Chapter 2 Organometallic Compounds of the Group 13 Elements B, Al, Ga, In, Tl

Norbert Auner

Institut für Chemie der Humboldt-Universität zu Berlin, Hessische Straße 1–2, D-10115 Berlin, Germany

2.1 Introduction

2.2 Boron Compounds

2.3 Aluminum Compounds

2.4 Gallium Compounds

2.5 Indium Compounds

2.6 Thallium Compounds

2.1 Introduction

Although the chemistry of the five elements in group 13 of the periodic table reveals more differences than similarities — this will be illustrated in the present chapter by the selected examples for the preparation of numerous, very different compounds — the chemistry of boron and its heavier congeners does indeed include a multitude of interesting substances exhibiting quite divergent chemical behavior and properties.

Each of the five elements in question — boron, aluminum, gallium, indium, and thallium — possesses an electronic ground state of three valence electrons with the configuration ns2np1. This is illustrated, for example, by the following similarities between the five elements: the dominance of the +3 oxidation state and the acceptor properties which mainly govern the structures of the derivatives.

Apart from these similarities, the chemistry of group 13 elements seems to be more characterized by the wide variety of structural and chemical non-conformity, as is already apparent from the elements themselves. Elemental boron, which occupies a position at the metal/non-metal frontier in the periodic table, exhibits mainly non-metallic properties such as strong interatomic bonds, high melting and sublimation temperatures, and a low electrical conductivity which increases with increasing temperature. The subsequent elements from aluminum to thallium are all relatively soft metals with the typical metallic shine, considerably higher conductivities for heat and electricity, and lower melting and boiling temperatures. The low melting point of gallium (29.78 °C) and the wide temperature range of liquid gallium (bp 2420 °C) are particularly worthy of mention.

Like its horizontal and diagonal neighbors, carbon and silicon in group 14, boron has a pronounced tendency to form covalent molecular compounds. On account of their acceptor character resulting from the above-mentioned valence shell structure, boron compounds often reveal electron-deficient, multicentered bonding.

The heavier congeners possess higher chemical reactivities at elevated temperatures in comparison with boron which has to be heated to much higher temperatures before reacting readily with nitrogen, oxygen, and most metals. Volatile hydrides or cluster compounds of aluminum, gallium, indium, and thallium are unknown in contrast to the numerous boranes and carbaboranes known today.

Aluminum, the most abundant metal in the lithosphere, is widely used either as the metal itself (in the industrialized countries, aluminum is second only to iron in its usage) or in the form of numerous compounds of importance in daily life and representing valuable tools in the hands of the chemist, for example alkylaluminum derivatives.

Recently, gallium and indium have attained considerable importance as sources of semiconductors such as gallium arsenide and indium antimonide. On account of the transmission of long wavelength light, thallium halides are used in several special infrared techniques. Aqueous solutions of thallium formate and malonate (Clerici’s solution) have been used for small-scale mineral separation on account of their high density of approximately 4.32 g · cm−3.

The objective of this chapter is to present some selected examples of detailed and reliable synthetic procedures for group 13 compounds already well known as valuable intermediates or the subjects of current research work. This combination of old and proven procedures which have mostly already been published in the previous German editions of this handbook — and hence successfully performed by countless students in the past — with new, competent results from ongoing investigations of group 13 compounds has, of necessity, resulted in a somewhat inhomogeneous presentation of the contributions. Furthermore, it has not been possible to cover the entire spectrum of the interesting compounds of these elements in the space available.

In the selection process, we have placed great importance on the inclusion of a broad range of experimental techniques and methods: this chapter thus contains both one-pot and multistage syntheses, simple addition/stirring/filtration/distillation procedures can be found together with more sophisticated anaerobic preparative methods. Thus, the presented procedures will be of interest and use for undergraduate student courses as well as for graduate and postgraduate research work.

2.2 Boron Compounds

Translations and adaptations from H.-J. Becher, Chapter 13, in G. Brauer (ed.), Handbuch der Präparativen Anorganischen Chemie, pp. 787–819, Vol. 2, 3rd edn., Ferdinand Enke Verlag, Stuttgart, 1978, are indicated by the symbol # after the name of the compound.

