Metallocarboranes synthesis

In this review, we treat in depth the synthesis, structures, properties, and reactions of -bonded metallocarboranes. Our survey is restricted to complexes of 2-carbon carboranes and to species that have between nine and fourteen total polyhedral vertices. Coverage of metal complexes of other heteroboranes is available in Grimes s book (41) and in Todd s review (93). The recent work of Grimes and his group has concentrated on metallocarboranes having fewer than nine vertices (42, 75, 76). 

Our approach to the subject has been to divide the metallocarboranes according to the size of the polyhedron. Starting with twelve-vertex compounds, which constitute the majority of the effort, we proceed to the larger polyhedra, so far unknown in the B H 2 and C2B 2H series, and then to the lower polyhedra. Further subdivisions within each polyhedral size include synthesis, structures, and properties of monometallic complexes, reactions of monometal lies, bimetallic preparations and reactions, and, in two instances, trimetallic compounds. 

Five major synthetic routes are now available for the preparation of metallocarboranes, although only one was well established in 1969. The recently developed synthetic methods have allowed the preparation of more complex and diverse compounds and have greatly expanded the field of metallocarborane chemistry. These preparative methods are discussed in some detail in this section, for the synthesis of all the known metallocarboranes have been accomplished by one or more of these routes. 

As mentioned previously, the first metallocarborane synthesized, (l,2-C2B9Hu)2Fe(II)2- (Fig. 3), was prepared in a manner similar to the synthesis of ferrocene, e.g., reaction of anhydrous FeCl2 with the nido-carborane dianion 7, S-C BgHn2, which itself was formed from 1,2-C2BioHi2... 

This method is useful for the synthesis of metallocarboranes containing two similar or different transition metal vertices (19, 23) but has not yet been explored to determine its full potential. 

Grimes and co-workers (76) subsequently reported the synthesis of several other metallocarboranes in a similar fashion. This method may proceed in a manner similar to thermal metal transfer and is, thus, included here. A new synthetic classification may be warranted if this preparative route proves to have widespread utility. 

The photochemical synthesis of the tungsten carbonyl metallocarborane [Eq. (18) ] is also effective in the preparation of the molybdenum carbonyl analog. The resulting air-sensitive complexes show chemical behavior similar to the cyclopentadienyl analogs, C5H5M(CO)3, in that they undergo protonation with anhydrous HOI and met.hylation with CH3I. They also react further with metal hexacarbonyls to afford bimetallic complexes 54) ... 

Other Applications. DBBS has been used for the synthesis of boronic acid-based calix[6]arene) (acts as a singlechain surfactant at the air-water interface), 14-vertex carborane, and 14-vertex metallocarborane, 14- and 15-vertex ruthenacarboranes, and organo-tricyanoborates as tectons. ... 

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