Syntheses, Structures and Properties of Two 2D Coordination Polymers from 5-(Pyridin-2-yl-methyl)aminoisophthalate①

LI Xio-Ju XU Xi-Hong GUO Xio-Fng

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Syntheses, Structures and Properties of Two 2D Coordination Polymers from 5-(Pyridin-2-yl-methyl)aminoisophthalate①

LI Xiao-Jua②XU Xia-HongaGUO Xiao-Fanga

a(Fujian Key Laboratory of Polymer Materials, College of Chemistry and Chemical Engineering, Fujian Normal University, Fuzhou 350007, China)

Hydrothermal reactions of 5-(pyridin-2-yl-methyl)aminoisophthalic acid (H2paip) with Mn(OAc)2·4H2O and Cu(NO3)2·3H2O produced two 2D complexes, [Mn(paip)]n·nH2O (1) and [Cu(paip)(H2O)]n(2). In complex 1, paip serves as a4-bridge, and its two carboxylate groups in2,2-bridging and chelating modes connect Mn(II) into 1D chains, which are further extended into a 2D layer through coordination of two chelating nitrogen atoms. However, paip in complex 2 acts as a3-bridge to link Cu(II) into a 2D layer, in which two carboxylate groups function in a monodentate mode, and hydrogen bonds between the coordinated water and carboxylate oxygen atoms further extend the 2D layers into a 3D supramolecular network. The frameworks of complexes 1 and 2 are stable up to 470 and 250 ℃, respectively. Magnetic measurement shows that complex 2 possesses a weak antiferromagnetic interaction.

carboxylate, copper(II), coordination polymer, crystal structure, manganese(II)

1 INTRODUCTION

The rational design and synthesis of metal-organic coordination polymers are of great interest owing to their intriguing structures and potential applications in luminescence, magnetism, catalysis,gas storage and separation[1-2]. Molecular self-assembly based on the principle of crystal engineering has proved to be an efficient approach for the formation of coor- dination polymers. The structures and properties of the final products are mainly dependent upon the structural characters of organic ligands and the coor- dination preference of metal ions[3-4]. In the context, many efforts have been devoted to the judicial selection of multidentate ligands containing nitrogen and carboxylate groups. As is well known, the ne- gative charge of carboxylate group may compensate the positive charge from metal ions and mitigate the effect of counterion on self-assembly process. Moreover, carboxylate group also possesses a va- riety of coordination modes and strong coordination ability to transition metal ions, which can produce various robust frameworks. The further coordination of charge-neutral nitrogen atoms to metal ions may satisfy the coordination geometries of metal ions, resulting in the formation of coordination polymers with beautiful aesthetics and useful properties[5-6]. Recently, Lin. have reported a series of coor- dination polymers using ligands containing pyridyl and carboxylate groups, and the complexes exhibited interesting nonlinear optical properties[7]. Despite great progress in coordination polymers was made, the mechanism of molecular assembly of organic ligands and metal ions is still unclear. Lots of other factors, such as temperature, concentration, solvents and pH value, also have important influence on the assembly process[8-10]. Therefore, the exact predic- tion and control of target products remain a formi- dable challenge.

In the construction of coordination polymers, 5-aminoisophthalate is a promising,-containing ligand[11]. Similar to isophthalate, its two carboxylate groups are predisposed at 120° at the central phenyl ring, which may link metal ions into discrete mascrocycles, zigzag chains, 2D and 3D networks. The amino group may either take part in coordina- tion in a nonlinear mode, or may serve as a hydrogen bonding donor to form a strong hydrogen bond. Interestingly, the amino group is also reactive and is ready to be modified to form amino-substituted isophtalates[12]. Many studies have demonstrated that the electronic and steric characters of five-positioned substitutents of isophthalate have important effect on the structures and functions of the target complexes, and various inert- and coordinated-groups were introduced into the 5-position of isophthalate as substituents[13]. Inspired by their excellent results, we are interested in 5-(pyridin-2-yl-methyl)ami- noisophthalate (paip) (Scheme 1)[14]. Its two,?- donor atoms in five-positioned substitutent of iso- phthalate are inclined to chelate with the metal ions to form a stable five-membered ring, usually resul- ting in the formation of low-dimensional structures. As a continuation of our study in coordination poly- mers from 5-substituted isophthalate[15], herein, we report the syntheses, structures and properties of two 2D coordination polymers, [Mn(paip)]n·nH2O (1) and [Cu(paip)(H2O)]n(2).

