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Keywords:

  • Hydrogen bonding;
  • Channels;
  • Water chains;
  • X-ray diffraction;
  • Fluorescence

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

Four complexes with supramolecular architectures, namely, MZCA·3H2O (1), [Zn(H2O)6]2+·[MZCA]2·[H2O]6 (2), [Mn(MZCA)2(H2O)4]·2H2O (3), and [Ni(MZCA)2(H2O)4]·2H2O (4) [MZCA = 3-(carboxymethyl)-2, 7-dimethyl-3H-benzo[d]imidazole-5-carboxylic acid], were synthesized and characterized by elemental analysis, IR spectroscopy, and single-crystal X-ray diffraction. Complexes 1 and 2 display a remarkable 3D network with 1D hydrophilic channels. Complexes 3 and 4 are isostructural and exhibit a 3D structure encapsulating 1D 24-membered ring microporous channels. The UV/Vis and fluorescent spectra were measured to characterize complexes 14. The thermal stability of complexes 24 were also examined.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

The rational design of microporous solids based on supramolecular architectures have attracted considerable interest in recent years, mainly due to the potential use of their resulting cavities and channels in nanotechnology, including shape- and size-selective molecular recognition,1 separation,2 ion exchange,3 storage, adsorption,4 catalysis,5 electronic and magnetic properties.6 However, the control of crystal and network structures in a predictable manner still remains an elusive task because of the delicate balance and competition between directional noncovalent interactions such as hydrogen bonds and nondirectional noncovalent interactions such as van der Waals (dispersive) packing forces. The formation of hydrogen-bonded and metal coordination networks with large cavities often results in self-interpenetration to fill the voids in the initial host structure.7

The understanding of different molecular interactions plays a major role in supramolecular chemistry and crystal engineering.8 Hydrogen bonding and π–π stacking interactions are by far the most well-studied interactions.9 They are employed to control the conformational and topological features of the molecular assembly in the solid state.10 Many scientists have been involved in the analysis of weak interactions focusing on benzimidazole derivatives due to their relevance to DNA, proteins, and other biological systems.11 During the last decade, the structure of benzimidazole carboxylate was widely used in the design of therapeutic agents, such as diuretic and natriuretic,12 antiparasitic, serotonin antagonist, antineoplastic and antiflarial,13 herbicidal, and antihypertensive compounds.14 The analysis of various interactions in drugs has attracted considerable interest for their wide-ranging antiviral activity and the possibility of forming supramolecular aggregates with transition metal ions. However, reports on the coordination behavior of benzimidazole carboxylate compounds remain scarce.15 To the best of our knowledge, supramolecular frameworks constructed by 3-(carboxymethyl)-2, 7-dimethyl-3H-benzo[d]imidazole-5-carboxylic acid (MZCA) have not yet been reported.

From a structural point of view, MZCA has three interesting characteristics: (i) the nitrogen atom in benzimidazole ring and the oxygen atoms in carboxylate group are both potential sites of hydrogen bonding interactions, given that the presence of solvent molecules, such as water or methanol, might serve as possible hydrogen-bond donors;16 (ii) due to the benzimidazole rings, we can predict that the π–π interactions might devote to the formation of crystal structures;17 (iii) the relatively large π-conjugated system in benzimidazole ring might contribute much to the desirable fluorescence property.

Herein, we report the syntheses and structures of four new compounds MZCA·3H2O (1), [Zn(H2O)6]2+·[MZCA]2·[H2O]6 (2), [Mn(MZCA)2(H2O)4]·2H2O (3), and [Ni(MZCA)2(H2O)4]·2H2O (4) [MZCA = 3-(carboxymethyl)-2, 7-dimethyl-3H-benzo[d] imidazole-5-carboxylic acid]. Complexes 1 and 2 both have remarkable hydrogen-bonded 3D networks encapsulating 1D hydrophilic channels. Complexes 3 and 4 are isostructural and both have unusual hydrogen-bonded 3D networks encapsulating 1D 24-membered ring microporous channels.

Results and Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

MZCA·3H2O (1)

The overall structure of the crystal for 1 with atom labeling is shown in Figure 1. One carboxylic group [O(1), C(1), O(2)] of MZCA is coplanar with the molecular plane: the dihedral angle between the benzimidazole ring and the O(1)/C(1)/O(2) plane is 3.3(4)°. The other carboxylic group [O(3), C(12), O(4)] is almost perpendicular to the molecular plane: the dihedral angle between the benzimidazole ring and the O(3)/C(12)/O(4) plane is 72.8(4)°. In the structure of MZCA, one carboxylic acid group is deprotonated and the nitrogen atom is protonated. Therefore, MZCA is in the inner-salt form. A similar feature has been reported in the complex N-(2-oxopyrrolidin-1-ylmethyl)-L-valine·2H2O.18

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Figure 1. Molecular structure of 1 showing 30 % probability displacement ellipsoids and the atom-numbering scheme.

