A wide variety of macrocyclic ligands able to interact with cations, anions, and neutral species are known. These have been isolated from natural resources or are the result of the synthetic developments which have taken place in the last 30 years.1a,b Among the latter, calixarenes (products of the baseinduced condensation of p-substituted phenols and formaldehyde) and their derivatives have received considerable attention in recent years. The chemistry of calixarenes has been extensively discussed in several books2a-c and review articles.2d-g There are several reasons for the current widespread interest in calixarenes. An important one is the remarkably simple way (single-step procedure) used for the synthesis of the parent compounds. In addition, lower and upper rim functionalization of these macrocycles has resulted in a massive expansion in the range of derivatives available.2e,g Within this context, calix(4)arenes are of particular interest. Several conformers (cone, partial cone, 1,2 alternate, and 1,3 alternate) have been isolated. A great deal of efforts have been directed toward calix(4)arene derivatives in “cone” conformation. Their particular molecular architecture with well-defined hydrophobic and hydrophilic regions makes these compounds suitable hosts for neutral and ionic species. Calixarenes are potential building blocks for the design of novel supramolecular systems. This is exemplified in the synthesis of calixarenes bridged across the lower and upper rim or indeed coupled between themselves or to other macrocycles.2g-i Calixarenes have found a wide range of applications. A good account on the industrial uses of these macrocycles prior to 1992 has been given by Perrin and Harris.2j However in the past few years several internationally significant patents have been published.2k-u These patents center mostly on the potential use of calixarenes to sequester metal ions and neutral species, although the ultimate use is fairly broad. The remarkable growth of the field of calixarene chemistry has been greatly motivated by the interest in finding derivatives able to enter into selective complexation with neutral or ionic species. A quantitative measure of the strength of interaction between two chemical species (macrocycle and guest) in a given solvent is provided by the stability constant. Therefore, the role of experimental thermodynamics in understanding the factors contributing to complex stability and selectivity is important and must not be underestimated. It should be explicity stated that the derivation of accurate thermodynamic data requires knowledge of the speciation in the systems under study. A significant point arising from this statement is that a high level of scrutiny and detail regarding the behavior of chemical species in a given medium is required in thermodynamic studies related to solution processes. It is only on the bases of accurate experimental data that the 2495 Chem. Rev. 1998, 98, 2495−2525

Thermodynamics of Calixarene Chemistry.

Anion selective recognition and optical/electrochemical sensing by novel transition-metal receptor systems

Durch Kondensation von p-tert-Butylphenol mit Formaldehyd sind in einem Schritt in guten Ausbeuten cyclische Oligomere erhaltlich, in denen je nach Reaktionsbedingungen vier, sechs oder acht Phenolbausteine uber Methylenbrucken verknupft sind. Die kelchartige Gestalt der stabilsten Konformation des Tetramers erklart den Namen Calixaren (calix = Kelch). In ahnlicher Weise liefert Resorcin bei der Kondensation mit einer Vielzahl von Aldehyden cyclische Tetramere mit dem gleichen Grundgerust (Resorcarene). In beiden Fallen erfordert die Herstellung keine Verdunnungsbedingungen, so das auch grosere Mengen leicht zuganglich sind. Uberdies konnen die Stammverbindungen vielfaltig modifiziert werden, wobei die phenolischen Hydroxygruppen und die Phenylringe zwei Ansatzpunkte bieten, die getrennt oder in Kombination genutzt werden konnen. Calixarene sind deshalb ideale Edukte fur die Synthese der unterschiedlichsten Wirtverbindungen und Bausteine fur die Konstruktion groserer molekularer Systeme mit definierter Struktur und Funktion. Die potentiellen Anwendungen reichen vom Einsatz als hochspezifische Liganden fur die Analytik, die Sensortechnik und die medizinische Diagnostik uber die Verwendung zur Decontaminierung von Abwassern bis hin zur Konstruktion kunstlicher Enzyme und zur Herstellung neuer Materialien fur die nichtlineare Optik oder fur ultradunne Schichten und Siebmembranen mit molekularen Poren.

