4(1), Topics in current chemistry. Elemental sulfur and sulfur-rich compounds II

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Top Curr Chem (2003) 231:127–152
DOI 10.1007/b13183
Inorganic Polysulfides S
n
2
and Radical Anions S
n
·
Ralf Steudel
Institut fr Chemie, Sekr. C2, Technische Universitt Berlin, 10623 Berlin, Germany
E-mail: steudel@schwefel.chem.tu-berlin.de
Abstract
Inorganic polysulfide anions S
n
2
and the related radical anions S
n
·
play an impor-
tant role in the redox reactions of elemental sulfur and therefore also in the geobiochemical
sulfur cycle. This chapter describes the preparation of the solid polysulfides with up to eight
sulfur atoms and univalent cations, as well as their solid state structures, vibrational spectra
and their behavior in aqueous and non-aqueous solutions. In addition, the highly colored
and reactive radical anions S
n
·
with
n
= 2, 3, and 6 are discussed, some of which exist in
equilibrium with the corresponding diamagnetic dianions.
Keywords
Sulfur chains · Molecular structures · Radical anions · Spectra · Redox reactions
1
Introduction ................................... 128
2 Preparation of Solid Polysulfides ..................... 130
2.1 Alkali Metal Polysulfides ........................... 130
2.2 Sulfur-Rich Polysulfides with Complex Univalent Cations...... 132
2.2.1 General ...................................... 132
2.2.2 Tetrasulfides ................................... 133
2.2.3 Pentasulfides ................................... 133
2.2.4 Hexasulfides ................................... 133
2.2.5 Heptasulfides................................... 134
2.2.6 Octasulfides ................................... 134
3
Properties of Solid Alkali Polysulfides .................. 134
4
Structures of Polysulfide Dianions .................... 135
5 Polysulfide Solutions ............................. 137
5.1 Aqueous Solutions ............................... 137
5.2 Non-Aqueous Solutions ............................ 141
5.2.1 Electrochemical Studies ............................ 141
6
Vibrational Spectra .............................. 142
7
Reactions in Solution ............................. 143
8 Polysulfide Radical Anions ......................... 145
8.1 General ...................................... 145
8.2
The Radicals S
2
·
and S
3
·
........................... 145
Springer-Verlag Berlin Heidelberg 2003
128
Ralf Steudel
8.3 The Radical S
4
·
................................. 147
8.4 The Radical S
6
·
................................. 148
8.5 Calculated Structures ............................. 148
References ........................................ 149
List of Abbreviations
crypt-2.2.2 N(C
2
H
4
OC
2
H
4
OC
2
H
4
)
3
N
en
Ethylenediamine
hmpa
Hexamethylphosphoric triamide
N
,
N
,
N
0
,
N
00
,
N
000
-pentamethyldiethylenetriamine
pmdeta
teeda
Tetraethylethylenediamine
tmeda
Tetramethylethylenediamine
1
Introduction
Inorganic polysulfides are ionic substances containing chain-like dianions
S
n
2
. Such ions are formed in numerous reactions, e.g., by oxidation of
monosulfide ions HS
in water or other polar solvents as well as by reaction
of aqueous monosulfide with sulfur-rich compounds including elemental
sulfur:
2HS
2e
!
S
2
2
þ
2H
þ
ð
1
Þ
2HS
þ
S
8
!
2S
2
5
þ
2H
þ
ð
2
Þ
Therefore, polysulfide ions play a major role in the global geological and
biological sulfur cycles [1, 2]. In addition, they are reagents in important in-
dustrial processes, e.g., in desulfurization and paper production plants. It
should be pointed out however that only sulfide, elemental sulfur and sulfate
are thermodynamically stable under ambient conditions in the presence of
water, their particular stability region depending on the redox potential and
the pH value [3]:
1
=
2S
8
þ
4H
2
O
Ð
3HS
þ
SO
2
4
þ
5H
þ
ð
3
Þ
On the other hand, the large activation energy for the formation of sulfate
from S
8
and water makes it possible to prepare polysulfides as well as other
reduced sulfur compounds as metastable products in aqueous solution at
ambient conditions.
