Identification of Reaction Conditions That Can Reproducibly Lead to a

CRYSTAL GROWTH & DESIGN

Identification of Reaction Conditions That Can Reproducibly Lead to a Particular Vertex Geometry: Quest for a Robust and Reproducible Metal-Carboxylate Noncluster-type SBU

2009 VOL. 9, NO. 8 3488–3496

Tannistha Basu,† Hazel A. Sparkes,‡ Manas Kumar Bhunia,† and Raju Mondal*,† Department of Inorganic Chemistry, Indian Association for the CultiVation of Science, Raja S. C. Mullick Road, Kolkata, 700032, India, and Department of Chemistry, UniVersity of Durham, South Road, Durham, DH1 3LE, U.K. ReceiVed February 16, 2009; ReVised Manuscript ReceiVed March 25, 2009

ABSTRACT: The present work reports the identification of a robust and reproducible metal-carboxylate noncluster-based secondary building unit (SBU) using flexible ligands, methylenebis(3,5-dimethylpyrazole), and homologous alkanedicarboxylic acids (HOOC(CH2)n-COOH). We have successfully synthesized a series of zinc(II) coordination polymers using a tailor-made SBU, designed with the proper utilization of the hydrogen bonds of a pyrazole-based ligand. We have identified a clear correlation between the type of auxiliary linker used and the vertex geometry that is present in the resultant network. This, in effect, can pave the way for the strategic control of the vertex geometry, which should enable us to design and predict the topology and dimensionality of a metal-organic framework. Introduction The synthesis and design of metal-organic frameworks (MOFs) have drawn enormous interest in recent years, owing to their intriguing structural diversities and potential application as functional materials.1 Depending on the architecture or the structural diversity, MOFs can offer potential application in areas such as gas-storage, molecular sensors, luminescence, nonlinear optics, etc.2 Getting the correct architecture or network topology is, therefore, at the heart of the synthesis of these polymeric materials. Over the past few decades, a large number of synthetic approaches, such as retrosynthesis, modular chemistry, point zero charge, or supramolecular chemistry, have been pursued to produce MOFs with the desired structures.3 One of the most successful approaches for generating MOFs would be reticular synthesis using a secondary building unit (SBU), a concept adopted from zeolite structure analysis.4 The strategy is to identify a robust and reproducible building block (SBU) as a vertex and subsequently to design a network by linking them with organic ligands of well-defined geometry. This, in effect, can pave the way for the strategic control of vertex geometry in the resulting MOFs, which should enable us to predict the topology of the resulting structure. Since its inception, a variety of representative SBUs have been identified and adopted to construct MOFs.5 However, even a cursory inspection of these structures reveals that most of the strategies adopted for generating MOFs are more or less “one-dimensional”, that is, by using a SBU based on metal-carboxylate cluster (MCC), with the “paddle-wheel” remaining the jewel in the crown.6 The stability of these rigid MCCs and consequently the reproducibility of the vertex geometry could be attributed as the major reason for this. Metal-carboxylate noncluster-based SBUs as a subject, on the other hand, have remained largely unexplored.7 Synthesis of MOFs by using metal-carboxylate nonclustertype SBUs essentially offers a two-step challenge: first, to restrict the formation of paddle-wheel type MCC units and second, to offset the extra stability gained from the rigidity of the linker * Corresponding author: Fax: 91-33-2473 2805; tel: 91-33-2473 4971; e-mail: [email protected]. † Indian Association for the Cultivation of Science. ‡ University of Durham.

Scheme 1

molecule. The latter issue can easily be resolved by using flexible linkers such as the ligand molecule of the present study, methylenebis(3,5-dimethylpyrazole) (H2MDP).8 Interestingly, H2MDP offers an amicable solution to the first problem, too; the carboxylate groups can bind to metal centers in three different modes, monodentate, bidentate chelating, and a monodentate but bonded to two metal ions (Scheme 1). It is this latter mode of binding that is mostly responsible for the formation of multinuclear “paddle-wheel” type SBUs. Any judicious design of MOFs by using a noncluster-based SBU would therefore aim to restrict this particular binding mode of the carboxylate groups and this is exactly where the H2MDP is really handy. In H2MDP, the protonated nitrogen atoms are ideally situated for forming hydrogen bonds with the second uncoordinated oxygen atom of the monodentate carboxylate group (Scheme 1 (IV)). This effectively locks the second uncoordinated oxygen atom and rules out other binding possibilities of the carboxylate group and subsequently diminishes the chance of forming a “paddlewheel” type cluster. Inspired by the above-mentioned considerations, we are interested in exploring the synthesis of polymeric networks by using non-MCC type SBUs in a mixed ligand system. The homologous alkanedicarboxylic acids (HOOC-(CH2)n-COOH) offer the simplest series of flexible ditopic linkers with varieties