Calixarene – Makrocyclen mit (fast) unbegrenzten Möglichkeiten

The Lewis acidic redox-active and photoactive ruthenium(II) bipyridyl moiety in combination with amide (CO−NH) groups has been incorporated into acyclic, macrocyclic, and lower rim calix[4]arene structural frameworks to produce a new class of anion receptor with the dual capability of sensing anionic guest species via electrochemical and optical methodologies. Single-crystal X-ray structures of (1)Cl and (11)H2PO4 reveal the importance of hydrogen bonding to the overall anion complexation process. In the former complex, six hydrogen bonds (two amide and four C−H groups) stabilize the Cl- anion and three hydrogen bonds (two amide and one calix[4]arene hydroxyl) effect H2PO4- complexation with 11. Proton NMR titration investigations in deuterated DMSO solutions reveal these receptors form strong and, in the case of the macrocyclic 5 and calix[4]arene-containing receptor 11, highly selective complexes with H2PO4-. Cyclic and square-wave voltammetric studies have demonstrated these receptors to electrochemica...

Synthesis and Characterization of Novel Acyclic, Macrocyclic, and Calix[4]arene Ruthenium(II) Bipyridyl Receptor Molecules That Recognize and Sense Anions

This review discusses molecular design principles of calixarene-type macrocycles for ion recognition and gives examples on the relationship between structure and selectivity, however without attempting to cover all the different approaches.

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Calixarene-Based Molecules for Cation Recognition

The complexing properties of some calix[4]arenes in which carboxylic acid ligating groups have been progressively attached to the lower rim through the phenolic hydrogen atoms have been measured with alkali and alkaline-earth metal cations in methano by means of potentiometric titrations. The acid-base characteristics of these compounds were first determined. the weak acidities generally observed may be explained by the formation of intramolecular hydrogen bonding.

Cation complexation by chemically modified calixarenes. 5. Protonation constants for calixarene carboxylates and stability constants of their alkali and alkaline-earth complexes

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.

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 binding abilities of a series of ionizable calixarenes towards three
lanthanides (Pr
 3+
, Eu
 3+
 and Yb
 3+
) and one
actinide (Th
 4+
) have been established in methanol by
potentiometric measurements. The calixarenes result from progressive
substitution of the phenolic hydrogens of
p-tert-butylcalix[4]arene by carboxylic acid functions.
Calixarene derivatives with mixed carboxylic and ester functional groups,
as well as
octa-O-carboxymethyl-p-tert-butylcalix[8]arene
and the two oxa-derivatives,
tetra-O-carboxymethyl-p-tert-
butyltetrahomodioxacalix[4]arene and
tri-O-carboxymethyl-p-tert-
butylhexahomotrioxacalix[3]arene have also been studied. The formation of
1 : 1 species partially protonated
[M(H
 z
 
L)] and totally deprotonated (ML) as well as
methoxo species [{ML(OMe)
 z
 
}] has been
established with lanthanides. Additional binuclear complexes and their
methoxo forms have been found in some cases. With thorium the mononuclear
ML and MHL complexes are mainly formed in addition to the corresponding
methoxo species at high pH. For a given ligand, the stability of the
complexes increases with the cationic charge: complexes of thorium are more
stable than those of lanthanides, which are themselves generally more
stable than the corresponding alkaline-earth- and alkali-metal complexes.
The predominance of electrostatic interactions in the binding is further
confirmed by a linear relationship between the stability of mononuclear
complexes (log β
 110
) and the total basicity of the ligands
(ΣpK
 ai
). The
p-tert-butylcalix[8]arene octaacid and the
p-tert-butylcalix[4]arene monoacid are respectively the
best complexing agents for the lanthanides and thorium. The monoacid
derivative shows significant Yb
 3+
/Eu
 3+
 and
Th
 4+
/Eu
 3+
 selectivities in appropriate pH
ranges.

Complexation of Pr3+, Eu3+, Yb3+ andTh4+ ions by calixarene carboxylates

p-tert-Butylcalix[4]arene derivatives 2 and 4 bearing one carboxy-lic acid, one ester and two phenolic groups on the lower rim form pH-dependent mononuclear or binuclear assemblies with alkali (excluding lithium) and alkaline earth cations. The structures of binuclear K+ complexes 5 and 6 of 2 and 4, respectively, have been determined by X-ray diffraction.

Pascale Schwinte

Papers

Cation complexation by chemically modified calixarenes. 5. Protonation constants for calixarene carboxylates and stability constants of their alkali and alkaline-earth complexes

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

Complexation of Pr3+, Eu3+, Yb3+ andTh4+ ions by calixarene carboxylates

A supramolecular assembly of a multifunctional calix[4]arene with alkali and alkaline earth cations in solution and in the solid state