Fluoro-Sulfur Anions - American Chemical Society

Chapter 9 Fluoro-Sulfur

Anions

Transition States, Intermediates, and Reagents

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

The addition offluorideions to lower coordinated sulfur derivatives and sulfur-nitrogen species with SN double bonds leads to a number of unusualfluoroanions. 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 effectivefluorideion donors, e.g. (Me2N)3S Me3SiF2~ (TASF) (2), Me N F" (3), or [(Me N) P] N F- (4), much progress has been achieved in the chemistry of fluorinated anions of the maingroup elements, especially during the last few years. +

+

4

+

2

3

2

Fluorosulfuranide Anions Sulfur tetrafluoride is known to act as both a fluoride ion donor and acceptor. Bartlett and Robinson were the first to show thatfluoroLewis acids abstract F" under formation of the trifluorosulfonium ion SF3 " (5), similar results were obtained independently by Seel and Detmer (6). Structure determinations were reported later by Bartlett's group (7). 4

Fluoride ion acceptor properties of SF4 werefirstsuggested 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 In Inorganic Fluorine Chemistry; Thrasher, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

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Fluoro-Sulfur Anions

149

F


F

F, Ίι

1

il'

s

Λ F

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).

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

150

INORGANIC FLUORINE CHEMISTRY: TOWARD THE 21ST CENTURY

In α , ω -bis(trifluorosulfhr)alkanes F3S-(CF2-)nSF3 a comparable, but intramo lecular 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):

CS + 4 F


2

183 Κ > F C(SF ) 30 mbar

2

2

3

2

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(SF ) = 166.4 (4) pm] and the equatorial bonds [d(SF ) = 156.2 (4) pm]. Within the limits of experimental error they are the same as in F3CSF3 [d(SF ) = 156.5 (8) pm; d(SF ) = 165.5 (5)] (16). Especially interesting in the structure of F C(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 axialfluorineof the opposite SF3-group. As can be seenfromthe Newman projection in Figure 1, the sulfur centers are attacked by these bridgingfluorinesin the equatorial plane, as in the model suggested from the NMR data of liquid SF4 or CF3SF3 (77). a

e

e

a

2

Similar to SF4 CF3SF3 acts as afluorideion 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 classicalfluorideion donor influorinechemistry is CsF; however, for the generation and stabilization of anions, ionicfluorideswith large organic counterions, e.g., Me4N (3) or [(Me2N)3P]2N (4) seem to be more useful. In "TAS-fluoride" (Me2N)3S Me3SiF2'(2) thefluorideis stabilized as the Me3SiF2" anion. Due to the very weak SiF bonds [d(SiF) = 176 pm] it is an extremely usefulfluoridatingagent (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): +

+

+

,SF FC 2

\

3

+

CsF

Cs®

SF

3

Θ

FC 2

SF

3

+

TASF

TASF®

According to the X-ray structure determination (Fig. 2), the two sulfur centers are symmetrically bridged by afluorideion; this distance d(SFfo) = 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 r



9. MEWS


151

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 fF C(SF3)2F]--anion (21). 2


152


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(trifluorosu!fur)perfluoroethane was isolated in 83% yield (23):

C1S-CF -CF -SC1 + 3 F Downloaded by UCSF LIB CKM RSCS MGMT on November 29, 2014 | http://pubs.acs.org Publication Date: April 29, 1994 | doi: 10.1021/bk-1994-0555.ch009

2

2

F S-CF -CF -SF 3

2

2

2

+ Cl

3

2

+H20/-2HF

BF /SO2 3

V S-CF —CF —SF y

S—CF —CF —S' 2

2

2

2

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 yieldfromCH3CN solution (24). +

ÇF -SF 2

+

CsF

^

3

Θ

I > CF -SFf

Cs®

2

ÇF -SF I CF -SF 2

2

3

ÇF -SF Q I J> C F —SF3 2

+

TASF

3

•

3

TAS

2

CF —SF I CF —SF4 2

+

2 TASF

E 4

(TAS®)

2

2

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.


9. MEWS

153


Crystals of the Cs salt which were suitable for an X-ray structure determination (Fig. 3) were obtainedfromCH3CN solution. The salt crystallizes with one molecule of the solvent (25). Due to the bridging of the fluoride, afive-memberedC2S2F 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 (SF )" - "d(SF )" = 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.


a

e

The structures of CF (SF ) , [CF (SF )2F]' and [(-CF2-SF )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): 2

SF

2

2

3

SF

C F SF 5

3

7

e 4

*·

TAS

3

C F SF 3

e 5

CF SF

3

+ TASF 2

3

4

CF SF 3

3

C F SF

4

C F SF

4

2

3

3

5

7

C

e

e

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

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

F

CF

59.8

15.7

44.8

20.0

3

C F 2

5

{35.0}

C3F7 32.4 12.0

(SF4) [ppm] 2

7 3j

J(FF) (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).



154


+

Figure 3. X-ray structure of Cs [(-CF -SF )2F]- · CH3CN. 2

3


9. MEWS


155

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

F

CF, 1 7 ^ 'CF

3

d^S'^—CF

3

I

FC 3


Θ

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 ) F

+

RY

R

V

R

+

Y

F RSF + RT + Y 3

F,

F θ

R—-S*C1) F

+ X—F

R—-S-—X

+

,Θ

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

They react asfluorinatingagents, 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 offluorosulfuramideanions (29-31). Especially interesting are the inorganic fluorosulfonyl and pentafluorosulfanyl derivatives, but Nperfluoroalkyl derivatives also react:


INORGANIC FLUORINE CHEMISTRY: TOWARD T H E 21ST CENTURY

156

Ο FSQ,-N=S=0

-

F

- ^

N

_ | _

0

F

ô

O J

FS02-N=SF

*•

2

F_

F

f^N_J^ A


TASF

e

1>F

TAS

Λ

O

FS02-N=S(0)F

F 2

F ~H^N-Î

O

+

FS0 -N=SF 2

-

4

F

Θ

IV

F *

~1^ JVN

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, onlyfluorosulfuramideanions were described in the literature with sulfur (VI) and coordination number 4 or 6, e.g. ( F S C ^ N " (32), FSO2NSF5(33), (SF ) N- (34), andNSF -NS0 F" (35). 5

2

2

2

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

^

_ J _ /

N

F +

F S—N=SF 5

F 2

S

5

\

© _ ^

N

l*F

F

R

TASF

TAS* 6

+

F S—N=S(0)F 5

2

•

P

5

S

\

N

F _ ^ ° F *

xrc +

F S-N=SF 5

4

•

F

5

S

^

F F ® ^ _ F

N

F'>


9. MEWS


157

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

F S—N=S(0)F 5

2

+

KOH

H 0 2

PI14PCI

F S—N—S0 F 5

2

9

Ph4P ® + HC1 + HF


ASF5/-AsF

1 6

CIF

CsF CI

H Cs

FS02-N-SF

I

I 5

FSO2-N—SF

FS02-N—SF

5

5

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

2


158


+

NC—N=SF

2

TASF

TAS θ

+

NC-N=S(0)F


V

F |N^C-N—Sf, ~ I^F F

2

2

'

Θ 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+ CF -N-C(0)F- or TAS+ N(CF ) " (65). +


3

3 2

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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Fluoro-Sulfur Anions - American Chemical Society

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