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Hydrogen bond lengths and angles are listed in Table 1. As shown in Figure 2a, the hydrogen bonds [O(5)–H(5E)···O(3) = 2.690(4) Å, O(2)–H(2)···O(5) = 2.548(4) Å] bridge molecules forming a 1D helical chain along the b axis, which is interconnected by the hydrogen bonds [N(2)–H(2A)···O(4) = 2.702(3) Å] and π–π stacking interactions, giving rise to a 2D helical architecture. Furthermore, the 2D structure is extended through the water molecules into a 3D network with 1D hydrophilic channels (Figure 2b), in which water chains containing acyclic water hexamers are observed. Each water chain consists of acyclic water hexamers, interlinked by hydrogen bonds [O(5)–H(5D)···O(6) = 2.666(4) Å, O(6)–H(6A)···O(7) = 2.760(5) Å, and O(7)–H(7A)···O(7′) = 2.886(10) Å] and short contacts [O(6)···O(6′) = 2.590(5) Å]. In the water hexamer, the average O···O distance is 2.771 Å. This distance is slightly longer than the corresponding values in ice Ih (2.759 Å)19 and the calculated value of 2.718 Å for cyclic water hexamer and comparable to that in water trapped in organic compound of DMNY·2.5H2O (2.776 Å, DMNY = 2, 4-bimethyl-5-aminobenzo[b]-1, 8-naphthyridine).20 However, it is shorter than that observed in the metal-organic framework of [Pr(pdc)-(Hpdc)(H2O)2]·4H2O (2.783 Å, pdc = pyridine-2, 6-dicarboxylic acid)21 and in liquid water (2.854 Å) and the ring water hexamer with a planar arrangement (2.905 Å).22 The water molecules O(6) and O(7) in the cluster are in a tetrahedral arrangement with three water–water interactions and one water–carboxylate interaction. Meanwhile, the water molecule O(5) involves one water–water interaction and two water–carboxylate interactions. Therefore, the water molecule O(5) acts as single-hydrogen-bond donor, the water molecule O(6) acts as single-hydrogen-bond donor and single-hydrogen-bond acceptor, and the water molecule O(7) functions as single-hydrogen-bond donor and double-hydrogen-bond acceptors in the water hexamer (Figure 2c).

Table 1. Hydrogen bonding parameters /Å,° for compounds 14.
 D–H···Ad(D–H)d(H···A)d(D···A)[ang](D–H···A)
1 a)O(2)–H(2)···O(5)0.821.752.548(4)165
 O(5)–H(5D)···O(6)i0.882.042.666(4)127
 O(5)–H(5E)···O(3)ii0.861.842.690(4)171
 O(6)–H(6A)···O(7)iii0.881.932.760(5)155
 O(6)–H(6B)···O(4)0.861.872.681(5)157
 O(7)–H(7A)···O(7)iv0.852.272.886(10)129
 O(7)–H(7B)···O(1)v0.892.212.874(4)130
 N(2)–H(2A)···O(4)vi0.861.842.702(3)177
2 b)OW(1)–HW(1)···O(1)0.761.932.655(3)159
 OW(2)–HW(2A)···O(2)0.861.872.708(4)164
 OQ(1)–HQ(1A)···O(3)0.751.992.732(4)174
 OQ(2)–HQ(2B)···O(3)0.822.022.795(4)158
 OQ(3)–HQ(3C)···O(2)0.852.072.914(4)177
 OW(1)–HW(1B)···O(1)i0.801.902.685(3)165
 OW(2)–HW(2B)···OQ(3)ii0.851.872.720(4)171
 OW(3)–HW(3A)···OQ(2)iii0.812.182.947(4)157
 OW(3)–HW(3B)···O (4)iv0.841.872.701(4)170
 OQ(1) –HQ(1B)···OW(1)v0.822.092.899(4)167
 OQ(2) –HQ(2A)···OQ(1)vi0.881.802.674(4)170
 OQ(3) –HQ(3B)···O(4)vii0.851.862.709(5)177
 N(2) –HN(2)···OQ(2)viii0.861.912.768(4)172
3 c)N(2)–H(2A)···O(7)i0.891.772.639(3)164
 O(5)–H(5D)···O(4)i0.851.902.728(3)163
 O(5)–H(5E)···O(4)ii0.851.862.680(3)161
 O(6)–H(6A)···O(3)iii0.862.012.826(3)159
 O(6)–H(6B)···O(2)0.861.872.723(3)170
 O(7)–H(7A)···O(3)iii0.851.852.673(3)164
 O(7)–H(7B)···O(2)iv0.851.962.793(3)166
4 d)OW(1)–HW(1A)···O(4)i0.961.782.737(3)173
 OW(1)–HW(1B)···O(4)ii0.891.802.684(3)170
 OW(2)–HW(2A)···O(2)0.861.982.716(3)143
 OW(2)–HW(2B)···O(3)iii0.871.982.846(3)170
 OW(3)–HW(3A)···O(3)iv0.901.812.680(3)161
 OW(3)–HW(3B)···O(2)v0.991.812.787(3)168
 N(1)–HN(1)···OW(3)0.921.762.630(3)157
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Figure 2. (a) Projection of the 2D structure of 1. Hydrogen atoms not involved in hydrogen bonding are omitted for clarity and hydrogen bonds are shown as dashed lines. (b) Space-filling diagram shows the hydrogen-bonded 3D network of 1 viewed along the c axis, with water molecules and all hydrogen atoms are omitted for clarity. (c) Projection of the 1D water chain and its coordination environment of 1. Dashed lines indicate hydrogen bonds.

