A cubane-type cluster is an arrangement of atoms in a molecular structure that forms a cube. Cubane itself is a hydrocarbon with chemical formula C8H8, having the carbon atoms at the corners of a cube and covalent bonds forming the edges. Other compounds having different elements in the corners, various atoms or groups bonded off the corners beyond the cube, or additional bonding among the atoms of the cube itself are all part of this class of structures. The structure may be a simple covalent compound or be a more macromolecular or supramolecular cluster compound. Many of these compounds have a high strain due to the 90° angles between the cube-edge bonds at each corner, though others are highly stable.
Derivatives of cubane include the high-performance explosives octanitrocubane and heptanitrocubane. Octaazacubane is a hypothetical allotrope of nitrogen with formula N8; the nitrogen atoms are the corners of the cube. Like the carbon-based cubane compounds, octaazacubane is predicted to be highly unstable, but does not enjoy the kinetic stability seen for its organic analogues.
Methyllithium, ethyllithium, and tert-butyllithium exist as clusters in solution, typically tetramers, with the formula [RLi]4. The individual RLi molecules are not observed. The four lithium atoms and the carbon from each alkyl group bonded to them occupy alternating vertices of the cube, with the additional atoms of the alkyl groups projecting off their respective corners.
Cubane clusters are common throughout inorganic chemistry. Prominent are members of the iron-sulfur clusters, the ferredoxins. They are pervasive in a nature. The four iron atoms and four sulfur atoms form an alternating arrangement at the corners. The whole cluster is typically anchored by coordination of the iron atoms, usually with cysteine residues. In this way, each Fe center achieves tetrahedral coordination geometry. In solution, 4Fe-4S clusters arise via dimerization of square-shaped 2Fe-2S precursors.
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