Two Zinc(II) Coordination Polymers Constructed with Rigid 1,4

Jul 26, 2008 - benzenedicarboxylic acid, bimx ) 1,4-bis(imidazol-1-ylmethyl)-2,3,5 ... (bimx) linkers were synthesized in aqueous media (hydrothermal ...
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CRYSTAL GROWTH & DESIGN

Two Zinc(II) Coordination Polymers Constructed with Rigid 1,4-Benzenedicarboxylate and Flexible 1,4-Bis(imidazol-1-ylmethyl)-2,3,5,6-tetramethylbenzene Linkers: From Interpenetrating Layers to Templated 3D Frameworks

2008 VOL. 8, NO. 9 3200–3205

Andreas Schaate, Stefan Klingelhöfer, Peter Behrens, and Michael Wiebcke* Institute of Inorganic Chemistry, Leibniz UniVersity HannoVer, Callinstraβe 9, 30167 HannoVer, Germany, and Center for Solid-State Chemistry and New Materials, Leibniz UniVersity HannoVer, HannoVer, Germany ReceiVed October 22, 2007; ReVised Manuscript ReceiVed May 28, 2008

ABSTRACT: Two new coordination polymers, [Zn(bdc)(bimx)] · 1.5H2O (1) and [Zn2(bdc)2(bimx)] · (H2bdc) (2) (H2bdc ) 1,4benzenedicarboxylic acid, bimx ) 1,4-bis(imidazol-1-ylmethyl)-2,3,5,6-tetramethylbenzene), containing both rigid (bdc) and flexible (bimx) linkers were synthesized in aqueous media (hydrothermal conditions) and characterized by single-crystal and powder X-ray diffraction, FT-IR spectroscopy, and thermogravimetric/differential thermal analysis. While 1 is an essentially dense material consisting of stacked composite layers each being built from two mutually interpenetrating two-dimensional square-grid frameworks, 2 possesses a single (non-interpenetrated) three-dimensional open framework (R-Po net) with wide channels that are filled with H2bdc template molecules. Formally, ring-penetrating bdc linkers are replaced by H2bdc molecules when going from the structure of 1 to the structure of 2. The occluded H2bdc molecules exhibit distinct interactions with the framework and can thus be regarded as template molecules or structure-directing agents. In line with these host-guest interactions, attempts to remove the template from as-synthesized 2 with retention of the framework structure by heat treatment or liquid exchange were not successful. These results give new impetus, however, to the recently discussed idea to use structure-directing effects, as they occur in zeolite synthesis, for the preparation of metal-organic framework materials. Introduction Micro- and mesoporous coordination polymers or metal organic frameworks (MOFs) are a new class of materials with periodic host structures and properties that cannot be obtained with purely inorganic porous materials such as zeolites and are therefore currently of great interest as new functional materials for potential applications in sorption, separation, catalysis, sensing, etc.1 Also, it is of particular importance that compared to zeolites, MOFs allow a more rational design and synthesis provided that essentially rigid ligands are used to link metal ions or clusters, which are also known as secondary building units (SBUs). This has been well established by Yaghi, O’Keeffe and co-workers for MOFs based on rigid carboxylate linkers.2 Mixtures of carboxylates and amines or imines have also been used for the construction of porous MOFs. For example, a number of isoreticular MOFs with three-dimensional (3D) open frameworks based on the R-Po (pcu) net3 with interesting sorption properties were constructed by using bidentate amine or imine pillar ligands to bridge two-dimensional (2D) squaregrid layers built from carboxylate-linked dinuclear paddle wheel SBUs.4,5 Naturally, such kind of rational design becomes increasingly more difficult when the conformational flexibility of the linkers increases. A comparatively simple flexible bidentate ligand is 1,4-bis(imidazol-1-ylmethyl)benzene (bix) which can adopt cis or trans conformations.6 With bix as the sole ligand a number of non-porous coordination polymers were synthesized some of which exhibit unusual types of entanglement (e.g., rotaxane-like catenation).7 More recently, bix was also combined with some polyfunctional carboxylate linkers in hydrothermal syntheses to yield MOFs that in general possess single or interpenetrating frameworks without or with only low porosity.8 * To whom correspondence should be addressed. E-mail: michael.wiebcke@ acb.uni-hannover.de.

