Ball-milling Mechanochemical Synthesis of Coordination Bonds: Discrete Units, Polymers and Porous Materials - Ball Milling Towards Green Synthesis: Applications, Projects, Challenges - page 165

previously, upon heating to 100 1C, these complexes lose water and form
coordination polymers.
A mechanochemical alternative to the synthesis of coordination polymers
from 4,4 0 -bipyridinium salts of FeCl 4 2- , CoCl 4 2- and ZnCl 4 2- by thermal
dehydrohalogenation (Figure 7.2b) is milling with an external base, such as
KOH. While adding an external base is likely to be energetically advan-
tageous to thermal dehydrohalogenation, this advantage is somewhat offset
by the need to remove the byproduct salt (e.g. KCl). 68,69
7.5.4 Synthesis of Coordination Polymers and MOFs from
Metal Oxides and Carbonates
So far, all discussed examples of acid-base reactions used to construct co-
ordination polymers involve soluble metal salts as precursors. However, as
mechanochemical reactivity is independent of the solubility of reactants, the
use of slightly soluble but cleaner and readily available carbonate or oxide
reactants is also possible. 36,52,66 With oxide reactants, the highly basic na-
ture of the oxide ion provides a strong driving force for the mechano-
chemical reaction. For metal carbonates, formation of CO 2 gas serves as an
additional reaction driving force.
Perhaps the first example of mechanochemical reactivity of metal oxides
against organic ligands was reported by Fern´ndez-Bertr´n, who obtained
coordination polymers of Ag( I ), Zn, Cd and Hg( II ) by manually grinding the
respective metal oxides with imidazole. 70 Analogous reactions with oxides of
other transition metals, Pb( II ) or Mg could not be achieved, suggesting that
the nd 10 configuration of the metal ion was important for the reactivity.
Construction of coordination polymers from metal carbonates was utilized
in the synthesis of the 2D framework Co(bipy)Cl 2 by LAG of cobalt( II ) car-
bonate with bipyridinium chloride (Figure 7.6a). 68 The product was identical
to the one constructed by kneading of CoCl 2 and bipy with water. Similarly,
LAG of the basic zinc carbonate with bipyridinium chloride gave a mixture of
two polymorphs of the [Zn(bipy)Cl 2 ] n polymer. This contrasts with the neat
grinding reaction of ZnCl 2 and bipy, which yields only one polymorph, thus
suggesting a surface templating effect in the reaction.
The reaction of ZnO with H 2 fum demonstrates rapid screening for co-
ordination polymers using LAG. 36,52 Different liquid phases were found to
steer the reaction towards the formation of different products (Figure 7.6b).
Quantitative yields, obtainable within 30-45 min milling, allowed the first
structural characterization of anhydrous zinc fumarate and a previously
unknown zinc fumarate dihydrate directly from powder X-ray diffraction
data. Grinding of equimolar amounts of ZnO and H 2 fum with three and four
equivalents of water selectively produced the tetrahydrate and the penta-
hydrate forms of zinc fumarate, respectively, known to form concomitantly
from solution. Formation of differently hydrated coordination polymers was
subsequently correlated to the activity of water in the grinding liquid,