Boron hydride clusters

Boron hydride clusters are compounds with the formula B_xH_y 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.^[1]

History

The development of the borane hydride clusters resulted from pioneering work by Alfred Stock, invented the glass vacuum line for their study.^[2] The structures of the boron hydride clusters were determined beginning in 1948 with the characterization of decaborane. William Lipscomb was awarded the Nobel prize in Chemistry in 1976 for this and many subsequent crystallographic investigations. These investigations revealed the prevalence of deltahedral structures, i.e., networks of triangular arrays of BH centers.

The bonding of the clusters ushered in Polyhedral skeletal electron pair theory and Wade's rules, which can be used to predict the structures of boranes.^[3] These rules were found to describe structures of many cluster compounds.

Chemical formula and naming conventions

Borane clusters are classified as follows, where n is the number of boron atoms in a single cluster:^[1]^[4]^[5]

Cluster type	Chemical formula	Example	Notes
hypercloso-	B_nH_n		Unstable; derivatives are known^[6]
closo-	[B_nH_n]²⁻	Caesium dodecaborate
nido-	B_nH_n+4	pentaborane(9)
arachno-	B_nH_n+6	pentaborane(11)
hypho-	B_nH_n+8		Only found in adducts

The International Union of Pure and Applied Chemistry rules for systematic naming is based on a prefix denoting a class of compound, followed by the number of boron atoms and finally the number of hydrogen atoms in parentheses. Various details can be omitted if there is no ambiguity about the meaning, for example, if only one structural type is possible. Some examples of the structures are shown below.

Borane
BH₃
Diborane(6)
B₂H₆
arachno-Tetraborane(10)
B₄H₁₀
Pentaborane(9)
B₅H₉
Decaborane(14)
B₁₀H₁₄
Octadecaborane(22)
B₁₈H₂₂
iso-B₁₈H₂₂

Hexaborate(6)
[B₆H₆]²⁻
Heptaborate(7)
[B₇H₇]²⁻
Octaborate(8)
[B₈H₈]²⁻
Nonaborate(9)
[B₉H₉]²⁻
Decaborate(10)
[B₁₀H₁₀]²⁻
Undecaborate(11)
[B₁₁H₁₁]²⁻
Dodecaborate(12)
[B₁₂H₁₂]²⁻

The naming of anions is illustrated by

octahydridopentaborate, [B₅H₈]⁻

The hydrogen count is specified first followed by the boron count. The -ate suffix is applied with anions. The ionic charge value is included in the chemical formula but not as part of the systematic name.

Bonding in boranes

Boranes are nonclassically–bonded compounds, that is, there are not enough electrons to form 2-centre, 2-electron bonds between all pairs of adjacent atoms in the molecule. A description of the bonding in the larger boranes was formulated by William Lipscomb. It involved:

3-center 2-electron B-H-B hydrogen bridges
3-center 2-electron B-B-B bonds
2-center 2-electron bonds (in B-B, B-H and BH₂)

Lipscomb's methodology has largely been superseded by a molecular orbital approach. This allows the concept of multi-centre bonding to be extended. For example, in the icosahedral ion [B₁₂H₁₂]²⁻, the totally symmetric (A_g symmetry) molecular orbital is equally distributed among all 12 boron atoms. Wade's rules provide a powerful method that can be used to rationalize the structures in terms of the number of atoms and the connectivity between them.

Multicluster boranes

Structure of the conjuncto boron hydride cluster [B₁₉H₂₂]⁻.^[7]

Although relatively rare, several multi-cluster boranes have been characterized. For example, reaction of a borane cluster with B₂H₆ (as a source of BH₃) can lead to the formation of a conjuncto-borane species in which borane cluster sub-units are joined by the sharing of boron atoms.^[8]

B₆H₁₀ + "BH₃" → B₇H₁₁ + H₂

B₇H₁₁ + B₆H₁₀ → B₁₃H₁₉ + H₂

Other conjuncto-boranes, where the sub-units are joined by a B-B bond, can be made by ultra violet irradiation of nido-boranes. Some B-B coupled conjuncto-boranes can be produced using PtBr₂ as catalyst.^[9]

Analogous to Wade's Rules, electron counting scheme has been developed to predict or rationalize multicluster boranes.

