Solvent-Driven Structural Diversities in ZnII Coordination Polymers

Article pubs.acs.org/crystal

Solvent-Driven Structural Diversities in ZnII Coordination Polymers and Complexes Derived from Bis-pyridyl Ligands Equipped with a Hydrogen-Bond-Capable Urea Backbone Subhabrata Banerjee, N. N. Adarsh, and Parthasarathi Dastidar* Department of Organic Chemistry, Indian Association for the Cultivation of Science, 2A & 2B Raja S. C. Mullick Road, Kolkata 700032, India S Supporting Information *

ABSTRACT: A new series ZnII metal−organic compounds [both coordination complexes (CCs) and coordination polymers (CPs)] derived from bis-pyridyl ligands equipped with hydrogen-bonding functionality such as urea have been synthesized and characterized by single crystal X-ray crystallography. Out of the six compounds reported herein, [{Zn(μ-3bpmu)Cl2}] (CP1), [{Zn(μ-3bpmu)Cl2}·benzene] (CP2), [{Zn(μ-3bpmu)Cl2}·p-xylene] (CP3), and [{Zn(μ4bpmu)Cl2}·p-xylene] (CP4) are coordination polymers, whereas [{Zn2(μ-3bpu)2Cl4}·p-xylene] (CC1) and [{Zn2(μ3bpu)2Cl4}·benzene] (CC2) are coordination complexes. The solvent of crystallization has profound effect on the supramolecular architecture of the resultant coordination compounds; while crystallization in the presence of aromatic solvents resulted in guest occluded crystals (CP2−CP4, CC1, and CC2), crystallization using only polar solvents produced crystals devoid of guests. Aromatic solvents appear to have induced the formation of metallamacrocycle in CC1 and CC2.

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hydrogen-bond-functionalized ligands,10a we intend to study the role of solvents (guests) and conformational flexibility driven ligating topology of two bis-pyridyl urea ligands. For this purpose, we have chosen N,N′-bis(3-pyridyl)methylurea (3bpmu) and N,N′-bis(4-pyridyl)methylurea (4bpmu) ligands, which are relatively less explored compared to their parent ligands, namely, N,N′-bis(3-pyridyl)urea (3bpu) and N,N′bis(4-pyridyl)urea (4bpu).11 It is understandable that these ligands are capable of displaying diverse ligating topology depending on the N−Cpyridyl, C−Cpyridyl, and N−Cmethylene bond rotation; the hydrogen-bonding backbone, namely, urea, may display either internetwork urea···urea synthon12 or urea− solvent (guest) interactions. We have deliberately chosen the metal salt ZnCl2, mainly because of the strong Zn−Cl coordination bond that leaves only two coordinating sites on the ZnII metal center available for coordination by the pyridyl ligands, thereby allowing the systematic study of the influence of ligating topology of the ligands and solvents (guests) on the resultant supramolecular architecture of the coordination compounds thus obtained. In this contribution, we report a new series of CPs/CCs (coordination complexes) (Scheme 1) derived from these ligands and ZnCl2 synthesized in the presence of various solvents. The resultant coordination compounds were charac-

INTRODUCTION Studies of coordination polymers1a,b (CPs) are important due to their various potential applications such as catalysis,2 selective anion3 and cation separation,4 gas adsorption,5 inclusion materials,6 magnetism,7 luminescence,8 and isomeric hydrocarbon separation.9 CPs are generally synthesized via spontaneous crystallization of organic ligands and appropriate metal salts. Various secondary interactions, such as hydrogen bonding, π−π stacking, and van der Waals interactions, along with the other parameters, such as the dynamic nature of metal−ligand coordination; coordination geometries of the metal center; nature and ligating topologies of the ligands; metal−ligand ratio;10a and nature, shape, and size of the counteranions;10b and various experimental conditions like temperature,10c pH,10d and solvents,10e−h play important roles in the reaction kinetics, thereby influencing immensely the final outcome, which is understandably often unpredictable. Among these various parameters that remarkably influence the final outcome in CP synthesis, both solvents (guests)10e−h and the nature and ligating topology of the ligands play a crucial role. While the various bulk properties of the solvents, such as polarity, viscosity, dielectric constants, and ionization potential,10e influence immensely the reaction kinetics in the CP synthesis, the flexibility and ligating topology of the ligand also play a major role in shaping up the final supramolecular architecture of CPs. As a part of our ongoing research activities pertaining to the synthesis, structure, and function of CPs derived from © 2012 American Chemical Society