Detailed procedures for the following compounds containing boron are given in ▶ Chapter 3 — “Commonly Used Starting Materials” compiled by W. A. Herrmann and C. Zybill in Volume 1 of this series:

• [bis(diisopropylamino)boryl]diazomethane (▶ p. 69)
• tetra-n-butylammonium tetrafluoroborate (▶ p. 81)
• trimethyloxonium tetrafluoroborate, triethyloxonium tetrafluoroborate (▶ p. 71)

Aluminum Boride #,1–3 — AlB12

Procedure

B2O3 (50 g), sulfur (75 g), and aluminum (100 g) are mixed and filled in a crucible of fire-clay. The reaction is initiated by ignition of a mixture of magnesium powder (7 g) and barium peroxide (BaO2, 5 g). When the mixture has cooled down, it is removed from the crucible, crushed to small pieces, and boiled in water until all Al2S3 has been hydrolyzed. The solid is then separated and the regulus particles are picked out. These particles are cleaned mechanically and heated with concentrated HCl until a black, glittering residue of AlB12 (α-modification) is obtained.

In the presence of powdered carbon (or silicon), the reaction yields β-AlB12. The carbon content of the product can reach an amount corresponding to the formula Al3C2B48. For the preparation, aluminum (130 g), sulfur (75 g), B2O3 (60 g) and graphite powder (2.5 g) are mixed and reacted in the same manner as described for α-AlB12.

Properties

α-AlB12 crystallizes in a tetragonal lattice (a = 1016, c = 1428 pm), density: 2.56 g · cm−3. β-AlB12 crystallizes in an orthorhombic lattice (a = 1234, b = 1263, c = 508 pm or a = 617, b= 1263, c = 1016 pm). β-AlB12 can be used for the preparation of other borides with well-defined crystal structures.

References

1 J. Kohn, G. Katz, A. A. Giardini, Z. Kristallogr. 111, 53 (1958).

2 V. I. Matkovich, J. Economy, R. F. Giese, J. Am. Chem. Soc. 86, 2337 (1964).

3 H. J. Becher, H. Neidhard, Acta Cryst. B 24, 280 (1968).

Diborane#,1–3 — B2H6

The synthesis of B2H6 from the reaction of lithium hydride with boron trifluoride diethyl etherate is fully described in the literature. In this section two alternative procedures are described.

3 LiAlH4 + 4 BF3 · Et2O → 2 B2H6 + 3 AlF3 + 3 LiF + 4 Et2O

Procedure

The synthesis is performed in the apparatus shown in ▶ Figure 2.1.

▶ Figure 2.1 Preparation of diborane: a = reaction flask with electromagnetic stirring; b = cold trap; f1 – f3: cold traps, sizes chosen according to the expected amount of diborane; v1 – v3 = mercury pressure release valves, tube height ~ 900 mm.

The apparatus is carefully evacuated and dried. Then it is filled with dry nitrogen and the stopcock between the reaction vessel and the trap system is closed. In a stream of nitrogen, the flask is charged with lithium aluminum hydride (8.5 g) and 120 mL of absolute diethyl ether and closed with a glass stopper. The dropping funnel is charged with 140 mL of freshly distilled boron trifluoride etherate. The cold finger and the trap f1 are cooled to −78 °C, the traps f2 and f3 to −196 °C. The stopcock between vessel and trap system is opened. In a continuous, slow stream of nitrogen the BF3 · Et2O is added dropwise to the vessel (first very slowly, after a while faster). Because of the heat of the reaction, which is high at the beginning of the addition, the flask is cooled with dry ice. After some time cooling is no longer necessary. The BF3 · Et2O is added during 5 – 6 h. After the addition has been completed a stream of nitrogen is passed through the apparatus and the stopcock between the vessel and the trap system is closed again. Volatile materials, which do not condense in the cooled traps are removed under vacuum....