Influence of Anion on the Coordination Mode of a Flexible Neutral

Influence of Anion on the Coordination Mode of a Flexible Neutral Ligand in Zn(II) Complexes: From Discrete Zero-Dimensional to Infinite 1D Helical Chains, 2D Nanoporous Bilayer Networks, and 3D Interpenetrated Metal-Organic Frameworks

CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 2 1095–1105

Raju Mondal,*,† Tannistha Basu,† Dipali Sadhukhan,† Tanmay Chattopadhyay,‡ and Manas kumar Bhunia† Department of Inorganic Chemistry, Indian Association for the CultiVation of Science, Raja S. C. Mullick Road, Kolkata, 700032, India, and Department of Chemistry, Calcutta UniVersity, 92, A. P. C. Road, Kolkata 700 009, India ReceiVed August 22, 2008; ReVised Manuscript ReceiVed NoVember 10, 2008

ABSTRACT: A series of Zn(II) complexes with a flexible ligand have been synthesized in the presence of different anions [Cl(1) and (4), CH3COO- (2), CF3COO- (3), Br- (5), I- (6), HCO2- (7), SO4- (8) and (9), and NO3- (10)]. Employment of different anions have resulted in different architectures, ranging from discrete zero-dimensional to infinite one-, two-, and three-dimensional (1D, 2D, and 3D) coordination polymers. Complexes 1, 2, and 3 exhibit binuclear metallocyclic zero-dimensional structures, while 4-7 form 1D helical networks. Among the halides, Cl- and Br- form interesting helical networks with the coexistence of both left-handed and right-handed helical chains, while the introduction of I- ion results in a water mediated right-handed helical network. Complex 7, on the other hand, represents an unique example of an alternative array of a left-handed and right-handed helical network, which can also be described as a hydrogen bonded 2D (4,4) network. Complexes 8 and 9 exhibit bilayer structures containing a distinct nanoporous void and channels within the networks. Furthermore, complexes 8 and 9 can be described as resulting from the structural transformation of 1 and 2, respectively, by replacing the terminally coordinated anions with bridging sulfate ions, whereas complex 10 exhibits a 2-fold interpenetrated diamondoid network assisted by the molecular recognition of the nitrate ions. Throughout the series, hydrogen bonds, anion-π interactions, and other intermolecular interactions involving the anions play some crucial role in stabilizing the resulting networks. Different conformations adopted by the flexible ligand further facilitate to achieve a suitable coordination environment around the metal center. Introduction The design and syntheses of porous metal-organic frameworks and coordination polymers have drawn enormous interest in recent years.1 Construction of coordination polymers with specific topologies are becoming increasingly popular in recent years by virtue of the possible design of materials with specific electronic, optic, magnetic, and catalytic properties.2 Although there are examples wherein the molecules are assembled in a predetermined fashion, prediction of the crystal structure is largely considered to be serendipitous. The structure of the resulting framework is primarily dependent upon the coordination geometry of the metal centers as well as on the functionality and coordinating ability of the bridging ligand.3 The Zn2+ ion, because of its spherical d10 configuration, is particularly suited for the construction of coordination polymers and networks. A flexible coordination environment associated with d10 configuration can lead to zinc complexes with variable geometries from tetrahedral through trigonal bipyramidal and square pyramidal to octahedral and often leads to severe distortion from the ideal polyhedron. This coupled with the general lability of the zinc complexes can lead to coordination polymers of varieties of architectures, ranging from one-, two-, to threedimensional (1D, 2D, to 3D) networks.3a One of several factors that have a great impact on the final structural topology of coordination polymer is the nature of the anion present.4 This is really important when the ligand system is neutral, wherein the essential anions can dictate the self* Corresponding author. Fax: 91-33-2473 2805. Tel: 91-33-2473 4971. E-mail: [email protected]. † Indian Association for the Cultivation of Science. ‡ Calcutta University.

assembly by either coordinating strongly to the vacant sites on the metal center or remaining as noncoordinating counterions and acting as templates. Anions can coordinate to the metal center in monodentate fashion as well as in a bridging mode.5 It has been observed that the bridging ability of the anion has a drastic effect on the dimensionality of the coordination polymer and often leads to varieties of intriguing architectures, ranging from discrete zero-dimensional (0D) to infinite 1D, 2D, and 3D frameworks. Anions, therefore, can be selectively used to increase or reduce the dimensionality of the resulting polymeric network. Furthermore, anions are in principle hydrogen bond acceptors. Noncovalent interactions such as hydrogen bonds, weak C-H · · · X (X ) O, N, π) interactions, and π-π stacking play an important role in the self-assembly process of coordination polymers.6 Thus, for a system containing hydrogen bond forming functional groups, the anions can have a great influence on the resulting crystal structure.7 Our recent work shows an interesting and beneficial use of the anions as part of the frameworks.8 We have reported the use of a flexible, neutral ligand methylenebis(3,5-dimethylpyrazole) (H2MDP) for the generation of 2D and 3D polymeric networks by employing benzene polycarboxylates as anions. On the other hand, employment of inorganic anions, such as chloride and thicyanate resulted in 0D and 1D structures.9 The potential of this ligand to produce interesting supramolecular arrays is now evaluated with the coordination of H2MDP to Zn(II) center in the presence of different anions. In particular, the influence of the anion on the conformation of the flexible H2MDP molecule with respect to the metal center is examined. Conformational freedom of a ligand can also have a great impact on the final structural topology of the coordination polymer.