Received: August 17, 2012 Revised: October 12, 2012 Published: October 17, 2012 6061

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Scheme 1

self-assembled via urea···urea synthon [N···O = 2.953(2) Å; ∠N−H···O = 152.4°] to form a 2D network (Figure 1). Crystal Structure of [{Zn(μ-3bpmu)Cl2}·benzene] (CP2). Crystals of CP2 belonged to the centrosymmetric monoclinic space group C2/c. The asymmetric unit was comprised of one ZnII metal center, two fully occupied ligands coordinated to the metal center, two chloride ion also coordinated to the metal center, and a solvate benzene molecule. Both the ligand molecules displayed highly nonplanar syn−anti conformation having dihedral angles of 76.1°−82.2° (involving the central urea moiety and the terminal pyridyl rings) and 34.03°−46.4° (involving the terminal pyridyl rings). Due to the extended coordination, the ligand ultimately led to the formation of 1D wavy coordination polymer. Such chains were self-assembled via urea···urea synthon [N···O = 2.813(9)−2.854(9) Å; ∠N−H···O = 146.8°−148.4°] to form a 2D sheet architecture wherein the 1D chains are kind of entangled, resulting in interstitial spaces within which the solvate benzene molecules were occluded. The solvate benzene molecules were found to be stabilized further via C−H···π interactions (2.8 Å) (Figure 2). Crystal Structure of [{Zn(μ-3bpmu)Cl2}·p-xylene] (CP3). Crystals of CP3 belonged to the centrosymmetric monoclinic space group P21/c. In the asymmetric unit, two crystallograpahically independent fragments of 1D coordination polymer and two lattice occluded p-xylene molecules were located. In each fragment, two ZnII metal centers coordinated by two chloride and two ligands via pyridyl N atoms were observed. The metal centers displayed distorted tetrahedral geometry. All the ligands in each fragment displayed highly nonplanar syn−anti and syn−syn conformation with the dihedral angle of 76.1°−89.9° (involving the central urea moiety and the terminal pyridyl rings) and 22.01°−61.3°

terized by single crystal X-ray diffraction (SXRD). The role of ligating topology and solvents (guests) on the final outcome of the supramolecular architectures of the crystalline products is discussed.

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RESULTS AND DISCUSSIONS Thus, to study the effect of solvents and the conformation flexibility driven ligating topology of the ligands 3bpmu and 4bpmu on the supramolecular structures of the resultant coordination compounds, we reacted these ligands in separate experiments with ZnCl2 in the presence of solvents like MeOH/benzene, MeOH/p-xylene, and MeOH/EtOH/H2O. In a typical experiment, a methanolic solution was layered on a solution containing the ligand in MeOH/aromatic solvent and the reaction mixture was allowed to settle down at room temperature. Plate- or block-shaped, colorless crystals were obtained after 3 days, which were subjected to SXRD (Table 1). Crystal Structure of [{Zn(μ-3bpmu)Cl2}] (CP1). The crystals of CP1 belonged to the centrosymmetric monoclinic space group P2/c. The asymmetric unit was comprised of one half-occupied Zn (located on a 2-fold axis) and one fully occupied 3bpmu and one chloride ion, both coordinated to the metal center. The ligand displayed a highly nonplanar syn−anti conformation with the dihedral angles of 89.6° and 89.6° (involving the central urea moiety) and the terminal pyridyl units of 79.13° (involving the terminal pyridyl rings). The metal center ZnII displayed distorted tetrahedral geometry wherein the ZnII center was coordinated by two pyridyl N atoms and two chloride ions. Due to the extended coordination of the ligand with the ZnII metal center, a wavy 1D coordination polymer was formed. The 1D chains were further 6062

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CCDC No. empirical formula formula weight crystal size, mm crystal system space group a, Å b, Å c, Å α, deg β, deg γ, deg volume, Å3 Z F(000) μ Mo Kα, mm‑1 temp, K Rint range of h, k, l θmin/max, deg reflections collected/unique/ observed [I > 2σ(I)] data/restraints/parameters goodness of fit on F2 final R indices [I > 2σ(I)] R indices (all data)

