Chemistry Reference
In-Depth Information
The first reported mechanosynthesis of a coordination polymer by design
appears to have been based on a ligand exchange reaction involving the
binuclear copper(
II
) acetate paddlewheel cluster: manual grinding of cop-
per(
II
) acetate monohydrate with 1,3-bis(4-pyridyl)propane (pn) produced the
water inclusion compound of a 1D polymer by mechanochemical replace-
ment of copper-coordinated water molecules with bridging pn ligands
(Figure 7.5c).
65
The product was isostructural to the product of an analogous
reaction in refluxing methanol, which yielded a methanol inclusion com-
pound. Isostructurality indicated that the water produced by mechano-
chemical ligand exchange probably became incorporated into the
mechanochemical product as a guest. This was also supported by the
13
C
MAS-NMR spectrum of the mechanochemical product, which was identical
to the one of the methanol solvate, with the exception of NMR signals of the
guest methanol molecules.
During manual grinding of silver(
I
) acetate (AgOAc) and dabco, the neutral
dabco ligand displaces the anionic acetate ligands coordinated to Ag
1
with
simultaneous absorption of water from air to form the product Ag(dab-
co)
2
(OAc)
5H
2
O. The structure consists of cationic [Ag(dabco)
2
(H
2
O)]
n
n1
chains with additional water molecules and acetate ions acting as inclu-
sion guests.
66
The formation of a hydrated product upon grinding in air
illustrates the influence that the surrounding atmosphere can have on
mechanosynthesis in the open mortar-and-pestle setup.
7.5.3 Acid-Base Reactions
Reactions mediated by proton transfer are a powerful tool in mechano-
synthesis of coordination compounds, particularly reactions of metal
acetates with organic acids, which yield acetic acid as a byproduct. An ex-
ample of such reactivity involves milling of hydrated nickel(
II
) acetate and
acetylenedicarboxylic acid (H
2
adc) to form the hydrated 3D coordination
polymer Ni(adc)(H
2
O)
2
.
43
The same product was also obtained by grinding of
H
2
adc with Ni(NO
3
)
2
6H
2
O, revealing that very basic anions like acetates are
not crucial for successful proton exchange in the solid state. Analogous re-
action with zinc acetate led to the formation of a previously unknown 3D
polymer Zn(adc)(H
2
O)
2
that is isostructural to the Ni(
II
) analogue, demon-
strating how mechanosynthesis can be used to discover new metal-organic
materials. A similar approach was used for the synthesis of coordination
polymers of alkaline earth metals, specifically by grinding-annealing of
magnesium or calcium acetate with H
2
adc
67
to yield Ca(adc) and
Mg(adc)
2H
2
O products, isostructural to Sr(adc) and Mn(adc)
2H
2
O.
The acidic ligand in reactions with metal acetates does not need to be a
carboxylic acid, as demonstrated by grinding of transition metal acetates
with HCNacac (Figure 7.2a).
40
Acetates of Fe(
II
), Co(
II
) and Ni(
II
) directly yield
non-porous 3D frameworks. However, hydrated acetates of Mn(
II
), Cu(
II
)or
Zn yield previously unknown discrete complexes Mn(CNacac)
2
2H
2
O,
Cu(CNacac)
2
H
2
O and Zn(CNacac)
2
H
2
O,
respectively. As mentioned
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