Coordination Polymers of Flexible Bis (benzimidazole) Ligand

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Coordination Polymers of Flexible Bis(benzimidazole) Ligand: Halogen Bridging and Metal 3 3 3 Arene Interactions Suman Samai and Kumar Biradha* Department of Chemistry, Indian Institute of Technology, Kharagpur-721302, India

bS Supporting Information ABSTRACT: A new organic ligand, namely 1,4-bis[2-(1-methylbenzimidazol-2-ylmethyl)]benzene (L), was synthesized, and its crystal structure has been determined. The ability of L to form coordination polymers was explored with various metal salts under different conditions. Six complexes of L—[Cu2(L)I2], 1; [Cu2(L)I2], 2; [Cd3(L)Cl6 3 2DMF], 3; [Cu(L)Cl2], 4; [Zn(L)I2 3 MeOH], 5; and [Cd(L)(NO3)2 3 H2O] 3 2THF, 6—have been synthesized by the reaction of L with the corresponding metal salts at room temperature, either by direct mixing or by a layering technique. Single crystal X-ray analyses revealed that complexes 1 and 2 are isomers of each other and that both contain Cu2I2 building units. The crystal structure of 1 contains a discrete unit that contains a strong Cu(I) 3 3 3 arene interaction, whereas, in 2, the Cu2I2 units act as a linear linker and form a one-dimensional zigzag chain. Complex 3 exhibits a two-dimensional layer in which the 1D polymeric chains of halide bridged Cd(II) ions are linked by the organic ligand L. The crystal structures of 4 and 5 contain a 1D zigzag chain and a discrete hydrogen bonded dimer, respectively. The crystal structure of complex 6 contains nitrate as a counterion and forms an interesting one-dimensional helical chain containing cavities that are occupied by THF molecules. In these crystal structures, the ligand L is found to exhibit four different conformations. The Cambridge Structural Database was used to rationalize the results obtained here and to provide some insight into the metal coordination geometries, the halide bridging of metal centers, as well as metal 3 3 3 arene interactions.

’ INTRODUCTION The quest for the design of coordination networks with novel properties demands the exploration of new ligands with a potential to form multidimensional networks by assembling with metal ions. Such metal organic materials have attracted significant attention over the last two decades due to their fascinating architectures with novel functional properties, such as ion exchange, gas storage, separation, host guest chemistry, optics, magnetism, catalysis, and photoluminescence.1 3 Although coordination bonds are the primary interactions in assembling such networks, several weak interactions also play a significant role in tailoring the geometry of the networks. The modularity in assembling the building units into various coordination networks can well be realized by fine-tuning the reaction conditions, such as metal ligand ratio, ligand structure, coordination nature of metal ion, solvent, pH, and temperature.4 The pyridyl or carboxylate functional groups have extensively been used for the construction of various coordination frameworks in the last two decades.5 On the other hand, the potential of imidazole containing ligands for the construction of coordination networks has been recently realized. The imidazole moieties have high affinity for coordinating to metals, and also several synthetic strategies are readily available in the literature for the synthesis of imidazole ligands containing various functional groups. Accordingly, a good number of metal organic frameworks (MOFs) have been reported recently by r 2011 American Chemical Society

using flexible bis(imidazole) ligands alone or with coligands such as carboxylic acids and sulfonic acids.6,7 Moreover, in several of these reports the conformational flexibility of the ligand was utilized to produce diverse supramolecular topologies such as helices, molecular boxes, rotaxanes, catenanes, polythreaded networks, and dimondoid networks.8 However, in many of these cases, one of the N atoms of the imidazole (Scheme 1) was connected to the C atom of the central moiety or spacer. Here we focus upon the ligand (L) that contains C2 of the imidazole connected with the spacer and that enables functionalization of one of the N H groups of the imidazole, while the imine N atom coordinates to the metal.8n,o The presence of such a site which is susceptible to functionalization allows fine-tuning of the properties of the cavities or material properties either by the pre- or postfunctionalizations. The benzimidazole was considered, as it decreases the number of imidazole moieties around the metal center due to its larger size and it allows the halide ions to bridge the metal centers. Given these reasons and in continuation of our ongoing research interest of exploring novel coordination polymers, here in this paper, we report the synthesis and crystal structures of Received: October 5, 2011 Revised: November 4, 2011 Published: November 07, 2011 5723

dx.doi.org/10.1021/cg2013254 | Cryst. Growth Des. 2011, 11, 5723–5732

Crystal Growth & Design Scheme 1

coordination polymers of a new flexible ligand, namely 1,4-bis[2-(1-methylbenzimidazol-2-ylmethyl)]benzene, L, with various metal halogen and nitrate salts. The halide ions are used as counterions, as they are known to increase the dimension of the networks by bridging the metal centers in various modes.9,4f In this study, we have obtained six metal complexes of L with Cu(I), Zn(II), Cu(II), and Cd(II). The Cu(I) complexes were found to be isomers to each other and are formed due to the exhibition of a different conformation of L. The geometries found in the complexes of L include discrete species, one-dimensional chains and helices, and two-dimensional layers.

