Published: November 07, 2011 r2011 American Chemical Society 5723 dx.doi.org/10.1021/cg2013254 | Cryst. Growth Des. 2011, 11, 5723–5732 ARTICLE pubs.acs.org/crystal Coordination Polymers of Flexible Bis(benzimidazole) Ligand: Halogen Bridging and Metal 333 Arene Interactions Suman Samai and Kumar Biradha* Department of Chemistry, Indian Institute of Technology, Kharagpur-721302, India b S Supporting Information ’ INTRODUCTION The quest for the design of coordination networks with novel properties demands the exploration of new ligands with a poten- tial 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 archi- tectures 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 func- tional 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 real- ized. 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 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 proper- ties 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 ABSTRACT: A new organic ligand, namely 1,4-bis[2-(1-methylbenzi- midazol-2-ylmethyl)]benzene (L), was synthesized, and its crystal struc- ture has been determined. The ability of L to form coordination poly- mers was explored with various metal salts under different conditions. Six complexes of L—[Cu 2 (L)I 2 ], 1; [Cu 2 (L)I 2 ], 2; [Cd 3 (L)Cl 6 3 2DMF], 3; [Cu(L)Cl 2 ], 4; [Zn(L)I 2 3 MeOH], 5; and [Cd(L)(NO 3 ) 2 3 H 2 O] 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 Cu 2 I 2 building units. The crystal structure of 1 contains a discrete unit that contains a strong Cu(I) 333 arene interaction, whereas, in 2, the Cu 2 I 2 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 333 arene interactions.
Transcript
Published: November 07, 2011
r 2011 American Chemical Society 5723 dx.doi.org/10.1021/cg2013254 | Cryst. Growth Des. 2011, 11, 5723–5732
ARTICLE
pubs.acs.org/crystal
Coordination Polymers of Flexible Bis(benzimidazole) Ligand:Halogen Bridging and Metal 3 3 3Arene InteractionsSuman Samai and Kumar Biradha*
Department of Chemistry, Indian Institute of Technology, Kharagpur-721302, India
bS Supporting Information
’ INTRODUCTION
The quest for the design of coordination networks with novelproperties demands the exploration of new ligands with a poten-tial to formmultidimensional networks by assembling with metalions. Such metal�organic materials have attracted significantattention over the last two decades due to their fascinating archi-tectures with novel functional properties, such as ion exchange,gas storage, separation, host�guest chemistry, optics, magnetism,catalysis, and photoluminescence.1�3 Although coordinationbonds are the primary interactions in assembling such networks,several weak interactions also play a significant role in tailoringthe geometry of the networks. The modularity in assemblingthe building units into various coordination networks can wellbe realized by fine-tuning the reaction conditions, such as metalligand ratio, ligand structure, coordination nature of metal ion,solvent, pH, and temperature.4 The pyridyl or carboxylate func-tional groups have extensively been used for the construction ofvarious coordination frameworks in the last two decades.5 On theother hand, the potential of imidazole containing ligands for theconstruction of coordination networks has been recently real-ized. The imidazole moieties have high affinity for coordinatingtometals, and also several synthetic strategies are readily availablein the literature for the synthesis of imidazole ligands containingvarious functional groups. Accordingly, a good number of metalorganic frameworks (MOFs) have been reported recently by
using flexible bis(imidazole) ligands alone or with coligands suchas carboxylic acids and sulfonic acids.6,7 Moreover, in severalof these reports the conformational flexibility of the ligand wasutilized to produce diverse supramolecular topologies such ashelices, molecular boxes, rotaxanes, catenanes, polythreadednetworks, and dimondoid networks.8 However, in many of thesecases, one of the N atoms of the imidazole (Scheme 1) wasconnected to the C atom of the central moiety or spacer. Herewe focus upon the ligand (L) that contains C2 of the imidazoleconnected with the spacer and that enables functionalization ofone of the N�Hgroups of the imidazole, while the imine N atomcoordinates to the metal.8n,o The presence of such a site which issusceptible to functionalization allows fine-tuning of the proper-ties of the cavities or material properties either by the pre- orpostfunctionalizations. The benzimidazole was considered, as itdecreases the number of imidazole moieties around the metalcenter due to its larger size and it allows the halide ions to bridgethe metal centers.
