Tetrahedral hosts

Fig.9.10 Tetrahedral hosts with Td symmetry, (a) Lehn s spherand, (b) Vogtle s spheraphane, (c) Saalfrank s metal-based cage.

Fig. 9.11 (a) Tetrahedral host with Th symmetry, X-ray crystal structure of... 

MD simulations of halide anions [X]- and their inclusion complexes [X] c [L4+] with a macrotricyclic tetrahedral host built from four quaternary ammonium sites dissolved in [C4mim][PF6] were carried out [118]. In the dry IL the uncomplexed halides were surrounded by four to five [C4mim]+ cations which bond via hydrogen bonding to facial coordination. The first solvation shell of [Cl]-, [Br], and [I] comprised of three to four cations next to four H20 molecules for the humid system. The solvation of the [L4+] host and of its [X]-1 [L4+] complex mainly involved anions in the dry IL, and additional H20 molecules in the humid IL. Free energy perturbation calculations predicted that in the dry liquid [F] is preferred over [Cl] , [Br] and [I]- in contrast to the aqueous solution where [L4+] was selective for [Cl]-. In the humid liquid no [F] /[C1] discrimination was observed, showing the importance of small amounts of water on the complexation selectivity [118]. 

Figure 13. (a) Tetrahedral host with Th symmetry, X-ray crystal structure of the [AglDsNidOe SC(Me)2CH(NH2)C02 i2CI] " ion showing the positions of the Ag, Ni, and S atoms, and (b) a volleyball, an object that possesses Th symmetry. 

The tetrahedral host [Ga4(23)6] (5.32) (adapted with permission from reference, copyright 2005, American Chemical Society). 

More recently Raymond, Bergman and coworkers used the chiral tetrahedral host [Ga4(23)g] (5.32) as a catalyst by itself to carry out some catalytic reactions such as ... 

The macrotricyclic spherand designed by Lehn was the first tetrahedral host (Fig. 4a). The bridgehead nitrogen atoms and ethyleneoxy units supply the threefold and twofold rotation axes, respectively. This molecule, and its tetraprotonated form, was shown to bind an ammonium and chloride ion, respectively. 

Figure 6 Activation of C-H bonds in aldehydes by an Mdium complex within a tetrahedral metal-organic host in aqueous solution, (a) Crystal structure of the host, viewed down the C2-axis of S5munetry (b) simplified view, showing the structure of one of the six identical ligands that comprise the edges of the tetrahedral host (c) schematic representation of host and (d) proposed mechanism for C-H bond activation by an encapsulated Ir complex.

It is interesting to note that catalytic turnover was possible in spite of the closed (as compared to the bowlshaped structures used by Fujita and Rebek, respectively) tetrahedral host structure. Apparently, catalytically active systems rely on an intricate balance of guest-binding kinetics and thermodynamics in order to achieve turnover. 

Figure 9 Proposed catalytic cycle for acid-catalyzed acetal deprotection inside a self-assembled tetrahedral host in basic media. For structural details of the tetrahedral host, see Figure 6(a) and (b). (Reproduced with permission from Ref. 41. American Chemical Society, 2009.)...

Mahata K, Frischmann PD, Wiirthner F. Giant electroactive M4L6 tetrahedral host self-assembled with Fe(II) vertices and perylene bsimide dye edges. J Am Chem Soc 2013 135 15656-61. 

For the alkali metal doped Cgo compounds, charge transfer of one electron per M atom to the Cgo molecule occurs, resulting in M+ ions at the tetrahedral and/or octahedral symmetry interstices of the cubic Cgo host structure. For the composition MaCgg, the resulting metallic crystal has basically the fee structure (see Fig. 2). Within this structure the alkali metal ions can sit on either tetragonal symmetry (1/4,1/4,1/4) sites, which are twice as numerous as the octahedral (l/2,0,0) sites (referenced to a simple cubic coordinate system). The electron-poor alkali metal ions tend to lie adjacent to a C=C double... 

The strained bicyclic carbapenem framework of thienamycin is the host of three contiguous stereocenters and several heteroatoms (Scheme 1). Removal of the cysteamine side chain affixed to C-2 furnishes /J-keto ester 2 as a possible precursor. The intermolecular attack upon the keto function in 2 by a suitable thiol nucleophile could result in the formation of the natural product after dehydration of the initial tetrahedral adduct. In a most interesting and productive retrosynthetic maneuver, intermediate 2 could be traced in one step to a-diazo keto ester 4. It is important to recognize that diazo compounds, such as 4, are viable precursors to electron-deficient carbenes. In the synthetic direction, transition metal catalyzed decomposition of diazo keto ester 4 could conceivably furnish electron-deficient carbene 3 the intermediacy of 3 is expected to be brief, for it should readily insert into the proximal N-H bond to... 

The structure of alumina on NiAl(l 1 0) was the subject of a surface X-ray diffraction study by Stierle et al. [46]. The model derived by Stierle et al. from the analysis of the X-ray diffraction data was based on a strongly distorted double layer of hexagonal oxygen ions, where the Al ions are hosted with equal probability on octahedral- and tetrahedral-coordinated sites the resulting film structure was closely related to bulk k-A1203. An attractive feature of Stierle s model was that it provided a natural explanation of the domain structure of the alumina overlayer, which is induced by a periodic row matching between film and substrate lattices. However, as pointed out recently by Kresse et al. [47], this structure model has two bonds with... 

Although the face-centred cubic structure of metals is close packed, it is still possible for atoms which are much smaller than the host metal atoms to fit into interstitial sites inside the structure, while maintaining the essential properties of metals such as electrical conductivity and heat transport. These interstitial sites are of two kinds. The octahedral interstitial sites have six metal atoms at equal distances from the site, and therefore at the apices of a regular octahedron. The tetrahedral interstitial sites have four nearest neighbour metal atoms at the apices of a regular tetrahedron. A smaller atom can just fit into the octahedral site if the radius ratio is... 

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