The First Step of the Oxidation of Elemental Sulfur: Crystal Structure of the Homopolyatomic Sulfur Radical Cation [S8].+

Article abstract of DOI:10.1002/anie.201703083

The oxidation of elemental sulfur in superacidic solutions and melts is one of the oldest topics in inorganic main group chemistry. Thus far, only three homopolyatomic sulfur cations ([S₄]²⁺, [S₈]²⁺, and [S₁₉]²⁺) have been characterized crystallographically although ESR investigations have given evidence for the presence of at least two additional homopolyatomic sulfur radical cations in solution. Herein, the crystal structure of the hitherto unknown homopolyatomic sulfur radical cation [S₈]^.+ is presented. The radical cation [S₈]^.+ represents the first step of the oxidation of the S₈ molecule present in elemental sulfur. It has a structure similar to the known structure of [S₈]²⁺, but the transannular sulfur???sulfur contact is significantly elongated. Quantum-chemical calculations help in understanding its structure and support its presence in solution as a stable compound. The existence of [S₈]^.+ is also in accord with previous ESR investigations.

Full text of DOI:10.1002/anie.201703083

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201703083
German Edition: DOI: 10.1002/ange.201703083
Sulfur
The First Step of the Oxidation of Elemental Sulfur: Crystal Structure
of the Homopolyatomic Sulfur Radical Cation [S8]C+
Janis Derendorf, Carsten Jenne,* and Mathias Keßler
Dedicated to Professor Jack Passmore
Abstract: The oxidation of elemental sulfur in superacidic
solutions and melts is one of the oldest topics in inorganic main
group chemistry. Thus far, only three homopolyatomic sulfur
cations ([S4]2+, [S8]2+, and [S19]2+) have been characterized
crystallographically although ESR investigations have given
evidence for the presence of at least two additional homopo-
lyatomic sulfur radical cations in solution. Herein, the crystal
structure of the hitherto unknown homopolyatomic sulfur
this intense color and that of similar solutions of other p-block
elements in oleum was one of the driving forces for
investigations of such systems in superacidic media.[7]
A
large number of homopolyatomic cations of sulfur and the
other p-block elements were discovered and structurally
characterized in the solid state, and have been reviewed
numerous times.[1,4b,8] Especially the impressive work by
Gillespie, Passmore, and co-workers led to the discovery of
the three presently known and crystallographically charac-
terized homopolyatomic sulfur cations [S4]2+,[9] [S8]2+,[9a,d,10]
+
+
radical cation [S8]C is presented. The radical cation [S8]C
represents the first step of the oxidation of the S8 molecule
present in elemental sulfur. It has a structure similar to the
known structure of [S8]2+, but the transannular sulfur···sulfur
contact is significantly elongated. Quantum-chemical calcula-
tions help in understanding its structure and support its
presence in solution as a stable compound. The existence of
and [S19]2+ [9d,11]
All known structurally characterized homo-
.
polyatomic sulfur cations are dications and were crystallized
with [AsF6]ꢀ or [SbF6]ꢀ counteranions.
In superacidic solutions, the situation is much more
complicated. As early as 1957, Symons and Ingram reported
evidence for the presence of paramagnetic compounds in the
blue solutions of sulfur in oleum.[12] In the 1970s, Gillespie and
co-workers thoroughly investigated sulfur cations in super-
acidic solutions (e.g., oleum, HSO3F).[10a,11,13] In contrast to
other main group elements such as phosphorus[2b,14] and
selenium,[15] studies of sulfur and its cations are severely
hindered by the absence of a useful nucleus for NMR
spectroscopic investigations. Therefore, the characterization
is limited to the interpretation of UV/Vis, IR, and electron
spin resonance (ESR) spectra of mixtures in solution or the
determination of crystal structures from isolated solids. The
oxidation of sulfur in liquid sulfur dioxide or superacidic
media with strong oxidizing agents gives colored solutions.
ESR spectra of these solutions show three different ESR
signals, namely a minor signal at g = 2.008 and two main
signals at g = 2.027 and 2.014, which change in intensity during
the addition of AsF5.[11] These signals obviously originate
from different sulfur radical cations in different oxidation
states, and these sulfur radicals have been discussed contro-
+
[S8]C is also in accord with previous ESR investigations.
H
omopolyatomic cations of the main group elements have
been one of the main themes in p-block chemistry in the
second half of the last century. These often very simple
compounds with unusual structures forced the development
of new models in chemical bonding.[1] Lately, the synthetic
strategies and methods that were very successful in the past
did not allow for further progress. Nevertheless, by using new
methods and techniques, spectacular results were still
obtained even recently. For instance, evidence for the first
homopolyatomic phosphorus cation [P9]+ was obtained by
using modern weakly coordinating anions instead of classical
hexafluorometallates such as [AsF6]ꢀ.[2] The first homopolya-
tomic antimony cation [Sb8]2+ was stabilized even by the
simple chloro anion [GaCl4]ꢀ.[3] Recently, ionic liquids as
reaction media[4] and electrochemical syntheses[5] were intro-
duced as new strategies for the field of homopolyatomic
cations of the heavier chalcogen elements.