Given these reasons and in continuation of our ongoingresearch interest of exploring novel coordination polymers, herein this paper, we report the synthesis and crystal structures of
Received: October 5, 2011Revised: November 4, 2011
ABSTRACT: A new organic ligand, namely 1,4-bis[2-(1-methylbenzi-midazol-2-ylmethyl)]benzene (L), was synthesized, and its crystal struc-ture has been determined. The ability of L to form coordination poly-mers was explored with various metal salts under different conditions. Sixcomplexes of L—[Cu2(L)I2], 1; [Cu2(L)I2], 2; [Cd3(L)Cl6 3 2DMF], 3;[Cu(L)Cl2], 4; [Zn(L)I2 3MeOH], 5; and [Cd(L)(NO3)2 3H2O] 3 2THF,6—have been synthesized by the reaction of L with the correspondingmetal salts at room temperature, either by direct mixing or by a layeringtechnique. Single crystal X-ray analyses revealed that complexes 1 and 2are isomers of each other and that both contain Cu2I2 building units.The crystal structure of 1 contains a discrete unit that contains a strongCu(I) 3 3 3 arene interaction, whereas, in 2, the Cu2I2 units act as a linearlinker and form a one-dimensional zigzag chain. Complex 3 exhibits atwo-dimensional layer in which the 1D polymeric chains of halide bridged Cd(II) ions are linked by the organic ligand L. The crystalstructures of 4 and 5 contain a 1D zigzag chain and a discrete hydrogen bonded dimer, respectively. The crystal structure of complex6 contains nitrate as a counterion and forms an interesting one-dimensional helical chain containing cavities that are occupied byTHF molecules. In these crystal structures, the ligand L is found to exhibit four different conformations. The Cambridge StructuralDatabase was used to rationalize the results obtained here and to provide some insight into the metal coordination geometries, thehalide bridging of metal centers, as well as metal 3 3 3 arene interactions.
coordination polymers of a new flexible ligand, namely 1,4-bis-[2-(1-methylbenzimidazol-2-ylmethyl)]benzene, L, with variousmetal halogen and nitrate salts. The halide ions are used as coun-terions, as they are known to increase the dimension of thenetworks by bridging the metal centers in various modes.9,4f Inthis study, we have obtained six metal complexes of Lwith Cu(I),Zn(II), Cu(II), and Cd(II). The Cu(I) complexes were foundto be isomers to each other and are formed due to the exhibitionof a different conformation of L. The geometries found in thecomplexes of L include discrete species, one-dimensional chainsand helices, and two-dimensional layers.
’RESULTS AND DISCUSSION
The ligand L was synthesized by a condensation reaction ofo-phenylenediamine and 1,4-phenylenediacetic acid10 followedby N-methylation with CH3I. The crystallization of compound Lin ethanol solution afforded crystals suitable for single crystalX-ray diffraction analysis. The reactions of L with various metalsalts have been carried out at room temperature by either a layerdiffusion technique or direct mixing. The reaction of L with CuIresulted in crystals of two complexes, 1 and 2, which containsimilar stoichiometries but different architectures. Our attemptsto produce single crystals of complexes with another Cu(I) halide(CuBr and CuCl) were unsuccessful. The reaction of L withCdCl2 resulted in crystals of complex 3. In all three complexesthe halogen atoms act as a bridge between the metal centers andincrease the dimensionality in 2 and 3. The reactions of L withCuCl2 and ZnI2 form complexes 4 and 5, respectively. In both ofthese complexes no halide bridging was observed. The reactionof L with Cd(NO3)2 afforded crystals of complex 6, which con-tains a one-dimensional helical chain. The crystal structures of Land complexes 1�6
were determined and analyzed in terms of the geometry of thecoordination networks, the conformation of the ligand, andthe utility of halide bridging in coordination networks. The perti-nent crystallographic details, hydrogen bonding parameters, andselected 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 theunit cell; the asymmetric unit contains half a molecule of L andonemolecule of water. The water molecules form one-dimensional
