Metal(II) Complexes Derived from Conformation Flexible Cyclic Imide

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Metal(II) Complexes Derived from Conformation Flexible Cyclic Imide Tethered Carboxylic Acids: Syntheses, Supramolecular Structures, and Molecular Properties Devendra Singh and Jubaraj B. Baruah* Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati, 781 039 Assam, India S Supporting Information *

ABSTRACT: A series of mononuclear M(II) complexes, namely, [Mn(L1)2(H2O)4] (1), [Cu(L1)2(pyr)2(H2O)2] (2), [Zn(L1)2(pyr)2(H2O)2] (3), [Cd(L1)2(pyr)2(H2O)2] (4), [Co(L2)2(pyr)2(H2O)2].pyr (5), [Cu(L2)2(pyr)2].2pyr.2H2O (6), and [Cd(L2)2(pyr)2(H2O)2].pyr (7) (where L1H = 4-(1,3-dioxo-1,3-dihydro-isoindol-2-ylmethyl)-cyclohexanecarboxylic acid, L2H = 4-(1,3-dioxo-1H,3H-benzo[de]isoquinolin-2-ylmethyl)cyclohexanecarboxylic acid, and pyr = pyridine) were synthesized and structurally characterized by elemental analyses, IR spectroscopy, thermogravimetric analyses, powder X-ray diffraction, and single crystal X-ray diffraction. Coordination environments around the metal centers and subtle differences in weak interactions affect the dimensionality and features of supramolecular architectures of these complexes. In the case of M(II) complexes of L1H, 1 shows a zigzag 3D architecture, whereas other complexes exhibit 3D channel-like structures containing the coordinated pyridine rings inside these channels. The coordination environments around the M(II) centers are the same in the complexes 5 and 7; however, the former is composed of 3D architecture containing channels and voids, whereas the latter reveals a 2D sheet structure. The 3D supramolecular architecture of complex 6 is sustained by helical channels, which are filled by lattice water and pyridine molecules. Solid state fluorescence emission properties of L1H and its M(II) complexes show resemblance to each other, whereas they have different characteristics in the case of L2H and its M(II) complexes. Cyclic voltammetry of redox-active ligand L2H and its complexes 5 and 6 are also studied.

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INTRODUCTION Design and synthesis of transition metal complexes with structural diversities have attracted considerable research interest as far as the dimensionality and topology of the species are concerned.1 The assemblies of metal−organic materials2 (MOMs) are not only governed by strong and highly directional coordination bonds but also directed by various weak noncovalent forces such as hydrogen bonding, π···π, C− H···π, and cation···π interactions.3 Especially, the less directional π···π interactions are widely explored to design superstructures with novel properties in the field of crystal engineering.4 Among the different MOMs, metal carboxylates have attracted much attention because of the versatile coordination binding modes of the carboxylate group along with hydrogen bond donor and acceptor properties in making complexes with various dimensionalities.5 In the case of carboxylate ligands, benzene-1,3-dicarboxylic acid,6 benzene1,4-dicarboxylic acid,7 and benzene-1,3,5-dicarboxylic acid8 have been widely utilized. Metal carboxylates with flexible ligands have also been studied for the formation of coordination networks possessing permanent porosity and high thermal stability, which are useful for ionic exchange,9 separation,10 chemisorption,11 gas storage,12 catalysis,13 magnetism,14 optoelectronics,15 and luminescence properties.16 © 2012 American Chemical Society

However, much work in this direction is required to have control over constructions of voids in a predictable manner.17 Amino acids and its derivatives with flexible coordination modes have been explored in the generation of different coordination frameworks with potential application in material science and biology.18 N-protected amino acids play a significant role in biology as they are a part of some natural proteins and peptides.19 We have discussed earlier the role of weak interactions in the formation of supramolecular networks of N-phthaloylglycinato and 4-carboxy-N-phthaloylglycinato complexes of transition metals.20 Recently, a series of copper(II) carboxylate dimers derived from propionic acid and butanoic acid tethered by the 1,8-naphthalimide unit is reported.21 The supramolecular networks in these dimers are sustained by strong π···π stacking interactions of electron deficient naphthalimide units. We were also interested to elucidate the role of weak interactions in the construction of supramolecular architectures of metal complexes that can be built from bifunctional ligands bearing both the carboxylate donor group and different π-stacking aromatic imide units. For Received: January 25, 2012 Revised: February 27, 2012 Published: March 2, 2012 2109

dx.doi.org/10.1021/cg300113f | Cryst. Growth Des. 2012, 12, 2109−2121

Crystal Growth & Design

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complexes 1−4. These complexes show identical infrared spectra with the absorption bands of asymmetric and symmetric vibrations of carboxyl groups of L1 respectively appearing at 1543−1576 cm−1 and 1395−1398 cm−1, which confirms the monodentate coordination modes of L1 in each of the complexes.22 In our previous studies, we observed that the carboxylate groups of ligands N-phthaloylglycine and 4carboxy-N-phthaloylglycine remained as monodentate carboxylates in their mononuclear transition M(II) complexes.20 Moreover, dinuclear Ag(I)23 and tetranuclear Sn(IV)24 complexes of ligand N-phthaloylglycine are also reported in which the carboxylate groups of ligands coordinate to the metal centers in monodentate fashion. Crystallographic analysis revealed that all these complexes possess inversion centers and appear as their halves in the crystallographic asymmetric units. Complex 1 is crystallized in the monoclinic P21/c space group. Octahedral geometry of six coordinated manganese(II) complex 1 is satisfied by two crystallographic equivalent oxygen atoms of two monodentate carboxylate groups of L1 (Mn1− O1, 2.136(2) Å) and two pairs of crystallographic equivalent oxygen atoms of four water ligands (Mn1−O5, 2.210(3) Å; Mn1−O6, 2.202(3) Å). In the crystal lattice of 1, both the oxygen atoms of coordinated carboxylate group take part in intermolecular hydrogen bonding interactions. The hydrogen bond parameters are listed in Table 1. The free oxygen atom of carboxylate group of L1 is involved in intermolecular bifurcated acceptor hydrogen bonding (Table 1) with one of the coordinated waters and cyclohexane ring via O5−H5A···O2 and C2−H2···O2 interactions, respectively (Figure 1a). The coordinated oxygen atom also forms an acceptor O6− H6C···O1 hydrogen bond with another coordinated water. Along with these interactions, one of the carbonyl oxygen atoms interacts with the cyclohexane ring via acceptor C5− H5···O4 interaction, making a 1D layered structural arrangement. Another carbonyl oxygen also interacts with the phthalimide ring via acceptor C11−H11···O3 interaction, which further induces C−H···π (dc6···π 3.38 Å) interactions between the methylene hydrogen atoms and phthalimide rings. Consequences of these later interactions grow the 1D layers to 2D sheet arrangements. Beside that, the phthalimide rings also interact with each other via C−H···π (dc11···π 3.53 Å) interactions, which resulted in the construction of a 3D zigzag