In order to study the influence of increasing conformational flexibility of the linkers on the construction of porous MOFs with mixed carboxylate-imine linkers we have chosen the bixrelated bidentate ligand 1,4-bis(imidazol-1-ylmethyl)-2,3,5,6tetramethylbenzene (bimx) which should have a lower tendency towards interpenetration due to the steric hindrance provided by the four methyl groups attached to the benzene ring. Using bimx in combination with rigid 1,4-benzenedicarboxylic acid (H2bdc, terephthalic acid) and applying hydrothermal conditions we have been able to prepare two new Zn(II) compounds, [Zn(bdc)(bimx)] · 1.5H2O (1) and [Zn2(bdc)2(bimx)] · (H2bdc) (2). While 1 is essentially a dense material containing interpenetrating 2D frameworks (in which bimx is however not the ringpenetrating linker), 2 possesses indeed a single (non-interpenetrated) 3D open framework with wide channels that are filled with H2bdc template molecules. We report here on the synthesis, crystal structure, and thermal behavior of 1 and 2. These compounds are, to the best of our knowledge, the first coordination polymers containing bimx as a linker molecule. Experimental Section General Methods and Materials. Powder X-ray diffraction (XRD) measurements were performed on a Stoe STADI-P diffractometer in transmisson geometry (linear position-sensitive detector, curved Ge(111) monochromater, CuKR1 radiation, λ ) 1.54060 Å). Thermogravimetry (TG) and difference thermal analysis (DTA) measurements were performed simultaneously on a Netzsch STA 429 thermoanalyzer. For this purpose, the samples were filled into alumina crucibles and heated under a flow of air or nitrogen at a rate of 5 °C/min up to 1000 °C. A Bruker TENSOR 27 instrument was used to collect FT-IR spectra in the range from 4000 to 400 cm-1 from samples diluted in KBr pellets. Hydrothermal syntheses were carried out in homemade stainless steel autoclaves with Teflon liners of 8 mL capacity. Commercial materials of reagent grade were used as received. Synthesis of bimx. 1,4-Bis(bromomethyl)-2,3,5,6-tetramethylbenzene was synthesized according to a known protocol.9 A reported

10.1021/cg701041k CCC: $40.75  2008 American Chemical Society Published on Web 07/26/2008

