Macrobicyclic ligands-cryptands

The addition of alkali or alkaline earth metal cations to a solution of (50) in chloroform gave a new set of peaks in the Hnmr spectrum indicative of complexation. Further studies using ion-selective electrodes (47), pH-metric titration (47) and x-ray (48) analysis showed that the complexes formed were very stable and that the metal cation guest is located within the central cavity 

Nomenclature. The lUPAC nomenclature applied to compound (50) is 4,7,13,16,21,24-hexaoxa-l, 10-diazabicyclo 8,8,8-hexacosane this is quite a mouthful Trivial names have been introduced and are generally used. The notation is very simple and is derived from three numbers, each designating the number of oxygen donor atoms in each bridge. The macrobicycle (50) in 

Stability and selectivity. The pH-metric titration method is usually used to determine the stability constants of cryptates (47). For alkali and alkaline earth metal cations, high stability constants are generally observed. As with the monocyclic polyethers, the most stable complex results when the ionic radius of the metal cation best matches the radius of the cavity formed by the cryptand on complexation. Because the cryptand host cavity is three-dimensional and spheroidal in shape, it is well adapted for a ball-like guest metal cation. Hence they have more pronounced recognition receptor 

Structures. Weiss et al (48) confirmed in 1970 from an x-ray structural study 

Structural modifications relating to selectivity changes. Altering (i) the lipophi-licity and (ii) the nature of the donor heteroatoms of these macrobicyclic host molecules can have profound effects on their selectivity behaviour towards metal cations. 

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Nickel(ll) complexes with macrobicyclic ligands (cryptands) 270... 

Ion transport by macrobicyclic ligands (cryptands) has been investigated using a bulk liquid membrane149. It was found that the relative transport rates are not proportional to complex stability. E.g. cryptand (Fig. 67 m = n = 1) which forms a very stable K+ complex, is an inefficient K+ carrier, because of the slow exchange rate of the complex167. On the other hand, cryptand (Fig. 68 3) which forms a more lipophilic and less stable complex, is quite an efficient K+ carrier. It appears that the cryptands display efficient carrier properties for those cations which form complexes with stability constants of about 10s in methanol. 

The 18-crown-6-cyclic polyether-KBr complex catalyses the homogeneous liquid phase molecular oxygen oxidation of ethylbenzene to the hydroperoxide. The macrobicyclic ligands (cryptands) are strong bases but the rates of proton transfer from ethyl nitroacetate to the free base cryptand and to the mono-protonated cryptand are ca, 10 and 100-fold smaller, respectively, than that of transfer to a normal base of similar basicity. This is attributed either to steric hindrance to proton transfer or to proton transfer occurring only to the thermodynamically unfavourable exo-nitrogen conformation. However, a large kinetic isotope effect = 3.9) is observed for the protonation of cryptand... 

Indeed, macrobicyclic ligands such as 7-9 form cryptates [Mn+ c cryptand], 10, by inclusion of a metal cation inside the molecule [1.26, 1.27, 2.17, 2.24-2.26]. The optimal cryptates of AC and AEC have stabilities several orders of magnitude higher than those of either the natural or synthetic macrocyclic ligands. They show pronounced selectivity as a function of the size complementarity between the cation... 

The topologies of various cryptands are shown in Fig. 1. Considering the macrobicyclic ligands, each bridgehead may be turned either inward or outward with respect to the molecular cavity (11,12,22). This leads to three topological isomers exo-exo, exo-endo, and endo-endo. Crystal structures of one representative of each have been described. [2.2.2] adopts the endo-endo conformation with both nitrogen lone pairs directed into the cavity, while its bis-borane derivative is exo-exo (23) and the mono-borane derivative of 11.1.11 has the exo-endo form (24) (Fig. 2), with the —BH 1 group outside the cavity. 

The cryptands were first prepared in 1969 and form a series of well-defined complexes (cryptates) with alkali and alkaline-earth cations. In this chapter the synthesis of the first cryptand, 8, a macrobicyclic ligand, will be described1,2. The schematic representation (Fig. 5.1) shows that one deals with a multi-step synthesis. The major drawback of this approach is the rather large number of synthetic steps, but the route offers the advantage of being able to construct unsymmetrical compounds (A B C). 

Synthesis of diaza-polyoxa-macrobicyclic compounds (cryptands) and spherical macrotricycles ligands (supercryptands) (see 1st edition). 

The synthesis of a new 6/s(phosphotriester) macrobicyclic polyether cryptand, 0=P[0(CH2)20(CH2)20(CH2)20]3P=0, called phosphocrypt, has been described <94JOC7695>. The synthesis of four 1,1 -binaphthyl-based macrocyclic 6/sphosphane ligands (e.g., 13), in both racemic and optically forms, has been reported <94CB1411> their square planar cfr-P2-Metal (Ni Pd) complexes have also been established <94CB1411,94HCA409>. 

Successful anion complexation was also achieved with larger macrobicyclic ligands. Thus, the macrobicycle 17 (Fig. 17). SI binds halide anions and the linear triatomic azide anion with a very high stability constant. The x-ray structure revealed an efficient shape and size complementarity between N3 and the cavity. A whole series of polyazacryptands was obtained in high yields by a simple Schiff base [2 + 3] condensation between the triaminotriethylamine (tren) and various dicarboxalde-hydes. The hexaimine macrobicycles prepared by this way lead, after reduction, to the polyaza cryptands, which in... 

Macrobicyclic ligands incorporating heterocyclic subunits. Vogtle (58) and Newkome (59) have successfully prepared cryptands containing respectively one (58) and three (59) pyridine subunits in the macrobicyclic framework (Figure 2.18). The ligand (58) forms complexes with many metal cations, and binuclear cobalt and copper complexes of (59) were isolated. 

Miscellaneous macrobicyclic ligands. Numerous other types of macrobicyclic cryptands have been synthesized in recent years, including ferrocene cryptands... 

In macrobicyclic cryptate complexes where the cation is more efficiently encapsulated by the organic ligand these ion pair interactions are diminished and the reactivity of the anion is enhanced. This effect is seen in the higher dissociation constant, by a factor of 104, of Bu OK in Bu OH when K+ is complexed by [2.2.2]cryptand (12) compared to dibenzo[18]crown-6 (2). The enhanced anion reactivity is illustrated by the reaction of the hindered ester methyl mesitoate with powdered potassium hydroxide suspended in benzene. 

Various approaches have been taken to the synthesis of effective luminescent materials, using a variety of large encapsulating antenna-containing ligands, including podands, calixarenes, macrocycles, and macrobicycles (cryptands). These have been divided into acyclic (sub-section A) and cyclic (sub-section B). Representative ligands and complexes will be presented and discussed. 

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... 

The macrobicyclic cryptands also bind AEC very strongly. Ligand 9 displays a unique and very high preference for Sr2+ and Ba2+ over Ca2+. Suitable structural modifications allow control over the M2+/M+ selectivity from preference for AEC to preference for AC binding [2.31]. 

The synthetic routes may often involve template directed condensations, a widely used reaction being the (carbonyl + amine) to imine condensation that efficiently leads to a variety of Schiff-base macrocycles [2.58-2.60, A.7, A.14], macrobicyclic cryptands [2.61-2.63] and lacunar cyclidene ligands [2.60, 2.64]. 

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