+
+
+
+
The first generation of a homopolyatomic cation of the
main group element sulfur was reported already in 1804,
when Buchholz observed an intense blue color upon dissolv-
ing elemental sulfur in oleum.[6] The quest for the origin of
versially. The sulfur radicals [S12]C , [S8]C , [S7]C , [S5]C , and
+
[S4]C were suggested to be the origin of these resonan-
ces.[10a,11,13,16] Eventually, the ESR signal at g = 2.014, which
appears at high concentrations of an added oxidizing agent,
+
was unambiguously assigned to [S5]C by ESR experiments
using 33S-enriched sulfur.[17] Further evidence for the presence
of additional currently unidentified sulfur cations comes from
the color of these superacidic solutions. Typically, solutions of
sulfur in a superacidic medium containing a large excess of an
oxidizing agent are blue. However, Gillespie and co-workers
discovered that solutions with a formal composition of
[S16][AsF6]2 are red and turn blue only after addition of
more oxidizing agent.[11,13] Conversely, a blue solution of
[S8][AsF6]2 takes up sulfur up to an approximate composition
[*] J. Derendorf, Prof. Dr. C. Jenne, Dr. M. Keßler
Anorganische Chemie
Fakultꢀt fꢁr Mathematik und Naturwissenschaften
Bergische Universitꢀt Wuppertal
Gaußstr. 20, 42119 Wuppertal (Germany)
E-mail: carsten.jenne@uni-wuppertal.de
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.201703083.
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of [S19][AsF6]2, and undergoes a color change from blue to red
when an approximate composition of [S16][AsF6]2 is
reached.[18] It was suggested that [S16]2+ may exist in equilib-
rium with [S8]C+ in solution.[10a] However, having determined
the crystal structure of [S19]2+, Gillespie rejected the existence
interpretation with a different charge distribution (a neutral
S8 ring co-crystallized with M+[B12Cl12]ꢀ (M = Na, Cs)) can be
excluded for several reasons. The conformation of the eight-
membered sulfur homocycle is very different (see below)
from the typical crown structure of neutral S8. The B12Cl12
cluster is undistorted and very close to Ih symmetry and thus
in accord with a [B12Cl12]2ꢀ dianion whereas a significant
Jahn–Teller distortion would be expected for the radical anion
[B12Cl12]Cꢀ.[23]
The crystal structure shows some disorder for the sulfur
ring, which was modeled over two sites (see Section S2). The
alkali metal cation in the structure was modeled to be
a mixture of Cs+ and Na+ with a freely refined ratio. The alkali
metal cations originate from incomplete metathesis reactions
during the preparation of the [Sx][B12Cl12] (x = 4, 8) salts (see
Section S1.7). Apparently, their presence is crucial for the
crystallization process.
of [S16]2+ [11]
.
Our previous success in the use of the weakly coordinating
anion [B12Cl12]2ꢀ for the stabilization of highly reactive
cations in the solid state prompted us to pursue a similar
approach to investigate homopolyatomic cations of sulfur.[19]
First attempts to perform simple metathesis reactions of the
soluble salts [S8][AsF6]2 and Na2[B12Cl12] in liquid sulfur
dioxide were disappointing because a mixture of the two
insoluble solids [S8][B12Cl12] and Na[AsF6] was obtained.
Replacing Na+ by [NBu4]+ led to the formation of the soluble
byproduct [NBu4][AsF6], which could be removed. However,
the remaining blue residue was still insoluble in liquid sulfur
dioxide and anhydrous hydrogen fluoride. The insoluble
material resisted all attempts to characterize it. The very low
solubility is due the high lattice energy of this salt, which
consists of a dication and a dianion. In addition, sulfur cations
are incompatible with many organic solvents, for instance,
they undergo cycloaddition reactions with nitriles;[20] there-
fore, we decided to use supercritical sulfur dioxide (Tc =
1588C, pc = 77.8 atm) as a medium for crystallization.[21]
Numerous attempts were made to obtain crystalline material
in supercritical sulfur dioxide starting from [S8][B12Cl12],
[S4][B12Cl12], or mixtures of both compounds as well as
mixtures of one of the salts with elemental sulfur (see the
Supporting Information, Section S1). Eventually, a crystalli-
zation attempt with a mixture of equimolar amounts of
[S8][B12Cl12] and [S4][B12Cl12] gave blue single crystals of the
composition Cs0.73Na0.27[S8][B12Cl12].[22] The structure contains
an eight-membered sulfur homocycle aside from alkali metal
cations and [B12Cl12]2ꢀ dianions (Figure 1 and Section S2). For
electroneutrality, the sulfur homocycle has to have a charge of
The obtained crystal structure represents the first example
of a crystallographically characterized homopolyatomic sulfur
+
radical cation. The [S8]C radical cation has an exo–endo
conformation similar to the structure of [S8]2+ but the
transannular sulfur···sulfur contact (S3···S7) is significantly
+ [9a]
elongated from 285 pm in [S8]2+ to 362 pm in [S8]C . The
experimentally determined structure of [S8]C+ resembles the
+
structure of the calculated lowest-energy isomer of [S8]C
(Figure 1).[24]
+
The elongated transannular sulfur···sulfur contact in [S8]C
compared to [S8]2+ can be explained by a simple treatment of
the frontier molecular orbitals (Figure 2). The highest molec-
ular orbital (HOMO) of [S8]2+ is bonding across the ring and
thus results in a shortening of the transannular contact.[8d,9a]
When a further electron is added to [S8]2+, the lowest
unoccupied orbital (LUMO) of [S8]2+ will be occupied. The
+
resulting singly occupied molecular orbital (SOMO) of [S8]C
is of antibonding nature across the ring, which causes
a significant elongation of the transannular sulfur···sulfur
+
+ 1 and thus has to be an [S8]C radical cation. An alternative
Figure 1. Experimental (X-ray) and calculated (PBE0/def2-TZVPP) structures of [S8]2+ (left) and [S8]C+ (right). Thermal ellipsoids set at 50%
probability.