10.1021/cg800923g CCC: $40.75  2009 American Chemical Society Published on Web 12/22/2008

1096 Crystal Growth & Design, Vol. 9, No. 2, 2009

Mondal et al. 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 1-10. Syntheses of [Zn2(H2MDP)2Cl4] · H2O (1) and [Zn(H2MDP)Cl2]n (4). A mixture of H2MDP (0.020 g, 1.0 mmol) and ZnCl2 (0.136 g,1.0 mmol) were added to 10 mL of distilled water in a 23 mL sealed Teflon-lined autoclave and heated to 180 °C for 18 h. Afterward, the autoclave was slowly cooled up to 25 °C. Two different shaped crystals (rhombohedral and plate) were obtained. They were washed with water and dried in air (4.1 mg, 20% yield and 2.4 mg, 12% yield for complex 1 and 4, respectively, based on H2MDP). Anal. Calcd. (found) for C22H36N8Cl4O2Zn2 (1) (%): C, 36.81 (36.75); H, 5.02 (5.12); N, 15.62 (15.55). IR (400-4000 cm-1): 3317s, 1630s, 1602s, 1585w, 1521m, 1472w, 1413vs, 1286vs, 1174s, 1042m, 1012m, 902w, 815m, 671vs. Anal. Calcd. (found) for C11H16N4Cl2Zn (4) (%): C, 38.76 (38.68); H, 4.69 (4.76); N, 16.44 (16.42). IR (400-4000 cm-1): 1557s,1527w, 1473m, 1440w, 1409w, 1382vs, 1276vs, 1211s, 1164m, 1012m, 808w, 767m, 676vs. Synthesis of [Zn2(H2MDP)2(OAc)4] · 3H2O (2). The process was similar to 1 except that ZnCl2 was replaced by Zn(OAc)2 · 2H2O (0.219 g, 1.0 mmol). Yellow needle-shaped crystals of 2 were obtained in 44% yield (based on H2MDP). Anal. Calcd. (found) for C30H50N8O11Zn2 (2) (%): C, 38.61 (38.68); H, 4.70 (4.76); N, 13.57 (13.50). IR (400-4000 cm-1): 1590s, 1564w, 1401m, 1387w, 1331vs, 1295w, 1192m, 1009m, 891w, 663m, 489vs. Synthesis of [Zn(H2MDP)Br2]n (5). The process was similar to 1 except that ZnCl2 was replaced by ZnBr2 (0.225 g, 1.0 mmol). Yellow needle-shaped crystals of 5 were obtained in 35% yield (based on H2MDP). Anal. Calcd (found) for C11H16N4Br2Zn (5) (%): C, 30.74 (30.68); H, 3.73 (3.69); N, 13.04 (13.14). IR (400-4000 cm-1): 1583s, 1525w, 1471m, 1441w, 1407w, 1278vs, 1172m, 1010m, 818w, 769m, 675vs. Synthesis of {[Zn(H2MDP)I2] · H2O}n (6). The preparation was analogous to 1, by using ZnI2 (0.319 g, 1.0 mmol) in place of ZnCl2. Deep yellow block shaped crystals of 6 (39% yield based on H2MDP) were obtained. Anal. Calcd. (found) for C11H18N4I2OZn (6) (%): C, 24.38 (24.33); H, 3.32 (3.27); N, 10.34 (10.31). IR (400-4000 cm-1): 3315s, 1589s, 1572w, 1531w, 1510w, 1472m, 1433w, 1409w, 1379vs, 1334vs, 1282vs, 1164m, 1009m, 810w, 767m, 667vs. Syntheses of {[Zn(H2MDP)(SO4)] · H2O}n (8) and [Zn(H2MDP)(SO4)]n (9). The process was similar to 1 except that ZnCl2 was replaced by ZnSO4.H2O (0.179 g, 1.0 mmol). Two different shaped crystals, block shaped crystals in 22% yield and needle-shaped crystals in 14% yield were obtained for complex 8 and 9, respectively. Anal. Calcd. (found) for C11H18N4O5SZn (8) (%): C, 34.40 (34.34); H, 4.69 (4.63); N, 14.59 (14.51). IR (400-4000 cm-1): 3125s, 1589s,