10.1021/cg900195f CCC: $40.75  2009 American Chemical Society Published on Web 05/07/2009

Metal-Carboxylate Noncluster-type SBU

Crystal Growth & Design, Vol. 9, No. 8, 2009 3489 Scheme 2

of angular disposition of coordinating sites. It has been observed that the angular disposition of the coordinating sites of the auxiliary ligand plays an important role in directing the network topologies, with a common trend of forming a network of higher dimensionality as the ligand becomes linear.8a,9 In this contribution, we report six new coordination polymers,10 {[Zn(Mal)(H2MDP)] · (H2O)0.1}n (1), [Zn(Succ)(H2MDP)]n (2), [Zn(Glu)(H2MDP)]n (3), {[Zn(Adip)(H2MDP)] · (H2O)}n (4), [Zn(Sub)(H2MDP)]n (5), and [Zn(Fum)(H2MDP)]n (6) (H2Mal ) malonic acid, H2Succ ) succinic acid, H2Glu ) glutaric acid, H2Adip ) adipic acid, H2Sub ) suberic acid, and H2Fum ) fumaric acid) (Scheme 2), which clearly outline a roadmap for the synthesis of MOFs using a metal-carboxylate noncluster-based SBU. Experimental Section Materials and General Methods. Methylene bis(3,5-dimethylpyrazole) (H2MDP) was synthesized via a published procedure.8 All reagents and chemicals were purchased from commercial sources and were used without further purification. FT-IR spectra were obtained on a Nicolet MAGNA-IR 750 spectrometer with samples prepared as KBr pellets. C, H, and N microanalyses were carried out with a 2400 Series-II CHN analyzer, Perkin-Elmer, USA. Syntheses of 1-6. Synthesis of [Zn(H2MDP)(Mal)]n (1). To a mixture of Zn(NO3)2, H2MDP, and malonic acid in a 1:1:1 mol ratio was added 7 mL of distilled water in a 23 mL sealed Teflon-lined autoclave, and the mixture was heated to 130 °C for 15 h. Afterward, the autoclave was slowly cooled to 25° C. Colorless block-shaped crystals were obtained. They were washed with water and dried in air (35% yield based on H2MDP). Anal. calcd (found) for C14H18N4O4Zn (1) (%): C, 45.09(45.05); H, 4.81 (4.86); N, 15.01(15.02). IR (400-4000 cm-1): 3182.33w, 3099.39w, 2977.89w, 1610.45vs, 1573.81m, 1398.30m, 1367.44s, 1292.22m, 1244s, 1213.14m, 1182.28m, 1049.20m, 800.40m, 709.76s, 634.54m, 586.32m,484.10m. Synthesis of [Zn(H2MDP)(Suc)]n (2). The procedure was similar to 1 except that malonic acid was replaced by succinic acid. Needle-shaped colorless crystals were obtained in 40% yield (based on H2MDP). Anal. calcd (found) for C15H20N4O4Zn (2): C, 46.75(46.72); H, 5.25(5.23); N, 14.49(14.53). IR (400-4000 cm-1): 3087.82w, 2929.67w, 1600.81w, 1564.16w, 1531.37w, 1431.08m, 1400.22w, 1382.87m, 1307.65s, 1203.50w, 1080.06m, 1029.92s, 975.91w, 875.62m, 825.48m, 719.40m, 636.47w, 482.17w. Synthesis of [Zn(H2MDP)(Glu)]n (3). The procedure was similar to 1 except that malonic acid was replaced by glutaric acid. Plateshaped colorless crystals were obtained in 33% yield (based on H2MDP).

