[acs symposium series] inorganic fluorine chemistry volume 555 (toward the 21st century) ||...

Chapter 9

Fluoro-Sulfur Anions Transition States, Intermediates, and Reagents

R. Mews

Institut für Anorganische und Physikalische Chemie, Universität Bremen, Leobenerstrasse NW2, D-28334 Bremen, Germany

The addition of fluoride ions to lower coordinated sulfur derivatives and sulfur-nitrogen species with SN double bonds leads to a number of unusual fluoro anions. The stability of those anions depends on the fluoride ion donor (CsF, TASF); especially with TAS + [(Me2N)3S+] as a counterion, isolable anions are generated, which might be regarded as stabilized transition states of SN2 reactions. A second approach to NS anions is the Si - Ν bond cleavage of silicon-nitrogen-sulfur derivatives. Useful reagents for further syntheses are generated from this approach.

Fluorinated sulfur anions and cations were often postulated as intermediates in sulfur chemistry and systematic investigations to generate, stabilize, and isolate these reaction intermediates were started in our group some time ago (7). With the availability of new and more effective fluoride ion donors, e.g. (Me2N)3S+Me3SiF2~ (TASF) (2), Me4N+F" (3), or [(Me2N)3P]2N+F- (4), much progress has been achieved in the chemistry of fluorinated anions of the maingroup elements, especially during the last few years.

Fluorosulfuranide Anions Sulfur tetrafluoride is known to act as both a fluoride ion donor and acceptor. Bartlett and Robinson were the first to show that fluoro Lewis acids abstract F" under formation of the trifluorosulfonium ion SF34" (5), similar results were obtained independently by Seel and Detmer (6). Structure determinations were reported later by Bartlett's group (7).

Fluoride ion acceptor properties of SF4 were first suggested by Tullock et al. (8); Christe and coworkers established the existence of the SF5 ' ion by IR spectroscopy (9), and Seppelt et al. recently succeeded in solving the crystal structure of Rb+SF5" (JO).

0097-6156/94/0555-0148$08.00/0 © 1994 American Chemical Society

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In Inorganic Fluorine Chemistry; Thrasher, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

9. MEWS Fluoro-Sulfur Anions 149

F

F

F ,

Λ F

Ί ι 1 i l '

s

These intrinsic donor-acceptor properties of SF4 should lead to intermolecular interactions in the condensed phase, resulting either in self-ionization or association. For the liquid state, several models are discussed in the literature. Seel and Gombler concluded from NMR-investigations that SF4 forms dimers or polymers with axial fluorines acting as bridges (77)

F F,, Ι

F

F

These associates are too unstable to be isolated; therefore, no structural information is available. Perfluoro alkyl derivatives, e.g., CF3SF3, show similar behavior (77).

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150 INORGANIC FLUORINE CHEMISTRY: TOWARD THE 21ST CENTURY

In α , ω -bis(trifluorosulfhr)alkanes F3S-(CF2-)nSF3 a comparable, but intramolecular interaction between the two SF3 groups might be expected, even in the gas phase. This interaction should depend on the number, n, of bridging atoms. The first member of this series, F3S-CF2-SF3 (12-14), is easily prepared by direct fluorination of CS2 at low temperature and low pressure (14):

The gas phase structure of F2C(SF3)2 (Fig. 1) was determined by electron diffraction (75). The two SF3-groups show the expected pseudo trigonal bipyramidal structure with a difference in the bond lengths of 10 pm between the axial [d(SFa) = 166.4 (4) pm] and the equatorial bonds [d(SFe) = 156.2 (4) pm]. Within the limits of experimental error they are the same as in F3CSF3 [d(SFe) = 156.5 (8) pm; d(SFa) = 165.5 (5)] (16). Especially interesting in the structure of F2C(SF3)2 is the small SCS angle of 108.2 (5)°; this results in a very short non bonded contact Fi—S' (Fι/·-S) = 266 pm between a sulfur and one axial fluorine of the opposite SF3-group. As can be seen from the Newman projection in Figure 1, the sulfur centers are attacked by these bridging fluorines in the equatorial plane, as in the model suggested from the NMR data of liquid SF4 or CF3SF3 (77).