Scheme 1. Coordination codes of paip

2 EXPERIMENTAL

2. 1 Materials and physical measurements

H2paip was synthesized according to literature methods[14]. All other chemicals were purchased from commercial supplies and used as received without further purification. IR spectra (KBr pellets) were recorded on a Magna750 FT-IR spectropho- tometer in the range of 400～4000 cm-1. Powder X-ray diffraction data were recorded on a PANay- tical X’pert pro X-ray diffractometer with graphite- monochromatized Curadiation (= 1.542 ?). Thermal stability studies were carried out on a NETSCHZ STA 449C thermoanalyzer at a heating rate of 10 ℃?min-1under N2atmosphere. C, H and N elemental analyses were determined on an EA1110 CHNS-0CE element analyzer. The polycrystalline magnetic susceptibility data were collected on a Quantum Design MPMS model 6000 magnetometer in the temperature range from 2 to 300 K.

2. 2 Synthesis

Synthesis of {[Mn(paip)]·H2O}n(1) A mixture of H2paip (68.1 mg, 0.25 mmol) and Mn(OAc)2·4H2O (61.3 mg, 0.25 mmol) in CH3CH2OH (3 mL) and H2O (9 mL) was placed in a Teflon-lined stainless steel vessel (30 mL), and then heated to 180 ℃ for 4 days. After being cooled to room temperature at a rate of 3 ℃?h-1, pale yellow crystals of 1 were obtained. The crystals were collected by filtration, washed with H2O and dried in air. Yield: 34.8 mg (41%). Elemental analysis (%): calcd. for C14H12N2O5Mn (343.19): C, 49.00; H, 3.52; N, 8.16. Found (%): C, 49.07; H, 3.93; N, 8.04. IR (KBr, cm-1): 3421(vw), 3289(w), 3075(vw), 2917(vw), 1612(s), 1559(vs), 1428(vs), 1382(vs), 1244(vw), 1155(vw), 1048(w), 1016(w), 782(m), 726(s), 611(w), 553(w), 458(vw).

Synthesis of [Cu(paip)(H2O)]n(2)A mixture of H2paip (68.1 mg, 0.25 mmol), Cu(NO3)2·3H2O (120.8 mg, 0.50 mmol) in CH3CH2OH (6 mL) and H2O (6 mL) was placed in a Teflon-lined stainless steel vessel (30 mL) followed by heating to 160 ℃ for 3 days. Cooling to room temperature at a rate of 3 ℃?h-1resulted in green acicular crystals of 2. The crystals were collected by filtration, washed with H2O and dried in air. Yield: 25.3 mg (29%). Elemental analysis (%): calcd. for C14H12N2O5Cu (351.80): C, 47.80; H, 3.44; N, 7.96. Found (%): C, 47.58; H, 3.64; N, 7.86. IR (KBr, cm-1): 3467(m), 3189(s), 2976(vw), 2901(vw), 1615(s), 1586(vs), 1574(s), 1489(vw), 1418(m), 1333(vs), 1106(vw), 1033(w), 917(vw), 772(m), 726(w), 623(vw), 531(w).

2. 3 X-ray crystal structural determination

The single crystals of complexes 1 and 2 were mounted on a glass fiber for X-ray diffraction analy- sis. Data were collected on a Rigaku AFC7R equip- ped with a graphite-monochromated Mo-radia- tion (= 0.71073 ?) from a rotating generator at 293(2) K. Intensities were corrected forfactors and empirical absorption using thescan technique. The structures were solved by direct methods using Siemens, and refined on2with full- matrix least-squares techniques using Siemens. All non-hydrogen atoms were refined anistropically. The hydrogen atoms of water in 2 were located from the difference Fourier map and refined isotropically. The positions of other hydro- gen atoms were generated geometrically (C–H bond fixed at 0.96 ?), assigned isotropic thermal parame- ters, and allowed to ride on their parent carbon atoms before the final cycle of refinement. Crystal data as well as details of data collection and refine- ment for complexes 1 and 2 are summarized in Table 1. The selected bond distances and bond angles are given in Table 2.