Anal. calcd (found) for C17H24N4O5Zn (3): C, 47.50(47.51); H, 5.61(5.63), N, 13.01(13.04). IR (400-4000 cm-1): 3090.68w, 2925.81w, 2854.45w, 1604.66m, 1562.23s, 1440.73s, 1384.79vs, 1290.29s, 1218.93w, 1193.85w, 1141.78w, 1085.85m, 1029.92s, 825.48m, 649.97m, 572.82m, 482.17m, 364.52s. Synthesis of [Zn(H2MDP)(Adi)]n (4). The procedure was similar to 1 except that malonic acid was replaced by adipic acid. Plate-shaped colorless crystals were obtained in 40% yield (based on H2MDP). Anal. calcd (found) for C17H24N4O5Zn (4): C, 47.50 (47.41); H, 5.51(5.63); N, 13.04 (13.12). IR (400-4000 cm-1): 3500.56w, 2923.88w, 2852.52m, 1745.46w, 1604.66m, 1566.09vs, 1402.15vs, 1384.79s, 1321.15m, 1294.15s, 1195.78m, 1027.99m, 894.91m, 813.90m, 754.12m, 692.40w, 651.89w, 592.11m, 553.53m, 489.89m, 412.74m. Synthesis of [Zn(H2MDP)(Sub)]n (5). To a mixture of Zn(NO3)2, H2MDP, and suberic acid in a 1:1:1 mol ratio was added 7 mL of distilled water in a 23 mL sealed Teflon-lined autoclave and the mixture was heated to 120 °C for 72 h. Afterward, the autoclave was slowly cooled to 25 °C. Colorless block-shaped crystals were obtained. They were washed with water and dried in air (35% yield based on H2MDP). Anal. calcd (found) for C19H30N4O5Zn (5): C, 49.77 (49.65); H, 6.61 (6.7); N, 12.23 (12.31). IR (400-4000 cm-1): 2935.46m, 2856.38w, 1697.24vs, 1562.23s, 1407.94s, 1384.79s, 1332.72m, 1294.15m, 1255.57m, 1191.93s, 1010.63w, 929.63m, 763.76m, 682.75m, 486.03m, 399.24m. Synthesis of [Zn(H2MDP)(Fum)]n (6). The procedure was similar to 1 except that malonic acid was replaced by fumeric acid. Blockshaped colorless crystals were obtained in 42% yield (based on H2MDP). Anal. calcd (found) for C15H18N4O4Zn (6): C, 47.11 (47.02); H, 4.75 (4.82); N, 14.66 (14.62). IR (400-4000 cm-1): 3402.20w, 2925.81m, 1614.31s, 1384.79s, 1296.08m, 1180.35m, 1016.42m, 688.54m. X-ray Crystallography. X-ray diffraction intensities for 1-2 and 4-6 were collected at 120 K on Bruker Smart 6K/Bruker APEX-2 CCD and diffractometer using Mo KR radiation and processed using SAINT. X-ray data for 3 was collected at room temperature on a Bruker APEX-2 CCD and diffractometer. The structures were solved by direct methods in SHELXS and refined by full matrix least-squares on F2 in SHELXL.11 Crystallographic data are summarized in Table 1, and CIF files for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre (CCDC). Deposition numbers are given in Table 1. Copies of the data can be obtained, free of charge, on application to the CCDC, 12 Union Road, Cambridge CB2 1EZ UK (Fax: 44 (1223) 336 033; e-mail: [email protected]).

Result and Discussion Complexes 1-6 were synthesized by a hydrothermal method under similar conditions using Zn(NO3)2, H2MDP, and the

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Table 1. Crystal Data and Structure Refinement Information for 1-6

empirical formula formula weight crystal system space group a/Å b/Å c/Å R/° β/° γ/° V/Å3 refls collected unique reflections obs reflections [I > 2σ(I)] R1 wR2 CCDC numbers

1

2

3

4

5

6

C14H18N4O4Zn,0.1(O) 373.29 monoclinic P21/n 10.7450(4) 12.0948(4) 12.7678(4) 90 102.533(1) 90 1619.75(10) 10630 3303 2870 0.0283 0.0702 719764

C15H20N4O4Zn 385.72 monoclinic I2/a 19.2673(7) 8.3095(3) 22.2571(13) 90 96.927(1) 90 3537.4(3) 11484 3616 2860 0.0402 0.1005 719766

C16H22N4O4Zn 399.75 orthorhombic Pbca 15.5048(4) 9.8540(2) 24.3284(6) 90 90 90 3717.00(15) 43147 3496 2690 0.0299 0.0775 719763

