This journal is c The Royal Society of Chemistry 2012 Chem. Commun.

Cite this: DOI: 10.1039/c2cc35543d

Stepwise construction of grid-type Cu(II)–Cd(II) heterometallic MOFs

based on an imidazole-appended dipyrrin ligandwzAntoine 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

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.

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 = Cl� or NCS�).

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

Fig. 1 Stepwise construction of a grid-type M1M2OF.

Laboratoire de Chimie de Coordination Organique, UMR CNRS7140, Universite de Strasbourg, F-67000, Strasbourg, France.E-mail: hosseini@unistra.fr, sbaudron@unistra.fr;Fax: +33 368851325; Tel: +33 368851323w 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 andcrystallographic data in CIF or other electronic format see DOI:10.1039/c2cc35543d

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Chem. Commun. This journal is c The Royal Society of Chemistry 2012

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 theN atom

of the imidazolyl units within the network. Consequently, although

the connectivity remains identical, the grids formed may be of

different shapes. RegardingM1 andM2, Cu(II) andCdX2 (X=Cl�

or NCS�) 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 ofMOFs 3 (X=Cl�) and 4 (X=NCS�)

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 groupC2/mwith one complex

2 on a glide plane, a CdCl2 unit on a mirror and a DMFmolecule

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

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

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

Fig. 2 Crystal structure of metallatecton 2. Hydrogen atoms and

solvent molecules have been omitted for clarity.

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 NCS� 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.

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.

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This journal is c The Royal Society of Chemistry 2012 Chem. Commun.

using the anionic nature of the bis-pyrrolic moiety. Self-assembly of

the latter offering two monodentate sites with diprotected CdIIX2

(X=Cl� orNCS�) salts leads to the formation of heterobimetallic

MOFs 3 and 4 displaying rhombic grid-type arrangements.

Although the two crystals are not isomorphous, they display the

same connectivity. The eclipsed stacking of grids leads to the

formation of channels occupied by solvent molecules. Interestingly,

both 3 and 4 are thermally stable porous materials up to 230 1C.

Thus, the proposed sequential construction strategy (Fig. 1), based

first on a classical coordination step followed by self-assembly

processes, is viable for the formation of heterobimetallic

MOFs. The application of this approach to other M1(dpm)2metallatectons and secondary metal salts M2X2 is currently

under investigation.

We thank theUniversite 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.

and the Ministere de l’Enseignement Superieur et de la Recherche

(PhD fellowship to A. B.) for financial support.

Notes and references

y Synthesis of complex 2: A THF (20mL) solution of ligand 1was added toa THF (30 mL) solution of Cu(OAc)2 (31.7 mg, 0.174 mmol). After stirringat room temperature for two hours, the solvent was removed under vacuumand the residue purified by column chromatography (SiO2, AcOEt, Rf =0.50). After washing with n-pentane (150 mL), complex 2was obtained as ared solid (104.4 mg, 94%). lmax(CH2Cl2)/nm (e/mol�1 L cm�1): 342(61000), 468 (167000), 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%. Synthesisof 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 afew days, large single-crystalline rods were obtained, 30% (60 mg),IR(ATR) n/cm�1: 1649, 1540, 1534, 1403, 1372, 1331, 1240, 1205, 1186,1176, 1055, 1039, 1018, 996, 892, 774, 733, 716, 645, 610. Synthesis ofM0MOF 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) bufferlayer. After a few days, large single-crystalline rods were obtained,30% (60 mg). IR(ATR) n/cm�1: 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 =7227.0(5) A3, T = 173(2) K, Z = 4, Dc = 1.495 g cm�3,m = 0.978 mm�1, 21 262 collected reflections, 8279 independent(Rint = 0.0320), GooF = 1.031, R1 = 0.0618, wR2 = 0.1464 forI > 2s(I) and R1 = 0.0835, wR2 = 0.1668 for all data. Crystal datafor 3: C75H59CdCl2Cu2N17O, M = 1524.77, monoclinic, space groupC2/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 g cm�3, m = 0.863 mm�1, 121 879 collected reflections,9818 independent (Rint = 0.0393), GooF = 1.108, R1 = 0.0459,

wR2 = 0.1340 for I > 2s(I) and R1 = 0.0542, wR2 = 0.1377 for alldata. 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 cm�3, m = 0.849 mm�1, 88 657 collectedreflections, 12 288 independent (Rint = 0.0371), GooF = 1.074, R1 =0.0351, wR2 = 0.1012 for I>2s(I) andR1 = 0.0464, wR2 = 0.1060 forall data.

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Fig. 6 Thermo-gravimetric analysis of MOFs 3 and 4.

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