Carborane

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Ball-and-stick model of o-carborane O-carborane-3D-balls.png
Ball-and-stick model of o-carborane

Carboranes (or carbaboranes) are electron-delocalized (non-classically bonded) clusters composed of boron, carbon and hydrogen atoms. [1] Like many of the related boron hydrides, these clusters are polyhedra or fragments of polyhedra. Carboranes are one class of heteroboranes. [2]

Contents

In terms of scope, carboranes can have as few as 5 and as many as 14 atoms in the cage framework. The majority have two cage carbon atoms. The corresponding C-alkyl and B-alkyl analogues are also known in a few cases.

Structure and bonding

Carboranes and boranes adopt 3-dimensional cage (cluster) geometries in sharp contrast to typical organic compounds. Cages are compatible with sigma—delocalized bonding, whereas hydrocarbons are typically chains or rings.

Like for other electron-delocalized polyhedral clusters, the electronic structure of these cluster compounds can be described by the Wade–Mingos rules. Like the related boron hydrides, these clusters are polyhedra or fragments of polyhedra, and are similarly classified as closo-, nido-, arachno-, hypho-, hypercloso-, iso-, klado-, conjuncto- and megalo-, based on whether they represent a complete (closo-) polyhedron or a polyhedron that is missing one (nido-), two (arachno-), three (hypho-), or more vertices. Carboranes are a notable example of heteroboranes. [2] [3] The essence, these rules emphasize delocalized, multi-centered bonding for B-B, C-C, and B-C interactions.

Structurally, they can be considered to be related to the icosahedral (Ih) [B12H12]2− via formal replacement of two of its BH− fragments with CH.

Isomers

Geometrical isomers of carboranes can exist on the basis of the various locations of carbon within the cage. Isomers necessitate the use of the numerical prefixes in a compound's name. The closo-dicarbadecaborane can exist in three isomers: 1,2-, 1,7-, and 1,12-C2B10H12.

Preparation

Carboranes have been prepared by many routes, the most common being addition of alkynyl reagents to boron hydride clusters to form dicarbon carboranes. For this reason, the great majority of carborane have two carbon vertices.

Monocarba derivatives

Monocarboranes are clusters with BnC cages. The 12-vertex derivative is best studied, but several are known.

Typically they are prepared by the addition of one-carbon reagents to boron hydride clusters. One-carbon reagents include cyanide, isocyanides, and formaldehyde. For example, monocarbadodecaborate ([CB11H12]−) is produced from decaborane and formaldehyde, followed by addition of borane dimethylsulfide. [4] [5] Monocarboranes are precursors to weakly coordinating anions. [6]

Dicarba clusters

Dicarbaboranes can be prepared from boron hydrides using alkynes as the source of the two carbon centers. In addition to the closo-C2BnHn+2 series mentioned above, several open-cage dicarbon species are known including nido-C2B3H7 (isostructural and isoelectronic with B5H9) and arachno-C2B7H13.

Structure of nido- C2B4H8, highlighting some trends: carbon at the low connectivity sites, bridging hydrogen between B centers on open face CAHBOR01.png
Structure of nido-C2B4H8, highlighting some trends: carbon at the low connectivity sites, bridging hydrogen between B centers on open face

Syntheses of icosahedral closo-dicarbadodecaborane derivatives (R2C2B10H10) employ alkynes as the R2C2 source and decaborane (B10H14) to supply the B10 unit.

Classification by cage size

The following classification is adapted from Grimes's book on carboranes. [1]

Small, open carboranes

This family of clusters includes the nido cages CB5H9, C2B4H8, C3B3H7, C4B2H6, and C2B3H7. Relatively little work has been devoted to these compounds. Pentaborane[9] reacts with acetylene to give nido-1,2-C2B4H8. Upon treatment with sodium hydride, latter forms the salt [1,2-C2B4H7]−Na+.

Small, closed carboranes

This family of clusters includes the closo cages C2B3H5, C2B4H6, C2B5H7, and CB5H7. This family of clusters are also lightly studied owing to synthetic difficulties. Also reflecting synthetic challenges, many of these compounds are best known as their alkyl derivatives. 1,5-C2B3H5 is the only known isomer of the five-vertex cage. It is prepared from the reaction of pentaborane(9) with acetylene in two operations beginning with condensation with acetylene followed by pyrolysis (cracking) of the product:

B5H9 + C2H2 →nido-2,3-C2B4H8 + BH3
C2B4H8 →closo-2,3-C2B3H5 + BH3

Intermediate-sized carboranes

Structure of 1,3- C2B7H13 (all unlabeled vertices are BH). 1,3-C2B7H13.svg
Structure of 1,3-C2B7H13 (all unlabeled vertices are BH).