Polysulfides are the key reactants in the high-density sodium-sulfur and
lithium-sulfur batteries [4] which are based on the following reversible re-
dox reaction taking place in the polysulfide melt:
 129
Inorganic Polysulfides S
n
2
and Radical Anions S
n
·
1
=
2S
8
þ
2e
Ð
S
2
4
ð
4
Þ
In melts and polar solvents polysulfide dianions are usually present as
mixtures of species of different chain-lengths as a result of the following
types of equilibria which are rapidly established even at 20 C [5]:
2S
2
4
Ð
S
2
3
þ
S
2
5
ð
5
Þ
2S
2
5
Ð
S
2
4
þ
S
2
6
ð
6
Þ
The chemistry of polysulfide dianions is closely related to that of the rad-
ical-monoanions S
n
·
since both types of anions are in equilibrium with each
other in solution and in high-temperature melts, e.g
.
:
S
2
6
Ð
2S
3
ð
7
Þ
S
2
5
Ð
S
2
þ
S
3
ð
8
Þ
Furthermore, polysulfide anions are subject to autoxidation if molecular
oxygen is present, e.g.:
S
2
4
þ
3
=
2 O
2
!
S
2
O
2
3
þ
1
=
4 S
8
ð
9
Þ
In solution this reaction is rather rapid but in the solid state autoxidation
takes place much slower. Nevertheless, commercial sulfides and polysulfides
of the alkali and alkali earth metals usually contain thiosulfate (and anions
of other sulfur oxoacids) as impurities [6]. For all these reasons the chemis-
try of polysulfides is rather complex, and some of the earlier studies on
polysulfides (prior to ca. 1960) are not very reliable experimentally and/or
describe erroneous interpretations of the experimental results.
Polysulfides have been prepared with many different types of cations,
both monoatomic like alkali metal ions and polyatomic like ammonium or
substituted ammonium or phosphonium ions. In this chapter only those
salts will be discussed in detail which contain univalent main-group cations
although a large number of transition metal polysulfido complexes have
been prepared [7–9].
A truly comprehensive review on the chemistry of inorganic (ionic) poly-
sulfides has never been published. Szekeres [10] as well as Hanley and Czech
[11] reviewed the classical analytical chemistry (titrimetric and gravimetric
analysis) of sulfur acids including sulfides and polysulfides in 1974 and
1970, respectively. Chivers reviewed the chemistry of polychalcogenide an-
ions including the radical monoanions (with the stress on the latter) in 1977
[12]. Hamilton critically reviewed the literature on aqueous polysulfide solu-
tions and proposed a speciation model of his own [13].
130
Ralf Steudel
Table 1
Properties of some ionic polysulfides
Density (g cm
3
)
(20 C)
Compound
Color (20 C)
Melting point [14, 15]
Na
2
S
2
Yellow
470€10 C (b)
Two allotropes
Na
2
S
4
Orange-yellow
290€5 C
2.08
Na
2
S
5
Brown-yellow
265€5 C
2.08
K
2
S
2
Pale yellow
487 C
1.973
K
2
S
3
Yellow-brown
302 C
2.102
K
2
S
4
Orange-yellow
154 C
K
2
S
5
Orange
206 C
2.128
K
2
S
6
Red
189 C
2.02
2
Preparation of Solid Polysulfides
2.1
Alkali Metal Polysulfides
The rather complex equilibrium phase diagrams of the sodium-sulfur sys-
tem [14] and of the potassium-sulfur system [15] have been critically re-
viewed by Sangster and Pelton in 1997. In the sodium-sulfur system the
compounds Na
2
S, a-Na
2
S
2
, b-Na
2
S
2
,Na
2
S
4
, and Na
2
S
5
exist but neither Na
2
S
3
nor polysulfides higher than the pentasulfide do [16]. Na
2
S
2
exists as a-form
below 160 C and as b-form above this temperature; both are of hexagonal
crystal symmetry. b-Na
2
S
2
melts incongruently at 470 C while the tetra- and
pentasulfides show congruent melting points (peritectic) [14]. As higher the
sulfur content as lower the melting points (see Table 1). Liquid sodium poly-
sulfides easily supercool and form relatively stable glasses. By Raman spec-
troscopy it was found that Na
2
S
4
and Na
2
S
5
can be obtained as several meta-
stable phases depending on the preparation conditions [17,18]. The thermo-
dynamically stable forms at ambient conditions are designated by a.
In the potassium-sulfur system the compounds K
2
S, K
2
S
2
,K
2
S
3
,K
2
S
4
,
K
2
S
5
, and K
2
S
6
exist and there are six eutectics [15]. All sodium and potassi-
um sulfides and polysulfides are hygroscopic and some of them form well
defined hydrates.