’ RESULTS AND DISCUSSION The ligand L was synthesized by a condensation reaction of o-phenylenediamine and 1,4-phenylenediacetic acid10 followed by N-methylation with CH3I. The crystallization of compound L in ethanol solution afforded crystals suitable for single crystal X-ray diffraction analysis. The reactions of L with various metal salts have been carried out at room temperature by either a layer diffusion technique or direct mixing. The reaction of L with CuI resulted in crystals of two complexes, 1 and 2, which contain similar stoichiometries but different architectures. Our attempts to produce single crystals of complexes with another Cu(I) halide (CuBr and CuCl) were unsuccessful. The reaction of L with CdCl2 resulted in crystals of complex 3. In all three complexes the halogen atoms act as a bridge between the metal centers and increase the dimensionality in 2 and 3. The reactions of L with CuCl2 and ZnI2 form complexes 4 and 5, respectively. In both of these complexes no halide bridging was observed. The reaction of L with Cd(NO3)2 afforded crystals of complex 6, which contains a one-dimensional helical chain. The crystal structures of L and complexes 1 6

were determined and analyzed in terms of the geometry of the coordination networks, the conformation of the ligand, and the utility of halide bridging in coordination networks. The pertinent crystallographic details, hydrogen bonding parameters, and selected bond lengths are given in Tables 1 3 respectively. Hydrogen Bonded Networks between L and Water. The ligand L crystallizes with two water molecules per ligand in the unit cell; the asymmetric unit contains half a molecule of L and one molecule of water. The water molecules form one-dimensional

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zigzag chains via O H 3 3 3 O hydrogen bonds. The L molecules join the water chains into a highly corrugated two-dimensional layer via O H 3 3 3 N hydrogen bonds (Figure 1a). These twodimensional layers are tightly packed in the lattice such that there exists weak C H 3 3 3 π interactions (3.058 Å) between central phenyl and benzimidazole rings (Figure 1b). Isomeric Coordination Complexes Based on the Cu2I2 Unit. The ligand L with CuI forms two types of crystals, 1 and 2, when crystallized from CH3CN THF and CH3CN nitrobenzene THF, respectively. Both complexes crystallized in the P1 space group. The single crystal diffraction analyses reveal that these two complexes are isomers to each other; they have the same molecular formula but exhibit different structures. The asymmetric unit consists of two copper atoms, two iodide ions, and one unit of L in 1, whereas, in 2, the asymmetric unit contains half a unit of ligand L and one unit each of copper and iodide ion. Interestingly, both the structures contain a Cu2I2 unit, formed through iodo-bridges, as observed in related Cu(I) iodides with bulky N-based donors.11 It was shown earlier by us and others that the Cu2I2 unit can be used as a four connected square planar node to build higher dimensional coordination networks.12 In contrast to those studies, the Cu2I2 unit was found to adopt a different geometry as well as connectivity in the crystal structures of 1 and 2. In 1, the ligand exhibits a bow shaped geometry such that two imidazole nitrogens from the same L ligand coordinate to a Cu2I2 unit, thus forming a zero-dimensional bowlike structure (Figure 2a). Each Cu(I) exhibits a distorted trigonal coordination geometry with two I atoms and one N atom. The Cu2I2 unit was placed under the central phenyl ring, and the iodine atoms of the Cu2I2 unit were pushed downward such that they are away from the phenyl ring. The Cu---Cu bond overlaps with the edge of the central phenyl ring with the C 3 3 3 Cu distances as short as 2.647 Å and 2.714 Å. We note here that this distance is much shorter than the sum of the van der Walls radii of Cu(I) and carbon atom (3.10 Å). The Cu2I2L units pack in the crystal lattice via weak C H 3 3 3 I and C H 3 3 3 π interactions (2.874 Å) (Figure 2b). A Cambridge Structural Database (CSD) search was conducted to understand the generality or uniqueness of the short Cu 3 3 3 C contacts observed in 1. It reveals that nine more Cu(I) complexes exhibit the short Cu 3 3 3 C contacts below 2.7 Å. The shortest Cu 3 3 3 C distance (2.270 Å) was observed in the Cu(I) complex of π-prismand;13 in this complex the Cu(I) was totally embedded by the three aryl rings making possible such a short contact. The next shortest Cu 3 3 3 C contacts (∼2.4 Å) were observed in the three Cu(I) complexes of 1,2,4,5-tetra(7-azaindolyl)benzene in which the ligand acts as a cleft with the benzene ring acting as a backbone.14 The others are the complexes of diarylbialkylphosphane, in which the Cu 3 3 3 C distances are ∼2.6 Å.15 It is worth mentioning that the calculations by Echavarren et al. on metal 3 3 3 arene interactions indicated that they are weak interactions with minimal or no covalent character and no electron transfer from arene to metal.15 In complex 2, the Cu2I2 unit is planar and acts as a linear linker to form a one-dimensional zigzag chain (Figure 3a) due to the fact that L adopts a divergent geometry by placing the two arms of benzimidazoles opposite to each other. Similar to 1, the Cu(I) centers in each Cu2I2 unit form a planar trigonal coordination geometry. The Cu 3 3 3 Cu distances in the Cu2I2 unit are almost similar in both the complexes (2.604 Å in 1 and 2.559 Å in 2) and are in agreement with the previously reported distances.16 The Cu2I2 unit was connected to imine nitrogen atoms more linearly 5724

