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M

any of the most abundant minerals on Earth belong to

the class of oxoanionic compounds, i.e. they contain

complex anions like silicate (SiO

4

4-

), phosphate (PO

4

3-

),

carbonate (CO

3

2-

), nitrate (NO

3

-

), and sulfate (SO

4

2-

). Parts of

these minerals are used, for example, as gem stones, fertilisers or

resource materials for the extraction of various metals. Structurally,

oxoanionic compounds exhibit an enormous diversity. This is

especially true for tetrahedral anions, which show a large number

of different co-ordination modes. The tendency of tetrahedral

anions to co-ordinate to a given metal centre is correlated to its

charge. Thus, SiO

4

4-

and PO

4

3-

are strongly co-ordinating and

linking ligands, whilst, for example, the perchlorate anion, ClO

4

-

(which does not occur in natural abundant minerals), is a so-

called ‘weakly co-ordinating anion’, e.g. it only shows a slight

tendency to enter the inner co-ordination sphere of a cation.

The structural chemistry of silicates and phosphates is getting

even more complicated because of frequent condensation under

the formation of larger anions, namely polysilicates like Si

2

O

7

6-

or

polyphosphates like P

3

O

10

5-

. Such a condensation is not possible

for the perchlorate anion, because this would lead to uncharged

Cl

2

O

7

molecules. SO

4

2-

is in between these extremes: on one hand

it co-ordinates not as extensively as silicates and phosphates, but

much better than the perchlorate anion; on the other hand it may

condense to larger anions (‘polysulfates’), but this tendency is

significantly lower than observed for silicates and phosphates.

An investigation into how the structural chemistry of the sulfate

ions fits between those of the well-known silicates and

phosphates on one hand, and the scarcely known perchlorates on

the other, was one of the driving forces when we started our work.

After some comments on sulfuric acid and synthesis, this

report shows some examples to illustrate the structural

diversity of sulfate chemistry. Subsequently, it will be shown

how slight derivatisations of the sulfate ions will even enrich

the structural features.

Reactions under harsh conditions: new pathways to

anhydrous sulfates

The initial compound for the preparation of sulfates is sulfuric acid,

H

2

SO

4

. Investigations into this acid began in the 17th Century

when the famous chemist Johann Glauber described how sulfate-

containing minerals called vitriols release a substance which he

called

spiritus vitrioli

that reacts with water to an acidic fluid.

Chemically,

spiritus vitrioli

was SO

3

and the acid obtained was

sulfuric acid. During the Industrial Revolution in England the so-

called ‘lead chamber process’ was invented by John Roebuck. This

process, as well as the double contact process, later established

by the German chemical company BASF, allowed for the production

of large amounts of sulfuric acid, which is nowadays the most

important inorganic substance in the chemical industry, and the

understanding of its chemistry is mandatory.

Our strategy to prepare new sulfates is the reaction of sulfuric

acid and its anhydride, SO

3

, under harsh conditions. For this

purpose the initial compounds were heated in sealed glass

ampoules to temperatures up to 450°C (Fig. 1). Such reaction

conditions have several advantages. Of utmost importance is that

Synthesis comes first – according to Professor Mathias SWickleder’s research

group at Gießen University, the development of preparative pathways is the

crucial step for gaining novel materials and new functions

Oxo for fun

58

I S S U E S E V E N

H O R I Z O N 2 0 2 0 P R O J E C T S : P O R TA L

www.horizon2020projects.com

P R O F I L E

C H E M I S T R Y

Fig. 1 For reactions under harsh conditions the starting materials were

sealed in thick-walled glass ampoules and heated up to 450°C

Fig. 2 Chelating SO

4

2-

ions in the structure of Pt

2

(SO

4

)

2

(HSO

4

)

2