this purpose, we have chosen two bifunctional ligands (L1H and L2H, Scheme 1) in which the carboxylate donor group is Scheme 1. Structures of Organic Ligands L1H and L2H

separated from π-stacking aromatic imide tectons through a flexible hydrophobic spacer and studied the structures of their supramolecular complexes upon incorporation of different metal centers. The assemblies of these metal−organic materials are governed partially or completely by supramolecular interactions. In addition, photoluminescence and electrochemical properties of these new complexes are investigated.

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RESULTS AND DISCUSSION Two conformational flexible methyl cyclohexane carboxylic acids tethered by phthalimide (L1H) and naphthalimide (L2H) units are synthesized by condensation reactions of trans-4(aminomethyl)cyclohexanecarboxylic acid with corresponding anhydrides (shown in Scheme 1). These two bifunctional ligands that contained both the carboxylate donor groups and π-stacking aromatic imide unit are incorporated into the construction of various transition metal complexes after deprotonation of the acid groups. A series of mononuclear metal carboxylates (1−4) of L1H with Mn(II), Cu(II), Zn(II), and Cd(II) is prepared under the solution state similar reaction conditions (Scheme 2). Each of these complexes other than the complex 1, has pyridine as ancillary ligands. To obtain uniform or similar compositions of complexes in this series of complexes, we had made numerous attempts to obtain suitable single crystals of 1 with pyridine for X-ray diffraction study. Which were not successful since the Mn(II) complex (1) was found to be soluble in water, whereas others were not, it was crystallized from water solvent for comparative study with other M(II) complexes. The carboxylate group of L1 coordinates to M(II) centers in monodentate coordination modes possessing 1:2 metal to ligand ratios in the square-pyramidal (for complex 2) and octahedral geometries (for complexes 1, 3, and 4) of

Scheme 2. Synthesis of Mononuclear Transition M(II) Complexes of L1

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Table 1. Hydrogen Bond Geometry (Å, deg) for Crystals 1, 2, 3, and 4 complex

D−H···A

dD−H

dH···A

dD···A

∠D−H···A

1

O5−H5A···O2 [x, 1 + y, z] O6−H6C···O1 [1 − x, 2 − y, −z] C2−H2···O2 [−x, −1 + y, z] C5−H5···O4 [−1 + x, y, z] C11−H11···O3 [−1 + x, 2 − y, −z] O5−H5A···O2 [−x, 1 + y, 1/2 − z] C11−H11···O2 [−x, y, 1/2 − z] C21−H21···O2 −1/2 + x, 1/2 − y, 1 − z] O5−H5A···O2 [−x, 2 −y, −z] O5−H5B···O1 [1 − x, 2 − y, −z] C2−H2···O2 [1 + x, y, z] C12−H12···O4 [−x, 1 − y, z] C15−H15···O2 [−1 + x, 2 − y, 1 − z] C18−H18···O3 [−1 + x, 2 − y, −z] O5−H5A···O1 [1 − x, 2 − y, −z] O5−H5B···O2 [−x, 2 − y, −z] C18−H18···O4 C2−H2···O2 C21−H21···O1 [−1 + x, y, z]

0.95 0.83 0.98 0.98 0.93 0.75 0.93 0.93 0.87 0.81 0.98 0.93 0.97 0.93 0.74 0.82 0.93 0.98 0.93

1.79 2.00 2.65 2.51 2.52 1.85 2.68 2.71 1.74 1.95 2.59 2.59 2.71 2.70 1.98 1.82 2.67 2.65 2.67

2.70 (4) 2.82 (4) 3.58(7) 3.29 (4) 3.34 (5) 2.60 (18) 3.47 (3) 3.63 (3) 2.58 (2) 2.76 (2) 3.45 (2) 3.39 (3) 3.52(2) 3.60(3) 2.72 (3) 2.63 (4) 3.57(1) 3.51(2) 3.60 (2)

160.0 167.8 160.4 136.1 147.8 170.3 142.5 172.2 162 170.6 147.1 143.7 142.8 165.4 169.9 170.5 164.8 147.6 175.7

