Calixarenes metal complexation

In a study, Cacciapaglia et al. (2006) successfully synthesized a pseudo-biomimetic catalyst that worked as a phosphodiesterase enzyme [37]. For that purpose, 1,2-, 1,3-, and 1,2,3-positions of the upper rim of calix[4]arene were functionalized with 1,5,9-triazacyclododecane ([12]ane-N3) moieties, which were able to coordinate with several metals (see Fig. 27.15). These calix[4]arene derivatives were complexed with Zinc(II) and copper(II) to produce effective catalysts for the cleavage reaction of phosphodiester as RNA models. As seen in Fig. 27.16, the reaction takes place over the calixarene-metal complexes that indeed clarifies the importance of the substitution pattern of ligated metal ions (1,2-vicinal or 1,3-distal) along with cahx[4]arene skeleton. 

Solution State of Metal Complex Calixarenes and Polymeric Calixarenes... 

Heteroaryl groups are present in the 4-pyridyl-ethynyl compounds81,99,100 and their extended variants with C=C-C6H4-C=C-pyr-4 ligands.81,99 These compounds can be quaternized with Mel or coordinated to a metal complex at their terminal pyridyl functions to give species addressed as molecular rods or molecular wires.81,99 Similar reactions were carried out with 3-phenanthrolinyl-ethynyl complexes,62 and special examples are also known with R based on calixarenes.51... 

Metalated container molecules can be viewed as a class of compounds that have one or more active metal coordination sites anchored within or next to a molecular cavity (Fig. 2). A range of host systems is capable of forming such structures. The majority of these compounds represent macrocyclic molecules and steri-cally demanding tripod ligands, as for instance calixarenes (42), cyclodextrins (43,44), and trispyrazolylborates (45-48), respectively. In the following, selected types of metalated container molecules and their properties are briefly discussed and where appropriate the foundation papers from relevant earlier work are included. Porphyrin-based hosts and coordination cages with encapsulated metal complexes have been reviewed previously (49-53) and, therefore, only the most recent examples will be described. Thereafter, our work in this field is reported. 

The smaller p -tert-butyl-calix [4] arenes have a rich coordination chemistry as well (65). Of these, however, only the upper-rim modified calixarenes seem to support metal complexes with confined binding sites (66), except in those cases where the lower-rim substituents form an appended cavity. Thus, Matt and coworkers have reported (67) a pocket-shaped calix[4]arene ligand L2 bearing two lower-rim [([Pg.410]

It has already been mentioned that metal complexes with confined binding pockets often display unusual chemical reactivities (see Section II). Thus, complexes of substituted hydrotris (pyrazolyl)borates, in which the substituents serve to from a hydrophobic binding pocket, have already been shown to exhibit enhanced chemical reactivity when compared with their unmodified analogs (282,283). Likewise, cyclodextrin and calixarene-based metallocavitands have been used as catalysts for selective organic transformations, and even as catalysts for reactions that... 

Roundhill, D. Max, Metal Complexes of Calixarenes Rovis, Tomislav, and Evans, David A., Structural and Mechanistic 43 533... 

HPNPP (101) was also used to probe the catalytic ability of zinc- and copper-containing calix[4]arenes that carried two or three [12]ane-N3 macrocycles on their upper rim. Cooperativity was found between the catalytically active metal complexes during phosphodiester transesterification provided that they were adjacent to each other, i.e. on proximal positions of the calixarene rim, whereas those on opposite... 

Another interesting and potentially very useful group of calixarene-based anion receptors is represented by systems with appended transition metal complexes of 2,2-bipyridine units. Technically, these systems utilise classical hydrogen bonding interactions of amidic/urea functions hence, from this point of view, they do not differ from many other receptors. On the other hand, the covalent attachment of bipyridine complexes of ruthenium(II) or rhe-... 

