Stepwise construction of grid-type Cu(ii)-Cd(ii) heterometallic MOFs based on an imidazole-appended dipyrrin ligand

Article abstract of DOI:10.1039/c2cc35543d

An imidazole-appended dipyrrin ligand yields, upon coordination to Cu(ii) cations, a linear metallatecton that self-assembles with Cd(ii) salts to afford 2D grid-type MOFs which, upon parallel stacking, lead to porous crystals offering 1D channels.

Full text of DOI:10.1039/c2cc35543d

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Stepwise construction of grid-type Cu(II)–Cd(II) heterometallic MOFs
based on an imidazole-appended dipyrrin ligandwz
´
Antoine Beziau, Stephane A. Baudron,* Dmitry Pogozhev, Audrey Fluck and
Mir Wais Hosseini*
´
Received 31st July 2012, Accepted 30th August 2012
DOI: 10.1039/c2cc35543d
An imidazole-appended dipyrrin ligand yields, upon coordination
to Cu(II) cations, a linear metallatecton that self-assembles with
Cd(II) salts to afford 2D grid-type MOFs which, upon parallel
stacking, lead to porous crystals offering 1D channels.
Metal–Organic Frameworks (MOFs) and Coordination Polymers
(CPs), hybrid architectures resulting from the self-assembly of
metal centres and organic bridging ligands, are promising
materials for applications in separation, storage, catalysis and
sensing, for example.1 The vast majority of these crystalline
architectures reported so far are of the homometallic type
Fig. 1 Stepwise construction of a grid-type M1M2OF.
comprising only one type of metallic centre (M), while the
heterometallic networks (MxOFs, x Z 2), based on two or
more metal cations, seem more synthetically challenging.2
They however constitute an interesting class of materials, since
their heterometallic nature provides diversity in their structural
and physical properties. Regarding heterobimetallic networks
(M1M2OFs), over the past few years, an efficient stepwise
approach based on the use of metalloligands2 or metallatectons3
has been developed. In this sequential strategy, metallatectons
are first prepared upon reaction of a first metal centre with
ligands bearing at least two different coordination poles (Fig. 1).
The generation of heterobimetallic M1M2OFs is achieved,
under self-assembly conditions, upon the combination of these
discrete species with a second metal centre. In this approach,
the ligand plays a key role in the synthetic strategy. While
many organic derivatives can be envisioned, dipyrrins (dpms),4
bis-pyrrolic ligands, forming monoanionic chelates under basic
conditions and readily functionalized, have demonstrated their
ability to form discrete and infinite heterobimetallic architectures.5–9
However, it is worth noting that, with only one exception,8 all the
reported dpm-based M1M2OFs involve Ag(I) salts (M1Ag) as a
secondary metal source.5–7 This might be due to the adaptability of
the coordination sphere around the d10 Ag(I) cation leading to a
broad range of possible connecting nodes and its propensity to
form Ag–p interactions with the pyrrolic system of dpm ligands.7
The combination of these two features leads to the formation of
complex architectures with a rather low degree of predictability.
For the formation of heterobimetallic M1M2OFs with pretargeted
arrangement, it would be interesting to apply the sequential
strategy described above and illustrated in Fig. 1 using a second
metal centre M2 with more defined coordination demands such as
coordination number and geometry. Let us focus here on the
formation of grid-type architectures which may be obtained upon
combining a metallic node (M2) displaying four free coordination
sites occupying the corners of a square with a linear metallatecton
based on M1 bearing two neutral monodentate binding sites
oriented in a divergent fashion. As M2, one may either use metal
centres adopting the square planar geometry (Pd(II) or Pt(II)) or an
octahedral coordination sphere with the two axial positions
blocked by auxiliary ligands. In the first case, the network will be
cationic in nature and, thus, for the sake of charge neutrality, the
crystal must contain anions. In the second case, if the two auxiliary
ligands were anionic, then the assembling nodes of the network
would be neutral.
Laboratoire de Chimie de Coordination Organique, UMR CNRS
´
We report herein the formation of heterobimetallic (Cu,Cd)
grid-type architectures resulting from the combination of a
CuII(dpm)2 metallatecton 2 bearing peripheral imidazolyl
groups (Fig. 1) and CdIIX2 salts (X = Clꢀ or NCSꢀ).
The design of ligand 17c is based on three parts, a dpm
moiety as a chelating unit, a phenyl group as a spacer and an
imidazole as a monodentate coordinating site. Although all
three parts are rigid, owing to the connectivity between them
7140, Universite de Strasbourg, F-67000, Strasbourg, France.