zigzag chains via O�H 3 3 3O hydrogen bonds. The L moleculesjoin the water chains into a highly corrugated two-dimensionallayer via O�H 3 3 3N hydrogen bonds (Figure 1a). These two-dimensional layers are tightly packed in the lattice such that thereexists weak C�H 3 3 3π interactions (3.058 Å) between centralphenyl and benzimidazole rings (Figure 1b).Isomeric Coordination Complexes Based on the Cu2I2
Unit. The ligand L with CuI forms two types of crystals, 1 and2, when crystallized from CH3CN�THF and CH3CN�nitro-benzene�THF, respectively. Both complexes crystallized in theP1 space group. The single crystal diffraction analyses revealthat these two complexes are isomers to each other; they havethe same molecular formula but exhibit different structures. Theasymmetric unit consists of two copper atoms, two iodide ions,and one unit of L in 1, whereas, in 2, the asymmetric unit containshalf a unit of ligand L and one unit each of copper and iodide ion.Interestingly, both the structures contain a Cu2I2 unit, formedthrough iodo-bridges, as observed in related Cu(I) iodides withbulky N-based donors.11 It was shown earlier by us and othersthat the Cu2I2 unit can be used as a four connected square planarnode to build higher dimensional coordination networks.12 Incontrast to those studies, the Cu2I2 unit was found to adopt adifferent geometry as well as connectivity in the crystal structuresof 1 and 2. In 1, the ligand exhibits a bow shaped geometry suchthat two imidazole nitrogens from the same L ligand coordinateto a Cu2I2 unit, thus forming a zero-dimensional bowlike struc-ture (Figure 2a). Each Cu(I) exhibits a distorted trigonal coordi-nation geometry with two I atoms and one N atom. The Cu2I2unit was placed under the central phenyl ring, and the iodineatoms of the Cu2I2 unit were pushed downward such that theyare away from the phenyl ring. The Cu---Cu bond overlaps withthe edge of the central phenyl ring with the C 3 3 3Cu distances asshort as 2.647 Å and 2.714 Å. We note here that this distance ismuch shorter than the sum of the van derWalls radii of Cu(I) andcarbon atom (3.10 Å). The Cu2I2L units pack in the crystal latticevia weak C�H 3 3 3 I and C�H 3 3 3π interactions (2.874 Å)(Figure 2b).A Cambridge Structural Database (CSD) search was con-
ducted to understand the generality or uniqueness of the shortCu 3 3 3C contacts observed in 1. It reveals that nine more Cu(I)complexes exhibit the short Cu 3 3 3C contacts below 2.7 Å. Theshortest Cu 3 3 3C distance (2.270 Å) was observed in the Cu(I)complex of π-prismand;13 in this complex the Cu(I) was totallyembedded by the three aryl rings making possible such a shortcontact. The next shortest Cu 3 3 3C contacts (∼2.4 Å) wereobserved in the three Cu(I) complexes of 1,2,4,5-tetra(7-azain-dolyl)benzene in which the ligand acts as a cleft with the benzenering acting as a backbone.14 The others are the complexes ofdiarylbialkylphosphane, in which the Cu 3 3 3C distances are∼2.6 Å.15 It is worth mentioning that the calculations byEchavarren et al. on metal 3 3 3 arene interactions indicated thatthey are weak interactions with minimal or no covalent characterand no electron transfer from arene to metal.15
In complex 2, the Cu2I2 unit is planar and acts as a linear linkerto form a one-dimensional zigzag chain (Figure 3a) due to thefact that L adopts a divergent geometry by placing the two armsof benzimidazoles opposite to each other. Similar to 1, the Cu(I)centers in each Cu2I2 unit form a planar trigonal coordinationgeometry. The Cu 3 3 3Cu distances in the Cu2I2 unit are almostsimilar in both the complexes (2.604 Å in 1 and 2.559 Å in 2) andare in agreement with the previously reported distances.16 TheCu2I2 unit was connected to imine nitrogen atoms more linearly
