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The SF5 anion
J . l n o r g Nucl. Chem., 1963 Vol. 25, pp. 1097 to 1098. Pergamon Press Ltd. Printed in Northern Ireland
THE SF5 ANION R. TUNDER and B. SIEGEL Laboratories Division, Aerospace Corporation, E1 Segundo, California (Received 21 February 1963; in revised fbrm 14 March 1963)
Abstract--Formation of a new type of anion, SF5 , is described. Salts of this anion with simple metal cations are not stable, but the tetramethylammonium salt is reasonably stable, exhibiting a small equilibrium dissociation pressure at room temperature. The tetramethylammonium salt of SF4C1decomposes readily at this temperature. Thesesalts of pentavalent sulphur were prepared by reacting liquid SF4 with the corresponding tetramethylammonium fluoride or chloride. SALTS containing anions of the type SX 5- have not been previously reported. As a general rule the compounds in which second period elements such as sulphur exhibit maximum coordination numbers are the fluorides of these elements. Thus the SF 5anion is the most probable anion of this type that should be capable of stable existence. However, while alkali metal salts of SeF 5- have been prepared by the reaction of SeF~ with alkali fluorides, {1~ the comparable reaction with SF 4 does not proceed. Bartlett and Robinson found that SF 4 will not complex with CsF. {2} We have confirmed the latter results, finding that SF 4 will not complex with CsF or BaF 2, even at temperatures down to --40°C. However, we have also found that SF 4 will react with (CH3)4NF to form an adduct whose stoichiometry approaches unity. In many complexing reactions reported for SF4, it reacts with fluorides that are strong fluoride ion acceptors, the SF 4 transferring a fluoride ion to form salts of the SF3+ cation, c2~ However, in the case of the SF4-(CH3)4NF reaction, the opposite is clearly true and (CH3)4NF furnishes a fluoride ion to SF 4 to form (CH3)4N+SFs-. The evidence for the formation of an ionic complex when SF 4 is reacted with (CH3)4NF is substantial. Despite the magnitude of the SF 4 used, if this reagent is in excess the stoichiometry of the solid product which is formed is about 0.9 SF4.1.0 (CH3)4NF. This stoichiometry is obtained from both the increase in weight of the solid reactant and the quantity of SF a consumed. That no side reaction occurs is confirmed by the infra-red spectrum of the residual gas phase, which is simply that of SF 4. The complex can be conveniently dissociated almost quantitatively to the original reactants by heating. However at 25°C, there is only a small equilibrium pressure o f 2-3 mm of SF 4 above the solid, as verified by the infra-red spectrum of the gas, upon standing for several days at ambient temperature the equilibrium pressure did not rise above this value. At 38°C this pressure rises to 7 mm and is only 19 mm at 60°C. The complex is also stable in solution and does not decompose when dissolved in dimethylformamide. Despite the solubility in dimethylformamide the complex is not soluble in most organic solvents and reacts readily with water, nitromethane and monomethylformamide (with active hydrogens). This solubility in a solvent of high dielectric constant indicates the polarity of the complex. Further its infra-red spectrum exhibits the major absorption peaks characteristic of the (CH3)4N+ cation (identical in its halide salts). The infra-red spectrum of the complex in dimethylformamide solution ~1~E. E. AYNSLEY,R. D. PEACOCKand P. L. ROBINSON,J. Chem. Soc. 1231 (1952). ~z~N. BARTLETTand P. L. ROBINSON,J. Chem. Soc. 3417 (1961). 1097
1098
R. TUNDER and B. SIEGEL
exhibits a new p e a k at 14.8/~ b u t the p r i n c i p a l S F 4 p e a k s at 11.3-11.5 a n d 13.7 # are a b s e n t f r o m the spectrum. This shift to lower frequencies is characteristic o f anions f o r m e d b y i n c o r p o r a t i o n o f fluorine ions into the structure o f lower covalent fluorides o f second p e r i o d elements.* A l t h o u g h the c o m p l e x (CH3)4NSF 5 certainly represents a new electronic configuration for sulphur c o m p o u n d s , n o t m a n y c o m p o u n d s o f this t y p e can be p r e p a r e d . Simple m e t a l cations a p p a r e n t l y c a n n o t f o r m stable salts o f S F s - , a n d c o m p l e x cations similar to (CH3)4N + are r e q u i r e d ; substitution o f the b u l k y C6H5 g r o u p for one o f the CH3 g r o u p s in (CH3)4N+ also prevents f o r m a t i o n o f a c o m p l e x . T h e r e a c t i o n between SF4 a n d (CH3)4NC1 leads to the f o r m a t i o n o f a very w e a k c o m p l e x which d e c o m p o s e s readily at 25°C; thus the SF4C1- a n i o n is n o t very stable. Details o f the c o m p l e x i n g reactions are given below. EXPERIMENTAL
Preparations of reactants Sulphur tetrafluoride, as obtained from the Du Pont Co., contained varying amounts of SiF4 and SOF~. It was purified by a method based on a suggestion by BARTLETTand ROBINSON'~. Equivalent amounts of SF4 and BF3 were reacted in a high vacuum line to form SFs+BF4-, from which the volatile impurities were distilled. The complex was broken by addition of dry dioxane which displaces SF4 and the latter was distilled at -60°C. Purity was verified by inspection of the infra-red spectrum of the SF,. Tetramethylammonium fluoride was prepared by reacting equivalent quantities of (CH3)4NCI with KF in methanol. The resulting (CHs)4NF was isolated from the supernatant and recrystallized from isopropanol; the absence of a bifluoride impurity was verified by the absence of a bifluoride frequency in the infra-red spectrum of the product. In previous experiments the (CHs)4NF had been prepared by the reaction between (CHz)4NOH and HF but this always led to the formation of a bifluoride impurity. CeHs(CH3)sNF was prepared by neutralization of the hydroxide with HF, but in this cage the infra-red spectrum of the product indicated the absence of a bifluoride impurity.
