Macrobicyclic cage

The remarkable physical properties exhibited by the divalent macrobicyclic cage complex [Co(sep)]2+ (29) are unparalleled in Co chemistry.219 The complex, characterized structurally, is inert to ligand substitution in its optically pure form and resists racemization in stark contrast to its [Co(en)3]2+ parent. The encapsulating nature of the sep ligand ensures outer sphere electron transfer in all redox reactions. For example, unlike most divalent Co amines, the aerial oxidation of (29) does not involve a peroxo-bound intermediate. 

Severin and coworkers reported (146) the reaction of tris(2-aminoethyl)amine and 4-formylphenylboronic acid with penta-erythritol to give, via multicomponent assembly, the boronic acid based macrobicyclic cage 35 (Fig. 25). The cage has the form of an ellipsoid with a diameter of 20.5 A and binds two Cud) ions in a fashion similar to the smaller tren-based cryptands. The reversible formation of boronic esters has also been employed to build other hollow structures such as nanotubes (147) and porous covalent organic frameworks (148,149). 

The final example by Severin et al. indicates as to where the dynamic covalent synthesis of superstructures may lead to." Driven by a previous observation that boronic ester-based macrocycles with pendent aldehyde groups could be functionalized with amines, they investigated the possibility of performing boronic ester and imine condensation reactions simultaneously. In the first attempt, a mixture of 3-formylphenylboronic acid 14, pentaerythritol 15, and 1,4-diaminobenzene 16 in tetrahydrofuran/toluene was heated in a flask equipped with a Dean-Stark trap (Figure 8a). Analysis of the formed products revealed the formation of macrocycle 17 as a result of a [4-F2-F2] condensation with a yield of 44%. Prompted by this success, they performed the polycondensation reaction on a mixture of 4-formylphenylboronic acid 18, pentaerythritol 15, and tris(2-aminoethyl)amine 19, rather than the linear 1,4-diaminobenzene (Figure 8b). With a remarkable yield of 82%, the macrobicyclic cage 20 was spontaneously formed as a result of the condensation of six boronic acid molecules, three pentaerythritol molecules, and two triamine molecules. A total of 18 covalent bonds were... 

Sepulchrates, the polyaza cage macrobicycles analogous to the cryptates, were first synthesized in 1977.178 The cobalt(lII) complex shown in Figure 3 (19) is the octaazasepulchrate analog of the [2.2.2] cryptand (Figure 3 16), and is commonly written [Co(sep)]3+ (sep = sepulchrate). 

The medium may have a marked effect on the shape of receptor molecules itself. Shape modifications could strongly influence their substrate binding properties, for instance in the case of amphiphilic cyclophane receptors subjected to hydrophobic-hydrophilic factors in aqueous solution. Such medium effects in action are visualized by the solid state structures of two different forms of the water-soluble hexasodium salt of the macrobicyclic cyclophane 66, which could be crystallized in two very different shapes an inflated cage structure 71 building up cylinders disposed in a hexagonal array and a flattened structure 72 stacked in molecular layers separated by aqueous layers in a lamellar arrangement [4.73]. These two... 

Macrobicyclic cascade cryptands have also been prepared such as the series of bis(tren) derived (tren = tris(2-aminoethyl) amine, see Section 4.4.3) compounds 5.9. The di-nickel(II) and di-copper(II) complexes of the cages with various spacers all bind a metal cation into each tren unit. These metal... 

Creaser II et al (1977) Sepulchrate a macrobicyclic nitrogen cage for metal ions. 1 Am Chem Soc 99 3181-3182... 

The redox potentials of macrobicyclic complexes vary over a wide range. They are much lower than those of metal aqua ions and are dependent on the central ion and on the nature of the cage. In this section, the dependence of the redox properties on the central ion is discussed. The redox potentials for several d-metal aqua ions and sarcophaginates are summarized in Table 32 and Fig. 42. Fig. 42 also shows ionization potentials Z n=i In and LFSEs [323]. 

Thus, the data on photochemical hydrogen production indicate that macrobicyclic complexes can be effective ETAs only in a narrow redox potential range (-400 mVexperiments demonstrated that cage complexes with such potentials are efficient for quenching of [Ru(bpy)3]2+ and [Ru(4,4 -Me2bpy)3] + cations (Table 63). 

When immobilized on polymer surfaces, cage complexes may be utilized for modifications of electrodes. The increase in the electron-transfer rate on such surfaces is governed by two factors a high rate of electron transfer in cobalt clathrochelates and the regular disposition of these complexes on the surface. The properties of immobilized macrobicyclic complexes have been considered in Refs. 94, 410, and 411. Cyclic voltammetry has been used to characterize the incorporation of a range of structurally different d-metal sarcophaginates and sepulchrates into Nafion polymer [412]. 

The introduction of intermediate and soft donor atoms like nitrogen and sulfur into a ligand architecture leads to a stronger preference for softer metal ions, such as the transition and precious metals. For example, the macrobicyclic tris(pyridine) cage 6 was shown to transport Ag+ with high selectivity into an organic phase [31] ... 

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