In the Laboratory edited by
the microscale laboratory
Arden P. Zipp SUNY–Cortland Cortland, NY 13045
The Preparation of TlBF4 Francisco J. Arnáiz Laboratorio de Química Inorgánica, Universidad de Burgos, 09001 Burgos, Spain Thallium tetrafluoroborate is a useful reagent in chemistry labs for preparing tetrafluoroborate salts via abstraction of coordinated halide ions (1). Although many of these species can be isolated as perchlorates by reacting the appropriate parent halocompound with silver perchlorate, the explosive nature of many perchlorate salts prompted a search for substitutes (2). TlBF4 is a good alternative to AgClO4, because: •
TlBF4 can be easily isolated in an anhydrous form (3) and is soluble in a variety of solvents, which provides the means to conduct reactions in nonaqueous media.
•
The anion BF4{ has the same charge and symmetry as ClO4{, similar size, and inferior coordinating ability. In addition, BF 4{ is not oxidizing, so it is compatible with reducing cations.
•
Tl + (like Ag +) forms halides that are only slightly soluble in common solvents, but Tl+ is superior to Ag + at least in two respects: (i) Tl+ is less oxidizing than Ag+ so that many species that are oxidized with silver salts react satisfactorily with TlBF4; (ii) Tl + lacks the affinity of Ag+ for neutral C-, N-, and P-donor bases, which prevents the decomposition of labile complexes via abstraction of ligands weakly coordinated to metallic centers.
TlBF4, which is not readily available as a commercial product (4), can be prepared by treating the metal or carbonate with aqueous tetrafluoroboric acid and concentrating the resulting solution until a solid product forms (5). The procedure outlined here is more appropriate for teaching purposes because: •
It saves time because the product can be isolated without evaporating the solvent. (The entire procedure requires less than 30 minutes, provided a scrupulously dry product is not required.)
•
It involves minor amounts of toxic chemicals and wastes.
•
The equipment is simple. (Only test tubes and Pasteur pipets are required to achieve the synthesis.)
Procedure CAUTION: Thallium compounds are very toxic and may cause alopecia. Fluoroboric acid is corrosive and neurotoxic. Concentrated hydrogen peroxide produces severe burns. Gloves should be worn throughout the process. In a 12–15-mL test tube are placed 0.50 g (2.5 mmol) of Tl turnings (prepared from a Tl rod with a pencil sharpener) and 0.70 g (2.8 mmol) of aqueous 35% HBF 4 (Pasteur pipet, weigh difference). To this mixture ~ 0.20 g (2.0 mmol) of 33% H2O 2 is added drop by drop, with gentle shaking after each addition (exothermic reaction, evolution of gas). The resulting colorless solution is treated with 1 g of 2,2dimethoxypropane (endothermic reaction) and 5 g of anhydrous diethyl ether, and the mixture is stirred for 5 min.
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(Pumping the solution with a Pasteur pipet is preferable to using a spin bar, which may be ineffective with a significant amount of dense precipitate.) The white microcrystalline solid is washed by decanting with diethyl ether (3 × 3 mL) and dried in vacuo. CAUTION: To protect against implosion, the tube connected to the flexible vacuum tubing is introduced into a bigger tube or flask. The yield is 0.55 g of TlBF4 (77%). About 0.15 g more product can be recovered by adding diethyl ether to the mixture of the discarded solution and washings. Anal. calcd for TlBF4 : Tl, 70.19. Found: Tl (as TlI), 69.3. The absence of Tl(III) and/or peroxo species in the product can be easily tested by treating a small sample of the solid with some drops of 1 M acetic acid and 1 M sodium iodide solutions. Discussion The reaction is conducted in a common glass container because HBF4 , although an acid stronger that HF, does not significantly attack glass during short periods. (Fluoride ions, required to form fluorosilicate complexes, are nearly absent in pure HBF4 , and their liberation from the coordinatively saturated boron in BF4{ proceeds very slowly.) Using a test tube facilitates the workup, not only in decanting and washing the precipitate, but also in avoiding the spills produced during the addition of hydrogen peroxide. The tube may be connected directly to the vacuum line to dry the product. The standard E° value for the half-reaction Tl+(aq) + e{ → Tl(s) in acid media ({0.336 V) indicates that Tl metal can be oxidized by H+(aq) to form TlBF 4. However, the addition of a strong oxidizing agent such as hydrogen peroxide accelerates the reaction considerably. Thus a TlBF 4 solution can be prepared in less than 5 minutes. The reaction, eq 3 , may be regarded as the sum of two rapid exothermic reactions, eqs 1 and 2: Tl + 1/2 H2O2 → TlOH
(1)
TlOH + HBF4 → TlBF4 + H2O
(2)
Tl + 1/2 H2O2 + HBF4 → TlBF4 + H2O
(3)
2,2-Dimethoxypropane is added to remove water by forming organic solvents, which are miscible with diethyl ether, according to eq 4: CH3-C(OCH3)2-CH3 + H2O → CH3-CO-CH3 + 2 CH3OH (4) Upon the addition of ether a single liquid phase results, from which TlBF4 precipitates readily. In case the ketal is unavailable, trimethyl and triethyl orthoformates may be used. Furthermore, precipitation of
Journal of Chemical Education • Vol. 74 No. 11 November 1997
In the Laboratory TlBF4 is possible through repeated treatment of the resulting aqueous solution with anhydrous diethyl ether (5-g portions, 4–5 times), stirring the mixture and discarding the ethereal layer. Acknowledgment I acknowledge the helpful comments of the editor.
Literature Cited 1. Arnáiz, F. J.; Aguado, R.; Martínez de Ilarduya, J. M. Polyhedron 1994, 13, 3257. 2. Grant, G. J.; Mauldin, P. L.; Setzer, W. N. J. Chem. Educ. 1991, 68, 605. 3. Möller, H.; Lutz, H. D. Z. Crystallogr. 1992, 201, 285. 4. Taylor, M. J.; Brothers, P. J. In Chemistry of Aluminium, Gallium, Indium and Thallium; Downs, A. J., Ed.; Chapman and Hall: London, 1993; p 112. 5. Nouveau Traité de Chimie Minérale, Vol. VI; Pascal, P., Ed; Masson: Paris, 1961; p 92.
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