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[Zn(H2O)6]2+·[MZCA]2·[H2O]6 (2)

The asymmetric unit of complex 2 contains one MZCA molecule, half a ZnII, three coordinated water molecules, and three lattice water molecules (Figure 3). The central zinc atom is hexacoordinated by six oxygen atoms from water molecules with bond length of 2.067(2)–2.129(2) Å, MZCA molecules connect zinc atoms by the hydrogen bonds between the carboxylic oxygen atoms from MZCA molecules and oxygen atoms from water molecules. Each MZCA molecule loses two protons from the carboxylic acid group and one of the two protons transfers to the nitrogen atom. Therefore, each MZCA molecule provides one negative charge. The dihedral angles between the benzimidazole ring and the carboxylic groups are 9.9(4)° for O(3)/C(11)/O(4) and 82.0(4)° for O(1)/C(10)/O(2), respectively, these values are larger than those observed in 1.

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Figure 3. Asymmetric unit of complex 2 showing 30 % probability displacement ellipsoids and the atom-numbering scheme.

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As shown in Figure 4a, one [Zn(H2O)6]2+ cation is attached to the other [Zn(H2O)6]2+ cation by hydrogen bonds with the water molecules as bridges [OW(3)–HW(3A)···OQ(2) = 2.947(4) Å, OQ(2)–HQ(2A)···OQ(1) = 2.674(4) Å, OQ(1)–HQ(1B)···OW(1) = 2.899(4) Å], forming a 1D linear metal–water chain along the a axis with the intermolecular Zn···Zn distance is 7.566 Å. Interestingly, the metal–water chain consists of the acyclic (H2O)4 cluster and the [Zn(H2O)4]2+ cation. This conformation contrasts with that in the {[Cd(dpp)(sip)(H2O)3]·0.5Cd(H2O)6·5H2O}n complex [dpp = 1, 3-bis(4-pyridyl)propane and NaH2sip = 5-sulfoisophthalic acid monosodium salt], in which the infinite chain consists of the water decamer and [Cd(H2O)4]2+.23

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Figure 4. (a) Projection of the 1D linear metal-water chain formed by [Zn (H2O)6]2+ cations in 2 viewed along the a axis. Hydrogen atoms not involved in hydrogen bonding are omitted for clarity and hydrogen bonds are shown as dashed lines. (b) Projection of the 2D network formed by MZCA molecules in 2. Hydrogen atoms not involved in hydrogen bonding are omitted for clarity and hydrogen bonds are shown as dashed lines. (c) Space-filling diagram shows the hydrogen-bonded 3D network of 2 viewed along the b axis, with [Zn(H2O)6]2+ cations, water molecules and all hydrogen atoms are omitted for clarity.

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As shown in Figure 4b, one MZCA molecule is attached to the other molecule via hydrogen bonds with the water molecules OQ(2) as bridges [OQ(2)–HQ(2B)···O(3) = 2.795(4) Å, N(2)–HN(2)···OQ(2) = 2.768(4) Å], forming a 1D chain along the b axis, which is further interlinked through water molecules OQ(1) by hydrogen bonds [OQ(1)–HQ(1A)···O(3) = 2.732(4) Å, OQ(2)–HQ(2A)···OQ(1) = 2.674(4) Å] and form a graph-set motif of R64(12). These chains are interconnected by water molecules OQ(3) via hydrogen bonds [OQ(3)–HQ(3A)···O(2) = 2.914(4) Å, OQ(3)–HQ(3B)···O(4) = 2.709(5) Å] and π–π stacking interactions, giving rise to a 2D structure, which are further interlinked to the [Zn(H2O)6]2+ cations by hydrogen bonds [OW(3)–HW(3B)···O(4) = 2.701(4) Å, OQ(1)–HQ(1B)···OW(1) = 2.899(4) Å, OW(2)–HW(2A)···O(2) = 2.708(4) Å] into a 3D network (Figure 4c). Interestingly, a hydrophilic channel is observed, being enclosed by the MZCA molecules. The [Zn(H2O)6]2+ cations and water molecules OQ(3) are accommodated in each channel.

