Geraldine Barrett

Cation complexation by chemically modified calixarenes. Part 10. Thioamide derivatives of p-tert-butylcalix[4]-, [5]- and [6]-arenes with selectivity for copper, silver, cadmium and lead. X-Ray molecular structures of calix[4]arene thioamide–lead( II ) and calix[4]arene amide–copper( II ) complexes

The effect of chemical modification of the lower rim of p-tert-butylcalix[4]-, [5]- and [6]-arenes has been analysed with respect to cation binding by thioamide podands. Extraction data for metal picrates from water into dichloromethane are discussed. CuII, PbII and AgI ions are extracted efficiently by all the thioamides studied, whereas extraction of CdII only reaches significant levels with the pentamer derivative. Unlike their calixarene amide counterparts, these thioamides have no affinity in extraction for either alkali or alkaline earth metals. The X-ray molecular structure of a thioamide–PbII(ClO4)2 complex and, for comparison, that of an amide–CuII(ClO4)2 complex have been determined. Both complexes exist in the cone conformation with the metal ion encapsulated by the heteroatoms on the lower rin. The crystal structure of the lead complex derivative 5·Pb(ClO4)2·EtOH·0.5 H2O, was solved by Patterson methods and refined by block diagonal least-squares analysis. The crystals are triclinic, space group P, a = 13.394(6), b = 13.459(6), c = 26.711(4) A, a = 78.33(2), β = 87.62(2), γ = 60.46(2)° with R = 0.108 for 4417 observed reflections. The Pb2+ is bonded to the four ethereal oxygen and four thiocarbonyl sulfur atoms, [Pb–O, 2.65(2) to 2.72(2), mean value 2.68(3) A; Pb–S, 2.82(1) to 2.95(1) with a mean of 2.91(3) A]. The crystal structure of the copper calixarene complex 2·Cu(ClO4)2·H2O ·1.4CH3OH·EtOH, was solved by direct methods and refined by block-diagonal least-squares methods. The crystals are tetragonal, space group P4/ncc a = 17.147(2), c = 28.054(4) A, with R = 0.079 for 1482 observed reflections. The Cu2+ resides on a four-fold axis and is bonded to the four carbonyl oxygen atoms [Cu–O, 1.926(6) A]. Four ethereal oxygen atoms are at a distance of 2.963(6) A from the metal ion.

Extraction and solution thermodynamics of complexation of alkali and alkaline-earth cations by calix[4]arene amides

The extracting properties of nine new calix[4]arene tetraamides towards the entire alkali cation series and strontium have been determined. One of the amides bearing propargyl (prop-2-ynyl) residues on the nitrogen atoms has also been tested in Complexation in methanol for the same cations. The enthalpies and entropies of Complexation of alkali and alkaline-earth cations by the diethyl and pyrrolidinyl tetraamides in methanol and of alkali cations by the diethyl tetraamide in acetonitrile have been determined for the first time from calorimetric measurements. The data are compared with data for calixarene esters and solvent effects are discussed in the light of earlier results.

Cation complexation by chemically modified calixarenes. Part 7. Transport of alkali cations by p-tert-butylcalix[n]arene esters and amides

The co-transport of alkali thiocyanates, at 20 °C, from an aqueous source solution into an aqueous receiving solution through a bulk liquid dichloromethane solution, is facilitated by p-tert-butylcalix[n]arene esters of lower rim calixarene acetic acids, named ethyl esters (n= 4, 5, 6, 8), by p-dealkylated calix[6]arene ethyl ester and the diethyl and pyrrolidine amides of a p-tert-butylcalix[4]arene. The transport rates show that all derivatives, except the octamer, are efficient and selective neutral ionophores for alkaline cations. The amides are better carriers than the esters and the pyrrolidinyl amide is more efficient for Li+, Na+ and K+ than the diethyl amide. Among the esters, the most efficient, by far, is the pentamer. The tetra- and penta-esters are selective for Na+, the hexaester for Cs+ and the tetraamides for K+. The Cs+/Na+ selectivity of the hexaester is increased by dealkylation in the para position. The same is shown to be displayed in extraction. The data are compared with existing stability constant values in methanol and extraction equilibrium constants for alkali picrates, from water to dichloromethane. New extraction data are provided for alkali thiocyanates.

Selective alkali-metal cation complexation by chemically modified calixarenes. Part 4. Effect of substituent variation on the Na+/K+ selectivity in the ester series and X-ray crystal structure of the trifluoroethyl ester

The cation complexing abilities of a series of p-tert-butylcalix[4]arenes bearing ligating ester groups in the cone conformation have been assessed by stability constant measurements in methanol and extraction studies from water into dichloromethane. The cations studied were Na+ and K+ and variations in the ester function (CO2R) included R = methyl, ethyl, n-butyl, tert-butyl, benzyl, phenyl, phenacyl, methoxyethyl, trifluoroethyl, methylthioethyl and prop-2-ynyl. The effect of replacing one or two ester functions in the tetraethyl ester by methyl ester, carboxylic acid, ketone and amide functions was also studied. Selectivities for Na+ relative to K+ in stability constants range from 2 to 2500, the phenacyl derivative having the highest selectivity. X-Ray diffraction analysis was used to probe the conformation of the trifluoroethyl ester 11. Crystals of 11 are monoclinic, space group P21/n, in a cell of dimensions a= 13.987(2), b= 16.194(3), c= 27.630(5)A; β= 98.70(1)°; R= 0.077 for 3172 observed data. The compound possesses a distorted cone conformation.