Zinc(II) Coordination Polymers procedure10 was modified to prepare bimx from this compound. Imidazole (2.69 g, 0.039 mmol) and KOH (8.87 g, 0.158 mmol) were dissolved in dimethylsulfoxide (170 mL) and the solution was stirred for 2 h at room temperature. Then, 1,4-bis(bromomethyl)-2,3,5,6tetramethylbenzene (6.32 g, 19.74 mmol) was added and stirring was continued for 3 h. After addition of water (170 mL) the aqueous solution was extracted with chloroform (4 × 170 mL) and the organic phase was dried above anhydrous Na2SO4. Thereafter, the solvent was removed on a rotary evaporator. The obtained white residue was kept at 80° C in vacuum (10-3 mbar) overnight, recrystallized from chloroform, and dried in a desiccator. The yield was 5.59 g (96 %). 1H NMR (CDCl3): δ 2.25 (s, 12H, -CH3), 5.23 (s, 4H, -CH2-), 6.80 (s, 2H, 4HIm), 7.04 (s, 2H, 5HIm), 7.33 (s, 2H, 2HIm). Synthesis of 1. Zn(NO3)2.6H2O (0.066 g, 0.222 mmol), H2bdc (0.037 g, 0.222 mmol), bimx (0.065 g, 0.222 mmol), and distilled water (4 mL, 222 mmol) were mixed to yield a suspension with a pH value of 4. The suspension was charged into an autoclave (degree of filling 50 %) and treated hydrothermally under autogeneous pressure and static conditions at 180° C for 66 h. Pale yellow crystals of 1 were separated by filtration, washed with water, and dried. The yield was 0.071 g (29 % based on Zn). Elemental analysis for C52H58N8O11Zn2: C 56.36, H 5.29, N 9.80 (calc: C 56.68, H 6.25, N 10.17). Experimental and simulated powder XRD patterns and an IR spectrum are presented in the Supporting Information. Synthesis of 2. Zn(NO3)2.6H2O (0.165 g, 0.555 mmol), H2bdc (0.138 g, 0.833 mmol), bimx (0.082 g, 0.278 mmol), and distilled water (4 mL, 222 mmol) were mixed to yield a suspension with a pH value of 4. The suspension was charged into an autoclave (degree of filling 50 %) and treated hydrothermally under autogeneous pressure and static conditions at 150° C for 43 h. Pale yellow crystals of 2 were separated by filtration, washed with DMF and water and dried. The yield was 0.141 g (85 % based on Zn). Elemental analysis for C21H18N2O6Zn: C 55.02, H 4.13, N 5.86 (calc: C 54.86, H 3.95, N 6.09). Experimental and simulated powder XRD patterns and an IR spectrum are presented in the Supporting Information. Single-Crystal X-ray Crystallography. Single crystals of 1 and 2 were glued on the tips of thin-walled glass capillaries. The X-ray diffraction measurements were performed on a Stoe IPDS single-axis diffractometer with graphite-monochromatized MoKR radiation (λ ) 0.71073 Å). For each crystal two data sets with different crystal orientations were collected and merged in order to obtain satisfactory completeness. All structure calculations were done with the WinGX software.11 Absorption corrections based on symmetry-equivalent reflections were applied to the intensity data using the MULABS option in the PLATON software package.12 The structures were solved by direct methods using the program SHELXS-97.13 Full-matrix leastsquares refinements including all data (F2) were performed using the program SHELXL-97.14 All non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms bonded to carbon atoms were geometrically constructed and allowed to ride on the respective carbon atoms. In the case of 1, the occupancy factors of the two distinct water oxygen positions were determined by Fourier synthesis and refinement. The hydrogen atoms of the fully-occupied position (O35) were localized on a difference Fourier map and further refined by applying a restraint on the O-H bond lengths. Hydrogen atoms of the half-occupied position (O36) could not be determined. In the case of 2, hydrogen atoms attached to oxygen atoms of H2bdc molecules were localized on a difference Fourier map and refined independently. After final refinement in space group P1j, two distinct carbon atoms (C4 and C5) of the imidazole rings possess comparatively large anisotropic displacement parameters. This was found for several data sets measured on different crystals. A likely reason is that the crystal structure actually deviates slightly from centrosymmetry, as was also indicated by the intensity statistics. However, attempts to reduce symmetry and refine the crystal structure in space group P1 (instead of P1j) by considering possible racemic twinning were not successful but were hampered by the known problems when refining a centrosymmetric structure in a non-centrosymmetric space group (high correlation coefficients, etc.). Crystal data and details of the final structure refinements for 1 and 2 are listed in Table 1.

Results and Discussion Syntheses. The new coordination polymers 1 and 2 containing rigid (bdc) and flexible (bimx) linkers were synthesized as pure-

Crystal Growth & Design, Vol. 8, No. 9, 2008 3201 Table 1. Crystal Data and Details of Structure Refinement for 1 and 2

empirical formula formula mass crystal system space group a/Å b/Å c/Å R/° β/° γ/° V/Å3 Z T/K Dcalc/Mg m-3 µ/mm-1 2Θmax reflns measured unique reflns, Rint no. params, restraints R1, wR2 [I > 2σ(I)] R1, wR2 [all data] max peak, hole/e Å3

1

2

C52H56N8O11Zn2 1099.79 triclinic P1j 9.152(3) 12.333(4) 13.340(6) 65.60(4) 71.20(4) 76.03(4) 1287.8(10) 1 298 1.418 0.999 25.61 35597 4757, 0.081 340, 2 0.032, 0.053 0.059, 0.061 0.30, -0.25