[Mn(MZCA)2(H2O)4]·2H2O (3) and [Ni(MZCA)2(H2O)4]·2H2O (4)

X-ray diffraction revealed that complexes 3 and 4 are isostructural. Both structures contain two MZCA molecules, one metal atom, four coordinated water molecules, and two lattice water molecules. Two MZCA molecules are almost perpendicular to each other; the dihedral angles between two benzimidazole rings are 69.43(8) and 68.87(8)° for 3 and 4, respectively. The coordination arrangement around the metal ion, comprising of two oxygen atoms belonging to two monodentate carboxylate groups from two MZCA located at the axial positions, and four oxygen atoms from water molecules located at the equatorial plane, can be described as a distorted octahedral. The average M–O bond lengths are 2.179 and 2.068 Å for 3 and 4, respectively. The overall structure of the crystal for 3 is shown in Figure 5. Similar to 2, each MZCA provides one negative charge. One carboxylic group [O(1), C(1), O(2)] of MZCA is coplanar with the benzimidazole ring: the dihedral angles between the benzimidazole ring and the O(1)/C(1)/O(2) plane are 3.4(4)° for 3 and 1.8(3)° for 4, these values are similar to those observed in 1, and smaller than those observed in 2. The other carboxylic group [O(3), C(12), O(4)] is almost perpendicular to the benzimidazole ring: the dihedral angles between the benzimidazole ring and the O(3)/C(12)/O(4) plane are 87.9(4)° for 3 and 88.1(4)° for 4, these values are larger than those observed in 1 and 2.

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Figure 5. Molecular structure of 3 showing 30 % probability displacement ellipsoids and the atom-numbering scheme. Symmetry code (A): –x, y, –z+1/2.

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As shown in Figure 6a, the molecules are connected to each other by hydrogen bonds with distances of O(5)–H(5D)···O(4) = 2.728(3) Å and O(6)–H(6A)···O(3) = 2.826(3) Å, resulting in 1D ribbons along the b axis and forming a graph-set motif of R22(8). On the other hand, there exist π–π stacking interactions in complexes 3 and 4 with distances of 3.497 and 3.482 Å, respectively (Figure S1, Supporting Information). Hydrogen bonds [O(7)–H(7B)···O(2) = 2.793(3) Å, N(2)–H(2A)···O(7) = 2.639 (3) Å] and π–π stacking interactions bridge molecules forming 1D chains along the a axis, and further interconnect the ribbons into 2D layer structures (Figure 6b), in which the grid-like 22-membered ring (22-MR) cavities is observed.

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Figure 6. (a) Projection of the 1D ribbon of 3 viewed along the b axis. Hydrogen atoms not involved in hydrogen bonding are omitted for clarity and hydrogen bonds are shown as dashed lines. (b) Projection of the 2D layer structure of 3 viewed along the b axis. Hydrogen atoms not involved in hydrogen bonding are omitted for clarity and hydrogen bonds are shown as dashed lines. (c) Projection of the 2D network of 3 viewed along the c axis, with all hydrogen atoms are omitted for clarity. Dashed lines indicate hydrogen bonds. (d) View of 3D network of 3 with 1D channel down the c axis.

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As shown in Figure 6c, hydrogen bonds [O(7)–H(7A)···O(3) = 2.673(3) Å, O(7)–H(7B)···O(2) = 2.793(3) Å, O(5)–H(5D)···O(4) = 2.728(3) Å] bridge molecules forming a 2D architecture, and form a graph-set motif of R86(24) which shares the C–O and M–O bond. The 2D structure is further extended into a 3D supramolecular network by hydrogen bonds [N(2)–H(2A)···O(7) = 2.639(3) Å, O(6)–H(6A)···O(3) = 2.826(3) Å, O(5)–H(5E)···O(4) = 2.680(3) Å]. The interesting feature, a 24-MR microporous channel running along the c axis, is observed in the 3D network (Figure 6d).

UV/Vis and Fluorescence Spectroscopy

The UV/Vis spectra of MZCA and complexes 24 in CH3OH were measured at room temperature (Figure S2, Supporting Information). MZCA shows a broad absorption at 270 nm. Because the absorption bands of complexes 24 are similar to that of the free MZCA ligand with a slight bathochromic shift, it should be assigned to the π[RIGHTWARDS ARROW]π* transition of the MZCA ligand.

The fluorescence properties of MZCA and complexes 24 in CH3OH were measured at room temperature (Figure S3, Supporting Information). It can be observed that the free MZCA has an emission at 335 nm when excited at 278 nm. In contrast, complexes 24 exhibit an intense broad emission peak at 341 nm when excited at 277, 280, and 281nm, respectively. Generally, the intraligand fluorescence emission wavelength is determined by the energy gap between the π and π* molecular orbitals of the free ligand, which is simply related to the extent of conjugation in the system. As for the metal complexes, the bathochromic shift of the band in contrast to that of ancillary ligand should be attributed to the intraligand (π–π*) fluorescent emission.