893880 C19 H20 Cl2 Zn N4 O 456.66 0.32 × 0.28 × 0.18 monoclinic C2/c 25.141(6) 9.081(2) 18.154(4) 90.00 103.459(3) 90.00 4030.9(16) 8 1872 1.500 100(2) 0.0358 −29/14, −10/9, −21/21 1.67/25.00 7245/3368/2697 3368/0/246 1.335 R1 = 0.0673 wR2 = 0.2080 R1 = 0.0914 wR2 = 0.2942

1298/0/97 1.137 R1 = 0.0218 wR2 = 0.0683 R1 = 0.0232 wR2 = 0.0700

CP2

893879 C13 H14 Cl2 Zn N4 O 378.55 0.32 × 0.20 × 0.12 monoclinic P2/c 12.1981(13) 4.6782(5) 15.3941(13) 90.00 122.302(6) 90.00 742.52(13) 2 384 2.016 100(2) 0.0199 −14/14, −5/5, −18/18 1.98/25.00 6505/1298/1231

CP1

Table 1. Crystallographic Parameters for CP1−CP4, CC1, and CC2

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11292/52/893 1.149 R1 = 0.0650 wR2 = 0.1337 R1 = 0.0927 wR2 = 0.1428

893881 C34 H38 Cl4 Zn2N8 O2 863.26 0.32 × 0.24 × 0.11 monoclinic P21/c 24.0201(11) 17.8559(8) 18.0012(8) 90.00 90.1630(10) 90.00 7720.7(6) 8 3536 1.561 100(2) 0.0779 −26/26, −19/19, −20/20 0.85/23.43 78068/11292/8132

CP3

8133/1/460 0.967 R1 = 0.0650 wR2 = 0.1337 R1 = 0.0361 wR2 = 0.0690

893882 C34 H38 Cl4 Zn2N8 O2 863.26 0.28X 0.14 × 0.09 monoclinic P2 13.0778(10) 9.3016(7) 16.7013(12) 90.00 107.963(2) 90.00 1932.6(2) 2 884 1.559 100(2) 0.0457 −16/16, −11/11, −21/21 1.28/27.76 21282/8133/6717

CP4

3508/0/380 1.026 R1 = 0.0359 wR2 = 0.0822 R1 = 0.0569 wR2 = 0.0922

893877 C26 H25 Cl4 Zn2N8 O2 754.08 0.32X 0.26 × 0.17 monoclinic P21/c 16.664(3) 13.496(2) 15.194(2) 90.00 114.342(4) 90.00 3113.4(8) 4 1524 1.923 100(2) 0.0664 −17/17, −13/13, −15/15 1.34/21.43 20402/3508/2623

CC1

3993/5/370 1.007 R1 = 0.0479 wR2 = 0.1388 R1 = 0.0746 wR2 = 0.1599

893878 C25 H23 Cl4 Zn2 N8 O2 740.05 0.25 × 0.18 × 0.13 monoclinic P21/c 16.7703(19) 13.4530(15) 15.1615(17) 90.00 114.853(2) 90.00 3103.8(6) 4 1492 1.927 100(2) 0.0713 −17/17, −14/14, −15/16 1.34/22.38 22256/3993/2845

CC2

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Figure 1. 2D sheet sustained by the urea···urea synthon in the crystal structure of CP1.

Figure 2. Crystal structure illustration of CP2: (a) 1D wavy polymeric chains displaying 2D sheet architecture sustained by urea···urea synthon and (b) the guest benzene molecules (orange in space-fill model) occupying the interstitial space within the 2D sheet.

