Syntheses, Structures and Catalytic Properties of Two Mn(II) and Cd(II) Coordination Polymers through in Situ Ligand Reaction①

ZHAO Su-Qin GU Jin-Zhong

a (College of Physics and Electronic Information Engineering,Qinghai University for Nationalities, Xining 810007, China)

b (College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, China)

ABSTRACT Two coordination polymers, namely [Mn(μ-Hcpia)(bipy)(H2O)2]n (1) and [Cd3(μ3-Hcpia)2(μ-Hbiim)2(μ-H2biim)(H2O)2]n (2), have been constructed hydrothermally using H2cbia (H2cbia = 5-(4?-cyanobenzoxy)isophthalic acid), bipy (bipy = 4,4?-bipyridine), H2biim (H2biim = 2,2?-biimidazole), and manganese or cadmium chlorides at 160 °C. Interestingly, the H3cpia (H3cpia = 5-(4?-carboxylphenoxy)isophthalic acid) ligand was generated by in situ hydrolysis of cyano group in H2cbia. The products were isolated as stable crystalline solids and were characterized by IR spectra, elemental analyses, thermogravimetric analyses (TGA), and single-crystal X-ray diffraction analyses. Both compounds crystallize in the triclinic system, space group P1. Compound 1 discloses a 1D linear chain of the 2C1 topological type. Adjacent chains are assembled into a 2D supramolecular sheet through O–H???O/N hydrogen bonds. Compound 2 features a 3D framework with a 3,4,4T25 topology. The luminescent and catalytic properties of two compounds were investigated. Compound 1 exhibits a superior catalytic activity in the cyanosilylation at room temperature.

Keywords: coordination polymer, tricarboxylic acid, luminescent properties, catalytic properties, in situ reaction; DOI: 10.14102/j.cnki.0254–5861.2011–3052

1 INTRODUCTION

During the past decades, the design and construction of functional coordination polymers have caught enormous attention of chemical researchers, not only because of their charming architectures and topologies, but also for their various potential applications in gas storage and separation,catalysis, magnetism, luminescence, and biomedicine[1-12].However, the design and composition of crystalline complexes with target structures as well as multifunction are of great important topics and are still the tremendous challenges because the assembly of coordination polymers is mainly affected by many factors, such as metal ions, organic and auxiliary ligands, metal-to-ligand ratio, solvent, and the reaction temperature[13-22]. The metal ions and organic ligands are the key to get intriguing topologies and functional materials. Multicarboxylate ligands are frequently used for the construction of coordination polymers because they can satisfy the charge-balance and provide diverse ligands and coordination modes[5,6,14,16,17,23,24]. Among such polycarboxylate blocks, semirigid biphen ligands are particularly intriguing, since they enable the formation of uncommon metal-organic networks or even topologically unique nets; at the same time, such ligands can also show interesting properties along with flexibility and conformational diversity[6,14,16,17,25].

In recent years, somein situligands or metal reactions have attracted increasing interest for the sake of obtaining new chemistry reactions, as well as the generation of novel coordination polymers[26]. As we know,in situligand may contain three benefits: Firstly,in situligand synthesis produces the possibility of novel ligand and simultaneous metal coordination. Secondly, in some cases,in-situligand synthesis can be used as a simple and friendly synthesis route.Thirdly,in situligand synthesis has been used to gain ligands hard to obtain through assembly synthetic routes[27,28]. It is hard to access the ligandsin situreaction by conventional methods. Solvothermal (containing hydrothermal) method can be a good choice. Up to now, solvothermal-microwave as well as ultrasound method has been observedin situreactions[29].

Considering these in mind, recently, we have begun to construct coordination polymers by use of the advantages ofin situligand reaction. On the basis of current research onin situligand reaction, the carboxylate-based ligands can bein situgenerated from the CN-containing ligands precursor[27,30,31].Thus, the precursor we selected is 5-(4?-cyanobenzoxy)isophthalic acid (H2cbia) due to its following characteristics: (1) It contains two carboxylate and one CN groups, which can form a tricarboxylate ligand throughin situligand reaction (Scheme 1); (2) Although its corresponding acid, 5-(4?-carboxylphenoxy)isophthalic acid(H3cpia), has been applied in the construction of coordination polymers[32-34], throughin situligand reaction, novel coordination polymers possessing different structures with reported results may be generated.