Scheme 1

This becomes even more important for flexible ligands, which, depending on the conformational requirement, can adopt different conformations by bending or rotation when coordinating to the metal center. The advantages of conformational freedom of the spacer have been successfully exploited by many in recent times leading to a surge in intriguing architectures with the use of flexible bridging ligands.10 However, correspondence between the conformational freedom of the spacers and coordination polymers, as a subject, has remained largely unexplored. The H2MDP molecule has a flexible methylene group, which allows H2MDP to have flexible conformations and better adaptability and thus provides various possible self-assembly and construction of polymeric networks. Indeed, the crystal structures reported herein demonstrate nicely how a flexible ligand may orient itself in different conformations to facilitate the formation of distinctly different interaction hierarchies that sustain a particular coordination polymer. Although the H2MDP molecule adopts different conformations, it can broadly be classified as cis and trans orientation with respect to metal coordination sites (Scheme 1). In this contribution, we report the syntheses and characterization of three 0D dinuclear complexes, [Zn2(H2MDP)2Cl4] · H2O (1), [Zn2(H2MDP)2(AcO)4] · 3H2O (2), and [Zn2(H2MDP)2(TFA)4] (3) (AcOH ) acetic acid, HTFA ) trifluoro acetic acid), three novel 1D helical networks, [Zn(H2MDP)Cl2]n (4), [Zn(H2MDP)Br2]n (5), {[Zn(H2MDP)I2] · H2O}n (6), one hydrogen bonded 2D (4,4) network, [Zn(H2MDP)(HCO2-)2]n (7), two nanoporous bilayer structures, {[Zn(H2MDP)(SO4)] · H2O}n (8), [Zn(H2MDP)(SO4)]n (9), and one novel 3D metal-organic framework [Zn(H2MDP)(NO3)2]n (10). Experimental Procedures Materials and General Methods. Methylene bis(3,5-dimethylpyrazole) (H2MDP) was synthesized via a published procedure.8 All reagents

Table 1. Crystal Data and Structure Refinement Information for 1-10 1 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

2

3

4

5

6

7

8

9

10

C11H18Cl2N4OZn C30H50N8O11Zn2 C30H32F12N8O8Zn2 C11H16Cl2N4Zn C11H16Br2N4Zn C11H18I2N4OZn C13H18N4O4Zn C11H18N4O5SZn C11H16N4O4SZn C22H32N10O6Zn 358.56

829.52

991.38

340.55

429.47

541.46

359.68

383.72

365.71

597.95

triclinic

orthorhombic

monoclinic

monoclinic

monoclinic

orthorhombic

orthorhombic triclinic

orthorhombic

tetragonal

P1j 8.5549(15) 9.5391(17) 9.5715(17) 85.746(3) 79.841(2) 73.507(2) 737.0(2) 11162

P212121 12.7651(12) 12.9336(12) 22.579(2) 90 90 90 3727.8(6) 46402

P21/n 8.5742(1) 14.5932(2) 15.0547(2) 90 91.288(1) 90 1883.24(4) 35488

P21/n 9.2787(4) 8.7453(3) 17.7348(7) 90 99.192(2) 90 1420.61(10) 35702

P21/n 9.5574(2) 8.7122(2) 18.1866(3) 90 100.459(1) 90 1489.16(5) 36232

P212121 9.9103(2) 12.8091(2) 13.3863(2) 90 90 90 1699.28(5) 23592

Pbca 13.4095(6) 14.0204(7) 16.6409(8) 90 90 90 3128.6(3) 36754

P1j 8.8415(1) 10.0385(1) 10.2123(1) 77.676(1) 67.268(1) 64.660(1) 754.298(14) 12801