dx.doi.org/10.1021/cg2013254 |Cryst. Growth Des. 2011, 11, 5723–5732

Crystal Growth & Design

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Table 1. Crystallographic Parameters for the Crystal Structures of L and 1 6 L

1

2

3

4

5

6

formula

C24H26N4O2

C24H22Cu2I2N4

C24H22Cu2I2N4

C30H36Cd3Cl6N6O2

C24H22Cl2CuN4

C25H26I2N4OZn

C32H40CdN6O9

mol wt

402.49

747.34

747.34

1062.55

500.90

717.67

765.10

T (K)

293(2)

293(2)

293(2)

293(2)

293(2)

293(2)

293(2)

system

monoclinic

triclinic

triclinic

triclinic

monoclinic

triclinic

monoclinic

space group

P21/c

P1

P1

P1

P21/n

P1

P21/c

a (Å)

5.5126(7)

8.724(2)

7.1455(6)

9.452(2)

5.7847(7)

10.847(3)

11.102(2)

b (Å)

11.076(1)

10.227(2)

8.7718(8)

10.371(1)

17.012(2)

10.990(4)

14.458(3)

c (Å) α (deg)

17.406(2) 90.00

14.472(3) 89.265(6)

10.4620(9) 67.487(2)

10.901(1) 112.507(6)

11.280(1) 90.00

11.076(4) 83.459(1)

23.518(4) 90.00

β (deg)

94.931(4)

78.105(5)

87.840(2)

104.459(9)

104.402(4)

85.689(1)

113.883(8)

γ (deg)

90.00

74.384(6)

78.498(2)

97.657(9)

90.00

84.363(1)

90.00

vol (Å3)

1058.9(2)

1215.7(4)

593.07(9)

924.1(2)

1075.2(2)

1299.0(7)

3451.7(11)

Z

2

2

1

1

2

2

4

Dcalc (mg/m3)

1.262

2.042

2.092

1.909

1.547

1.830

1.472

R1 (I > 2σ(I))

0.0577

0.0290

0.0345

0.0561

0.0584

0.0776

0.0598

wR2 (on F2, all data)

0.1593

0.0942

0.1115

0.1550

0.1374

0.1561

0.1565

Table 2. Hydrogen Bonding Parameters for the Crystal Structures of L and 1 6a complex

interactions

L

O3 3 3N C H 3 3 3 I#1 C H 3 3 3 I#2 C H 3 3 3 Cl#3 C H 3 3 3 Cl#4

1 2 3 4

5 6

a

C H 3 3 3 Cl#5 C H 3 3 3 Cl#6 C H 3 3 3 Cl#7 C H 3 3 3 I#8 O 3 3 3 N#9

C H 3 3 3 O#10 C H 3 3 3 O#11 C H3 3 3O C H 3 3 3 O#12 C H 3 3 3 O#13 C H3 3 3O

H 3 3 3 A (Å)

D 3 3 3 A (Å)

D H 3 3 3 A (deg)

2.881(3) 3.23

4.04

148

3.18

3.96

144

2.96 3.03

3.68 3.98

136 169

2.87

3.69

144

2.92

3.75

150

2.81

3.67

154

3.12

4.00

157

2.595(16) 2.34

3.28

165

2.43 2.58

3.32 3.14

161 119

2.60

3.42

148

2.55

3.38

149

2.41

3.35

163

Symmetry operators: (#1) 1 + x, y, z; (#2) x, y, 1 + z; (#3) 1 x, y, 1 z; (#4) 2 x, y, 1 z; (#5) 1/2 + x, 3/2 y, 1/2 + z; (#6) 1 + x, y, z; (#7) x, 1 y, z; (#8) 1 + x, y, 1 + z; (#9) 1 x, 1 y, 1 z; (#10) 3 x, 1/2 + y, 3/2 z; (#11) 2 x, y, 1 z; (#12) 1 + x, 1/2 y, 1/2 + z; (#13) 3 x, y, 2 z.

