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architectures of intermediate porosity. Such transformation is reminiscent
of the Ostwald's rule of stages in which the less stable frameworks, expected
to be of low density, transform into more stable, increasingly dense struc-
tures. The role of the salt additive in ZIF mechanosynthesis by ILAG is dif-
ferent than that in the synthesis of pillared MOFs. While the synthesis of
pillared MOFs using ILAG clearly reveals an anion-related effect, ZIF syn-
thesis strongly depends on the use of weakly acidic ammonium salts.
54,79
The topological control in ILAG synthesis of ZIFs appears to arise from the
ability of salt additives to control the rate of spontaneous interconversion of
porous and non-porous structures, rather than from specific structure-
templating effects.
79
The described mechanochemical methodologies have obvious advantages
over solvothermal approaches used for porous MOF synthesis in terms of
time (minutes versus days), temperature (near room-temperature versus 100-
150 1C) and reactant choice (metal oxides versus soluble salts). Further ad-
vantages are the absence of bulk solvent and a wide choice of catalytic liquid
phases, including ethanol, which is one of the more preferred solvents from
a green chemistry perspective. The only waste product of the oxide-based
reaction is water. The functional nature of porous MOFs obtained
mechanochemically was recently investigated by Yuan and co-workers, who
found that the Brunauer-Emmett-Teller (BET) surface area of the well-
known, commercially relevant MOF HKUST-1 obtained by neat milling was
comparable to that of samples obtained by conventional routes.
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A different
study by Schlesinger and co-workers compared neat and liquid-assisted
mechanosynthesis to room-temperature and reflux solution synthesis, sol-
vothermal, microwave-assisted, sonochemical, as well as electrochemical
syntheses.
80
The comparison revealed that BET surface area and specific
pore volume of HKUST-1 samples are increased if the reaction is conducted
by LAG, resulting in a material whose porous properties are comparable to
those of products obtained by solvothermal or microwave-assisted reactions.
Similar observations have been made for the mechanochemically made
MOF-14.
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7.6.2 Pharmaceutical Derivatives and Metallodrugs
Mechanochemistry is becoming increasingly popular in pharmaceutical
materials science for the synthesis of, as well as screening for, new solid
forms of active pharmaceutical ingredients (APIs).
13,82
Whereas most such
work has focused on making and screening for molecular materials, such
as polymorphs, co-crystals, solvates and salts, there is a growing interest
in mechanochemical derivatization of APIs using metal ions. Such use of
coordination chemistry for API modification has mostly targeted the
improvement in a specific property of a drug, such as solubility, with
few studies directed towards making metallodrugs,
83
i.e. metal-organic
materials whose pharmacological activity arises from biologically active
metal ions.
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