Carboranes Metallocarboranes

Carboranes, metallocarboranes Timofeeva 1996 Carboxylic acids, esters Allinger 1992a... 

Waders rule polyboranes, carboranes, metallocarboranes AND transition-metal CLUSTERS... 

The structure of the bimetallic 10-vertex cluster was shown by X-ray diffraction to be (84). When the icosahedral carborane l,2-C2BioHi2 was used, the reaction led to the first supraicosahedral metallocarboranes with 13- and 14-vertex polyhedral structures (85)-(89). Facile isomerism of the 13-vertex monometallodicarbaboranes was observed as indicated in the scheme above (in which = CH and O = BH). 

Cu ( j -C5H5)2] is not. Likewise, Fe and Ni carborane derivatives are extremely stable. Conversely, metallocarboranes tend to stabilize lower oxidation states of early transition elements and complexes are well established for Ti", Zr , Hf , V , Cr and Mn" these do not react with H2, N2, CO or PPh3 as do cyclopentadienyl derivatives of these elements. 

To conclude, we shall mention some metal-atom reactions with boranes (172) and carboranes (173). When cobalt atoms reacted with pentaboraneO) and cyclopentadiene, a number of new metalloborane clusters were formed (172), two of which were 65115003(17-05115)3 and cyclopentyl-B5H40o2(i7-05H5)3. Possible structures for the former are shown in Fig. 42. The reaction of cyclopentadiene, pentaboraneO), and 2-butyne with cobalt atoms yielded the metallocarborane species illustrated in Fig. 43 (173). 

Metallocarboranes and metalloboranes, discussed in 6.5.4, may be prepared from metal halides and carborane anions. Selected examples of complexes formed by thermal—or otherwise—induced polyhedral rearrangements of existing metallocarboranes are given in a few equations, and some other individual compounds are listed in tables. 

Neutral carboranes and boranes react with transition-metal complexes forming metallocarboranes or metalloboranes, respectively. However, most metallocarboranes and metalloboranes are prepared from transition-metal halides and anionic carborane and borane species ( 6.5.3.4) or by reacting metal atoms and neutral boranes and carboranes. These reactions are oxidative addition reactions ( 6.5.3.3). 

Because the complexes listed in Tables 1-8 are all prepared with metal halides and carborane anions, only one example for each type of metallocarborane (number of B atoms) is given in the following representative equations for clarity, the skeletal framework in the equations have numbered positions and H atoms are omitted ... 

Carboranes also offer the capacity for Co111—C bond formation. Perhaps the best-known example of a mixed cyclopentadienyl-metallocarborane sandwich complex is the cobalt(III) compound (Cp)Co(l,2-C2B9Hn), which serves as a precursor for other species.544... 

Metalloboranes, 4 172 exopolyhedral, 4 208-210 main group element, 4 207-208 transition element, 4 205-207 Metallo-carbohedrene clusters, 4 648 Metallocarboranes, 4 170 as catalysts, 4 217-218 economic aspects, 4 229 exopolyhedral, 4 215-216 f-block element, 4 225-226 host-guest chemistry-carborane anticrowns, 4 216-217 structural systematics, 4 176-179 transition metal, 4 210-215 Metallocene catalysis, MAO in, 16 92-93. 

However, separation of the carborane cage from The phos-phazene ring or chain by a methylene spacer group allows metals to be Inserted into the open face of the carborane. These syntheses were accomplished by the reaction routes shown in Schemes 3 and A. High polymeric analogues of these transformations have also been accomplished following polymerization of XXX. The rhodium-bound cyclophosphazenes and polyphosphazenes are catalysts for the hydrogenation of 1-hexene. In this, they show a similar behavior to metallocarboranes linked to polystyrene... 

Although most known metallocarboranes have only 1 metal atom per polyhedron, the existence of an increasing number of metallocarboranes with 2 or more metal atoms per cluster emphasizes the close relationship between metal clusters and borane clusters. Since they can be synthesized from cZoao-carboranes by replacing BH units by metal-carbonyl or metal-cyclopentadienyl residues, carborane clusters can effectively be used as disposable templates on which to fabricate... 