in 2 compared to that in 1 (Cu 3 3 3 Cu N angles: 158.08° and 161.54° in 1 and 173.36° in 2). The one-dimensional chains pack via C H 3 3 3 π interactions between the N-methyl C H and the π cloud of the phenyl ring of benzimidazole moieties and also via weak C H 3 3 3 I interactions to form a two-dimensional layer (Figure 3b and d). The CSD was searched to understand the coordination number of two for Cu2I2 unit rather than usual coordination number of four. The CSD search shows that there exists a total of 58 complexes containing a Cu2I2 unit coordinating two or four N atoms. Out of 58, only 11 structures were found to exhibit two coordination. Interestingly, in all these 11 structures, the N atom comes

from either bulky ligands (substituted benzimidazoles) or sterically hindered N-heterocycles containing the substitution near the N atom.17 These studies reveal that the steric factors play a major role in determining the coordination number of the Cu2I2 unit. Two-Dimensional Network via Halide Bridging. In the crystal structure of 3, the asymmetric unit is constituted by half of the ligand L, one and a half Cd(II) atoms, three Cl ions, and one coordinated DMF molecule. The two symmetry independent Cd(II) atoms exhibit different coordination environments: one is six coordinated (A) with octahedral geometry, and the other is five coordinated (B) with square pyramidal geometry. These two Cd(II) atoms are bridged by chloride ions such that they form a one-dimensional chain with a 3 3 3 BBABB 3 3 3 pattern (Figure 4a). The six coordinated Cd(II) center coordinates to four Cl ions in equatorial positions (Cd Cl: 2.639 and 2.613 Å) and two O atoms of DMF molecules in the axial positions (Cd O: 2.299 Å), and it sits on inversion symmetry. The five coordinated Cd(II) center contains four chloride ions (Cd Cl: 2.488, 2.508, 2.686, and 2.736 Å) in equatorial positions and one imine nitrogen of L in the axial position, resulting in a distorted square pyramidal geometry around the metal center. The equatorial Cl ions do not lie in plane, as evidenced by the N Cd Cl angles: 93, 92, 113, and 117°. In the one-dimensional chain, each octahedron shares two of its edges with two square pyramids, whereas each square pyramid shares two of its edges with one octahedron and one with a neighboring square pyramid. These one-dimensional chains are linked by the L units to form two-dimensional layers (Figure 4b). These layers pack on each other via bifurcated C H 3 3 3 Cl, aromatic face-to-face π 3 3 3 π (centroid to centroid distance 3.845 Å), and C H 3 3 3 π interactions (2.829 Å) (Figure 4c). A CSD search on the chloride bridged Cd(II) complexes shows that there are 32 structures containing polymeric CdCl2 chains in which the Cd(II) octahedrons are edge shared. However, only two structures were found to exhibit edge sharing of the Cd(II) octahedron and pbp.18,19 Interestingly, both these structures do not contain a CdCl2 polymeric chain but just contain three Cd(II) atoms in series (pbp...oh....pbp) interconnected through halide bridges (“pbp” and “oh” refer to the pentagonal bipyramid and octahedron geometries, respectively). 5725



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Table 3. Bond Distances (Å) and Angles (deg) around Metal Center(s) for Complexes 1 6a 1 M X

2

3

4

2.6568(9)

2.5428(7)

2.686(2)

2.6599(9)

2.5902(7)

2.508(2)

2.5664(8)

2.256(15)

5

6

2.574(2) 2.556(2)

2.489(2)

2.5667(9)

2.736(2) 2.613(2) 2.639(2)

M N

1.964(4)

1.970(4)

2.240(5)

1.996(3)

2.027(11)

2.274(5)

2.017(11)

2.256(4) 2.364(5)a

1.975(3) M O

2.300(5)

2.348(6)c 2.460(5)c 2.463(5)c X M X

105.88(2)

119.03(3)

105.78(3)

86.09(6)

180.00

114.73(8)

93.99(6) 175.10(6) 30.63(7) 91.47(7) 84.38(6) 180.00(0) 95.02(6)

N M N

180.00

122.16(16) 159.63(16)b

O M O

86.76(17)b 81.85(19)b 81.56(17)c 77.83(19)c 50.71(18)c N M X

O M X

119.39(11)

123.69(11)

93.08(15)

88.70(11)

117.1(3)