Herein, we report the synthesis, crystal structures,luminescence and catalysis of Mn(II) and Cd(II) coordination polymers with H3cpia ligands.

Scheme 1. H3cpia ligand formed via the in situ reaction

2 EXPERIMENTAL

2. 1 General procedures

All chemicals and solvents were of AR grade and used without further purification. Carbon, hydrogen and nitrogen were determined using an Elementar Vario EL elemental analyzer. IR spectra were recorded using KBr pellets and a Bruker EQUINOX 55 spectrometer. Thermogravimetric analysis (TGA) was performed under N2atmosphere with a heating rate of 10 K/min on a LINSEIS STA PT1600 thermal analyzer. Excitation and emission spectra were recorded on an Edinburgh FLS920 fluorescence spectrometer using the solid samples at room temperature. Powder X-ray diffraction patterns (PXRD) were measured on a Rigaku-Dmax 2400 diffractometer using CuKαradiation (λ= 1.5406 ?); the X-ray tube was operated at 40 kV and 40 mA. The data collection ranged from 5o to 45o. Solution1H NMR spectra were recorded on a JNM ECS 400 M spectrometer.

2. 2 Synthesis of compound 1

A mixture of MnCl2?4H2O (0.040 g, 0.2 mmol), H2cbia(0.057 g, 0.2 mmol), bipy (0.031 g, 0.2 mmol), NaOH (0.016 g, 0.4 mmol), and H2O (10 mL) was stirred at room temperature for 15 min, and then sealed in a 25 mL Teflon-lined stainless-steel vessel, and heated at 433 K for 3 days, followed by cooling to room temperature at a rate of 10 K·h-1. Yellow block-shaped crystals of 1 were isolated manually, and washed with distilled water. Yield: 55% (based on H2cbia). Anal. Calcd. (%) for C25H20MnN2O9: C, 54.86; H,3.68; N, 5.12. Found (%): C, 55.02; H, 3.70; N, 5.09. IR (KBr,cm-1): 3307m, 3092w, 1678w, 1612s, 1560s, 1502w, 1458w,1414m, 1371s, 1318w, 1253w, 1168w, 1103w, 1072w, 1042w,1002w, 976w, 910w, 853w, 814w, 782w, 730w, 680w, 624w.?OH3307 and 3092,?(CO2H) 1678,?as(CO2) 1612 and 1560,?s(CO2) 1414 and 1371.

2. 3 Synthesis of compound 2

A mixture of CdCl2?H2O (0.040 g, 0.2 mmol), H2cbia(0.057 g, 0.2 mmol), H2biim (0.027 g, 0.2 mmol), NaOH(0.016 g, 0.4 mmol), and H2O (10 mL) was stirred at room temperature for 15 min, and then sealed in a 25 mL Teflon-lined stainless-steel vessel, and heated at 433 K for 3 days, followed by cooling to room temperature at a rate of 10 K·h-1. Colourless block-shaped crystals of 2 were isolated manually, and washed with distilled water. Yield: 50% (based on H2cbia). Anal. Calcd. (%) for C48H36Cd3N12O16: C, 41.96;H, 2.64; N, 12.23. Found (%): C, 41.73; H, 2.65; N, 12.17. IR(KBr, cm-1): 3442w, 3128w, 1664w, 1608m, 1565s, 1509w,1453w, 1402s, 1369s, 1318w, 1258m, 1218w, 1171w, 1143w,1107w, 1004w, 976w, 944w, 900w, 860w, 801w, 773w, 729w,693w, 638w. ?OH3442 and 3128,?(CO2H) 1664,?as(CO2)1608 and 1565,?s(CO2) 1402 and 1369.

