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construction of open pillared structures by addition of bridging ligands
such as dabco or bipy.
95
Such behavior is considerably different from
solution-based chemistry, as the mechanochemically induced transfor-
mations do not readily occur by simple immersion of reactant materials in
the bulk liquid.
95,96
7.7.2 Synthesis of Solid Solutions
Although the isostructurality of components is often considered to be a
very important factor in the synthesis of solid solutions, their formation via
conventional solution-based methods is often hindered by the difference in
solubility of individual components, which leads to the preferential crys-
tallization of separate components. The potential advantage of mechano-
chemistry for the construction of solid solutions of isostructural
coordination polymers has been demonstrated by James and co-workers
who constructed close-packed and open-framework materials by LAG re-
actions of rare earth metal carbonates and trimesic acid.
97
In particular,
LAG of each individual metal carbonate in the presence of water quanti-
tatively yielded isostructural close-packed materials consisting of 1D co-
ordination polymers.
1
In contrast, LAG with DMF resulted in the formation
of isostructural lanthanide MOFs based on an open 3D architecture
(Figure 7.12b). The isostructurality of the 3D MOF products suggested the
possibility to form solid solutions. Indeed, grinding of different pairs of
lanthanide metal carbonates (e.g. pairsSm-Gd,Eu-Gd,Tb-GdandDy-Gd)
with trimesic acid afforded mixed-lanthanide 3D open frameworks iso-
structural to the ones obtained using individual metal carbonate reactants
(Figure 7.12c).
97
The ability to mechanochemically form solid solutions of coordination
polymers was exploited for the synthesis of metal-organic materials with
controlled crystallographic lattice parameters and physical properties.
Mechanochemical liquid-assisted grinding of anhydrous CoBr
2
with bipy
results in the quantitative formation of the 2D sheet coordination polymer
Co(bipy)Br
2
.
98
The coordination polymer is isostructural to Co(bipy)Cl
2
,
which is obtained by analogous reaction of anhydrous CoCl
2
(Figure 7.4a).
Correspondingly, LAG of different mixtures of anhydrous CoCl
2
and CoBr
2
yields solid solutions of the two coordination polymers, with overall com-
position Co(bipy)Br
2-x
Cl
x
. The solid solutions are homogeneous at length
scales detectable by XRPD and UV/Vis spectroscopy, with the Br/Cl ratio
determined by the relative amounts of CoBr
2
and CoCl
2
in the reaction
mixture.
98
The coordination polymer solid solutions are isostructural to the
single phases Co(bipy)Cl
2
and Co(bipy)Br
2
, but with lattice parameters and
UV/Vis absorption properties determined by the Br/Cl ratio. Specifically,
the crystallographic lattice parameters and UV/Vis absorption of Co-
(bipy)Br
2-x
Cl
x
were found to change in a continuous and linear fashion with
x over the range 0-2.
98
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