C17H24N4O5Zn 429.77 tetragonal P41212 13.4503(3) 13.4503(3) 21.2120(8) 90 90 90 3837.48(19) 34245 4759 4330 0.0266 0.0610 719761

C19H30N4O5Zn 459.84 orthorhombic P21212 18.3051(5) 18.6004(5) 13.0542(4) 90 90 90 4444.7(2) 65562 8785 7687 0.0420 0.0978 719765

C15H18N4O4Zn 383.70 monoclinic P21/n 8.5731(3) 15.4267(5) 13.0059(4) 90 98.754(1) 90 1700.05(10) 19648 3472 3033 0.0281 0.0717 719762

Scheme 3. A Schematic Diagram of SBU-I

corresponding acid in a 1:1:1 molar ratio. All the complexes were characterized by elemental analysis, IR spectroscopy, and X-ray single crystal diffraction analyses. For the present series, the zinc atoms adopt four coordination number and exhibit tetrahedral geometry except in 5, wherein one of the zinc atoms adopts a coordination number of 5. Metal-Carboxylate Noncluster Based SBU. The vertex geometry of the resulting network can be driven by one of the competing functional groups, viz., pyrazole and carboxylate; the one which has greater influence over the other would be the determinant. For the present series, as expected, the carboxylate groups adopted a binding mode as depicted in Scheme 1(IV) and, in a way, ruled out the possibility of a metalcarboxylate cluster based SBU formation. As a result of that, the flexible acid molecules have much lesser dominance over the conformational preference of the H2MDP molecules and behave like mere linker molecules. On the other hand, the H2MDP molecule has a natural tendency to form a M2L2-type metallocyclic motif8b (M ) Zn, L ) H2MDP) and is of major significance in this structural series and henceforth referred to as SBU-I (Scheme 3). Structural Description. {[Zn(mal)(H2MDP)] · 0.1 (H2O)}n (1). The crystal structure of 1 has an interesting two-dimensional (2D) noninterpenetrating (4,4) grid network. However, unlike most of the 2D (4,4) grid networks, it is not planar but undulated, resulting from the inherent bent conformations of the malonate and H2MDP molecules. The dimension of the grid can be assigned as 12.095 × 10.745 Å (based on Zn · · · Zn distance). However, the effective pores in the actual structure are further reduced due to the significant offset stacking of adjacent 2D layers, while the noninterpenetration of the network can be attributed to the severe steric hindrance caused by the methyl groups. The most novel aspect of the structure is the undulated nature of the grid network. A closer inspection of the structure reveals that the undulation of the network is a manifestation of

the parallel stacking of an alternative array of helical strands of malonate and H2MDP molecules. Both malonate and H2MDP molecules form right-handed helices joined together at alternate metal centers; that is, each side of the grid is basically half of the pitch of the individual helix. It is noteworthy that the (4,4) grid framework with helical units constructed from mixed ligands are rare and can be considered as resulting from the ring-opening polymerization of the SBU-I.7a,12 This is further supported by the fact that the crystal packing can also be envisaged as an infinite one-dimensional chain with the malonate molecules linking from both sides of the SBU-I when viewed along the b axis. SBU-Based Syntheses of One-, Two-, and Three-Dimensional (1D, 2D, and 3D) MOFs. The conformation of the linker molecule and the angle between coordinating sites are considered to be the key factors for the dimensionality of the resultant

Figure 1. Crystal packing of 1 (a) along the c axis showing the (4,4) grid network comprised of helical units. Individual helical chains of malonate and H2MDP molecules are highlighted in yellow and green, respectively. (b) Along the b axis illustrating the ring-opening polymerization of SBU-I.


Figure 2. Crystal structure of 1 showing the offset staking of undulated (4,4) grids.

Crystal Growth & Design, Vol. 9, No. 8, 2009 3491

Figure 4. Crystal packing of 3 showing the rare herringbone type (4,4) grid topology.

Figure 3. (a) Representative unit of SBU-I. (b) Crystal packing of 2 along the a axis showing a 1D ladder type network by using SBU-I as vertex. (c) View of 2 illustrating the parallel stack of nanoporous channels along the b axis.

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Figure 5. A space-filling model of 3 showing the nanoporous channels resulting from the offset stacking of the (4,4) grids.