PXRD and Thermogravimetric Analysis

To confirm the phase purity of the complexes, the original samples are characterized by X-ray powder diffraction (PXRD) at room temperature. The peak positions simulated from the single-crystal X-ray data of complexes are in good agreement with those observed (Figure S4–S7, Supporting Information).

In order to examine the thermal stability of the complexes, thermal gravimetric (TG) analyses are carried out for 24 (Figure S8, Supporting Information). The TG curve of complex 2 exhibits two weight loss stages. The first weight loss of 27.2 % (calcd. 27.8 %) from 30 to 165 °C corresponds to the loss of six lattice water molecules and six coordinated water molecules. The residue remains stable up to 298 °C and then upon further heating decomposes to unidentified products. The TG curve of complex 3 exhibits two main weight loss steps. The first with a value of 16.2 % (calcd. 16.4 %) from 30 to 86 °C corresponds to the loss of two lattice water molecules and four coordinated water molecules. The dehydrated compound remains stable up to 313 °C and then upon further heating decomposes to unidentified products. The thermogravimetric studies of complex 4 indicate two main steps of weight losses. The first step starts at approximately 100 °C and completes at approximately 210 °C. The observed weight loss of 16 % is corresponding to the loss of two lattice water molecules and four coordinated water molecules (calcd. 16.3 %). The dehydrated compound remains stable up to 300 °C and then upon further heating decomposes to unidentified products.

Conclusions

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

In this work, the ligand 3-(carboxymethyl)-2, 7-dimethyl-3H-benzo[d]imidazole-5-carboxylic acid (MZCA) was employed to construct coordination polymers with ZnII, MnII, and NiII in solutions. The MZCA molecules in 1 form 1D helical chains through hydrogen bonds. These chains are further extended through the water molecules and π–π stacking interactions into 3D networks with 1D hydrophilic channels. The MZCA anions in 2 form interesting 2D network by hydrogen bonds and π–π stacking interactions, which are further interlinked to the [Zn(H2O)6]2+ cations by hydrogen bonds into three dimensions with 1D hydrophilic channels. Complexes 3 and 4 are isostructural. The M(MZCA)2(H2O)4 (M = Mn, Ni) molecules in complexes 3 and 4 form 1D ribbons, which are further extended through hydrogen bonds and π–π stacking interactions into a 3D network containing a 24-membered ring microporous channel.

Experimental Section

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

Materials and General Methods: The ligand 3-(carboxymethyl)-2, 7-dimethyl-3H-benzo[d]imidazole-5-carboxylic acid (MZCA) was prepared according to the pathway shown in Scheme 1. Other commercially available chemicals were of analytical grade and were used without further purification. Powder X-ray diffraction measurements were carried out with a Bruker D8 Focus X-ray diffractometer to check phase purity. Elemental analyses (C, H, N) were determined with an Elementar vario EL elemental analyzer. UV/Vis spectra were measured with a GBC Cintra 10e UV/Vis spectrophotometer in MeOH solution. Photoluminescence analyses were performed with a RF-5301PC fluorescence spectrometer in MeOH solution. Thermogravimetric analyses were conducted on a ZRY-2P Thermal Analyses using a heating rate of 20 K·min–1 from room temperature to 800 °C under nitrogen. The IR spectra were recorded with a Nicolet-AVATAR 360 FT-IR spectrometer using KBr pellets in the 4000–400 cm–1 regions. 1H NMR spectra were recorded with a Varian 500 Bruker spectrometer in [D6]DMSO or CDCl3.

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Scheme 1. Synthetic pathway for MZCA.

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Synthesis of MZCA: To a solution of 1a (70 g, 0.386 mol) in CH3OH (200 mL) was slowly added concentrated H2SO4 (20 mL). The solution was heated to reflux for 24 h and cooled to room temperature, to yield a yellow precipitate. Intermediate 2a was obtained by filtration and crystallization from methanol (54.7 g, 72.6 %). M.p: 78–79 °C. IR (KBr): equation image = 3005 (br), 1740 (m), 1620 (m), 1590 (m), 1520 (m), 1350 (m) cm–1. 1H NMR (CDCl3, 500 MHz): δ = 8.03(br., 1 H, Ar–H), 7.9(m, 2 H, Ar–H), 3.97(s, 3 H, CH3O), 2.63(s, 3 H, PhCH3).

Pd/C (0.4 g, 5 %) was added to a methanol solution (150 mL) of 2a (7 g, 35.9 mmol) under hydrogen. The mixture was stirred at room temperature for 6 h and afterwards passed through Celite. The filtrate was concentrated to give a brown solid of 3a (5.286g, 89.1 %). M.p:188–121 °C. IR (KBr): equation image = 3452 (br), 3370 (m), 1686 (m), 1597 (m) cm–1. 1H NMR (CDCl3, 500 MHz): δ = 7.76(s, 1 H, Ar–H), 7.74(d, 3JH, H = 8.21Hz, 1 H, Ar–H), 6.64 (d, 3JH, H = 8.20Hz, 1 H, Ar–H), 4.15(br., 2 H, NH2), 3.85(s, 3 H, CH3O), 2.17(s, 3 H, PhCH3).

