Angela F. Danil de Namor

Solution thermodynamics of amino acid–18-crown-6 and amino acid–cryptand 222 complexes in methanol and ethanol. Linear enthalpy–entropy compensation effect

Stability constants, free energies, enthalpies and entropies of complexation of amino acids with synthetic macrocyclic ligands (18-crown-6 and cryptand 222) in methanol and ethanol are reported. No significant variations are found in the free energies of complexation of the various amino acids with these two ligands as a result of a remarkable enthalpy–entropy compensation effect. Free energies of transfer of amino acids and recently isolated amino acid–18-crown-6 and amino acid–cryptand 222 complexes have been mostly obtained from solution thermodynamic data of the free and complexed amino acids in the appropriate solvents. Analysis of transfer-free-energy data indicates that in the amino acid–macrocyclic complex, most of the amino acid molecule is fully exposed to the reaction medium. Previous suggestions on the linear enthalpy–entropy relationship for complexation reactions involving crown ethers and cryptands with metal and organic cations in different reaction media are discussed. The implications of the results obtained for amino acid–macrocyclic complexation reactions to processes of biological importance are considered.

Solubility of electrolytes in 1,2-dichloroethane and 1,1-dichloroethane, and derived free energies of transfer

The solubility of 27 1:1 electrolytes in 1,2-dichloroethane and 25 1:1 electrolytes in 1,1-dichloroethane has been determined. Combination of the solubility values with association constants, and correction for activity coefficients by the extended Debye–Huckel theory, yields standard free energies of solution of the ionic species (M++ X–). From like data in water, free energies of transfer from water to the dichloroethanes have been calculated and have been split into single-ion values through the assumption that ΔGt°(Ph4P+, Ph4As+)=ΔGt°(Ph4B–). It is shown that the free energy of anions (Cl–, Br–, I– and ClO4–) in the dichloroethanes is much higher in value than in water and in dipolar approtic solvents (DMSO, DMF and MeCN). The free energy of most cations (Na+, K+, Rb+, Cs+, Me4N+, Et4N+, Pr4N+ and Bu4N+) is also higher in value in the dichloroethanes than in the dipolar approtic solvents. Both anions and cations are invariably higher in free energy by ∼1 kcal mol–1 in 1,1-dichloroethane than in 1,2-dichloroethane. It is concluded that the (Ph4P+, Ph4As+)=(Ph4B–) assumption yields single-ion free energies of transfer to or from the dichloroethanes that are chemically reasonable.

Solution thermodynamics of geometrical isomers of pyridino calix(4)arenes and their interaction with the silver cation. The X-ray structure of a 1:1 complex of silver perchlorate and acetonitrile with 5,11,17,23-tetra-tert-butyl-[25,26,27,28-tetrakis(2- pyridylmethyl)oxy]calix(4)arene

The solubility and derived standard Gibbs energies of solution of three geometrical isomers of pyridinocalix(4)arenes {5,11,17,23-tert-butyl-[25,26,27,28-tetrakis-(n-pyridylmethyl) oxy]-p-tert-butylcalix(4)arene, with n=2, 3, 4} in a variety of solvents at 298.15 K are reported. Solvation effects are assessed from the standard transfer Gibbs energies of these ligands, using acetonitrile as the reference solvent. For the 2-pyridyl derivative in alcohols as the aliphatic chain length of the alcohol increases, the extent of solvent–ligand interaction increases, whereas for the 3- and the 4-pyridyl derivatives the opposite is observed. The protonation constants for the 4-pyridyl derivative in methanol at 298.15 K are reported. The results, compared with corresponding values for the 3-pyridyl derivative show that the basic character of these ligands follows the sequence; 4-pyridyl>3-pyridyl>2-pyridyl calix(4)arene. Thermodynamic parameters for the complexation of these macrocycles (2 and 4-pyridyl derivatives) and the silver cation in acetonitrile show that, as the distance between the phenolic oxygens and the pyridyl nitrogens increases, the strength of complexation decreases. Thermodynamic data for the complexation of silver and the 2-pyridylcalix(4)arene derivative are compared with corresponding values for sodium and this ligand in the same solvent. The results suggest that the 2-pyridyl derivative provides phenolic oxygens and pyridyl nitrogens as the active sites for complexation with silver. This is corroborated by 1H NMR measurements of this ligand and silver in CD3CN at 298 K.The crystal structure of a 1:1 monoacetonitrile and silver complex of the 2-pyridyl derivative with perchlorate as the counter-ion has been determined from XRD data. The substance crystallises in the tetragonal space group P4/n with a=15.580(2) A, c=13.082(2) A, and z=2. The macrocycle is sited on a fourfold symmetry axis and consists of two conical parts, one hydrophobic, filled with an acetonitrile molecule, the other hydrophilic, encapsulating the Ag+ ion through the ethereal oxygen [d(Ag···O)=2.923(3) A] and pyridine nitrogen atoms [d(Ag···N)=2.483(5) A]. The coordination around the Ag+ ion has the form of a distorted Archimedean square antiprism. The ability of the macrocycle to complex the silver cation in its hydrophilic cavity is demonstrated.