Pnna 22.833(4) 14.036(2) 8.6926(15) 90 90 90 2785.8(8) 28106

I41/acd 13.4139(7) 13.4139(7) 31.309(3) 90 90 90 5633.5(7) 35322

4250

8526

7396

9154

8849

3107

2956

4282

3546

1407

3808

7286

5759

6917

6129

3014

2214

3902

2484

1197

0.0251 0.0632 698837

0.0766 0.2170 698841

0.0359 0.0891 698844

0.0312 0.0693 698836

0.0321 0.0640 698835

0.0232 0.0555 698839

0.0333 0.0846 698838

0.0250 0.0626 698843

0.0535 0.1221 698842

0.0297 0.0780 698840

Flexible Neutral Ligand in Zn(II) Complexes

Crystal Growth & Design, Vol. 9, No. 2, 2009 1097

Table 2. Selected Bond Length (Å) and Angles (°) for 1-10

1527w, 1512w, 1469m, 1438w, 1419w, 1379vs, 1299vs, 1191m, 1120m, 1062m, 894w, 750m, 619vs. Anal. Calcd. (found) for C11H16N4O4SZn (9) (%): C, 36.09 (36.19); H, 4.38 (4.28); N, 15.31 (15.36). IR (400-4000 cm-1): 1571s, 1527w, 1413m, 1386w, 1296vs, 1228w, 1172m, 1093m, 1064m, 1035m, 983w, 626m, 597vs. Synthesis of [Zn(H2MDP)(NO3)2]n (10). The preparation was analogous to 1, by using ZnNO3 · 6H2O (0.297 g, 1.0 mmol) in place of ZnCl2. Colorless prism shaped crystals of 10 were obtained in 44% yield. Anal. Calcd. (found) for C22H32N10O6Zn (10) (%):C, 44.15 (44.21); H, 5.35 (5.33); N, 23.41(23.49). IR (400-4000 cm-1): 3184w, 1581s, 1552w, 1483m, 1384w, 1305vs, 1178m, 1087m, 1060m, 1016m, 821w, 767m, 671vs. Syntheses of [Zn2(H2MDP)2(TFA)4] (3) and [Zn(H2MDP)(HCO2-)2]n (7). The complexes 3 and 7 were prepared adopting a similar procedure. In a methanolic solution of H2MDP (0.020 g,1.0 mmol), ZnCO3 (0.125 g,1.0 mmol) was added and stirred for 30 min with heating. After that, trifluroacetic acid and formic acid were added to the solution until effervesion ceased to prepare the complexes 3 and 7, respectively and the mixture was stirred for another 30 min. Then the solution was cooled and filtered and the colorless filtrate part was

kept in a CaCl2 desiccator in dark. After a few days colorless cube shaped crystals of complex 3 and 7 were separated out. (10.2 mg, 50.1% yield and 10 mg, 50% yield and for complex 3 and 7 respectively based on H2MDP). Anal. Calcd. (found) for C30H32F12N8O8Zn (3) (%): C, 36.31 (36.25); H, 3.23 (3.19); N, 11.29 (11.26). IR (400-4000 cm-1): 3132s, 1676s, 1583w, 1537w, 1471m, 1444w, 1382w, 1296vs, 1203vs, 1159m, 1141m, 1035m, 844w, 798m, 727vs. Anal. Calcd. (found) for C13H18N4O4Zn (7) (%): C, 43.37 (43.34); H, 5.00 (4.94); N, 15.57(15.59). IR (400-4000 cm-1): 3205s, 1596s, 1564w, 1477m, 1408w, 1338vs, 1296vs, 1193m, 1043m, 1018m, 896w, 813w, m, 665vs. X-ray Crystallography. X-ray diffraction intensities for 1-6 and 8-10 were collected at 120 K and for 7 at room temperature on a Bruker APEX-2 CCD diffractometer using Mo KR radiation. Data were processed using the Bruker SAINT package and the structure solution and refinement procedures were performed using SHELX97.11 The structures were solved by direct methods and refined with the fullmatrix least-squares on F2. Crystal and structure refinement data are summarized in Table 1. Crystallographic data 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


Mondal et al.

Figure 1. Crystal packing of 1 along the a axis showing the closepacked M2L2 units. Note that the hydrogen and methyl carbon atoms are omitted for clarity. 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 All the complexes were characterized by elemental analysis, IR spectroscopy, and X-ray single crystal diffraction analyses. Although all the reactions of metal ions with the H2MDP ligand were carried out under similar conditions in a molar ratio of 1:1, two type of crystals were isolated for both sulfate and chloride ions. Hydrothermal reaction of ZnCl2 with H2MDP in a 1:1 ratio afforded two distinctly different types of crystals, large rhombohedral shaped deep yellow colored crystals and colorless plate shaped crystals. Single crystal structure analysis reveals that the yellow colored crystals are of 0D dimeric units of 1 [Zn2(H2MDP)2Cl4] · H2O, whereas the colorless crystals are those of 1D coordination polymer of 4 [Zn(H2MDP)Cl2]n. Similarly, two different types of crystals were obtained from the hydrothermal reaction of ZnSO4 with H2MDP, block shaped yellow colored crystals of 8 and needle shaped colorless crystals of 9. Structural Description. Dinuclear Zero-Dimensional Structures. [Zn2(H2MDP)2Cl4] · H2O (1). Single crystal X-ray diffraction analysis shows that complex 1 has a 0D dinuclear structure. Two H2MDP molecules form a closed loop by coordinating to two zinc atoms resulting in a M2L2 type metallocyclic motif (M ) Zn, L ) H2MDP), while the remaining two sites of the tetrahedral metal center are occupied by chloride ions. Topologically, the crystal structure can be envisaged as a close-packing of nanoporous rings (Figure 1), linked via water molecules with the formation of two cooperatively hydrogen bonded centerosymmetric square motifs of O-H · · · Cl bonds. While further support comes from two cooperative N-H · · · O bonds (Figure 2). The dimension within the ring can be defined by through-space Zn · · · Zn distance of ca 8 Å and C · · · C distance between two methylene group of H2MDP of ca 7 Å. It is noteworthy that acceptor capabilities of halogen atoms with respect to hydrogen bonding are controversial and unactivated halogen atoms are generally deemed to be poor acceptors.12 However, halogen atom can act as a good hydrogen bond acceptor when they are activated or bonded to a metal center.13 In the present context, the halogen atoms are bonded to metal center and therefore can be considered as activated. The crystal structure of 1, therefore, shows a nice example where the activated chlorine atoms dictate the 3D supramolecular structure with the formation of O-H · · · Cl bonds. The significance of the acceptor capabilities of chlorine is even more prominent by

Figure 2. Crystal structure of 1 highlighting the hydrogen bond acceptor capability of chlorine atom in connecting M2L2 units with the formation of two cooperative centrosymmetric hydrogen bonded square motifs of OH · · · Cl bonds.

