Competition of methyltrioxorhenium (MTO) with osmium tetroxide (OsO4) for pyridines binding: Ligand binding assay

Competition of methyltrioxorhenium (MTO) with osmium tetroxide (OsO₄) toward L = pyridine and its derivatives, based on the equilibrium constant for the reaction OsO₄·L + MTO = MTO·L + OsO₄, has been measured. A successful correlation of log K_eq with the Hammett σ constants of the substituents on the ligands was realized. A negative reaction constant, obtained for the reactions, shows that a more positive charge expands on the pyridine nitrogen in the complex MTO·L as compared with the complex OsO₄·L. So, the rhenium center acts as a better electron acceptor than osmium center. The thermodynamic parameters have been obtained and an excellent linear relationship was observed between the enthalpy and entropy of the reactions.     

Lewis acidity of methyltrioxorhenium(VII) (MTO) based on the relative binding strengths of N-donors

This article presents a sigma acceptor strength scale for methyltrioxorhenium(VII) (MTO), one of the most versatile and useful high oxidation state organometallics ever described. The spectrophotometric titration of MTO with a series of N-donor bases in CCl(4) gives formation constants (K(f)) and enthalpies for the adduct formation reactions. An excellent linearity of log K(f) with respect to the Hammett sigma constants of the substituents on the ligands was observed. The resulting rho constant is proposed to be a good indication of the Lewis acidity of MTO. The enthalpies of adduct formation of N-donors with MTO also fit the ECW model to predict the values of E(A) and C(A) parameters for MTO. The parameters can be used to predict an acidity scale for MTO. These parameters also allow the chemists to predict and correlate quantitatively the enthalpies of MTO.Lewis base interactions. Significant chemical insights result from the fit of the data to the ECW model.     

Polymeric catalyst with polymerization-enhanced Lewis acidity for CO2-based copolymers

Ring-opening copolymerization of CO₂ and epoxides is a promising way to manufacture high value-added materials. Despite a variety of catalyst systems have been reported, the reaction is still limited by low activity and polymer selectivity. Herein, a strategy of polymerization-enhanced Lewis acidity is reported to construct a series of highly efficient polymeric aluminum porphyrin catalysts (PAPCs). The characterization of the coordination equilibrium constant (K_eq) showed significantly enhanced Lewis acidity of PAPC (K_eq = 18.2 L/mol) compared to the monomeric counterpart (K_eq = 6.4 L/mol), accompanied with increased turnover frequency (TOF) from 136 h⁻¹ to 5500 h⁻¹. Through detailed regulation of Lewis acidity, the highly Lewis acidic PAPC-OTs displayed a record high TOF of 30,200 h⁻¹ with polymer selectivity of up to 99%.     

Spectrophotometric determination of acidity constants of 4-(2-pyridylazo)resorcinol in binary methanol-water mixtures

The acidity constants of 4-(2-pyridylazo)resorcinol (PAR, Scheme 1) in binary mixtures of methanol-water at 25 °C and an ionic strength of 0.1 M have been determined spectrophotometrically. DATa ANalysis (DATAN) program applied for determination of acidity constants. As percent of methanol increases in solvent mixtures the pK_a constants also increased. There is linear relationship between acidity constants and the mole fraction of methanol in the solvent mixtures. Effect of solvent composition on acidity constants and pure spectrum of each component are also discussed.     

An imidazolidin-1-ol, nitrone and oxadiazinane ring-chain-ring tautomeric dynamic combinatorial library

A dynamic combinatorial library (DCL) based on benzylidene exchange reactions between imidazolidin-1-ol, nitrone and oxadiazinane (INO) ring-chain-ring tautomers at room temperature is created. The probable mechanism of the reaction is discussed based on Hammett type correlation analyses. The equilibria in the DCL are defined as interconversion of INOAr-X to INOAr-Y as a result of reactions with the corresponding aldehydes Y-ArCHO and X-ArCHO and are shown to depend on the σ constants of the substituents and are described by a simple equation log K_XY = ρσ_dif + log K_XY(X = Y). The prediction of K_XY for any INOAr-X and INOAr-Y interconversion requires only the experimental values of ρ and the known Hammett σ constants of the substituents. The effect of Zn(II) and CF₃SO₃H on the DCL equilibria is also reported.     