Calix[4]Pyrrole Derivative: Recognition of Fluoride and Mercury Ions and Extracting Properties of the Receptor-Based New Material

A calix[4]pyrrole derivative, namely, meso-tetramethyl tetrakis (4-phenoxy methyl ketone) calix[4]pyrrole, 1, was synthesized and structurally (1H NMR) and thermodynamically characterized. The complexing properties of this receptor with a wide variety of anions and cations in dipolar aprotic media (acetonitrile, propylene carbonate, and dimethyl sulfoxide) were investigated through 1H NMR and conductance studies. The former technique was used to assess whether or not complexation occurs and if so to identify the active sites of interaction of 1 with ions. The composition of the complexes was established by conductance measurements. It was found that in dipolar aprotic solvents, 1 interacts only with two polluting ions (fluoride and mercury). The complexation thermodynamics of 1 and these ions in these solvents is reported. The medium effect on the binding process involving the fluoride ion is discussed taking into account the solvation properties of reactants and the product. Complexes of moderate stability are found. Given that this is an important factor to consider for the recycling of the loaded material in extraction processes, 1 was treated with formaldehyde in basic medium leading to the production of a calix[4]pyrrole based material able to extract fluoride and mercury (II) ions from water. Thus the optimum conditions for the extraction of these ions from aqueous solutions were established. The material is easily recyclable using an organic acid. Final conclusions are given.

Calixarene and Resorcarene Based Receptors: From Structural and Thermodynamic Studies to the Synthesis of a New Mercury(II) Selective Material

Materials used in current technological approaches for the removal of mercury lack selectivity. Given that this is one of the main features of supramolecular chemistry, receptors based on calix[4]arene and calix[4]resorcarene containing functional groups able to interact selectively with polluting ions while discriminating against biologically essential ones were designed. Thus two receptors, a partially functionalized calix[4]arene derivative, namely, 5,11,17,23-tetra-tert-butyl [25-27-bis(diethyl thiophosphate amino)dihydroxy] calix[4]arene (1) and a fully functionalized calix[4]resorcarene, 4,6,10,12,16,18,22,24-diethyl thiophosphate calix[4]resorcarene (2) are introduced. Mercury(II) was the identified target due to the environmental and health problems associated with its presence in water Thus following the synthesis and characterization of 1 and 2 in solution ((1)HNMR) and in the solid state (X-ray crystallography) the sequence of experimental events leading to cation complexation studies in acetonitrile and methanol ((1)H NMR, conductance, potentiometric, and calorimetric measurements) with the aim of assessing their behavior as mercury selective receptors are described. The cation selectivity pattern observed in acetonitrile follows the sequence Hg(II) > Cu(II) > Ag(I). In methanol 1 is also selective for Hg(II) relative to Ag(I) but no interaction takes place between this receptor and Cu(II) in this solvent. Based on previous results and experimental facts shown in this paper, it is concluded that the complexation observed with Cu(II) in acetonitrile occurs through the acetonitrile-receptor adduct rather than through the free ligand. Receptor 2 has an enhanced capacity for uptaking Hg(II) but forms metalate complexes with Cu(II). These studies in solution guided the inmobilization of receptor 1 into a silica support to produce a new and recyclable material for the removal of Hg(II) from water. An assessment on its capacity to extract this cation from water relative to Cu(II) and Ag (I) shows that the cation selectivity pattern of the inmobilized receptor is the same as that observed for the free receptor in methanol. These findings demonstrate that fundamental studies play a critical role in the selection of the receptor to be attached to silicates as well as in the reaction medium used for the synthesis of the new decontaminating agent.