Figure 3. (a) 1D wavy polymeric chains forming a 2D network through the urea···urea synthon and (b) packing diagram viewed down the b axis showing occluded p-xylene molecules (green in space-fill model) within the interstitial space in CP3.

independent 1D chains recognized each other via the urea···urea synthon [N···O = 2.838(3)−3.082(3) Å; ∠N− H···O = 143.1°−150.6°] and were oriented in tilted fashion with respect to each other, thereby forming a 3D network structure having continuous channels down the b-axis. The solvate p-xylene molecules were located within the channel further sustained by C−H···π interactions (2.8 Å) (Figure 4). Thus, the results revealed that the urea···urea synthon is quite reliable in these series of structures; most interestingly, urea synthon is also observed in CP1, which was crystallized from a hydrogen-bond-competing solvent like MeOH. It may be noted that all these crystals were highly unstable outside the mother liquor at room temperature, presumably because of the rapid solvent loss. Powder X-ray diffraction (PXRD) patterns of CP1−CP3 under various conditions revealed interesting results; it is seen that the PXRD patterns of CP2 and CP3 as bulk matched exactly with the simulated pattern of CP1. These results might indicate a phase transition during the loss of

(involving the terminal pyridyl rings). Interestingly, urea···urea synthon [N···O = 2.836(8)−2.902(8) Å; ∠N−H···O = 138.4°−151.7°] is highly conserved in this structure as well, allowing the 1D chains to self-assemble to a 2D network. The guest p-xylene molecules were found to be occluded within the interstitial space (Figure 3). Crystal Structure of [{Zn(μ-4bpmu)Cl2}·p-xylene] (CP4). The space group of CP4 was found to be the noncentric monoclinic space group P2. The asymmetric unit was comprised of two independent fragments of 1D coordination polymer. In one fragment, two ZnII metal centers both located on a 2-fold axis were found to be bridged by a fully occupied ligand via pyridyl N−Zn coordination. Each ZnII metal center was also coordinated by two chloride ions displaying distorted tetrahedral geometry. The ligand 4bpmu displayed a syn−syn conformation with a dihedral of 78.2°−87.9° (involving the central urea moiety and the terminal pyridyl rings) and ∼84.8° (involving the terminal pyridyl rings). The crystallographically 6064

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Figure 4. Crystal structure illustration of CP4: (a) criss-cross propagation of 1D polymeric chains sustained by urea···urea synthon and (b) packing diagram displaying open channels occluded with p-xylene guest molecules (orange in space-fill model).

center of symmetry. The ZnII center displayed a distorted tetrahedral geometry. Ligand displayed a syn−syn conformation. Both the ligands were reasonably nonplanar, displaying dihedral angles of 19.5°−38.7° (involving the central urea moiety and the terminal pyridyl rings) (11.1°, 19.8°) (between terminal pyridyl ring). The structure may be described as a metallamacrocycle. The urea···urea synthon [N···O = 3.013(5)−3.041(5) Å; ∠N−H···O = 156.4°−157.8°] was found to be responsible for holding two such metallamacrocycles, which were further self-assembled via N−H···Cl [N···Cl = 3.221(4)−3.551(4) Å; ∠N−H···Cl = 151.6°−172.8°] interactions, resulting in a 1D chain. The 1D chains were further self-assembled via N−H···Cl interactions, resulting in an overall 2D network. The solvent molecules (p-xylene) were located within the interstitial space of the parallely packed 2D networks (Figure 6). PXRD patterns of CC1 and CC2 under various conditions revealed that the bulk PXRD patterns in both the cases were found to be near superimposable with that of the simulated pattern of the 1D coordination polymer of 3bpu published earlier by us (CP5).13 Thus, there is an apparent structural change from metallamacrocycle to 1D coordination polymer just because of the solvent loss. However, the possibility of occasional formation of the metallamacrocycle while synthesizing CC1 and CC2 could not be ruled out. Due to the severely unstable nature of the crystals of CC1 and CC2, it was not possible for us to investigate this possibility further. However, such a huge energy expensive structural change from metallamacrocyle to 1D coordination polymer is unlikely under the current understanding of the subject (Figure 7).

lattice occluded solvents. Thus, during solvent loss, there seemed to be a rearrangement of the 1D polymeric chains to attain close packing as observed in CP1 (Figure 5).

Figure 5. PXRD patterns of CP1−CP3 under various conditions.

