Metallocarboranes reactions

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). 

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). 

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). 

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... 

Similar polyhedral expansion reactions have been used to synthesize metallocarboranes containing 3 metal atoms, e.g., the icosahedral (CsHsColsCaByHj (80), or containing two different metals (71,175), e.g., the bicapped square-antiprismatic CsHsCoCByHgNiCsHs (175). 

Metallocarbenoids, in addition reactions, for C-C bond formation, characteristics, 10, 320 Metallocarboranes, with Zr(IV), 4, 982... 

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. 

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... 

The reaction conditions for this type of preparation generally involve nonaqueous solvents, such as tetrahydrofuran (THF) or diethyl ether, and rigorous exclusion of air and water. Some metallocarboranes, however, may be prepared in high yield by reaction of a metal salt and the nido monoanion, C2B9H12-, in strong aqueous base ... 

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. 

Limited investigations of the chemical reactions of metallocarboranes have been reported. In general, these complexes are much less reactive than the analogous metallocenes, yet several unique new species have been prepared from the unsubstituted metallocarboranes. 

This protonated metallocarborane is unique in that it undergoes ready substitution at polyhedral boron atoms, whereas the unprotonated species is unreactive in the absence of acid. Reaction of (1,2-C2B9Hn)2FeH- with Lewis bases such as dialkyl sulfides results in the loss of H2 and the formation of boron-substituted complexes in good yields ... 

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 ... 

The first cyclopentadienyl metallocarborane to be synthesized emphasized the analogy between metallocene chemistry and metallocarborane chemistry. This preparation simply involved the reaction of stoichiometric amounts of the two ligands with a transition metal halide ... 

This complex, along with its iridium analog, has proven to be an effective catalyst for the isomerization and hydrogenation of olefins (80), for H—D exchange at BH vertices (60), and for other organic reactions. Catalysis by metallocarboranes is discussed in detail in Section X. 

The discovery of the thermal metal transfer reaction (27) afforded a third preparative route to bimetallic twelve-vertex metallocarboranes. This method involves the pyrolysis of eleven-vertex cfoso-metallocarboranes or cobalticinium salts of eleven-vertex cowimo-metallocarboranes and results in the production of several isomeric, closed, neutral, twelve-vertex bimetallocarboranes [Eqs. (9) and (10) Fig. 7]. Yields are reasonable in... 

Only one trimetallic twelve-vertex metallocarborane has been reported. This species, (C5H5)3Co3C2B7H9, arose as a side product during the polyhedral expansion of 2-C5H6-2-Co-l,6-C2B7H9 with Co(II) and C6H5 (25, 28). The isolation of this trimetallic complex suggests that the polyhedral expansion reaction may be extended to bimetallic substrates to produce novel metal-rich polyhedra. 

Reactions of monometallic eleven-vertex metallocarboranes have been discussed in previous sections and may be summarized briefly as (a) polyhedral expansion to bimetallic twelve-vertex complexes and (6) thermal metal transfer to bimetallic twelve-vertex compounds. Polyhedral contraction to ten-vertex monometallocarboranes is discussed in Section VII. 

The original preparation of ten-vertex metallocarboranes involved the deprotonation of the nido-carboranc 6,8-C2B7H13, followed by reaction with a transition metal halide. Additional hydrogen gas was liberated during the complexation, and cZoso-metallocarborane compounds were formed (38) ... 

The chemical reactions of monometallic ten-vertex metallocarboranes have been examined (40). Fricdel-Crafts acylation of C5H6Co(l, 6-C2B7H9) produced a monosubstitution product. Attack occurred on the boron atom farthest removed from the polyhedral carbon vertices. No substitution on the cyclopentadienyl ring was observed. 

The first metallocarborane of this geometry to be synthesized was an unexpected product. In an attempt to prepare a ten-vertex manganese carbonyl complex, the C2B7Hii2 ion, discussed in Section VII, was reacted with BrMn(CO)5. Surprisingly, the only metal-containing compound isolated from the reaction mixture had just 6 boron atoms (36, 50). The course of the reaction may be outlined as follows ... 

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,... 

Although the hydridorhodacarborane is formally a rhodium (III) derivative, it functions as a facile catalyst in alkenc isomerization, hydrogenation, hydroformylation, and hydrosilylation reactions 80). This catalyst system is extremely stable and may be recovered quantitatively from alkene isomerization and hydrogenation reactions. In addition to these reactions, the hydridorhodacarborane is very effective in the catalysis of deuterium exchange at terminal BH positions 59). These discoveries may soon lead to industrially useful metallocarborane catalysts. 

By the time the alkylation studies were started, the reactivity of the dicarbollide anions toward electrophilic agents had been studied mainly on insertion reactions of boron, or heteroatom, or transition metal into the place of the missing vertex of the icosahedron, restoring its structure. A broad area of metallocarboranes was developed as a result of these studies, which was the subject of many articles and reviews. 

It was the perceptive recognition that the open pentagonal faces of these dianions were structurally and electronically equivalent to the pentahapto cyclopentadienide anion (rj -CsHs)" (Fig. 6.21) that led to the discovery of the metallocarboranes and the development of some of the most intriguing and far-reaching reactions of the carboranes. These are considered in the next section. 

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