R. S. Kumeev

Selective Na+/K+ Effects on the Formation of α-Cyclodextrin Complexes with Aromatic Carboxylic Acids: Competition for the Guest

We investigated the effects of K(+) and Na(+) ions on the formation of α-cyclodextrin complexes with ionized aromatic carboxylic acids. Using solution calorimetry and (1)H NMR, we performed the thermodynamic and structural investigation of α-cyclodextrin complex formation with benzoic and nicotinic acids in different aqueous solutions containing K(+) and Na(+) ions as well as in pure water. The experiments show that the addition of sodium ions to solution leads to a decrease in the binding constants of the carboxylic acids with α-cyclodextrin as compared to pure water and solutions containing potassium ions. From another side, the effect of potassium ions on the binding constants is insignificant as compared to pure water solution. We suggest that the selectivity of cation pairing with carboxylates is the origin of the difference between the effects of sodium and potassium ions on complex formation. The strong counterion pairing between the sodium cation and the carboxylate group shifts the equilibrium toward dissociation of the binding complexes. In turn, the weak counterion pairing between the potassium cation and the carboxylate group has no effect on the complex formation. We complemented the experiments with molecular modeling, which shows the molecular scale details of the formation of cation pairs with the carboxylate groups of the carboxylic acids. The fully atomistic molecular simulations show that sodium ions mainly form direct contact pairs with the carboxylate group. At the same time, potassium ions practically do not form direct contact pairs with the carboxylate groups and usually stay in the second solvation shell of carboxylate groups. That confirms our hypotheses that the selective formation of ion pairs is the main cause of the difference in the observed effects of sodium and potassium salts on the guest-host complex formation of α-cyclodextrin with aromatic carboxylic acids. We propose a molecular mechanism explaining the effects of salts, based on competition between the cations and α-cyclodextrin for binding with the ionized carboxylic acids.

Preparation of 1-butyl-3-methylimidazolium salts and study of their phase behavior and intramolecular intractions

Seven organic salts of 1-butyl-3-methylimidazolium with anions Br−, BF4−, NO3−, SO42−, HSO4−, SCN−, PO43− were prepared. Structure of these compounds is elucidated and purity is confirmed. The products are characterized by melting point, thin layer chromatography, data of elemental analysis, cromatomass-, NMR and IR spectroscopy. All these compounds are ionic liquids, five are low temperature ones. Principal thermal characteristics are found that allow accounting for the phase behavior of the prepared compounds at their application. Existence of intramolecular and intermolecular interactions between the heterocyclic anion and inorganic cation in by means of the formation of hydrogen bond is established.

Synthesis and spectral properties of cobalt(II) and cobalt(III) tetraarylporphyrinates

Reactions of 5,10,15,20-tetraphenylporphin, 5,10,15,20-tetra(4′-methoxyphenyl)porphyrin, and 5,10,15,20-tetra(4′-chlorophenyl)porphyrin with cobalt(II) acetate in dimethylformamide were studied by spectrophotometry. The corresponding cobalt(II) porphyrinates were synthesized and identified. The corresponding cobalt porphyrinates in +3 oxidation state were obtained by reaction of cobalt(II) 5,10,15,20-tetraphenylporphyrinate and cobalt(II) 5,10,15,20-tetra(4′-methoxyphenyl)porphyrinate with 2,3-dichloro-5,6-dicyano-p-benzoquinone in chloroform. The oxidation of cobalt(II) 5,10,15,20-tetra(4′-chlorophenyl)porphyrinate with hydrochloric acid in dimethylformamide leads to cobalt(III) porphyrinate.

Thermodynamics of mixed-ligand complexation of mercury(II) ethylenediaminetetraacetate with histidine and lysine in aqueous solution

The formation of mixed-ligand complexes HgEdtaIm2−, HgEdtaL3−, HgEdtaHL2−, and (HgEdta)2L5− (L is histidine, lysine; Im is imidazole) was studied by calorimetry, pH-metry, and NMR spectroscopy. The thermodynamic parameters (logK, ΔrG0, ΔrH, ΔrS) for the reactions of complex formation at 298.15 K and ion strength of 0.5 (KNO3) were determined. The most likely coordination mode for the complexone and amino acid in the mixed complexes was identified.

Molecular recognition of aromatic carboxylic acids by hydroxypropyl-γ-cyclodextrin: experimental and theoretical evidence

Inclusion complex formation of hydroxypropyl-γ-cyclodextrin with benzoic, nicotinic and isomeric aminobenzoic acids in water was studied by calorimetry, 1H NMR, densimetry and molecular modeling. It was observed that hydroxypropyl-γ-cyclodextrin selectively interacts with the considered acids forming stable inclusion complexes of 1 : 2 stoichiometry with benzoic and p-aminobenzoic acids, which exist in aqueous solution predominantly as neutral molecules. The binding affinity of hydroxypropyl-γ-cyclodextrin to m-aminobenzoic and nicotinic acids having the zwitterionic structure is considerably lower and a 1 : 1 inclusion complex is formed only with the former. The binding mode and thermodynamic parameters of complex formation were evaluated. It was shown that they strongly depend on the structure and ionization state of the acids. An efficient molecular modeling approach for simulating the encapsulation process for 1 : 1 and 1 : 2 stoichiometries was developed and implemented. A good agreement between experimental and theoretical results was demonstrated.

