Study Notes on Metallaboranes and Carboranes
Metallaboranes are metals that contain B clusters. In some cases, metal is bonded to a borohydride ion through hydrogen bridges. Generally, more robust groups of Metallaboranes have direct meta-boron bonds. An example of Metallaborane is closo-[B11H11 AlCH3]2– which is prepared by the interaction of the acidic hydrogens in Na2[B11H13] with trimethylaluminum: When B5H9 is heated with Fe(CO)5, a metallated analogue of pentaborane is formed. Boranes are very reactive to metal reagents due to which an attack can take place at several points on a polyhedral cage.
Carbaboranes is a large family of clusters that contain both C and B atoms. One of the interesting reactions of carboranes is the conversion of decaborane (14) to closo- 1,2-B10C2H12. The first reaction in this preparation is the displacement of H2 molecules from decaborane by a thioether:
B10H14 + 2SEt2 B10H12(SEt2)2 + H2
The loss of two H atoms in this reaction will be compensated by the donation of electron pairs by the added thioethers, so the electron count will remain unchanged. The product of the reaction is then converted to the carboranes by the addition of an alkyne:
B10H12 (SEt2)2 + C2H2 B10C2H12 + 2SEt2 + H2
The four π electrons of ethyne will displace two thioether molecules (two two-electron donors) and an H2 molecule (which leaves with two additional electrons). The net loss of two electrons correlates with the change in structure from a Nido starting material to the closo product. The C atoms are in adjacent (1, 2) positions indicating their origin from ethyne. This closo-carborane survives in the air due to which it can be heated without decomposition. At 500°C, in an inert atmosphere, it undergoes isomerization into 1,7-B10C2H12 (35), which in turn isomerizes at 700°C to the 1,12- isomer.
The H atoms attached to carbon in closo-B10C2H12 are very mildly acidic, as a result, it is possible to lithiated these compounds with butyllithium:
B10C2H12 + 2LiC4H9 B10C2H10Li2 + 2C4H10
These dilithio-carboranes are good nucleophiles and undergo many of the reactions that are characteristic of organolithium reagents
Thus, a wide range of carborane derivatives can be synthesised. For example, reaction with CO2 gives a carborane dicarboxylic acid:
Similarly, I2 leads to the diiodocarboranes and COCl yields B10C2H10 (NO)2.
Although 1,2-B10C2H12 is very stable, the cluster can be partially fragmented in strong base, and then deprotonated with NaH to yield nido[B9C2H11]2-:
2B10C2H12 + 2EtO- + 4EtOH 2[B9C2H11]- 2B(OEt)3 + 3H2
Na[B9C2H12] + NaH Na2[B9C2H11] + H2
The importance of these reactions is that nido -[B9C2H11]2- is an excellent ligand. In this role, it mimics the cyclopentadienyl ligand ([C5H5]- which is widely used in organometallic chemistry.
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