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Dicarbollide

In organometallic chemistry, a dicarbollide is an anion of the formula [C<sub>2</sub>B<sub>9</sub>H<sub>11</sub>]<sup>2-</sup>. Various isomers exist, but most common is 1,2-dicarbollide derived from ortho-carborane. These dianions function as ligands, related to the cyclopentadienyl anion. Substituted dicarbollides are also known such as [C<sub>2</sub>B<sub>9</sub>H<sub>10</sub>(pyridine)]<sup>−</sup> (pyridine bonded to B) and [C<sub>2</sub>R<sub>2</sub>B<sub>9</sub>H<sub>9</sub>]<sup>2-</sup> (R groups bonded to carbon).

Synthesis of dicarbollides

Dicarbollides are obtained by base-degradation of 12-vertex dicarboranes. This degradation of the ortho derivative has been most heavily studied. The conversion is conducted in two-steps, first "deboronation" and second deprotonation:

C<sub>2</sub>B<sub>10</sub>H<sub>12</sub> + NaOEt + 2 EtOH → Na<sup>+</sup>C<sub>2</sub>B<sub>9</sub>H<sub>12</sub><sup>−</sup> + H<sub>2</sub> + B(OEt)<sub>3</sub>
Na<sup>+</sup>C<sub>2</sub>B<sub>9</sub>H<sub>12</sub><sup>−</sup> + NaH → Na<sub>2</sub>C<sub>2</sub>B<sub>9</sub>H<sub>11</sub> + H<sub>2</sub>

The dianion derived from dicarboranes, [C<sub>2</sub>B<sub>9</sub>H<sub>11</sub>]<sup>2-</sup>, are nido clusters. Three isomers exist. Most commonly studies is the 7,8-isomer, with two adjacent carbon centers on the rim. 7,9- has non-adjacent carbon centers on the rim. It is derived by degradation of meta-. 2,9- has only one carbon center on the rim. It is derived by degradation of para-.

Coordination compounds

A variety of complexes - a subset of metallaborane - are known with one or two dicarbollide ligands. An example of a 1:1 complex is [Mn(CO)<sub>3</sub>(η<sup>5</sup>-7,8-C<sub>2</sub>B<sub>9</sub>H<sub>11</sub>)]<sup>−</sup>.

Most heavily studied are complexes with two dicarbollide ligands, especially sandwich complexes. Thus, these are prepared by salt metathesis reactions, as illustrated by the synthesis of the ferrocene analogue:

2 Na<sub>2</sub>C<sub>2</sub>B<sub>8</sub>H<sub>11</sub> + FeCl<sub>2</sub> → Na<sub>2</sub>[Fe(C<sub>2</sub>B<sub>8</sub>H<sub>11</sub>)<sub>2</sub>] + 2 NaCl

These bisdicarbollide dianions are often readily oxidized. Fe(III), Co(III), Ni(III), and Ni(IV) derivatives are known. In some cases, the oxidation induces rearrangement of the C<sub>2</sub>B<sub>9</sub> cage to give complexes where the carbon centers are nonadjacent.

Precursor to other carboranes

Diprotonation of [C<sub>2</sub>B<sub>9</sub>H<sub>11</sub>]<sup>2−</sup> gives the neutral carborane C<sub>2</sub>B<sub>9</sub>H<sub>13</sub>. Pyrolysis of this nido cluster gives closo-C<sub>2</sub>B<sub>9</sub>H<sub>11</sub>. Chromate-oxidation of [C<sub>2</sub>B<sub>9</sub>H<sub>12</sub>]<sup>−</sup> results in deboronation, giving the C<sub>2</sub>B<sub>7</sub>H<sub>13</sub>. This carborane features two CH<sub>2</sub> vertices.

Homogeneous catalysis

The clam-shell dicarbollide complex (Cp*)(C<sub>2</sub>B<sub>9</sub>H<sub>11</sub>)ZrCH<sub>3</sub> catalyzes alkene polymerization.

References