Crown ether functionalised complexes

Since the early work of Pedersen, the crown ethers have become very popular in cation complexation chemistry. Their versatile binding ability and their almost infinite synthetic malleability have led to an enormous variety of functionalised and derivatised crowns and crown analogues with varying selectivity for an enormous variety of guest species, Figure 3.7. The analogy of the K+-binding ability of 3.4... 

The detection of aromatic carboxylates via the formation of ternary complexes using lanthanide ion complexes of functionalised diaza-crown ethers 30 and 31 has been demonstrated [134]. Like the previous examples, these complexes contained vacant coordination sites but the use of carboxylic acid arms resulted in overall cationic 2+ or 1+ complexes. Furthermore, the formation of luminescent ternary complexes was possible with both Tb(III) and Eu(III). A number of antennae were tested including picolinate, phthalate benzoate and dibenzoylmethide. The formations of these ternary complexes were studied by both luminescence and mass spectroscopy. In the case of Eu-30 and Tb-30, the 1 1 ternary complexes were identified. When the Tb(III) and Eu(III) complexes of 30 were titrated with picolinic acid, luminescent enhancements of 250- and 170-fold, respectively, were recorded. The higher values obtained for Tb(III) was explained because there was a better match between the triplet energy of the antenna and a charge transfer deactivation pathway compared to the Eu(III) complex. 

The earliest recognised examples of synthetic supramolecular structures were the complexes formed from crown ethers and metal cations [19]. Since then numerous macrocycles have been synthesised. Representative examples are the cryptands [20], These differ from crown ethers in that the former contains a tridimensional cavity while the latter are characterised by a hole. Similarly, calix[4]arenes are compounds with a cup -like structure that through lower rim functionalisation gives rise to a hydrophilic and a hydrophobic cavity, thus allowing the reception of ionic species in the former and neutral species in the latter. Most of the above mentioned macrocycles are known for their capability to serve as cation receptors. 

The development of CTOwn ether functionalised imidazolinm salts starts from the consideration that it is possible to link one polyether chain with two imidazolinm nnits at the end points. Since a transition metal can coordinate two imidazoUnm salts in trans fashion [131,162,208], two of these (poly)ether fnnctionaUsed bis-carbenes can form a macrocychc crown ether type hgand system with two transition metal carbene linkages. In favonrable cases, a pincer type C,0,C ms-coordination to the same transition metal is conceivable, bnt may not be very likely when the great affinity of late transition metals to NHC ligands and the aversion of these same late transition metals to ether donor Ugands is taken into account. However, hanilabile stabilisation of transition metal complexes in catalytic processes can certainly be hoped for. 

Functionalised conjugated polymers such as polythiophenes were studied from the point of view of the detection and transduction of chemical and physical information into an optical or electrical signal. Their ionochromism (reversible change of colour in the presence of ions), photochromism (reversible change of colour on exposure to light), affinity chromism (tendency to colour change) and electroluminescence of polythiophene complexes with crown ethers and other solutions are discussed in detail [295]. 

Synthetic developments in the area of Supramolecular Chemistry are currently leading to a massive production of new macrocycles. The driving force for this continuous growth is the search for selective hosts to target a particular neutral or ionic species. There is no doubt that the impact produced by the discovery of macrocyclic ligands such as the crown ethers [1] and the cryptands [2] resulted from their cation complexation and this prompted us to consider the thermodynamic characterisation of these systems (mainly cryptands) which has been extensively reported [3-5]. Calixarenes, an important class of macrocyclic compounds, are products of the base-catalysed condensation reaction of p-substituted phenols and formaldehyde [6, 7]. These compounds are characterised by their low solubilities in most solvents, although until recently [8], no quantitative data has been reported. Functionalisation of the lower or upper rim of parent calix[n]arenes has... 

Diaza[18]crown-6 (2.20) can also be readily functionalised with two side arms, instead of just one as in 2.21, to form double lariat ethers termed bibracchial lariat ethers (BiBLEs), such as 2.24. This modification results in the possibility of the side arms covering both faces of the macrocycle (Figure 2.6(a)). A particularly striking example is the complexation of sodium in a BiBLE that contains two thymine groups as the side arms (Figure 2.6(b)). 

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