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PROFILE
M AT E R I A L S & C H E M I S T R Y
Sulfates
Sulfuric acid and sulfates belong to the longest known chemical species.
Nevertheless, their chemistry is still full of surprising results and fascinating structures
M
any of the most abundant minerals on Earth belong to
the class of sulfates, i.e. they contain the SO42- anion.
Among these minerals the so-called ‘vitriols’ were of the
utmost importance for a long time as starting material for the
fabrication of sulfuric acid, H2SO4. The procedure of gaining
sulfuric acid from vitriols can be traced back to the 17th Century
when the famous chemist Johann Glauber explained that the
heating of vitriols led to a substance which he called spiritus
vitrioli and which gives an oily liquid through the reaction of water
named oleum.
Chemically, spiritus vitrioli is sulfur trioxide, SO3, and the preparation
of SO3 is still the first step in the production of sulfuric acid,
although nowadays the process is based on elemental sulfur in the
so-called ‘double contact process’. In addition, the term oleum is
also still used today for a mixture of H2SO4 and SO3. Sulfuric acid is
one of the most important chemical substances in industrial
chemistry. It is a very strong Brønsted acid, a good electrolyte and a
quite strong oxidiser. Besides the abovementioned sulfates the
protonated variety HSO4-, called hydrogensulfates, is known for a
large number of metals. However, the connection of sulfate groups
to form larger arrays like chains or even networks of higher
dimensionality is very rare. This is in strong contrast to other
mineralic acids like phosphoric or silicic acid, H3PO4 and ‘H4SiO4’,
which show a strong tendency to condensation under formation of
polyphophoric and polysilicic acid.
Therefore, many of the naturally abundant phosphates are
polyphosphates, and almost all the frequently occurring silicates
are in fact polysilicates displaying a fascinating structural
chemistry with one, two and three-dimensional networks of linked
[SiO4] tetrahedra. The observation that condensation increases
with decreasing charge of the central atom in the tetrahedron is
Fig. 2 The tris(disulfato)palatinate(IV) [Pt(S2O7)3]2- with chelating
S2O72- ions
reasonable with respect to electrostatic arguments: the overall
charge of the tetrahedron growth in the series [SO4]2-, [PO4]3-,
[SiO4]4- and condensation is a simple way to compensate the
negative charge located at the oxygen atoms. Following this trend,
however, the condensation of H2SO4 under formation of
polysulfuric acids should also be possible, even if the tendency is
not very high. Indeed, the most simple polysulfuric acid, that is
disulfuric acid (H2S2O7), is well-known and also its salts
containing the disulfate anion S2O72- have been reported.
Long-chained polysulfuric acids, however, are not known up to
know. Our research, which started some 20 years ago, addresses
different topics. On one hand we explore the reactivity of sulfuric
acid, sulfur trioxide and mixtures of both under harsh conditions.
On the other hand we are interested in the structural chemistry of
the sulfate anion in combination with different metals, and finally
we aim at the characterisation of the hitherto unknown
polysulfuric acids and their compounds.
Reactions
Fig. 1 Chelating anions in the structure of Pt2(SO4)2(HSO4)2
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Reactions with H2SO4, oleum, and SO3 under harsh conditions
were carried out in sealed glass ampoules containing the
respective starting materials. If concentrated H2SO4 is used the
ampoules could be heated up to 450°C without issue. For a
mixture of H2SO4 and SO3 the maximum temperature is 250°C
and neat SO3 should not be heated above 120°C. Under such
conditions highly unusual reactivities are seen. One striking
example is the oxidation of elemental platinum with sulfuric acid
at 300°C under formation of Pt2(SO4)2(HSO4)2. This reaction is
especially noteworthy because platinum is usually regarded as
unreactive against acids. That’s why it is used as metal for
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Fig. 3 Single crystals of Pd(S2O7)
various lab ware. The structure of Pt2(SO4)2(HSO4)2 contains
[Pt2] dumbbells co-ordinated by four chelating sulfate (and
hydrogensulfate) ions to give the so-called ‘paddlewheel motif’
(Fig. 1). In subsequent experiments we have very often found
this structural motif, and only in rare cases were we able to
oxidise the metal to its tetravalent state within the complex
anion [Pt(S2O7)3]2- which shows the platinum atom in
co-ordination of three chelating disulfate groups (Fig. 2).
However, this oxidation state can only be achieved if SO3-rich
oleum is used in the reaction.
Fig. 4 Structure of the molecular disulfate Re2O4Cl2(S2O7)2
Interestingly, analogous reactions using the platinum congener
palladium as reaction partner did not run at all. Only if pure SO3
is used can the metal be oxidised, leading to beautiful crystals of
Pd(S2O7) (Fig. 3). The compound exhibits a Pd2+ ion in octahedral
co-ordination of oxygen atoms, leading to the paramagnetic
behaviour of the compound. Moreover, below 11.6 Kelvin a
ferromagnetic ordering is observed, the first example of such
behaviour for a Pd2+ compound.
their salts with polysulfate anions [SnO3n+1]2- of different lengths.