C21H18N2O6Zn 459.74 triclinic P1j 8.473(3) 10.154(3) 13.397(3) 68.21(3) 78.64(3) 77.46(4) 1036.1(6) 2 298 1.474 1.225 27.60 24605 4804, 0.062 274, 0 0.030, 0.052 0.057, 0.059 0.37, -0.35

phase materials (as comparisons between the experimental and calculated powder XRD patterns show, see Supporting Information) under mild hydrothermal conditions at temperatures between 150 and 180° C (pH 4). In optimized syntheses the reaction mixtures contained Zn(II), H2bdc and bimx in the same molar ratios as found in the final products, that is, 1:1:1 and 2:3:1 for 1 and 2, respectively. An interesting point here is that H2bdc, which is only scarcely soluble in water at room temperature, assumes the additional function of a template or structure-directing agent for the crystallization of a MOF with a 3D open framework structure, 2, when present in excess to Zn(II) and bimx. On the contrary, 1 is an essentially dense material consisting of interpenetrating layers with the bdc linkers being the ring-penetrating moieties. Crystal Structures. In the structure of 1 there occur one distinct Zn cation, two distinct bdc anions, one distinct bimx molecule, and two distinct water positions. As is shown in Figure 1, the Zn cation is coordinated by two monodentate carboxylate groups (via atoms O23 and O29) from two bdc anions and by two imidazole nitrogen atoms (N1, N20) from two bimx

Figure 1. Crystal structure of 1. Coordination environment around a Zn center. Atomic labelling is given for atoms in the asymmetric unit. Displacement ellipsoids correspond to the 50% probability level. Hydrogen atoms are omitted. Color code: zinc, green; carbon, black; nitrogen, blue; oxygen, red.

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Figure 2. Conformations of the bimx linkers in 1 (a) and 2 (b). Color code: zinc, green; carbon, black; nitrogen, blue. Table 2. Geometrical Parameters Describing the Conformations of the bimx Linkers in 1 and 2 1

2

161.68 127.60 25.43

180.0 18.91 18.91

Angle (°) between Aromatic Ringsa,b X-Y Y-Z/Y-X′

70.16 76.58

82.34 82.34

Distance (Å) between N Donors Atomsa,b N1 · · · N20/N1 · · · N1′

10.654

10.292

Figure 3. Crystal structure of 1. (a) Single puckered layer (same color code for atoms as in Figure 1). (b) Schematic representation of a composite layer, consisting of two interpenetrating layers.

a,b

Torsion Angle (°) N3-C6-C17-N18/N3-C6-C6′-N3′ C2-N3-C6-C7 C19-N18-C17-C10/C2′-N3′-C6′-C7′

a

For the labelling of atoms and aromatic rings see Figure 2. b Atoms and rings that are generated by an inversion operation are indicated by the superscript ′.

molecules. The variation of the bond angles (93.29(8)-122.73(9)°) indicates a significant distortion from ideal tetrahedral geometry; the Zn-O and Zn-N bond lengths are in the typical ranges. The bdc anions both have Ci symmetry. The bimx molecule of C1 symmetry adopts a trans conformation (Figure 2a) as demonstrated by the torsion angle N3-C6-C17-N18 (161.68°). The two imidazole rings are differently oriented with respect to the central benzene ring, and the two donor N1 and N20 atoms are 10.654 Å apart from each other. Geometrical parameters describing the conformation of the bimx molecule are listed in Table 2. The Zn cations are linked by bimx units into linear chains, which are further joined together by bdc units to form layers that extend parallel to the a,c-plane and contain only fourmembered rings with the sequence Zn-bdc-Zn-bimx-Zn-bdcZn-bimx (Figure 3a). Thus, the layers are based on the common 2D, 4-connected 44 net (sql net).3,15 Two distinct four-membered rings exist, a square-shaped ring and a parallelogram-shaped ring. The layers are strongly puckered allowing for an interpenetration of two such parallel layers (symmetrically related by inversion centers) to form a composite layer (Figure 3b). Penetration of the rings occurs exclusively by bdc linkers (Figure 4) of the second layer. Such composite layers are stacked along the b-axis in AA sequence to generate a rather dense crystal structure. Quite remarkable are the strongly different environments which the penetrating bdc units encounter in the two distinct four-membered rings. Typical aromatic-aromatic (face-to-face or edge-to-face) interactions are clearly seen in the case of the parallelogram-shaped ring (Figure 4b) with offset face-to-face contacts16 between the benzene ring and two adjacent imidazole