Structures

This family of clusters includes the closo cages C2B6H8, C2B7H9, C2B8H10 and C2B9H11 and their derivatives. Isomerism is well established in this family:

  • 2,3- and 2,4-C2B4H8
  • 2,3- and 2,4-C2B5H7
  • 1,2- and 1,6-C2B6H8
  • 1,10-, 1,6-, and 1,2-C2B8H10 [8]
  • 1,2 and 1,3-C2B9H13.

Syntheses

Carboranes of intermediate nuclearity are most efficiently generated by degradations from larger clusters. In contrast, smaller carboranes are usually prepared by building-up routes, e.g. from pentaborane + alkyne, etc. For example ortho-carborane can be degraded to give [C2B9H12]−, [9] which can be manipulated with oxidants, protonation, and thermolysis.

[C2B9H12]− + Fe3+ → C2B8H12 + "B+" + Fe2+
[C2B9H12]− + H+ → C2B9H13
C2B9H13 → C2B9H11 + H2

Chromate oxidation of 11-vertex clusters results in deboronation, giving C2B7H13. From that species, other clusters result by pyrolysis, sometimes in the presence of diborane: C2B6H8, C2B8H10, and C2B7H9. [1]

In general, isomers having non-adjacent cage carbon atoms are more thermally stable than those with adjacent carbons. Thus, heating tends to induce mutual separation of the carbon atoms in the framework.

Icosahedral carboranes

The icosahedral charge-neutral closo-carboranes, 1,2-, 1,7-, and 1,12- C2B10H12 (informally ortho-, meta-, and para-carborane) are particularly stable and are commercially available. [10] [11] The ortho-carborane forms first upon the reaction of decaborane and acetylene. It converts quantitatively to the meta-carborane upon heating in an inert atmosphere. Producing meta-carborane from ortho-carborane requires 700 °C, proceeding in ca. 25% yield. [1]

[CB11H12]− is also well established.

Reactions

The metalation of carboranes is illustrated by the reactions of closo-C2B3H5 with iron carbonyl sources. Two closo Fe- and Fe2-containing products are obtained, according to these idealized equations:

C2B3H5 + Fe2(CO)9 → C2B3H5Fe(CO)3 + Fe(CO)5 + CO
C2B3H5Fe(CO)3 + Fe2(CO)9 → C2B3H5(Fe(CO)3)2 + Fe(CO)5 + CO

Base-induced degradation of carboranes give anionic nido derivatives, which can also be employed as ligands for transition metals, generating metallacarboranes, which are carboranes containing one or more transition metal or main group metal atoms in the cage framework. Most famous are the dicarbollide, complexes with the formula M2+[C2B9H11]2−, where M stands for metal. [12]

Research

Dicarbollide complexes have been investigated for many years, but commercial applications are rare. The bis(dicarbollide) [Co(C2B9H11)2]− has been used as a precipitant for removal of 137Cs + from radiowastes. [13]

The medical applications of carboranes have been explored. [14] [15] C-functionalized carboranes represent a source of boron for boron neutron capture therapy. [16]

The compound H(CHB11 Cl 11) is a superacid, forming an isolable salt with protonated benzene cation, [C6H7]+ (benzenium cation). [17] The formula of that salt is [C6H7]+[CHB11Cl11]−. The superacid protonates fullerene, C60. [18]

See also

Related Research Articles

<span class="mw-page-title-main">Boron hydride clusters</span>

Boron hydride clusters are compounds with the formula BxHy or related anions, where x ≥ 3. Many such cluster compounds are known. Common examples are those with 5, 10, and 12 boron atoms. Although they have few practical applications, the borane hydride clusters exhibit structures and bonding that differs strongly from the patterns seen in hydrocarbons. Hybrids of boranes and hydrocarbons, the carboranes are also well developed.