Similar to SF4 CF3SF3 acts as a fluoride ion donor towards strong Lewis acids (ASF5, SbF5) (77,75). Under the same conditions, F2C(SF3)2 forms only monocations even with a large excess of Lewis acid, due to a strong interaction of the two sulfur centers (19).

The classical fluoride ion donor in fluorine chemistry is CsF; however, for the generation and stabilization of anions, ionic fluorides with large organic counterions, e.g., Me4N + (3) or [(Me2N)3P]2N+ (4) seem to be more useful. In "TAS-fluoride" (Me2N)3S+Me3SiF2'(2) the fluoride is stabilized as the Me3SiF2" anion. Due to the very weak SiF bonds [d(SiF) = 176 pm] it is an extremely useful fluoridating agent (20). Fluoride ions will be transferred to all acceptors better than Me3SiF. In the investigation described herein, CsF and TASF were used exclusively. Both reagents form only monoanions with F2C(SF3)2 (21):

183 Κ CS 2 + 4 F 2 > F 2 C(SF 3 ) 2

30 mbar

,SF3 + CsF Cs® SF3 Θ

F 2C \

SF3 TASF®

F 2C

+ TASF

According to the X-ray structure determination (Fig. 2), the two sulfur centers are symmetrically bridged by a fluoride ion; this distance d(SFfor) = 211,7 (1) pm is much longer than d(SF(2))/d(SF(4)) = 170,0(2) pm and d (SF(3)) = 160,7(2) pm

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Figure l.(a) Gas phase structure of F2C(SF3)2 (15) (b) Newman projection along one CS bond (c) Newman projection along the S...S' direction

Figure 2. X-Ray structure of the fF2C(SF3)2F]--anion (21).

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Despite the almost ideal pseudo square pyramidal geometry (according to the bond angles around the sulfur atoms), the difference in SF bond lengths between "axial" and "equatorial" bonds in the anion is similar to that in the parent neutral molecule. Here also the incoming (bridging) fluoride attacks the pseudo trigonal bipyramidal centers in the equatorial plane, almost exactly bisecting the "C(l) -S- lone pair" angle.

From the direct fluorination of (CF2SC1)2 (22) l,2-bis(tri-fluorosu!fur)perfluoroethane was isolated in 83% yield (23):

C1S-CF 2-CF 2-SC1 + 3 F 2 F 3 S - C F 2 - C F 2 - S F 3 + C l 2

BF 3 /SO2

S—CF 2 —CF 2 —S' V y

+H20/-2HF

S-CF 2 —CF 2 —SF 3

When compared to the methane derivative, the ethane derivative is much more sensitive against moisture. Since the mutual interaction of the SF3 groups should have a stabilizing effect, this indicates a lower interaction between the SF3 groups in the ethane than in the methane derivative.

The compound (-CF2-SF3)2 reacts with ASF5 and SbF5 to give stable salts with monocations, e.g., [(-CF2-SF2)2F]+MF6~. With CsF only monoanions are formed, while with excess TASF even a dianion precipitates in quantitative yield from CH3CN solution (24).

Ç F 2 - S F 3 Θ

+ CsF ^ I > Cs® C F 2 - S F f

Ç F 2 - S F 3 Ç F 2 - S F 3 Q I + T A S F • I J > T A S

C F 2 - S F 3 C F 2 —SF3

C F 2 — S F 4

E

+ 2 T A S F I ( T A S ® ) 2

C F 2 — S F 4

This indicates that TASF is superior to CsF in its fluoride donor property. It also indicates that towards TASF both -CF2-SF3 groups react independently.

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Crystals of the Cs salt which were suitable for an X-ray structure determination (Fig. 3) were obtained from CH3CN solution. The salt crystallizes with one molecule of the solvent (25).