Figure 3. View of 2 along the c axis showing the role of water molecules in connecting M2L2 units.

the fact that the supramolecular framework of 1 is primarily sustained by O-H · · · Cl bonds despite the presence and possibility of “stronger” O(N)-H · · · O bonds. [Zn2(H2MDP)2(OAc)4] · 3H2O (2). X-ray single crystal structure of 2 reveals a 0D structure with the formation of a similar M2L2 motif. The discrete M2L2 units are interconnected by hydrogen bonds via water molecules through the coordinated acetate ions (Figure 3). While the pyrazole moiety forms an intramolecular N-H · · · O to support the framework. [Zn(H2MDP)(TFA)2]2 (3). Crystal structure of 3 is again 0D, and the formation of the M2L2 motif is a recurring theme. However, there is one exception with respect to the other two 0D structures: the asymmetric unit does not contain any water



Figure 4. Crystal packing of 3 along the a axis. Notice that the M2L2 units are directly linked to each other by N-H · · · O bonds.

molecules. In other words, the crystal structure is devoid of any external hydrogen bond linker molecule. Consequently, hydrogen bonds involving the pyrazole ring and oxygen atom of TFA molecule play an important role in 3D supramolecular network formation. One of the pyrazole N-H proton forms an intramolecular N-H · · · O bond, whereas the other one forms an intramolecular N-H · · · O bond as well as an intermolecular N-H · · · O bond which connects the M2L2 motifs to generate a 3D structure (Figure 4). One-Dimensional Helical Networks. The conformation of the ligand molecule and the angle between coordinating sites are considered to be the key factors for helical network formation.14 It has been observed that flexible and V-shaped exobidentate ligands, such as 4,4′-oxybis(benzoic acid), can be used effectively for the construction of helical networks.14a Interestingly, the H2MDP molecule possesses structural characteristics that are considered to be prerequisites for helical network construction: (a) the H2MDP molecule is also a V-shaped exobidentate ligand. Furthermore, the two pyrazole moieties of the H2MDP molecule are linked with a methylene group. This invokes more flexibility for H2MDP, which can adopt a favorable conformation when coordinating to the metal center. (b) The angle between the coordinating sites is ca 110°, which enhances the possibility of formation of helical framework. Indeed, our previous result8 shows that H2MDP molecule can be used for constructing helical units of a coordination polymer in a mixed ligand system containing bent anionic O-donor ligand. These observation prompts us to explore the possibility of helical networks with H2MDP molecule. As such, we report herein four novel 1D helical coordination polymers, 4-7, constructed from H2MDP and Zn(II) center in the presence of different anions. [Zn(H2MDP)Cl2]n (4). In comparison to the M2L2 metallocyclic motif of 1, complex 4 features an interesting 1D helical network sustained by π-π stacking and weak interactions (Figure 5). These infinite helical chains are extended along the crystallographic b axis and the distance of the repeating parts of the helices is 8.745 Å. Interestingly, a closer inspection of the structure reveals a coexistence of two kind of helical networks, a left-handed and a right-handed helical network, arranged in an alternative array. The crystal structure of 4 further illustrates the importance of halogen atom as hydrogen bond acceptor with the formation

Figure 5. Crystal packing of 4 showing (a) the alternative array of left-handed and right-handed helical 1D networks along the b axis and (b) the importance of N-H · · · Cl and π-π interactions (red lines) between the helices in order to sustain the architecture along the b axis.

of an intrahelix N-H · · · Cl bond as well as an interhelix N-H · · · Cl bond. Two such complementary interhelix N-H · · · Cl bonds play an important role in sustaining the helical arrangement. While a very strong metal-polarized π-π stacking further promotes the supramolecular connectivity among these parallel strands of helices. The pyrazole moieties of the convex part of one helix come very close to the concave part of the adjacent helix and adopts a favorable “head-to-tail” arrangement with respect to the metal center and results in a very strong faceto-face π-π interaction15 (centroid to centroid distance is 3.472 Å, while the shortest C · · · C distance is 3.397 Å). It is interesting to note that these intermolecular interactions are also extended in an alternative left-handed and right-handed helical manner along the b axis. The crystal packing of 4, therefore, represents an unique case where two sets of left-handed and right-handed helices, one covalently bonded and another with intermolecular interactions, run along the same directions. [Zn(H2MDP)Br2]n (5). The crystal structure of 5 is isostructural with 4, forming a similar alternative array of left-handed and right-handed helical 1D coordination polymer (Figure 6a). The only difference is relatively longer separation of π-π interaction (centroid to centroid distance is 3.509 Å, while the shortest C · · · N distance is 3.418 Å). {[Zn(H2MDP)I2] · H2O}n (6). Introduction of iodide as an anion results in an interesting water mediated 1D helical polymeric network. In stark contrast to the alternative array of left-handed and right-handed helices in 4 and 5, the crystal structure of 6 features a right-handed infinite helical network extended along the crystallographic b axis (Figure 6). Interestingly, the H2MDP molecule adopts a trans conformation with the planes of the two pyrazole rings inclined by 89.81°. Subsequently, the Zn · · · Zn separation across H2MDP became much larger (10.21 Å) and results in a helix of much longer