α-Cyclodextrin/aminobenzoic acid binding in salt solutions at different pH: dependence on guest structure.

Influence of Na(+) and K(+) cations on α-cyclodextrin guest-host complex formation with isomeric aminobenzoic acids was examined at different pH and temperature of 298.15 K by (1)H NMR and calorimetry methods. More pronounced influence of Na(+) on inclusion complex formation of α-CD with aminobenzoic acid anions compare to the effects of Na(+) on α-CD complex formation with zwitterionic aminobenzoic acid molecules was revealed. For the first time, the dependence of salt effects on the structure, ionization and the hydration state of the guest molecule was demonstrated and analysed on the basis of the obtained thermodynamic parameters of complex formation and calculated free energy of hydration of different ionized forms of aminobenzoic acids.

Thermodynamic characteristics of the formation of α- and β-cyclodextrin complexes with lumichrome, lumazine, and uracil in aqueous solution

Interactions of α- and β-cyclodextrins with lumichrome and its structural fragments, lumazine and uracil, were studied by means of solubility and 1H NMR spectroscopy. α-Cyclodextrin was found to have a weak complexing ability toward the studied compounds. It was established that β-cyclodextrin forms stable complexes with lumichrome and does not complex with lumazine and uracil. It was shown that only the benzene ring of lumichrome penetrates the β-cyclodextrin cavity, leading to a substantial increase in the solubility of lumichrome in water. We concluded that β-cyclodextrin complexation with lumichrome is highly exothermic due to the van der Waals interactions and hydrogen bonding between polar groups of the reagents.

Cyclodextrin–benzoic acid binding in salt solutions: Effects of biologically relevant anions

Inclusion complex formation of benzoic acid with α-, β- and γ-cyclodextrins in water and in 0.2 M solutions of inorganic salts (KCl, KBr, KH2PO4 and K2SO4) has been studied by means of 1H NMR at 298.15 K. Binding constants have been determined and role of biologically active inorganic anions in the inclusion complex formation has been revealed. It has been shown that effects of the anions are determined not only by changing the ionic strength. More pronounced influence of Br- and H2PO4- compared with Cl- and SO4(2-) is caused by specific ion-molecular interactions, occurrence of which depends on the physical-chemical properties of the anions as well as on the binding mode of cyclodextrins with benzoic acid. Competing interactions of cyclodextrin-anion were observed in the presence of KBr, while the ternary complex formation was detected upon addition of KH2PO4.

Numerical simulation of the solvate structures of acetylsalicylic acid in supercritical carbon dioxide containing polar co-solvents

Hydrogen-bonded complexes of acetylsalicylic acid with polar co-solvents in supercritical carbon dioxide, modified by methanol, ethanol, and acetone of 0.03 mole fraction concentration, are studied by numerical methods of classical molecular dynamics simulation and quantum chemical calculations. The structure, energy of formation, and lifetime of hydrogen-bonded complexes are determined, along with their temperature dependences (from 318 to 388 K at constant density of 0.7 g cm−3). It is shown that the hydrogen bonds between acetylsalicylic acid and methanol are most stable at 318 K and are characterized by the highest value of absolute energy. At higher supercritical temperatures, however, the longest lifetime is observed for acetylsalicylic acid–ethanol complexes. These results correlate with the known literature experimental data showing that the maximum solubility of acetylsalicylic acid at density values close to those considered in this work and at temperatures of 318 and 328 K is achieved when using methanol and ethanol as co-solvents, respectively.

Synthesis and Properties of β-Brominated Metal Complexes of meso-Triphenylcorrole

Spectral properties and chemical stability of Mn(III), Mn(IV), Fe(III), Fe(IV), and Cu(III) complexes of β-octabromotriphenylcorrole [(β-Br)8(ms-Ph)3Cor], synthesized from β-unsubstituted compounds by their reaction with molecular bromine, were studied. Cyclic voltammetry, electron microscopy, and X-ray spectral microanalysis were used to obtain electrochemical characteristics of metal corroles M(β-Br)8(ms-Ph)3Cor and gain insight into the surface texture of active catalysts on the basis of metal corroles. The electron-acceptor β-bromine substitution in the MCor macrocycle shifts the equilibrium in electron-donor solvents to lower oxidation states of the metals and also stabilizes manganese and destabilizes copper complexes in the protondonor medium HOAc-H2SO4. The electrocatalytic activity of the complexes in the reduction of molecular oxygen depends on the nature of the ligand and increases in the order Mn ≤ Cu ≪ Fe in the case of β-octabrominated macrocycles. The character of distribution of active centers on the surface of the catalysts was established for the first time.