From neutral to ionic species: amine–p-tert-butylcalix(n)arene (n= 6, 8) interaction. Electrochemical, thermodynamic and structural studies in benzonitrile

Solubilities and derived Gibbs energies, enthalpies and entropies of solution of p-tert-butylcalix(n)arenes (n= 4, 6, 8) in benzonitrile at 298.15 K are reported. An important consequence of amine–calixarene interactions in solution is the generation of new electrolytes. Thus, using two different calixarenes (cyclic hexamer and octamer) and various amines, conductance measurements are performed in order to derive equilibria data for the process involving two neutral species to give an electrolyte, as a result of a proton-transfer reaction from the calixarene to the amine. The results obtained by conductivity are checked by using a combination of pKa values for p-tert-butylcalix(n)arene and various amines in benzonitrile calculated from potentiometric data. Thermodynamic parameters for these processes obtained from titration calorimetry reflect the distinctive properties of cryptands relative to other cyclic and aliphatic amines, suggesting that cryptand 222 is likely to host the proton released by the cyclic octamer in its cavity. 13C NMR studies on these systems revealed that the charge on the calixarene anion is delocalised as a result of a dynamic interchange of the remaining protons among the oxygen atoms present in the oligomeric molecule. Finally, the extraction of amines from aqueous solutions by calixarenes is analysed by taking into account the individual parameters that contribute to the overall process. It is shown that the transfer of amines from water to benzonitrile is favoured in the presence of calixarenes. Although the main contribution to the extraction process results from ion-pair formation in solution, the selective extraction of these amines by calixarenes is largely controlled by the transfer Gibbs energy of the amine from water to the non-aqueous phase.

Thermodynamic studies of cryptand 222 and cryptates in water and methanol

Enthalpies of solution in water and in methanol are reported for a number of cryptate electrolytes ([M+222]+X–) and in conjunction with data on X– yield values for the enthalpy of transfer, ΔH°t, of [M+222] ions from water to methanol. Enthalpies of complexing of cations with cryptand 222 have been determined in water and methanol; when combined with known values of ΔH°t(M+) and the presently determined value of ΔH°t(222), they yield, via a thermodynamic cycle, values of ΔH°t for the ions [M+222] where M+= Li+, Na+, K+, Rb+, Cs+ and Ag+. The two methods of obtaining the ΔH°t values are in good agreement with each other.Solubility measurements on the perchlorates of [Na+222] and [K+222] yield values of ΔG°t([M+222]) from water to methanol. These values are also obtained from a thermodynamic cycle involving known values of ΔG°t(M+), the free energies of complex formation in water and methanol and the presently determined value of ΔG°t(222). The direct values and the cycle values are again in good agreement.ΔH°t, ΔG°t and ΔS°t values for transfer of the complexed ions [M+222] vary considerably with the central cation M+(M+= Li+, Na+, K+, Rb+, Cs+ and Ag+) and it is clear that the surrounding cryptand does not isolate the central ion from the environment. Also, various single-ion assumptions that require the constancy of ΔG°t([M+222]) and ΔH°t([M+222]) with M+ are not valid for the water to methanol transfer.Partition coefficients for the hypothetical extraction process M+(aq)+222(aq)→[M+222](methanol) have been obtained and it is shown that by comparison with the simple partition M+(aq)→ M+(methanol), greatly enhanced cation selectivities are observed. The largest selectivity enhancement occurs with the ions Ag+ and Li+, where the complex extraction equilibrium favours the extraction of Ag+ by a factor of 4 × 109 over the simple partition.Ion-pair partition coefficients have also been obtained for the species [Na+222]ClO–4 and [K+222]ClO–4; extraction of the latter is favoured by a factor of 1.6 × 102. This may be compared to a factor of 2.7 × 102 in favour of the potassium salt when extracted as the pair of ions ([M+222]+ ClO–4) and to a factor of 0.61 for extraction as the uncomplexed K+ and Na+ cations.