Figure 6. (a) A space filling model of 5 showing the alternative array of left-handed (red) and right-handed (green) helices. (b) View of 6 showing infinite helical chain of 6 along the b axis. (c) Crystal packing of 6 showing how the helical chains are linked with the O-H · · · I · · · H-O chain.

pitch length with the distance of the repeating unit value of 13.386 Å. Unlike 4 and 5, the asymmetric unit of 6 contains one water molecule which plays a pivotal role linking helices with hydrogen bonds. The water molecule sits in between two iodide ions of the adjacent helices and connects them vertically with two O-H · · · I bonds and results in a linear O-H · · · I · · · H-O chain (Figure 6c). A bifurcated O-H · · · π interaction and a N-H · · · O bond is formed with the uncoordinated nitrogen atom, as if to support the architecture horizontally. [Zn(H2MDP)(HCOO-)2]n (7). Similar to 4 and 5, complex 7 also features an alternative array of left-handed and righthanded 1D helical chains with a similar coordination environment around the metal center (Figure 7). However, there are some subtle differences between complex 7 and two previous helical networks (4 & 5). The H2MDP molecule adopts a trans conformation and as a result of that the angle between two subsequent zinc atoms of a helix from the central methylene carbon opens up considerably (138.47°) compared to that of 4 and 5 (103.48° and 104.26°, respectively). This has some direct consequences on the helicity of the network, and indeed the helix straightens up with a much longer pitch length. Subsequently, the Zn · · · Zn distance across the H2MDP becomes longer (10.524 Å), while the distance of the repeating unit (16.64 Å) becomes almost double compared to those of 4 and 5. As if

Mondal et al.

to complement this, the zinc atoms within a helix move further away from each other making a Zn · · · Zn · · · Zn angle of 104.48°, which is in stark contrast to ca. 59° for the other two helical networks. Interestingly, complex 7 can also be described as a 2D (4,4) layer structure wherein the corresponding nodes are generated via hydrogen bonds and intermolecular interactions among the organic parts surrounding the two juxtaposed metal centers (Figure 7a). In other words, the nodes result from the selfassembly of the pyrazole rings and formate ions surrounding the metal centers of two adjacent helical rings which come very close to each other at every alternate metal center. Such centrosymmetric self-assembly is instigated by two N-H · · · O bonds between the pyrazole moieties and the uncoordinated oxygen atom of one of the formate anions, which also forms an interesting CdO · · · π interaction16 (3.572 Å) facilitated by the orientation of the ring. The uncoordinated oxygen atom of the second formate anion, on the other hand, just sits above the pyrazole moiety and forms another strong CdO · · · π interaction (3.299 Å) (Figure 7c). Role of Anions in Generating Higher Dimensional Network. For 1-7, the anions are coordinated to the metal center as terminal monodentate ligands and, as such, have little effect on increasing the dimensionality of the resulting networks. However, higher denticity or bridging ability of an anion can lead to a higher dimensional network. In this context, crystal structure of 7 can be considered as an interesting “missing link”. The formate ion in 7 is coordinated to the metal center in monodentate fashion and results in a 1D helical network. Interestingly, the same structure can also be envisaged as a 2D (4,4) grid topology by considering two juxtaposed hydrogen bonded metal coordination environments as a node. Clearly, the difference in nomenclature for the network is subjective. While the first one is a covalently bonded polymeric network, the later one is a hydrogen bonded supramolecular topology. Apparently, a bidentate or bridging mode of the formate ion would have led to a higher dimensional covalently bonded polymeric network. Indeed, this is what happens for the crystal structures of 8 and 9 with the sulfate as anion. The most novel aspect of these two structures is that they can easily be correlated with two previously discussed 0D structures (1 and 2). In other words, crystal structures of 8 and 9 are not just higher dimensional networks resulting from varying anions; instead, they can be better described in the form of structural transformation from a particular lower dimensional network. {[Zn(H2MDP)SO4] · H2O}n (8). Crystal structure of 8 features an interesting 1D porous bilayer network (Figure 8) which can be easily linked to the 0D structure of 1. One can hardly overlook the topological resemblance between the crystal structures of 1 and 8 with the formation of similar M2L2 motif. However, unlike 1, the M2L2 units in 8 are not discrete but are interlinked with covalent bonds of two sulfate ions which adopt a bridging mode and result in another dimeric loop, and by doing so generates a higher dimensional network (Figure 10a). Subsequently, centrosymmetric square motifs of O-H · · · O hydrogen bonds involving water molecules and sulfate ions promotes the interlayer supramolecular connectivities and led to the formation of a 3D supramolecular structure. For both structures, the orientations are facilitated by two familiar N-H · · · O bonds between a water molecule and the pyrazole moiety (Figure 8). The overall topology led to a bilayer nanoporous structure. As illustrated in Figure 9, a space-filling model of the crystal



