Chemistry Reference
In-Depth Information
employed ratio of AgCl and etu. Indeed, investigations of mechanochemical
co-crystallisation,
22
as well as of inorganic
23
and organic syntheses,
24
indi-
cate that excellent control over reaction stoichiometry is a general property
of mechanochemical reactions (Figure 7.1c).
Mechanochemical reactions can also provide coordination complexes
with stereoselectivity that is either different, or superior, to that observed in
solution. A notable example is the neat grinding reaction of platinum(
II
)
chloride with triphenylphosphine (PPh
3
), which yields exclusively the cis-
complex Pt(PPh
3
)
2
Cl
2
.
25
This contrasts with thermal reactions, which also
provide the trans-isomer. Besides demonstrating superior reaction selectivity
by grinding, this outcome also infers a difference between mechanochemical
and thermally-induced reactions. Another example of improved control over
reaction stereochemistry by switching from solution to mechanochemical
synthesis is the oxidative addition of bromine onto the organometallic
complex tricarbonyl(cyclopentadienyl)rhenium(
I
), CpRe(CO)
3
(Cp
ΒΌ
cyclo-
pentadienide anion).
26
Variation of mechanochemical reaction parameters
enabled the reaction to be either optimized for selective and quantitative
synthesis of the trans-isomer of the complex CpRe(CO)
2
Br
2
or to yield the cis-
isomer as the major (70%) product. The analogous reaction in solution
normally gives the two isomers in ca. 50 : 50 ratio.
However, probably the most important practical advantages of mechano-
chemistry in coordination chemistry, which can be identified as the major
driving force for the rapid development of this area, address the synthesis
of microporous MOFs. The rapid development of MOFs,
27
particularly for
applications in hydrogen storage,
28
carbon dioxide sequestration,
29
light
harvesting,
30
gas separation
31
and molecular sensing,
32
as well as their re-
cent commercialization and large-scale manufacture (e.g. the Basolite
s
series of products)
33
has highlighted the need to develop new techniques
for their discovery and synthesis in a scalable, atom- and energy-ecient
manner. Considerations
33,34
of safety, cost and environmental impact dic-
tate that thermally sensitive, toxic and often costly organic solvents and
metal precursors should be avoided. It is particularly desirable to replace the
hazardous or corrosive metal nitrates and chlorides with safer sulfates or
oxides.
33,34
Indeed, using metal oxides as starting materials is expected to
greatly improve synthetic procedures, as the only reaction byproduct would
in many cases be water and, as oxides are primary products in the course
of mineral and metal processing, such starting materials are inexpensive.
Unfortunately, low solubilities of metal sulfates and oxides, especially in
organic solvents, present a severe limitation to their use in solvent-based
MOF syntheses. Such limitations do not exist in mechanochemical
procedures in which the solvent is either not present or is added in
sub-stoichiometric, catalytic amount.
35
Consequently, mechanochemistry
permits the synthesis of MOFs from slightly soluble substances, such as
metal oxides or carbonates, at room temperature. The recently introduced
mechanochemical techniques of liquid-assisted grinding
35
and ion- and
liquid-assisted grinding
36
are particularly attractive for such clean syntheses
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