2

3

4

supramolecular architecture of complex 1 as viewed along the a axis (Figure 1b). The complex 2 having composition [Cu(L1)2(pyr)2(H2O)] is crystallized in a monoclinic C2/c space group. Square pyramidal geometry around the one Cu(II) center of five coordinated complex 2 is satisfied by two crystallographic equivalent oxygen atoms of two monodentate carboxylates groups of L1 (Cu1−O1, 1.976(11)Å), two crystallographic equivalent nitrogen atoms of two pyridine ligands (Cu1−N2, 2.029(16)Å) and one oxygen atom of water ligand (Cu1−O5, 2.151(19)Å). The equatorial positions of square pyramidal geometry are occupied by two carboxylate groups and two pyridine ligands at trans disposition to each other, respectively, whereas the water ligand takes the axial position (Figure 2a). The free oxygen atom of the coordinated carboxylate group of L1 remains involved in the trifurcated acceptor hydrogen bonding with coordinated water, coordinated pyridine, and with phthalimide ring via O5−H5A···O2, C21−H21···O2, and C11−H11···O2 interactions (Table 1), respectively. Simultaneously, the carbonyl oxygen atom and the coordinated pyridine hydrogen atom interact with phthalimide ring via O···π (do4···π, 3.16 Å) and C−H···π (dc17···π, 3.68 Å) interactions, which create a 2D layered structural arrangement. The phthalimide rings of these 2D layers are further connected by strong π···π interactions; constructing a 3D supramolecular architecture containing channels among the 2D layers of 2. The coordinated pyridine molecules takes position inside the channels as viewed along the b axis (Figure 2b). The complex 3 having composition [Zn(L1)2(pyr)2(H2O)2] is crystallized in a triclinic P1̅ space group. The complex 3 has a near octahedral geometry around the Zn(II) metal center with two monodentate carboxylate groups of crystallographic equivalent L1 (Zn1−O1, 2.102(11) Å) and two water ligands (Zn1−O5, 2.103(12)Å) in trans orientation to each other, respectively, occupying the equatorial positions. The remaining two crystallographic equivalent pyridine ligands (Zn1N−2, 2.267(15) Å) occupy the axial positions in the octahedral geometry (Figure 3a). The free oxygen of the carboxylate group forms a bifurcated acceptor hydrogen bond with the cyclohexane ring via two C−H···O interactions, namely, C2−

H2···O2 and C15−H15A···O2 interactions (Table 1), whereas the coordinated oxygen atom interacts simultaneously with the coordinated water through acceptor O5−H5B···O1 interaction, making a 1D zigzag layered structure. Further, both the carbonyl oxygen atoms of L1 interact with coordinated pyridine and aromatic phthalimide ring via C18−H18···O3 and C12− H12···O4 interactions, overall making a 3D supramolecular network creating channels in the lattice. Similar to the structure of 2, the coordinated pyridine molecules are situated inside the channels as viewed along the a axis (Figure 3b). The complex 4 also crystallizes in the triclinic P1̅ space group. Structural features of 4 are similar to those of 3, exhibiting some minor differences in intermolecular hydrogen bonding interactions (Figure 4). In contrast to the structure of 3, the free oxygen atom of the carboxylate group of L1 interacts with the cyclohexane ring via only an acceptor C2−H2···O2 interaction (Table 1), whereas the coordinated oxygen atom engages in a bifurcated acceptor hydrogen bond interacting with coordinated water and coordinated pyridine via two different O5−H5A···O1 and C21−H21···O1 interactions in the crystal lattice of 4. Three mononuclear transition metal complexes of conformational flexible methyl cyclohexane carboxylic acid, that is tethered by a naphthalimide unit (L2H), are also synthesized under solution state similar reaction conditions. The reaction conditions used in the preparation of mononuclear Co(II), Cu(II), and Cd(II) complexes are summarized in Scheme 3. The M(II) to ligand (L2) ratios in all these complexes are 1:2 where carboxylate groups of L2 show monodentate (for complex 5 and 7) and chelating coordination modes (for complex 6) in the octahedral geometries of complexes 5−7. The infrared spectra of the complexes 5 and 7 are similar to each other showing the characteristic bands of carboxyl groups of L2 at 1588−1589 cm−1 and 1438−1440 for the asymmetric and symmetric vibrations, respectively. Complex 6 exhibits the absorption bands of asymmetric and symmetric vibrations of a carboxyl group at 1589 cm−1 and 1490 cm−1, respectively. These values suggest the monodentate coordination modes of L2 in complexes 5 and 7, whereas it is a chelating coordination mode in complex 6.22 The four carboxylate groups from four 2111



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Figure 1. (a) Part of crystal lattice of complex 1 showing weak interactions; (b) 3D zigzag supramolecular architecture of complex 1.

separate ligands coordinate with two Cu(II) atoms in bridging coordination modes in earlier reported dimeric Cu(II) paddlewheel complexes of flexible N-(3-propanoic acid)-1,8naphthalimide and N-(4-butanoic acid)-1,8-naphthalimide ligands.21 The M(II) complexes of L2H also possess inversion centers and appear as their halves in the crystallographic asymmetric units. The complex 5 is crystallized in the monoclinic P21/c space group. The octahedral geometry of six coordinated complex 5 is satisfied by two crystallographic equivalent pyridine ligands (Co1−N2, 2.228(6)Å) coordinated axially, along with two monodentate carboxylate groups of two crystallographic equivalent L2 (Co1−O2, 2.095(4)Å) and two water ligands (Co1−O5, 2.082(4) Å) coordinated equatorially (Figure 5a). Both the lattice and coordinated pyridine molecules are disordered in the crystal structure of complex 5. Coordinated pyridine molecules are disposed in the lattice such that the two carbon atoms at each 2 and 3 position of the pyridine ring are shared with half occupancies. The guest pyridine molecule is

also disordered across an inversion center sharing the nitrogen and carbon atoms at 1 and 4 position of pyridine ring with half occupancies. The free oxygen atom of the coordinated carboxylate group takes part in bifurcated acceptor hydrogen bonding with cyclohexane ring via two C2−H2···O1 and C19− H19A···O1 interactions (Table 2), whereas the coordinated oxygen atom forms O5−H5B···O2 hydrogen bond with the coordinated water molecule. The hydrogen bond parameters are listed in Table 2. The 1D layer formed by these interactions further assemble in another dimension by C−H···π (dc9···π, 3.57 Å) interactions experienced between the naphthalimide rings and coordinated pyridine molecules. This arrangement leads to the formation of channels in discrete 2D arrays of complex 5 in the lattice. Apart from that, naphthalimide rings also interact with each other via intermolecular C14−H14···O3 interactions, further generating a 3D supramolecular network, which is sustained by large solvent voids as viewed along the b axis (Figure 5b). The voids created between the discrete 2D layers are filled by disordered lattice pyridine molecules. 2112



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Figure 2. (a) Various weak interactions in the crystal lattice of complex 2; (b) 3D supramolecular channel-like network of complex 2 containing coordinated pyridine rings along the b axis (shown by circles).

