Cryptand crystallization

Analysis of solid-state structures has played an important role in understanding crown and cryptand conformation. A variety of interactions between these heteromacrocyclic receptors and metal cations, ammonium ions, and other species have been revealed. The earliest work in this area revealed that crowns and cryptands crystallize in conformations in which one or more methylene group(s) rotate inward to fill the molecular void, if this is permitted by the overall size of the macroring. In fact, molecules such as 12-crownM are remarkably congested and have little or no internal cavity (i.e., hole). This is apparent in Figure 8, which shows space-filling molecular models of 12-crown-4 (left) and 15-crown-5 (right). 

Auffinger and Wipff used AMBER 3.0 to compute the relative free energies of complexation of Na, K, and Rb cations by the cryptand 222 in water and also in methanol.The mutations of K to Na and to Rb" were carried out in water and in methanol starting from a known K -cryptand crystal structure. The computed selectivity is exaggerated by about 3.5 kcal/mol for Na /K and by about 1.2-2.0 kcal/mol for K /Rb in water and in methanol, respectively. The authors blamed this overestimation on several approximations in the force field such as the absence of parameters for cage polarization as well as on insufficient sampling times. These results and the other studies reported by the authors question the predictive power of FEP calculations when restricted to short simulation times and when the experimental starting structures are not available. 

The Na is 555 pm from the nearest N and 516 pm from the nearest O, indicating that it is a separate entity in the structure. Potas-sides, rubidides and caesides have similarly been prepared. The same technique has been used to prepare solutions and even crystals of electrides, in which trapped electrons can play the role of anion. Typical examples are [K(cryptand)]+e and [Cs(18-crown-... 

Ms " clusters have 12 framework bonding electrons as has [BsHs]- (p. 161) the anions are also isoelectronic with the well-known cation [Bis]. Similarly, the alloy NaSn. 2.23 reacts with cryptand in ethylenediamine to give dark-red crystals of [Na(ciypt)]4 [Sng] the anion is the first example of a C41, unicapped Archi-median antiprism (Fig. 10. lOc) and differs from the >3/, structure of the isoelectronic cation [Bis] + which, in the salt Bi+[Bi9] +[HfCl6]5 (p. 591), features a tricapped trigonal prism, as in [BgHg] " (p. 153). The emerald green species [Pb9] , which is stable in liquid NH3 solution, has not so far proved amenable to isolation via ciyptand-complexed cations. 

CRYOENZYMOLOGY CRYPTANDS CRYPTATE EFFECT CRYPTIC CATALYSIS CRYPTIC STEREOCHEMISTRY CRYSTAL FIELD SPLITTING LIGAND FIELD SPLITTING CRYSTAL FIELD THEORY Crystal growth,... 

Cryptands, 42 122-124, 46 175 nomenclature, 27 2-3 topological requirements, 27 3-4 Cryptate, see also Macrobicyclic cryptate 12.2.2], 27 7-10 applications of, 27 19-22 cylindrical dinuclear, 27 18-19 kinetics of formation in water, 27 14, 15 nomenclature, 27 2-3 spherical, 27 18 stability constants, 27 16, 17 Crystal faces, effect, ionic crystals, in water, 39 416... 

Complexes of alkali metal ozonides with cryptand[2.2.2] and various crown ethers have been prepared in order to increase the solubility range of these compounds in solvents other than NH3. The first structural analysis reported for such complexes was that of ([18-crown-6]Rb)03 NH3 (334), carried out by single-crystal XRD. The Rb+ cation is part of... 

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... 

Fig. 1. The inclusion of a pentamethylene-diammonium guest cation in an uncharged cryptand 51 a formula, b crystal structure...

X-Ray crystallography has provided a large amount of data on the structures of crown ethers, cryptands and their complexes in the crystal (B-78M152102). The uncomplexed... 

An example of the exo-exo configuration is seen with the bis(borane-amine) derivative of [2.2.2]cryptand (Figure 7c) in which both of the large borohydride groups point away from the cavity. In contrast, the mono(borane-amine) derivative of [l.l.l]cryptand takes up an endo-exo structure in the crystal (78NJC615). 

Figure 7 Crystal structures of (a) uncomplexed [2.2.2]cryptand (12) (b) potassium complex of [2.2.2]cryptand (c) bis-borohydride derivative of [2.2.2]cryptand (reproduced with permission from 76JCS(P2)423 and 73AX(B)383)...

The crystal structures of a number of cryptands with bridgehead carbon, as opposed to nitrogen, atoms have also been reported (81CC729). 

Figure 8 Crystal structures of (a) complex between spherical cryptand (13) and NH<+ (b) complex between tetraprotonated (13) and CF (reproduced with permission from 76CC533)...

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... 

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