Figure 4. Crystal structure of 1. (a) and (b) The two distinct fourmembered rings in a single layer with ring-penetrating bdc moieties of the second interpenetrated layer (same color code for atoms as in Figure 1).

rings (centroid-to-centroid separation at 3.669 Å, shortest distance between two non-hydrogen atoms at 3.397 Å). For the larger square-shaped ring, no such defined interactions exist (Figure 4a). The small voids left within and between the composite layers are filled with guest water molecules. Each water molecule in the fully-occupied position (O35) donates two O-H · · · O and accepts two C-H · · · O hydrogen bonds. The bonding situation of the water molecules in the half-occupied position (O36) is not as well defined; carboxylate oxygen atoms (O31) are potential acceptors of a hydrogen bond.

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Figure 5. Crystal structure of 2 The SBU with atomic labelling given for atoms in the asymmetric unit. Displacement ellipsoids correspond to the 50% probability level. Hydrogen atoms are omitted. Color code: zinc, green; carbon, black; nitrogen, blue; oxygen, red.

In the structure of 2 there occur one distinct Zn cation, two distinct bdc anions, one distinct H2bdc molecule, and one distinct bimx molecule. As shown in Figure 5, pairs of Zn cations are bridged by two µ-η1:η1-carboxylate groups (O12, O14) from two bdc anions into dimeric SBUs of Ci symmetry. The distorted tetrahedral coordination at each Zn center is completed by one monodentate carboxylate group (O18) from a bdc anion and one imidazole nitrogen atom (N1) from a bimx molecule. The bond angles vary between 101.15(7) and 122.34(7)°; the Zn-O and Zn-N bond lengths are in the typical ranges. The “acidic” C-H groups (at C2) of the two imidazole rings participate in weak C-H · · · O hydrogen bonds with the carboxylate O14 atoms of the central shair-shaped ring of the SBU (distance C2...O14 at 3.294(4) Å, angle C2-H2 · · · O14 at 140°; see Figure S13 in the Supporting Information). This is an as yet rather rare type of dinuclear SBU; we are aware of two other 2D coordinationpolymers,[Zn2(H2mdip)2(4,4′-bipy)]and[Zn2(mdip)(bix).H2O] (H4mdip ) methylenediisophthalic acid, 4,4′-bipy ) 4,4′-bipyridine), which contain such a SBU.8c The bdc and bimx units all have Ci symmetry. The bimx molecule adopts a trans conformation (Figure 2b) with the torsion angle N3-C6-C6′-N3′ at 180° (′ indicates an atom obtained after transformation by an inversion operation). The two imidazole rings are oriented nearly perpendicular with respect to the central benzene ring, and the two donor N1 and N1′ atoms are 10.292 Å apart from each other. Thus, the conformation of the bimx molecule in 2 deviates considerably from that of the bimx molecule in 1 (compare the geometrical parameters listed in Table 2). The 6-connecting SBUs are linked by bdc units into nearly planar layers, which are further cross-linked by the bimx molecules into a 3D framework of R-Po-net topology. Thus, 2 is isoreticular to a number of porous MOFs with layer-pillar frameworks built from carboxylate-amine/imine-linked paddle wheel SBUs reported recently.4,5 Due to the angular orientation of the donor atoms in the bimx linkers the framework of 2 is rather distorted and of low symmetry. This can be seen in Figure

Figure 6. Crystal structure of 2. (a) Topological unit cell representing the connectivity in the 3D open framework. (b, c, d) The three distinct four-membered rings in the framework. The view is the same as for the topological unit cell, except that ring C is rotated by ca. 90° normal to the sheet (same color code for atoms as in Figure 5).