Complexing reactions A large excess of SF4 was distilled onto approximately one gram of dry (CH3),NF. By maintaining the reaction at --40 to --20°C, the SF4 was kept in the liquid state and (CHs),NF was essentially dissolved in excess SF~. Detection of reaction was noted by distilling the excess SF, and reweighing the solid. It was observed that a gradual increase in weight of the solid reactant occurred, which ceased after four hours. At that point the increase in weight of the solid (originally (CH~)~NF) corresponded to approximately 0.9 SF4:I.0 (CH3)~NF. That was indeed the stoicheiometry of the product was further verified by independent measurements of the SF4 consumed during the reaction. The infra-red spectrum of the SF4 before and after the complexing reaction was essentially unchanged. Together with the stoicheiometric determinations this verified that the complex was simply (CH3),NF.(SF,)0.9. Under similar conditions there was no indication of complex formation between SF4 and CsF or BaFz. (CH3)4NCI was also reacted with SF4 but no indication of strong complexing occurred; some weight increase was observed but the product decomposed readily at room temperature, again releasing the SF4. A complexing reaction between SF4 and CeHs(CH~)sNF was attempted, but there was no weight increase at all. Infra-red spectra were taken in the range 2-15 #. The spectrum of the solid product was taken in a KBr pellet and in dimethylformamide solution. * In the 2-15 micron range, there is one principal frequency at 9.7 # for SiFt; t3~ In Na2SiF., the peak has shifted to 13"9/~.,4~ We have observed that [(CH3)~N)]2SiF6 also exhibits an absorption peak at 13.9 #. t3~ D. C. McKEAN, J. Chem. Phys. 24, 1002 (1956). (4) F. A. MILLERand C. H. WILLIAMS,Analyt. Chem. 24, 1253 (1952).
THE SF5 ANION R. TUNDER and B. SIEGEL Laboratories Division, Aerospace Corporation, E1 Segundo, California (Received 21 February 1963; in revised fbrm 14 March 1963)
Abstract--Formation of a new type of anion, SF5 , is described. Salts of this anion with simple metal cations are not stable, but the tetramethylammonium salt is reasonably stable, exhibiting a small equilibrium dissociation pressure at room temperature. The tetramethylammonium salt of SF4C1decomposes readily at this temperature. Thesesalts of pentavalent sulphur were prepared by reacting liquid SF4 with the corresponding tetramethylammonium fluoride or chloride. SALTS containing anions of the type SX 5- have not been previously reported. As a general rule the compounds in which second period elements such as sulphur exhibit maximum coordination numbers are the fluorides of these elements. Thus the SF 5anion is the most probable anion of this type that should be capable of stable existence. However, while alkali metal salts of SeF 5- have been prepared by the reaction of SeF~ with alkali fluorides, {1~ the comparable reaction with SF 4 does not proceed. Bartlett and Robinson found that SF 4 will not complex with CsF. {2} We have confirmed the latter results, finding that SF 4 will not complex with CsF or BaF 2, even at temperatures down to --40°C. However, we have also found that SF 4 will react with (CH3)4NF to form an adduct whose stoichiometry approaches unity. In many complexing reactions reported for SF4, it reacts with fluorides that are strong fluoride ion acceptors, the SF 4 transferring a fluoride ion to form salts of the SF3+ cation, c2~ However, in the case of the SF4-(CH3)4NF reaction, the opposite is clearly true and (CH3)4NF furnishes a fluoride ion to SF 4 to form (CH3)4N+SFs-. The evidence for the formation of an ionic complex when SF 4 is reacted with (CH3)4NF is substantial. Despite the magnitude of the SF 4 used, if this reagent is in excess the stoichiometry of the solid product which is formed is about 0.9 SF4.1.0 (CH3)4NF. This stoichiometry is obtained from both the increase in weight of the solid reactant and the quantity of SF a consumed. That no side reaction occurs is confirmed by the infra-red spectrum of the residual gas phase, which is simply that of SF 4. The complex can be conveniently dissociated almost quantitatively to the original reactants by heating. However at 25°C, there is only a small equilibrium pressure o f 2-3 mm of SF 4 above the solid, as verified by the infra-red spectrum of the gas, upon standing for several days at ambient temperature the equilibrium pressure did not rise above this value. At 38°C this pressure rises to 7 mm and is only 19 mm at 60°C. The complex is also stable in solution and does not decompose when dissolved in dimethylformamide. Despite the solubility in dimethylformamide the complex is not soluble in most organic solvents and reacts readily with water, nitromethane and monomethylformamide (with active hydrogens). This solubility in a solvent of high dielectric constant indicates the polarity of the complex. Further its infra-red spectrum exhibits the major absorption peaks characteristic of the (CH3)4N+ cation (identical in its halide salts). The infra-red spectrum of the complex in dimethylformamide solution ~1~E. E. AYNSLEY,R. D. PEACOCKand P. L. ROBINSON,J. Chem. Soc. 1231 (1952). ~z~N. BARTLETTand P. L. ROBINSON,J. Chem. Soc. 3417 (1961). 1097
