Cryptate structure

The binuclear Co(n) cryptate (monohydroxo bridged) was also found to combine reversibly with dioxygen yielding a doubly-bridged cascade complex containing both hydroxo and peroxo bridges within the cryptate structure. 

A number of X-ray crystal determinations have made the principles of lanthanide cryptate structural chemistry fairly clear. In [La(N03)2(2,2,2-cryptate)][La(N03)6] (Figure 8), the La3+ ion is 12-coordinated with two bidentate nitrate ions coordinating in two of the three spaces between the cryptate chains the third space is thus too compressed to be occupied also.508 [Sm(N03)(2,2,2-cryptate)][Sm(N03)5(H20)] shows only one such space occupied511 and the structure of [Eu(C104)2,2,2-cryptate](C104)2MeCN is similar to the samarium cryptate.512,513 Intemuclear distances in these complexes are shown in Table 10. 

The alkalides. The first crystalline alkalide to be prepared in this manner was [Na+(2.2.2)].Na. This salt is obtained as shiny, gold-coloured crystals (Dye etal., 1974). The 23Na nmr spectrum yields a narrow upfield signal for the Na- ion (Dye, Andrews Ceraso, 1975) the X-ray structure indicates close-packed sodium cryptate cations with Na" anions occupying octahedral holes between the cryptate layers (Tehan, Barnett Dye, 1974). 

The proportion of the /rans-O-alkylated product [101] increases in the order no ligand < 18-crown-6 < [2.2.2]-cryptand. This difference was attributed to the fact that the enolate anion in a crown-ether complex is still capable of interacting with the cation, which stabilizes conformation [96]. For the cryptate, however, cation-anion interactions are less likely and electrostatic repulsion will force the anion to adopt conformation [99], which is the same as that of the free anion in DMSO. This explanation was substantiated by the fact that the anion was found to have structure [96] in the solid state of the potassium acetoacetate complex of 18-crown-6 (Cambillau et al., 1978). Using 23Na NMR, Cornelis et al. (1978) have recently concluded that the active nucleophilic species is the ion pair formed between 18-crown-6 and sodium ethyl acetoacetate, in which Na+ is co-ordinated to both the anion and the ligand. 

In comparison, both the free ligand and the dinuclear Cu(I) cryptate of an analogous macrobicyclic structure possessing a diphenylmethane group as a central unit display only two resonances for the CH2CH2 fragment, as is the case here only for the complexes 91 and 92. This points to the special conformation features of the free macrobicycles 89 and 90. 

F. Fages, J.-P. Desvergne, H. Bouas-Laurent, J.-M. Lehn, J. P. Konopelski, P. Marsau, and Y. Barrans, Synthesis and fluorescence emission properties of a bis-anthracenyl macrotricyclic ditopic receptor. Crystal structure ofits dinuclear rubidium cryptate, J. Chem. Soc., Chem. Commun. 655(1990). 

For potassium zeolites, cryptofix 222 and cryptofix 222BB, for example, can be used. The structures together with the stability constants Ks of the complexes (cryptates) of cryptofix 222 and cryptofix 222BB with potassium are shown in... 

Table 2 Structures of Potassium Selective Cryptands Cryptofix 222 and Cryptofix 222BB and the Stability Constants of the Matching Cryptates...

In the case of ligands E, F and H the chelate/cryptate nature of the complexes will depend on whether or not the cation is contained between a branch and a ring or between two rings, or included inside a ring. Of course, in such cases, the above definitions, which concern the limiting cases, are less clear and classification may have to await a crystal structure determination. Finally, complexes formed by inclusion of a cation in a cavity delimited by a monocyclic, bicyclic or tricyclic structure may... 

From various observations it has been inferred that most AC and AEC complexes formed by the ligands of type 6—45 are 1 1 inclusion complexes, cryptates 34), in which the cation is held in the central cavity of the ligand molecule 34, 61, 106). This has been amply confirmed by several crystal structure determinations which also provided fundamental information about the shape of the ligand in the complex. 

Fig. 7. Crystal structure and conformation of a) its RbSCN cryptate (from Ref. (103) and (125)) b) the free macrobicyclic ligand [2.2.2], 30...

Modem work on these and related bare post-transition element clusters began in the 1960s after Corbett and coworkers found ways to obtain crystalline derivatives of these post-transition element clusters by the use of suitable counterions. Thus, crystalline derivatives of the cluster anions had cryptate or polyamine complexed alkali metals as countercations [8]. Similarly, crystalline derivatives of the cluster cations had counteractions, such as AlCLj, derived from metal halide strong Lewis acids [9]. With crystalhne derivatives of these clusters available, their structures could be determined definitively using X-ray diffraction methods. 

The hexaethylenetetraamine 12 has been created as a proton cryptate and proven by its X-ray structure<99ACIE956>. Related but larger macrocyclic cages have been formed in order to encapsulate metal ions<99ACIE959>. 

Figure 6. X-ray crystal structure of the iodide cryptate of the macrotricyclic quaternary ammonium receptor 16." ...

The X-ray crystal structures of the F", Cl , and Br" cryptates of 19-6H demonstrate the inclusion of one of the halide anions in an unsymmetrical fashion. In the case of the small fluoride ion complex a tetrahedral coordination environment is observed for the guest anion with a mean N(H) - P hydrogen-bonding distance of 2.72(8) A. The CP and Br" cryptates exhibit octahedrally coordinated halide ions situated more centrally within the host framework with N(H) - X" distances in the ranges 3.19-3.39 A (X = CP) and 3.33-3.47 A (X = Br ). It is noteworthy that the hydrogen-bonded distances for the anion within the cryptand host are longer by up to ca 0.15 A than those for the other anions in the lattice, suggesting a particularly... 

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