Figure 8. (A) A comparative depiction of SBU-I and linker molecules of 2-4 to emphasize the role of conformational freedom of the linker molecules in directing the dimensionality of the resulting network. (B) The SBU-Is of 2-4 are superimposed to illustrate how the extended flexible homologous linkers adopt different conformations when coordinating to metal center. The succinate, glutarate, and adipate chains are shown in red, yellow, and light blue, respectively.

Figure 6. Crystal structure of 4 showing 3D diamondoid network formation with SBU-I as vertex.

Figure 7. A space filling model of 4 illustrating 3-fold interpenetration of the diamondoid network.

network.8 Furthermore, there is a trade off between pore size and the nature of interpenetration in a network as the length of the linker increases.1a Three polymeric networks containing succinate (2), glutarate (3), and adipate (4) as linker molecules are in excellent agreement with these considerations and result in interesting one-, two-, and three-dimensional (1D, 2D, and 3D) frameworks, respectively.

One-Dimensional Network. The crystal structure of [Zn(Succ)(H2MDP)]n (2) reveals an interesting 1D polymeric network with SBU-I as vertex. Apparently, the succinate molecule with a torsion angle of 67.19°, shows more flexibility than does malonic acid. The additional flexibility reduces the influence of the succinate molecules over the H2MDP molecules, which adopt a favorable M2L2 type self-aggregation and result in SBU-I formation. The succinate molecules, in turn, act as linkers and clamp the SBU-I from both ends and lead to a ladder-like 1D polymeric network. When viewed along the crystallographic b axis, the resultant crystal structure resembles a parallel stack of nanochannels, connected to each other via weak C-H · · · O bonds. The dimensions of the nanochannels are those of the SBU-I, that is, the through-space Zn · · · Zn distance of ca. 8 Å and C · · · C distance between the two methylene groups of H2MDP of ca. 7 Å. Two-Dimensional Network. For glutaric acid, with increasing chain length and more linear disposition of coordinating sites, one would expect a higher dimensional network with increasing porosity. Indeed, complex 3 ([Zn(Glu)(H2MDP)]n) with a more or less linear conformation of glutarate as a linker molecule exhibits an interesting 2D framework. On the other hand, a longer chain length invokes more flexibility in the system and favors the formation of SBU-I which when considered as a vertex results in a (4,4) grid framework with glutarate as a linker. One of the most novel aspects of complex 3 is the formation of a rare herringbone-type (4,4) grid topology. Among the 2D coordination polymers (4,4) square grid geom-


Crystal Growth & Design, Vol. 9, No. 8, 2009 3493 Table 2. Selected Bond Length (Å) and Angles (°) for 1-10a Compound 1 Zn1-O1 Zn1-O3 Zn1-N3 Zn1-N1

1.9522(14) 1.9494(14)i 2.0139(17)ii 2.0219(16)

Zn1-O1 Zn1-O3 Zn1-N3 Zn1-N1

1.928(2) 1.939(2)iii 2.007(2)iv 2.014(2)


1.9179(17) 1.9287(18) 2.008(2) 2.0095(19)


1.9341(15) 1.9378(14)v 2.0027(16) 2.0082(17)

O3-Zn1-O1 O3-Zn1-N3 O1-Zn1-N3 O3-Zn1-N1 O1-Zn1-N1 N3-Zn1-N1

118.69(6)i 113.32(6)i 109.31(6)ii 101.09(6)i 103.29(6) 110.08(7)ii

Compound 2 O1-Zn1-O3 O1-Zn1-N3 O3-Zn1-N3 O1-Zn1-N1 O3-Zn1-N1 N3-Zn1-N1

104.72(9)iii 111.96(10)iv 115.29(9)iii 111.69(10) 111.64(9)iii 101.78(10)iv

Compound 3 O1-Zn1-O4 O1-Zn1-N1 O4-Zn1-N1 O1-Zn1-N3 O4-Zn1-N3 N1-Zn1-N3

101.66(9) 116.21(8) 110.19(9) 112.53(8) 110.03(8) 106.18(8)

Compound 4

Figure 9. (a) View of a single 1D chain of 5 based on SBU-I and subarate molecule as linker. (b) Crystal packing of 5 showing molecular fabric type network formation.