1098
R. TUNDER and B. SIEGEL
exhibits a new p e a k at 14.8/~ b u t the p r i n c i p a l S F 4 p e a k s at 11.3-11.5 a n d 13.7 # are a b s e n t f r o m the spectrum. This shift to lower frequencies is characteristic o f anions f o r m e d b y i n c o r p o r a t i o n o f fluorine ions into the structure o f lower covalent fluorides o f second p e r i o d elements.* A l t h o u g h the c o m p l e x (CH3)4NSF 5 certainly represents a new electronic configuration for sulphur c o m p o u n d s , n o t m a n y c o m p o u n d s o f this t y p e can be p r e p a r e d . Simple m e t a l cations a p p a r e n t l y c a n n o t f o r m stable salts o f S F s - , a n d c o m p l e x cations similar to (CH3)4N + are r e q u i r e d ; substitution o f the b u l k y C6H5 g r o u p for one o f the CH3 g r o u p s in (CH3)4N+ also prevents f o r m a t i o n o f a c o m p l e x . T h e r e a c t i o n between SF4 a n d (CH3)4NC1 leads to the f o r m a t i o n o f a very w e a k c o m p l e x which d e c o m p o s e s readily at 25°C; thus the SF4C1- a n i o n is n o t very stable. Details o f the c o m p l e x i n g reactions are given below. EXPERIMENTAL
Preparations of reactants Sulphur tetrafluoride, as obtained from the Du Pont Co., contained varying amounts of SiF4 and SOF~. It was purified by a method based on a suggestion by BARTLETTand ROBINSON'~. Equivalent amounts of SF4 and BF3 were reacted in a high vacuum line to form SFs+BF4-, from which the volatile impurities were distilled. The complex was broken by addition of dry dioxane which displaces SF4 and the latter was distilled at -60°C. Purity was verified by inspection of the infra-red spectrum of the SF,. Tetramethylammonium fluoride was prepared by reacting equivalent quantities of (CH3)4NCI with KF in methanol. The resulting (CHs)4NF was isolated from the supernatant and recrystallized from isopropanol; the absence of a bifluoride impurity was verified by the absence of a bifluoride frequency in the infra-red spectrum of the product. In previous experiments the (CHs)4NF had been prepared by the reaction between (CHz)4NOH and HF but this always led to the formation of a bifluoride impurity. CeHs(CH3)sNF was prepared by neutralization of the hydroxide with HF, but in this cage the infra-red spectrum of the product indicated the absence of a bifluoride impurity.
Complexing reactions A large excess of SF4 was distilled onto approximately one gram of dry (CH3),NF. By maintaining the reaction at --40 to --20°C, the SF4 was kept in the liquid state and (CHs),NF was essentially dissolved in excess SF~. Detection of reaction was noted by distilling the excess SF, and reweighing the solid. It was observed that a gradual increase in weight of the solid reactant occurred, which ceased after four hours. At that point the increase in weight of the solid (originally (CH~)~NF) corresponded to approximately 0.9 SF4:I.0 (CH3)~NF. That was indeed the stoicheiometry of the product was further verified by independent measurements of the SF4 consumed during the reaction. The infra-red spectrum of the SF4 before and after the complexing reaction was essentially unchanged. Together with the stoicheiometric determinations this verified that the complex was simply (CH3),NF.(SF,)0.9. Under similar conditions there was no indication of complex formation between SF4 and CsF or BaFz. (CH3)4NCI was also reacted with SF4 but no indication of strong complexing occurred; some weight increase was observed but the product decomposed readily at room temperature, again releasing the SF4. A complexing reaction between SF4 and CeHs(CH~)sNF was attempted, but there was no weight increase at all. Infra-red spectra were taken in the range 2-15 #. The spectrum of the solid product was taken in a KBr pellet and in dimethylformamide solution. * In the 2-15 micron range, there is one principal frequency at 9.7 # for SiFt; t3~ In Na2SiF., the peak has shifted to 13"9/~.,4~ We have observed that [(CH3)~N)]2SiF6 also exhibits an absorption peak at 13.9 #. t3~ D. C. McKEAN, J. Chem. Phys. 24, 1002 (1956). (4) F. A. MILLERand C. H. WILLIAMS,Analyt. Chem. 24, 1253 (1952).