Displacement of neutral nitrogen donors by chloride in [Pt(SNS)(R-py)](ClO4)2 (SNS = 2,6-bis(methylthiomethyl)pyridine; R-py = pyridines): A reactivity comparison between meta- and para-substituted pyridines

The kinetics of the process [Pt(SNS)(R-py)]²⁺ + Cl⁻ → [Pt(SNS)Cl]⁺ + R-py {SNS = 2,6-bis(methylsulfanylmethyl)pyridine; R-py = meta- or para-substituted pyridines covering a wide range of basicity} were studied in methanol at 25 °C. The reactions obey the usual two-term rate law observed in the substitution reactions of square-planar d⁸ complexes. The plots of log k₂ {k₂ = second-order rate constants} against the pK_a of the heterocycles conjugate acids highlighted a different sensitivity of the two groups of N-donors to changes in basicity, thepara-substituted pyridines (4R-py) showing a weaker dependence on pK_a than the meta-substituted (3R-py). The results have been explained on the basis of a π-acidity difference between 3R-py and 4R-py which influences the reaction ground state.     

Thermochemistry of adducts of some bivalent transition metal bromides with aniline

The compounds [MBr₂(an)₂] (where M is Mn(II), Fe(II), Co(II), Ni(II), Cu(II) or Zn(II); an = aniline) were synthesized and characterized by melting points, elemental analysis, thermal studies, and electronic and IR spectroscopy. The enthalpies of dissolution of the adducts, metal(II) bromides and aniline in methanol, aqueous 1.2 M HCl or 25% (v/v) aqueous 1.2 M HCl in methanol were measured. The following thermochemical parameters for the adducts have been determined by thermochemical cycles: the standard enthalpies for the Lewis acid/base reactions (Δ_rH°), the standard enthalpies of formation (Δ_fH°), the standard enthalpies of decomposition (Δ_DH°), the lattice standard enthalpies (Δ_MH°) and the standard enthalpies of the Lewis acid/base reactions in the gaseous phase (Δ_rH°(g)). The mean bond dissociation enthalpies of the M(II)-nitrogen bonds () and the enthalpies of formation of the adducts from the ions in the gaseous phase: M²⁺_(g) + Br⁻_(g) + an_(g) → [MBr₂(an)₂]_(g), (Δ_fiH°) have been estimated.     

Thermochemistry of adducts of some bivalent transition metal bromides with pyridine

The compounds [MBr₂(py)₂] (where M is Mn(II), Co(II), Ni(II), Cu(II) or Zn(II); py = pyridine) were synthesized and characterized by melting points, elemental analysis, thermal analysis and electronic and IR spectroscopy. The enthalpies of dissolution of the adducts, metal(II) bromides and pyridine in 25% (v/v) 1.2 M aqueous HCl in methanol were measured and by using thermochemical cycles, the following thermochemical parameters for the adducts have been determined: the standard enthalpies for the Lewis acid/base reactions (Δ_rH^θ), the standard enthalpies of formation (Δ_fH^θ), the standard enthalpies of decomposition (Δ_DH^θ), the lattice standard enthalpies (Δ_MH^θ) and the standard enthalpies of the Lewis acid/base reactions in the gaseous phase (Δ_rH^θ(g)). The mean bond dissociation enthalpies of the M(II)-nitrogen bonds have been estimated as well as the enthalpies of the adducts formation in the gaseous phase.     

Development of a chemometric correlation technique to estimate acid-base descriptors for cationic acids in non-aqueous media

We propose a procedure for estimating acid-base constants in organic solvents or mixture of solvents from the corresponding pK_a values in aqueous medium and from certain properties of the organic solvents that characterize them. To accomplish this, we developed and validated a chemometric correlation for the calculation of the acid-base constants of different cationic acids in a broad variety of non-aqueous solvents. The parameters chosen for building the model were as follows: the acid-base constant of the compound in aqueous medium and those corresponding to the polarity-polarizability, basicity and acidity scales of the solvent. The results of the fitting were significant (p < 0.01), with a root mean error in cross-validation of 18%, with no overfitting. The prediction of the acid-base constants for an external set of compounds had a mean absolute prediction error value of less than 0.8 pK_a units.     

Modelling of acid-base equilibria in binary-solvent systems: A comparative study

A new approach to modelling of some binary (hydro-organic, HL/H₂O + B) acid-base systems with organic co-solvent B fully miscible with water, is suggested and applied for the determination of acidity parameters pK for some weak acids HL. The models are designed to get the pK = pK(x) relationships (x—mole fraction of B in H₂O + B) from results of pH titrations made in such systems and for the determination of pK_B = pK(1) for HL in pure B. The Redlich-Kister equation, together with its asymmetric extensions, and the Legendre functions with orthogonal polynomials, appeared to be suitable for such purposes.     

