A metallocarbohedryne (met-car) is any one of a family of chemical compounds with the generic molecular formula M8C12, where M is a transition metal such as titanium, vanadium, zirconium, niobium, hafnium, molybdenum, chromium, or iron. These compounds have similar properties and a similar molecular structure, with the eight metal atoms at the corners of a somewhat distorted cube, and the twelve carbon atoms, in pairs, placed diagonally across the faces of the cube. The structure can also be described as two intersecting tetrahedra of metal atoms, with the carbon atoms placed in pairs along the edges of one tetrahedron. They have been extensively studied in the gas phase, and sometimes dispersed in solid materials, but so far have not been produced in bulk or in solution. Nevertheless, they have attracted interest because of their stability and symmetry, a relatively low ionization potential, delayed extraction, and possibly interesting magnetic properties. Some authors suggest that they may eventually find applications in electronics and catalysis. The name is also used for the corresponding cations M8Cn+12 and anions M8Cn-12. The first papers used the name metallo-carbohedrene (with or without the hyphen) for this type of compound.
History The earliest known member of this family is the cation Ti8C+12, discovered by Guo, kerns, and Castleman in 1992 while researching the dehydrogenation of various hydrocarbons (including methane, acetylene, ethylene, benzene, and propylene) with titanium atoms, in the gas phase. Although fullerenes like C60 were already known, that may have been the first cage-like molecule with metal atoms replacing carbon at some corners of the mesh. They observed that the cluster would bind eight ammonia molecules, indicating that the eight titanium atoms were exposed. They also observed the analogous cations with vanadium, zirconium, or hafnium substituted for titanium, the corresponding neutral molecules, and the anion V8C−12.
Synthesis Metallocarbohedrynes can be readily generated by vaporizing the desired metal with a laser, in an atmosphere containing the suitable hydrocarbon. The technique can produce mixed clusters, such as Ti8-xZrxC12. They have been also detected, at a concentration of 1% or less, in the soot generated by an electric arc between two Ti-C electrodes.
Structure The structure of these clusters has been extensively investigated since their discovery. At first, the 20 atoms of Ti8C+12 were conjectured to be arranged as the vertices of a dodecahedron, with the titanium atoms at the corners of a cube, and two carbon atom pairs, on opposite faces, aligned with each set of four parallel edges of the cube. This structure was conjectured to be analogous to that of the hypothetical dodecahedral fullerene C20. However, this claim was soon disputed by Linus Pauling who proposed an alternative arrangement—with the titanium atoms still at the corners of a cube, but with the carbon atoms pushed inwards so as to be nearly coplanar with the faces of that cube.
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