E-mail: hosseini@unistra.fr, sbaudron@unistra.fr;
Fax: +33 368851325; Tel: +33 368851323
w This article is part of the ChemComm ‘Metal–organic frameworks’
web themed issue.
z Electronic supplementary information (ESI) available: Temperature-
dependent PXRD of MOF 3. CCDC 894060–894062. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c2cc35543d
c
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Fig. 2 Crystal structure of metallatecton 2. Hydrogen atoms and
solvent molecules have been omitted for clarity.
through s bonds, the ligand as well as metallatecton 2 offer
different rotamers. However, the latter may adopt two extreme
conformations differing by the syn or anti orientation of the N atom
of the imidazolyl units within the network. Consequently, although
the connectivity remains identical, the grids formed may be of
different shapes. Regarding M1 and M2, Cu(II) and CdX2 (X = Clꢀ
or NCSꢀ) salts appeared as interesting candidates in light of their
reported use for the preparation of MOFs upon assembly with
pyridine and imidazole based ligands and metallatectons.10–12
Complex 2 was prepared in 94% yield by reaction of two
equivalents of dpm ligand 1, prepared as described,7c with
Cu(OAc)2 in THF.y Single crystals were obtained by slow diffusion
of n-pentane vapours into a CHCl3 solution of 2. The latter
crystallizes as a solvate, (2)2(CHCl3)3, in the monoclinic space
group C2/c with one complex and a CHCl3 molecule in general
position and an additional CHCl3 molecule on a two-fold axis. The
Cu(II) cation is surrounded by four N atoms (dCu–N in the
1.949(3)–1.961(3) A range) belonging to two dpm units 1 and
adopts a pseudo-tetrahedral geometry (dpm chelates tilted by
53.71) (Fig. 2). The Cu–N distances are comparable to those
observed for other reported Cu(dpm)2 complexes.13
Large single crystals of MOFs 3 (X = Clꢀ) and 4 (X = NCSꢀ)
were obtained by slow diffusion of a MeOH solution of an excess
of CdX2 salt into a DMF solution of complex 2. Compound 3
crystallizes in the monoclinic space group C2/m with one complex
2 on a glide plane, a CdCl2 unit on a mirror and a DMF molecule
with half-occupancy in general position. Compound 4 crystallizes
in the monoclinic C2/c space group with the metallatecton and
the Cd(NCS)2 unit (with one of the two thiocyanate anions
disordered over two positions and the Cd cation on an inversion
center) on glide planes. For both compounds, the phenyl group of
one ligand 1 is disordered over two positions. It should be noted
that both crystals contain solvent molecules (DMF, MeOH,
H2O). However, owing to the high positional disorder, the
SQUEEZE command was used.14 The Cu(II) cation remains
tetracoordinated with a similar environment to the one observed
for the starting complex 2 (average dCu–N = 1.944 A and average
tilt angle between the dpm moieties = 62.41). The Cd(II) cation is
in an octahedral environment with two anions occupying the
apical positions (dCd–X = 2.679 for 3, 2.374 A for 4) and four
N atoms of imidazolyl groups belonging to four different
metallatectons 2, occupying the corners of the square (average
Fig. 3 Portions of grid-type networks formed upon combining metall-
atecton 2 with CdCl2 (3) (a) or Cd(NCS)2 (4) (b). H atoms and solvent
molecules have been omitted for clarity. Note that only one position of
the disordered Ph group and of the NCSꢀ anion (for 4) is depicted.
Fig. 4 Stacking of the grids and resulting 1D channels along the c
axis in 3. H atoms and solvent molecules have been omitted for clarity.
grids are stacked along the c axis in an almost eclipsed
arrangement leading thus to the formation of channels filled
with solvent molecules (Fig. 4). The Cd atoms belonging to two
consecutive grids are separated by 10.17 and 9.89 A for 3 and 4
respectively. The solvent accessible volume, as calculated by
PLATON,14 amounts to 30 and 26% of the unit cell volume.
In both cases, the purity of the crystalline materials was
established by PXRD which revealed a perfect match between
the simulated and observed patterns (Fig. 5).
The thermal behaviour of both MOFs was investigated by
TGA (Fig. 6). In the 25–200 1C range, weight losses of 15.2
and 15.1% corresponding to the removal of solvent molecules
located in the channels were observed for 3 and 4 respectively.
Between 200 and 250 1C, a plateau is observed before decomposi-
tion occurs. Interestingly, temperature-dependent PXRD experi-
ments on compound 3 (see ESIz) revealed that it remains
crystalline up to 230 1C, albeit with a loss of crystalline quality.
In conclusion, the combination of an imidazolyl-appended dpm
ligand 1 with Cu(II) leads to the formation of a neutral complex 2
dCd–Nimid = 2.320 A). As expected from the proposed construction
strategy (Fig. 1), in both cases, a rhombic grid-type architecture is
obtained (Fig. 3).
Within the grid, the Cd cations are separated by 26.35 and
26.49 A for 3 and 4 respectively. Although 3 and 4 are not
isomorphous, nevertheless they display the same connectivity.
For 3, the grid type architecture is chiral and the crystal is
formed by packing, in a consecutive manner, of 2D networks
with opposite chirality. For 4, owing to the presence of a centre
of symmetry, the grid is achiral. In both structures, consecutive
Fig. 5 Simulated (a and c) and observed (b and d) PXRD patterns for
3 (left) and 4 (right). Note that the single-crystal (used for the simulation)
and powder data have been collected at different temperatures (173 vs.