116.75(11)

117.02(11)

112.62(15)

91.30(11)

111.8(3)

131.98(11)

116.66(15)

132.86(10)

91.77(15) 84.98(6) 92.55(13)

103.3(3)

87.46(13)

108.5(3)

89.45(15) 90.55(15) O M N

99.5(5)

92.05(18)a 87.29(16)a 95.33(16)c 85.10(17)c 152.29(17)c 135.52(17)c 101.63(17)c 103.44(18)c

a

a

M = metal atom, X = halogen, N = nitrogen atom from L, O = oxygen atoms in complex 3 from DMF molecule. In 6 , = oxygen from water molecule; b = one of the oxygens from water and another from nitrate; c = oxygen from nitrate.

Therefore, the CSD studies indicate that the geometry of the CdCl2 polymeric chain observed in 3 is unique and unprecedented. Zero- and One-Dimensional Networks with No Halide Bridging. The complexation reaction of L with CuCl2 and ZnI2 produced complexes 4 and 5, respectively. Complex 4 was crystallized in a monoclinic lattice with a P21/n space group.

The asymmetric unit contains half a unit each of ligand L and Cu(II) and one chloride ion. The Cu(II) atom exhibits a square planar geometry, with two imine nitrogens from two L ligands and two chloride ions, and sits on an inversion center. This type of coordination results in the linking of metal centers by L into a one-dimensional zigzag chain (Figure 5a). It is interesting to note here that Cu(II) exhibits a less prominent square planar 5726


Crystal Growth & Design geometry due to the fact that two bulky groups such as benzimidazoles block the axial sites. The packing of the chains

Figure 1. Illustrations for the crystal structure of L: (a) hydrogen bonded two-dimensional layer; (b) packing of the layers in the crystal lattice.

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in the crystal lattice is governed by a plethora of C H 3 3 3 Cl interactions and aromatic stacking interactions (Figure 5b and c). The CSD search on CuCl2 complexes suggests that in the majority of the complexes, the Cu(II) atoms are bridged by Cl anions only when the ligands are large and chelating. The percentage of complexes containing Cl anion bridging Cu(II) ions in CuCl2 complexes, found to be around 29% (960 out of 3331 complexes), indicates that this phenomenon is not so common. Complex 5 was crystallized in the triclinic P1 space group, and its asymmetric unit contains one unit each of Zn(II), ligand L, and methanol and two iodide ions. The Zn(II) exhibits a tetrahedral geometry with the coordination of two iodide ions, one imine nitrogen, and one methanol O atom. One end of the ligand does not coordinate with the metal but is involved in the O H 3 3 3 N hydrogen bond with the coordinated MeOH. We note here that the coordination of the MeOH is preferred over the coordination with the other end of the ligand due to the bulky nature of the benzimidazole groups. Two of these complexes form a centrosymmetric dimer via Me O H 3 3 3 N hydrogen bonds (Figure 6a). These dimers pack in a crystal lattice via C H 3 3 3 I, C H 3 3 3 π, and π 3 3 3 π interactions (Figure 6b). Our observation of no bridging in the ZnI2 complex is in agreement with the CSD statistics; nine structures contain halide bridges out of 285 ZnI2 complexes. One-Dimensional Helices with Guest Inclusion. The nitrate ion is also known to act as a bridge between metal ions in several coordination polymers. In order to understand the importance of a halide ion in the formation of a one-dimensional polymeric CdCl2 chain via μ2-halide bridges in 3, single crystals of complex 6 containing nitrate in place of chloride ions were grown. The composition of complex 6 was found to be totally different

Figure 2. Illustrations for the crystal structure of 1: (a) the geometry of the molecular complex, note Cu(l) 3 3 3 arene interactions; (b) packing of these units via C H 3 3 3 π and C H 3 3 3 I interactions.

Figure 3. Illustrations for the crystal structure of 2: (a) one-dimensional zigzag chain by linking the Cu2I2 units with L; (b) two-dimensional layer via C H 3 3 3 π and C H 3 3 3 Cl interactions; (c) packing of the layers in the crystal lattice. 5727



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Figure 4. Illustrations for the crystal structure of 3: (a) one-dimensional chain of CdCl2 units via chlorine bridged with six and five coordinated Cd(II) centers; (b) linking of the one-dimensional CdCl2 chains by L to form a two-dimensional layer; (c) packing of layers in the crystal lattice via weak interactions.

