Metal-free reduction of the greenhouse gas sulfur hexafluoride, formation of SF5 containing ion pairs and the application in fluorinations

Article abstract of DOI:10.1039/c7gc00877e

A protocol for the fast and selective two-electron reduction of the potent greenhouse gas sulfur hexafluoride (SF₆) by organic electron donors at ambient temperature has been developed. The reaction yields solid ion pairs consisting of donor di

Full text of DOI:10.1039/c7gc00877e

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Metal-free reduction of the greenhouse gas sulfur
hexafluoride, formation of SF5 containing ion pairs
and the application in fluorinations†
Cite this: DOI: 10.1039/c7gc00877e
Magnus Rueping,
*
a,b Pavlo Nikolaienko,‡a,b Yury Lebedev‡a,b and Alina Adamsc
Received 22nd March 2017,
Accepted 17th April 2017
A protocol for the fast and selective two-electron reduction of the potent greenhouse gas sulfur
hexafluoride (SF6) by organic electron donors at ambient temperature has been developed. The reaction
yields solid ion pairs consisting of donor dications and SF5-anions which can be effectively used in fluori-
nation reactions.
DOI: 10.1039/c7gc00877e
rsc.li/greenchem
sulfides and fluorides have been published. Such procedures
generally require harsher conditions or/and elemental alkali
Introduction
Sulphur hexafluoride is a non-flammable, odourless and col- metals.10 Polycrystalline silicon, tin oxide and organic polymer
ourless gas of high density.1 The unique physical and chemical layers react with sulphur hexafluoride at high temperature.11
properties of SF6 make it suitable in specialized electrical More recently, protocols for the SF6 activation under milder
equipment, commercial products as well as in scientific and conditions were reported. These methods require transition
industrial processes. Due to its great arc-quenching ability, metals as catalysts or reducing agents and the products of SF6
almost 80% of all SF6 produced is used as an insulator and fire decomposition are fluorides, sulfides or/and silicon and phos-
suppression agent in high-voltage circuit breakers.2 Another phorus fluoro-derivatives.12a–f In addition, the reduction with
application of SF6 consists in its use as a blanketing, sound- tetrakis(dimethylamino)ethylene under UV-light irradiation is
and thermo-insulating agent in industry3 and as a contrast described.13
agent for ultrasound imaging in medicine.4 Furthermore,
sulphur hexafluoride is applied in the determination of venti-
lation efficiency as well as in environmental modeling.5
However, despite the mentioned applications, SF6 is a
potent greenhouse gas included in the Kyoto Protocol.6,7
According to the World Meteorological Organization, the
atmospheric lifetime of SF6 is ca. 3200 years and its global
warming potential is 22 450 times higher than that of CO2.8
Therefore, besides the necessity to control the level of SF6
emissions and the development of more efficient trapping
techniques,9 a methodology to decompose this gas is in
demand. Thus, since 1980s several reports on SF6 activation
followed by the total reduction to compounds containing futile
Results and discussion
Based on our interest in developing metal-free reactions, we
wondered whether purely organic molecules without UV
irradiation would be able either to decompose this greenhouse
gas at room temperature or to selectively activate it in an
efficient manner, allowing simultaneous use of the reduction
products in fluorination procedures. In particular, we were
−
also interested in the preparation of the SF5 anion14 from
sulphur hexafluoride which would potentially allow the for-
mation of SF5 containing organic molecules which are of con-
siderable interest as well.13,15 Taking into account the fact that
strong reducing agents react with SF6 we became interested in
the use of organic electron donors.
aKing Abdullah University of Science and Technology (KAUST), KAUST Catalysis
Center (KCC), Thuwal, 23955-6900 Saudi Arabia.
E-mail: magnus.rueping@Kaust.edu.sa
Organic electron donors16 are easily accessible on the mul-
tigram scale and redox reactions can be performed in regular
organic solvents with common laboratory glassware. Hence,
2,2′-bipyridyl based organic electron donors 1a,16c 1b and a
member of viologens,17 octyl-4,4′-bipyridine 1c (Fig. 1) were
chosen as reducing agents. Next to the 2,2′- and 4,4′-bipyridine
derivatives we were also interested in evaluating other electron
bRWTH Aachen University, Institute of Organic Chemistry, Landoltweg 1,
D-52074 Aachen, Germany
cRWTH Aachen University, Institut für Technische und Makromolekulare Chemie,
Worringerweg 1, D-52074 Aachen, Germany
†Electronic supplementary information (ESI) available: Experimental pro-
cedures, and full characterization of the products and spectra. See DOI: 10.1039/
c7gc00877e
‡These authors contributed equally to this work.
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Fig. 1 Selected donors 1a–c for the SF6-activation.
donors including TMBI (tetramethylbisimidazolidine) 1d18
and TDAE (tetrakis-dimethylaminoethylene) 1e.19
Interestingly, exposure of 1a and 1b in n-hexane or toluene
to SF6 at ambient temperature led to an immediate colour
change. The deep-purple colour of the starting material dis-
appeared completely within minutes and the formation of a
brown microcrystalline precipitate was observed (2a and 2b,
Scheme 1).
