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a few examples of void-space containing networks formed by
the aggregation of MCs held together by weak interac-
[
12,13]
tions.
As with the first example, the interactions estab-
lished in these architectures do not provide sufficient structural
stabilization for the lattices to exhibit significant gas uptake
properties. Also, these (pseudo)-porous extended frameworks
were formed by serendipitous spatial arrangement of the MC
units, rather than from the rational design and control of the
interactions established between the building blocks.
In this paper, we present two novel MC networks, one of
which is the first permanent PCP made of metallacrowns that
is resistant to solvent evacuation and capable of adsorbing
gas. In this architecture, 12-MC-4 units were assembled using
II
Cu ions and a pyridinic analogue of salicylhydroxamic acid as
3À
the ligand (hinHA , Scheme 1a). We aimed to connect these
MC units through in-plane metal-ion bridges to form a chess-
board-like fourfold arrangement (Scheme 1b). Hence, the
ligand was designed to provide the MC with pyridine-like moi-
eties pointing towards the exterior of the scaffold, in order to
increase the propensity for forming peripheral coordination
bonds. These bonds were designed to impose proper distance
[
8a]
between MC units to generate voids within the lattice.
The ligand H hinHA (3-hydroxyisonicotine hydroxamic acid)
3
was synthesized in moderate yield from the carboxylic acid
[15]
precursor using activation–deprotection strategies. The reac-
II
II
tion of Cu salts with H hinHA selectively provided a {Cu [12-
3
2À
MCCuII, hinHA-4]} species, as observed by mass spectrometry
ESI-MS, Figure S8, Supporting Information). By modifying
(
reaction conditions, we were able to isolate two distinct MC
networks.
Network 1 (Figure 1) was isolated by reacting H hinHA with
Cu acetate in a 1:1.75 ratio in a dilute 2:1 DMF/pyridine
Figure 1. a) Representation of one layer of the porous network 1, viewed
along the c axis. One MC unit is shown in a space-filling representation.
3
II
Taking into consideration van der Waals radii, the 2D pore section measures
solution, followed by slow evaporation of the solvent.
The structure is composed of stacked two-dimensional
2
1
2.68.9 , accounting for 36% of the total cell volume. b) Encapsulation
of a coordinated pyridine molecule within the hydrophobic pyridinic pocket
forms p–p pillars holding the layers together.
II
II
coordination networks with the formula {[Cu (AcO)Py] {Cu [12-
2
MC
-4]}} . As shown in Figure 1, single units of the
CuII, hinHA
II
n
2À
{
Cu [12-MCCuII, hinHA-4]} metallacrown are bridged by additional
II
Cu ions coordinated to the peripheral pyridinic moieties of
MC units. In this nonplanar configuration, the prochiral MC
units gain chirality, as one can distinguish the clockwise or an-
II
two neighboring MC units. The bridging Cu nodes have the
II
+
formula [Cu (AcO)Py] : their coordination geometry can be de-
scribed as distorted square planar, in which the two MC pyri-
dinic moieties coordinate to the linking metals in a trans fash-
ion. The remaining positions are occupied by a monodentate
acetate ion and a solvent pyridine molecule (Figure 1b). The
result is the formation of a chessboard-like arrangement of 12-
MC-4 units, in which the voids have a rectangular shape. Inter-
estingly, each layer is not planar, but rather has a wave-like
topology, as the result of the bowl-shaped MC units.
ticlockwise rotation of the (Cu–O–N) cyclic repetition of the
4
MC ring, with respect to the concave face of the molecule (Fig-
[
16]
ure S12, Supplementary Information). Since the crystal lattice
is formed by the wave-like repetition of only one of the two
possible enantiomers, a homochiral coordination polymer
which crystallizes in the chiral space group P42 2 is obtained.
1
The four pyridine molecules coordinated to the convex face
of an MC subunit form an aromatic pocket that encapsulates
the lone pyridine of the neighboring 2D layer (Figure 1b). This
columnar stacking of interpenetrated pyridines creates p–p pil-
lars, which provide stability to the assembly and allow the rec-
tangular void spaces of each layer to be perfectly stacked, one
above the other, to form void channels that run along the
entire crystal (Figure 2). Since the charge balance is provided
II
In the MC unit, each of the five Cu ions are coordinated
axially by one solvent molecule of pyridine, and exhibit a dist-
orted square-pyramidal coordination geometry (Figure S14,
Supporting Information). Due to steric interactions, the four
ring ions are all coordinated by the pyridine molecules on the
II
II
+
MC convex face, while the central Cu ion is coordinated by
by the bridging [Cu (AcO)Py] units, no extra counter ions,
which may potentially occupy the void spaces, are required. By
taking into consideration the van der Waals radii, the estimat-
the fifth pyridine on the opposite face (Figure 1b).
One structural consequence of this pyridine distribution is
the disruption of planarity leading to the bowl shape of the
2
ed channel cross section measures 12.68.9 and accounts
Chem. Eur. J. 2016, 22, 6482 – 6486
6483
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