Multi-cluster descriptors^[10]
Prefix	Meaning	Example
klado-	branched clusters
conjuncto-	conjoined clusters
megalo-	multiple conjoined clusters

Lewis acid/base behavior

Some function as electron donors owing to the relative basic character of the B−H_terminal groups. Boranes can function as ligands in coordination compounds.^[11] Hapticities of η¹ to η⁶ have been found, with electron donation involving bridging H atoms or donation from B-B bonds. For example, nido-B₆H₁₀ can replace ethene in Zeise's salt to produce trans-Pt(η²-B₆H₁₀)Cl₂.^[11]

They can also act as Lewis acids, with concomitant opening of the cluster. An example involving trimethylphosphine:

B₅H₉ + 2 P(CH₃)₃ → B₅H₉·2P(CH₃)₃

Brønsted acid/base behavior

Some higher boranes, especially those with bridging hydrogen atoms, can be deprotonated with a strong base. An example:

B₅H₉ + NaH → Na[B₅H₈] + H₂

Acidity increases with the size of the borane, with B₁₀H₁₄ having a pK_a value of 2.7:^[12]

B₅H₉ < B₆H₁₀ < B₁₀H₁₄ < B₁₆H₂₀ < B₁₈H₂₂

In general, bridging hydrogen protons tend to be lost before terminal ones.^[13]

Aufbau reactions

Structure of [(CH₃)₄N⁺]₂[Fe(C₂B₉H₁₁)₂]⁺, showing only one Me₄N⁺.^[14]

For the boron hydride chemist, one of the most important reactions is the building up process by which smaller boron hydride clusters add borane to give larger clusters. This approach also applies to the synthesis of metallaboranes,

Hydroboration

Reminiscent of the behavior of diborane and its adducts, higher boranes participate in hydroboration. When boron hydrides add an alkyne, the carbon becomes incorporated into the cluster, producing carboranes, e.g. C₂B₁₀H₁₂.^[15]

Applications

Some cobalt derivatives of carboranes have been commercialized for sequestering ¹³⁷Cs from radioactive waste.^[16]

Boranes have a high specific energy of combustion compared to hydrocarbons, making them potentially attractive as fuels or igniters. Intense research was carried out in the 1950s into their use as jet fuel additives, but the effort did not lead to practical results.

Aspirational uses

Because ¹⁰B has a very high neutron-capture cross section, boron-hydride derivatives have often been investigated for applications in Neutron capture therapy of cancer.^[17]

¹⁰B + ¹n → (¹¹B*) → ⁴He + ⁷Li + γ (2.4 Mev)

References

^ ^a ^b Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8. pp 151-195
^ Stock, Alfred (1933). The Hydrides of Boron and Silicon. New York: Cornell University Press.
^ Fox, Mark A.; Wade, Ken (2003). "Evolving patterns in boron cluster chemistry" (PDF). Pure Appl. Chem. 75 (9): 1315–1323. doi:10.1351/pac200375091315. S2CID 98202127.
^ Cotton, F. Albert; Wilkinson, Geoffrey; Murillo, Carlos A.; Bochmann, Manfred (1999), Advanced Inorganic Chemistry (6th ed.), New York: Wiley-Interscience, ISBN 0-471-19957-5
^ Lipscomb W. N. Boron Hydrides. Benjamin, New York (1963).
^ Peymann, Toralf; Knobler, Carolyn B.; Khan, Saeed I.; Hawthorne, M. Frederick (2001). "Dodeca(benzyloxy)dodecaborane, B₁₂(OCH₂Ph)₁₂: A Stable Derivative of hypercloso-B₁₂H₁₂". Angew. Chem. Int. Ed. 40 (9): 1664–1667. doi:10.1002/1521-3773(20010504)40:9<1664::AID-ANIE16640>3.0.CO;2-O. PMID 11353472.
^ Londesborough, Michael G. S.; Bould, Jonathan; Baše, Tomáš; Hnyk, Drahomír; Bakardjiev, Mario; Holub, Josef; Císařová, Ivana; Kennedy, John D. (2010). "An Experimental Solution to the "Missing Hydrogens" Question Surrounding the Macropolyhedral 19-Vertex Boron Hydride Monoanion [B19H22]−, a Simplification of its Synthesis, and its Use as an Intermediate in the First Example of syn-B18H22 to anti-B18H22 Isomer Conversion". Inorganic Chemistry. 49 (9): 4092–4098. doi:10.1021/ic901976y. PMID 20349936.
^ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8. p. 162
^ Sneddon, L.G. (2009). "Transition metal promoted reactions of polyhedral boranes and carboranes". Pure and Applied Chemistry. 59 (7): 837–846. doi:10.1351/pac198759070837. S2CID 55817512.
^ Bould, Jonathan; Clegg, William; Teat, Simon J.; Barton, Lawrence; Rath, Nigam P.; Thornton-Pett, Mark; Kennedy, John D. (1999). "An approach to megalo-boranes. Mixed and multiple cluster fusions involving iridaborane and platinaborane cluster compounds. Crystal structure determinations by conventional and synchrotron methods". Inorganica Chimica Acta. 289 (1–2): 95–124. doi:10.1016/S0020-1693(99)00071-7.
^ ^a ^b Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. p. 177, "The concept of boranes as ligands". ISBN 978-0-08-037941-8.
^ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8. p. 171
^ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
^ Kang, H. C.; Lee, S. S.; Knobler, C. B.; Hawthorne, M. F. (1991). "Syntheses of Charge-Compensated Dicarbollide Ligand Precursors and Their Use in the Preparation of Novel Metallacarboranes". Inorganic Chemistry. 30 (9): 2024–2031. doi:10.1021/ic00009a015.
^ 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.
^ Chaudhury, Sanhita; Bhattacharyya, Arunasis; Goswami, Asok (2014). "Electrodriven Selective Transport of Cs+ Using Chlorinated Cobalt Dicarbollide in Polymer Inclusion Membrane: A Novel Approach for Cesium Removal from Simulated Nuclear Waste Solution". Environmental Science & Technology. 48 (21): 12994–13000. Bibcode:2014EnST...4812994C. doi:10.1021/es503667j. PMID 25299942.
^ Sauerwein, Wolfgang; Wittig, Andrea; Moss, Raymond; Nakagawa, Yoshinobu (2012). Neutron Capture Therapy. Berlin: Springer. doi:10.1007/978-3-642-31334-9. ISBN 978-3-642-31333-2.