The complex 6 is crystallized in the monoclinic P21/c space group, which has a distorted octahedral geometry around the Cu(II) metal center with two trans chelating carboxylate groups of two crystallographic equivalent L2 ligands (Cu1−O1, 1.951(3) Å, Cu1−O2, 2.760(3) Å) at equatorial positions and two crystallographic equivalent pyridine ligands (Cu1−N2, 2.002(3)Å) at axial positions (Figure 6a). One of the carbonyl oxygen atoms of L2 interacts with the cyclohexane ring via C2− H2···O4 interactions, whereas another carbonyl oxygen participates in a bifurcated acceptor hydrogen bonding with the cyclohexane ring and lattice pyridine molecule via two C− H···O interactions, namely, C19−H19B···O3 and C28− H28···O3 interactions creating 2D helical chains of complex 6 in the crystal lattice (Table 2). The lattice pyridine molecule further interacts with one of the hydrogen atoms of the lattice water molecule via O5−H5A···N5 interactions. Another hydrogen atom of the lattice water molecule also encloses the O5−H5B···O2 hydrogen bond with one of the oxygen atoms of the coordinated carboxylate group. The oxygen atom of this lattice water simultaneously engages in bifurcated acceptor hydrogen bonding with a coordinated pyridine molecule via two C−H···O interactions, namely, C21−H21···O and C28−

H28···O3 interactions. Consequence of these interactions emerges in the formation of 3D helical supramolecular network containing channels in the crystal lattice of 6 as viewed along the c axis (Figure 6b). The channels created in the lattice are filled by lattice pyridine molecules; the pyridine molecules also interact with each other via C−H···π (dc27···π, 3.64) interactions in the lattice. The lattice water molecules remains packed in between the 2D helical chains of this supramolecular architecture. The complex 7 is crystallized in the triclinic P1̅ space group. The structure of complex 7 is similar to the complex 5 having the same coordination environment around the Cd(II) metal center (Cd1−N2, 2.397(2) Å, Cd1−O1, 2.289(16) Å, and Cd1−O5, 2.302(19) Å) (Figure 7a). In the structure of 7, a guest pyridine molecule is also disordered across an inversion center as it is found in the case of complex 5. The coordinated oxygen atom of the carboxylate group contributes in bifurcate acceptor hydrogen bonding with a coordinated water and a coordinated pyridine molecule via O5−H5B···O2 and C− H···O interactions, respectively. The free oxygen atom of the carboxylate group also participates in bifurcated hydrogen bonding with the cyclohexane ring via two C−H···O 2113



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Figure 3. (a) Intermolecular interactions in a part of the crystal lattice of complex 3; (b) 3D supramolecular channel-like network containing coordinated pyridine molecules (purple) of complex 3 viewed along the a axis.

Figure 4. Intermolecular weak interactions in the lattice of complex 4.

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PXRD Patterns and Thermogravimetric Analyses of the Complexes. The PXRD patterns of complexes 1−7 are shown in Figure S1−S7 in the Supporting Information. All of the peaks of the seven compounds can be indexed nearly to their respective simulated PXRD patterns, which indicate that each of the seven compounds is pure phase. The small deviations in some cases are attributed to loss of solvent molecules leading to loss of crystallinity. To examine the thermal stability of the seven complexes, thermogravimetric analyses (TGAs) were carried out (Figure S8−S14, Supporting Information). The samples were heated up to 500 °C under N2 atmosphere. For the complex 1, one step weight loss due to the four coordinated water molecules takes place in the temperature range 65−120 °C corresponding to a weight loss of 9.46% (calcd 10.2%). Decomposition of organic ligands began at 275 °C for complex 1 and ended at about 460 °C. The TGA curve of complex 2 shows weight loss of coordinated water and pyridine molecules in two steps. In the first step, 11.28% weight loss occurs, corresponding to one molecule of pyridine and one molecule of water between 80 and 125 °C (calcd 12.00%). In the second step, 10.26% weight loss occurs from the residue of the first step for the remaining coordinated pyridine molecule

Scheme 3. Synthesis of Mononuclear Transition M(II) Complexes of L2

interactions, which are C19−H19B···O2 and C2−H2···O2 interactions (Table 2). These interactions make a stacked arrangement of units of complex 7 above each other and assemble the structure into an infinite 1D polymeric layered array. The naphthalimide rings in such 1D layers further interact with each other via short H···H contacts to generate 2D zigzag supramolecular sheet architecture in the crystal lattice of 7 as viewed along the b axis (Figure 7b).

Figure 5. (a) Weak intermolecular interactions in the crystal lattice of complex 5; (b) 3D supramolecular architecture of 5 viewed along the b axis showing arrangements of coordinated and lattice pyridine molecules; (c) space filling model of 5, shown without the lattice pyridine molecules. 2115



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Table 2. Hydrogen Bond Geometry (Å, deg) for Crystals 5, 6, and 7 complex

D−H···A

dD−H

dH···A

dD···A

∠D−H···A

5

O5−H5A···O1 O5−H5B···O2 [x, −1 + y, z] C2−H2···O1 [x, 1 + y, z] C19−H19···O1 C14−H14···O3 O5−H5A···N5 [−x, 1 − y, 1 − z] O5−H5B···O2 [x, 1/2 − y, 1/2 − z] C2H−2···O4 [1 − x, −y, 1 − z] C19−H19···O3 (−x, −1/2 + y, 3/2 − z) C28−H28···O3 C21−H21···O5 O5−H5A···O1 [1 − x, 1 − y, 1 − z] O5−H5B···O2 [2 − x, 1 − y, 1 − z] C19−H19B···O2 C2−H2···O2