6 which displays the triclinic (topological) unit cell having the SBUs at the corners and the linkers on the edges (cell constants: a′ ) 10.154, b′ ) 11.727, c′ ) 14.327 Å, R′ ) 61.18°, β′ ) 77.41°, γ′ ) 44.85°; the setting of this cell differs from the conventional crystallographic unit cell). Also shown in Figure 6 are the three distinct four-membered rings that occur in the framework. Around ring A, the sequence of SBUs and linkers is SBU-bdc-SBU-bimx-SBU-bdc-SBU-bimx (Figure 6b), around ring B, it is SBU-bdc-SBU-bdc-SBU-bdc-SBU-bdc (Figure 6c), and around ring C, it is SBU-bdc-SBU-bimx-SBU-bdc-SBUbimx (Figure 6d). The void volume left by this open framework amounts to appoximately half the crystal volume (a value of 49.0 % was estimated with PLATON software)12 and might therefore allow

3204 Crystal Growth & Design, Vol. 8, No. 9, 2008

Figure 7. Crystal structure of 2. 3D open framework with enclosed H2bdc template molecules. The view is parallel the elliptical channels (same color code for atoms as in Figure 5).

the mutual interpenetration of two such frameworks with R-Po topology. Clearly, this is hindered by the size of the bimx linkers, or more precisely by the presence of their methyl groups, which cannot penetrate four-membered rings (compare the structure of 1) as well as by the distortion of the framework. The distorted framework generates channels along the a axis with an elliptical cross-section of ca. 8.9 × 11.7 Å2, as estimated from atom-to-atom distances (Figure 7). The channels run through an alternating succession of the two larger rings (Figure 6b,c); the normal of each such ring is tilted against the channel axis by an angle of ca. 22°. The H2bdc molecule which possesses Ci symmetry is located in the center of a distorted ring of type A (Figure 6b). Its surroundings have some resemblance with the environment of that bdc linker in the structure of 1 that penetrates the parallelogram-shaped Zn-bdc-Zn-bimx-Zn-bdc-Zn-bimx ring (Figure 4b). However, the benzene ring of the H2dbc molecule in 2 is tilted against the two adjacent imidazole rings by an angle of 32.1°, resulting in centroid-to-centroid distances of 4.489 Å. Such values exclude significant aromatic-aromatic interactions. Clearly, the H2bdc molecule interacts predominantly via its two carboxyl groups with the host framework. Each carboxyl group donates via its protonated O24 atom a strong O-H · · · O hydrogen bond to a framework carboxylate O20 atom that does not coordinate to a Zn cation (distance O24 · · · O20 at 2.644(2) Å, angle O24-H24 · · · O20 at 168(3)°) and accepts via its non-protonated O26 atom a weak C-H · · · O hydrogen bond from a methylene group of the bimx molecule (distance O26 · · · C6 at 3.500(4) Å, angle O26 · · · H6B-C6 at 168°). This is shown in Figure S14 (Supporting Information). Thermal Behavior. TG curves for 1 and 2 measured in dynamic air atmosphere are displayed in Figure 8. In the case of 1, the first endothermic step of mass loss at temperatures up to 100 °C corresponds to the escape of water molecules (exp./ calc. mass loss: 4.8/4.9%), while the following two exothermic steps between ca. 340 and 535 °C can be assigned to the decomposition of dehydrated 1 with removal of the organic moieties (exp./calc. mass loss: 78.4/80.3 %). The final residue was ZnO. In the case of 2, the first endothermic step at temperatures between ca. 240 and 345 °C does not show a clear plateau. However, the mass loss agrees approximately with the release of the H2bdc molecules (exp./calc. mass loss: 16.7/ 18.1%). The second exothermic step between ca. 345 and 500 °C corresponds to the removal of the organic framework moieties (exp./calc. mass loss: 67.3/64.2%). The final residue was ZnO. A similar TG curve without a well-developed plateau for the first step was recorded in dynamic nitrogen atmosphere (not shown).

