Rigid macrocycles

Macrocyclic effect and specific character of complex formation with rigid macrocyclic ligands such as porphyrins and phthalocyanins 97MI8. 

Structural data on ruthenium porphyrins shows that the Ru-N (porphyrin) distance is relatively unaffected by changing the oxidation state, as expected for a metal atom inside a fairly rigid macrocyclic ring (Table 1.11). 

Synthesis of sterically rigid macrocycles by the use of pressure-induced repetitive Diels-Alder reactions [84]... 

In order to construct a hydrophobic three-dimensional cavity that is in-tramolecularly limited in space, we have prepared cage-type cyclophanes by linking macrocyclic rings. First we prepared a macropolycyclic host, which is constructed with two rigid macrocyclic skeletons of different size, tetraaza[3.3.3.3]paracyclophane as the larger one and tetraazacyclotetradecane as the smaller one, and four flexible hydrocarbon chains that connect the two macrocycles [40]. The flexibility of four hydrocarbon chains connecting the two macrocycles allows the induced-fit host-guest interaction in aqueous media. 

The two inner protons are attached on the two diagonal nitrogens but rapidly migrate between the two possible positions. The thermodynamic parameters, AG = 41.4 0.8 kJmol AH = 42.3 3.3 kJmol-1, AS = 0 1.3 JK.-1 mol-1, suggest a more rapid motion of the pyr-rolic protons (Is-1 at — 99°C) in these rigid macrocycles relative to those in porphyrin (AG = 46-50 kJ mol1).1174... 

Another interesting class of anion receptors based upon protonated nitrogen atoms are the expanded porphyrin macrocycles such as 4.17 (diprotonated sapphyrin) and compound 4.18. The tetrapyrrole porphyrin macrocycles are excellent hosts for metal cations such as Fe2+ and Mg2+ (e.g. haemoglobin and chlorophylls, Sections 2.3-2.5) however, their cavity dimensions are too small to accommodate anions. Conversely, expanded porphyrins such as 4.17 comprising five or more pyrrole residues present a rigid macrocyclic cavity about 5.5A in diameter, in which (particularly when protonated) the Nff... 

Another example of a rigid macrocycle used as the central core of a den-drimer is the porphyrin. First and second generation phenylene-based dendritic porphyrins with 8 and 16 long alkyl chains on their periphery have been studied (Fig. 80). Only the second generation exhibits mesomorphic behavior with the formation of a rectangular columnar mesophase upon heating (Cr 39 Coir 110 I) [334]. A stable supramolecular 1 1 complex was formed between the second generation and Ceo, which led to the enhancement of the mesophase stabiUty of the formed (imidentified) columnar phase (Cr 99 Col 250 I). 

Chart 3. Rigid macrocyclic tetra-thiaethers used in the investigation of the Cu /Cu redox change. All the stereoisomers deriving from different configurations of cyclohexyl rings have been studied. 

A large, macrocyclic ligand such as phthalocyanine is a very stable ligand with extended u-delocalisation and rigid macrocyclic structure. It can form metal complexes, [M(Pc)] (Pc = phthalocyanine), with many metals (M = Ni, Pt, Co, Fe, Cu, etc.) and the planarity of the molecule is retained... 

The facile condensation reaction between formaldehyde and phenols or their derivatives provides a major route into rigid macrocycles used in supramolecular chemistry. Calixarenes, the best-known class of phenol-derived macrocycles, are prepared this way, as are spherands and their relatives. Cyclotriveratrylene, however, is an excellent exemplar of the molecular basket type of ligand and has been known for the best part of a century. The basic cyclotriveratrylene synthesis is shown in Figure 3.1. The original procedure by Mrs. Gertrude Maud Robinson [1] has since been refined by others and many variations are known [2,3]. 

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