[G&E-1] Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8. pp 151-195

[2] Stock, Alfred (1933). The Hydrides of Boron and Silicon. New York: Cornell University Press.

[3] Fox, Mark A.; Wade, Ken (2003). "Evolving patterns in boron cluster chemistry" (PDF). Pure Appl. Chem. 75 (9): 1315–1323. doi:10.1351/pac200375091315. S2CID 98202127.

[4] Cotton, F. Albert; Wilkinson, Geoffrey; Murillo, Carlos A.; Bochmann, Manfred (1999), Advanced Inorganic Chemistry (6th ed.), New York: Wiley-Interscience, ISBN 0-471-19957-5

[5] Lipscomb W. N. Boron Hydrides. Benjamin, New York (1963).

[6] Peymann, Toralf; Knobler, Carolyn B.; Khan, Saeed I.; Hawthorne, M. Frederick (2001). "Dodeca(benzyloxy)dodecaborane, B₁₂(OCH₂Ph)₁₂: A Stable Derivative of hypercloso-B₁₂H₁₂". Angew. Chem. Int. Ed. 40 (9): 1664–1667. doi:10.1002/1521-3773(20010504)40:9<1664::AID-ANIE16640>3.0.CO;2-O. PMID 11353472.

[7] Londesborough, Michael G. S.; Bould, Jonathan; Baše, Tomáš; Hnyk, Drahomír; Bakardjiev, Mario; Holub, Josef; Císařová, Ivana; Kennedy, John D. (2010). "An Experimental Solution to the "Missing Hydrogens" Question Surrounding the Macropolyhedral 19-Vertex Boron Hydride Monoanion [B19H22]−, a Simplification of its Synthesis, and its Use as an Intermediate in the First Example of syn-B18H22 to anti-B18H22 Isomer Conversion". Inorganic Chemistry. 49 (9): 4092–4098. doi:10.1021/ic901976y. PMID 20349936.

[8] Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8. p. 162

[9] Sneddon, L.G. (2009). "Transition metal promoted reactions of polyhedral boranes and carboranes". Pure and Applied Chemistry. 59 (7): 837–846. doi:10.1351/pac198759070837. S2CID 55817512.

[10] Bould, Jonathan; Clegg, William; Teat, Simon J.; Barton, Lawrence; Rath, Nigam P.; Thornton-Pett, Mark; Kennedy, John D. (1999). "An approach to megalo-boranes. Mixed and multiple cluster fusions involving iridaborane and platinaborane cluster compounds. Crystal structure determinations by conventional and synchrotron methods". Inorganica Chimica Acta. 289 (1–2): 95–124. doi:10.1016/S0020-1693(99)00071-7.

[BoranesAsLigands-11] Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. p. 177, "The concept of boranes as ligands". ISBN 978-0-08-037941-8.

[12] Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8. p. 171

[13] Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.

[14] Kang, H. C.; Lee, S. S.; Knobler, C. B.; Hawthorne, M. F. (1991). "Syntheses of Charge-Compensated Dicarbollide Ligand Precursors and Their Use in the Preparation of Novel Metallacarboranes". Inorganic Chemistry. 30 (9): 2024–2031. doi:10.1021/ic00009a015.

[15] 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.

[16] Chaudhury, Sanhita; Bhattacharyya, Arunasis; Goswami, Asok (2014). "Electrodriven Selective Transport of Cs+ Using Chlorinated Cobalt Dicarbollide in Polymer Inclusion Membrane: A Novel Approach for Cesium Removal from Simulated Nuclear Waste Solution". Environmental Science & Technology. 48 (21): 12994–13000. Bibcode:2014EnST...4812994C. doi:10.1021/es503667j. PMID 25299942.

[17] Sauerwein, Wolfgang; Wittig, Andrea; Moss, Raymond; Nakagawa, Yoshinobu (2012). Neutron Capture Therapy. Berlin: Springer. doi:10.1007/978-3-642-31334-9. ISBN 978-3-642-31333-2.

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