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New synthesis of K2[TcF6]
Polyhedron Vol. 4, No. 6, p. 1067, IVES Printed in Great Britain
0
NEW SYNTHESIS
Laboratorium
0277-5387/85 $3.00 + .OO 1985 Pcrgamon Press Ltd
OF K2pcF6]
R. ALBERT0 and G. ANDEREGG fur Anorganische Chemie, ETH Zurich, CH-8092 Zurich, Switzerland (Received 19 October 1984; accepted 9 November
Abstract-A new high yield synthesis of K,[“TcF,] 40% aqueous HF as solvent.
[“TcBrJ2-
is given. It is based on eqn (1) and uses
+ 6F- + 6Ag+ + [“TcF,]~-
The hexahalogenotechnetate(IV) anions have been known for a long time.’ Those with X = Cl, Br, I can be obtained by the reduction of pertechnetate in the corresponding aqueous hydrogen halide solutions. On the other hand, up to now K,[TcF,] was prepared by analogy with the synthesis of K2[ReF,],2 i.e., from K,[TcBr,] in a melt of KHF, (T = 22o”C), followed by an aqueous work UP.~This method is lengthy and rather complicated, if radioactive substances are used, and the yields are unsatisfactory. During the course of our work we became interested in a Tc(IV) starting material which is stable towards hydrolysis over the largest possible pH range. The hexahalogenotechnetate ions with X = Cl, Br, and I, however, are hydrolysed very rapidly in aqueous solution to TcO,. Since TcFg- fulfils these expectations, we developed a simpler and better synthesis. We find that TcFg- is formed in a good yield by reacting TcBrg- with a 40% HF solution. No hydrolysis takes place at this hydrogen ion concentration and the substitution of Br- by Fin the Tc(IV) complex is fully accomplished. After bromide ions are removed from the solution by precipitation with a silver(I) solution, a red intermediate with an absorption band at 552 nm is produced very rapidly. The decrease of the absorbancy of the solution of the red species is accompanied by a parallel increase of the amount of the [TcF,]‘produced. At room temperature the half-life of this reaction is about 110 h, at 80°C 100 min. After 24 h we obtain a solution containing [TcF,12- in 98% yield, the rest, the Tc, being in the form of TcO; . EXPERIMENTAL 0.4 g (6.09 x 10m4 mol) of K,[TcBr,] were suspended in 15 cm3 of a400/, HF solution. 3.69 mm01
1984)
+ 6AgBr
(1)
Ag+ in the form of a 1.95 M AgF solution in 40% HF were dropped to the mixture. The black precipitate, which formed initially, changed to white AgBr and the solution became intensely red in colour. After 10 hr the AgBr precipitate was filtered off, dried and weighed. It corresponded to 99% of the calculated value. 14 h later the reaction was complete. The solution was cooled to room temperature, 45 cm3 of EtOH were carefully added to the reaction mixture and on stirring K,ncF,] precipitated as a white powder (0.158 g, yield 89%).
Characterization
UV/VIS: 1= 352 nm, E = 15.9; 1= 291 mn, E = 20.4. IR : Tc-Fst 574 cm-’ ; R, = 0.42 (CH,CN : H,O = 2.5: 1 v/v). Elemental analysis
Found: K: 25.2 F: 38.3 Tc: 31.7%. Calc. for K,TcF, - H,O : K : 25.3 F : 36.9 Tc : 32.0%. “Tc was determined by liquid scintillation; K+ by complex formation with Kryptat 2.2.2 (Merck, Darmstadt), F- potentiometrically by titration with La(NO,), solutions and a LaF, electrode. Acknowledgements-We thank the Stiftung Entwicklungsfonds fiir Seltene Metalle for financial support (R.A.) of these investigations and Professor R. Gut and Dr. K. May (Radiochemie) for technical help.
REFERENCES 1. J. Dalziel, N. S. Gill, R. S. Nyholm and R. D. Peacock, J. Chem. Sot. 19%X,4012. 2. E. Weise, Z. Anorg. Allgem. Chem. 1956,283,377. 3. K. Schwochau and W. Heer, Angew. Chem. 1963,75,95.