The various factors involved in the extraction of alkali metal picrates by calixarene ester derivatives in the mutually saturated water–dichloromethane solvent system

The distribution of alkali metal picrates in the mutually saturated water–dichloromethane solvent system at 298.15 K was investigated. From distribution data, the partition and the distribution constants of these salts in this solvent system and the ion-pair formation constants of alkali metal picrates in water saturated dichloromethane were calculated. For salts containing the smaller alkali metal cations, significant differences were found between the partition and the transfer Gibbs energies in this solvent system. The former data are referred to the mutually saturated solvents whereas the latter involve water and dichloromethane in their pure state. In order to (i) formulate an equation which is representative of the extraction process taking place when an aqueous solution containing the free metal ion salt is equilibrated with a solution of calixarene ester in dichloromethane and (ii) gain information regarding the behaviour of calixarene ester salts in non-aqueous media, conductance measurements were performed in acetonitrile and pure and wet dichloromethane at 298.15 K. Whenever possible, the ion-pair formation constants and the limiting molar conductances were calculated from the analysis of data using a well established treatment. The results obtained are discussed. From distribution experiments in the presence of ethyl p-tert-butylcalix[4]arene tetraethanoate in the mutually saturated solvents, the distribution and extraction constants for alkali metal picrates were calculated. Good agreement was found between the distribution constants derived from data in the absence and presence of the ligand. The individual processes involved in the overall extraction of cations by ethyl p-tert-butylcalix[4]arene tetraethanoate were assessed. The results were compared with corresponding data for the mutually saturated water–benzonitrile solvent system. It is concluded that the higher extraction constants obtained in the latter solvent system are mainly attributed to the more favourable distribution constants in water–benzonitrile relative to water–dichloromethane. Based on the experimental evidence shown in this paper, some of the statements made in the literature regarding these systems are discussed. The effect of the solvent on the selective extraction of alkali metal cations from water is discussed.

Cyclodextrin–monosaccharide interactions in water

Stability constants and derived Gibbs energies, enthalpies and entropies of complexation of D-monosaccharides with α- and β-cyclodextrin¶ in water at 298.15 K obtained by titration microcalorimetry are reported. The results show that α-cyclodextrin interacts with D-glucose, D-fructose, D-xylose, D-mannose and D-galactose. No heat was evolved with D-arabinose. However, β-cyclodextrin is able to recognise aldopentoses (D-xylose and D-arabinose) but not aldohexoses. 13C NMR studies on these systems are discussed.

Free energies of transfer of 1 : 1 electrolytes from water to nitrobenzene. Partition of ions in the water + nitrobenzene system

Free energies of solution of 1 : 1 electrolytes in nitrobenzen have been obtained from solubility measurements. Free energies of transfer for these electrolytes from water to nitrobenzene were obtained by combining their free energies of solution in the organic solvent and in water. By using the convention suggested by Parker, the single-ion free energies of transfer for 8 cations and 6 anions have been obtained. These are compared with single-ion free energies of transfer obtained by calculation and with single-ion free energies of partition previously reported. Close agreement is found between the observed single-ion free energies of transfer from water to nitrobenzene and the single-ion free energies of partition.On the basis of the experimental evidence shown in this work, it seems that in the partition of 1 : 1 electrolytes from water to nitrobenzene most of the ions are unhydrated in nitrobenzene. The influence of the cation as well as the anion in the partition of 1:1 electrolytes from water to nitrobenzene is discussed.