Ball-milling Mechanochemical Synthesis of Coordination Bonds: Discrete Units, Polymers and Porous Materials - Ball Milling Towards Green Synthesis: Applications, Projects, Challenges - page 160

terephthalate and diazabicyclo[2.2.2]octane (dabco), by milling zinc oxide,
terephthalic acid, dabco and a space-filling agent, were unsuccessful without
the addition of small amounts of ionic salts (few molar percent with respect
to the metal). The addition of small amounts of nitrate salts (e.g. KNO 3 or
NH 4 NO 3 ) to the reaction mixture led to the quantitative formation of the
porous framework based on square-grid zinc terephthalate sheets within
45 min (Figure 7.3d). 53 Ionic additives containing the sulfate ion (e.g. Na 2 SO 4
or (NH 4 ) 2 SO 4 ) selectively and quantitatively produced the supramolecular
isomer of this MOF, based on zinc terephthalate sheets with a hexagonal
(Kagome) topology (Figure 7.3d).
These unexpected structure-directing effects were accompanied by the
inclusion of salts in the MOF porous structure, as revealed by solid-state
NMR and infrared spectroscopy. 53 Subsequently, ILAG was used to
enable the mechanochemical oxidation of cobalt and ruthenium complexes,
and for the conversion of ZnO into ZIFs, 54 a family of porous MOFs with
topologies analogous to those of zeolites (Figure 7.3b). 55 In the context of
pharmaceutically-relevant materials, ILAG also enabled a rapid and
environmentally-friendly synthesis of the metallodrug bismuth subsalicylate, 56
active component of the popular gastrointestinal drug Pepto-Bismol s .
7.4 Characterization of Mechanochemical Products
Low solubility, possibilities of solvate formation, polymorphism or dissoci-
ation/association upon dissolution make it dicult to reliably characterize
products of metal-organic mechanosynthesis in any other way except in the
solid form obtained immediately from the reaction. In that way mechano-
synthesis of coordination compounds (and, indeed, of co-crystals held by
non-covalent interactions) is different from organic mechanosynthesis
where the product and reaction mixtures can often be analyzed by con-
ventional solution NMR spectroscopy after extraction from the solid reaction
mixture. Consequently, products of metal-organic mechanosynthesis are
characterized primarily by X-ray powder diffraction (XRPD), thermogravi-
metric analysis (TGA), reflectance FTIR spectroscopy and solid-state NMR
spectroscopy. Although the latter two methods can immediately provide
extensive information on the chemical composition of the product, full
structural characterization very often depends on the ability to obtain an
identical product by single-crystal growth from solution. In such cases,
structural characterization is readily achieved via single-crystal X-ray dif-
fraction. In cases where reaction in solution provides a different product
than mechanosynthesis, it is, in principle, possible to obtain single crystals
from solution by seeding it with fine powder of the grinding product. 57 Such
a seeding strategy provides single crystals of the grinding product through
heterogeneous nucleation.
If crystallization from solution consistently fails to provide single crystals
of the desired phase, a possible alternative is crystal structure solution from
XRPD data. 58 Recent advances in laboratory technology and software have