Pd(OAc)2-catalyzed cross-coupling of polyfluoroarenes with simple aromatics in imidazolium ionic liquids (ILs) without oxidant and additive and with recycling/reuse of the IL

Polyfluoroarenes can be cross-coupled with simple aromatics (benzene, toluene, and anisole), in good isolated yields, by using Pd(OAc)₂ dissolved in imidazolium ILs [(bmim)PF₆ and (bmim)BF₄] as solvent, without the need for an oxidant and an additive. The reaction is catalyzed by HOAc and it is subject to a primary isotope effect (K_H/K_D = 4.87). Competitive cross-coupling reactions of 1,2,4,5-tetrafluorobenzene with benzene/toluene, benzene/anisole, and anisole/toluene gave K_B/K_T = 5.1, K_B/K_A = 5.7, and K_A/K_T = 5.0, respectively, indicative of a remote substituent effect on Pd insertion into the phenyl C-H bond. Mild reaction conditions, simple product isolation and recycling/reuse of the IL are additional advantages of this method.     

The Proton Dissociation of Bio-Protic Ionic Liquids: [AAE]X Amino Acid Ionic Liquids

[AAE]X composed of amino acid ester cations is a sort of typically “bio-based” protic ionic liquids (PILs). They possess potential Brønsted acidity due to the active hydrogens on their cations. The Brønsted acidity of [AAE]X PILs in green solvents (water and ethanol) at room temperature was systematically studied. Various frameworks of amino acid ester cations and four anions were investigated in this work from the viewpoint of structure–property relationship. Four different ways were used to study the acidity. Acid dissociation constants (pK_a) of [AAE]X determined by the OIM (overlapping indicator method) were from 7.10 to 7.73 in water and from 8.54 to 9.05 in ethanol. The pK_a values determined by the PTM (potential titration method) were from 7.12 to 7.82 in water. Their Hammett acidity function (H₀) values (0.05 mol·L⁻¹) were about 4.6 in water. In addition, the pK_a values obtained by the DFT (proton-transfer reactions) were from 7.11 to 7.83 in water and from 8.54 to 9.34 in ethanol, respectively. The data revealed that the cationic structures of [AAE]X had little effect and the anions had no effect on the acidity of [AAE]X. At the same time, the OIM, PTM, Hammett method and DFT method were reliable for determining the acidic strength of [AAE]X in this study.     

A method for rapid screening of interactions of pharmacologically active compounds with albumin

We determine the association constants for ligand–protein complex formation using the flow injection method. We carry out the measurements at high flow rates (F = 1 mL min⁻¹) of a carrier phase. Therefore, determination of the association constant takes only a few minutes. Injection of 1 nM of the ligand (10 μL of 1 μM concentration of the ligand solution) is sufficient for a single measurement. This method is tested and verified for a number of complexes of selected drugs (cefaclor, etodolac, sulindac) with albumin (BSA). We obtain K = 4.45 × 10³ M⁻¹ for cefaclor, K = 1.00 × 10⁵ M⁻¹ for etodolac and K = 1.03 × 10⁵ M⁻¹ for sulindac in agreement with the literature data. We also determine the association constants of 20 newly synthesized 3β- and 3α-aminotropane derivatives with potential antipsychotic activity – ligands of 5-HT_1A, 5-HT_2A and D₂ receptors with the albumin. Results of the studies reported here indicate that potential antipsychotic drugs bind weakly to the transporter protein (BSA) with K ≈ 10²–10³ M⁻¹. Our method allows measuring K in a wide range of values (10²–10⁹ M⁻¹). This range depends only on the solubility of the ligand and sensitivity of the detector.     

Linear free energy relationships and kinetic isotope effects reveal the chemistry of the Ado 2′-OH group

Using kinetic isotope effects (KIE) and Hammett correlations, we show that the main role of the adenosine 2′-OH group on deprotonation by the non nucleophilic base DBU during external acyl group transfer is to generate enhanced electron density on the attacking nucleophile through ionization. The small primary KIEs (1.2 and 1.6) and the large Hammett reaction constants (+2.25 and +3.19) obtained for the ethanolysis of 2′/3′-O-p-substituted benzoyl 5′-O-trityl adenosines and 2′-deoxyadenosines are consistent with an A_N + D_N reaction mechanism. The implications of our results are discussed in terms of chemical contributions of the 2′-OH group in the ribosome catalysis of peptide bond formation.     