Chromogenic ligands for lithium based on calix[4]arene tetraesters bearing nitrophenol residues

Three novel chromogenic calix[4]arene tetraesters bearing nitrophenol residues which dramatically change their UV–VIS absorbance spectra upon complexation with metal ions have been investigated. Upon addition of metal perchlorates such as lithium or sodium to a solution of the calixarene in tetrahydrofuran, in the presence of a base, a colour change from near colourless to yellow occurs. The colour density is dependent on metal perchlorate concentration and can be monitored by UV–VIS spectroscopy. All three compounds were found to be lithium selective with a 10–40 fold selectivity for lithium against sodium. No colour change was noted in the absence of a base. From two-phase studies, it has been demonstrated that all three ligands have the ability to extract metal ions from an aqueous phase into an immiscible organic phase (butan-1-ol) which had been made basic by the addition of the lipophilic base tridodecylamine, with a colour change from near colourless to yellow occurring in the organic phase. The deprotonated calixarene metal ion complex was found to diffuse into the aqueous phase with time, with the rate of diffusion being strongly dependent on the lipophilic nature of the calixarene backbone.

Novel chromogenic ligands for lithium and sodium based on calix[4]arene tetraesters

Two new chromogenic calix[4]arene tetraesters bearing nitrophenol residues display a striking cation-selective shift in absorbance maximum from the ultraviolet to the visible region in the presence of base upon addition of lithium ion or, to a lesser extent, sodium ion; the metal salt causes a change from colourless to yellow, the colour density being concentration dependent.

Cation complexation by chemically modified calixarenes. Part 6. Alkali and silver(I) cation complexation by p-tert-butylcalix[5]arene derivatives and X-ray structure determination of a pentaester

The preparation of two p-tert-butylcalix[5]arene esters and a p-tert-butylcalix[5]arene carboxylic acid are reported. The cone conformation in solution at 20 °C has been established for all three compounds by 1H NMR spectroscopy. Crystals of pentaethyl ester 2 were studied by X-ray diffraction and the molecule was found to possess a distorted cone conformation in the solid state. Crystals of 2 are triclinic, space group P with two molecules in a cell of dimensions a= 11.656(2), b= 14.912(3). c= 23.044(5)A; α= 79.23(2), β= 78.31 (2), γ= 67.52(2)°. The structure was solved by direct methods and refined by least squares to R= 0.084 for 6967 reflections. One phenolic ring (B) is tilted inwards at the upper rim of the cone; the others are tilted outwards, but by varying amounts. The pendant side-chains also adopt very different conformations, resulting in a range of inter-chain O ⋯ O distances around the cavity, from 3.35 A upwards. The binding properties of the esters towards alkali cations have been assessed by picrate extraction experiments and stability constant determinations. The main conclusions are: (a) both pentamer esters are much more effective in extraction than either their tetramer or hexamer counterparts; (b) the nature of the alkoxy residue in the ester influences the extent of extraction, the tert-butyl ester being more effective than the ethyl ester; (c) extraction favours the larger cations with little discrimination between K+, Rb+, and Cs+; (d) the trends in extraction are mirrored in stability constants with the tert-butyl derivative forming the stronger complexes.

Selective monohydrolysis of bridged and unbridged calix[4]arene esters and its inhibition by alkali ion. Evidence for hydronium ion complexation

Several bridged and unbridged calix[4]arene tetraester derivatives are hydrolysed by trifluoroacetic acid in chloroform with the loss of one, and only one, ester group to form triester monoacids. In bridged tetraethyl ester 11, the group that is hydrolysed is that on the aromatic moiety not carrying the methylene bridge. The hydrolysis process is inhibited by the presence of alkali metal cations, especially sodium. Evidence is presented in favour of a hydrolytic process involving hydronium ion complexation within the hydrophilic oxygenated cavity of the calixarene receptor. Crystals of the triester monoacid 5 were studied at 123 K and are monoclinic, space group P21/n, with four molecules in a cell of dimensions a= 13.554(6), b= 20.095(9), c= 20.658(17)A, β= 102.27(5)°. The structure was solved by direct methods and refined by full-matrix least-squares calculations; R= 0.063 for 2017 observed reflections. The calix[4]arene has a distorted cone conformation similar to that found in other closely related derivatives. The hydroxy group of the carboxylic acid moiety is hydrogen bonded [O O 2.722(10)A] to a proximal ethereal oxygen and not to carbonyl oxygen atoms.

The solution thermodynamics of ethyl p-tert-butylcalix[4]arene tetraacetate and its alkali metal complexes in acetonitrile and methanol

The first calorimetric studies of ethyl p-tert-butylcalix[4]arene tetraacetate and alkali metal cations in acetonitrile and in methanol are reported; the results suggest that acetonitrile enters the hydrophobic cavity of the ligand producing a synergistic effect which makes the hydrophilic cavity better preorganised to interact with cations in this-reaction medium.

Polymeric calixarenes. Synthesis, polymerisation and Na+ complexation of a calix[4]arene methacrylate

A calix[4]arene methacrylate has been prepared from p-tert-butylcalix[4]arene and polymerised under free radical conditions: both monomer and polymer form stable complexes with sodium thiocyanate.