Schaate et al.

Figure 8. TG curves for compound 1 (red) and compound 2 (blue) measured in dynamic air atmosphere.

With the aim to remove the H2bdc molecules from assynthesized 2 and produce a material with permanent microporosity, 2 was heated in air and in vacuum to various temperatures. However, all these attempts were not successful, and it was found that the template molecules could not be removed from 2 without collapse of the framework structure. As an example from these experiments, XRD patterns along with FT-IR spectra taken from samples of 2 that had been kept in air at 300 °C for 1 and 12 h, respectively, are provided in the Supporting Information. The XRD patterns reveal increasing amorphization with ongoing removal of the H2bdc molecules, which can be identified in the IR spectra by their characteristic band at 1707 cm-1 (νas(CO)).17 Also, it was not possible to exchange H2bdc for p-xylene when contacting 2 with p-xylene/ DMF mixtures (H2bdc is soluble in DMF). This was attempted in the hope that volatile p-xylene could be removed more easily by thermal treatment from an exchanged material. Role of H2bdc in the Synthesis of 2. A rather tight fixation of the H2bdc molecules in 2 is revealed by both the crystal structure and our unsuccessful attempts to remove the acid molecules form the as-synthesized material with retention of the framework structure. This might be taken as an indication that during the assembly process of 2, H2bdc functioned as a template or structure-directing agent rather than a mere void filler or pore stabilizer that becomes included in a crystallizing solid without direct influence on the type of framework structure formed.18 The latter appears to be the role of (non-coordinating) guest molecules in most MOF syntheses, in particular when the more rigid linkers are used.2b In this sense, the template function of H2bdc would result from comparatively strong guest-host interactions (hydrogen bonds) along with the low solubility of the acid in the polar solvent (water) and, in addition, some conformational flexibility of the linker to allow adaptation of a framework to the size and shape of the template. Interestingly, Kwon and co-workers recently reported5,19 on their new strategy to synthesize microporous MOFs with mixed carboxylate-imine linkers under hydrothermal conditions in the presence of hydrophobic molecules (e.g., benzene, naphthalene), reasoning that in the polar medium at higher temperature and pressure those molecules could become trapped in the crystallizing MOF framework due to favorable hydrophobic interactions between the guest molecules and the organic linkers. However, only a limited number of MOFs could be successfully prepared following this stategy, and it was stated that one reason might be that compatibility of size and shape between a guest and a host framework have to be satisfied, but that the conditions for achieving this are very hard to predict. It should be noted that

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those authors apparently used rigid linkers in most cases. Our results now support Kwon’s strategy but suggest the use of combinations of rigid as well as flexible linkers for discovering new templated MOFs. In addition, consideration of more specific template-framework interactions (such as hydrogen bonds) might be promising, as has been demonstrated recently by Cahill and co-workers20 for the syntheses of templated lanthanide-carboxylate coordination polymers. Conclusions Two new coordination polymers containing rigid (bdc) and flexible (bimx) linkers were synthesized in aqueous media (hydrothermal conditions). While 1 is an essentially dense material consisting of mutually interpenetrating layers, 2 possesses a single (non-interpenetrated) 3D open framework with wide channels that are filled with H2bdc template molecules. Formally, ring-penetrating bdc linkers are replaced by H2bdc molecules when going from the structure of 1 to the structure of 2. The flexibility of the mixed-ligand framework constructions, as seen in the crystal structures of 1 and 2, and the rather tight fit of the H2bdc template in the channels of 2 suggest that using combinations of rigid and flexible linkers along with hydrophobic molecules (under hydrothermal conditions) might be a promising strategy for discovering new templated MOFs.

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Acknowledgment. The authors are grateful to Birgit Beisse and Falk Heinroth for performing TG/DTA measurements. Supporting Information Available: X-ray crystallographic data (CIF format); powder XRD patterns, and FI-IR specta for 1 and 2; structural drawings for 2; powder XRD patterns and FI-IR spectra for 2 after heat treatment. This material is available free of charge via the Internet at http://pubs.acs.org.

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