S-F and S-C activation of SF6 and SF5 derivatives at rhodium: Conversion of SF6 into H2S

Article abstract of DOI:10.1002/anie.201308254

The degradation of SF₆ and SF₅ organyls by S-F and S-C bond-activation reactions at [{Rh(μ-H)(dippp)}₂] under mild conditions is reported. Fluorido and thiolato species were identified as products or intermediates, and were characterized by X-ray diffraction analysis and multinuclear NMR spectroscopy. An unprecedented cyclic process for the conversion of the potent greenhouse gas SF₆ into H₂S was developed. Activation of the stable greenhouse gas SF₆: The rhodium hydrido complex [{Rh(μ-H)(dippp)}₂] effected defluorination at the sulfur atom of SF₆ and organic SF₅ compounds under mild conditions. The reduction of SF₆ in the presence of HSiEt₃ led exclusively to the thiolato complex [Rh₂(μ-H)(μ-SSiEt ₃)(dippp)₂] and FSiEt₃ (see Scheme). A cyclic process was developed for the conversion of SF₆ into H₂S.

Full text of DOI:10.1002/anie.201308254

Angewandte
Chemie
DOI: 10.1002/anie.201308254
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S F Activation
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S F and S C Activation of SF6 and SF5 Derivatives at Rhodium:
Conversion of SF6 into H2S**
Lada Zꢀmostnꢀ, Thomas Braun,* and Beatrice Braun
À
Abstract: The degradation of SF6 and SF5 organyls by S F and
bis(4-(pentafluorosulfanylphenyl))sulfane in very low yield
S C bond-activation reactions at [{Rh(m-H)(dippp)}2] under
(4%).[8e] The reaction of SF5CH2Br with n-butyllithium gave
À
[8g]
=
mild conditions is reported. Fluorido and thiolato species were
identified as products or intermediates, and were characterized
by X-ray diffraction analysis and multinuclear NMR spectros-
copy. An unprecedented cyclic process for the conversion of
the potent greenhouse gas SF6 into H2S was developed.
F4S CH2 through an initial lithiation of the a-carbon atom.
À
À
So far, the cleavage of either S F or S C bonds in aromatic
SF5 compounds at transition-metal complexes has, to the best
of our knowledge, not been observed. Note that a series of
organometallic transformations of SF5 compounds, such as
palladium-catalyzed cross-coupling reactions, are known in
[5a,6f,7c,9]
À
S
F6 is widely regarded as a chemically extremely stable
which the C SF5 moiety remains stable.
gas.[1] SF6 has a high density, is nontoxic and nonflammable,
and exhibits a high dielectric constant, which makes it ideally
suited for some specific applications, for example, as a gaseous
dielectric compound for high-voltage-power applications.[1,2]
However, SF6 is also known as one of the most powerful
greenhouse gases.[3] The activation of SF6 at transition-metal
complexes has only been described by Ernst and co-workers,
at low-valent Ti, V, Cr, and Zr complexes, as well as at
[Ni(PMe3)4] and [Fe(C5H5)(C6H3tBu3)].[4] The formation of
fluorido complexes or polyfluoride anions was observed in all
cases. Sulfur-containing products could only be identified in
the reactions of SF6 with [Cr(C5Me5)2] and [Ti(1,3-tBu2C5H3)-
Herein, we show that carbon-bound SF5 groups are not
stable in the presence of rhodium hydrido species. We report
the unprecedented defluorination of aryl SF5 compounds and
CF3SF5 by S F and S C bond-activation reactions under mild
conditions. Furthermore, a remarkable reduction of SF6 at
[{Rh(m-H)(dippp)}2] (1; dippp = 1,3-bis(diisopropylphospha-
nyl)propane) in the presence of HSiEt3 led exclusively to
[Rh2(m-H)(m-SSiEt3)(dippp)2] (8), FSiEt3, and H2. A unique
cyclic process for the conversion of SF6 into H2S was
developed.
À
À
The treatment of [{Rh(m-H)(dippp)}2] (1) with PhSF5
(0.3 equiv) for 120 h at 508C gave the fluorido complex
[{Rh(m-F)(dippp)}2] (2) and the binuclear hydrido thiolato
complex [Rh2(m-H)(m-SPh)(dippp)2] (3a) in a 2.5 :1 ratio,
along with the formation of H2 (Scheme 1). CH3C6H4SF5 and
CF3SF5 were activated in a similar manner at 1 to yield the
fluorido complex 2 and the hydrido thiolato complexes 3b
and 3c, respectively, in a 2.5 :1 ratio (Scheme 1). The reaction
of 1 with the aromatic SF5 compounds could be accelerated by
the use of an excess (10 equivalents) of the SF5 substrates to
enable full conversion within 48 h at room temperature.