Due to the bridging of the fluoride, a five-membered C2S2F ring is formed with significantly different F(l) - S(l) (222.8 pm) and F (1) - S(2) (206.7 pm) distances. Although the geometry around the sulfur centers is almost pseudo octahedral, the remaining SF distances are very different ( Δ = "d (SFa)" - "d(SFe)" = 10 pm), resembling the situation in a pseudo trigonal bipyramid. With the approach of the fluoride ion, negative charge is transferred to the sulfur centers, resulting in a lengthening of the SF bond distances.

The structures of CF 2 (SF 3 ) 2 , [CF2(SF3)2F]' and [(-CF2-SF3)2F]" establish four experimental points (two from the last structure) on the reaction coordinate for the nucleophilic attack of F" on pseudo pentacoordinated sulfur (IV) centers. This transition state is attained in the ftuorosulfuranide anions (23, 26):

SF 4 SF 5

e

CF 3 SF 3 CF 3SF 4

e

+ TASF *· TAS C

C 2 F 5 S F 3 C 2F 5SF 4

e

C 3 F 7 S F 3 C 3F 7SF 4

e

The ^F-NMR-data f o r the perfluoralkyl derivatives indicate an apical position for the Rp groups (See Table I):

Table I. l^F-NMR data of fluorosulfuranide anions

F C F 3 C 2 F 5 C3F7

59.8 15.7 {35.0} 32.4 (SF4) [ppm]

44.8 20.0 12.0 2J(FF) 7 3 j(FF) P^]

The perfluorethyl and -isopropyl derivatives readily decompose. Possible decomposition mechanisms are β-fluoride transfer from carbon to sulfur under formation of perfluoro alkene s and SF5" or heterolytic cleavage of the C-S bond to give Rp" and SF4 as the primary products (26).

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Figure 3. X-ray structure of Cs+[(-CF2-SF3)2F]- · CH3CN.

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Although CF3 groups seem to increase the F" acceptor properties of sulfur centers, (CF3)2SF2 surprisingly does not interact with F".

CF,

'CF3

F Θ F

1 7 ^ d ^ S ' ^ — C F 3

F3C I

A possible explanation for this behavior is, that in anions the CF3 group avoids the energetically unfavorable 3c-4e-bond system (26).

The chemistry of the sulfuranide ions is rather disappointing. No examples are yet known for a successful use as a nucleophile:

F Θ

^ S V J ) + R Y

F F

R V R + Y

RSF3 + RT + Y

F, F θ

R—-S*C1) + X—F F F

R—-S-—X + , Θ

R=F,Rf;X=F,Cl, Br

They react as fluorinating agents, thus only with oxidizing agents is the RSFzj-moiety preserved (27,28).

Fluorinated Sulfuramide Anions Sulfur nitrogen double bond systems with electron-withdrawing substituents will add fluoride ions under formation of fluorosulfuramide anions (29-31). Especially interesting are the inorganic fluorosulfonyl and pentafluorosulfanyl derivatives, but N-perfluoroalkyl derivatives also react:

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156 INORGANIC FLUORINE CHEMISTRY: TOWARD T H E 21ST CENTURY

Ο

FSQ,-N=S=0 - F - ^ N _ | _ F

0 ô O

FS02-N=SF 2 *• F _ f ^ N _ J ^

J F e

A 1>F TASF Λ TAS

O

FS02-N=S(0)F2

F ~ H ^ N - Î F Θ

O I V F *

+ FS0 2 -N=SF 4 - F ~ 1 ^ N J V - F

In these examples, the FSO2 groups (with sulfur (VI) and coordination number 4) are connected to both sulfur (IV) and sulfur (VI) centers with different coordination numbers. Previously, only fluorosulfuramide anions were described in the literature with sulfur (VI) and coordination number 4 or 6, e.g. ( F S C ^ N " (32), FSO2NSF5-(33), (SF 5) 2N- (34), andNSF2-NS02F" (35).

