by reaction with the OH radical. Acetone was identified and quantified as a product of the OH radical reaction by gas chromatography, with a formation yield of 0.29 ± 0.04. In situ atmospheric pressure ionization tandem mass spectrometry (API‐MS) analyses indicated the formation of additional products of molecular weight 166 (dicarbonyl), 182 (hydroxydicarbonyl), 186, 187, 213 (carbonyl‐nitrate), 229 (hydroxycarbonyl‐nitrate), and 243. A reaction mechanism leading to the formation of acetone is presented, as are pathways for the formation of several of the additional products observed by API‐MS. © 2000 John Wiley and Sons, Inc. Int J Chem Kinet 33: 56–63, 2001     

Pulse radiolysis study of the formation and the reactivity of baicalin radical anion,and in comparison with rutin,quercetin and acyrlate ester radical anions in ethanol

The reaction of solvated electrons with baicalin in N₂-saturated ethanol has been studied by pulse radiolysis. The results show that a solvated electron can add to baicalin and generate a baicalin radical anion with a maximum UV absorbance peak at 360 nm. Its molar extinction coefficient at this wavelength is 1.3×10⁴ M⁻¹ cm⁻¹. The rate constant for the build-up of the baicalin radical anion is 1.3(±0.4)×10¹⁰ M⁻¹ s⁻¹. Decay of the radical anion is induced by a proton transfer reaction and a recombination reaction, which involves a pseudo-first-order reaction with rate constant 2.6(±0.4)×10³ s⁻¹ and a second-order reaction with rate constant 1.3(±0.2)×10⁹ M⁻¹ s⁻¹. The effect of acetaldehyde on the decay of the baicalin radical anion was also investigated. Electron transfer between the baicalin radical anion and acetaldehyde was not observed, probably due to the low rate of electron transfer between the baicalin radical anion and acetaldehyde. Reactivity of the rutin, quercetin, baicalin and ethyl acrylate radical anions are also compared.     

A Study of Magnetic Relaxation in Dysprosium(III) Single-Molecule Magnets

Although the development of single-molecule magnets (SMMs) is rapid, there are only two families of high energy barrier (U_eff) dysprosium(III) SMMs known so far: the cyclopentadienyl (Cp) family with a sandwich structure and the pentagonal-bipyramidal (PB) family with D_5h symmetry. These high-barrier SMMs, which usually possess U_eff>500 cm⁻¹ allow the separate study of the four magnetic relaxation paths, namely, direct, quantum tunnelling, Raman and Orbach processes, in detail. Whereas the first family is chemically more challenging to modify the Cp rings, it is shown herein that the latter family, with the common formulae [DyX¹X²(L_eq)₅]⁺, such as X¹/X²=⁻OCMe₃, ⁻OSiMe₃, ⁻OPh, Cl⁻ or Br⁻; L_eq=THF/pyridine/4-methylpyridine, can be readily fine-tuned with a range of axial and equatorial ligands by simple substitution reactions. This allows unambiguous confirmation that the U_eff mainly depends on the identity of X¹ and X², rather than on L_eq. More importantly, the fitted parameters are barrier dependent. If X¹ is an O donor and X² is a halide, 500<U_eff<600 cm⁻¹, log τ_0avg (s)=−10.66, log C_avg (s⁻¹ K⁻ⁿ)= −5.05, n_avg=4.1 and T_H=9 K (in which τ₀ is the pre-exponential factor for the Orbach relaxation process, C and n are parameters used to describe Raman relaxation, and T_H is the highest temperature at which magnetic hysteresis is observed). For cases in which both X¹ and X² are O donors, 900<U_eff<1300 cm⁻¹, log τ_0avg (s)=−11.63, log C_avg (s⁻¹ K⁻ⁿ)= −6.03, n_avg=4.1 and 18<T_H<25 K. Based on these results, it can be further concluded that U_eff not only has a linear correlation to the axial Dy−X bond lengths, but also to T_H for these PB SMMs. This represents the first systematic study of a family of lanthanide SMMs and derives the first magneto-structural correlation in Dy SMMs.     

Tunable diode laser studies of the reaction of Cl atoms with CH3CHO

The reaction Cl + CH₃CHO → HCl + CH₃CO (1) was studied using flash photo‐lysis / tunable diode laser absorption spectroscopy to monitor the production of HCl. The rate coefficient, k₁, was measured to be (7.5 ± 0.8) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ at 298 K. HCl (v = 0) and HCl^† (v = 1) were measured directly in this study and the yields of HCl (v = 0, 1, >1) for the reaction of Cl with CH₃CHO were determined to be 0.44 ± 0.15, 0.56 ± 0.15, and <0.04, respectively. The rate coefficient for the quenching of HCl^† (v = 1) by CH₃CHO was k_17e = (4.8 ± 1.2) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 766–775, 1999     

An Octa-Urea [Pd2L4]4+ Cage that Selectively Binds to n-octyl-α-D-Mannoside

Designing compounds for the selective molecular recognition of carbohydrates is a challenging task for supramolecular chemists. Macrocyclic compounds that incorporate isophtalamide or bisurea spacers linking two aromatic moieties have proven effective for the selective recognition of all-equatorial carbohydrates. Here, we explore the molecular recognition properties of an octa-urea [Pd₂L₄]⁴⁺ cage complex ( 4 ). It was found that small anions like NO₃⁻ and BF₄⁻ bind inside 4 and inhibit binding of n-octyl glycosides. When the large non-coordinating anion ‘BAr^F’ was used, 4 showed excellent selectivity towards n-octyl-α-D-Mannoside with binding in the order of K_a≈16 M⁻¹ versus non-measurable affinities for other glycosides including n-octyl-β-D-Glucoside (in CH₃CN/H₂O 91 : 9).     