Fig. 2 (Top) The 19F NMR spectrum of the 2b [{bis-(t-Bu)Py
(propylene)}2+F−; SF5−] in acetonitrile-d3 at −30 °C; (bottom) the solid
state 19F NMR spectrum of 2b [{bis-(t-Bu)Py(propylene)}2+F−; SF5−] at a
spinning rate of 30 kHz.
The in situ 19F{1H} NMR analysis of the supernatant solu-
tion showed, except for the residual SF6, no fluorine contain-
ing materials. Subsequently, the solution was filtered off and
the solvent was removed to dryness; however, no residue was
obtained. The isolated products 2a and 2b were found to be
of SOF2 and formation of SO2F−.21 In order to establish the
identity of the composition in solution and in bulk, solid state
19F-NMR spectroscopy analysis of 2b was performed (Fig. 2,
bottom). The spectrum gives rise to two broad singlet signals
appearing at positions identical to those found in solution (δ =
+60.2 ppm SF5−; δ = −150 ppm F−). Noteworthily, the intensity
of the signal at δ = +60.2 ppm decays with time. Thus with 25
kHz rotational speed at ambient temperature after 24 h about
60% of the peak intensity vanished, whereas at 40 °C the
signal disappeared completely after 12 h. These results are
explained by the thermal instability of the SF5− anion.
1
poorly soluble in most common organic solvents. The H- and
13C NMR spectra, recorded in acetonitrile-d3, DMSO-d6 and
DMF-d7, revealed products 2a and 2b as the reaction is driven
by the restoration of aromaticity and formation of the corres-
ponding 2,2′-bipyridynium dications 1a2+ and 1b2+. Ambient
temperature 19F NMR spectra showed signals of F− along with
the signals arising from the solvent decomposition by an anhy-
drous fluoride anion (e.g. DF2−). The 19F NMR spectra of the
salts 2a and 2b in acetonitrile-d3 and DMF-d7 recorded at
−30 °C revealed two broad singlets at δ = +60.5 ppm and δ =
−142.8 ppm, respectively (Fig. 2, top). These data are in an
excellent agreement with the previously described signals of
−
Previously, IR and Raman spectra of salts containing SF5
have been reported. Yet, no Raman spectra for 2a and 2b
samples could be recorded due to intensive fluorescence.
However, IR spectroscopy provided us with an additional
−
SF5 and F− anions in acetonitrile-d3. Low temperature 19F
NMR measurement performed in liquid SO2 with 1,2-difluoro-
benzene as an internal standard (δ = −138.8 ppm) gave rise to
the signal of fluoride (δ = −140 ppm) and a singlet signal at δ =
+72.2 ppm corresponding to thionyl fluoride, SOF2.20 This
result may be rationalized by the decomposition of SF5− to SF4
and F− and the following reaction of SF4 with SO2. Upon evap-
oration of sulphur dioxide and addition of acetonitrile-d3 the
19F NMR spectrum (see the ESI†) at 298 K shows the absence
−
−
evidence for the presence of SF5 anions. The SF5 anion in
CsSF5 adopts a square-pyramidal geometry14d of C4V sym-
metry, giving rise to three characteristic absorbance maxima in
the IR spectrum at 795, 525 and 469 cm−1. These data are in
excellent agreement with our results (Fig. 3).
Dioctylviologen (C8V) 1c also reacted with SF6 to give a blue-
green paramagnetic microcrystalline precipitate. However, only
one signal of fluoride anions (δ = −160 ppm) and no evidence
for the SF5 anion in the 19F NMR spectra were observed.
Taking into consideration the colour of the product and its
paramagneticity we assume that incomplete oxidation of violo-
gen occurred and the corresponding radical cation C8V•+ was
formed.17a Further experiments with other electron donors,
such as TMBI 1d and TADE 1e, did not show any reaction.
In order to understand the correlation between the activity
of the organic electron donors toward SF6 activation and their
redox potential, compounds 1a–e were subjected to cyclic
voltammetry (CV) studies. The results are presented in Table 1.
Scheme 1 Reduction of SF6 by organic electron donors.
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Scheme 2 Proposed mechanism for the SF6 reduction. D – organic
e-donor.
Fig. 3 The IR spectra of 2b (upper line) and [1b]2Br− (see the ESI†)
(bottom line). Bands attributed to the {bis(t-Bu)Py(propylene)}2+ dication
and to the SF5−-anion are denoted with (o) and (*), respectively.
Scheme 3 Organic electron donor mediated deoxyfluorination.
the donor radical-cation and SF6•−–radical-anion couple
Table 1 RedOx-potentials (vs. SCE) of donors 1a–e measured by CV
(Scheme 2). The dissociation pathway of the resulting anion-
Organic electron donor E11=2 (V)
E21=2 (V) Reaction with SF6
•−
radical SF6 is not clear and can lead either to the fluorine-
−
1a
−1.12
−0.98
−0.81
−0.67
—
Observed
radical and the SF5 anion (path b) or the fluoride-anion and
•
1b
−0.71
−0.37
—
Observed
Observed
Not observed
Not observed
the SF5 -radical (path a). However, the latter is considered to
C8V (1c)
TMBI (1d)
TDAE (1e)
be more favourable.
