Felix J. Sueros Velarde

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.

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.

Lower rim calix(4)arene ketone derivatives and their interaction with alkali metal cations. Structural and thermodynamic (solution and complexation) characterisation of the tetraphenyl ketone derivative and its sodium complex

A modified method for the synthesis of 5,11,17,23-tetra-tert-butyl-25,26,27,28-tetra(benzoyl)methoxycalix(4)arene (1) and 5,11,17,23-tetra-tert-butyl-25,26,27,28-tetra(acetoyl)methoxycalix(4)arene (2) using 18-crown-6 as phase transfer catalyst is reported. Ligand 1 was structurally (1H NMR and X-ray crystallography) and thermodynamically characterised. An account of the steps required for the formulation of an equation representative of the complexation process involving metal cations and macrocycles is given using as an illustrative example the interaction of 1 with the Na(I) cation. Thus, detailed thermodynamics of this system are first reported taking into account the solution properties of the reactants and the product in various solvents with the aim of assessing quantitatively the medium effect on the complexation process. The sodium:monoacetonitrile:1 complex was isolated and its molecular structure was determinated from X-ray diffraction data. The calix now adopts a more symmetric cone conformation than the free ligand and its hydrophobic cavity is filled with an acetonitrile solvent molecule. The hydrophilic pocket hosts the sodium cation which is in an eight-fold coordination with the four ethereal oxygen atoms [average d(Na–O) = 2.45(4) A] and the four carbonyl oxygen atoms [average d(Na–O) = 2.51(4) A] forming the corners of a distorted Archimedean square antiprism. The key structural features relevant to the complexation process are discussed. 1H NMR and conductance measurements have been used to assess the interaction of 1 and 2 with alkali metal cations. Final conclusions are given. Attention is drawn about the relevance of detailed thermodynamic studies on cation complexation processes involving calixarene derivatives. This is discussed within the context of selectivity and the factors controlling it in solution processes.

New lower rim calix [4]arene amine derivatives: synthesis, characterisation and acid–base properties. Potential anion and cation binders

The synthesis of new lower rim calix[4]arene derivatives containing aliphatic and alicyclic amines is reported. 1H and 13C NMR characterisation shows that in solution these compounds exist in a cone conformation. UV spectrophotometry was used to investigate the interaction of these amines with the proton. These studies reveal that four protons are taken up for each calix[4]arene unit. The acid–base properties of these macrocyclic amines were investigated potentiometrically using methanol as the reaction medium. From these data, the equilibrium constant for each of the dissociation processes involved was derived. Diagrams showing the percentages of the individual species as a function of the solution pH are given. This information is of fundamental importance for investigating the use of these ligands as cation or anion binders. The higher basic character of aliphatic and alicyclic p-tert-butylcalix[4]arene amines relative to pyridinocalix[4]arene is demonstrated. The implication of these results on their affinity for the proton is discussed. Phase-transfer studies on the water–dichloromethane solvent system provide quantitative evidence that the presence of the calix[4]arene amino derivative in the organic phase leads to an 84% extraction of HAuCl4 from aqueous solutions. The 1H NMR spectrum of the gold(III) adduct is very similar to that of the protonated calix[4]arene amine derivative suggesting that the gold(III) containing anion is transferred by this ligand to the organic phase via an ion-pair mechanism involving the protonated ligand and these anions. The attachment of amino functional groups in the lower rim of p-tert-butylcalix[4]arenes provides a suitable arrangement for interaction with toxic metal cations (Cd2+, Pb2+ and Hg2+) while alkali and alkaline metal cations (Na+, K+ and Ba2+) are discriminated. The potential use of these ligands for the removal of toxic metal cations from contaminated sources is emphasised.

Lower rim calix[4] arene derivatives : complexation with silver in non-aqueous media and extraction of this cation in the water-nitrobenzene solvent system

The interaction of lower rim calix[4]arene containing aliphatic and alicyclic amines as pendant arms and the silver cation in a variety of solvents has been investigated. Conductance measurements indicate that the stoichiometry of the metal-ion complexes is 1:1 (ligand:metal cation) in these solvents. Using silver electrodes, direct potentiometric titrations were carried out at 298.15 K to derive the stability constants of these ligands and the silver cation in these solvents. The results are compared with previously reported data for this cation and a calix[4]arene derivative containing two different pendant arms alternately arranged, one of which contains an aliphatic amine substituent while the remaining one is a methylsulfanyl substituent. The extraction properties of calix[4]arene amine derivatives for the silver cation was investigated, from distribution data for silver(I) in the water–nitrobenzene solvent system at 298.15 K in the absence and in the presence of the ligand. Distribution data in the absence of the ligand were used to derive the partition, Kp, and the distribution, Kd, equilibrium constants for the silver(I) perchlorate in the mutually saturated solvents as well as the ion-pair formation constant, Ka, for this salt in nitrobenzene (saturated with water). Kd values were also obtained from distribution data for the silver salt in the presence of five different ligands. Equilibrium extraction constants, Kex, for the various ligands also derived from these data are reported. Good agreement is found between the Kd values derived from experimental work carried out in the presence of the various ligands as well as in their absence.