Kinetics and Mechanism of Radical‐Chain Oxidation of 1,2‐Substituted Ethylene and 1,4‐Substituted Butadiene‐1,3

A combination of microvolumetry, the rotating sector method, ESR, ¹H NMR, and IR allowed to establish a detailed mechanism of liquid‐phase oxidation of vinyl compounds X₁CH=CHX₂ and X₁CH=CH–CH=CHX₂ (X₁ and X₂—a polar substitute: С₆Н₅–, CO–, СOO–) initiated by azobisisobutyronitrile. A distinctive feature of the mechanism is the fact that the oxidation chain is carried out by a low‐molecular hydroperoxide radical joining the π‐bond. For nine compounds in the temperature range of 303–353 K, relative chain propagation and termination rate constants were measured (k ₂•k ₃^−0.5). Absolute values of k ₂ were obtained for diphenylethylene (110 L·mol⁻¹·s⁻¹), ethyl ether of trans‐phenyl‐pentadiene acid (13 L·mol⁻¹·s⁻¹), and methyl ether of trans‐phenyl‐pentadiene acid (14.2 L·mol⁻¹·s⁻¹) at T = 323 K. For the same conditions, 10⁻⁸k ₃ were calculated for diphenylethylene (0.87 L·mol⁻¹·s⁻¹) and methyl ether of trans‐phenyl‐pentadiene acid (1.21 L·mol⁻¹·s⁻¹). A cyclic mechanism of the oxidation chain termination on introduced antioxidants (stable nitroxyl radicals of the piperidine series ( > NO^●) and the transition metal compounds (Meⁿ )) was established. The inhibition factor (f ) showing how many reaction chains are terminated by the one particle of the antioxidant is equal to 10². The cyclic chain termination is caused by the following reactions: HO₂^● + > NO^● → NOH + O₂, HO₂● + NOH → >NO^● + H₂O₂ (for >NO^●) and HO₂^● + Meⁿ → Me^{n +1} + HO₂^●−, HO₂^● + Me^{n +1} → Meⁿ + H⁺ + O₂ (for Meⁿ ).     

Synthetic Receptors with Micromolar Affinity for Chloride in Water

A water-soluble coordination cage was obtained by reaction of Pd(NO₃)₂ with a 1,3-di(pyridin-3-yl)benzene ligand featuring a short PEG chain. The cavity of the metal-organic cage contains one nitrate anion, which is readily replaced by chloride. The apparent association constant for chloride binding in buffered aqueous solution is K_a=1.8(±0.1)×10⁵ M⁻¹. This value is significantly higher than what has been reported for other macrocyclic chloride receptors. The heavier halides Br⁻ and I⁻ compete with binding or self-assembly, but the receptor displays very good selectivity over common anions such as phosphate, acetate, carbonate, and sulfate. A further increase of the chloride binding affinity by a factor of 3 was achieved using a fluorinated dipyridyl ligand.     

Tunability of anion binding strength based on acyl-thiourea receptors containing isatin group

Some acyl-thiourea derivatives containing isatin group were synthesized and their interactions with anions were investigated using UV–vis spectroscopy and ¹H NMR titrations in DMSO and DMSO-d₆, respectively. These compounds have a same molecular framework, functionalising with different groups lead to different anion binding strength of these receptors. Receptor 1 showed a higher binding affinity for AcO⁻ than for F⁻, due to the cooperative multiple hydrogen bond interactions of AcO⁻ with the acyl-thiourea group and N–H group in the indole unit of receptor 1. Displacing the N–H proton in the indole unit with –CH₃ group, receptor 2 showed no obviously discriminative responses for F⁻, AcO⁻ and H₂PO₄⁻ due to lack of such additional binding. In the case of receptor 3, which was functionalised with strong electron-withdrawing group, it showed selectively chromogenic response for F⁻ based on double deprotonation of the receptor in DMSO, whereas AcO⁻ and H₂PO₄⁻ induced single deprotonation only.     

Intra- and inter-chain fluctuations in entangled polymer melts in bulk and confined to pore channels

It is known that topological restraints by “chain entanglements” severely affect chain dynamics in polymer melts. In this field-cycling NMR relaxometry and fringe-field NMR diffusometry study, melts of linear polymers in bulk and confined to pores in a solid matrix are compared. The diameter of the pore channels was 10 nm. It is shown that the dynamics of chains in bulk dramatically deviate from those observed under pore constraints. In the latter case, one of the most indicative signatures of the reptation model is verified 28 years after its prediction by de Gennes: The frequency and molecular mass dependencies of the spin-lattice relaxation time obey the power law T_! ∝ M⁰ v^3/4 on a time scale shorter than the longest Rouse relaxation time τ_R. The mean squared segment displacement in the pores was also found to be compatible to the reptation law < r²>∝ M^−1/2t^1/2 predicted for τ_R < t < τ_d, where τ_d is the so-called disengagement time. Contrary to these findings, bulk melts of entangled polymers show frequency and molecular mass dependencies significantly different from what one expects on the basis of the reptation model. The data can however be described with the aid of the renormalized Rouse theory.