Carborane

Carboranes (or carbaboranes) are electron-delocalized (non-classically bonded) clusters composed of boron, carbon and hydrogen atoms.

Carboranes and boranes adopt 3-dimensional cage (cluster) geometries in sharp contrast to typical organic compounds.

Like the related boron hydrides, these clusters are polyhedra or fragments of polyhedra, and are similarly classified as closo-, nido-, arachno-, hypho-, hypercloso-, iso-, klado-, conjuncto- and megalo-, based on whether they represent a complete (closo-) polyhedron or a polyhedron that is missing one (nido-), two (arachno-), three (hypho-), or more vertices.

[2][3] The essence, these rules emphasize delocalized, multi-centered bonding for B-B, C-C, and B-C interactions.

Structurally, they can be considered to be related to the icosahedral (Ih) [B12H12]2− via formal replacement of two of its BH− fragments with CH.

Geometrical isomers of carboranes can exist on the basis of the various locations of carbon within the cage.

[6] Dicarbaboranes can be prepared from boron hydrides using alkynes as the source of the two carbon centers.

In addition to the closo-C2BnHn+2 series mentioned above, several open-cage dicarbon species are known including nido-C2B3H7 (isostructural and isoelectronic with B5H9) and arachno-C2B7H13.

Syntheses of icosahedral closo-dicarbadodecaborane derivatives (R2C2B10H10) employ alkynes as the R2C2 source and decaborane (B10H14) to supply the B10 unit.

Isomerism is well established in this family: Carboranes of intermediate nuclearity are most efficiently generated by degradations from larger clusters.

In contrast, smaller carboranes are usually prepared by building-up routes, e.g. from pentaborane + alkyne, etc.

The icosahedral charge-neutral closo-carboranes, 1,2-, 1,7-, and 1,12- C2B10H12 (informally ortho-, meta-, and para-carborane) are particularly stable and are commercially available.