2. 4 Structure determination

Two single crystals of the title compounds were mounted on a Bruker CCD diffractometer equipped with a graphitemonochromatic CuKα(?= 1.54178 ?) radiation using aφ-ωscan mode at 293(2) K. The structures were solved by direct methods with SHELXS-97[35]and refined by full-matrix least-squares techniques onF2with SHELXL-97[36]. All non-hydrogen atoms were refined anisotropically. All hydrogen atoms (except those bound to water molecules)were placed in the calculated positions with fixed isotropic thermal parameters and included in structure factor calculations in the final stage of full-matrix least-squares refinement. The hydrogen atoms of water molecules were located by difference Fourier maps and constrained to ride on their parent O atoms. Detailed crystallographic data and structural refinements of compounds 1 and 2 are listed in Table 1. The selected important bond parameters are given in Table 2. The hydrogen bonds in crystal packing of compounds 1 and 2 are listed in Tables 3 and 4.

Table 1. Crystal Data and Structure Refinement for 1 and 2

Table 2. Selected Bond Lengths (?) and Bond Angles (°) for 1 and 2

Table 3. Geometrical Parameters of Hydrogen Bonds for 1

2. 5 Catalytic cyanosilylation of aldehydes

In a typical test, a suspension of an aromatic aldehyde(0.50 mmol, 4-nitrobenzaldehyde as a model substrate),trimethylsilyl cyanide (1.0 mmol), and catalyst (typically 3 mol%) in dichloromethane (2.5 mL) was stirred at room temperature. After a desired reaction time, the catalyst was removed by centrifugation, followed by an evaporation of the solvent from the filtrate under reduced pressure to give a crude solid. This was dissolved in CDCl3and analyzed by1H NMR spectroscopy for quantification of products(Fig. 11). To perform the recycling experiment, the catalyst was isolated by centrifugation, washed with dichloromethane, dried at room temperature, and reused.The subsequent steps were performed as described above.

3 RESULTS AND DISCUSSION

3. 1 Crystal structure of 1

X-ray crystallography analysis reveals that compound 1 crystallizes in the triclinic system space groupP1. As shown in Fig. 1, the asymmetric unit of 1 bears one crystallographically unique Mn(II) atom, oneμ-Hcpia2–block, one bipy moiety, and two H2O ligands. The five-coordinated Mn(1) atom exhibits a distorted trigonal bipyramidal{MnNO4} environment, which is occupied by two carboxylate O donors from two differentμ-Hcpia2–blocks,two O atoms from two H2O ligands, and one N atom from the bipy moiety. The Mn–O and Mn–N bond distances are 2.054(5)～2.225(5) and 2.245(6) ?, respectively; these are within the normal ranges observed in related Mn(II)compounds[14,17]. In 1, the Hcpia2–ligand adopts the coordination mode I (Scheme 2) with two COO–groups being monodentate. In the Hcpia2–ligand, a dihedral angle(between two aromatic rings) and a C–Oether–C angle are 81.08 and 118.26°, respectively. The bipy moiety takes a terminal coordination mode (mode III, Scheme 2). Theμ-Hcpia2–ligands connect Mn atoms to give a 1Dlinear chain of the 2C1 topological type (Figs. 2 and 3). Adjacent chains are assembled into a 2Dsupramolecular sheet through O–H???O/N hydrogen bonds (Fig. 4).

Fig. 1. Coordination environments of the Mn(II) atom in compound 1.The hydrogen atoms are omitted for clarity except in COOH group (Symmetry code: i: x–1, y, z)

Fig. 2. Perspective view of the 1D chain along the b axis. The H2O ligands are omitted for clarity

Fig. 3. Topological representation of two 1D chains in 1 with the 2C1 topology; Mn centers (green balls),centroids of μ-Hcpia 2? linkers (gray). View along the b axis

Fig. 4. Perspective view of the 2D sheet along the ac plane. Dashed lines present the hydrogen bonds

Scheme 2. Coordination modes of the Hcpia2–, bipy and H2biim/Hbiim– ligands in compounds 1 and 2