The preparation of anhydrous Na
2
S
2
,Na
2
S
4
,Na
2
S
5
,K
2
S
2
,K
2
S
3
,K
2
S
4
,K
2
S
5
,
and K
2
S
6
has been described in detail by Fehr et al. [16, 19–21]. These pro-
cedures are based on the following reactions (M: Na or K):
2M
þ
1
=
4S
8
!
M
2
S
2
ð
10
Þ
M
2
S
þ x
=
8S
8
!
M
2
S
xþ
1
ð
11
Þ
2MHS
þ x
=
8S
8
!
M
2
S
xþ
1
þ
H
2
S
ð
12
Þ
M
2
S
4
þ
2M
!
2M
2
S
2
ð
13
Þ
 131
Inorganic Polysulfides S
n
2
and Radical Anions S
n
·
M
2
S
x
þ
1
=
8S
8
!
M
2
S
xþ
1
ð
14
Þ
Na
2
S
2
,K
2
S
2
,Na
2
S
4
,K
2
S
4
, and K
2
S
5
may be prepared from the elements in
liquid ammonia according to Eq. (10) in a special apparatus which allows
the strict exclusion of moisture and oxygen. The alkali metals are soluble in
liquid ammonia and reduce the sulfur stoichiometrically. After evaporation
of the solvent the product is homogenized by heating under vacuum to a
temperature just below the melting point.
Anhydrous Na
2
S
2
and K
2
S
2
may also be prepared according to Eq. (11),
e.g., by heating of the components in an evacuated glass ampoule to 500 C
until a homogeneous melt is obtained which is then allowed to cool slowly.
High-melting glass should be used for the ampoule.
Sodium disulfide for the in situ preparation of organic disulfanes R
2
S
2
may also be prepared from the elements in 1,2-dimethoxyethane at 70 C in
the presence of catalytic amounts of an aromatic hydrocarbon or ketone
[22].
The reaction at Eq. (12) allows the preparation of Na
2
S
4
and K
2
S
5
from the
alkali metals, hydrogen sulfide and sulfur in anhydrous ethanol (ROH). First
the metal is dissolved in the alcohol with formation of ethanolate (MOR)
and hydrogen. Bubbling of H
2
S into this solution produces the hydrogen sul-
fide (MHS). To obtain the polysulfide the solution is refluxed with the calcu-
lated amount of elemental sulfur. After partial evaporation of the solvent
and subsequent cooling the product precipitates.
The alcoholic solution of Na
2
S
4
prepared as described above may be re-
duced to Na
2
S
2
by addition of the calculated amount of sodium and refluxing
under pure nitrogen; see Eq. (13).
K
2
S
6
is obtained if K
2
S
5
and sulfur are heated in an evacuated glass am-
poule to 220–280 C for several hours followed by cooling to 20 C within
10 h; see Eq. (14).
The mechanism of the reaction of solid a-Na
2
S
2
with S
8
has been studied
by Raman spectroscopy [18]. The reaction begins at the melting temperature
of S
8
(120 C) and the primary product is a-Na
2
S
4
. Near 160 C the remain-
ing a-Na
2
S
2
first transforms to b-Na
2
S
2
which also reacts with S
8
to a-Na
2
S
4
.
If Na
2
S is used as a starting material it first reacts with S
8
to Na
2
S
2
. Heating
of a-Na
2
S
4
to 500 C followed by cooling to 120 C results in a glassy materi-
al which on annealing at this temperature forms crystalline g-Na
2
S
4
as a
metastable phase which melts at ca. 230 C. If a mixture of Na
2
S and a-Na
2
S
4
is heated to 200 C the Raman lines of b-Na
2
S
2
can be observed:
2Na
2
S
þ
Na
2
S
4
!
3Na
2
S
2
ð
15
Þ
Na
2
S
3
is not stable in the solid state, and cooling of a melt of composition
Na
2
S
3
leads to an eutectic mixture of Na
2
S
2
and Na
2
S
4
[18, 23]. However, the
Raman spectra of melts of this composition show a line at 462 cm
1
which
has been assigned to S
3
2
ions [18], and at low temperatures in liquid am-
monia a metastable phase of Na
2
S
3
has evidently been prepared by the fol-
lowing reaction [24]:
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