<span class="mw-page-title-main">Decaborane</span> Chemical compound

Decaborane, also called decaborane(14), is the inorganic compound with the chemical formula B10H14. It is classified as a borane and more specifically a boron hydride cluster. This white crystalline compound is one of the principal boron hydride clusters, both as a reference structure and as a precursor to other boron hydrides. It is toxic and volatile, giving off a foul odor, like that of burnt rubber or chocolate.

<span class="mw-page-title-main">Pentaborane(9)</span> Chemical compound

Pentaborane(9) is an inorganic compound with the formula B5H9. It is one of the most common boron hydride clusters, although it is a highly reactive compound. Because of its high reactivity with oxygen, it was once evaluated as rocket or jet fuel. Like many of the smaller boron hydrides, pentaborane is colourless, diamagnetic, and volatile. It is related to pentaborane(11).

<span class="mw-page-title-main">Organoboron chemistry</span> Study of compounds containing a boron-carbon bond

Organoboron chemistry or organoborane chemistry studies organoboron compounds, also called organoboranes. These chemical compounds combine boron and carbon; typically, they are organic derivatives of borane (BH3), as in the trialkyl boranes.

In chemistry the polyhedral skeletal electron pair theory (PSEPT) provides electron counting rules useful for predicting the structures of clusters such as borane and carborane clusters. The electron counting rules were originally formulated by Kenneth Wade, and were further developed by others including Michael Mingos; they are sometimes known as Wade's rules or the Wade–Mingos rules. The rules are based on a molecular orbital treatment of the bonding. These rules have been extended and unified in the form of the Jemmis mno rules.

<span class="mw-page-title-main">Boron compounds</span>

Boron compounds are compounds containing the element boron. In the most familiar compounds, boron has the formal oxidation state +3. These include oxides, sulfides, nitrides, and halides.

<span class="mw-page-title-main">Octadecaborane</span> Chemical compound

Octadecaborane is an inorganic compound, a boron hydride cluster with chemical formula B18H22. It is a colorless flammable solid, like many higher boron hydrides. Although the compound has no practical applications, its structure is of theoretical and pedagogical interest.

John David Kennedy is a chemist and emeritus professor of inorganic chemistry at the University of Leeds. He works in the area of polyhedral borane chemistry.

<span class="mw-page-title-main">Dicarbollide</span> Chemical compound

In organometallic chemistry, a dicarbollide is an anion of the formula [C2B9H11]2-. Various isomers exist, but most common is 1,2-dicarbollide derived from ortho-carborane. These dianions function as ligands, related to the cyclopentadienyl anion. Substituted dicarbollides are also known such as [C2B9H10(pyridine)]− (pyridine bonded to B) and [C2R2B9H9]2- (R groups bonded to carbon).

<span class="mw-page-title-main">E. D. Jemmis</span> Indian theoretical chemist

Eluvathingal Devassy Jemmis is a professor of theoretical chemistry at the Indian Institute of Science, Bangalore, India. He was the founding director of Indian Institute of Science Education and Research, Thiruvananthapuram (IISER-TVM). His primary area of research is applied theoretical chemistry with emphasis on structure, bonding and reactivity, across the periodic table of the elements. Apart from many of his contributions to applied theoretical chemistry, an equivalent of the structural chemistry of carbon, as exemplified by the Huckel 4n+2 Rule, benzenoid aromatics and graphite, and tetrahedral carbon and diamond, is brought in the structural chemistry of boron by the Jemmis mno rules which relates polyhedral and macropolyhedral boranes to allotropes of boron and boron-rich solids. He has been awarded Padma Shri in Science and Engineering category by the Government of India.

In chemistry, the Jemmis mno rules represent a unified rule for predicting and systematizing structures of compounds, usually clusters. The rules involve electron counting. They were formulated by E. D. Jemmis to explain the structures of condensed polyhedral boranes such as B20H16, which are obtained by condensing polyhedral boranes by sharing a triangular face, an edge, a single vertex, or four vertices. These rules are additions and extensions to Wade's rules and polyhedral skeletal electron pair theory. The Jemmis mno rule provides the relationship between polyhedral boranes, condensed polyhedral boranes, and β-rhombohedral boron. This is similar to the relationship between benzene, condensed benzenoid aromatics, and graphite, shown by Hückel's 4n + 2 rule, as well as the relationship between tetracoordinate tetrahedral carbon compounds and diamond. The Jemmis mno rules reduce to Hückel's rule when restricted to two dimensions and reduce to Wade's rules when restricted to one polyhedron.