Potentiometric determination of the total acidity of humic acids by constant-current coulometry

A straightforward method for both the quantitative and the equilibrium analysis of humic acids in solution, based on the combination of potentiometry with coulometry, is presented. The method is based on potentiometric titrations of alkaline solutions containing, besides the humic acid sample, also NaClO₄ 1 M; by means of constant current coulometry the analytical acidity in the solutions is increased with a high precision, until the formation of a solid phase occurs. Hence, the total acid content of the macromolecules may be determined from the e.m.f. data by using modified Gran plots or least-squares sum minimization programs as well. It is proposed to use the pK_w value in the ionic medium as a check of the correctness of each experiment; this datum may be readily obtained as a side-result in each titration. Modelling acid-base equilibria of the HA samples analysed was also performed, on the basis of the buffer capacity variations occurring during each titration. The experimental data fit, having the least standard deviation, was obtained assuming a mixture of three monoprotic acids (HX, HY, HZ) having about the same analytical concentration, whose acid dissociation constants in NaClO₄ 1 M at 25 °C were pK_HX = 3.9 ± 0.2, pK_HY = 7.5 ± 0.3, pK_HZ = 9.5 ± 0.2, respectively. With the proposed method the handling of alkaline HA solutions, the titration with very dilute NaOH or HCl solutions and the need for the availability of very small volumes of titrant to be added by microburettes may be avoided.     

Protolytic properties of 2-acylthioacetamides

Concentration acidity of 2-acylthioacetamides dissolved in DMSO-H₂O was measured by the method of pH-metric titration. The data obtained were extrapolated for the determination of pK _a values of 2-acylthioacetamides in water and DMSO. We found that in aqueous solutions the acidity of thioacetarylamide (pK _a = 5.75?C6.01) was almost independent of the nature of substituents in their N-phenyl rings, whereas in DMSO solution the nature of substituents significantly affects their ionization constants (pK _a = 7.88?C11.70). The graphs were displayed and the acidity of N-aryl-3-oxobutanethioamides dependence on the Hammett constants of meta- and para-substituents in the phenyl rings derived. Water is shown to be a leveling, and DMSO a differentiating solvent for 2-acylthioacetamides.     

Formation of pseudo[3]rotaxanes containing calix-bis-crowns and secondary ammonium ions and their thermodynamic stabilities in a solution: preorganization by second macrocycle and nonallosteric behavior exhibited by large crown cavities

This Letter describes the formation of pseudo[3]rotaxanes containing calix[4]-bis-crowns, exhibiting a 1,3-alternate conformation and large crown cavities, and secondary ammonium ions. The first and second association constants of pseudo[3]rotaxane formation are moderate (K₁ = 175, K₂ = 100 M⁻¹) and are higher than that of the corresponding pseudo[2]rotaxane (K = 24 M⁻¹), consisting of a calix[4]-mono-crown and the same secondary ammonium ion.     

24-Thiacycloartanol, a potent mechanism-based inactivator of plant sterol methyltransferase

The rationally designed substrate mimic of cycloartenol, 24-thiacycloartanol (24-TC) 4A, and its corresponding sulfonium salt 5A were tested against the recombinant sterol methyltransferase (SMT) from Glycine max (soybean). Analog 4A was found to irreversibly inactivate the enzyme generating competitive- and time-dependent inhibition of activity accompanied by a K_i value of 2 μM and k_inact of 0.3 min⁻¹. Analog 5A, a presumptive high-energy intermediate of the SMT catalyzed reaction, was found to be a reversible, non-competitive-type inhibitor generating a K_i value of 55 nM.     

Surface characterization of poly(2,2,3,3,3-pentafluoropropyl methacrylate) by inverse gas chromatography and contact angle measurements

A fluorinated methacrylic homopolymer, poly(2,2,3,3,3-pentafluoropropyl methacrylate) (PPFPMA) was synthesized by a free radical polymerization reaction. The dispersive component of the surface energy () of PPFPMA was determined by contact angle measurements and inverse gas chromatography (IGC). An extensive surface characterization was conducted by means of IGC. Surface characterization demonstrated that PPFPMA has low value, even at 35 °C and is a Lewis amphoteric polymer with predominantly basic character, as confirmed by the Lewis acidity and basicity constants K_A and K_B, respectively. The values of obtained by IGC are slightly higher than those obtained by the contact angle method. This trend can be attributed to the fact that IGC evaluates, primarily, high energy sites of a surface.     

Study on ion–solvent interactions of alkali metal salts in pure methanol and its binary mixtures with ethane-1,2-diol by a conductometric technique

Electrolytic conductivities of some sodium salts (NaI, NaSCN, CH₃COONa) have been studied in 0, 25, 50 and 75 mass% ethane-1,2-diol + methyl alcohol mixtures at 293.15, 298.15 and 303.15 K. The limiting molar conductivity (Λ°), the association constant (K_A) and the distance of closest approach of ion (R) have been evaluated using the Fuoss conductance equation (1978). The association constant (K_A) decreases with temperature while it tends to decrease in the order: 0 mass% > 25 mass% > 50 mass% > 75 mass% ethane-1,2-diol + methyl alcohol mixtures. Thermodynamic parameters ΔH°, ΔG° and ΔS° along with the Walden products (Λ°η) are obtained and discussed. The results have been interpreted in terms of ion–solvent interactions and structural changes in the mixed solvents.