0.82 0.94 0.98 0.97 0.93 1.05 0.90 0.98 0.97 0.93 0.93 0.76 0.83 0.97 0.98

1.83 1.79 2.52 2.71 2.63 1.87 2.28 2.52 2.62 2.61 2.67 1.95 1.80 2.70 2.64

2.59 (6) 2.72 (6) 3.40 (8) 3.52 (9) 3.48 (3) 2.87 (6) 2.73 (6) 3.34 (6) 3.59(7) 3.42 3.43(9) 2.70 (3) 2.62 (3) 3.51(6) 3.48(3)

152.2 174.9 148.5 141.5 151.7 157.8 110.6 140.7 176.3 146.0 140.3 171.9 110.6 141.1 143.3

6

7

Figure 6. (a) Part of crystal lattice of 6 showing weak interactions; (b) 3D helical supramolecular network containing channel like structures in the lattice viewed along c axis; (c) space filling model of lattice shown omitting the water and pyridine molecules.

between 115 and 185 °C (calcd 11.1%). Further weight loss in the temperature range 285−400 °C is accounted for the decomposition of organic ligands. Complex 3 shows a gradual weight loss of 21.78% between 55 and 250 °C corresponding to the loss of the two coordinated water and two coordinated pyridine molecules (calcd 23.3 wt %). Decomposition of organic ligands for this complex occurs in the temperature range 315−450 °C. Similarly, the TGA curve of complex 4 also shows a gradual weight loss of 20.33% between 65 and 275 °C corresponding to the loss of the two coordinated water and two

coordinated pyridine molecules (calcd 22.0 wt %), followed by the degradation of organic ligands in the temperature range 330−470 °C. In the case of complex 5, the first weight loss of 8.0 wt % in the range of 95−170 °C corresponds to the loss of one lattice pyridine molecule (calcd 7.9 wt %). The second loss occurs in the range 220−330 °C corresponding to the weight loss of 20.36% of the residue from the first step due to the loss of coordinated two water and two pyridine molecules (calcd 20.4%). The decomposition of organic ligands occurred at 2116



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Figure 7. (a) Weak interactions in a part of crystal lattice of complex 7; (b) 2D supramolecular zigzag sheets where 1D supramolecular layers are sustained by H···H contacts.

about 375 °C. The TGA curve of complex 6 shows a continuous weight loss of 25.58% between 115 and 250 °C corresponding to the loss of the two lattice water and two lattice pyridine molecules along with two coordinated pyridine molecules (calcd 25.8 wt %). Further weight loss in the temperature range 280−450 °C is attributed to the decomposition of organic ligands. For the complex 7, the first step within the range 130−170 °C corresponds to a weight loss of 8.74% (calcd 7.9%) due to the loss of one lattice pyridine molecule, and the second step, 200−270 °C, is due to the loss of coordinated two water and two pyridine molecules (weight loss of 19.98% of the residue from the first step; calcd 21.0%). The organic framework of complex 7 starts to decompose when the temperature is higher than 370 °C. Photoluminescence Properties of the M(II) Complexes. Metal carboxylates have been reported to have the ability to adjust the emission wavelength of organic materials through incorporation of metal centers. Such observations are common for the d10 metal centers.25 Since, L1H and L2H have fluorescence active sites, we investigated the photoluminescence properties of M(II) complexes 1−7 along with the free organic ligands L1H and L2H in the solid state at room temperature. The emission spectra of the free organic ligand L1H and its four metal complexes with Mn(II), Cu(II), Zn(II), and Cd(II), respectively, are shown in Figure 8. Upon excitation at 280 nm, three photoluminescence emission bands at 390 nm, 410 nm,

Figure 8. Solid state photoluminescence spectra of free organic ligand L1H and its four M(II) complexes (1−4) at room temperature (λex = 280 nm).

and 472 nm are observed for free organic ligand L1H. The fluorescence emission of the metal complexes 1−4 differ in intensities of emission peaks (λex = 280 nm in each case) from the fluorescence emission of the parent ligand L1H. The emission peaks obtained for L1H and its different M(II) complexes may be attributed to the intraligand π → π* transition.26 However, the relative differences in intensities of the three peaks in the emission spectra of L1H and complexes 1−4 indicate that the intraligand transitions of L1H have been 2117



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ligand L2H shows a one-electron reversible redox couple in the negative side of the cyclic voltammogram. However, the metal complexes 5 and 6 dissolved in dimethylformamide show peaks for two sets of reversible redox couples (E1 and E2) of which one set corresponds to the ligand. The electrochemical properties of the complexes and the ligand are listed in Table 3. The second set of quasi-

enhanced or weakened to different extents because of the introduction of different metal ions in these structures. The emission spectra of the free organic ligand L2H and its three metal complexes with Cu(II), Co(II), and Cd(II), respectively (5−7), are shown in Figure 9. L2H shows a

Table 3. Electrochemical Parameters of L2H and Complexes 5 and 6 ligand/ complex

E11/2 (mV) (ipc/ ipa)

ΔEp1 (mV)

E21/2 (mV) (ipc/ ipa)

ΔEp2 (mV)

L2H 5 6

−1229 (1.07) −1145 (1.16) −1235 (1.01)

87 194 77

−1246 (1.33) −1591 (0.97)

198 198

reversible peak (E2) in these complexes is assigned to the cationic species [ML]+ generated by dissociation of the ligand. Because of the low solubility of the metal complexes, we could not record the cyclic voltamograms of the complexes in other solvents. This study shows that, upon complexation, redox potential of a ligand moves toward more −ve side and that such processes are dependent on the central metal ion; for example, in the case of copper and cobalt complexes, the former has a second redox potential at a higher value than that of the latter.