Acetyl chloride (12 mL) was added dropwise to a solution of 3a (10 g, 60.5 mmol) in CH2Cl2 (100 mL) and Et3N (22 mL). The solution was allowed to cool to 0–8 °C during the addition, and stirred at r t for 5 h. After filtration, the mixture was washed with water, NaHCO3 aqueous solution, water, dried (Na2SO4), and the solvent was removed in vacuo to afford the crude product, which was recrystallized from EtOAc to give a white solid of 4a (9.804 g, 78.2 %). M.p:130–133 °C. IR (KBr): equation image = 3289 (br), 2941 (m), 1719 (m), 1656 (m), 1527 (m), 891 (m) cm–1. 1H NMR ([D6]DMSO, 500 MHz): δ = 9.31(s, 1 H, NH), 7.80(s, 1 H, Ar–H), 7.77 (d, 2 H, 3JH, H = 9.60Hz, Ar–H), 3.83 (s, 3 H, –COOCH3), 2.28(s, 3 H, –COCH3), 2.12 (s, 3 H, Ar–CH3).

To a solution of fuming HNO3 (10.2 mL) at –15 °C was slowly added 4a (3 g, 14.5 mmol). After stirring for 1 h, the mixture was poured into ice water to yield a white precipitate, which was filtered and washed with ice water, NaHCO3 aqueous solution, and water, and dried to afford the crude product. This was recrystallized from EtOAc to give a white solid of 5a (2.29 g, 62.6 %). m.p:174–176 °C. IR (KBr): equation image = 3259 (br), 2953 (m), 1723 (s), 1667 (m), 1579 (m), 1534 (m), 1512 (m) cm–1. 1H NMR ([D6]DMSO, 500 MHz): δ = 10.13 ( s, 1 H, NH), 8.17(s, 1 H, Ar–H), 8.15(s, 1 H, Ar–H), 3.89(s, 3 H, –COOCH3), 2.38(s, 3 H, –NHCOCH3), 2.09(s, 3 H, Ar–CH3).

Pd/C (0.1 g, 5 %) was added to the methanol solution (20 mL) of 5a (1.35 g, 5.4 mmol) under hydrogen. The mixture was stirred at room temperature for 7 h. The solution was passed through Celite. The filtrate was concentrated to give a white solid of 6a (0.85 g, 71.5 %). IR (KBr): equation image = 3415 (br), 3244 (m), 2948 (m), 1704 (m), 1649 (m), 1512 (m) cm–1.

To a solution of glacial AcOH (30 mL) was added 6a (3 g, 13.5 mmol), the mixture was heated to reflux for 2 h and afterwards cooled to room temperature. The solvent was removed under reduced pressure to yield a yellow oil, which was dissolved in water (10 mL). Ammonia was added until a pH of about 9 to yield a white precipitate, which was filtered and dried to afford the product of 7a (1.7 g, 61.6 %). M.p:174–176 °C. Elemental analysis for C11H12O2N2: calcd. C 64.69; H 5.92; N 13.72 %; found: C 64.71; H 5.89; N 14.73 %. IR (KBr): equation image = 3422 (br), 3282 (br), 1727 (m), 1682 (m), 1616 (m) cm–1. 1H NMR ([D6]DMSO, 500 MHz): δ = :7.96(s, 1 H, Ar–H), 8.10(s, 1 H, Ar–H), 3.96(s, 1 H, –COOCH3), 2.70(s, 3 H, –NHCOCH3), 2.63(s, 3 H, –Ar–CH3).

To a solution of 7a (0.78 g, 3.819 mmol) in THF (50 mL) at 0 °C was added NaH (0.6 g, 25 mmol). The mixture was stirred at room temperature for 1 h, afterwards BrCH2COOEt (0.8 mL) was added and stirring was continued for 2 h. The solvent was removed under reduced pressure to give a white solid, which was purified by column chromatography (silica; petroleum ether/ethyl acetate; 20:1) to afford 8a (0.77 g, 69.4 %). M.p: 104–107 °C. IR (KBr): equation image = 3392 (br), 1734 (s), 1701 (s), 1520 (m), 1424 (m), 1350 (m), 1279 (m), 1217 (m), 1021 (m), 766 (m) cm–1. 1H NMR ([D6]DMSO, 500 MHz): δ = 7.84(s, 1 H, Ar–H), 7.81(s, 1 H, Ar–H), 4.88(s, 2 H, –CH2–COOC2H5), 4.27 (q, 2 H, 3JH, H = 7.10Hz, –COOCH2CH3), 3.96 (s, 3 H, –COOCH3), 2.69 (s, 3 H, imidazole–CH3), 2.65(s, 3 H, Ar–CH3), 1.31(t, 3 H, 3JH, H = 7.10Hz, –COOCH2CH3).

