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Figure 2.15
Illustration of the helical nanochannel-like structure of
MOROF-2
. (Reprinted with permission
from[56].Copyright2004RoyalSocietyofChemistry.)Afull-colourversionofthisfigureappearsintheColour
Platesectionofthis topic.
The first coordination polymer shows a paramagnetic non-interpenetrated supramolecular network with an
unprecedented (6
3
(6
8
8
1
)
·
·
)
topology. Additionally,
MOROF-2
exhibits helical nanochannels of dimen-
4 A and a void volume of 54.5 % along with antiferromagnetic interactions (Figure 2.15).
In contrast,
MOROF-3
exhibits a (6,3)-helical network with large 17
sions 13
.
2
×
9
.
8 A 1-D channels and a bulk
magnetic ordering below 1.8 K. This was the first reported example of a nanochannel-like structure that
in addition to exhibiting an unusual (6,3) helical coordination network showed mixed ferromagnetic and
antiferromagnetic interaction between the cobalt(II) ions and the
13
radicals.
The hexacarboxylic PTM radical with six carboxylic acid groups at the
meta
positions of the three phenyl
rings have been complexed with copper(II) ions to obtain a three-dimensional coordination polymer. The
resulting 3-D structure can be described as two interpenetrating cubic nets. Magnetic properties of this
metal radical organic open framework have been studied showing unexpected metal - radical ferromagnetic
interactions.
61
.
5
×
6
.
Pure Organic Radical Open Frameworks
In addition to their demonstrated utility as ligands, carboxylic
acid functionalized PTMs can also self-assemble into purely organic radical open frameworks (POROFs)
with porous structures and interesting magnetic properties. Thus, radicals
12
and
13
were used to obtain
extended hydrogen bonded networks, which in principle would show such a combination of properties.
The advantages of these radicals were considerable: (i) their trigonal symmetry provided a typical template
for getting channels held together by hydrogen bonds through the carboxylic groups; (ii) the molecular
bulkiness and rigidity of PTM radicals was expected to prevent a close packing of molecular units; and (iii)
besides their structural control, hydrogen bonds have been shown to favour magnetic coupling between
bound radical molecules (Section 'Transmission of Magnetic Interactions Through Hydrogen Bonds').
62
These expectations were confirmed in the hydrogen bonded self-assemblies of
12
and
13
by the formation
of robust porous extended networks. Indeed, the dicarboxylic
12
radical crystallizes in a robust porous
2-D extended network (
POROF-1
) with weak antiferromagnetic interactions.
63
The framework shows 1-D
5 A in diameter) and larger hydrophobic cavities, where a
sphere 10 A in diameter can fit inside them. The combination of supercages and windows gives way to
solvent accessible voids in the crystal structure that amount up to 31 % of the total volume of the unit cell
tunnels formed by narrowed polar windows (
≈
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