Figure 9. Solid state photoluminescence spectra of free organic ligand L2H and its three M(II) complexes (5−7) at room temperature (λex = 310 nm).

strong broad photoluminescence emission band at 470 nm (λex = 310 nm), which may be assigned as intraligand π → π* transition of the naphthalimide ligand.26 The photoluminescence band in the emission spectra of complexes 5, 6, and 7 are blue-shifted as compared with that of free L2H and observed at 388, 422, and 407 nm, respectively, which may be ascribed to the ligand-to-metal charge transfer (LMCT) transitions27 (λex = 310 nm in each case). The emission spectra of complexes 5−7 are also different to each other, which is probably due to the differences of electronic configurations of the M(II) centers and different coordination modes of ligands around them as observed in the complexes with L1H. Electrochemical Studies. Naphthalimide derivatives are generally electroactive, and they show one electron reversible redox couple.28 The naphthalimide units separated by conjugated moieties show interesting semiconducting properties.29 Thus, we studied the cyclic voltammetry of free organic ligand L2H and its Co(II) and Cu(II) complexes. Representative cases of cyclic voltammograms of L2H and complex 6 are shown in Figure 10. The cyclic voltammogram of complex 5 is given as Supporting Information (Figure S17). As expected, the

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CONCLUSIONS By using two conformational flexible bifunctional ligands that contain carboxylate as the donor group and a π-stacking aromatic imide unit, we have prepared a series of metal− organic materials in which the building blocks are organized in supramolecular assemblies by various hydrogen bonds as well as C−H···π and π···π stacking interactions. In these complexes, the carboxylate groups of L1 and L2 are found to coordinate with M(II) centers in monodentate coordination modes, except in the complex 6, which has a chelating carboxylate group of L2. It is observed that both the highly directional coordinate bonds and weak noncovalent forces play a significant role to assemble these complexes in diverse supramolecular architectures. The solid state photoluminescence shows that the emission properties of M(II) complexes of L1H are due to the intraligand transitions, whereas M(II) complexes of L2H demonstrate LMCT transitions in their emission spectra. The cyclic voltammograms of naphthalimide complexes in DMF

Figure 10. Cyclic voltammograms of ligand (a) L2H and (b) complex 6 in 10−4 DMF solution (scan speed 100 mV/s). 2118



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Table 4. Crystallographic Parameters of 1−7 compound no.

1

2

5

6

7

formulas

C32H40N2O12Mn

C42 H46 N4O10Cd 879.24 triclinic P1̅ 5.5386(2) 13.1183(4) 15.2664(4) 112.863(2) 94.311(2) 100.602(2) 991.36(6) 1 1.473 0.615 454 7557 2368 44.22 −5 ≤ h ≤ 5 −13 ≤ k ≤ 13 −16 ≤ l ≤ 16 97.5 2399/0/267

C60H60N6O10Cu

C55H54N5O10Cd

699.60 monoclinic P21/c 5.4520(4) 6.3891(4) 45.711(3) 90.00 91.796(3) 90.00 1591.48(19) 2 1.460 0.483 734 21 936 2709 50.00 −6 ≤ h ≤ 6 −7 ≤ k ≤ 7 −53 ≤ l ≤ 54 99.9 2796/0/224

C42 H46N4 O10Zn 832.20 triclinic P1̅ 5.4784(10) 13.0745(4) 15.1085(6) 112.077(2) 94.582(2) 100.6590(10) 972.18(5) 1 1.421 0.697 436 14 419 3094 50.00 −6 ≤ h ≤ 6 −15 ≤ k ≤ 15 −17 ≤ l ≤ 17 98.9 3398/0/263

C55H40N5O10Co

formula wt. crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z density (Mgm−3) abs. coeff. (mm−1) F(000) total no. of reflections reflections, I > 2σ(I) max 2θ (deg) ranges (h, k, l)

C42H44N4 O9Cu 812.36 mnoclinic C2/c 30.798(2) 5.6340(4) 24.491(2) 90.00 114.536(6) 90.00 3865.8(5) 4 1.396 0.627 1700 20 843 3591 54.12 −38 ≤ h ≤ 39 −7 ≤ k ≤ 7 −31 ≤ l ≤ 31 99.4 4229/0/254

989.85 monoclinic P21/c 19.0706(19) 5.3626(5) 25.392(3) 90.00 104.218(7) 90.00 2517.30(4) 2 1.306 0.404 1024 13 034 1262 34.32 −15 ≤ h ≤15 −4 ≤ k ≤ 4 −21 ≤ l ≤ 21 99.9 1515/0/372

1088.69 monoclinic P21/c 15.0683(7) 19.5440(10) 9.2955(5) 90.00 103.023(3) 90.00 2667.10(2) 2 1.356 0.476 1142 38 853 2884 50.00 −17 ≤ h ≤17 −22 ≤ k ≤23 −11 ≤ l ≤ 11 100.0 4679/0/357

1057.44 triclinic P1̅ 5.5409(2) 13.2051(4) 17.7936(6) 75.104(2) 81.318(2) 78.586(2) 122 639(7) 1 1.432 0.511 547 13 435 4083 50.00 −6 ≤ h ≤ 6 −15 ≤ k ≤ 15 −20 ≤ l ≤ 21 97.5 4224/0/339

1.382 0.0543 0.0566

1.054 0.0331 0.0388

1.078 0.0304 0.0335

1.111 0.0293 0.0296

1.075 0.0402 0.0514

1.019 0.0584 0.1098

1.027 0.0390 0.403

complete to 2θ (%) data/rstraints/ parameters goof (F2) R indices [I > 2σ(I)] R indices (all data)