Figure 7. Crystal structure of 7 showing (a) the hydrogen bonded 2D (4,4) layer network, (b) alternative array of left-handed and right-handed 1D helical networks along the b axis (c) the hydrogen bond and intermolecular interactions that are involved in generating the node of the (4,4) network.

Figure 8. Crystal packing of 8 illustrating how the layers are interlinked with a centrosymmtric square motif of O-H · · · O bonds, supported by N-H · · · O bonds. Note that the methyl carbon atoms and hydrogen atoms are omitted for clarity.

packing clearly shows the nanoporous channels running along the crystallographic a axis with a dimension of ca. 7.2 × 8.5 Å. [Zn(H2MDP)(SO4)]n (9). Crystal structure of 9, on the other hand, can be envisaged resulting from the structural transformation of 2. A structural comparison between 2 and 9 indicates that the driving force behind this transformation is the replace-

Figure 9. A space-filling model of 8 showing distinctly visible nanoporous channels running along the a axis.

ment of the water molecules with the bridging sulfate ion (Figure 10b). Topologically, with respect to the M2L2 units the sulfate ions in 9 occupy exactly the same position as that of water molecules in 2. As illustrated by a schematic diagram (Scheme 2), for both structures the M2L2 units can be considered as the basic building blocks. These basic units are then linked together either by hydrogen bonds or covalent bonds to generate the crystal structures of 2 and 9, respectively. In other words, crystal


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Figure 10. (a) A superimposed picture of packing diagrams of 1 (yellow) and 8 (green) and (b) a comparative depiction of crystal packing of 2 and 9 to emphasize the role of bridging sulfate ion in structural transformations to higher dimensional networks.

Scheme 2. A Schematic Illustration of the Structural Transformation from 2 to 9a

a

The blue dotted line represents hydrogen bonds, while the solid orange line represents the covalent bonds formed by sulphate ions.

structure of 9 features the same chassis as that of the 0D network of 2 but results in a 2D network by replacing hydrogen bonds with covalent bonds. Similar to 8, the crystal structure of 9 also exhibits nanoporosity in the structure. A single layer network contains a window of a dimension of 14.949 × 11.738 Å (based on Zn · · · Zn). However, unlike 8, the effective pores in the actual structure of 9 are further reduced due to the significant offset stacking of adjacent two 2D single layers (Figure 11). It is interesting to note here that the M2L2 units in 2 form a similar kind of offset stacking. 3D Metal-Organic Framework Assisted by the Molecular Recognition of Anions. [Zn(H2MDP) (NO3)2]n (10). Crystal structure of 10 represents a nice example of formation of a 3D

metal-organic framework assisted by the molecular recognition of anions.17 Anions are potential hydrogen bond acceptors.4 Therefore, hydrogen bonds, in principle, can be utilized for designing or tuning the topology of a molecular network. Although hydrogen bonds are not normally as strong as coordinate-covalent bonds, their combined influence can make a huge impact on the resulting network. Apparently, in 1-9 hydrogen bonds and intermolecular interactions involving the anions play some interesting roles in stabilizing the corresponding networks. Notwithstanding, the role of nitrate ion in 10 is unique. It is noteworthy that this is the only structure in this series where the anion is not coordinated to the metal center. The



Figure 11. View of 9 showing the off-set arrangement of 2D layer networks.

Figure 14. A space filling model showing two interpenetrating diamondoid units of 10. Notice how the nitrate ions (shown in ball and stick model) occupy the channels inside the framework.

Figure 12. Crystal packing of 10 showing the hydrogen bond assisted 3D metal-organic framework formation. Notice the role of nitrate ion in “stitching” the interpenetrating diamondoid units.