These polysulfates have been obtained either in isolated form or
as co-ordinating ligands in complexes. For example, in the noble
metals complexes [Au(S3O10)2]- and [Pd(S4O13)2]2- (Fig. 5), the
noble metal atoms are attached by two chelating trisulfate and
tetrasulfate ions, respectively. Compared to the isolated species
observed, for example, in Pb(S3O10) and (NO2)2[S4O13] the
co-ordination of the polysulfate anions leads to a significant
change of the bond lengths within the anions. Nevertheless,
theoretical calculations reveal that the stability of polysulfate ions
decreases with increasing chain length so that the preparation of
higher polysulfates is really challenging. Up to now we were only
able to extend the series up to a hexasulfate, prepared as the
potassium salt K2(S6O19) (Fig. 6).
Condensation
Derivatisation
The formation of Pd(S2O7) shows that higher sulfates, i.e
polysulfates with linked [SO4] tetrahedra, are to be expected if
SO3-rich media are used. This observation does not only hold for
reactions involving noble metals. The rhenium compound
Re2O4Cl2(S2O7)2 is another intriguing example. It is the first
molecular disulfate known so far and has been obtained from
oleum and ReCl5 (Fig. 4). The compound results from the reaction
of ReCl5 and oleum. Also the preparation of a large number of
[M(S2O7)3]2- type complexes containing silicon, germanium, tin,
and titanium shows the generality of the assumption. It is
worthwhile mentioning that all of these complexes have the same
structure as the abovementioned platinum compound.
The successful preparation of compounds with structures
stamped by the linkage of [SO4] tetrahedra raised the question of
if such a linkage could also be realised by using chemically
different tetrahedra, for example [SO4] and [SiO4], [SO4] and
[PO4], or [SO4] and [BO4]. While we were not successful for the
It is obvious that the formation of disulfates in oleum as a
reaction medium can be seen as the result of the presence of
H2S2O7 in the reaction mixture which was gained from the
reaction of H2SO4 and SO3. In this picture the formation of the
hitherto unknown long-chained polysulfuric acids H2SnO3n+1 (with
n being an integer number) should be possible under very high
SO3 concentrations. Even though we were, up to now, not able to
prepare the neat acids, we could gain a remarkable number of
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Fig. 5 The [Pd(S4O13)2]2- ion
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Fig. 6 The unique hexasulfate anion S6O192-
first two combinations, we recently obtained a fascinating
compound resulting from the linkage of sulfate and borate
tetrahedra. The unique compound has the composition B2S2O9
and exhibits a layer type network of corner-connected [BO4] and
[SO4] groups. The connection leads to a structural motif that is
known from a number of layered silicate minerals (Fig. 7). With
respect to the manifold properties of the latter it will be interesting
to develop potential applications for the boron sulfate, maybe as
sorption materials.
The few examples discussed might clearly show that sophisticated
synthesis may lead to a plethora of new compounds with unusual
structures and properties, even for such an obviously well-known
class of compounds like the sulfates. This richness might even be
increased if various derivatives of the [SO4] anion are taken into
account. The simplest derivatisation of sulfate ions is the formal
substitution of one oxygen atom for an OH, NH2, CH3 or CF3 group.
This leads to the formation of a hydrogensulfate, amidosulfate,
methanesulfonate and trifluoromethanesulfonate anion,
respectively (Fig. 8). These well-known anions are very similar to
each other as they all have a tetrahedral shape and the same
charge of -1.
Because the hydrogensulfate ion is always present in sulfuric acid
it is clear that hydrogensulfates may readily occur in reaction with
sulfuric acid. The abovementioned example of Pt2(SO4)2(HSO4)2
shows that this is indeed the case. The preparation of
amidosulfates is somewhat special because the related
Fig. 8 Derivatives of the sulfate anion
amidosulfuric acid is a solid due to its zwitterionic character.
Contrastingly, methanesulfonic and trifluoromethanesulfonic acid
(‘triflic acid’) are liquids with properties very similar to sulfuric
acid. That means that our typical preparation methods can be
easily applied, and an abundance of new anhydrous compounds
of these acids could be gained.
Because one vertex of the tetrahedral anion cannot be used for
co-ordination, i.e. the CH3 and the CF3 group, the co-ordination
behaviour is very different from that of the sulfate (and also
hydrogensulfate) ion. In general this leads to a weaker linkage of
the anions, and very often chain or layer type structures are seen.
For example, the palladium methanesulfonate Pd(CH3SO3)2 has a
chain structure with the anions acting as bidentate-bridging
ligands. These chains are only connected in the crystal structure by
weak interactions leading often to the mechanical lability of the
compounds. This is even more true for trifluoromethanesulfonates
(triflates) because the CF3 group is even more weakly interacting. A
nice example is the structure of Zr(CF3SO3)4, which shows chains
of Zr4+ ions linked by eight bidentate-bridging triflate groups. The
rods formed in that way are ‘decorated’ by CF3 groups so that no
interactions can occur and the rods can be easily shifted with
respect to each other.
Summarising, it can be stated that the investigation of long-known
sulfuric acid and its derivatives is still a fruitful topic in inorganic
and materials chemistry.
Professor Mathias S Wickleder
Inorganic Functional Materials
Institute of Inorganic and Analytical Chemistry
Justus Liebig University, Giessen, Germany
+49 641 99 34100 / 34101
Fig. 7 The layer type boron sulfate B2S2O9. The structure is
strongly related to layer-type silicates, bearing interesting properties
of this compound
mathias.wickleder@anorg.chemie.uni-giessen.de
https://www.mwickleder.de
Reproduced by kind permission of Pan European Networks Ltd, www.paneuropeannetworks.com
© Pan European Networks 2016
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