To investigate further the effect of solvent on supramolecular architecture, we did a separate study in which ligand 3bpmu was replaced by 3bpu (reduced flexibility, devoid of −CH2− group but same positional isomer with respect to pyridine) and the ligand was allowed to react with ZnCl2 in the same way as in the case of the synthesis of CP1−CP4. When the ligand 3bpu was reacted with ZnCl2 in presence of aromatic nonpolar solvents such as p-xylene and benzene, crystals designated as CC1 and CC2, respectively were obtained. The following is a description of the single crystal structure of CC1; the structure of CC2 is identical with that of CC1, as they are isomorphous. Crystal Structure of [{Zn2(μ-3bpu)2Cl4}·p-xylene] (CC1). CC1 crystallized in the centric monoclinic space group P21/c as a colorless, plate-shaped crystal. The asymmetric unit contained two full ligands, two ZnII atoms, two chloride atoms, and half of the p-xylene molecule located around a

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CONCLUSIONS Flexibility-driven ligating topology and its effect on the structures could be seen in the crystal structures presented herein. Figure 8 clearly demonstrates the ligating topology 6065

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Figure 8. Different ligating topology of the ligands leading to different metal organic compounds.

of a phase transition from CP2/CP3 to CP1 after removal of aromatic guests. Isolation of CC1/CC2 in the presence of aromatic solvent in the otherwise identical reaction conditions for CP5 might indicate the influence of solvent in promoting the macrocycle formation in CC1/CC2.

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EXPERIMENTAL SECTION

Materials and Methods. All chemicals were commercially available and used without further purification. PXRD patterns were recorded on a Bruker AXS D8 Advance Powder (Cu Kα1 radiation, λ = 1.5406 Å) diffractometer. The SXRD data sets were collected on Bruker Smart Apex2 (Mo Kα radiation, λ = 0.7107 Å). The elemental analysis was carried out using a Perkin-Elmer 2400 Series-II CHN analyzer. FT-IR spectra were recorded using Perkin-Elmer Spectrum GX Synthesis of the Ligands. The ligands 3bpmu,14a 4bpmu,14b and 3bpu,14c reported previously, were synthesized following the literature procedures. Except CP1, the crystals of the rest of the coordination compounds were found to be unstable outside the corresponding mother liquors; therefore, no consistent elemental analysis data could be obtained. CP1, [{Zn(μ-3bpmu)Cl2}]. The coordination polymer was synthesized by layering a methanolic solution of 3bpmu (24.2 mg, 0.1 mmol) over an aqueous ethanolic solution of ZnCl2 (6.8 mg, 0.05 mmol). After 1 week, colorless, blocked-shaped crystals were obtained. Anal. Calcd for C13H14Cl2N4OZn: C, 41.26; H, 3.70; N, 14.81. Found: C. 41.55; H, 3.71; N, 14.43. FT-IR (KBr pellet): 3348s, 3059w, 3016w, 1629s (urea CO stretch), 1608s, 1577s (s, urea N−H bend), 1525m, 1485s, 1435s, 1377m, 1292s, 1257s, 1220s, 1188s, 1105m, 1062m, 1049m, 1033m, 945w, 786s, 700s, 640s, 432m, 420m (cm−1). CP2, [{Zn(μ-3bpmu)Cl2}·benzne]. The coordination polymer was synthesized by layering a methanolic solution of 3bpmu (24.2 mg, 0.1 mmol) over an aqueous methanolic benzene solution of ZnCl2 (6.8 mg, 0.05 mmol). After 2 days, colorless, block-shaped crystals were obtained. CP3, [{Zn(μ-3bpmu)Cl2}·p-xylene]. The coordination polymer was synthesized by layering a methanolic solution of 3bpmu (24.2 mg, 0.1 mmol) over an aqueous methanolic p-xylene solution of ZnCl2 (6.8 mg, 0.05 mmol). After 2 days, colorless, plate-shaped crystals were obtained. CP4, [{Zn(μ-4bpmu)Cl2}·p-xylene]. The coordination polymer was synthesized by layering a methanolic solution of 4bpmu (24.2 mg, 0.1 mmol) over an aqueous methanolic p-xylene solution of ZnCl2 (6.8

Figure 6. (a) Discrete metallamacrocycle, (b) 1D network of the metallamacrocycle sustained by both urea···urea synthon and N− H···Cl interactions, and (c) occlusion of p-xylene (green in space-fill model) in the crystal of CC1.