etries are the most common and rectangular grids are known for systems containing mixed ligands of unequal lengths.13 Notwithstanding, a (4,4) grid with a herringbone topology has hitherto not been reported, and to the best of our knowledge, this is the first report of an herringbone-type (4,4) grid with a single flexible linker (glutarate) molecule. Apparently, an interesting tilted orientation of the SBU-I is the driving force behind this herringbone topology. Another interesting aspect of the framework would be the large pore size of the grid, with a dimension of ca. 19 × 18 Å (based on Zn · · · Zn distance). Moreover, what makes the crystal structure of 3 even more interesting is that it has a open network with no guest molecules inside the cavity. Although several grid structures are reported with large dimensions, most of them contain rigid linkers such as bipyridine and the cavities are usually filled with guest molecules.14 In addition, there are hardly any examples of open framework structures with spaces as large as 3 and constructed from flexible linkers. However, the effective pores in the actual structure are reduced considerably due to offset stacking of two adjacent 2D single layers. Nonetheless, a space-filling model of the crystal packing clearly shows the nanoporous channels running along the crystallographic b axis. Three-Dimensional Network. The networks obtained so far are intriguing and prompted us to explore the possibility of constructing MOFs by using SBU-I with higher members of this acid series. The crystal structure of 4 ([Zn(Adip)(H2MDP)]) would be a nice example of constructing 3D MOF with the strategic control of vertex geometry. The adipate molecule adopts a more or less linear conformation, and formation of the SBU-I is a recurring theme. From a topological point of view, the crystal structure features a diamondoid network by using SBU-I as vertex. However, the network appears in an elongated shape with a dimension of 71.48 × 55.98 × 19.02 Å. Apparently, the larger sizes of the secondary building unit and the linker molecule can be attributed as the major reason for such a huge network formation. It is quite remarkable to see that a framework of such a huge dimension is constructed


110.00(7)v 103.51(7) 111.64(7)v 111.12(7) 106.91(7)v 113.70(6)

Compound 5 Zn1-O4vi Zn1-O1 Zn1-N3 Zn1-N1 Zn2-O1A Zn2-O3Avii Zn2-N1A Zn2-N3A

O4vi-Zn1-O1 O4vi-Zn1-N3 O1-Zn1-N3 O4vi-Zn1-N1 O1-Zn1-N1 N3-Zn1-N1 O1A-Zn2-O3Avii O1A-Zn2-N1A O3Avii-Zn2-N1A O1A-Zn2-N3A O3Avii-Zn2-N3A N1A-Zn2-N3A

1.941(3) 1.955(3) 1.996(4) 2.041(4) 1.953(3) 1.983(3) 2.011(4) 2.041(4)

112.05(14) 117.04(15) 117.81(15) 99.70(15) 107.03(15) 99.98(16) 121.72(17) 117.20(17) 112.52(18) 104.45(16) 94.71(15) 100.00(15)

Compound 6 Zn1-O3 Zn1-O1 Zn1-N3 Zn1-N1

1.9385(15) 1.9559(13) 2.0239(16)viii 2.0296(16)


109.85(6) 120.40(7)viii 111.28(6)viii 101.06(7) 113.42(6) 100.02(6)viii

a Symmetry operations: (i) -x + 3/2, y + 1/2, -z + 1/2; (ii) -x + 5/2, y - 1/2, -z + 1/2; (iii) x, y + 1, z; (iv) -x + 1/2, y, -z; (v) x + 1/2, -y + 3/2, -z - 1/4; (vi) -x, -y + 1, z; (vii) -x, -y, z; (viii) -x + 1/2, y + 1/2, -z + 1/2.

with a highly flexible linker molecule. The diamondoid network undergoes 3-fold interpenetration, supported by hydrogen bonds with uncoordinated water molecules, which significantly reduces the pore size within the cavity. The supramolecular connectivity between the interpenetrating networks is further promoted by π-π interactions (centroid to centroid distance ) 3.97 Å, shortest C · · · C distance ) 3.63 Å) between the pyrazole moieties of the SBU-I. Conformational Freedom of Linker and the Dimensionality. Conformational freedom of a linker molecule can have a significant impact on the final structural topology and dimensionality of the coordination polymer. This becomes even more important for flexible linkers, such as the acid molecules of the

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Figure 10. Crystal structure of 6 showing (a) the diamondoid network and (b) 3-fold interpenetration of the network in a space filling model.