293 K). The difference in intensities results from preferential orientation.
c
Chem. Commun.
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wR2 = 0.1340 for I > 2s(I) and R1 = 0.0542, wR2 = 0.1377 for all
data. Crystal data for 4: C74H52CdCu2N18S2, M = 1496.64, monoclinic,
space group C2/c (no. 15), a = 23.1765(8), b = 29.6240(9), c =
14.4219(4) A, b = 121.9580(10)1, V = 8401.0(5) A3, T = 173(2) K,
Z = 4, Dc = 1.184 g cmꢀ3, m = 0.849 mmꢀ1, 88657 collected
reflections, 12288 independent (Rint = 0.0371), GooF = 1.074, R1 =
0.0351, wR2 = 0.1012 for I > 2s(I) and R1 = 0.0464, wR2 = 0.1060 for
all data.
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metallatectons and secondary metal salts M2X2 is currently
under investigation.
We thank the Universite de Strasbourg, the Institut Universitaire
´
de France, the International centre for Frontier Research in
Chemistry (icFRC), the Marie Curie Est Actions FUMASSEC
Network (Contract No. MEST-CET-2005-020992), the C.N.R.S.
Chem., 2010, 49, 11231–11239; (e) A. Beziau, S. A. Baudron and
´
M. W. Hosseini, Dalton Trans., 2012, 41, 7227–7234.
and the Ministere de l’Enseignement Superieur et de la Recherche
´
(PhD fellowship to A. B.) for financial support.
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Notes and references
y Synthesis of complex 2: A THF (20 mL) solution of ligand 1 was added to
a THF (30 mL) solution of Cu(OAc)2 (31.7 mg, 0.174 mmol). After stirring
at room temperature for two hours, the solvent was removed under vacuum
and the residue purified by column chromatography (SiO2, AcOEt, Rf =
0.50). After washing with n-pentane (150 mL), complex 2 was obtained as a
red solid (104.4 mg, 94%). lmax(CH2Cl2)/nm (e/molꢀ1 L cmꢀ1): 342
(61 000), 468 (167 000), 502 (8400). Found: C, 55.59; H, 3.83; N, 14.09%.
Anal. calcd for C75H55Cl9Cu2N16: C, 55.38; H, 3.41; N, 13.78%. Synthesis
of M0MOF 3: in a test tube, a DMF (6 mL) solution of complex 2 (70 mg,
0.11 mmol) was layered with a MeOH (10 mL) solution of CdCl2 (40 mg,
0.22 mmol) separated by a DMF–MeOH (1/1, 3 mL) buffer layer. After a
few days, large single-crystalline rods were obtained, 30% (60 mg),
IR(ATR) n/cmꢀ1: 1649, 1540, 1534, 1403, 1372, 1331, 1240, 1205, 1186,
1176, 1055, 1039, 1018, 996, 892, 774, 733, 716, 645, 610. Synthesis of
M0MOF 4: in a test tube, a DMF (8 mL) solution of complex 2 (34 mg,
0.054 mmol) was layered with a MeOH (15 mL) solution of Cd(NCS)2
(50 mg, 0.22 mmol) separated by a DMF–MeOH (1/1, 5 mL) buffer
layer. After a few days, large single-crystalline rods were obtained,
30% (60 mg). IR(ATR) n/cmꢀ1: 2049, 1673, 1535, 1403, 1371, 1330,
1304, 1239, 1205, 1186, 1176, 1056, 1038, 1020, 995, 892, 774, 734, 716,
652, 611. Crystal data for (2)2(CHCl3)3: C75H55Cl9Cu2N16, M =
1626.55, monoclinic, space group C2/c (no. 15), a = 37.4262(12),
b = 8.9610(3), c = 28.2529(16) A, b = 103.2970(10)1, V =
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7227.0(5) A3,
T
=
173(2) K,
Z
=
4, Dc
=
1.495
g
cmꢀ3
,
m = 0.978 mmꢀ1, 21 262 collected reflections, 8279 independent
(Rint = 0.0320), GooF = 1.031, R1 = 0.0618, wR2 = 0.1464 for
I > 2s(I) and R1 = 0.0835, wR2 = 0.1668 for all data. Crystal data
for 3: C75H59CdCl2Cu2N17O, M = 1524.77, monoclinic, space group
C2/m (no. 12), a = 14.5401(12), b = 30.0895(11), c = 19.4195(5) A,
b = 97.7360(10)1, V = 8418.8(4) A3, T = 173(2) K, Z = 4, Dc
1.203 121 879 collected reflections,
cmꢀ3 0.863 mmꢀ1
9818 independent (Rint = 0.0393), GooF = 1.108, R1 = 0.0459,
=
g
,
m
=
,
14 A. L. Spek, PLATON, The University of Utrecht, Utrecht,
The Netherlands, 1999.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun.