carborane

[Blend of CARBON and BORANE.]

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A cluster compound containing both carbon (C) and boron (B) atoms as well as hydrogen (H) atoms external to the framework of the cluster. A cluster compound is one with insufficient electrons to allow for classical two-center two-electron bonds between all adjacent atoms. Sometimes the term carborane is used as a synonym for closo-1,2-C₂B₁₀H₁₂, commonly referred to as ortho-carborane. Carboranes are of interest because of their nonclassical bonding, their relatively high thermal stability, and their ability, when containing the ¹⁰B isotope, to capture neutrons efficiently. See also Borane.

Parent clusters from which carborane structures are determined. The diagonal lines define series of related closo, nido, and arachno structures.

The structures of carboranes are based upon a series of three-dimensional, cagelike geometric shapes possessing triangulated faces; such shapes are termed delta polyhedra. The structure for any given carborane may be predicted by determining the framework electrons, by determining the number of electrons involved in bonding the boron and carbon atoms of the cluster framework together, and by using Wade's rule. Wade's rule states that a cluster containing n framework electrons will be derived from a delta polyhedron containing (n − 2)/2 vertices, the parent cluster. Once this parent cluster has been determined, the geometry of the cluster framework may be predicted by clipping off vertices from the parent cluster until a polyhedron whose number of vertices is equivalent to the sum of boron and carbon atoms in the cluster framework is obtained.

Carboranes are placed, according to their structure, into several classifications. The most common classifications are closo (closed), nido (nestlike), and arachno (cobweb) (see illustration). If a carborane's framework structure is that of a closed delta polyhedron, the carborane is said to be a closo-carborane. If a carborane's framework structure is that of a closed delta polyhedron minus one or two vertices, the carborane is said to be a nido- or arachno-carborane, respectively.

The bonding within a carborane can be thought of in terms of localized atomic orbitals forming both classical two-center two-electron bonds and nonclassical three-center two-electron bonds. Each vertex boron and carbon atom can be thought of as being sp³ hybridized, with three of these hybrid orbitals of each vertex atom employed in framework bonding. Employing this simple approach, the bonding within carboranes can be represented by employing resonance structures. For example, nido-C₂B₄H₈ exhibits three resonance structures. The existence of resonance structures implies a delocalization of electron density throughout the cluster framework. Indeed, carboranes exhibit a high degree of electron density delocalization, and their bonding can be described more accurately by employing molecular orbital theory. See also Chemical bonding; Delocalization; Molecular orbital theory; Resonance (molecular structure).

The typical synthesis of a carborane involves the reaction of a boron hydride cluster, containing only boron and hydrogen, with an alkyne. The resulting carborane contains two carbon atoms in its skeletal structure, a dicarbon carborane. The dicarbon carboranes, because of their relative ease of preparation, have been the most widely studied group of carboranes. In particular, closo-1,2-C₂B₁₀H₁₂, the most readily available carborane, has been extensively studied. Other common groups of carboranes include the monocarbon and tetracarbon carboranes. See also Alkyne.

Ball-and-stick model of o-carborane

Ball-and-stick model of the carborane acid anion. (acidic proton not displayed)

Colour scheme:
hydrogen − white,
chlorine − green,
boron − pink,
carbon − black.

A carborane is a cluster composed of boron and carbon atoms. Like many of the related boranes, these clusters are polyhedra and are similarly classified as closo-, nido-, arachno-, hypho-, etc. based on whether they represent a complete (closo-) polyhedron, or a polyhedron that is missing one (nido-), two (arachno-), or more vertices.

Interesting examples of carboranes are the extremely stable icosahedral closo-carboranes.^[1]

A prominent example is the charge-neutral C₂B₁₀H₁₂ or o-carborane with the prefix o derived from ortho, which has been explored for use in a wide range of applications from heat-resistant polymers to medical applications. This compound is called super aromatic because it obeys Huckel's rule and exhibits high thermal stability. At 420 °C o-carborane converts to the meta isomer. In comparison, benzene requires a >1000 °C to induce skeletal rearrangement. Like arenes, carboranes also undergo electrophilic aromatic substitution.

Another important carborane is the negatively charged CHB₁₁H₁₂⁻, which has been used to make solid superacids.

The carborane superacid H(CHB₁₁Cl₁₁)^[2] is one million times stronger than sulfuric acid.^[3][4] The reason for this high acidity is that the acid anion CHB₁₁Cl₁₁⁻ is very stable and substituted with electronegative substituents. H(CHB₁₁Cl₁₁) is the only acid known to protonate C₆₀ fullerene without decomposing it.^[5][6] Additionally, it is the only known anion capable of forming a stable, isolable salt with protonated benzene, C₆H₇⁺.