3. 2 Crystal structure of 2

Fig. 5. Coordination environments of the Cd(II) atoms. The hydrogen atoms are omitted for clarity except in COOH and NH groups(Symmetry codes: i: –x+2, –y+2, –z; ii: x+1, y, z; iii: x, y+1, z–1; iv: –x+2, –y+2, –z; v: –x+2, –y+3, –z)

The asymmetric unit of compound 2 contains two crystallographically unique Cd(II) atoms (Cd(1) with full occupancy and Cd(2) with half occupancy), oneμ3-Hcpia2–block, one Hbiim–moiety, a half of H2biim, and one H2O ligand. As depicted in Fig. 5, the six-coordinate Cd(1) atom features a distorted octahedral {CdN2O4} environment, which is filled by three carboxylate O atoms from twoμ3-Hcpia2–blocks, one O donor from the H2O ligand, one N atom from the H2biim, and one N atom of one Hbiim–moiety. The Cd(2)center is also six-coordinated and displays a distorted octahedral {CdN4O2} geometry. It is taken by two carboxylate O atoms from twoμ3-Hcpia2–blocks and four N donors from two Hbiim–ligands. The bond lengths of Cd–O are in the 2.230(2)～2.495(3) ? range, while the Cd–N bonds are 2.226(3)～2.388(3) ?, being comparable to those found in some reported Cd(II) compounds[14,25,28]. In 2, the Hcpia2–block acts as aμ3-linker (mode II, Scheme 2), in which three carboxylate groups adopt monodentate or bidentate modes.Besides, theμ3-Hcpia2–ligand is considerably bent, showing a dihedral angle of 68.14° (between two aromatic rings) and the C–Oether–C angle of 120.67°. The H2biim and Hbiim–moieties exhibit aμ-bridging coordination mode (modes IV and V, Scheme 2). Theμ3-Hcpia2–,μ-H2biim andμ-Hbiim–ligands multiply connect Cd(II) centers to form a 3Dframework (Fig. 6). The 3Dmetal-organic framework is assembled from the 4-connected Cd(1)/Cd(2) nodes,3-connectedμ3-Hcpia2–blocks, 2-connectedμ-H2biim andμ-Hbiim–linkers. As a result, a complex trinodal 3,4,4-connected net with a 3,4,4T25 topology[37-39]is generated (Fig. 7). The network is described by a point symbol of (62.84)(63)2(64.82)2, wherein the (62.84), (63), and(64.82)2notations correspond to Cd2,μ3-Hcpia2–, and Cd1 nodes, respectively.

Fig. 6. View of 3D metal-organic framework along the ac plane. The H2O ligands are omitted for clarity

Fig. 7. Topological representation of a trinodal 3,4,4-connected metal-organic framework in 2 with the 3,4,4T25 topology; view along the b axis;4-connected Cd nodes (turquoise balls), centroids of 3-connected μ3-Hcpia2– blocks (gray), centroids of 2-connected μ-H2biim and μ-Hbiim– linkers(blue). View of 3D metal-organic framework along the ac plane. The H2O ligands are omitted for clarity

3. 3 Thermal analysis

To determine the thermal stability of polymers 1 and 2,their thermal behaviors were investigated under nitrogen atmosphere by thermogravimetric analysis (TGA). As shown in Fig. 8, compound 1 loses its two coordinated water molecules in the 413～473 K range (exptl, 6.4%; calcd.6.6%), followed by the decomposition at 485 K. For 2, one weight loss (exptl, 2.4%; calcd. 2.6%) in the 449～508 K region corresponds to a removal of two H2O ligands;decomposition of the sample occurs only at 561 K.

Fig. 8. TGA curves of compounds 1 and 2

3. 4 Luminescent properties

The excitation and emission spectra of 5-(4?-cyanobenzoxy)isophthalic acid (H2cbia) and polymer 2 were measured in the solid state at room temperature (Figs. 9 and 10). The uncoordinated H2cbia shows a weak photoluminescence with an emission maximum at 404 nm (λex= 320 nm). In contrast,compound 2 displays the significantly more intense emission bands with the maxima at 379 nm (λex= 320 nm),respectively. All bands can be assigned to the intraligand(π*→n orπ*→π) emission[14,17]. The luminescence enhancement in the coordination compounds can be attributed to the binding of ligands to the metal centers,which effectively increases the rigidity of the ligand and reduces the loss of energy by radiationless decay[25,28].

