Carboranes isomerization

Whilst it is very encouraging that our recent work with metallacarborane isomerization has provided experimental support for the conclusions of previous theoretical studies on carborane isomerization, the inclusion of metal vertices can clearly have a moderating effect. This is dramatically demonstrated by examples of metal-dependency. Thus, reaction of 3 with Pt(PR3)2 " instantaneously produces a 1,2 1,7 Ccage isomerization, whereas reaction with Ni(PR3)2 " results in one... 

The discovery (116) of the base-promoted degradation of the isomeric closo-Q,. . cages provided one of the most important carborane anion systems, the isomeric nido-Q,. ][ , anions,... 

The three isomeric icosahedral carboranes (76-78) are unique both in their ease of preparation and their great stability in air, and consequently their chemistry has been the most fully studied. The 1,2-isomer in particular is available on the multikilogram scale. It is best prepared in bulk by the direct reaction of ethyne with decaborane in the presence of a Lewis base, preferably Et2S ... 

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). 

This review will restrict itself to boron-carbon multiple bonding in carbon-rich systems, as encountered in organic chemistry, and leave the clusters of carboranes rich in boron to the proper purview of the inorganic chemist. Insofar as such three-dimensional clusters are considered at all in these review, interest will focus on the carbon-rich carboranes and the effect of ring size and substituents, both on boron and carbon, in determining the point of equilibrium between the cyclic organoborane and the isomeric carborane cluster. A typical significant example would be the potential interconversion of the l,4-dibora-2,5-cyclohexadiene system (7) and the 2,3,4,5-tetracarbahexaborane(6) system (8) as a function of substituents R (Eq. 2). 

The discovery of polyhedral boranes and polyhedral heteroboranes, which contain at least one atom other than in the cage, initiated a new era in boron chemistry.1-4 Most commonly, of the three commercially available isomeric dicarba-closo-dodecaborane carboranes(l,2-, 1,7-, and 1,12-), the 1,2-isomer 1 has been used for functionalization and connection to organic molecules. The highly delocalized three-dimensional cage bonding that characterizes these carboranes provides extensive thermal and kinetic stabilization as well as photochemical stability in the ultraviolet and visible regions. The unusual icosahedral geometry of these species provides precise directional control of all exopolyhedral bonds. 

A B-Ge-B derivative is first obtained which thereafter isomerizes to the carborane. 

Many derivatives of isomeric 2,3-dicarba-nido-hexaborane(8) are known, available from the reaction of pentaborane(9) with alkynes [59, 60]. In contrast, the carboranes of type 43 with non-adjacent carbon atoms are rare. 

Some interesting photochemical reactions involving metalloboranes and metallo-carboranes have been reported. These include synthesis, isomerization and complexa-tion reactions. Gaines and Hildebrandt have used the following process to synthesize a metalloborane ... 

They have also demonstrated a reversible photochemical conversion from a tridentate to a bidentate B3H8 (Fig. 14). Fehlner has achieved a photochemical carborane synthesis by irradiating mixtures of B4H8Fe(CO)3 and dimethylacetylene. Products of the reactions include (CH3)4C4B4H4 and (CH3)8C8B4H4. Leyden and co-workers have observed two examples of metalloborane isomerization reactions initiated by light ... 

In the early 1960s it was recognized (6,187) that there were bonding similarities between the pentagonal face of the isomeric [nirio-(y HjJ2- ions and the well-known cyclopentadienide ion (Cp ) [CydJ- (Fig. 16). The isomeric nido-C2B9H1 J2- ions, which are commonly known as dicarbollide ions, and many other carborane anions, form stable complexes with most of the metallic elements. Indeed nearly all metals can be combined with polyborane hydride clusters to produce an apparently limitless variety of cluster compounds. 

An extensive series of stannoles results from trialkylboranes and dialkynylstannanes, notably if the alkyne substituent is H, Bu or Me3Si (Scheme 199) (78JOM( 148)137). However, if it is methyl the isomeric l-bora-4-stannacyclohexadiene is formed along with the stan-nacyclopent-3-ene (Scheme 200) (78JOM(153)153>. The lability of the tin-carbon bond makes the compounds useful synthetic intermediates, notably in the preparation of mWo-carboranes (Scheme 201) (77JOM(132)213). 

The acetylene may be unsubstituted (R = H) or substituted, in which case the reaction proceeds even more readily. The resulting compound is known as 1.2-dicarbtw/aY -dodecaborane(l2), or the ortho carborane, and is isoelectronic and isostructur.il with [BI2HI2]2-. It is stable to both heat and air, but it isomerizes at high temperatures to the 1,7 ( meta or neo isomer) and the 1,12 ( para isomer) (Fig. 16.51). The mechanism of isomerization, thought to be intramolecular, has been discussed for many years.133... 

Explicitly included are species with carbonyl (>CO) groups such as the isomeric phthalimide (VIII) and isatin (IX), and with thiocarbonyl (>CS) groups such as the isomeric benzo-l,2-dithol-3-thione (X) and benzo-l,3-dithiol-2-thione (phenylene trithiocarbonate) (XI). Conversely, species with the isoelectronic >BF (and the related >BH) are ignored. Thus the question of the aromaticity in carboranes never arises in this chapter, even had we been explicitly interested in three-dimensional aromaticity (another issue we will ignore here). 

A third isomeric species (II) has been isolated when B7C2H,, 2 is reacted with cobalt(ll) chloride at 70° C. This compound has only one carborane (CH) resonance in the proton NMR, suggesting that the two carbon atoms occupy equivalent positions in the cage. The favored structure of the three possible choices is illustrated in Fig. 10 (II). This choice is based on the... 

Fig. 1. Structures and numbering of the three isomeric icosahedral carboranes, and the degradation of 1,2-C.2BioHi2 to 7,8-C2B9Hir. The bridging hydrogen is shown in one of the two equivalent bridging positions.

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