Contents 1 Dicarbadodecaborane 1.1 History 1.2 Dicarbollide 1.3 Carborynes 2 Footnotes 3 External links

Dicarbadodecaborane

The most heavily studied carborane is C₂B₁₀H₁₂, m. p. 320 C. It is often prepared from the reaction of acetylene with decaborane. A variation on this method entails the use of dimethyl acetylenedicarboxylate to give C₂B₁₀H₁₀(CO₂C H₃)₂, which can be degraded to the C₂B₁₀H₁₂.^[7]

History

The 1,2-closo-dicarbadodecaboranes (usually simply called carboranes), were reported simultaneously by groups at Olin Corporation and the Reaction Motors Division of Thiokol Chemical Corporation working under the U.S. Air Force and published in 1963.^{[8][9][10][11][12][13][14][15][16]} Heretofore, decaborane derivatives were thought to be thermally unstable and reactive with air and water. These groups demonstrated the unprecedented stability of the 1,2-closo-dodecaborane group, presented a general synthesis, described the transformation of substituents without destroying the carborane cluster, and demonstrated the ortho to meta isomerization.

Dicarbollide

Numerous studies have been made on derivatives of the so-called dicarbollide anion, [B₉C₂H₁₁]²⁻. This anion forms sandwich compounds with many metal ions and some exist in otherwise unusual oxidation states. The dianion is a nido cluster prepared by degradation of the parent dicarborane:^[17]

B₁₀C₂H₁₂ + 3 CH₃OH + KOH → KB₉C₂H₁₂ + B(OCH₃)₃ + H₂O + H₂

Carborynes

Carboryne, or 1,2-dehydro-o-carborane, is an unstable derivative of ortho-carborane with the formula B₁₀C₂H₁₀. The hydrogen atoms on the C2 unit in the parent o-carborane are missing. The compound resembles and is isolobal with benzyne.^[18][19][20] A carboryne compound was first generated in 1990 starting from o-carborane. The hydrogen atoms connected to carbon are removed by n-butyllithium in tetrahydrofuran and the resulting lithium dianion is reacted with bromine at 0°C to form the bromo monoanion.

Heating the reaction mixture to 35 °C releases carboryne, which can subsequently be trapped with suitable dienes:

such as anthracene (to afford a trypticene-like molecule) and furan in 10 to 25% chemical yield.

Carborynes react with alkynes to benzocarboranes ^[21][22] in an adaptation of the above described procedure. O-carborane is deprotonated with n-butyllithium as before and then reacted with dichloro-di(triphenylphosphino) nickel to a nickel coordinated carboryne. This compound reacts with 3-hexyne in an alkyne trimerization to the benzocarborane.

Single crystal X-ray diffraction analysis of this compound shows considerable bond length alternation in the benzene ring (164.8 pm (!) to 133.8 pm) ruling out aromaticity.

