Chemistry Reference
In-Depth Information
manipulated by varying the conditions of neat milling, specifically the im-
pact force and temperature. While milling of ZnCl
2
with one or two
equivalents of cnge selectively provides the 1D polymer Zn(cnge)Cl
2
or the
discrete (0D) complex Zn(cnge)
2
Cl
2
, respectively, milling of CdCl
2
and cnge
in a 1 : 1 stoichiometric ratio provides the 3D structure Cd(cnge)Cl
2
. How-
ever, neat milling of CdCl
2
and cnge in the 1 : 2 stoichiometric ratio provides
only a mixture of Cd(cnge)Cl
2
with excess ligand. The expected 1D polymer
Cd(cnge)
2
Cl
2
could be obtained mechanochemically, but only through
harsher grinding conditions, i.e. by the use of heavier grinding media or by
milling at a higher temperature. The diculty in forming 1D Cd(cnge)
2
Cl
2
was related to the higher dimensionality and, hence, kinetic stability of
Cd(cnge)Cl
2
.
20
7.5.2 Ligand Exchange
An example of a mechanochemical ligand exchange reaction is ball milling
of cis-Pt(PPh
3
)
2
Cl
2
with potassium carbonate, giving rise to the neutral car-
bonato complex cis-Pt(PPh
3
)
2
CO
3
(Figure 7.5a).
25
The Coville group has
conducted extensive studies of solvent-free ligand exchange reactions on
organometallic complexes
12
and, in some cases, established that the re-
actions are mediated by a low melting eutectic phase (Figure 7.5b).
63,64
Figure 7.5 Mechanochemical reactions of ligand exchange: (a) synthesis of cis-
Pt(PPh
3
)
2
CO
3
by grinding of mechanochemically obtained cis-
Pt(PPh
3
)
2
Cl
2
with K
2
CO
3
.
25
(b) Thermal microscopy reveals that the
ligand exchange reaction between touching crystals of Mn(CO)
4
(PPh
3
)Br
and PPh
3
(on the left of each image) proceeds through an intermediate
eutectic melt phase although the temperature remains below the melting
point of any of the reactants.
63
For comparison, the right side of each
image shows a separate crystal of PPh
3
and the organometallic reactant.
Adapted with permission from ref. 63. (c) Mechanochemical replacement
of water ligands on the copper(
II
) acetate paddlewheel unit with the
bridging ligand pn yields a 1D coordination polymer.
65
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