Figure 5. Illustrations for the crystal structure of 4: (a) one-dimensional zigzag chain; (b) two-dimensional layer; (c) three-dimensional packing through C H 3 3 3 Cl and aromatic stacking interactions.

from that of 3. The asymmetric unit contains one each of Cd(II), ligand, and water molecule and two each of nitrates and THF molecules. The Cd(II) atom exhibits seven coordination with four O atoms from two nitrates that are cis coordinated, two imine N atoms of a benzimidazole group that are cis coordinated, and one water molecule (Figure 7a). The cis coordination as well as the twisted conformation of the ligand generates a onedimensional helix with a pitch length of 7.99 Å (Figure 7b). Within the helices, two guest THF molecules were accommodated and hydrogen bonded with the coordinated water molecule (Figure 7a). The crystal lattice contains left-handed and right-handed helices which interact with each other via face-to-face π 3 3 3 π stacking interactions, between two benzimidazole moieties (the centroidto-centroid distance is 3.829 Å), to form a two-dimensional layer

(Figure 7c). Weak C H 3 3 3 O hydrogen bonding interactions (involving the central phenyl ring and the neighboring NO3 ion; 2.629 Å, 3.492 Å, 154.61°) and edge-to-face π 3 3 3 π stacking interactions (arising due to the benzimidazole and central phenyl ring; 3.050 Å) between two such layers were found to be responsible for the generation of 3D packing (Figure 7d). Conformations of L. The ligand L was found to exhibit four different conformations in the crystal structures of 1 6 which can be categorized as cis cis, trans trans, cis trans, and trans cis; the first cis/trans descriptor indicates the orientation of the benzimidazole arm with respect to the central phenyl ring, and the second cis/trans descriptor represents the direction of the imine nitrogen atoms of the benzimidazole arm with respect to each other (Figure 8). It is worth noting that complexes 1 and 2 5728



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Figure 6. Illustrations for the crystal structure of 5: (a) O H 3 3 3 N hydrogen bonded dimeric unit, (b) packing of dimeric units via weak interactions in the crystal lattice.

Figure 7. Illustrations for the crystal structure of 6: (a) hydrogen bonded THF molecules with the coordinated water molecule; (b) one-dimensional helical chain; (c) two-dimensional layer (guest THF molecules are shown in space filling mode); (d) packing of the layers in the crystal lattice.

exhibit supramolecular isomerism due to the different conformations of L. In 1, the conformation is cis cis, leading to a cleftlike geometry and therefore a discrete complex. On the other hand, in 2, the geometry of L is trans trans (divergent) and leads to a one-dimensional network. A similar trans trans conformation of L with some minor variations was also found in complexes 3 and 4. The conformation of L in 5 can be described as a trans cis conformation, which leads to the formation of a hydrogen bonded dimer of the complex. Although the conformation of L in 6 was classified by us as cis trans, the imidazole moieties are placed such that they are gauche to each other along the central CH2 C6H4 CH2 moiety, whereas in the other structures these moieties exhibit either eclipsed or anti geometries. For example, the nonbonding torsions of imidazole C(2) CH2--CH2 C(2) are near to ∼180 in the crystal structures of L and

2 4; it is about 52° in 6, which gives some twisting nature to the ligand. This twisting nature as well as the cis coordination to Cd(II) in 6 helped in the formation of the helices containing channels that are occupied by THF molecules. The IR spectra of all the metal complexes show a significant deviation from that of the ligand L. The CdN stretching frequency of ligand L appeared at 1500 cm 1. For all the coordination complexes, the CdN stretching appeared at higher wavenumber than in the range of 1507 cm 1 to 1519 cm 1. This type of blue shift clearly indicates that the imine nitrogen atoms derived from the ligand L are coordinated with the metal ions. Moreover, in complex 6, a sharp band at 1384 cm 1 was observed due to the νO N O(sym) stretching of the coordinated nitrate group in addition to the band at 1508 cm 1 for the coordinated CdN stretching. 5729



Figure 8. Conformations of the ligand L observed in the crystal structures of 1 6.