Footnotes

1. ^ Eluvathingal D. Jemmis (1982). "Overlap control and stability of polyhedral molecules. Closo-Carboranes". J. Am. Chem. Soc. 104: 7017–7020. doi:10.1021/ja00389a021. 
2. ^ Note that the image the acidic proton is not the one bonded to the carborane but that it is the counterion not displayed
3. ^ George A. Olah, et. al. Superacid Chemistry, 2nd ed., Wiley, p. 41.
4. ^ That is, the concentration of H⁺ in a solution of the carborane superacid is a million times higher than in a solution of sulfuric acid.
5. ^ Mark Juhasz, Stephan Hoffmann, Evgenii Stoyanov, Kee-Chan Kim, Christopher A. Reed (2004). "The Strongest Isolable Acid". Angewandte Chemie International Edition 43: 5352–5355. doi:10.1002/anie.200460005. 
6. ^ Christopher A. Reed (2005). "Carborane acids. New "strong yet gentle" acids for organic and inorganic chemistry" (Full article (reprint)). Chem. Commun. 2005: 1669–1677. doi:10.1039/b415425h. http://www.reedgrouplab.ucr.edu/publications/Chem%20Comm%202005%201669.pdf. 
7. ^ Kutal, C. R.; Owen, D. A.; Todd, L. J. (1968). "Closo-1,2-dicarbadodecaborane(12)". Inorganic Syntheses 11: 19–23. doi:10.1002/9780470132425.ch5. 
8. ^ T. L. Heying, J. W. Ager, S. L. Clark, D. J. Mangold, H. L. Goldstein, M. Hillman, R. J. Polak, and J. W. Szymanski (1963). "A New Series of Organoboranes. I. Carboranes from the Reaction of Decaborane with Acetylenic Compounds". Inorganic Chemistry 2: 1089–1092. doi:10.1021/ic50010a002. 
9. ^ H. Schroeder, T. L. Heying, J. R. Reiner (1963). "A New Series of Organoboranes. II. The Chlorination of 1,2-Dicarbaclosododecaborane(12)" Inorganic Chemistry". Inorganic Chemistry 2: 1092–1096. doi:10.1021/ic50010a003. 
10. ^ T. L. Heying, J. W. Ager, S. L. Clark, R. P. Alexander, S. Papetti, J. A. Reid, S. I. Trotz (1963). "A New Series of Organoboranes. III. Some Reactions of 1,2-Dicarbaclosododecaborane(12) and its Derivatives". Inorganic Chemistry 2: 1097–1105. doi:10.1021/ic50010a004. 
11. ^ S. Papetti, T. L. Heying (1963). "A New Series of Organoboranes. IV. The Participation of the 1,2-Dicarbaclosododecaborane(12) Nucleus in Some Novel Heteratomic Ring Systems". Inorg. Chem. 2: 1105–1107. doi:10.1021/ic50010a005. 
12. ^ M. M. Fein, J. Bobinski, N. Mayes, N. Schwartz, M. S. Cohen (1963). "Carboranes. I. The Preparation and Chemistry of 1-Isopropenylcarborane and its Derivatives (a New Family of Stable Closoboranes)". Inorg. Chem. 2: 1111–1115. doi:10.1021/ic50010a007. 
13. ^ M. M. Fein, D. Grafstein, J. E. Paustian, J. Bobinski, B. M. Lichstein, N. Mayes, N. N. Schwartz, M. S. Cohen (1963). "Carboranes. II. The Preparation of 1- and 1,2-Substituted Carboranes". Inorg. Chem. 2: 1115–1119. doi:10.1021/ic50010a008. 
14. ^ D. Grafstein, J. Bobinski, J. Dvorak, H. Smith, N. Schwartz, M. S. Cohen, M. M. Fein (1963). "Carboranes. III. Reactions of the Carboranes". Inorg. Chem. 2: 1120–1125. doi:10.1021/ic50010a009. 
15. ^ D. Grafstein, J. Bobinski, J. Dvorak, J. E. Paustian, H. F. Smith, S. Karlan, C. Vogel, M. M. Fein (1963). "Carboranes. IV. Chemistry of Bis-(1-carboranylalkyl) Ethers". Inorg. Chem. 2: 1125–1128. doi:10.1021/ic50010a010. .
16. ^ D. Grafstein, J. Dvorak (1963). "Neocarboranes, a New Family of Stable Organoboranes Isomeric with the Carboranes". Inorg. Chem. 2: 1128–1133. doi:10.1021/ic50010a011. 
17. ^ Plešek, J.; Heřmánek, S.; Štíbr, B. Inorganic Syntheses, 1983, volume 22, pages 231-124.
18. ^ Henry L. Gingrich, Tirthankar Ghosh, Qiurong Huang, and Maitland Jones (1990). "1,2-Dehydro-o-carborane". J. Am. Chem. Soc. 112 (10): 4082–4083. doi:10.1021/ja00166a080. 
19. ^ E. D. Jemmis and B. Kiran (1997). "Structure and Bonding in B10X2H10 (X = C and Si). The Kinky Surface of 1,2-Dehydro-o-Disilaborane". J. Am. Chem. Soc. 119 (19): 4076–4077. doi:10.1021/ja964385q. 
20. ^ B. Kiran, A. Anoop, E. D. Jemmis (2002). "Control of Stability through Overlap Matching: closo-Carboranes and closo-Silaboranes". J. Am. Chem. Soc. 124: 4402–4407. doi:10.1021/ja016843n. 
21. ^ Liang Deng, Hoi-Shan Chan, and Zuowei Xie (2006). "Nickel-Mediated Regioselective [2 + 2 + 2] Cycloaddition of Carboryne with Alkynes". J. Am. Chem. Soc. 128 (24): 7728–7729. doi:10.1021/ja061605j. 
22. ^ Eluvathingal D Jemmis and Anakuthil Anoop (2004). "Theoretical Study of the Insertion Reactions of Benzyne- and Carboryne- Ni Complexes". MHPCC Application Briefs: 51. http://www.mhpcc.hpc.mil/research/appbriefs/2004/2004MHPCCAppBriefs.pdf. 

External links

• The Reed Group
• Material Safety Data Sheet

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