Another class of metal-employing anion receptors is represented by structure 24 [23]. Its function is based on the incorporation of positively charged transition metal complexes directly into the calixarene skeleton. Such calixarenes with enhanced electron deficiency of the aromatic walls provide well-preor-ganised cavities suitable for anion inclusion. The corresponding rhenium [24], ruthenium, rhodium or iridium complexes of this type were prepared and studied for anion recognition [25,26]. 

The area covered is very widespread and the role of the calixarene molecules reach from a simple platform or skeleton on which to assemble chiral centers to an inherent part of the chiral structure. Biologically active molecules or derivatives are involved as well as artificial ligands and their metal complexes. Chiral calixarenes have been used as stationary phases in analytical separations or as host molecules in sensors. Basic properties of calixarenes, such as their conformational stabilities, have been studied with chiral derivatives as well as more... 

Upper rim substitution is typically more synthetically challenging than the lower rim, but can confer very useful properties on the macrocycle. For example, water soluble / -sulfonatocalixarenes have been synthesized and found to be highly water soluble, unlike the hydrophobic p-t-butyl derivatives. The sulfonated calixarenes form a variety of metal complexes where the metal is usually bound to the sulfonato groups of the calixarene. 

Beer, Balzani and coworkers [96] have recently realized an anion sensor based on the same subunits, which operates through an intramolecular electron transfer between an appended luminescent metal complex and a quinone fragment, both included within a calixarene framework. Systems 58 and 59 display a remarkable... 

An alternative and often facile route to appropriately functionalised ICPs, that avoids the synthetic problems outlined in (ii) above, is the use of sulfonated species containing the desired molecular recognition/receptor site as the dopant anion for the conducting polymer chains. For example, calixarene-containing polypyrroles [34] and polyanilines [35] for selective metal ion detection have recently been prepared via the use of sulfonated calixarenes as dopant anions. We have similarly found that the incorporation of metal complexing agents such as sulfonated 8-hydroxyquinoline as dopants in polypyrroles provides a simple route to metal ion-selective ICPs [36]. 

Figure 80 Crystal structure of (HN(C2H5)3)2[(UO2)2(C88Hi08O8)(OH)]-(N(C2H5)3)2-(H2O)3-(CH3CN)4 and of the calixarene ligand with the metal complex removed for clarity (Thuery, Keller et al. Acta Crystallogr.,...

Polycarboxylate crown ethers such as (205) are suitable ligands for potentiometric studies of mixed-metal complexes of Al3+ and alkali or alkaline-earth cations.303 A similar (+)-18-crown-6-tetracarboxylic acid, chemically immobilized on a chiral stationary phase (CSP), can selectively recognize both enantiomers of some analytes.304 Calixarene polycarboxylates such as (206) and (207) are useful ligands toward alkali-305,306 and also transition-metal ions,307 308 with applications in... 

There are some 3,000 publications reporting calixarenes, many of which can be obtained by searching databases under the term calix. In addition, several books have been written on calixarenes, and metal complexes are described in all of them.1-6 Reviews on metal complexes of calixarenes has been published,7-10 and a book published in 2001 also makes numerous references to them.11 Among the other calixarenes are the calix[4]arene thiol (7) and the thiacalix[4]arene (8). Related compounds are the oxacalix[3]arene (9) and the azacalix[3]arene (10), both of which have been used as complexants for metal ions. 

The unsubstituted para-t-butyl calixarenes themselves complex metals via their aryloxide groups. Since aryloxide complexes are frequently oligomeric, the simple calixarenes do not give monomeric complexes. Aryloxides are hard ligands, therefore they readily form complexes with oxo-philic hard metal ions such as alkali metals, early transition metals, lanthanides, and actinides. Complexation is often inferred because the calixarene acts as a carrier for the metal ion from an aqueous to an organic phase. With the /wa-/-butylcalix[ ]arenes in alkaline solution, a value of n = 6 gives the best carrier for lithium(I), sodium(I), and potassium(I), with a value of n 8 giving the best carrier for rubidium(I) and caesium(I).15,16 Titanium(IV) complexes have been characterized,17-19 as well as those of niobium(V) and tantalum(V).20-22 These complexes are classified as... 

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