To a solution of 8a (0.72 g, 2.48 mmol) in CH3OH (20 mL) was added 10 % NaOH (20 mL). The mixture was heated to reflux for 3 h and next cooled to room temperature. Water (10 mL) was added followed by HCl (0.5 M) until the solution reached pH 7–8 to yield a white precipitate, which was filtered and dried to afford the product of MZCA (0.527 g, 85.6 %). Elemental analysis for C12H12N2O4: calcd. C 58.06; H 4.87; N 11.28 %; found: C 58.03; H 4.89; N 11.31 %. IR (KBr): equation image = 3407 (br), 1738 (m), 1686 (s), 1557 (m), 1516 (m), 1427 (s), 1346 (m), 1276 (m), 1209 (s) and 769 (m) cm–1. 1H NMR ([D6]DMSO, 500 MHz): δ = 13.32(br., 1 H, Ar–COOH), 11.67 (br., 1 H, –CH2–COOH), 7.93 (s, 2 H, Ar–H), 7.62 (s, 1 H, Ar–H), 5.17 (s, 2 H, –CH2–COOH), 2.52 (s, 3 H, Ar–CH3).

Synthesis of MZCA·3H2O (1): A water solution (3 mL) of Zn(NO3)2·6H2O (0.0297 g, 0.1 mmol) was added dropwise to CH2Cl2–CH3OH (1:4) solution (5 mL) of MZCA (0.0248 g, 0.1 mmol). The resulting mixture was filtered and colorless block-shaped crystals of 1 were obtained from the filtrate at room temperature after a few days in 73 % Yield (based on MZCA). Elemental analysis for C12H18N2O7: calcd. C 47.68; H 6.00; N 9.27 %; found: C 47.66; H 5.97; N 9.31 %. IR (KBr): equation image = 3420 (w), 2925 (w), 2533 (br), 1707 (s), 1649 (s), 1625 (s), 1363 (m), 1268 (s), 1220 (m), 1174 (s), 771 (m) cm–1.

Synthesis of [Zn(H2O)6]2+·[MZCA]2·[H2O]6 (2): A solution of Zn(NO3)2·6H2O (0.0297 g, 0.1 mmol) in CH3OH–H2O (1:1, 2 mL) was added dropwise to a CH2Cl2–CH3OH (1:1, 8 mL) solution of MZCA (0.0248 g, 0.1 mmol). The pH of the resulting mixture was adjusted to 4.5 with a CH3OH solution (2 mL) of pyridine (8 μL). The reaction mixture was filtered and the colorless block-shaped crystals of 2 were obtained from the filtrate at room temperature after a few days in 35 % Yield (based on MZCA). Elemental analysis for C24H46N4O20Zn: calcd. C 37.15; H 5.97; N 7.22 %; found: C 37.11; H 5.94; N 7.26 %. IR (KBr): equation image = 3448 (br), 1637 (m), 1569 (w), 1384 (vs), 1323 (w), 792 (v) cm–1.

Synthesis of [Mn(MZCA)2(H2O)4]·2H2O (3): To a CH3CH2OH solution (8 mL) of MZCA (0.0248 g, 0.10 mmol), CH3OH–H2O (1:1) solutions (10 mL) of MnSO4 (0.0151 g, 0.10 mmol) and Mg(ClO4)2 (0.0446 g, 0.20 mmol) were added dropwise with constant stirring. The pH of the mixture was adjusted to 5 with a 0.1 M NaOH solution, the resulting mixture was stirred at room temperature for 0.5 h. The reaction mixture was filtered and the colorless block-shaped crystals of 3 were obtained from the filtrate at room temperature after a few days in 65 % Yield (based on MZCA). Elemental analysis for C24H34MnN4O14: calcd. C 43.84; H 5.21; N 8.52 %; found: C 43.81; H 5.19; N 8.56 %. IR (KBr): equation image = 3335 (br), 1622 (s), 1567 (m), 1396 (s), 1313 (m), 794 (m) cm–1.

Synthesis of [Ni(MZCA)2(H2O)4]·2H2O (4): A solution of NiCl2·6H2O (0.0238 g, 0.1 mmol) in CH3CH2OH–H2O (2:1, 6 mL) was added dropwise with constant stirring to a CH3CH2OH–H2O (2:1, 6 mL) solution of MZCA (0.0248 g, 0.1 mmol). The pH of the mixture was adjusted to 5–6 with a CH3OH solution (2 mL) of pyridine (16 μL), The resulting mixture was heated at reflux for 1 h, cooled to room temperature, and stirred at room temperature for 4 h. Afterwards the reaction mixture was filtered and the green block-shaped crystals of 4 were obtained from the filtrate at room temperature after a few days in 62 % Yield (based on MZCA). Elemental analysis for C24H34N4NiO14: calcd. C 43.59; H 5.18; N 8.47 %; found: C 43.56; H 5.16; N 8.51 %. IR (KBr): equation image = 3424 (br), 1630 (m), 1568 (m), 1396 (m), 1313 (w), 793 (m) cm–1.