The reaction of pentafluorosulfanylimino derivatives with TASF is similar to that of the FSO2 species and results in the formation of stable salts:

Ο + F 5 s - N = S = 0 ^ F 5 S ^ N _ J _ /

F © + F 5 S — N = S F 2

F 5 S \ N _ ^

l *F F R

TASF TAS* F 6

+ F 5S—N=S(0)F 2 • P 5 S \ N _ ^ °

F *

xrc F F ® + F 5 S - N = S F 4 • F 5 S ^ N ^ _ F

F ' >

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The first isolation of the F5S-N-SC>2F"-anion resulted from the hydrolysis of F5SNSOF2 in the presence of large cations (33):

F5S—N=S(0)F2 + H 2 0 KOH

PI14PCI F5S—N—S02F 9 Ph4P ® + HC1 + HF

ASF5/-AsF6

1

CsF

Cs FS02 -N-SF 5

H I

FSO2-N—SF5

CIF

CI I

FS02-N—SF5

In this anion two kinetically rather inert sulfur centers are present. Arsenic pentafluoride exclusively abstracts F" from the pentafluorosulfanyl group (36), while an excess of ASF5 leads to the formation of rather labile FS02NSF3+AsF6~ (37). The tetrafluorosulfur imide is a useful starting material for introducing the FSO2-N-SF5 group, either via the Cs-salt, the free amine or the A/-chloroamine (37).

Addition of Fluoride Ions to Bifunctional SN Systems

As seen before, sulfur-nitrogen double bond systems readily add F" at the sulfur center. If this sulfur center competes with other functional groups, e.g. - CN, -C(0)R (with multiply bonded carbon), -S1R3 (with a center having the possibility of coordination expansion), when different reactive sites are in the same molecule, then fluoride ion addition by TASF might answer the question of relative acceptor properties of these different groups. With TASF as fluoride ion donor NMR spectroscopic investigations in homogeneous solution are possible. With this method the site of primary attack and reaction mechanisms might be elucidated.

TASF reacts with NC-NSF 2 (37,38) and NC-NS(0)F2 (39,40) in quantitative yield to give remarkably stable, colorless salts (41):

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+ NC—N=SF2

TASF TAS θ Θ

F F + NC-N=S(0)F 2 | N ^ C - N — S f , <« • N=C=N—Sf,

V ' 2 ~ I^F - I^F F F

F" is added exclusively to the sulfur centers. Structure determinations will show, whether these anions might be regarded as Af-cyano trifluor ο sulfur- and -oxotrifluorosulfur salts, respectively, or as trifluorosulfur / oxotrifluorosulfur carbodiimide salts. The NMR spectra of these N-cyano derivatives fit well into the series of other trifluorosulfur- and oxotrifluorosulfur amide anions as well as into the system of pentacoor-dinated sulfiir(VI)- and pseudo-pentacoordinated sulfur(IV) derivatives, as shown in Table Π (30, 36,41-46) and Table m (30, 41, 47-50).

Oxotrifluorosulfur imide anions RNS(0)F3" are isoelectronic with the tetrafluorosulfhrimides RN=SF4 Similar structures are expected with the substituent R in an axial position and inequivalent axial bonded fluorine substituents Fa, Fa' (see Table Π). This inequivalence of Fa and Fa has been established, e.g., for CH3N=SF4 (44) and FN=SF4 (57) by NMR in solution and by gas phase structure determinations. In the NMR experiments of RNS(0)F3" only one signal is found for Fa and Fa due to rapid exchange (either inversion at the nitrogen or rotation around the NS-bond).

In the JV-acyl fluorosulfur imides RpC(0)NS(F)RF (Rp=F, CF3) two competing centers for the fluoride ion are also present. Many years ago the reaction of HgF2 with FC(0)NSF2 was reported to give Hg(NSF2)2 and OCF2, with the mercurial serving as a source for pure NSF (52, 53).