’ CONCLUSIONS In summary, the new ligand L was shown to form six coordination complexes (1 6) with diversified geometries. In the crystal structures, the ligand L was found to exhibit four different conformations due to the inherent conformational flexibility. Five out of the six complexes studied contain halides (I or Cl) as counterions. The halogen bridge was found to be present in three structures out of five. The counteranion I exhibited bridging of Cu(I) units to form a discrete Cu2I2 unit in 1 and 2, whereas, in 3, the Cl anions bridge Cd(II) centers to form a one-dimensional polymeric CdCl2 chain which has an unprecedented geometry. Complex 1 exhibits a relatively new and unexplored type of metal 3 3 3 arene interaction between the Cu2I2 unit and the central phenyl ring. The CSD search on Cu(I) complexes reveals that there exists nine more such examples with Cu 3 3 3 C(arene) distance below 2.7 Å. However, none of these contain a bimetallic Cu(I) unit but contain only a single Cu(I) interacting with arene. Complexes 1 and 5 exhibit discrete geometries, whereas complexes 2, 4, and 6 exhibit one-dimensional networks in their crystal structures. On the other hand, complex 3 has a twodimensional layer in which Cd(II) exhibits two types of coordination environments, namely square pyramidal and octahedral coordination geometries, via halide bridging and L-coordination. The observed two coordination of the Cu2I2 unit and the halide bridging of the metal centers were examined thoroughly using CSD. These studies indicated that the larger size of the benzimidazole unit is largely responsible for such events. ’ EXPERIMENTAL SECTION All the chemicals were used as received without further purification. 1,4-Phenylenediacetic acid was purchased from Aldrich Chemicals. Other chemicals were bought from a local chemical company. FTIR spectra were recorded with a Perkin-Elmer Instrument Spectrum Rx Serial No. 73713. 1H NMR (200 MHz) and 13C NMR (50 MHz) spectra were recorded on a BRUKER-AC 200 MHz spectrometer. Elemental analyses were carried out with a Perkin-Elmer Series II 2400, and melting points were taken using a Fisher Scientific melting point apparatus cat. No. 12-144-1. CSD Analysis. The structures are retrieved from CSD version 5.32 (August 2011 update).20 The CSD search for metal 3 3 3 arene

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interactions was carried out by specifying the nonbonded interactions between the Cu(I) atom and one of the aromatic carbon atoms. The complexes containing Cu2I2 units were searched by defining the Cu2I2 unit such that it is connected to only N atoms, and the resultant structures were analyzed manually to get the required information regarding coordination of the Cu2I2 unit with an N atom. For the chlorine bridged CdCl2 complex, all the structures with a CdCl2 unit were analyzed manually, and only polymeric chains via chloride bridging were considered. With CuCl2 and ZnI2, all the resultant structures with and without halide bridging were retrieved and analyzed manually. Single Crystal X-ray Determination. All the single crystal data were collected on a Bruker-APEX-II CCD X-ray diffractometer that uses graphite monochromated Mo Kα radiation (λ = 0.71073 Å) at room temperature (293 K) by the hemisphere method. The structures were solved by direct methods and refined by least-squares methods on F2 using SHELX-97.21 Non-hydrogen atoms were refined anisotropically, and hydrogen atoms were fixed at calculated positions and refined using a riding model. The H atoms attached to the O atom or N atoms are located wherever possible and refined using the riding model. Synthesis of Ligand. Nonmethylated compound (Bn-IM) was synthesized by following the literature procedure.10 3 g (15.4 mmol) of 1,4-phenylenediacetic acid and 3.34 g (30.9 mmol) of o-phenylenediamine were mixed with a sufficient amount of polyphosphoric acid to make a pasty mass. The mixture was then heated slowly to 230 240 °C, and the resulting solution was stirred for 3 4 h at 240 °C ((3 °C) and allowed to cool to about 100 °C. Then the viscous crude solution was poured in a thin steam into a large volume of rapidly stirred cold water. The insoluble residue was collected by filtration, washed with water, and made into a slurry with an excess of 10% sodium carbonate solution. The alkaline slurry was filtered and washed thoroughly with water so that no alkali was present with the crude product. Then the crude product was dried at 60 °C and recrystallized from a hot DCM/MeOH mixture (1:1) subsequent to treatment with a small amount of activated charcoal. Yield: 3.2 g (61.5%). M. P. > 300 °C. 1H NMR (200 MHz, d6-DMSO) δ 4.35 (s, 4H), 7.31 7.37 (m, 4H), 7.41 (s, 4H), 7.60 7.65 (m, 4H); 13C NMR (50 MHz, d6-DMSO) δ 33.62, 114.83, 124.30, 130.08, 135.14, 135.22, 153.72. Elemental analysis for C22H18N4: Exptl C 78.88%, H 5.49%, N 16.92%; Calcd C 78.10%, H 5.32%, N 16.56%. Synthesis of Ligand L. To a stirred mixture of 3.0 g (8.8 mmol) of BnIM and 40 mL of dry THF in a 100 mL round-bottom flask fitted with a side arm and maintained under nitrogen atmosphere was added 0.213 g of sodium hydride (95%) over 1 h, followed by the dropwise addition of 2.51 g (17.6 mmol) of methyl iodide over 1 h. The reaction mixture was stirred overnight, quenched with water, and then added to 400 mL of water. After stirring for 1/2 h, a pale-yellow precipitate was collected by filtration, washed repeatedly with water, and dried in a vacuum for 24 h. The crude product was dissolved in 100 mL of ethyl alcohol containing 1 g of activated charcoal, refluxed for 15 min, filtered hot, and kept for slow evaporation at room temperature. After 3 to 4 days, pale-yellow crystals suitable for X-ray diffraction appeared and were collected via filtration. Yield: 2.70 g, 83%. M. P. 218 220 °C. 1H NMR (200 MHz, d6DMSO) δ 3.68 (s, 6H), 4.27 (s, 4H), 7.16 7.22 (m, 4H), 7.24 (s, 4H), 7.49 7.60 (m, 4H); 13C NMR (50 MHz, d6-DMSO) δ 30.41, 33.20, 110.49, 119.05, 121.99, 122.35, 129.52, 135.71, 136.42, 142.69, 154.31. Elemental analysis for C24H22N4: Exptl C 77.95%, H 6.12%, N 15.57%; Calcd C 78.68%, H 6.01%, N 15.30%. Preparation of [Cu2(L)I2], 1. 0.015 g of ligand L (0.04 mmol) was dissolved in 3 mL of THF and was taken in a test tube. 4 mL of CH3CN was carefully layered on top of the above solution. The CH3CN (1 mL) solution of CuI (0.008 g, 0.04 mmol) was layered carefully on top of the above solution. Colorless block shaped crystals were isolated after 1 week in 30% yield. Elemental analysis for Cu2C24H22N4I2: Exptl C 39.41%, H 3.01%, N 7.76%; Calcd C 38.65%, H 2.95%, N 7.51%. 5730



