Carbo rane

When one considers the potential high-energy release on rupture of a carborane unit, together with the thermodynamic stability of combustion products, it is hardly surprising that there is a body of literature that reports on the use of carbo-ranes within propellant compositions. Their use in energetic applications is to be expected when the enthalpy of formation (AH/) data for the products of combustion for boron are compared to those of carbon. Thermodynamic data for the enthalpy of formation of o-carborane and of typical boron and carbon combustion products is shown in Table 4. Measurements of the standard enthalpy of combustion32 for crystalline samples of ortho-carborane show that complete combustion is a highly exothermic reaction, AH = — 8994 KJmol. ... 

Considering the success of the condensation route to carboranes in the hydride bath (vide supra), other alkynylboranes than diethyl(propyn-l-yl)borane might be equally suitable. By heating a mixture of bis(diethylboryl) ethyne (65) and excess of (Et2BH)2 ( hydride bath ) at 110-120 °C, l,2,3,4-tetraethyl-5,6,7,8-tetracarba-mdo-octaborane(8) 64c was obtained by distillation in ca. 20% yield as a colorless liquid, stable to air and H2O (Scheme 3.2-35) [87]. Possible intermediates in this reaction can be proposed as 67 and 68, where 67 results from double hydroboration of bis(diethylboryl)ethyne (65), which dimerizes to 68 and finally yields the carbo-rane 64c by elimination of Et3B. 

Metal-borane and metal-carborane a- and -complexes are boranes or carbo-ranes having metal substituents external to the cage and bound to it by single (cr) or bridging (p,) bonds, as in /2,-[Cu(PPh3)2]B4H8 (Fig. 5-17e). 

Metallacarbaboranes (or metallacarboranes) are compounds that contain cluster polyhedra comprising carbon, boron, and metal atoms in various combinations. The first metallacarbaborane clusters were prepared by Hawthorne in 1965, and were derived from the icosahedral closo-carbaborane l,2-C2BioHi2 see Boron Polyhedral Carbo-ranes) by the replacement of one BH vertex with a metal center. Compounds such as [Fe(C2B9Hn)2]" (n = 1 or 2) can be represented as the commo-metallacarbaborane (1) or as... 

In addition to classical substituents such as NH2, NHR, NR2, OR, SR in various combinations, in recent years some more exotic groups have been attached to phosphazene rings. Among the novel cyclic phosphazenes the carbo-rane substituted derivatives are worth mentioning. Other unusual derivatives include chiral cyclophosphazenes, cyano-, ferrocenylphenoxy-, and ferrocenylhydrazone-cyclophosphazenes. Another interesting development is the synthesis of crown ethers with chlorocyclophosphazene subunits (Scheme 54). ... 

A number of organolanthanide complexes with carbo-ranes either bonded to the lanthanide metals via one carbon atom or with an open side as Cp analogs have been... 

Present information indicates that all the c/osnborane anions and carbo-ranes are fully triangulated polyhedra, as shown in Fig. 8-10. 

A basic theme of the chemistry highlighted in this article is the exploitation of the distinctive features of icosahedral and planar carbo-rane units in order to generate new types of stable molecular structures that are generally inaccessible via conventional organic or orga-nometallic approaches. Many other examples of creative and imaginative synthesis in... 

Probably the best known of the anticrowns are the mercuracarborands, e.g. 2.82 and 2.83 (Figure 2.17). As with the fluorinated complexes, the carbo-rane substituents act as elaborate electron-withdrawing groups and increase the Lewis acidity of the mercury(ii) ions. Most crown ethers exhibit one-to-one host guest stoichiometry, but most anticrowns form 2 1 complexes on size-fit grounds. One exception to this rule is the chloride complex of 2.83 in which the chloride ion binds within the centre of the host cavity with a long Hg-Cl distance of 2.94 A. 

Hawthorne group for their trimeric mercury-linked carbo-rane compound." ... 

Rt-TCEP Rtx-440 Stx-500 l,2,3-Tris(cyanoeth-oxy)propane Restek Exclusive Phenylpoly- carbo-rane-siloxane TCEP... 

Phosphonates and phosphonothioates which are separated from the ortto-carborane core by a methylene bridge were also reported. In these compounds, the electronic influence of the carbo-rane on the phosphonate group is low. Their synthesis starts from the Grignaid reagent (l-bromomagnesiomethyl)-ort/io-carborane and diethyl chlorophosphite (Scheme 2.3). Transformation into phosphonates 3 and 4 as well as into phosphonothioate 5 was achieved in the same manner as reported for the above-mentioned carborane derivatives. 

Similar to fluorination, iodination of o-carborane (1) has been carried out primarily through electrophilic iodination or by nucleophilic deboronation followed by reconstruction of the carbo-rane cage with BI3 (Scheme 6.2) (Yamazaki et al., 2005). Depending on the stoichiometric quantity of molecular iodine (Ij), the electron-rich borons 9 and 12 of 1 can be iodinated easily in the presence of AICI3 to give 9-iodo-o-carborane (17) and/or 9,12-diiodo-o-carborane (18) (Li et al., 1991 Yamazaki et al., 2005), whereas the reaction of compound 14 with boron triiodide (BI3) afforded 3-iodo-o-carborane (19, 74% yield) (Yamazaki et al., 2005). 

Kongpricha, S. and Schroeder, H. 1969. Icosahedral carboranes. XII. Direct fluorination of o-, m-, andp-carbo-rane. Inorg. Chem., 8, 2449-52. 

Zakharkin, L. I., Kahnin, V. N., and Podvisotskaya, L. S. 1970. Comparative reactivity of o-, m-, andp-carbo-ranes. Izv. Akad. Nauk SSSR, Ser. Khim., 6,1297-302. 

---