First report on electrochemical, thermodynamic and structural aspects of the interaction of p-tert-butylcalix[8]arene and cryptands in benzonitrile

p-tert-Butylcalix[8]arene interacts with cryptands 22 and 222 in benzonitrile to give 1 : 1 electrolytes, and thermodynamic, electrochemical and structural aspects of this interaction are reported for the first time; the results suggest that cryptand 222 hosts a proton from the calixarene in its cavity.

X-Ray diffraction studies and solution thermodynamics of 5,7,17,23-para-tert-butyl-25,26,27,28-tetra(diethylamine)ethoxy- calix(4)arene

The crystal structure of 5,7,17,23-para-tert-butyl-25,26,27,28-tetra(diethylamine)ethoxycalix(4)arene has been determined from X-ray diffraction data. The substance crystallises in the space group P21/a with a = 14.103(2), b = 34.250(5), c = 14.486(2) A, β = 105.15(1)°, and Z = 4. The macrocycle consists of two distorted conical parts, one hydrophobic the other hydrophilic, linked through their smaller rims. The larger bore of the hydrophobic cavity has transversal dimensions of about 5.90 × 5.90 A while the hydrophilic cavity reaches apertures of 5.06 × 10.39 A at the aminoethyl nitrogen atom positions. The solubility of this ligand at several temperatures in various solvents has been determined and the data were used to derive its solution Gibbs energies in these solvents at 288.15, 298.15 and 313.15 K. The enthalpy of solution for this calix(4)arene derivative in five solvents was measured calorimetrically at 298.15 K. Thermodynamic parameters of transfer are calculated using benzonitrile as the reference solvent. It is shown that the transfer Gibbs energy among these solvents is almost 0 kJ mol−1 as a result of an enthalpy–entropy compensation effect, where the most significant enthalpic and entropic changes are found in transfers to protic solvents.

Reaction of tetrakis[(3-pyridylmethyl)oxyl] p-tert-butylcalix(4)arene with KAuCl4 and K2PtCl6. New pyridinocalix(4)arene adducts of gold(III) and platinum(IV)

Abstract Tetrakis[(3-pyridylmethyl)oxy] p-tert -butylcalix(4)arene prepared from p-tert -butylcalix(4)arene and 3-chloromethylpyridine under basic conditions was characterized by 1 H and 13 C NMR. Protonation constants of this ligand in methanol at 298.15 K derived from potentiometric data are reported. Au III and Pt IV adducts of tetrakis[(3-pyridylmethyl)oxyl] p-tert -butylcalix(4)arene have been isolated. It is concluded that this calixarene derivative displaces chloride ions from the coordination spheres of the central atoms in AuCl 4 − and PtCl 6 − species, engaging itself in direct interaction with Au III and Pt IV .

The first crystal structure of a complex from a lower rim calix(4)arene alicyclic amino derivative: {tetrakis[1-(ethyl)piperidinium]oxyl}p-tert-butylcalix(4)arene-bis-[tetrachloroaurate(III)]dichloro dihydrate

Abstract Protonated lower rim calix(4)arene amino derivatives are able to interact with anions. This statement is corroborated by the first crystal structure of {tetrakis[1-(ethyl)piperidinium]oxyl}p-tert-butylcalix(4)arene-bis-[tetrachloroaurate(III)]dichloro dihydrate which shows that the protonated nitrogen atoms of the four piperidino pendant groups interact via hydrogen bond formation with two chloride ions and a water molecule. Two AuCl4− anions are located outside the hydrophilic cavity.

Synthesis, characterisation and X-ray diffraction studies of new lower rim calix[4]arene derivatives containing mixed donor atoms

New lower rim p-tert-butylcalix[4]arene derivatives containing tetrathiophene functional groups as well as derivatives with two different pendant arms alternately arranged, one of which is in all cases a methylsulfanyl substituent whilst the other is either a tertiary amine (aliphatic of alicyclic) or a methylthiophene or an amide substituent, have been synthesised and characterised by 1H and 13C NMR spectroscopy in chloroform at 298 K. The use of phase transfer catalysis in the synthesis of calixarene derivatives is discussed. X-Ray diffraction studies of 5,11,17,23-tetrakis-(1,1-dimethylethyl)-25,26,27,28-tetrakis-[2-(thienyl)methoxy]calix[4]arene 1a and 5,11,17,23-tetrakis-(1,1-dimethylethyl)-25,27-bis[(2-methylsulfanyl)ethoxy]-26,28-bis[2-(diethylamino)ethoxy]calix[4]arene 2b are reported.