One particularly interesting category of metallocarborane is that in which a single metal atom is shared between two polyhedra that have a vertex in eommon. In effect, the metal is sandwiched between two nido-carborane residues. Examples are shown in Fig. 17. For such commo compounds, the metal can be assumed to contribute three AO s to the skeletal bonding of each polyhedron, when the (n + 1) rule for closo clusters is found to be obeyed. For example, the isoelectronic... 

Applied to icosahedral carboranes as the starting materials, it leads to metallocarborane based on 13- or 14-vertex polyhedra, e.g. (35, 70),... 

By no means do all metallocarboranes have the metal atoms occupying vertices of the basic polyhedra. Apart from many derivatives in which o-bonded metal residues occupy exo sites attached to particular skeletal atoms, several metalloboranes and -carboranes are known in which the metal occupies an edge-bridging site, effectively replacing a bridging hydrogen atom of the parent borane. Many are nido species related to BeHio, for example, the /x-silyl and /i-germyl carboranes. 

Fig. 27. Examples of isostructural carboranes and metallocarboranes that are skele-tally isoelectronic (255).

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). 

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... 

Although a number of different metallocarboranes have been synthesized in high yield by this reaction (18, 24), the actual chemistry is frequently much more complex than implied by Eqs. (5) and (6). For example, products containing greater and fewer numbers of boron atoms than present in the carborane starting materials have been isolated from polyhedral expansion reactions. 

The polyhedral expansion reaction appears to be a general synthetic method for metallocarboranes all the known doso-carboranes have been found to produce metal-containing compounds when subjected to the re-duction-complexation operations of this synthetic scheme. Moreover, metallocarboranes containing more than one transition metal may be prepared by the polyhedral expansion of monometallocarboranes. Examples of this synthetic route will be described in following sections. 

It should be noted that in the polyhedral expansion process, as idealized in Eqs. (5) and (6), the product mctallocarborane has one more vertex than was present in the carborane starting material—hence the origin of the descriptive phrase polyhedral expansion. By contrast, wThen metallocarboranes are prepared by reaction with O2B9H112- ions, which are prepared from the icosahedral C2B10H12 carboranes, twelve-vertex metallocarboranes result. 

Synthetic polyhedral subrogation for the preparation of polymetallo-carboranes from monometallocarboranes is an offshoot of polyhedral contraction in that, after degradative removal of a BH vertex, a transition metal ion is reacted with the m do-metallocarborane produced rather than with an oxidizing agent. In this way, a new transition metal vertex is incorporated into the polyhedral framework without a change in the number of vertices between reactant and product (Fig. 6) ... 

Spencer, Green, and Stone (88) have recently described a new synthetic approach to metallocarboranes in which an organomctallic transition metal complex is thermally reacted with a neutral cZoso-carborane, resulting in the incorporation of the metal into the polyhedron ... 

Degradation of the icosahedral 1,2- and 1,7-C2Bi0Hi2 isomers with strong base to produce the nido eleven-vertex anions 7,8- and 7,9-CoB9Hi2-has been previously discussed and is an important route to the preparation of twelve-vertex monometallocarboranes. The discovery that similar reactions could be performed on metallocarboranes led to the isolation of novel chains of metal atoms bridged by carborane groups and to the development of the polyhedral contraction and polyhedral subrogation reactions. 

The following reaction sequence leading to these complexes is believed to be similar to the stepwise reactions in which metallocarborane complexes are formed from neutral cZoso-carboranes ... 

Although mixed carborane-cyclopentadienyl complexes, predominantly those of cobalt, are the metallocarboranes that have received the most... 

The relative reactivities of the various BH groups present in each carborane were such that those BH groups nearest the carbon vertices were favored for attack. Figure 28 illustrates this point. Reaction of the B-v-bondcd metallocarboranes with CO led to the regeneration of the carborane and the formation of Vaska s compound, hans-IrCl(CO) (PPh3)2,... 

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