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Angewandte
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addition of EtOAc or CH2Cl2 at room temperature. Both
types of materials underwent conversion into hexagonal
crystals of 3 when heated in H2O at 1208C for 1 h. Notably,
despite several attempts, we were unable to synthesize 3
directly by the treatment of 5PIA with different CuII salts [for
example, Cu(NO3)2, Cu(OAc)2].
the cages are coordinated to water molecules, and Cu atoms
at the outer sites are coordinated to methanol molecules.
Compound 1 was found to be soluble in DMF, N,N-
diethylformamide (DEF), and dimethyl sulfoxide (DMSO),
whereas Cu(pi) remained insoluble in almost all solvents.
TEM imaging indicated the presence of polyhedral MOP
particles of approximately 3–4 nm in diameter in the solution
of 1 in DMF (see Figure S22 in the Supporting Information).
In solution in DMF, 1 and 2 showed the same UV/Vis
spectrum, which was different from that of Cu(NO3)2 in
solution (see Figure S17), and thus confirmed the presence of
the Cu–Cu paddle-wheel structure in solution.
¯
Compound 1 crystallized in the Im3m space group in the
cubic crystal system. The secondary building unit (SBU) of
the structure consists of a Cu–Cu paddle-wheel center
coordinated to four units of 5-(propargyloxy)isophthalate
ions and two water molecules. Twelve such paddle-wheel units
form a cuboctahedral structure in which the core cavity is
accessible through eight triangular and six square faces
(Figure 1a). All edges of the polyhedron are decorated with
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Compound 3 crystallized in the P3 space group in the
trigonal crystal system and contains a similar Cu–Cu paddle-
wheel SBU to that of 1. However, half of the carboxylate
functionalities around each Cu–Cu paddle-wheel SBU are
pointed in one direction, whereas the other half are pointed in
the opposite direction (see Figure S6). There are two types of
pores present in the extended framework of 3. The larger
pores are similar in size to the triangular face of the MOP and
are hydrophilic in nature, whereas the smaller pores originate
from unfolding of the MOP and are hydrophobic in nature as
they contain the propargyloxy groups inside (see Figure S7).
The bulk-phase purity of all aforementioned materials
was confirmed by comparison of the experimentally observed
powder X-ray diffraction (PXRD) patterns with the patterns
simulated from the single-crystal structures. All PXRD peaks
of as-synthesized 1 and 3 matched exactly with the respective
simulated patterns (see Figure S8). However, 2 showed poor
crystallinity, and its PXRD pattern was different from those
of 1, Cu(pi), and 3 (see Figure S9a). MALDI-TOF analysis of
2 showed a peak at m/z 7261.02 (see Figure S13b), which
corresponds to the molecular weight of one MOP unit
(M+48H). Other studies, such as elemental analysis, FTIR
spectroscopy, and solubility studies (see Figure S2), suggested
that the chemical composition, metal–ligand binding, and
chemical behavior of 2 were similar to those of 1. DSC studies
(see Figure S14) indicated that 2 is less crystalline in nature
than 1. The formation of nascent crystals of 1 from 2
suggested that 2 has another arrangement of MOP units with
less periodicity. When 2 is dissolved, the arrangement of MOP
units collapses, and discrete MOP cages/units remain in
solution. Under suitable conditions, these MOP units rear-
range themselves into a more stable periodic arrangement to
form the crystalline form, 1 (see Figure S1).
Both 1 and 2 underwent conversion into hexagonal
crystals of 3 when heated in water. Microscopic images
collected at different time intervals showed that crystals of
1 in water first lose their transparency and are then converted
into aggregates of hexagonal MOF crystals 3 (Figure 2b). On
the other hand, the less crystalline flakelike material 2 clearly
showed conversion into hexagonal MOF crystals (Figure 2d).
Time-dependent PXRD patterns of water-treated crystals of
1 and 2 also indicated their conversion into crystals of 3
(Figure 2a,c). As-synthesized crystals of 1 displayed coherent
PXRD patterns with peak-to-peak matching with the simu-
lated PXRD pattern. These peaks were broader at the initial
stage of treatment with water, thus suggesting a decrease in
crystallinity owing to the rupture of the periodic MOP crystal
Figure 1. Structural comparison of 1 and Cu(pi) in terms of the
coordinating solvent (highlighted by the blue ring) in their paddle-
wheel SBU (a,c) and their packing in different space groups (b,d).
a propargyloxy moiety that points outwards. Each single Cu–
Cu paddle-wheel SBU is connected to the adjacent SBUs
through 5-(propargyloxy)isophthalate ions. All carboxylate
functionalities around each of the Cu–Cu paddle-wheel SBUs
of 1 point in the same direction (either upwards or down-
wards) and thus form a spherical assembly (Figure 1a).
A structurally similar MOP, Cu(pi), which is isomorphous
to 1, was previously synthesized by the reaction of Cu-
(NO3)2·3H2O with 5PIA.[6] Cu(pi) differs from 1 in terms of
the solvent molecules coordinated to the Cu–Cu paddle-
wheel SBU (Figure 1a,c) and overall crystal packing (Fig-
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ure 1b,d). Unlike 1, Cu(pi) crystallizes in the Pa3 space group,
and thus, the packing generates a nonporous assembly
(Figure 1d). Each Cu–Cu paddle-wheel SBU of both MOPs
has two axial sites available for solvent coordination. In 1,
H2O molecules coordinate to both of these sites, whereas in
the case of the Cu(pi) structure, Cu atoms at the inner sites of
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Angew. Chem. Int. Ed. 2013, 52, 13755 –13759