2FC(0)NSF2 + HgF2 ^ Hg(NSF2)2 + 2 OCF 2

FC(Q)NS(F)CF3 + HgF2 * ?

Similarly, FC(0)NS(F)CF3 was expected to give Hg [NS(F)CF3]2 and from this "CF3SN" should be generated. But from this reaction no conclusive results were obtained.

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9. MEWS FluoroSulfur Anions 159

Table II: 19F-NMR Data of Pentacoordinated N-S(VI)-F-Species

F ® F « F Θ „ F F F θ „ F

\=c F >-k R \ I F

F Me Et C H 3 C 2 F 5 FS0 2 (CF2)CF FS0 2 SF 5 NC Me

F a x 63.8 64.5 76.3/ 76.5/ 101.8/ 100.4 100.9 97.8 94.3 76.8 73.9 75.1 74.4

68.3 69.2 66.6 75.2 73.1 77.4 71.9 66.5

2JFF 220.6 218.4 194.0/ 194.8/ 206.0/ 44.0 151.9 148.1 149.4 163.1 201.0 200.6 211.0

(42) (42) (44) (45) (36) (30) (50) (30) (41) (46)

Table III: 19F-NMR Data of Pseudo - Pentacoordinated N-S(IV)-F-Species

F ρ Θ ρ 0 F θ

R/R' Me/Me Me/CF3 C 2 F 5 FS0 2 CN C 2 F 5

S F a x 59.4 66.9 64.0(br) 69.7 66.3 30.0 27.6

S F e q 20.2 4.1 56.0(br) 59.4 48.05

2JFF 58.0 32.8 - 47.0 70.2

3JFF - . . . 18.0 17.5

(47,48) (49) (30) (30) W 00) PÔT

3

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TASF + FC(0)NSF2 • TAS®OCF3° + NSF

TASF + F3CC(0)NSF2 • TAS® OCF 2-CF 3

0 + NSF

Ο Il F Θ

X I /C\ + FC(0)NSFCF3 F Ν — S ^

I CF3 F

TASF TAS (

Ο

F3C-C(0)NSFCF3 • F3CT F Θ

1-CF3

In the reaction of TASF and FC(0)NSF2 or F3CC(0)NSF2 no intermediates could be detected; the final products were NSF and TAS-perfluoroalkoxides. These results indicate that OCF2 and F3C-C(0)F are better F" acceptors than NSF (41).

In contrast to the difluorosulfur imides, reaction of TASF with the corresponding S-trifluoromethyl monofluorosulfur imides leads to reasonably stable anions with pseudo pentacoordinated sulfur (41). It is not unlikely that in both systems the sulfur centers are primarily attacked by F" under formation of RC(0)NSF3"and RC(0)NSF2CF3", respectively. There are no profound theoretical investigations on the different stabilities of these two types of anions. Nevertheless, for decomposition to occur fluoride ion transfer from sulfur to carbon is neccessary. If this occurs only from an equatorial position, then the difference in stability could be explained.

In the reaction of TASF with R3Si-N=S species the primary attack of F" could occur either at the sulfur or at die silicon atom Even in low temperature experiments no intermediates were detected; facile cleavage of the Si-N-bond was observed (54, 55).

+ Me 3Si—N=S=0 • N S O θ

TASF TAS®

+ Me3Si—N=S(0)F2 NS(0)F 2

G

The NSOF2" anion (isoelectronic with NSF3) is known from the literature to show a broad singlet in the 1 9 F-NMR (δ = 77.8 ppm) (56,57). With TAS + as a counterion 2 J( 1 4 N-F) = 17.0 Hz is observed similar to NSF 3 (δ = 70.0 ppm, 2 J ( 1 4 N - F) = 26.4 Hz (58)).

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Me3Si—NSN—SCF3 NSN-SCF3

TASF TAS'

Me3Si—NSN—SiMe3 NSN-SiMe3

The primary products of TASF addition to the bifunctional methane derivatives F 2C(NSF 2)2 (38) and F2C(NSF2)NS(0)F2 (59) are unstable and decompose at temperatures above -30 C to give F 3 CNSF 2 and TAS +NSF 2" or TAS+NS(0)F2", respectively.