Compound 2 was described previously.[10] Complexes 3a and
3b were synthesized independently by the treatment of 1 with
the corresponding thiophenols RSH (1 equiv; Scheme 2).[11]
The 31P{1H} NMR spectrum of 3a displays two doublets of
multiplets at d = 45.9 and 38.9 ppm as part of an AA’BB’XX’
spin system (X,X’ = Rh). In the 1H NMR spectrum, the
resonance for the hydrido ligand appears as a multiplet at d =
À10.14 ppm; it simplifies to a triplet in the phosphorus-
decoupled spectrum (1JRh,H = 21.8 Hz). The NMR spectro-
scopic data of 3b and 3c are very similar to those of 3a. The
molecular structures of 3a and 3b in the solid state were
determined by X-ray crystallography (Figure 1; the structure
of 3b resembles that of 3a: see the Supporting Informa-
tion).[12] Complex 3a exhibits a nearly planar Rh2(m-H)(m-
SPh) moiety with a trigonal-pyramidal geometry of the
sulfido ligand. Each rhodium center shows a slightly distorted
square-planar coordination sphere. The distance between the
rhodium atoms is 2.8850(2) ꢀ.
(6,6-dmch)(PMe3)]
(6,6-dmch = 6,6-dimethylcyclohexa-
dienyl), which yielded, among other compounds, [{Cr-
(C5Me5)(m2-F)}3(m3-S)]+ [Cr(C5Me5)(F)3]À or SPMe3, respec-
tively.
Some of the properties of SF6 are maintained in its organic
SF5 derivatives.[5] The SF5 group is characterized by its high
electronegativity, its high lipophilicity, and its significant steric
demand. Its high stability and chemical inertness in organic
compounds are generally accepted as characteristic fea-
tures.[5–7] Compounds which contain a SF5 building block are
of increasing importance because of their various applications
as bioactive compounds, liquid crystals, and advanced poly-
mer materials.[5,6] Studies on the reactivity of the SF5 moiety in
organic derivatives are extremely rare.[7a,c,8] A nucleophilic
attack by an azide anion at (pentafluorosulfanyl)propyl
tosylate[8a] and a base-induced dehydropentafluorosulfanyla-
tion at olefinic double bonds to give alkynes[8d] have been
reported. However, in these examples, the fate of the SF5
group is not known. A defluorination of the SF5 group upon
the treatment of 1-bromo-4-(pentafluorosulfanyl)benzene
with n-butyllithium led to a mixture of products, including
[*] L. Zꢀmostnꢀ, Prof. Dr. T. Braun, Dr. B. Braun
Department of Chemistry, Humboldt-Universitꢁt zu Berlin
Brook-Taylor-Strasse 2, 12489 Berlin (Germany)
E-mail: thomas.braun@chemie.hu-berlin.de
[**] We thank the Solvay Fluor GmbH for a gift of SF6.
The reaction of 1 with the SF5 substrates was monitored by
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201308254.
1
31P, 19F, and H NMR spectroscopy. In each case, the hydrido
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Scheme 1. S F and S C activation of pentafluorosulfanyl compounds at [{Rh(m-H)(dippp)}2] (1); a: R=Ph, b: R=C6H4CH3, c: R=CF3.
Mechanistically, we tentatively suggest that the reaction of
the dihydrido complex 1 with a SF5 organyl compound leads
À
initially to the cleavage of a S F bond with the concomitant
generation of HF and a rhodium tetrafluorosulfanyl inter-
mediate [Rh2(m-H)(m-SF4R)(dippp)2]. HF can in turn react
with hydrido compounds, such as 1, to give H2, as well as
fluorido complexes, such as 4 and subsequently 2, although
the reverse reaction is also conceivable (see below).[10a,15] The
formation of fluorido complexes by the protonation of
hydrido compounds with HF is in accordance with the
observation that the reaction of 1 with PhSF5 in the presence
of Cs2CO3/NEt3 led to smaller amounts of 2. Note that 3a–c
did not react with Et3N·3HF. Rhodium fluorido complexes
can alternatively be formed by the migration of fluorine
atoms from sulfur to a rhodium center; the resulting
complexes can subsequently also yield HF in the presence
of a hydrido ligand or H2 as a hydrogen source.[16] H2 may
Scheme 2. Synthesis of the hydrido thiolato complexes 3a,b; a: R=Ph,
b: R=C6H4CH3.
