Carborane residue

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... 

Figure 3.23. Orientations possible for a V-shaped ( 2 ) ML2 unit (M = Pd or Pt L = CNR or PR3) over the open pentagonal face of a nido carborane residue Orientations depend on C atom positions in the face and the frontier orbitals involved.

Reactions of this type have been used to prepare an enormous number of derivatives. One of the principal motivations in developing this chemistry has been to incorporate the thermally stable carborane residue into high polymers, and this has been accomplished, notably in some very stable ones incorporating 1,7-B10C2H10 residues with siloxane linkages. 

The reaction of an olide complex with a strong base can form a new kind of complex containing two metal atoms and three carborane residues in which the carborane residue at the center has two boron atoms removed (Fig. 19). The central ion is called a canistide ion from the Spanish word for basket. ... 

M]Mercuracarborands-n (macrocyclic compounds with m-member cycle con-stracted from n o-carborane residues connected with the bridge Hg atoms) as... 

On heating in air at 10°C per min, poly(m-carborane-siloxane) shows typically only 4% mass loss at 450°C and 7% mass loss at 600°C (see Fig. 4). In comparison, siloxanes without carborane units, show an approximate 50% mass loss at 450°C. As a consequence of the relatively high boron and carbon content of these materials, pyrolysis is expected to generate ceramic residues of boron carbide/silicon carbide. 

The solution is cooled to room temperature and is washed with a few milliliters of benzene into a single-necked flask. The solvent is removed with a rotary evaporator connected to a water aspirator vacuum gentle heat is supplied from a steam bath. The residue is cooled to room temperature before air is admitted. About 200 ml. of hexane is added and stirred with the residue to extract most of the carborane. The brownish tar which remains undissolved is allowed to settle and the solution is decanted. A second extraction of the tar with 40 ml. of hexane converts the residue to a solid which is removed by filtration. The solid is washed on the filter with an additional 40 ml. of hexane. The combined hexane extracts are filtered and then washed in a separatory funnel with four 100-ml. portions of a chilled aqueous 10% sodium hydroxide solution, followed by four 100-ml. portions of water. After the yellow hexane solution has been dried over anhydrous magnesium sulfate and filtered, the solvent is removed by use of a rotary evaporator connected to a water aspirator. The carborane is washed with a small amount of pentane into a 300-ml. single-necked flask which is attached to an alembic column as pictured in Fig. 13. 

The distilling flask, collection flask, and column are continuously evacuated with a high-vacuum system. When the bulk of the pentane and residual hexane have distilled away, the temperature of a silicone oil bath surrounding the distillation flask is raised from room temperature to 125° over about a one-hour period. When the distillation rate diminishes appreciably, the temperature is slowly raised to 150° and maintained there until no more liquid is obtained. The distillation flask is cooled to room temperature before air is admitted to the system. The distilled product weighs 86.5 g. (89% yield checkers report 89 %t) and melts at 32 to 32.5°. In this state of purity, (bromomethyl)-carborane is suitable for most uses, but it may be further purified by crystallization from pentane or methanol. For example, crystals obtained by chilling a solution of 86.5 g. of the car-... 

The halogen atoms remaining can then be replaced by organic residues such as trifluoroethoxy units. High polymers can also be prepared by ring-opening polymerization of the chlorocyclo-phosphazene, XXVIII. Compounds of this type can be converted to nldo-carboranes in the presence of base, but these do not form metallo-derivatlves, presumably for sterlc reasons (29). 

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... 

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. 

A solution of methyl carborane (6.32 mmol) dissolved in 20 ml of diethyl ether was cooled to —78°C and then treated with 4.20 ml n-BuLi (6.72 mmol 1.6M in n-hexane) and stirred for 30 minutes. The mixture was warmed to ambient temperature, stirred for 4 hours, and treated with cyclohexene oxide (6.80 mmol) at 0°C. It was then stirred for 6 hours and quenched with 10 ml of water. The organic phase was separated, and the aqueous phase extracted twice with 25 ml of diethyl ether. The combined organics were dried using MgS04 and concentrated. The residue was re-crystallized in n-hexane, and the product was isolated in 86%yield, MP= 101-103°C... 

The Step 3 product (4.00 mmol) was dissolved in 100 ml of THF and then cooled to —78°C and treated with 2.7ml of n-BuLi (4.32mmol 1.6M in hexane). It was stirred for 30 minutes, followed by 4 additional hours of stirring at ambient temperature, and then concentrated. The residue was washed twice with 15 ml of n-hexane, dissolved in 150 ml of THF containing 1% Merrifield s resin (2.0 g 3.94 mmol Cl), and stirred at ambient temperature for two days. The mixture was refluxed for 4 hours and then quenched with 3.0 ml of methanol and re-concentrated. The residue was washed twice with 10 ml of deionized water and twice with 20 ml of n-hexane. The solid was dried, and 2.40 g polystyrene-supported ortho-carborane were isolated as a pale yellow solid. 

In the presence of air at 70°C, the reaction of the iminophosphorane 5 with meta carborane 2 resulted in deboronation to yield the H2NP(NMe2)3+ salt 10 of the nido anion 7,9-C2B9Hi2" and boric acid.5 When isolated in the solid state, the salt 10 degraded further to boric acid residues in the presence of air. Salts containing the 7,9-C2B9Hi2 anions are characteristically air-sensitive solids.9,11... 

As an alternative to the reductive generation of a nido borane or carborane cluster from a closo parent, one can regard a nido species as the product of decapitation or deboronation of a closo parent. Formal removal of a B kf unit, normally from a high connectivity site in a closo borane or carborane, followed by protonation of the anionic residue around the open face generated, leaves a neutral nido residue. 

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