3

4

using a Bruker Nonius SMART CCD diffractometer equipped with a graphite monochromator. The SMART software was used for data collection and also for indexing the reflections and determining the unit cell parameters; the collected data were integrated using SAINT software. The structures were solved by direct methods and refined by full-matrix least-squares calculations using SHELXTL software.30 All the non-H atoms were refined in the anisotropic approximation against F2 of all reflections. The H-atoms, except those attached to nitrogen and oxygen atoms, were placed at their calculated positions and refined in the isotropic approximation; those attached to nitrogen and oxygen were located in the difference Fourier maps and refined with isotropic displacement coefficients. Crystallographic data collection was done at room temperature, and the data are tabulated in Table 4. The CCDC numbers of the crystals 1−7 are 854227− 854233. Synthesis and Characterization of Compounds and Their M(II) Complexes. Compound L1H. A solution of phthalic anhydride (0.740 g, 5 mmol) and trans-4-(aminomethyl)cyclohexanecarboxylic acid (0.785 g, 5 mmol) in acetic acid (20 mL) was refluxed for 3 h. The reaction mixture was cooled to room temperature, poured into ice cooled water (50 mL), and stirred for 15 min. A white colored crystalline product was obtained. This was filtered and dried in open air. Yield: 85%. IR (KBr, cm−1): 3406 (w), 2915 (m), 2854 (m), 2528 (w), 1769 (w), 1712 (s), 1596 (s), 1538 (w), 1433 (m), 1399 (s), 1360 (m), 1331(m), 1304 (m), 1249 (m), 1223 (m), 1197 (s), 1158 (m), 1061(m), 983 (w), 805 (m), 772 (m). 1H NMR (400 MHz, CDCl3): 7.82 (dd, 2H, J = 2.8 Hz), 7.69 (dd, 2H, J = 3.2 Hz), 3.52 (d, 2H, J = 6.8 Hz), 2.24 (t, 1H, J = 12.0 Hz), 2.00 (d, 2H, J = 10.8 Hz), 1.78 (d, 3H, J = 6.8 Hz), 1.37 (q, 2H, J = 8.4 Hz), 1.07 (q, 2H, J = 10.4 Hz). 13C NMR (CDCl3): 168.9, 150.4, 145.9, 134.1, 132.2, 123.4, 121.8, 43.9, 43.2, 36.6, 30.0, 28.5. ESI-MS: 288.156 [M + H+]. Compound L2H. A solution of 1,8-naphthalic anhydride (0.990 g, 5 mmol) and trans-4-(aminomethyl)cyclohexanecarboxylic acid (0.785

showed a couple of ligand centered redox processes for undissociated and dissociated ligands.

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EXPERIMENTAL SECTION

General. All reagents and solvents were obtained from commercial sources. The elemental analyses were performed on a Perkin-Elmer PE 2400 II CHN analyzer. The IR spectra were recorded on a PerkinElmer-Spectrum One FT-IR spectrometer with KBr disks in the range 4000−400 cm−1. The 1H NMR and 13C NMR spectra were recorded using a Varian Mercury plus 400 MHz instrument. Chemical shifts were reported in parts per million (ppm) relative to internal TMS (0 ppm). Electrospray ionization mass (ESI-MS) spectra were recorded on a Waters (Micromass MS Technologies) Q-Tof Premier mass spectrometer. Powder X-ray Diffraction data were collected on a Bruker D2 diffractometer in Bragg−Brentano θ−θ geometry with Cu Kα radiation (λ = 1.5418 Å) on a glass surface of an air-dried sample using a secondary curved graphite monochromator. Diffraction patterns were collected over a 2θ range of 5−50° at a scan rate of 2° min−1. Thermogravimetric analyses (TGA) were performed on a Mettler Toledo TGA/SDTA 851e module. Samples were placed in open alumina pans in the temperature range 25−500 °C and were purged with a stream of dry N2 flowing at 100 mL min−1. The UV−vis and solid state fluorescence emission spectra were recorded on a Perkin-Elmer-Lambda 750 UV−vis spectrometer and Perkin-Elmer-LS 55 fluorescence spectrometer, respectively, at room temperature. The cyclic-voltammetric experiments were performed on a CHI660A electrochemical work-station with a three electrodes system. A platinum electrode and glassy carbon electrode were used as auxiliary and working electrodes, respectively, with a Ag/AgCl electrode as the reference electrode. The voltammogram were recorded by dissolving adequate amounts of complexes (10−4 M) in dry DMF. The experiments were performed with tetra butyl ammonium perchlorate as the supporting electrolyte under nitrogen atmosphere. Structure Determination. The X-ray single crystal diffraction data were collected at 296 K with MoKα radiation (λ = 0.71073 Å) 2119