À
induce additional S F bond-cleavage steps at Rh, including
HF formation once again, as well as the generation of
rhodium complexes with low-valent fluorinated sulfur-con-
taining ligands. Overall, the reduction of the sulfur-containing
ligands occurs formally by conversion of the hydrido ligands
Figure 1. Molecular structure of 3a (ORTEP, ellipsoids are set at 50%
probability). All hydrogen atoms except for the bridging hydride are
omitted for clarity. Selected bond lengths [ꢂ] and angles [8]: Rh1–S1
2.3129(10), Rh2–S1 2.3131(11), S1–C1 1.785(2), C1-S1-Rh1 112.32(13),
Rh1-S1-Rh2 77.167(15).
À
into HF or H2. No rhodium intermediates containing a S F
moiety could be detected by NMR spectroscopy. This result
À
indicates that the first S F bond-cleavage step is the rate-
determining step.
Evidence for the fluorido thiolato complexes [Rh2(m-F)(m-
SR)(dippp)2] as intermediates was provided by the reactions
of 1 with RSF5 in the presence of neohexene, which acts as
a trapping agent for H2.[17] The transformation yielded the
complex 2 as well as [Rh2(m-F)(m-SR)(dippp)2] (6a–c), in each
case in a 1.5 :1 ratio, along with HF and neohexane
(Scheme 1). Apparently, H2 was removed in part and does
not convert 6a–c into 3a–c. Furthermore, no formation of the
tetrahydrido complex 5 was observed. Indeed, in an inde-
pendent reaction, complex 6a was converted into 3a and HF
by treatment with H2. Compounds 6a–c were characterized in
solution by NMR spectroscopy and LIFDI MS. The reso-
nances for the bridging fluorido ligands appear in the
19F{1H} NMR spectra at high field (d = À390.8 ppm for 6a).
fluorido complex [Rh2(m-H)(m-F)(dippp)2] (4) was detected as
an intermediate, as well as considerable amounts of HF. The
binuclear tetrahydrido compound [Rh2(H)(m-H)3(dippp)2] (5)
was also present and presumably serves as the resting state
after the reaction of 1 with H2.[13] Complex 4 was charac-
terized by NMR spectroscopy and liquid injection field
desorption ionization mass spectrometry (LIFDI MS). The
signal for the bridging hydrogen atom appears at d =
À10.43 ppm in the 1H NMR spectrum. The 19F{1H} NMR
spectrum shows a multiplet for the fluorido ligand at d =
À355.6 ppm, which is a characteristic chemical shift for
rhodium fluorido complexes.[10a,14]
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It is known that fluorido complexes can be highly reactive.
Reactions with silanes, such as HSiEt3, as a hydrogen source
often lead to the formation of hydrido compounds.[10a,15] Thus,
it was demonstrated that [{Rh(m-F)(dippp)}2] (2) can be
converted into the h2-silane complex [Rh(H)(h2-HSiEt3)-
(dippp)] (7). The latter undergoes transformation into the
binuclear hydrido complex 1 on a slower timescale.[10a]
Remarkably, the reaction of 1 with RSF5 in the presence of
À
À
HSiEt3 resulted in both S Fand C S cleavage reactions of the
SF5 compounds to yield 3a–c as well as the silylthiolato
complex 8 in a 1:2.5 ratio (Scheme 1). RH and H2 were
generated concomitantly, and FSiEt3 was obtained as the only
fluorinated product. The use of H2 as a reductant, instead of
HSiEt3,[18] resulted predominantly in the formation of the
Figure 2. Molecular structure of 8 (ORTEP, ellipsoids are set at 50%
À
tetrahydrido complex 5, which is not suitable for S F
activation.
probability). All hydrogen atoms except for H1 are omitted for clarity.
Selected bond lengths [ꢂ] and angles [8]: Rh1–S1 2.3600(5), Rh2–S1
2.3816(5), S1–Si1 2.1586(7), Rh1-S1-Rh2 75.908(15).
Complexes 3a,b showed no reactivity towards HSiEt3.