2
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Table 1: Reaction enthalpies and free energies [kJmolꢀ1] in the gas phase
and SO2 solution determined by quantum-chemical calculations (PBE0/
def2-TZVPP).
Oxidation
state
Equation
DH(gas)
DHðSO
DGðSO
2Þ
2Þ
+1/8
[S8]C+Q[S7]C+ +1/8 S8
[S8]C+Q[S6]C+ +2/8 S8
[S8]C+Q[S5]C+ +3/8 S8
24.6
43.8
49.4
112.8
201.5
17.6
31.5
26.4
77.9
40.7
11.2
21.6
6.8
118.8
47.8
+
+
[S8]C Q[S4]C +4/8 S8
[S8]C+Q4/8 [S8]2+ +4/8 S8
Figure 2. Frontier molecular orbitals of [S8]2+ and [S8]C+.
+1/7
+1/6
[S7]C+Q2/3 [S8]C+ +1/3 [S5]C+
ꢀ8.2
ꢀ8.8
ꢀ9.0
+
contact. The experimental structures of [S8]2+ and [S8]C were
+
+
+
[S6]C Q1/3 [S8]C +2/3 [S5]C
ꢀ10.9
ꢀ6.9
ꢀ13.9
ꢀ9.5
ꢀ17.1
ꢀ12.6
well reproduced by DFT calculations (Figure 1).
[S6]C+Q1/2 [S5]C+ +1/2 [S7]C+
[S5]C+Q3/10 [S6]2+ +2/5 [S8]C+
+
The homopolyatomic sulfur radical cation [S8]C repre-
+1/5
108.3
20.9
41.2
sents the first step of the oxidation of S8, and was expected to
show an ESR signal. Therefore, we revisited previously
published ESR data of homopolyatomic sulfur cations.
Solutions of sulfur in superacidic media containing small
amounts of a strong oxidizer showed a signal at g = 2.027 in
ESR spectroscopic investigations.[11] The signal at g = 2.014,
+
S8, is actually the formation of the hitherto unknown [S8]C
radical cation.
+
assigned to [S5]C , appeared only in the presence of large
amounts of strong oxidizers.[17] Symons and co-workers have
investigated the sulfur radicals formed by radiation of S8 in
frozen trichlorofluoromethane solution by ESR spectrosco-
py.[25] They observed two different signals, which were
Acknowledgements
This work was supported by the Deutsche Forschungsge-
meinschaft (DFG). We are grateful to Dr. Thilo Ludwig for
assistance with the reactions in supercritical sulfur dioxide.
+
assigned to two different forms of [S8]C . The initially
formed radical with gav = 2.041 is transformed into a second
radical with gav = 2.027 upon annealing. Taking into account
the crystal structure data presented above, Symonsꢀ experi-
ments can be reinterpreted as follows: Irradiation of S8 in
Conflict of interest
+
frozen solution gives [S8]C in crown conformation, which is
The authors declare no conflict of interest.
transformed into the lowest-energy isomer with an exo–endo
conformation, as in the crystal structure, upon annealing. The
g value of the relaxed form of the [S8]C radical cation in
Keywords: crystal structures · homopolyatomic cations ·
radicals · sulfur · weakly coordinating anions
+
Symonsꢀ report is identical to that of the sulfur radical found
in initial stages of the oxidation of sulfur in superacidic media.
Therefore, it is very likely that the first step of the oxidation of
sulfur in superacidic media also corresponds to the formation
+
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+
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+
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+
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+
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Manuscript received: March 24, 2017
Version of record online: && &&, &&&&
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Communications
Sulfur
A radical step: Crystallographic evidence
is presented for the new homopolyatomic
+
J. Derendorf, C. Jenne,*
M. Keßler
sulfur radical cation [S8]C , which is the
&&&& — &&&&
product of the first step of the oxidation
of elemental sulfur. These results are
supported by quantum-chemical calcula-
tions and confirm previously published
ESR data.
The First Step of the Oxidation of
Elemental Sulfur: Crystal Structure of the
Homopolyatomic Sulfur Radical Cation
[S8]C+
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