in 2 compared to that in 1 (Cu 3 3 3Cu�N angles: 158.08� and161.54� in 1 and 173.36� in 2). The one-dimensional chains packviaC�H 3 3 3π interactions between the N-methyl C�H and theπ cloud of the phenyl ring of benzimidazole moieties and also viaweak C�H 3 3 3 I interactions to form a two-dimensional layer(Figure 3b and d).The CSDwas searched to understand the coordination number
of two for Cu2I2 unit rather than usual coordination number offour. The CSD search shows that there exists a total of 58 com-plexes containing a Cu2I2 unit coordinating two or four N atoms.Out of 58, only 11 structures were found to exhibit two coordina-tion. Interestingly, in all these 11 structures, the N atom comes
from either bulky ligands (substituted benzimidazoles) or steri-cally hindered N-heterocycles containing the substitution near theN atom.17 These studies reveal that the steric factors play a majorrole 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 halfof the ligand L, one and a half Cd(II) atoms, three Cl ions, andone coordinated DMF molecule. The two symmetry indepen-dent Cd(II) atoms exhibit different coordination environments:one is six coordinated (A) with octahedral geometry, and theother is five coordinated (B) with square pyramidal geometry.These two Cd(II) atoms are bridged by chloride ions such thatthey form a one-dimensional chain with a 3 3 3BBABB 3 3 3 pattern(Figure 4a). The six coordinatedCd(II) center coordinates to fourCl ions in equatorial positions (Cd�Cl: 2.639 and 2.613 Å) andtwo O atoms of DMF molecules in the axial positions (Cd�O:2.299 Å), and it sits on inversion symmetry. The five coordinatedCd(II) center contains four chloride ions (Cd�Cl: 2.488, 2.508,2.686, and 2.736 Å) in equatorial positions and one iminenitrogen of L in the axial position, resulting in a distorted squarepyramidal geometry around the metal center. The equatorial Clions do not lie in plane, as evidenced by the N�Cd�Cl angles:93, 92, 113, and 117�. In the one-dimensional chain, each octa-hedron shares two of its edges with two square pyramids, whereaseach square pyramid shares two of its edges with one octahedronand onewith a neighboring square pyramid. These one-dimensionalchains are linked by the L units to form two-dimensional layers(Figure 4b). These layers pack on each other via bifurcatedC�H 3 3 3Cl, aromatic face-to-face π 3 3 3π (centroid to centroiddistance 3.845 Å), and C�H 3 3 3π interactions (2.829 Å)(Figure 4c).A CSD search on the chloride bridged Cd(II) complexes
shows that there are 32 structures containing polymeric CdCl2chains in which the Cd(II) octahedrons are edge shared. How-ever, only two structures were found to exhibit edge sharing ofthe Cd(II) octahedron and pbp.18,19 Interestingly, both thesestructures do not contain a CdCl2 polymeric chain but justcontain three Cd(II) atoms in series (pbp...oh....pbp) inter-connected through halide bridges (“pbp” and “oh” refer to thepentagonal bipyramid and octahedron geometries, respectively).
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
Therefore, the CSD studies indicate that the geometry ofthe CdCl2 polymeric chain observed in 3 is unique andunprecedented.Zero- and One-Dimensional Networks with No Halide
Bridging. The complexation reaction of L with CuCl2 andZnI2 produced complexes 4 and 5, respectively. Complex 4was crystallized in a monoclinic lattice with a P21/n space group.
The asymmetric unit contains half a unit each of ligand L andCu(II) and one chloride ion. The Cu(II) atom exhibits a squareplanar geometry, with two imine nitrogens from two L ligandsand two chloride ions, and sits on an inversion center. This typeof coordination results in the linking of metal centers by L intoa one-dimensional zigzag chain (Figure 5a). It is interesting tonote here that Cu(II) exhibits a less prominent square planar
Table 3. Bond Distances (Å) and Angles (deg) around Metal Center(s) for Complexes 1�6a
aM=metal atom, X = halogen, N = nitrogen atom from L, O = oxygen atoms in complex 3 from DMFmolecule. In 6 ,a = oxygen from water molecule;b = one of the oxygens from water and another from nitrate; c = oxygen from nitrate.
