Carborane

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Ball-and-stick model of o-carborane

Carboranes 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]

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[edit]

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-, etc., 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.

Isomers[edit]

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[edit]

Carboranes have been prepared by many routes, the most common being addition of alkynyl reagents to boron hydride clusters to form dicarbon carboranes. For example, the high-temperature reaction of pentaborane(9) with acetylene affords several closo-carboranes as well as other products:

nido-B5H9 + C2H2 closo-1,5-C2B3H5, closo-1,6-C2B4H6, 2,4-C2B5H7

When the reaction is conducted at lower temperatures, an open-cage carborane is obtained:

nido-B5H9 + C2H2 nido-2,4-C2B4H8

Other procedures generate carboranes containing three or four cage carbon atoms (refs. [1],[4],[5][clarify])

Monocarba derivatives[edit]

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[edit]

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[7]

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[edit]

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

Small, open carboranes[edit]

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 Na[1,2-C2B4H7].

Small, closed carboranes[edit]

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 this five-vertex cage.

Intermediate-sized carboranes[edit]

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

This family of clusters includes the closo cages C2B6H8, C2B7H9, C2B8H10, and C2B9H11.

Isomerism is well established in this family: 1,2- and 1,6-C2B6H8, and 2,3- and 2,4-C2B5H7, as well as in open-cage carboranes such as 2,3- and 2,4-C2B4H8 and 1,2 and 1,3-C2B9H13. In general, isomers having non-adjacent cage carbon atoms are more thermally stable than those with adjacent carbons, so that heating tends to induce mutual separation of the carbon atoms in the framework.

Carboranes of intermediate nuclearity are most efficiently generated by degradations starting with carborane as summarized in these equations, starting from C2B9H12:

C2B9H12 + H+ → C2B9H13
C2B9H13 → C2B9H11 + H2

In contrast, smaller carboranes are usually prepared by building-up routes, e.g. from pentaborane + alkyne.

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

Icosahedral carboranes[edit]

The icosahedral charge-neutral closo-carboranes, 1,2-, 1,7-, and 1,12-C2B10H12 (respectively ortho-, meta-, and para-carborane in the informal nomenclature) are particularly stable and are commercially available.[8][9] The 1,2-isomer forms first upon the reaction of decarborane and acetylene. It converts quantitatively to the 1,7-isomer upon heating in an inert atmosphere. The formation of the 1,2- to 1,12-isomer requires 700 °C, proceeding in ca. 25% yield.[1]

[CB11H12] is also well established.

Reactions[edit]

Base-induced degradation of carboranes give anionic nido derivatives, which can 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 M[C2B9H11]2-.[10]

Research[edit]

Dicarbollide complexes have been evaluated for many applications 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.[11]

The medical applications of carboranes have been explored.[12][13] C-functionalized carboranes represent a source of boron for boron neutron capture therapy.[14]

The compound H(CHB11Cl11) is a superacid, forming an isolable salt with protonated benzene, C6H+
7
.[15] It protonates fullerene, C60.[16]

See also[edit]

References[edit]

  1. ^ a b c d Grimes, R. N., Carboranes 3rd Ed., Elsevier, Amsterdam and New York (2016), ISBN 9780128018941.
  2. ^ a b 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.{{cite journal}}: CS1 maint: uses authors parameter (link)
  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. doi:10.1107/S0365110X66001531.
  8. ^ 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.
  9. ^ Spokoyny, A. M. (2013). "New ligand platforms featuring boron-rich clusters as organomimetic substituents". Pure and Applied Chemistry. 85 (5): 903–919. doi:10.1351/PAC-CON-13-01-13. PMC 3845684. PMID 24311823.
  10. ^ 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.{{cite journal}}: CS1 maint: uses authors parameter (link)
  11. ^ 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.{{cite journal}}: CS1 maint: uses authors parameter (link)
  12. ^ 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.{{cite journal}}: CS1 maint: uses authors parameter (link)
  13. ^ 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
  14. ^ 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.
  15. ^ 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.
  16. ^ 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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