Scheme 4

present study, which depending on the conformational requirement, can adopt different conformations by bending, twisting, or rotating when coordinating to the metal center.15 Indeed, the crystal structures of 2-4 demonstrate nicely how similar flexible homologous linkers may orient themselves in different conformations to facilitate the formation of distinctly different interaction hierarchies that sustain a network with a particular dimensionality. In the present study, the vertex geometry of SBU-I consists of four extended ditopic linker molecules each of which coordinates to a neighboring SBU-I. Apparently, the topology and the dimensionality of the resultant network primarily depend on the relative spatial disposition of the neighboring SBU-Is to which the other ends of the extended linkers are coordinated.16

Since the linker molecules used herein are long chain homologous ditopic acids, it is the conformational freedom of the linker molecules that can change the relative disposition of the SBUIs through bending or twisting of the alkane chain. As illustrated in Figure 8, the pairs of succinate and glutarate molecules on each metal center protrude along the same plane; that is, the four metal centers on the extended ends are coplanar, reducing the SBU-I to a square planar vertex. The succinate molecules protrude almost in a straight line and connect to the metal centers of parallel stack of SBU-Is and result in a 1D chain, while the glutarate molecules divert away from each other and link to four neighboring SBU-Is in such a way that a 2D grid network is formed. The alkane backbone of the adipate molecule shows a significant distortion from linearity. The other end of the adipates coordinate to the neigboring SBU-Is in such a manner that the central SBU-I act as a tetrahedral vertex and result in a diamondoid network in a comprehensible manner. SBU-I Based Networks with Higher Members of the Acid Series. Complexes 2-4 exhibit an interesting trend of forming higher dimensional networks by using SBU-I as a vertex with increasing chain length of the flexible ditopic acids. The results are encouraging and prompt us to explore the possibility of generating networks with the higher members of the alkanedicarboxylic acid series. Reproducibility of the vertex geometry (SBU-I) is obviously our primary concern; notwithstanding, it would be interesting to see the effects of longer chain length and flexibility on the resulting networks. While longer chain lengths are expected to increase the pore size within the framework, it may also increase the interpenetration,1a and it will be interesting to determine whether any of the abovementioned networks are repeated for a higher member of this series.10


The asymmetric unit of 5 ({[Zn(Sub)(H2MDP)]n · (H2O)2}) consists of two independent Zn(II) units resulting from the different coordinating modes of two subarate molecules. One of the carboxylate groups of the subarate molecules coordinates to the metal center in a bidentate chelating mode (Scheme 1(I)), while the remaining three carboxylate groups adopt a coordination mode similar to that of Scheme 1(IV). However, both the crystallographically independent units lead to a similar 1D polymeric network with SBU-I as vertex. From a topological point of view, the 1D network of 5 closely resembles that of 2 as two subarate molecules conjoin two SBU-I units from two sides and result in an infinite 1D polymeric network. Topological similarity between 2 and 5 can be rationalized from the similar conformational orientation of the two acid molecules. The resulting network can be envisaged as molecular fabrics supported by the hydrogen bonds from two noncoordinating water molecules which reside in the distinct channels formed inside the framework. Role of Flexible Ligand on the SBU-I Formation. The crystal structures of complexes 2-5 are in excellent agreement with our assumption that SBU-I formation is favored by increasing the flexibility of the linker molecule. However, our previous results8 showed that rigid linker molecules favored networks with the metal center as vertex and not the SBU-I. This is consistent with the crystal structure of 1, where the two carboxylate groups which are linked by only one carbon atom in a geminal fashion, are less flexible, and result in a metalcenter vertex geometry. To assess the role of rigidity in restricting SBU-I formation, we have carried out a comparative structural study using rigid fumaric acid vis-a`-vis flexible succinic acid as linker molecules. If flexibility of the auxiliary ligand is the main driving force for the SBU-I formation, then fumarate should form a network devoid of SBU-I. The crystal structure of complex 6 ([Zn(Fum)(H2MDP)]n) seems to support this hypothesis with the rigid fumaric acid restricting the formation of SBU-I and resulting in a 3-fold interpenetrated diamondoid network with the metal center as the vertex (Figure 10). The dimension of the network is 25.72 × 22.43 × 20.18 Å. Interestingly, the conformational flexibility of the H2MDP molecule is an important issue here and should be discussed in a little more detail. It was observed earlier8b that the H2MDP molecule usually adopts two different conformations, a “bent” cis conformation and a “linear” trans conformation (Scheme 2). Furthermore, while the cis conformation is primarily responsible for 1D and 2D networks formation, the 3D MOFs (e.g., diamondoid) are favored by a linear trans conformation of the H2MDP molecules. In the present context, the rigid fumarate molecules, as expected, perturb the metallocyclic self-assembly (SBU-I), which would favor cis conformations of the H2MDP molecule, and adopts a trans conformation, resulting in a diamondoid network. A comparison of diamondoid network of 6 with 4 further displays the interplay of conformational freedom and flexibility of the linker molecules and their combined influence on the resulting crystal structures. Both the structures feature a 3-fold interpenetrated diamondoid network but consist of different vertex geometries. Apparently, the vertex geometry of the resulting network is a direct manifestation of the conformation adopted by the H2MDP molecule with cis giving SBU-I and trans giving a metal center. This in turn is directed by the flexibility of the auxiliary linker molecule; that is, the SBU-I is

