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Tetrahydroborates

Tetrahydroborates, [BH₄]⁻, are a class of complex hydrides containing boron and hydrogen. Tetrahydroborates are an extensively used reagent for both organic and inorganic synthesis.^[1] They have the ability to reduce carbonyls with bridging hydride ligands^[1] and form unusual covalent complexes with several transition metals^[1]. Tetrahydroborates are used as reducing agents, starting compounds for the synthesis of organometallic derivatives, precursors for the production of borides, hydrides, as well as other inorganic materials, and catalysts for hydrogenation, isomreisation, oligomerisation, and polymerisation.^[2]

Tetrahydroborates

General
Systematic name	Tetrahydroborate Ion
Molecular formula	BH₄^-
Molar mass	14.84 g/mol
Structure
Molecular Shape	Tetrahedral (T_d)
Coordination	[Ion, One, Two, or Three Bridges (η¹, η², or η³)]
Related compounds
Common BH₄ Complexes	TiBH₄ Zr(BH₄)₄ Hf(BH₄)₄ Ti(BH₄)₃(Et₂O)
Except where noted otherwise, data are given for materials in their standard state (at 25°C, 100 kPa) Infobox disclaimer and references

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1 History
2 Structure
3 Physical Properties
4 Uses
5 Preparation
6 Decomposition
7 References

History

The first details of pure alkali metal tetrahydroborates appeared in the literature in 1940 by Schlesinger and Brown. They synthesized lithium tetrahydroborate (Li[BH₄]) by means of a reaction with ethyl lithium and diborane (B₂H₆).^[3]

The equation Schlesinger and Brown used to produce high yields of tetrahydroborates in etheral solvents under suitable conditions was:

2MH + B₂H₆ → 2M[BH₄] (M = Li, Na, K, etc.)^[4]

Structure

Tetrahydroborates bond in the tetrahedral structure with boron in the center and the hydrogens located at the four corners.^[5] The structure of the tetrahydroborates occurs through bridging hydrogen atoms.^[6] The hydrogens can bind to the metal in three different orientations: monodentate, one bond; bidentate, two bonds; and tridentate, three bonds.^[7] (η¹, η², or η³)^[2] There is an interconversion between the three different orientations. The preferred coordination mode is strongly affected by the nature of the metal and its oxidation state.^[1]

Physical Properties

The properties of M[BH₄] are strongly determined by the difference in electronegativity of the metal cation and the boron anion.^[5] It is critical that the negative charge is localized on the boron atom in order for the anion to be stable.^[5]

Alkali Metal Tetrahydroborates: Ionic, white, crystalline, high-melting-point solids, sensitive to moisture, not sensitive to oxygen^[5]

Group 3 and Transition Metal Tetrahydroborates: Covalently bonded; liquids or sublimable solids^[5]

Alkaline Earth Tetrahydroborates: Intermediate between ionic and covalent^[5]

Uses

Catalysis

Tetrahydroborates have been found to be effective catalysts for polymerization, oligomerization, and hydrogenation of olefins.^[1]

Homogeneous Catalysis

Reduce the central metal to a lower oxidation state allowing the metal to undergo oxidative addition reactions.^[1]
Provide a source of hydrogen.^[1]
Provide a coordination sphere for the metal atom using fluctations of the bonding mode.^[1]
Activation ligand for other ligands bound to that metal.^[1]

Hydrogen Storage Materials

Tetrahydroborates have the potential to store hydrogen for mobile applications because of their large gravimetric and volumetric hydrogen density for hydrogen.^[5] Hydrogen is difficult to transport and store because of it is gas at room temperature. In order to transport and store hydrogen, chemical combinations with metals must be made. Tetrahydroborates offer a means of transport and storage of hydrogen because they have the highest gravimetric hydrogen density known today.^[5]

CVD Precursors

CVD is chemical vapor deposition. Transition metal tetrahydroborates are volatile and thermolyze under mild condition giving hydrogen and diborane.^[8] This characteristic of tetrahydroborates makes them excellent precursors for CVD.^[8] Zirconium and hafnium diborides, ZrB₂ and HfB₂, can be prepared through CVD of the tetrahydroborate, Zr(BH₄)₄ and Hf(BH₄)₄.^[8]

M(BH₄)₄ → MB₂ + B₂H₆ + 5H₂^[9]

The diboride, prepared by CVD, are used as coatings because of their hardness, high melting point, strength, resistance to wear and corrosion, and good electrical conductivity.^[8] The diboride films produced can be used on a variety of substances including glass, copper, aluminum, and steel.^[8]

Preparation

Example of one preparation of a tetrahydroborate complex:

TiCl₄ + LiBH₄ → Ti(BH₄)₃(Et₂O) in diethyl ether^[10]

Decomposition

Examples of decomposition of tetrahydroborates:

M(BH₄)₄ → MB₂ + B₂H₆ + 5H₂^[9]
M(BH₄) → MB + 2H₂^[11]
M(BH₄)₂ → MB₂ + 4H₂^[11]

References

^ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ Barone, V.; Dolcetti, G.; Lelj, F.; Russo, N. Inorg. Chem. 1981, 20, 1687–1691.
^ ^a ^b Makheav, V.D. Russ. Chem. Rev. 2000, 69, 727-746.
^ Schlesinger, H.C.; Brown, H.R. J. Am. Chem. Soc. 1940, 62, 3429-3435.
^ Schlesinger, H.C.; Brown, H.R.; Hoekstra, L.R. J. Am. Chem. Soc. 1953, 75, 199–204.
^ ^a ^b ^c ^d ^e ^f ^g ^h Zuttel, A.; Borgschulte, A.; Orimo, S. Scripta Materialia 2007, 56, 823–828.
^ Kaesz, H.D. Chem. Br. 1973, 9, 344.
^ Marks, T.J.; Kolb, J.R. Chem. Rev. 1977, 77, 263.
^ ^a ^b ^c ^d ^e Jensen, J.A.; Gozum, J.E.; Pollina, D.M.; Girolami, G.S. J. Am. Chem. Soc. 1988, 110, 1643–1644.
^ ^a ^b James, B.D.; Smith, B.E. Synth. React. Inorg. Metal-Org. Chem. 1974, 4, 461–465.
^ Jenson, J.A.; Wilson, S.R.; Girolami, G.S. J. Am. Chem. Soc. 1988, 110, 4977–4982.
^ ^a ^b Dilts, J.A.; Ashby, E.C. Inorg. Chem. 1972, 11, 1230.

Categories: Hydrides | Anions

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Tetrahydroborates". A list of authors is available in Wikipedia.