Figure 7. PXRD patterns of CC1, CC2, and CP6 under various conditions.

observed in these structures. The less flexible 3bpu showed less diversity in the ligating topology, whereas its higher analogues (3bpmu and 4bpmu) showed various ligating topologies. The effect of solvents could be observed in CP1−CP3; while CP1, which was crystallized in absence of aromatic solvent, displayed a 2D array of 1D coordination polymers sustained by the urea···urea synthon, CP2 and CP3, which were crystallized in the presence of aromatic solvent, displayed completely different self-assembly wherein the 1D coordination polymeric chains were packed in entangled fashion sustained by the urea···urea synthon and allowing the guest molecules to be occluded within the interstitial space. PXRD data revealed the occurrence 6066

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Kitagawa, S.; Ng, S. W. Cryst. Growth Des. 2005, 5, 837. (d) Chai, X. C.; Zhang, H. H.; Zhang, S.; Cao, Y. N.; Chen, Y. P. J. Solid State Chem. 2009, 182, 1889. (e) Li, C.-P.; Du, M. Chem. Commun. 2011, 47, 5958. (f) Li, C.-P.; Wu, J.-M.; Du, M. Chem.Eur. J. 2012, 18, 12437. (g) Chen, X.-D.; Zhao, X-H; Wu, J.-M.; Du, M Chem.Eur. J. 2009, 15, 12974. (h) Du, M.; Li, C.-P.; Wu, J.-M.; Guo, J.-H.; Wang, G.-H. Chem. Commun. 2011, 47, 8088. (11) CSD version 5.33 (November 2011): 3bpmu, 8 hits; 4bpmu, 10 hits; search strategy, any transition metal with the corresponding ligand. (12) Desiraju, G. R. Angew. Chem., Int. Ed. 1995, 34, 2311. (13) Kumar, D. K.; Das, A.; Dastidar, P. CrystEngComm. 2006, 8, 805. (14) (a) Nguyen, T. L.; Fowler, F. W.; Lauher, J. W. J. Am. Chem. Soc. 2001, 123, 11057. (b) Diaz, P.; Tovilla, J. A.; Ballester, P.; BenetBuchholz, J.; Vilar, R. Dalton Trans. 2007, 3516. (c) Kumar, D. K.; Jose, D. A.; Das, A.; Dastidar, P. Chem. Commun. 2005, 8, 4059.

mg, 0.05 mmol). After 2 days, colorless, plate-shaped crystals were obtained. CC1, [{Zn2(μ-3bpu)2Cl4}·p-xylene]. The coordination polymer was synthesized by layering a methanolic solution of 3bpu (21.2 mg, 0.1 mmol) over an aqueous methanolic p-xylene solution of ZnCl2 (6.8 mg, 0.05 mmol). After 2 days, colorless, block-shaped crystals were obtained. CC2, [{Zn2(μ-3bpu)2Cl4}·benzene]. The coordination polymer was synthesized by layering a methanolic solution of 3bpu (21.2 mg, 0.1 mmol) over an aqueous methanolic benzene solution of ZnCl2 (6.8 mg, 0.05 mmol). After 2 days, colorless, plate-shaped crystals were obtained. X-ray Crystallography. X-ray single crystal data were collected using Mo Kα (λ = 0.7107 Å) radiation on a BRUKER APEX II diffractometer equipped with CCD area detector. Data collection, data reduction, and structure solution/refinement were carried out using the software package of APEX II. The structures of CP1−CP4, CC1, and CC2 were solved by direct methods, respectively, and refined in a routine manner. In all cases, nonhydrogen atoms were treated anisotropically. Whenever possible, the hydrogen atoms were located on a difference Fourier map and refined. In other cases, the hydrogen atoms were geometrically fixed. CCDC Nos. 893878−893882 are the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html [or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB21EZ, UK; fax: (+44) 1223−336−033; or [email protected]].

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ASSOCIATED CONTENT

* Supporting Information S

OPTEP diagram and hydrogen-bonding parameters. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; parthod123@rediffmail.com. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Department of Science & Technology (DST), New Delhi, India, for financial support. S.B. and N.N.A. thank IACS for research fellowships. Single crystal X-ray diffraction was performed at the DST-funded National Single Crystal Diffractometer Facility at the Department of Inorganic Chemistry, IACS

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REFERENCES

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