1067
0
NEW SYNTHESIS
Laboratorium
0277-5387/85 $3.00 + .OO 1985 Pcrgamon Press Ltd
OF K2pcF6]
R. ALBERT0 and G. ANDEREGG fur Anorganische Chemie, ETH Zurich, CH-8092 Zurich, Switzerland (Received 19 October 1984; accepted 9 November
Abstract-A new high yield synthesis of K,[“TcF,] 40% aqueous HF as solvent.
[“TcBrJ2-
is given. It is based on eqn (1) and uses
+ 6F- + 6Ag+ + [“TcF,]~-
The hexahalogenotechnetate(IV) anions have been known for a long time.’ Those with X = Cl, Br, I can be obtained by the reduction of pertechnetate in the corresponding aqueous hydrogen halide solutions. On the other hand, up to now K,[TcF,] was prepared by analogy with the synthesis of K2[ReF,],2 i.e., from K,[TcBr,] in a melt of KHF, (T = 22o”C), followed by an aqueous work UP.~This method is lengthy and rather complicated, if radioactive substances are used, and the yields are unsatisfactory. During the course of our work we became interested in a Tc(IV) starting material which is stable towards hydrolysis over the largest possible pH range. The hexahalogenotechnetate ions with X = Cl, Br, and I, however, are hydrolysed very rapidly in aqueous solution to TcO,. Since TcFg- fulfils these expectations, we developed a simpler and better synthesis. We find that TcFg- is formed in a good yield by reacting TcBrg- with a 40% HF solution. No hydrolysis takes place at this hydrogen ion concentration and the substitution of Br- by Fin the Tc(IV) complex is fully accomplished. After bromide ions are removed from the solution by precipitation with a silver(I) solution, a red intermediate with an absorption band at 552 nm is produced very rapidly. The decrease of the absorbancy of the solution of the red species is accompanied by a parallel increase of the amount of the [TcF,]‘produced. At room temperature the half-life of this reaction is about 110 h, at 80°C 100 min. After 24 h we obtain a solution containing [TcF,12- in 98% yield, the rest, the Tc, being in the form of TcO; . EXPERIMENTAL 0.4 g (6.09 x 10m4 mol) of K,[TcBr,] were suspended in 15 cm3 of a400/, HF solution. 3.69 mm01
1984)
+ 6AgBr
(1)
Ag+ in the form of a 1.95 M AgF solution in 40% HF were dropped to the mixture. The black precipitate, which formed initially, changed to white AgBr and the solution became intensely red in colour. After 10 hr the AgBr precipitate was filtered off, dried and weighed. It corresponded to 99% of the calculated value. 14 h later the reaction was complete. The solution was cooled to room temperature, 45 cm3 of EtOH were carefully added to the reaction mixture and on stirring K,ncF,] precipitated as a white powder (0.158 g, yield 89%).
Characterization
UV/VIS: 1= 352 nm, E = 15.9; 1= 291 mn, E = 20.4. IR : Tc-Fst 574 cm-’ ; R, = 0.42 (CH,CN : H,O = 2.5: 1 v/v). Elemental analysis
Found: K: 25.2 F: 38.3 Tc: 31.7%. Calc. for K,TcF, - H,O : K : 25.3 F : 36.9 Tc : 32.0%. “Tc was determined by liquid scintillation; K+ by complex formation with Kryptat 2.2.2 (Merck, Darmstadt), F- potentiometrically by titration with La(NO,), solutions and a LaF, electrode. Acknowledgements-We thank the Stiftung Entwicklungsfonds fiir Seltene Metalle for financial support (R.A.) of these investigations and Professor R. Gut and Dr. K. May (Radiochemie) for technical help.
REFERENCES 1. J. Dalziel, N. S. Gill, R. S. Nyholm and R. D. Peacock, J. Chem. Sot. 19%X,4012. 2. E. Weise, Z. Anorg. Allgem. Chem. 1956,283,377. 3. K. Schwochau and W. Heer, Angew. Chem. 1963,75,95.
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