geometry due to the fact that two bulky groups such asbenzimidazoles block the axial sites. The packing of the chains
in the crystal lattice is governed by a plethora of C�H 3 3 3Clinteractions and aromatic stacking interactions (Figure 5b and c).The CSD search on CuCl2 complexes suggests that in themajority of the complexes, the Cu(II) atoms are bridged byCl anions only when the ligands are large and chelating. Thepercentage of complexes containing Cl anion bridging Cu(II)ions in CuCl2 complexes, found to be around 29% (960 outof 3331 complexes), indicates that this phenomenon is not socommon.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 atetrahedral geometry with the coordination of two iodide ions,one imine nitrogen, and one methanol O atom. One end of theligand does not coordinate with the metal but is involved in theO�H 3 3 3N hydrogen bond with the coordinated MeOH. Wenote here that the coordination of the MeOH is preferred overthe coordination with the other end of the ligand due to the bulkynature of the benzimidazole groups. Two of these complexesform a centrosymmetric dimer via Me�O�H 3 3 3N hydrogenbonds (Figure 6a). These dimers pack in a crystal lattice viaC�H 3 3 3 I, C�H 3 3 3π, and π 3 3 3π interactions (Figure 6b).Our observation of no bridging in the ZnI2 complex is in agree-ment with the CSD statistics; nine structures contain halidebridges out of 285 ZnI2 complexes.One-Dimensional HeliceswithGuest Inclusion.The nitrate
ion is also known to act as a bridge between metal ions in severalcoordination polymers. In order to understand the importance ofa halide ion in the formation of a one-dimensional polymericCdCl2 chain via μ2-halide bridges in 3, single crystals of complex6 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 theseunits via C�H 3 3 3π 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 viaC�H 3 3 3π and C�H 3 3 3Cl interactions; (c) packing of the layers in the crystal lattice.
Figure 1. Illustrations for the crystal structure ofL: (a) hydrogen bondedtwo-dimensional layer; (b) packing of the layers in the crystal lattice.
from that of 3. The asymmetric unit contains one each of Cd(II),ligand, and water molecule and two each of nitrates and THFmolecules. The Cd(II) atom exhibits seven coordination withfour O atoms from two nitrates that are cis coordinated, twoimine N atoms of a benzimidazole group that are cis coordinated,and one water molecule (Figure 7a). The cis coordination aswell as the twisted conformation of the ligand generates a one-dimensional helix with a pitch length of 7.99 Å (Figure 7b). Withinthe helices, two guest THF molecules were accommodated andhydrogen bonded with the coordinated water molecule (Figure 7a).The crystal lattice contains left-handed and right-handed heliceswhich interact with each other via face-to-face π 3 3 3π stackinginteractions, between two benzimidazole moieties (the centroid-to-centroid distance is 3.829 Å), to form a two-dimensional layer
(Figure 7c). Weak C�H 3 3 3O 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 3π stackinginteractions (arising due to the benzimidazole and central phenylring; 3.050 Å) between two such layers were found to beresponsible 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 whichcan be categorized as cis�cis, trans�trans, cis�trans, and trans�cis; the first cis/trans descriptor indicates the orientation of thebenzimidazole arm with respect to the central phenyl ring, andthe second cis/trans descriptor represents the direction of theimine nitrogen atoms of the benzimidazole arm with respect toeach other (Figure 8). It is worth noting that complexes 1 and 2
Figure 5. Illustrations for the crystal structure of 4: (a) one-dimensional zigzag chain; (b) two-dimensional layer; (c) three-dimensional packingthrough C�H 3 3 3Cl and aromatic stacking interactions.
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 weakinteractions.
exhibit supramolecular isomerism due to the different conforma-tions of L. In 1, the conformation is cis�cis, leading to a cleftlikegeometry and therefore a discrete complex. On the other hand, in2, the geometry of L is trans�trans (divergent) and leads to aone-dimensional network. A similar trans�trans conformation ofL with some minor variations was also found in complexes 3 and4. The conformation of L in 5 can be described as a trans�cisconformation, which leads to the formation of a hydrogenbonded dimer of the complex. Although the conformation of Lin 6 was classified by us as cis�trans, the imidazole moieties areplaced such that they are gauche to each other along the central�CH2�C6H4�CH2 moiety, whereas in the other structuresthese moieties exhibit either eclipsed or anti geometries. Forexample, 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 theligand. This twisting nature as well as the cis coordination toCd(II) in 6 helped in the formation of the helices containingchannels 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 stretchingfrequency of ligand L appeared at 1500 cm�1. For all the coor-dination complexes, the CdN stretching appeared at higherwavenumber than in the range of 1507 cm�1 to 1519 cm�1. Thistype of blue shift clearly indicates that the imine nitrogen atomsderived from the ligand L are coordinated with the metal ions.Moreover, in complex 6, a sharp band at 1384 cm�1 was observeddue to the νO�N�O(sym) stretching of the coordinated nitrategroup in addition to the band at 1508 cm�1 for the coordinatedCdN stretching.