<span class="mw-page-title-main">Carborane acid</span> Class of chemical compounds

Carborane acidsH(CXB
11Y
5Z
6) (X, Y, Z = H, Alk, F, Cl, Br, CF3) are a class of superacids, some of which are estimated to be at least one million times stronger than 100% pure sulfuric acid in terms of their Hammett acidity function values (H0 ≤ –18) and possess computed pKa values well below –20, establishing them as some of the strongest known Brønsted acids. The best-studied example is the highly chlorinated derivative H(CHB
11Cl
11). The acidity of H(CHB
11Cl
11) was found to vastly exceed that of triflic acid, CF
3SO
3H, and bistriflimide, (CF
3SO
2)
2NH, compounds previously regarded as the strongest isolable acids.

<span class="mw-page-title-main">Azaborane</span>

Azaborane usually refers a borane cluster where BH vertices are replaced by N or NR. Like many of the related boranes, these clusters are polyhedra and can be classified as closo-, nido-, arachno-, etc..

<span class="mw-page-title-main">Dodecaborate</span> Chemical compound

The dodecaborate(12) anion, [B12H12]2−, is a borane with an icosahedral arrangement of 12 boron atoms, with each boron atom being attached to a hydrogen atom. Its symmetry is classified by the molecular point group Ih.

<span class="mw-page-title-main">1,2-Dimethyldiborane</span> Chemical compound

1,2-Dimethyldiborane is an organoboron compound with the formula [(CH3)BH2]2. Structurally, it is related to diborane, but with methyl groups replacing terminal hydrides on each boron. It is the dimer of methylborane, CH3BH2, the simplest alkylborane. 1,2-Dimethyldiborane can exist in a cis- and a trans arrangement. 1,2-Dimethyldiborane is an easily condensed, colorless gas that ignites spontaneously in air.

Heteroboranes are classes of boranes in which at least one boron atom is replaced by another elements. Like many of the related boranes, these clusters are polyhedra and are similarly classified as closo-, nido-, arachno-, and hypho-, according to the so-called electron count. Closo- represents a complete polyhedron, while nido-, arachno- and hypho- stand for polyhedrons that are missing one, two and three vertices.

<span class="mw-page-title-main">Carboryne</span>

In organoboron chemistry, a carboryne is an unstable derivative of ortho-carborane with the formula B10C2H10. They are also called 1,2-dehydro-o-carboranes. The hydrogen atoms on the C2 unit in the parent o-carborane are missing. The compound resembles and is isolobal with benzyne. 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.

<i>ortho</i>-Carborane Chemical compound

ortho-Carborane is the organoboron compound with the formula C2B10H12. The prefix ortho is derived from ortho. It is the most prominent carborane. This derivative has been considered for a wide range of applications from heat-resistant polymers to medical applications. It is a colorless solid that melts, without decomposition, at 320 °C

<span class="mw-page-title-main">Metallaborane</span>

In chemistry, a metalloborane is a compound that contains one or more metal atoms and one or more boron hydride. These compounds are related conceptually and often synthetically to the boron-hydride clusters by replacement of BHn units with metal-containing fragments. Often these metal fragments are derived from metal carbonyls or cyclopentadienyl complexes. Their structures can often be rationalized by polyhedral skeletal electron pair theory. The inventory of these compounds is large, and their structures can be quite complex.

Dicarbollylcobaltate(III) anion is a dicarbollide cluster compound containing cobaltic cation (III) as a metal center. The dicarbollylcobaltate(III) anion can be abbreviated to [COSAN]− or [CoD]−. The center cobaltic cation is sandwiched by two dicarbollide clusters, so that it can be regarded as the carboranyl version of Cp2Co+.