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Preparation of [Cu2(L)I2], 2. The procedure is the same as above, except that, instead of THF, here we have dissolved the ligand L in a THF and nitrobenzene mixture (1:1). Here also colorless blocked shape crystals were isolated after 10 days in 25% yield. Elemental analysis for Cu2C24H22N4I2: Exptl C 39.41%, H 3.01%, N 7.76%; Calcd C 38.65%, H 2.95%, N 7.51%. Preparation of [Cd3(L)Cl6 3 2DMF], 3. The mixing of a methanol (1 mL) solution of L (0.015 g, 0.04 mmol) with a methanol solution (1 mL) of CdCl2 3 2.5H2O (0.01 g) resulted in a white precipitate which was dissolved by the addition of a few drops of a DMF/H2O mixture (1:1). The filtered solution was kept for slow evaporation at ambient temperature. Colorless block shaped crystals were separated after 2 weeks in 40% yield. Elemental analysis for Cd3C30H36N6O2Cl6: Exptl C 34.25%, H 3.43%, N 7.81%; Calcd C 33.89%, H 3.38%, N 7.90%. Preparation of [Cu(L)Cl2], 4. The procedure is the same as described for 1, except that, instead of THF, here the ligand L was dissolved in a THF and chloroform mixture (1:3). Blue crystals were produced after 10 days in 50% yield. Elemental analysis for CuC24H22N4Cl2: Exptl C 56.44%, H 4.44%, N 10.64%; Calcd C 57.00%, H 4.40%, N 11.20%. Preparation of [Zn(L)I2 3 MeOH], 5. 0.015 g of ligand L (0.04 mmol) was dissolved in 2 mL of methanol, and 0.013 g of ZnI2 was also dissolved in 2 mL of methanol in a separate vial. The mixing of these two solutions resulted in a dirty white precipitate which was dissolved by the addition of excess methanol with gentle heating. Then the filtered solution was kept for crystallization at room temperature. Colorless needle like crystals develop after 2 days in 65% yield. Elemental analysis for ZnC25H26N4OI2: Exptl C 42.19%, H 3.68%, N 7.67%; Calcd C 41.72%, H 3.6%, N 7.78%. Preparation of [Cd(L)(NO3)2 3 H2O] 3 2THF, 6. The mixing of a THF (1 mL) solution of L (0.015 g, 0.04 mmol) with a THF (1 mL) solution of Cd(NO3)2 3 4H2O (0.013 g, 0.04 mmol) resulted in a white precipitate which was dissolved by the addition of 1 mL of acetone with gentle heating. The filtered solution was kept for slow evaporation at ambient temperature. Colorless block shape crystals were generated after 3 5 days in 55% yield. Elemental analysis for CdC32H40N6O9: Exptl C 51.56%, H 5.31%, N 11.21%; Calcd C 50.23%, H 5.23%, N 10.98%.

’ ASSOCIATED CONTENT H NMR and 13C NMR spectra of the ligand, IR-spectra for the ligand and complexes, and crystallographic information for the crystal structures of L and complexes 1 6 (CIF). This material is available free of charge via the Internet at http://pubs.acs.org.

bS

Supporting Information.

1

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Fax: +91-3222-282252. Telephone: +91-3222-283346.

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Coordination Polymers of Flexible Bis (benzimidazole) Ligand

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