−
−0.78; −0.62a −0.46b
The fact that viologen C8V does not form the SF5 anion
product can be rationalized by the value of its redox-potentials
(Table 1). The first EredOx is sufficient to activate SF6 but the
second one is too low for the reduction of the SF5-radical.
Notably, E21=2 is sufficient for the reduction of fluorine-
radicals23 and C8V should give an SF5-anion containing
product in the case of path b. However, this is not observed,
supporting the notion that path a is more probable. Finally,
after the second SET from D•+ (for 1a, b) an ion pair consisting
All measurements were done in acetonitrile vs. Ag/Ag+(CH3CN)-couple
quasi-reference electrode with ferrocene (EredOx = 0.073 V) as an
internal standard. The values are recalculated and stated versus
Saturated Calomel Electrode (vs. SCE). a Two anodic peaks. b One
cathodic peak.
Starting with donor 1a we carried out the measurements in
acetonitrile and the Ag/Ag+ reference electrode (for details see
the ESI†). According to the literature data,16c 1a shows one
reversible two-electron oxidation wave with E1/2 = −1.23 V vs.
SCE in DMF due to the similarity of ERedOx for each SET-
process. The same behaviour was observed for 1a in aceto-
−
of a dication, a F− and an SF5 anion, [D2+][F−,SF5−], is
formed, which can be detected.
nitrile and the redox potential was determined with E1/2
=
−1.12 V vs. SCE. The donor 1b had not been measured before
and two reversible one-electron waves were obtained with
E11=2 = −0.98 V and E12=2 = −0.71 V vs. SCE. Following this obser-
vation, we also measured the redox-potentials of dioctylviolo-
gen 1c (E11=2 = –0.81 V; E12=2 = −0.37 V vs. SCE); TMBI 1d (E11=2
=
−0.67 V vs. SCE) and TDAE 1e. Two anodic peaks (E1pa = −0.78
V; E2pa = −0.62 V vs. SCE) and one cathodic peak (Epc = −0.46 V
vs. SCE) were observed. From these results, we conclude that
the electron donor needs to have a redox potential of about
E1/2 = −0.8 V vs. SCE in order to activate SF6.
The oxidation path of the electron donors is known and it
proceeds through SET processes via the formation of a radical
cation. However, so far, mechanistic investigations of SF6
reduction are rare and have been mainly performed in a
gaseous phase or with solvated electrons.22 Based on these
studies and our observations we propose the following mech-
anism for the sulphur hexafluoride reduction: the SET process
Scheme 4 Deoxyfluorination of benzylic alcohols, aldehydes and
from the electron donor D to SF6 results in the formation of carboxylic acids with 2a and 2b.
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During our NMR studies, we recorded the 19F-NMR spectra
of 2a and 2b in methanol-d4. Interestingly, we observed a
septet signal which was assigned to the formation of CD3F (δ =
−276.0 ppm, sept, J2(D,F) = 7.1 Hz) (Scheme 3).24
This result can be explained by a decay of the formed
[D2+][F−,SF5−] ion pair in solution and the formation of
[D2+][2F−] and sulphur tetrafluoride (SF4), which is known as a
deoxy-fluorinating reagent.25
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Given that the salt [D2+][F−,SF5−] can be considered as a
solid, safe and easy to handle method of storage of SF4, we
decided to evaluate this new procedure by applying the
method to the fluorination of benzyl alcohols, aldehydes and
carboxylic acids. Our proof-of-concept studies are shown in
Scheme 4. Generally, the reaction occurred with all three sub-
strate classes and provided the benzyl fluorides from the
corresponding alcohols (Scheme 4a), aryl-difluoromethane
from benzyl aldehyde (Scheme 4b) and acid fluoride from the
carboxylic acid (Scheme 4c).26
Conclusions
In summary, we report the use of organic electron donors for
the activation of the greenhouse gas, sulphur hexafluoride.13
Bipyridine-based organic electron donors were found to react
fast and selectively with SF6 at ambient temperature in non-
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−
consisting of donor dications [D2+] and fluoride [F−] and [SF5
]
−
anions. The presence of the SF5 anion was confirmed by
NMR- and IR-spectroscopy analysis. The salts can be isolated
and also be applied as fluorinating reagents. This was demon-
strated by the deoxofluorination of alcohols, aldehydes as well
as carboxylic acids. Thus, SF6 is a readily available and stable
precursor for the otherwise more difficult to handle deoxo-
fluorinating reagent, sulphur tetrafluoride. Given the simpli-
city of the procedure it can be used for both, the decompo-
sition of one of the most potent greenhouse gases and at the
same time the generation of a powerful fluorinating reagent.
Further applications of this metal-free activation of SF6 as well
as demonstration to reuse and recycle the electron donors are
currently part of our further research.
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Acknowledgements
We thank Dr Christoph Rauber for low temperature 19F-NMR
spectra measurements. We would also like to thank Professor
A. C. Filippou and Dr J. Tirree for the opportunity to record
solid state IR spectra under an inert atmosphere.
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