What are boron clusters?
Although the boron hydrides have been known to science for over a century, their complex cluster structures, such as the 12-vertex icosahedron B12H12, were elucidated decades later. It was calculated that this closed icosahedral species would need to be stabilized by a doubly negative charge. This was experimentally verified, only years later, when Hawthorne and Pitochelli isolated salts of B12H122–. Soon afterwards, neutral compounds with the empirical formula C2B10H12 were produced. The bloom of this chemistry was catalyzed by the military research of smaller boron hydrides.
We have already mentioned the icosahedral dodecaborate anion B12H122– (Figure 1, 1), one of the most stable molecules known to science. This class of molecule, of general formula BxHy, is electron-deficient and is only possible through the delocalization of electron density in three-center two-electron (3c‒2e) bonds. We call these molecules the boranes. Carboranes, on the other hand, are molecules where one or more boron vertices are replaced with a C‒H group. Carbons in these clusters are six-coordinated and their hybridization is roughly sp1.6. Monocarborane, so-called ”CB11”, is an anion with the molecular formula CB11H12– (2). The most well-studied, however, is neutral ortho-carborane (1,2-C2B10H12, 5) and its isomers meta– (1,7-C2B10H12, 6) and para-carborane (1,12-C2B10H12, 7). Last but not least is the class of boron clusters (BC) called metallacarboranes. In this case, we are dealing with carboranes acting as ligands where one of the BH vertices is removed (3) and replaced with one or more transition metals. Probably the most studied metallacarborane is cobalt bis(dicarbollide), shortly abbreviated as CoSAN (Cobalt Sandwich Anion) (4). Important characteristics of the aforementioned structures are similar, yet slightly different to those of the aromatic hydrocarbons. Such differences can be seen in their space requirements (planar versus cage), bonding (σ and π versus 3c‒2e), and aromaticity (2D versus 3D). Carbon-containing clusters are also known to possess a strong amphiphilic character.
Although huge development in the chemistry of BC has been made, it is still incomparable to classical organic synthesis. Fortunately, in recent years the popularity and utilization of BC in organic synthesis have been growing.
Boron clusters in the synthesis
Due to their aromatic character, the reactions of boranes are based on aromatic substitutions. These reactions have been broadly studied since the 1960s with huge developments being made. Carboranes are the most studied BC. Their chemistry is dramatically different compared to boranes due to their heteroatomic composition. The carbon atoms have a higher electronegativity compared to boron atoms, making them electron-withdrawing resulting in the C‒H bonds being highly polar. These hydrogens readily react with alkaline metals and thus make substitutions at the carbon(s) relatively straightforward. This area is extensively studied, and the overall scheme is the same – deprotonation of C‒H vertex followed by subsequent in situ attacks on the electrophilic reagent. Reactions using catalysis are emerging as well, whether it is direct C‒H activation or via carboryne (carborane analog of benzyne) cycloadditions. Worth mentioning is recent work by Xie et al. describing fused carborane-carbo-/hetero-cycles. These studies confirmed the aromatic character of these carborane-cyclic structures and therefore, they are highly applicable for other further studies.
On the other hand, B‒H bonds are inert to deprotonation reactions and, together with the fact that they are present in 10 vertices within 4 different sites, makes substitution reactions challenging. Certain B‒H vertices can be activated into B‒X (X = Br, I) with Lewis acid catalysts or through cage reconstruction, and then further functionalized. This was tested by Spokoyny et al. who presented Pd-catalyzed formation of B‒N, B‒C, and B‒O bonds. Last, but not least are the B‒H metal-catalyzed cross-coupling activations. This class of reactions have most extensively been investigated by the Xie laboratory. In recent years Xie et al. have presented successful examples of regioselective borylations, enantioselective substitutions, regioselective direct alkylations/arylations at different sites with Grignard reagents,  and many more (see examples in ref.).
Compared to carboranes, metallacarborane chemistry is similar. Various derivatives can be prepared by complexation of deprotonated nido-carborane (3) synthons around transition metals or by direct substitutions at the B8,8´ or C1,2 and C1´,2´ vertices. The main obstacle is disubstitution at the C-site which generates a mixture of three possible isomers (1,2´ syn, 1,1´ anti, and 1,2 vicinal).
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