Macropolycyclic ligands

Macropolycyclic ligands, 2,942 classification, 2,917 metal complexes binding sites, 2, 922 cavity size, 2,924 chirality, 2, 924 conformation, 2,923 dimensionality, 2, 924 electronic effects, 2, 922 shaping groups, 2,923 structural effects, 2,922 molecular cation complexes, 2,947 molecular neutral complexes, 2,952 multidentate, 2,915-953 nomenclature, 2,920 Macro tetrolide actins metal complexes, 2,973 Macrotricycles anionic complexes, 2,951 cylindrical... 

Multidentate Macrocyclic and Macropolycyclic Ligands 22 Ligands of Biological Importance... 

A number of organic molecules capable of efficiently operating as phase-transfer catalysts is now available. The reaction mechanism both for soluble and polymer-supported systems is completely understood and the factors ruling the reactivity are recognised. The drawback of soluble catalysts is their difficult separation from the reaction products which in the case of the expensive macropolycyclic ligands imposes severe limitations in their use on a large scale. The cheap and easy to synthesize ammonium quaternary salts, providing they are stable under the reaction conditions, represent the catalysts of choice. 

Many new Ni(II) complexes of aza-type macrocycles have been synthesized, and their redox chemistries have been explored. In particular, complexes of macropolycyclic ligands and bismacrocyclic ligands have been prepared. Complexes with uncommon oxidation states of nickel (Nim and Ni1 complexes) have also been synthesized by employing a specially designed macrocycle, and their characteristic spectroscopic properties and X-ray structures reported. These nickel(II) complexes... 

For the monocyclic complexes 1-7, oxidation to Ni(III) occurs at +0.90-+0.93 V and reduction to Ni(I) at -1.46--1.55 V vs SCE. However, in the macropolycyclic ligand complexes 9-14, oxidation and reduction occur at + 1.25-+1.60 V and at -0.94-—1.40 V vs SCE, respectively. That is, electrochemical oxidation of Ni(II) complex to Ni(III) species is easier for the monocyclic complexes, whereas electrochemical reduction to Ni(I) is easier for the macropolycyclic complexes. The anodic shifts in both oxidation and reduction potentials for Ni(II) macropolycyclic complexes in part may be attributed to the tertiary nitrogen donors of the ligands. 

Macropolycyclic ligands containing intramolecular cavities of a three-dimensional nature are referred to as cryptands. The bicyclic cryptands (73) exist in three conformations with respect to the terminal nitrogen atoms, exo-exo, endo-exo and endo-endo 6 these forms can rapidly interconvert via nitrogen inversion but only the endo-endo form has been found in the crystal structures of a variety of complexes372 and for the free ligand ([2.2.2], 73, m = n = / = l).449 In their complexes with alkali and alkaline earth cations, the cryptands exhibit an enhanced stability over the crown ethers and coronands dufe to the macrobicyclic, or cryptate, effect.33 202... 

Multidentate Macrocyclic and Macropolycyclic Ligands 21.3.5.3 Macrotricycles 21.3.5.3.1 Cylindrical macrotricycles... 

Although some scattered examples of binding of alkali cations (AC) were known (see [2.13,2.14]) and earlier observations had suggested that polyethers interact with them [2.15], the coordination chemistry of alkali cations developed only in the last 30 years with the discovery of several types of more or less powerful and selective cyclic or acyclic ligands. Three main classes may be distinguished 1) natural macrocycles displaying antibiotic properties such as valinomycin or the enniatins [1.21-1.23] 2) synthetic macrocyclic polyethers, the crown ethers, and their numerous derivatives [1.24,1.25, 2.16, A.l, A.13, A.21], followed by the spherands [2.9, 2.10] 3) synthetic macropolycyclic ligands, the cryptands [1.26, 1.27, 2.17, A.l, A.13], followed by other types such as the cryptospherands [2.9, 2.10]. 

Numerous macrocyclic and macropolycyclic ligands featuring subheterocyclic rings such as pyridine, furan or thiophene have been investigated [2.70] among which one may, for instance, cite the cyclic hexapyridine torands (see 19) [2.39] and the cryptands containing pyridine, 2,2 -bipyridine (bipy), 9,10-phenanthroline (phen) etc. units [2.56,2.57,2.71-2.73]. The [Na+ c tris-bipy] cryptate 20 [2.71] and especially lanthanide complexes of the same class have been extensively studied [2.74, 2.75] (see also Sect. 8.2). 

Macrocycle obtained by the condensation of phenol rings with formaldehyde and amenable to substitution both on the phenolic functions and on the para positions of the phenol rings Macropolycyclic ligand forming a cage Metal ion complex with a cryptand... 

Because the definition of metal ion encapsulation or its absence is, to a certain extent, ambiguous, we first describe the conventional criteria that we use to restrict the scope of the compounds to be considered. The major signs of complexation (formation of a complex) with an encapsulated metal ion are (a) a three-dimensional cavity (capsule, cage) produced by a macropolycyclic ligand and (b) metal ion coordinating heteroatoms in this cavity that isolate this metal ion from the environment. 

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