Coordination cages

Metalated container molecules can be viewed as a class of compounds that have one or more active metal coordination sites anchored within or next to a molecular cavity (Fig. 2). A range of host systems is capable of forming such structures. The majority of these compounds represent macrocyclic molecules and steri-cally demanding tripod ligands, as for instance calixarenes (42), cyclodextrins (43,44), and trispyrazolylborates (45-48), respectively. In the following, selected types of metalated container molecules and their properties are briefly discussed and where appropriate the foundation papers from relevant earlier work are included. Porphyrin-based hosts and coordination cages with encapsulated metal complexes have been reviewed previously (49-53) and, therefore, only the most recent examples will be described. Thereafter, our work in this field is reported. 

The encapsulation of classical and organometallic transition-metal complexes to yield molecules of the type complex in a complex is a very attractive research area. A variety of inclusion complexes of this type has been reported. Of relevance for this review are reports on the encapsulation of coordinatively unsaturated transition-metal complexes inside self-assembled coordination cages (120), cyclodextrins (121,122), and cucurbiturils (123). 

Recently, Bergman and Raymond reported on the stabilization of highly reactive organometallic species inside self-assembled [M4L6] coordination cages (120). The very reactive ion [CpRu(cis-l,3,7-octatriene)]BF4, for example, decomposes rapidly in aqueous solution (within minutes), but is stable for weeks in the same solvent when encapsulated in the tetrahedral [Ga4(L13)6]12- cage... 

The encapsulation of reactive organometallic complexes is not restricted to the anionic [Ga4(L13)6]12- cages. Thus, Fujita and coworkers were able to generate the coordinatively unsaturated complex [Cp Mn(CO)2] (Cp = C5H4Me) within a self-assembled [M Le] coordination cage 28 (Fig. 20) (132). Photoirradiation of solid 27 gave complex 28, the crystal structure of which confirms the presence of the unsaturated pyramidal [CpMn(CO)2] fragment. The direct observation of such intermediates is... 

Fig. 19. Stabilization of [CpRul cis-1,3,7-octatriene)]+ in a tetrahedral [Ga4(L13)6]12 coordination cage (120).

Fujita and coworkers have also reported the encapsulation of multimeric porphyrin assemblies in the box-shaped cavities of ternary Pd6 coordination cages. Two types of cofacial porphine dimers A and B could be stabilized (133). In the smaller [Pd6(L14)2(L15)6]12+ cage 29, whose diameter is 10.4 A (Fig. 21), two porphyrin molecules can be stackedo directly on top of each other with an interplane distance of 3.4 A. In the larger cage 30, an additional molecule of L14 is intercalated between the two porphyrin bases. All complexes were found to be water-soluble in contrast to other 7i-stacked porphyrin dimers. The encapsulation... 

Fig. 21. The two types of cofacial porphyrin dimers A and B and the chemical structures of the coordination cages 29 and 30 (133).

Recently, it has been found that the outcome of some cycloadditions can be altered remarkably when performed inside the cavity of cyclodextrins (288), self-assembled molecular capsules (289), or coordination cages (290). This fact intrigued us greatly and stimulated our interest in the Diels-Alder reactivity of the calixarene-like [M2(L19)(L )]+ complexes bearing unsaturated carboxylate coligands L (215). 

Seidel, S. R. Stang, P. J. High-symmetry coordination cages via self-assembly. Acc. Chem. Res. 2002, 35, 972-983. 

Figure 1.11 Fujita s self-assembled coordination cage, which is prepared by simple mixing of an exo-tridentate organic ligand and end-capped Pd (I I) ion in a 4 6 ratio. Cache optimized structure of the ternary complex anthracene-maleimide metallocage. Molecular structure ofthesyn-isomerofthe 1,4-Diels-Alder adduct.

Yoshizawa, M. and Fujita, M. (2005) Self-assembled coordination cage as a molecular fiask. Pure Appl. Chem., 77 (7), 1107-1112. 

Argent, S. P., Adams, H., Riis-Johannessen, T., etal, Coordination chemistry of tetradentate N-donor ligands containing two pyrazolyl-pyridine units separated by a 1,8-naphthyl spacer Dodecanuclear and tetranuclear coordination cages and cyclic helicates. Inorg. Chem. 2006, 45, 3905-3919. 

Fujita, M., Yoshizawa, M., Takeyama, Y., Kusukawa, T., Cavity-directed, highly stereoselective [2+2] photodimerization of olefins within self-assembled coordination cages. Angew. Chem. Int. Ed. 2002, 41, 1347-1349. 

Generation and stabilization of cyclic siloxane trimer in a self-assembled coordination cage. From M. Yoshizawa, T. Kusukawa, M. Fujita, and K. Yamaguchi. J. Am. Chem. Soc. 122, 6311-12, (2000). 

In this chapter, we cover the metal-directed self-assembly of 3D architectures using macrocycles as molecular scaffolds. Particular emphasis will be given to the synthetic strategies leading to coordination cages and to multitopic macrocyclic receptors, as well as to their complexation properties. 

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