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Address: Aiko Sugaya

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During the past decade, metallosupramolecular architectures such as coordination rings, cages, and tubes have attracted considerable attention because of their structural characteristics and potential applications, which include the recognition and inclusion of small molecules and ions. [1,2] In many cases, metallosupramolecules have been synthesized by means of metal-assisted assembly of organic molecules with several metal-binding sites, [1] which leads to the formation of thermodynamically stable assembled structures. On the other hand, examples of metallosupramolecular species created by the self-assembly of one kind of building block are relatively rare, [2] although this method is advantageous for the reversible control of assembled and disassembled structures by external factors. One way to achieve this is to design coordination compounds with vacant acceptor sites at a metal center, together with free donor groups that come to bind to another metal center in response to external factors. A mercury(II) coordination compound with free carboxyl groups is a potential candidate for creating a reversible self-assembly system because 1) a mercury(II) ion can adopt several coordination geometries, such as two-coordinated linear, three-coordinated T-shape, and four coordinated tetrahedron; 2) a mercury(II) ion has a good affinity not only for soft sulfur donors but also for hard oxygen donors;[3] and 3) the donating ability of carboxyl groups toward a metal center can be controlled by changing the pH of the solution. Thus, as part of our research project on the rational construction of chiral metallosupramolecular architectures based on thiolato metal complexes, [4] we newly pre III Hg II Co III ) 6 metallocage structure (4 Cr ) in the presence of Cr 3 + , encapsulating an aqua chromiumA C H T U N G T R E N N U N G (III) ion in its cavity. To our surprise, this (L L L L -Co III Hg II Co III ) 6 metallocage compound was exclusively isolated when the mesolike D L L L isomer of 1 was treated under the same conditions. Herein we report on these remarkable results, together with the incorporation of another aqua metal ion in the metallocage structure (Scheme 1).Compound D L D L -1 was isolated as dark-red needle crystals from the 2:1 reaction ofand HgA C H T U N G T R E N N U N G (ClO 4 ) 2 in aqueous HClO 4 . The IR spectrum of D L D L -1 showed a relatively sharp C=O stretching band at 1725 cm À1 , [6] which suggests the presence of COOH groups.[7] Single-crystal X-ray analysis demonstrated that D L D L -1 is an expected S-bridged Co III Hg II Co III trinuclear complex with two pendent COOH groups, D