Issue Date: May 21, 2007
FROM THREE SIMPLE ingredients has emerged a complex metal cage structure with arresting symmetries. Although its beauty stands out, the molecule also has shown some intriguing magnetic properties, says Zhiping Zheng of the University of Arizona, one of the molecule's architects.
In the synthesis, Zheng and a team of collaborators from China combined La(NO3)3, Ni(NO3)2, and iminodiacetic acid (IDA) in an aqueous solution, sealed the mixture in a Teflon-coated stainless steel container, and subjected it to a roughly weeklong program of heating and slow cooling. Crystallographic analysis revealed a stunning architecture: a pair of nested, cagelike metal spheres bridged by IDA (J. Am. Chem. Soc., DOI: 10.1021/ja0726198).
The outer Ni(II) sphere's face of pentagons and triangles forms an icosidodecahedron (32 sides), while the inner La(III) sphere is a perfect dodecahedron (12 sides). Classified as a Kepler solid, or Keplerate, after the 16th-century astronomer who modeled the heavens with polyhedra, this metal cluster couldn't be more symmetric. Both sets of metal cages possess icosahedral symmetry, the highest possible symmetry for molecules.
In addition to "the beauty of the chemical structure, this work presents some cool and potentially useful new chemistry," Zheng notes. More specifically, when the researchers chilled the Keplerate to cryogenic temperatures (16 K), they observed ferromagnetic interactions between the outer cage's Ni(II) ions.
Zheng and collaborator Lasheng Long of China's Xiamen University now are looking deeper into their Keplerate's magnetic properties and expect to study other Keplerates featuring d- and f-block elements. "Because of the different electronic structures of other transition-metal and lanthanide elements, a spectrum of diverse magnetic properties may be anticipated," Zheng says. "New magnetic phenomena may be observed and new materials discovered."
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