Cage-type structures

The tetrameric zinc anion [Zn4(//-ScPh)r,(ScPh)4]2 has been structurally characterized as the first example of a Zn4Se4 adamantane cage-type structure.594 A further example was characterized by Bochmann and co-workers as part of a study of polymetallic zinc selenophenolates, in this case with chlorides replacing the terminal selenophenolates, [Zn4(/u-SePh)6Cl4]. Octameric zinc species were also characterized with either chloride or phosphine ligands resulting in anionic and neutral complexes respectively, [Zn8(/x-SePh)12Cl4]2 and Zn8(yu-SePh)12(SePh)2 (P Pr3)2.301... 

The more general technique for window-size analysis is based on the study of adsorption isotherms. The conventional methods of pore-size analysis assuming cylindrical pore channels, such as the Barret-Joyner-Halenda (BJH) method, were shown not to be appropriate for cage-type structures. 

When an aqueous solution of a-cyclodextrin is placed in contact with an ethereal iodine solution, reddish-brown crystals are formed at the surface having the composition (C6H,o05)6 l2-4H20. X-ray diffraction analysis indicates that the iodine molecule is colinear with the axis of the cyclodextrin. The closest contacts in the perpendicular direction to this axis take place between one of the iodine atoms and the C-5 and C-6 atoms, and the other iodine atom and the 0-4 oxygen atom. The environment is thus hydrophobic for the first, and hydrophilic for the second. The stacking on top of each other of the molecules in the crystal is such that the two apertures of the cavities of each cyclodextrin molecule are clogged by neighbouring molecules. This creates a cage-type structure (McMullan et al. 1973). 

In oxides or sulfides of low-valent phosphorus with their cage-type structures, like P4O6 or P4S3, the phosphorus atoms have very significant donor character and consequently the molecules can be introduced as ligands into metal complexes. Classical examples are the 1 1 complexes with nickel carbonyls, (P406)Ni(C0)3 (57) or (P406)[Ni(C0)3]2, in which one or two P-donor sites are occupied by Ni(C0)3 acceptors. 

In reactions with metal or metalloidal halides, primary amines, because of their difunctionality, tend to yield more complex molecules, such as cyclic, cage, or polymeric structures. For example, cage-type structures of the type P4(NCH3)e and As4(NCH3)e have been prepared by methylaminolysis of phosphorus(III) or arsenic(III) chloride, respectively, and cyclic structures of formula (HNBC1)3 or (RBNC1)4 have been prepared from boron halides and ammonium chloride or mono-alkylamines. 

Fig. 5. (a) The herring bone pattern cage type structure in a schematic (bottom) and in a more realistic (top) presentation, the latter taken from the a-cyclodextrin-(H20)-4H20 structure [13]. In the schematic drawing, the a-cyclodextrin cavities are indicated by hatching. 

The cage type structures are preferred when small, molecular guests like water, methanol, small carboxylic acids, rare gases are included while for large molecular... 

When the homologous series of carboxylic acids from acetic acid to valeric acid are included by a-cyclodextrin, a change from cage to channel type structure is observed acetic, propionic, butyric acids form cage type structures but valeric acid is too large to fit into a cage and is therefore accommodated in a channel type structure [11],... 

For methanolic solutions, the two nitrogens are apparently equivalent, but there is broadening of alternate lines (the / = 1 components) indicating a rapid fluctuation between asymmetric forms. So the asymmetry is also induced, but the solvent flip occurs far more rapidly in methanol. It is noteworthy that methanol cannot develop clathrate cage type structures. 

Any type of open, porous structure, allowing intimate contact between the solid and aqueous phase, is likely to yield a hydrous phase. In the interaction between water and aluminum, for instance, transmission electron microscopy studies indicate that the initial product consists of very fine fibrils, ca. 3.5 nm thick similarly, Selwood has postulated a threadlike or strand structure to account for the unusually high degree of dispersion evident from magnetic susceptibility studies of various hydrous metal oxide systems. A further obvious possibility here is that open, three-dimensional, cage-type structures (as found, for instance, in zeolites) may be involved. 

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