X-ray Crystallography: Crystal diffraction intensities for complexes 1 and 3 were collected with a Bruker SMART 1000 CCD diffractometer with graphite monochromatized Mo-Kα radiation (λ = 0.71073 Å) at 294(2) K. Semi-empirical absorption corrections were applied using the SADABS program. The structures were solved by direct methods and refined by full-matrix least-squares on F2 using the SHELXS-97 and SHELXL-97 programs.24 All non-hydrogen atoms were refined anisotropically. Hydrogen atoms bound to carbon atoms were placed in calculated positions and allowed to ride at distances of 0.93 Å (C–H aromatic), 0.96 Å (CH3) and 0.97 Å (CH2); whereas those of the aqua molecules were first located in difference Fourier maps, and then fixed at the calculated positions and included in the final refinement. Crystal diffraction intensities for complexes 2 and 4 were collected with a Rapid-AUTO diffractometer with graphite monochromatized Mo-Kα radiation (λ = 0.71073 Å) at 293(2) K. Abscor-Empirical Absorption Corrections were based on Fourier Series Approximation. The structures were solved by direct methods and refined by full-matrix least-squares on F2 using the SHELXS-97 and SHELXL-97 programs. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms bound to carbon atoms were placed in calculated positions and allowed to ride at distances of 0.93 Å (C–H aromatic), 0.96 Å (CH3) and 0.97 Å (CH2); whereas those of the aqua molecules were first located in difference Fourier maps, and then fixed at the calculated positions and included in the final refinement. The crystallography details for complexes 14 are presented in Table 2. Selected bond lengths and angles are listed in Tables S1–S4 (Supporting Information).

Table 2. Crystal data and details of structure determination of compounds 14.
 1234
  1. a

    a) R1 = Σ||Fo|–|Fc||/Σ|Fo|. b) wR2 = {Σ[w(Fo2Fc2)2]/Σ[w(Fo2)2]}1/2.

Empirical formulaC12H18N2O7C24H46N4O20ZnC24H34MnN4O14C24H34N4NiO14
Formula weight302.28776.02657.49661.26
Crystal systemmonoclinictriclinicmonoclinicmonoclinic
Space groupC2/cPequation imageC2/cC2/c
Crystal size /mm0.22 × 0.18 × 0.100.50 × 0.15 × 0.100.26 × 0.22 × 0.160.50 × 0.20 × 0.08
a24.678(4)7.5659(15)22.391(4)22.223(4)
b7.3698(19)9.757(2)11.369(2)11.413(2)
c16.045(4)11.634(2)11.821(2)11.731(2)
α9095.82(3)9090
β100.367(4)98.05(3)102.535(3)103.43(3)
γ9094.63(3)9090
V32870.5(12)842.1(3)2937.5(9)2894.0(10)
Z8144
Dc /g·cm–31.3991.5301.4871.518
μ /mm–10.1160.8190.5240.746
θ range /°2.58–26.431.78–27.481.86–26.372.55–27.48
Reflns collected /unique7821/29376392/36538093/299511390/3308
GOF on F21.0221.0050.9300.966
Rint0.03690.05640.03180.0594
R1a), wR2b) [I > 2σ(I)]0.0618, 0.17180.0488, 0.10930.0469, 0.13180.0441, 0.0945
R1, wR2 (all data)0.1079, 0.20790.0813, 0.11700.0723, 0.15520.0786, 0.1002
Largest diff. peak, hole /e·Å–30.436, –0.3910.336, –0.3780.376, –0.5730.354, –0.400

Crystallographic data (excluding structure factors) for the structures in this paper have been deposited with the Cambridge Crystallographic Data Centre, CCDC, 12 Union Road, Cambridge CB21EZ, UK. Copies of the data can be obtained free of charge on quoting the depository numbers CCDC-288684 http://www.ccdc.cam.ac.uk/data_request/cif , -284063 http://www.ccdc.cam.ac.uk/data_request/cif , -602559 http://www.ccdc.cam.ac.uk/data_request/cif and -290069 http://www.ccdc.cam.ac.uk/data_request/cif . (Fax: +44-1223-336-033; E-Mail: deposit@ccdc.cam.ac.uk, http://www.ccdc.cam.ac.uk)

Supporting Information (see footnote on the first page of this article): Diagram of π···π stacking interactions in complexes 3 and 4 (Figure S1), UV/Vis spectra for 14 (Figure S2), fluorescence spectra for 14 (Figure S3), PXRD patterns for 14 (Figures S4–7), thermogravimetric analyses (TGA) for 24 (Figure S8), and selected bond lengths and angles for compounds 14 (Tables S1–S4).

Acknowledgements

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

This work is supported by the National Natural Science Foundation of China (No. 21071022) and the Fundamental Research funds for the Central Universities. We especially thank Ms. LiQin Xiong for her excellent work and kind help in preparing this paper.

Supporting Information

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

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