References

  1. 1 2 3 4 Grimes, R. N., Carboranes 3rd Ed., Elsevier, Amsterdam and New York (2016), ISBN   9780128018941.
  2. 1 2 Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. p. 181–189. ISBN   978-0-08-037941-8.
  3. ↑ The Wade–Mingos rules were first stated by Kenneth Wade in 1971 and expanded by Michael Mingos in 1972: They are sometimes known as simply "Wade's rules".
  4. ↑ W. H. Knoth (1967). "1-B9H9CH− and B11H11CH−". J. Am. Chem. Soc. 89 (5): 1274–1275. doi:10.1021/ja00981a048.
  5. ↑ Tanaka, N.; Shoji, Y.; Fukushima, T. (2016). "Convenient Route to Monocarba-closo-dodecaborate Anions". Organometallics. 35 (11): 2022–2025. doi:10.1021/acs.organomet.6b00309.
  6. ↑ Reed, Christopher A. (2010). "H+, CH3+, and R3Si+Carborane Reagents: When Triflates Fail". Accounts of Chemical Research. 43 (1): 121–128. doi:10.1021/ar900159e. PMC   2808449 . PMID   19736934.
  7. ↑ G. S. Pawley (1966). "Further Refinements of Some Rigid Boron Compounds". Acta Crystallogr. 20 (5): 631–638. Bibcode:1966AcCry..20..631P. doi:10.1107/S0365110X66001531.
  8. ↑ Štíbr, Bohumil (2018). "Recent aspects of the ten-vertex dicarbaborane chemistry". Journal of Organometallic Chemistry. 865: 4–11. doi:10.1016/j.jorganchem.2018.01.010. S2CID   102645157.
  9. ↑ Plešek, J.; Heřmánek, S.; Štíbr, B. (1983). "Potassium dodecahydro-7, 8-dicarba- nido -undecaborate(1-), k[7, 8-c 2 b 9 h 12], intermediates, stock solution, and anhydrous salt". Potassium Dodecahydro-7,8-Dicarba-nido-Undecaborate (1-), K[ 7,8-C2B9H13], Intermediates, Stock Solution, and Anhydrous Salt (PDF). Inorganic Syntheses. Vol. 22. p. 231-237. doi:10.1002/9780470132531.ch53. ISBN   978-0-471-88887-1.
  10. ↑ Jemmis, E. D. (1982). "Overlap Control and Stability of Polyhedral Molecules. Closo-Carboranes". Journal of the American Chemical Society. 104 (25): 7017–7020. doi:10.1021/ja00389a021.
  11. ↑ Spokoyny, A. M. (2013). "New Ligand Platforms Featuring Boron-Rich Clusters as Organomimetic Sbstituents". Pure and Applied Chemistry. 85 (5): 903–919. doi:10.1351/PAC-CON-13-01-13. PMC   3845684 . PMID   24311823.
  12. ↑ Sivaev, I. B.; Bregadze, V. I. (2000). "Chemistry of Nickel and Iron Bis(dicarbollides). A Review". Journal of Organometallic Chemistry. 614–615: 27–36. doi:10.1016/S0022-328X(00)00610-0.
  13. ↑ Dash, B. P.; Satapathy, R.; Swain, B. R.; Mahanta, C. S.; Jena, B. B.; Hosmane, N. S. (2017). "Cobalt Bis(dicarbollide) anion and its Derivatives". J. Organomet. Chem. 849–850: 170–194. doi:10.1016/j.jorganchem.2017.04.006.
  14. ↑ Issa, F.; Kassiou, M.; Rendina, L. M. (2011). "Boron in drug discovery: carboranes as unique pharmacophores in biologically active compounds". Chem. Rev. 111 (9): 5701–5722. doi:10.1021/cr2000866. PMID   21718011.
  15. ↑ Stockmann, Philipp; Gozzi, Marta; Kuhnert, Robert; Sárosi, Menyhárt B.; Hey-Hawkins, Evamarie (2019). "New keys for old locks: carborane-containing drugs as platforms for mechanism-based therapies". Chemical Society Reviews. 48 (13): 3497–3512. doi: 10.1039/C9CS00197B . PMID   31214680. Open Access logo PLoS transparent.svg
  16. ↑ Soloway, A. H.; Tjarks, W.; Barnum, B. A.; Rong, F.-G.; Barth, R. F.; Codogni, I. M.; Wilson, J. G. (1998). "The Chemistry of Neutron Capture Therapy". Chemical Reviews. 98 (4): 1515–1562. doi:10.1021/cr941195u. PMID   11848941.
  17. ↑ Olah, G. A.; Prakash, G. K. S.; Sommer, J.; Molnar, A. (2009). Superacid Chemistry (2nd ed.). Wiley. p.  41. ISBN   978-0-471-59668-4.
  18. ↑ Reed Christopher A (2013). "Myths about the Proton. The Nature of H+ in Condensed Media". Acc. Chem. Res. 46 (11): 2567–2575. doi:10.1021/ar400064q. PMC   3833890 . PMID   23875729.
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