Figure 6. Illustrations for the crystal structure of 5: (a) O�H 3 3 3Nhydrogen bonded dimeric unit, (b) packing of dimeric units viaweak interactions inthe crystal lattice.
Figure 7. Illustrations for the crystal structure of 6: (a) hydrogen bonded THF molecules with the coordinated water molecule; (b) one-dimensionalhelical chain; (c) two-dimensional layer (guest THF molecules are shown in space filling mode); (d) packing of the layers in the crystal lattice.
In summary, the new ligand L was shown to form six coor-dination complexes (1�6) with diversified geometries. In thecrystal structures, the ligand L was found to exhibit four differentconformations due to the inherent conformational flexibility.Five out of the six complexes studied contain halides (I or Cl) ascounterions. The halogen bridge was found to be present in threestructures out of five. The counteranion I� exhibited bridging ofCu(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-dimensionalpolymeric CdCl2 chain which has an unprecedented geometry.Complex 1 exhibits a relatively new and unexplored type ofmetal 3 3 3 arene interaction between the Cu2I2 unit and the centralphenyl ring. The CSD search on Cu(I) complexes reveals thatthere exists nine more such examples with Cu 3 3 3C(arene)distance below 2.7 Å.However, none of these contain a bimetallicCu(I) unit but contain only a single Cu(I) interacting with arene.Complexes 1 and 5 exhibit discrete geometries, whereas com-plexes 2, 4, and 6 exhibit one-dimensional networks in theircrystal structures. On the other hand, complex 3 has a two-dimensional layer in which Cd(II) exhibits two types of coordi-nation environments, namely square pyramidal and octahedralcoordination geometries, via halide bridging and L-coordination.The observed two coordination of the Cu2I2 unit and the halidebridging of the metal centers were examined thoroughly usingCSD. These studies indicated that the larger size of the benzi-midazole 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. FTIRspectra were recorded with a Perkin-Elmer Instrument Spectrum RxSerial No. 73713. 1H NMR (200 MHz) and 13C NMR (50 MHz)spectra were recorded on a BRUKER-AC 200 MHz spectrometer. Ele-mental analyses were carried out with a Perkin-Elmer Series II 2400, andmelting points were taken using a Fisher Scientific melting pointapparatus 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
interactions was carried out by specifying the nonbonded interactionsbetween the Cu(I) atom and one of the aromatic carbon atoms. Thecomplexes containing Cu2I2 units were searched by defining the Cu2I2unit such that it is connected to only N atoms, and the resultant struc-tures were analyzed manually to get the required information regardingcoordination of the Cu2I2 unit with an N atom. For the chlorine bridgedCdCl2 complex, all the structures with a CdCl2 unit were analyzed man-ually, and only polymeric chains via chloride bridging were considered.With CuCl2 and ZnI2, all the resultant structures with and without halidebridging 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 thatuses graphite monochromated Mo Kα radiation (λ = 0.71073 Å) atroom temperature (293 K) by the hemisphere method. The structureswere solved by direct methods and refined by least-squares methodson F2 using SHELX-97.21 Non-hydrogen atoms were refined aniso-tropically, and hydrogen atoms were fixed at calculated positionsand refined using a riding model. The H atoms attached to the Oatom or N atoms are located wherever possible and refined using theriding model.