Fig. 9. Solid-state excitation spectra of H2cbia and compound 2 at room temperature

Fig. 10. Solid-state emission spectra of H2cbia and compound 2 at room temperature

3. 5 Catalytic cyanosilylation of aldehydes

Given the potential of manganese(II) and cadmium(II)coordination compounds to catalyze the organic reactions[40-42], we explored the application of 1 and 2 as heterogeneous catalysts in the cyanosilylation of 4-nitrobenzaldehyde as a model substrate to give 2-(4-nitrophenyl)-2-[(trimethylsilyl)oxy]acetonitrile. Typical tests were carried out by reacting a mixture of 4-nitrobenzaldehyde,trimethylsilyl cyanide (TMSCN), and a Mn or Cd catalyst in dichloromethane at room temperature (Scheme 3, Table 5).Such effects as reaction time, catalyst loading, solvent composition, catalyst recycling, and finally substrate scope were investigated

Table 5. Mn and Cd-Catalyzed Cyanosilylation of 4-Nitrobenzaldehyde with TMSCN

Scheme 3. Mn-catalyzed cyanosilylation of 4-nitrobenzaldehyde (model substrate)

Upon using compound 1 as the catalyst (3 mol-%), a high conversion of 87% of 4-nitrobenzaldehyde into 2-(4-nitrophenyl)-2-[(trimethylsilyl)oxy]acetonitrile was reached after 12 h in dichloromethane at room temperature (Table 5,entry 7).

Fig. 11. Example of integration in the 1H-NMR spectrum for the determination of cyanosilylation reaction products (Table 5, Entry 7)

The results show that compound 1 is more active than 2.Although a relationship between structure and catalytic activity in the present study can not be clearly established, the highest conversation shown by compound 1 may eventually be associated to its 1D structure for easily accessible metal centers, together with the higher Lewis acidity of the manganese sites[43].

We also compared the activities of catalyst 1 in the reactions of other substituted aromatic and aliphatic aldehydes with trimethylsilyl cyanide, and the corresponding cyanohydrin derivatives were produced in yields ranging from 66 to 81% (Table 6). Aryl aldehydes bearing strong electron-withdrawing substituents (e.g., nitro and chloro)exhibited the higher reactivities (Table 6, entries 2～5),which may be related to an increase in the electrophilicity of the substrate. Aldehydes containing electron-donating groups(e.g., methyl) showed lower reaction yields (Table 6, entry 7),as expected. An ortho-substituted aldehyde showed lower reactivity, possibly as a result of steric hindrance.

Table 6. Cyanosilylation of Various Aldehydes with TMSCN Catalyzed by 1

To examine the stability of 1 in the cyanosilylation reaction,we tested the recyclability of this heterogeneous catalyst. For this purpose, upon completion of a reaction cycle, we separated the catalyst by centrifugation, washed it with CH2Cl2, and dried it at room temperature before further use.We repeated recycled catalyst 1, and the catalytic system maintained the higher activity over at least five consecutive cycles (the yields are 85, 85, 83, and 82% for the second to fifth run, respectively). According to the PXRD data (Fig. 12),the structure of 1 is essentially preserved after five catalytic cycles.

Fig. 12. PXRD patterns for 1: simulated (red), before (blue) and after (black) catalysis

4 CONCLUSION

In summary, we have synthesized two Mn(II) and Cd(II)coordination polymers based on an tricarboxylate ligand generatedin situreaction. Compound 1 discloses a 1Dchain structure, which is assembled to a 2Dsupramolecular network through O–H???O/N hydrogen bonds. Compound 2 features a 3Dframework with a 3,4,4T25 topology. The luminescent and catalytic properties of both compounds were investigated. Compound 1 revealed a superior catalytic activity in the cyanosilylation at room temperature.