Figure 13. Crystal structure of 10 showing the intermolecular interactions formed in the molecular recognition of nitrate ion.

metal-bound anion has a restricted stereoelectronic option of forming intermolecular interactions, whereas a “free” anion can influence a framework by instigating a preferable supramolecular recognition. The tetrahedral metal center in 10 is coordinated by four H2MDP molecules, which is in sharp contrast to previous structures of two metal coordinated H2MDP molecules. This,

in a way, enhances the chance of a 3D metal-organic framework formation for 10. It has been observed that a fourcoordinated atom linked by ditopic linkers usually forms a 3D network.18 Indeed, the resultant framework of 10 is threedimensional and can be assigned as a diamondoid network by considering the zinc center as a node. It is imperative to note that the conformation of the H2MDP molecule also plays a crucial role here. It has been postulated that a tetrahedral metal usually leads to a 2D structure when coordinated to a ligand with bent configuration but generates 3D frameworks when ligands with linearly aligned coordinating sites are used.18b The linker of interest, the H2MDP molecule, adopts a trans conformation in 10 with two coordinating sites very close to a linear alignment. The dimension of framework is 31.309 × 24.421 × 18.970 Å. A diamondoid network with such a large cavity usually tends to undergo interpenetration, and crystal structure of 10 is not an exception. A closer inspection of the structure reveals a 2-fold interpenetration of the network (Figure 12). However, the most intriguing and novel aspect of the framework is the participation of the nitrate ions as “supramolecular glue”6a between the two interpenetrating diamondoid networks in the form of hydrogen bonds and intermolecular interactions. The nitrate ions adopt an interesting orientation with respect to the two interpenetrating diamondoid networks. One of the oxygen atom of the nitrate ion forms two N-H · · · O bonds with the two pyrazole moieties from each of the interpenetrating networks (Figure 13). While the remaining two oxygen atoms of the nitrate ion sit just on top of the two other pyrazole rings, one from each network, and form strong anion-pi interactions.19 The overall topology can be envisaged as an interesting 3D metal-organic framework where the diamondoid networks are finely knitted together with nitrate ions6c which occupy the distinct channels formed inside the framework20 (Figure 14). Conformational Freedom of H2MDP Molecule. A structural comparison of 1-10 shows the influence of conformational freedom of the H2MDP molecule on the resulting network. Apparently, depending on the coordination requirements, the


Figure 15. The individual H2MDP molecules of 1-10 are superimposed to illustrate the conformational freedom of the flexible ligand. Notice that the three trans conformations corresponding to 6, 7, and 10 (green, blue, and orange, respectively) are markedly different than others. Table 3. Conformational Parameters of the H2MDP Molecule structure

θ (°)

φ (°)

d (Å)

conformation

1 2 3 4 5 6 7 8 9 10

110.77 104.17 106.82 110.71 111.23 120.72 128.32 112.53 98.84 123.04

70.79 87.40 84.80 88.33 83.33 89.81 82.00 63.97 77.80 77.03

8.597 8.116 8.817 8.823 8.870 10.211 10.524 8.584 7.886 10.308

cis cis cis distorted cis distorted cis trans trans cis distorted cis trans

H2MDP molecule adopts different conformations via bending, stretching, or twisting of the rings. A superimposed picture of H2MDP molecules (Figure 15) corresponding to 1-10 will put this into perspective. Each of these molecules adopts different conformations, required for a particular network formation, and trans conformations corresponding to 5, 6, and 10 (green, blue, and orange, respectively) can be identified easily because of their larger angular distortion. The conformation of H2MDP molecules are almost identical for the isostructural networks of 4 and 5 (white and yellow, respectively). The conformational variations for H2MDP can broadly be quantified by three parameters: (i) the angular tilt between two coordinating sites (N1 and N3) with respect to the methylene carbon atom, θ, (ii) the dihedral angle between the pyrazole rings, φ and (iii) through space Zn · · · Zn distance across the H2MDP molecule, d (Table 3). The θ and d values are significantly larger for trans conformations, leading to the helical structures with longer pitch lengths. On the other hand, conformations with smaller φ values tend to generate M2L2 metallocyclic structures. Almost linear orientation of coordinating sites in 10 facilitate the formation of the diamondoid network. Conclusions In summary, we have successfully synthesized 10 interesting Zn(II) complexes constructed from a flexible neutral ligand in the presence of various anions. The employment of different anions leads to diverse structures, from discrete

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0D structures to infinite 1D chains, 2D bilayer, and 3D metal-organic framework. The crystal structures in this paper show that the anions as well as the conformational freedom of the ligand have an important influence on the resulting networks. For instance, halides and monocarboxylates coordinate to the metal center as monodentate terminal ligand and lead to M2L2 metallocylic motif or 1D helical structures, facilitated by the V-shaped conformation of the ligand molecule. Sulfate ions, on the other hand, adopt a bridging mode and transform the 0D structures to 1D and 2D bilayer structures in a comprehensible manner. A 3D metal-organic framework is formed when the tetrahedral metal center is not coordinated by any anion but with four ditopic ligand molecules with linear alignment of the coordinating sites. One of the promising features of 2D and 3D networks of this series is the presence of distinct void and channels within the networks. The potential of this ligand to produce porous networks using suitable noncoordinating or coordinating anions with bridging properties is currently underway in our laboratory. Supporting Information Available: X-ray crystallographic files in CIF format for 1-10. This information is available free of charge via the Internet at http://pubs.acs.org.

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.

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CG800923G

Influence of Anion on the Coordination Mode of a Flexible Neutral

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