Cryptate ligand structures

Many Class II complexes are known for which the EPR signal shows localization occurring at low temperatures (169,170) but only one (167) other synthetic example in which the seven-line signal is still evident at 77 K is known. Delocalization is most evident in systems in which the ligand imposes very similar geometry at both copper centers and the small, dinuclear cryptate achieves this very effectively. The properties observed are those of the encapsulated [Cu(1.5)-Cu(1.5)] unit and are independent of the details of ligand structure. 

Fig. 2. Structures of some cryptate ligands (stability constants of their complexes...

Fig. 26) (230). This led to a Mn11 complex having a square prismatic geometry. The second complex is composed of a cryptate ligand that yields a complex with a cubic symmetry (Fig. 27) (231). The third structure is built up from two tetradentate tripodal amine ligands about the Mn I ions (Fig. 26) (232). 

The species that results from the reductive electrocrystallization of the sodium complex of tris-bipy cryptate, the schematic structure of which is shown in figure 2, has been called a "cryptatium." The name was chosen to express the dual nature of its procedence, since it is part cryptate and part sodium metal. It must be stressed that this cryptatium material is electroneutral, thus forming an expanded atom structure. Figure 2 also shows the schematic structure of an electride and that of a simple sodium atom, to illustrate two additional and extreme situations. In one, the electride, the complexation of the metal ion by the cryptand is so strong that the electron is essentially expelled from its interaction with the cationic center. In the other, the simple sodium atom, the outermost electron resides in an s orbital of the metallic center. Cryptatium thus represents an in-between situation, where the electron is not totally expelled from its interaction with the cation but it does not reside on the cation either, but rather on the ligand. The result is an expanded-metal type structure, where the electron is localized in the ligand structure. 

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. 

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...

Mono- and bimetallic lanthanide complexes of the tren-based macrobicyclic Schiff base ligand [L58]3- have been synthesized and structurally characterized (Fig. 15), and their photophysical properties studied (90,91). The bimetallic cryptates only form with the lanthanides from gadolinium to lutetium due to the lanthanide contraction. The triplet energy of the ligand (ca. 16,500 cm-1) is too low to populate the terbium excited state. The aqueous lifetime of the emission from the europium complex is less than 0.5 ms, due in part to the coordination of a solvent molecule in solution. A recent development is the study of d-f heterobimetallic complexes of this ligand (92) the Zn-Ln complexes show improved photophysical properties over the homobinuclear and mononuclear complexes, although only data in acetonitrile have been reported to date. 

Bkouche-Waksman, I. Guilhem, J. Pascard, C. Alpha, B. Deschenaux, R. Lehn, J.-M. 110. Crystal structures of the lanthanum(III), europium(III), and terbium(III) cryptates of tris(bipyridine) macrobicyclic ligands. Helv. Chim. Acta 1991, 74,1163-1170. 

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 results (Table 10) show that the cryptands could act to produce carrier-mediated facilitated diffusion and there was no transport in the absence of the carrier. The rate of transport depended upon the cation and carrier, and the transport selectivity differed widely. The rates were not proportional to complex stability. There was an optimal stability of the cryptate complex for efficient transport, logKs 5, and this value is similar to that for valinomycin (4.9 in methanol). [3.2.2] and [3.3.3] showed the same complexation selectivity for Na+ and K+ but opposing transport selectivities. The structural modification from [2.2.2] to [2.2.C8] led to an enhanced carriage of both Na+ and K+ but K+ was selected over Na+. The modification changes an ion receptor into an ion carrier, and indicates that median range stability constants are required for transport. Similar, but less decisive, results have been found in experiments using open-chain ligands and crown ethers.498... 

Alkali metal anions have also been generated as a result of cryptand stabilization of the corresponding cation. Cryptands were found to enhance the solubility of zerovalent alkali metals in various organic solvents.156-157 Initially, the solutions apparently contain the cryptate cation and solvated electrons together with free ligand. When more metal is dissolved, metal anions, M , are formed.158 Dye and co-workers have isolated gold-colored crystals of [Na+ c 2.2.2]Na 159160 and the crystal structure has been determined.161,162 Anion clusters such as Sb] , Pb2 and Sn," have been isolated as crystalline salts of the [2.2.2] cryptate counterion [2.2.2].162,163... 

The non-complementarity between the ellipsoidal 33-6H+ and the spherical halides results in much weaker binding and appreciable distortions of the ligand, as seen in the crystal structures of the cryptates 35 where the bound ion is F , Cl-, or Br-. In these complexes, F- is bound by a tetrahedral array of hydrogen bonds whereas Cl- and Br- display octahedral coordination (Fig. 4). Thus, 33-6H+ is a molecular receptor for the recognition of linear triatomic species of a size compatible with the size of the molecular cavity [3.11]. 

Metz, B., Rosalky, J. M., Weiss, R., [3] Cryptates - X-ray crystal-structures of chloride and ammonium ion complexes of a spheroidal macrotricyclic ligand. J. Chem. Soc., Chem. Commun. 1976, 533-534. 

Dietrich, B., Dilworth, B., Lehn, J.-M., etal, Anion cryptates Synthesis, crystal structures, and complexation constants of fluoride and chloride inclusion complexes of polyammonium macrobicyclic ligands. Helv. Chim. Acta 1996, 79, 569-587. 

---