Monitoring of the reaction of 1 with RSF5 and HSiEt3 by
NMR spectroscopy revealed the presence of complexes 7 and
[Rh2(H)(m-H)3(dippp)2] (5). The dihydrido complex 1 did not
react with the silane HSiEt3 to give 7.[10a] It is therefore likely
that [Rh(H)(h2-HSiEt3)(dippp)] (7) serves as an intermediate
in the generation of 8. Indeed, in an independent reaction, 7
was generated by the treatment of 2 with HSiEt3 and then
treated with PhSF5 in the presence of HSiEt3 to yield
exclusively 8, with concomitant elimination of FSiEt3, H2,
and benzene (Scheme 3b). The fluorido complex 2 showed no
reactivity towards PhSF5, but in the presence of HSiEt3, 8 was
formed only.
example, in [{Zn(SSitBu3)2}2] or [{(tBu3SiS)Fe}2(m-
SSitBu3)2].[19]
In an extraordinary reaction, complex 1 proved to be
highly reactive towards SF6. In the presence of HSiEt3, the
treatment of 1 with SF6 gave 8, FSiEt3, and H2 as the only
products after 72 h at 508C (Scheme 4). In analogy to the
Scheme 4. Activation at SF6 and a cyclic process for its conversion into
H2S.
reactions with the SF5 compounds, the h2-silane hydrido
complex 7 was detected as an intermediate during the course
of the reaction, as well as the tetrahydride 5. Independent
reactions revealed that the treatment of 7 or 2 in the presence
of HSiEt3 gives 8 selectively (Scheme 3a). We did not observe
any formation of [Rh2(m-H)(m-SF)(dippp)2], in analogy to the
generation of 3a–c from 1. It is likely that a m-SF species
would be highly unstable, but might be converted into 8 by
loss of HF and the addition of HSiEt3. This reactivity would
explain the formation of complex 8 as the only product in the
reaction starting from 1. In the absence of HSiEt3, full
conversion of 1 was also observed upon treatment with SF6
gas (1 atm) at room temperature for 1 day: The fluorido
complex 2 was formed in approximately 70% yield according
to the 31P NMR spectra. However, we were not able to
À
Scheme 3. S F bond-activation reactions at 2 and 7.
The high-field region of the 1H NMR spectrum of 8
displays a multiplet centered at d = À11.46 ppm for the
bridging hydride. In the 29Si{1H} NMR spectrum of 8,
a broad multiplet for the silicon atom is observed at d =
22.1 ppm. The molecular structure of 8, which was determined
by X-ray diffraction, differs from the structure of 3a, as it is
characterized by a bent geometry of the Rh2(m-H)(m-SSiEt3)
moiety, with a hinge angle q of 142.9(6)8 (Figure 2).[12] The
geometry around the rhodium centers is close to square-
À
planar. The S Si distance is 2.1586(7) ꢀ and is comparable to
those found in other silylthiolato-bridged complexes, for
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characterize any sulfur-containing species. LIFDI MS studies
indicated the formation of several rhodium–sulfur clusters,
most of which appeared to be trinuclear.
A reaction of complex 8 with HCl (3 equiv) yielded the
chlorido compound [{Rh(m-Cl)(dippp)}2] (9)[20] along with H2,
ClSiEt3, and H2S. A reaction of 8 with DCl gave D2S.
Treatment of the chlorido complex 9 with an allyl Grignard
reagent afforded [Rh(h3-C3H5)(dippp)] (10), which was con-
[13]
verted into the hydrido compound 1 by the addition of H2
(Scheme 4). These reactions complete a cyclic process for the
transformation of SF6 into H2S.
In conclusion, we have shown that the binuclear rhodium
hydrido complex 1 effects the defluorination of SF5 organyl
compounds and of SF6 under mild conditions to give sulfido
species. Notably, the oxidation state of the rhodium centers
remains + I, whereas the sulfur atom is formally reduced by
oxidation of the hydrido ligands. Studies on chemical trans-
formations of SF6 are generally scarce and often involve
reductive reaction conditions.[1,4,21] An initial electron-trans-
fer step is also conceivable for the reaction of 1 and 7 with SF6
and the SF5 compounds. However, it is alternatively feasible
that the first activation step occurs at the highly reactive 14-
electron complex [Rh(H)(dippp)], which could be generated
from 1 or 7.[10,13] The transformation of thiolato complex 8
into H2S paves the way for an unprecedented cyclic process
for the conversion of SF6 into H2S. We believe that these
results open up new opportunities for the degradation of SF6
and even its transformation into valuable reagents. SF6 and
CF3SF5 have been identified as greenhouse gases with
enormous global-warming potential.[3]
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Received: September 20, 2013
Published online: January 22, 2014
Keywords: fluorido complexes ·
.
À
pentafluorosulfanyl compounds · rhodium · S F activation ·
sulfur hexafluoride
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PhSH; the molecular structure was determined by X-ray
crystallography (see the Supporting Information).
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