Crystal Growth & Design, Vol. 9, No. 8, 2009 3495

formed when the linker is flexible with the metal centers acting as nodes when the linker is rigid. Conclusion The impetus for the work presented herein was to identify a robust and reproducible metal-carboxylate noncluster type SBU. Accordingly, we have successfully synthesized a series of polymeric networks using a tailor-made SBU, designed with the proper utilization of the hydrogen bonding of a pyrazole-based ligand. There is a clear correlation between the type of auxilary linker molecule used (flexible or rigid) and the vertex geometry (SBU-I or metal center) that is present in the resultant network (Scheme 4). We have demonstrated our strategy of using flexible ditopic acids rather than rigid ones for generating a polymeric network using SBU-I. We have also shown that the conformational freedom of the linker molecules plays a crucial role in determining both the dimensionality and topology of the final structure. In conclusion, we have successfully identified the reaction conditions that can reproducibly lead to a particular vertex geometry. The next obvious task of designing a network with desired topology and dimensionality by linking them with an organic linker of well-defined geometry is currently under investigation in our laboratory. Acknowledgment. R.M. gratefully acknowledges the financial support of the SERC Fast Track Proposal for Young Scientists Scheme (SR/FTP/CS-36/2007), Department of Science and Technology (DST), India. We also thank DST for National Single Crystal X-ray Diffractiometer Facility at the Department of Inorganic Chemistry, IACS. Supporting Information Available: X-ray crystallographic files in CIF format for 1-6. This information is available free of charge via the Internet at http://pubs.acs.org.

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(7)

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(9) (10)


J. L. C.; Yaghi, O. M. Microporous Mesoporous Mater. 2004, 73, 3–14. (a) Zhang, L.; Lu¨, X.-Q.; Chen, C.-L.; Tan, H.-Y.; Zhang, H.-X.; Kang, B.-S. Cryst. Growth Des. 2005, 5, 283–287. (b) Qi, Y.; Che, Y.; Luo, F.; Batten, S. R.; Liu, Y.; Zheng, J. Cryst. Growth Des. 2008, 8, 1654– 1662. (a) Mondal, R.; Bhunia, M. K.; Dhara, K. CrystEngComm 2008, 10, 1167–1174. (b) Mondal, R.; Basu, T.; Sadhukhan, D.; Chattopadhyay, T.; Bhunia, M. K. Cryst. Growth Des. 2009, 9, 1095–1105. (c) Kruger, P. E.; Moubaraki, B.; Fallon, G. D.; Murray, K. S. J. Chem. Soc., Dalton Trans. 2000, 713–718. Evans, O. R.; Lin, W. Acc. Chem. Res. 2002, 35, 511–522. Hydrothermal reaction of a mixture of Zn(NO3)2, H2MDP, and azelaic acid (H2Aze) in a 1:1:1 mol ratio afforded a colorless crystalline material. Unfortunately, the complex gave small crystals of extremely poor diffraction quality, and the unit cell parameters derived from these small crystals were approximately 12 × 13 × 19 Å. The resulting X-ray data recorded were of very low resolution but suggest that the chemical connectivities shown schematically in Scheme 4 for the stoichiometry ({[Zn(Aze)(H2MDP)]n · (H2Aze)2}) are overall correct

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CG900195F

Identification of Reaction Conditions That Can Reproducibly Lead to a

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