F 2 C \

N = S F 2

N = S F 2

CF3NSF2

TASF

r ^

Θ .

Θ

TAS ©

F 2 C = N — SF 3 + TAS NSF2 Θ

F 2 C V

N = S F 2

" N = S F 2

II o

+ TASF

N = ' S \ H Λ

O

Θ

TAS .θ

CF3NSF2 F 2 C = N - S F 3 + TAS®NSOF 2°

The first step involves the attack of fluoride ion at die sulfur centers. Ια both systems only one signal is observed for the sulfur-bonded fluorine atoms (with a very broad line in the latter example). In the rapid exchange of F" between the two sulfur centers, a six-membered CN2S2F" heterocycle with pseudo pentacoordinated sulfur atoms should be the transition state. As with the previously described results, the bridging

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position should be occupied by axial fluorine ligands. The first step in the decomposition will be F2C=NSF3 and TAS+NSF2" or TAS+NS(0)F2", respectively. F2C=NSF3 will rearrange via a 1,3-fluorine atom shift to give the thermodynamically more favorable CF3NSF2.

The formation of CF 3 NSF 2 and TAS+NSOF2" instead of CF 3NS(0)F 2 and TAS+NSF2"' does not suggest that the NSF2 group is a better acceptor than the NS(0)F2 group. Rather it merely indicates that NSOF2" is a better leaving group than NSF2". There are several examples in the literature for the decomposition of bis<<ffluorosulfurimide) derivatives, e.g., OC(NSF2>2 (60) and 02S(NSF2)2 (60) under formation of NSF and FC(0)NSF2 and FSO2NSF2, respectively. Our investigations show that these decompositions might be F" catalyzed.

Another example of the elucidation of reaction mechanisms comes from the chemistry of thiatriazines. In the reaction of 1,3,5-trifluoro-dithiatriazines with silylamines the CF bond is attacked first. With an excess of sirylamine, exchange also occurs at one or both sulfur atoms (61):

Carbon containing fluoro triazines react with TASF to give stable cyclic anions [(Cs+C3N3F4~ is known from the literature (62)], with attack occurring exclusively at the carbon. With (NSF)3 the corresponding cyclic anion N3S3F4" is the primary product, but this readily decomposes to give TAS+NSF2" (63). These cyclic anions might be regarded as stabilized S^2- transition states for a nucleophilic attack at the ring systems.

MB3SiNMe2/-]vfe3SiF

NMe2 NMe2

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9. MEWS FluorO'Sulfur Anions 163

Conclusions Due to the availability of unusual fluoride ion transfer reagents new pathways for the generation, stabilization, and structural investigation of previously unknown fluorosulfur-, fluoro sulfur-nitrogen-, and sulfur-nitrogen anions have been developed. With these tools a "mechanistic-synthetic" chemistry is possible, leading to "stabilized transition states", starting points for further preparative work. This chemistry is not

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restricted to sulfur chemistry, as examples from other main group elements have been previously reported e.g., from >C=0 (64) and >C=N- systems (65) stable TAS salts have been generated. Structural investigations with TAS+ as a counterion are often not completely satisfying, since the C§ symmetry of the cation leads to disordering of the anion. Exanqdes are TAS+ CF3-N-C(0)F- or TAS+ N(CF3)2" (65).

Acknowledgment I want to thank my coworkers Prof. Dr. U. Behrens, Dr. S.-J. Shen, G Knitter, Dr. N. Hamou, Dr. W. Heilemann, Dr. T. Meier, Dr. D. Viets, Dr. A. Waterfeld and specially E. Lork for their enthusiastic engagement in this work. The help of Dr. P. G. Watson in preparing this manuscript and support by the University of Bremen (F.N.K), Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie is gratefully acknowledged.

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