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g, 5 mmol) in N,N-dimethylformamide (15 mL) was refluxed for 5 h. The reaction mixture was cooled to room temperature, poured into ice cooled water (30 mL), and stirred for 15 min. A brown colored precipitate of the product was formed, which was filtered and air-dried. Yield: 82%. IR (KBr, cm−1): 3501 (s), 2946 (s), 2855(m), 1725 (s), 1695 (s), 1651 (s), 1590 (m), 1442 (w), 1389 (w), 1354 (m), 1316 (w), 1255 (m), 1237 (m), 1198 (m), 1176 (m), 1074 (w), 1030 (w), 973 (w), 935 (w), 776 (m). 1H NMR (400 MHz, CDCl3): 8.57 (d, 2H, J = 7.2 Hz), 8.19 (d, 2H, J = 8.0 Hz), 7.73 (d, 2H, J = 7.2 Hz), 4.05 (d, 2H, J = 6.8 Hz), 2.26 (t, 1H, J = 9.2 Hz), 2.00 (d, 2H, J = 13.2 Hz), 1.82 (d, 3H, J = 14.0 Hz), 1.34 (q, 2H, J = 12.0 Hz), 1.18 (q, 2H, J = 11.2 Hz). 13C NMR (CDCl3): 164.8, 134.2, 131.6, 127.2, 122.8, 45.9, 43.0, 36.2, 30.1, 28.4. ESI-MS (m/e): 338.185 [M + H+]. Complex 1. To a solution of L1H (0.144 g, 0.5 mmol) in methanol, manganese(II) acetate tetrahydrate (0.061 g, 0.25 mmol) was added, and the reaction mixture was stirred for about 30 min at room temperature. The precipitate obtained in the reaction mixture was filtered and dissolved in water. Colorless block crystals of the product 1 were obtained after five days. Yield: 65%. Elemental Anal. Calcd. for C32H40N2O12Mn: C, 54.94; H, 5.76; N, 4.00%. Found: C, 54.90; H, 5.70; N, 4.10%. IR (KBr, cm−1): 3270 (w), 2941 (s), 1774 (w), 1712 (s), 1543 (s), 1425 (s), 1397 (s), 1361 (s), 1277 (m), 1155 (w), 1055 (m), 926 (w), 766 (w), 723 (m), 621 (w). Complex 2. Needle-shaped blue crystals of the product 2 were obtained by the reaction of L1H (0.144 g, 0.5 mmol) and copper(II) acetate monohydrate (0.050 g, 0.25 mmol) with the similar procedure as used for 1, with the only difference being its recrystallization solvent (pyridine). Yield: 68%. Elemental Anal. Calcd. for C42H44N4O9Cu: C, 62.10; H, 5.46; N, 6.90%. Found: C, 62.07; H, 5.43; N, 6.96%. IR (KBr, cm−1): 3150 (w), 2926 (s), 2859 (m), 1772 (w), 1705 (s), 1605 (m), 1576 (s), 1446 (w), 1395 (s), 1360 (m), 1052 (m), 931 (w), 723 (m), 692 (w). Complex 3. Colorless good quality crystals of the product 3 were obtained by the reaction of L1H (0.144 g, 0.5 mmol) and zinc(II) acetate dihydrate (0.055 g, 0.25 mmol) with a similar procedure as that used for 2. Yield: 72%. Elemental Anal. Calcd. for C42H46N4O10Zn: C, 60.61; H, 5.57; N, 6.73%. Found: C, 60.54; H, 5.56; N, 6.80%. IR (KBr, cm−1): 3214 (w), 2928 (s), 2859 (m), 1771 (w), 1704 (s), 1553 (m), 1434 (m), 1398 (s), 1361 (m), 1278 (w), 1151 (w), 1055 (w), 935 (w), 719 (m), 531 (w). Complex 4. Brown colored block crystals of the product 4 were obtained by the reaction of L1H (0.144 g, 0.5 mmol) and cadmium(II) acetate dihydrate (0.065 g, 0.25 mmol) with a similar procedure as that used for 2. Yield: 73%. Elemental Anal. Calcd. for C42H46N4O10Cd: C, 57.37; H, 5.27; N, 6.37%. Found: C, 57.32; H, 5.21; N, 6.48%. IR (KBr, cm−1): 3148 (w), 2926 (s), 2858 (m), 1772 (w), 1704 (s), 1562 (s), 1435 (w), 1396 (s), 1361 (m), 1277 (m), 1151 (w), 1054 (w), 934 (w), 720 (m), 625 (w). Complex 5. To a solution of L2H (0.167 g, 0.5 mmol) in N,Ndimethylformamide and ethanol (1:3), cobalt(II) acetate dihydrate (0.062 g, 0.25 mmol) was added. After stirring this reaction mixture for about 30 min at room temperature, a brown color precipitate was obtained in the reaction mixture, which was filtered and dissolved in pyridine. Needle-shaped crystals of the product 5 were obtained from the pyridine solution. Yield: 55%. Elemental Anal. Calcd. for C55H55N5O10Co: C, 65.73; H, 5.52; N, 6.97%. Found: C, 65.21; H, 5.16; N, 7.22%. IR (KBr, cm−1): 3408 (w), 2922 (s), 2852 (m), 1698 (m), 1657 (s), 1589 (s), 1555 (m), 1440 (s), 1415 (s), 1385 (s), 1346 (s), 1284 (w), 1237 (m), 1177 (w), 1070 (w), 936 (w), 779 (m), 699 (w). Complex 6. Needle-shaped blue crystals of the product 6 were obtained by the reaction of L2H (0.167 g, 0.5 mmol) and copper(II) acetate monohydrate (0.050 g, 0.25 mmol) with a similar procedure as that used for 5. Yield: 58%. Elemental Anal. Calcd. for C60H60N6O10Cu: C, 66.19; H, 5.55; N, 7.72%. Found: C, 66.03; H, 5.42; N, 7.90%. IR (KBr, cm−1): 3418 (w), 2924 (s), 2853 (m), 1698 (m), 1659 (s), 1589 (s), 1490 (s), 1386 (s), 1344 (s), 1281 (w), 1237 (m), 1178 (w), 1073 (w), 933 (w), 819 (w), 779 (m), 698 (w). Complex 7. Brown colored block crystals of the product 7 were obtained by the reaction of L2H (0.167 g, 0.5 mmol) and

cadmium(II) acetate dihydrate (0.065 g, 0.25 mmol) with a similar procedure as that used for 5. Yield: 60%. Elemental Anal. Calcd. for C55H55N5O10Cd: C, 62.41; H, 5.24; N, 6.62%. Found: C, 62.35; H, 5.28; N, 6.77%. IR (KBr, cm−1): 3434 (w), 2924 (s), 2853 (m), 1698 (m), 1656 (s), 1588 (m), 1537 (s), 1438 (s), 1385 (s), 1349 (s), 1284 (w), 1238 (m), 1177 (w), 1071 (w), 936 (w), 779 (m), 698 (w).

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ASSOCIATED CONTENT

S Supporting Information *

CIF files, tables of bond length and angles, simulated and experimental PXRD patterns, TGA curves, UV−vis spectra, and cyclic voltammogram. 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]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Department of Science and Technology, New Delhi (India) for financial support. D.S. is thankful to the Council of Scientific and Industrial Research, New Delhi (India) for a senior research fellowship.

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REFERENCES

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