Synthesis of Ligand. Nonmethylated compound (Bn-IM) wassynthesized 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-phenylene-diamine were mixed with a sufficient amount of polyphosphoric acid tomake 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) andallowed to cool to about 100 �C. Then the viscous crude solution waspoured in a thin steam into a large volume of rapidly stirred cold water.The insoluble residue was collected by filtration, washed with water, andmade into a slurry with an excess of 10% sodium carbonate solution. Thealkaline slurry was filtered and washed thoroughly with water so that noalkali was present with the crude product. Then the crude product wasdried at 60 �C and recrystallized from a hot DCM/MeOHmixture (1:1)subsequent to treatment with a small amount of activated charcoal.Yield: 3.2 g (61.5%). M. P. > 300 �C. 1HNMR (200MHz, d6-DMSO) δ4.35 (s, 4H), 7.31�7.37 (m, 4H), 7.41 (s, 4H), 7.60�7.65 (m, 4H); 13CNMR (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%, H5.49%, 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) ofBn-IM and 40 mL of dry THF in a 100 mL round-bottom flask fitted with aside arm and maintained under nitrogen atmosphere was added 0.213 gof sodium hydride (95%) over 1 h, followed by the dropwise addition of2.51 g (17.6 mmol) of methyl iodide over 1 h. The reaction mixture wasstirred overnight, quenched with water, and then added to 400 mL ofwater. After stirring for 1/2 h, a pale-yellow precipitate was collected byfiltration, washed repeatedly with water, and dried in a vacuum for 24 h.The crude product was dissolved in 100 mL of ethyl alcohol containing1 g of activated charcoal, refluxed for 15 min, filtered hot, and kept forslow evaporation at room temperature. After 3 to 4 days, pale-yellowcrystals suitable for X-ray diffraction appeared and were collected viafiltration. Yield: 2.70 g, 83%. M. P. 218�220 �C. 1H NMR (200 MHz, d6-DMSO) 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) wasdissolved in 3 mL of THF and was taken in a test tube. 4 mL of CH3CNwas 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 theabove solution. Colorless block shaped crystals were isolated after 1week in 30% yield. Elemental analysis for Cu2C24H22N4I2: Exptl C39.41%, H 3.01%, N 7.76%; Calcd C 38.65%, H 2.95%, N 7.51%.
Figure 8. Conformations of the ligand L observed in the crystal struc-tures of 1�6.
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 aTHF and nitrobenzene mixture (1:1). Here also colorless blocked shapecrystals were isolated after 10 days in 25% yield. Elemental analysis forCu2C24H22N4I2: 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 whichwas dissolved by the addition of a few drops of a DMF/H2O mixture(1:1). The filtered solution was kept for slow evaporation at ambienttemperature. Colorless block shaped crystals were separated after2 weeks in 40% yield. Elemental analysis for Cd3C30H36N6O2Cl6: ExptlC 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 aTHF and chloroform mixture (1:3). Blue crystals were produced after10 days in 50% yield. Elemental analysis for CuC24H22N4Cl2: Exptl C56.44%, H 4.44%, N 10.64%; Calcd C 57.00%, H 4.40%, N 11.20%.Preparation of [Zn(L)I2 3MeOH], 5. 0.015 g of ligand L (0.04 mmol)
was dissolved in 2 mL of methanol, and 0.013 g of ZnI2 was alsodissolved in 2 mL of methanol in a separate vial. The mixing of these twosolutions resulted in a dirty white precipitate which was dissolved by theaddition of excess methanol with gentle heating. Then the filteredsolution was kept for crystallization at room temperature. Colorlessneedle like crystals develop after 2 days in 65% yield. Elemental analysisfor ZnC25H26N4OI2: Exptl C 42.19%, H 3.68%, N 7.67%; Calcd C41.72%, H 3.6%, N 7.78%.Preparation of [Cd(L)(NO3)2 3H2O] 3 2THF, 6. The mixing of a THF
(1 mL) solution of L (0.015 g, 0.04 mmol) with a THF (1 mL) solutionof Cd(NO3)2 3 4H2O (0.013 g, 0.04 mmol) resulted in a white pre-cipitate which was dissolved by the addition of 1 mL of acetone withgentle heating. The filtered solution was kept for slow evaporation atambient temperature. Colorless block shape crystals were generatedafter 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%, N10.98%.
’ASSOCIATED CONTENT
bS Supporting Information. 1H NMR and 13C NMR spec-tra of the ligand, IR-spectra for the ligand and complexes, andcrystallographic information for the crystal structures of L andcomplexes 1�6 (CIF). Thismaterial is available free of charge viathe Internet at http://pubs.acs.org.
We gratefully acknowledge financial support from DST andDST-FIST for the single-crystal X-ray facility. S.S. thanks UGCfor a senior research fellowship.
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