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

a= ∑|F| – |F|/∑|F|.b= ∑[(F2–F2)2]/∑[(F2)2]1/2

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

Symmetry transformations used to generate the equivalent atoms for 1: (A) –, –, –; (B)–1,,; (C) –,, –+1/2; for 2: (A) –+1, –+2, –+1; (B) –+1,–1/2, –+1/2

3 RESULTS AND DISCUSSION

3. 1 Structural description of[Mn(paip)]n·nH2O (1)

Single-crystal X-ray structural analysis shows that complex 1 crystallizes in the monoclinic space group2/. As shown in Fig. 1, the asymmetric unit con- sists of one crystallographically independent Mn(II)ion and one paip. Mn(II) is in a distorted octahedral geometry. It is coordinated by two chelating nitrogen atoms, two chelating carboxylate oxygen atoms and two2,2-carboxylate oxygen atoms from different paip. The equatorial plane is defined by pyridyl nitrogen atom, two chelating carboxylate oxygen atoms and one-coordinated oxygen atom of2,2-carboxylate. The mean deviation of Mn(II) from the equatorial plane is 0.0147 ?. The amino nitrogen atom and-coordinated oxygen atom of the other2,2-carboxylate occupy the axial posi- tions with O(2C)–Mn–N(1A) bond angle being 165.37(11)°. The Mn(1)–N(1A) bond in 2.426(3) ? is longer than Mn(1)–N(2A) of 2.223(3) ?, sug- gesting that the pyridyl nitrogen atom possesses strong coordination ability than the amine nitrogen atom. The bond distance of 2.426(3) ? is slightly large, but it is comparable with the reported Mn–N bond distances[16]. The Mn–O bond distances from chelating carboxylate oxygen atoms are larger than that from2,2-carboxylate oxygen atoms, which are probably ascribed to the effect of steric hindrance around Mn(II). The longest Mn–O bond distance is 2.339(3) ?, which is comparable with those in Mn(II) coordination complexes containing chelating car- boxylate groups[16]. Paip bridges four Mn(II) ions through its chelating carboxylate,2,2-carboxylate and,-chelating donor atoms in five-positioned substitutent (Scheme 1a). The pyridyl ring is highly twisted with respect to the phenyl ring with the dihedral angle between them being 81.1o. It should be mentioned that two2,2-carboxylate groups from different paip bridge two equivalent Mn(II) centers to form a dinuclear Mn(II)-carboxylate unit. The Mn???Mn distance is 4.258 ?, which is comparable with those in the coordination complexes consisting of dinuclear Mn(II)-carboxylate units[16]. The di- nuclear Mn(II) units are connected by chelating carboxylate group of paip into a 1D chain, in which the closest Mn???Mn distance is 7.846 ?. Further coordination of chelating nitrogen atoms in paip results in the formation of a corrugated 2D layer (Fig. 1b), and such 2D layers are arranged in an offset packing mode along theaxis (Fig. 1c).

Fig. 1a. Coordination environment of Mn(II) with the thermal ellipsoids at 50% probability in complex 1

Fig. 1b. 2D layer structure in complex 1

Fig. 1c. Packing diagram viewed along theaxis in complex 1

3. 2 Structural description of[Cu(paip)(H2O)]n (2)

Single-crystal X-ray structural analysis shows that complex 2 crystallizes in the monoclinic space group21/. It is isostructural with the reported Zn(II), Co(II) and Ni(II) complexes from paip, but the coordination geometry of Cu(II) and coordination mode of paip are different[13]. As shown in Fig. 2a, Cu(II) is in a distorted square-pyramidal geometry, while metal ions in the reported paip complexes are in a distorted octahedral coordination arrangement. In complex 2, two chelating nitrogen atoms and two monodentate carboxylate oxygen atoms from different paip comprise the equatorial plane, with the mean deviation of Cu(II) from the equatorial plane to be 0.1343 ?. Water molecule occupies the apical position with the Cu–O(1W) bond distance being 2.257(4) ?, which is longer than the Cu–O and Cu–N bond distances in the equatorial plane. The Cu–O and Cu–N bond distances are similar to the reported typical values in Cu(II) coordination poly- mers[8, 16]. Different from that in 1, paipbridges three metal ions through its two monodentate carboxylate and two chelating nitrogen atoms (Scheme 1b). However, two carboxylates in the reported paip complexes adopt the chelating and monodentate modes, respectively. As a result, paip connects Cu(II) ions into a 2D corrugated layer (Fig. 2b), which is further extended into a 3D supramolecular network by strong hydrogen bonds among coordinated water molecules and carboxylate oxygen atoms (O(1W)– H(1W1)???O(1)i2.836(6) ?, O(1W)–H(1W2)???O(2)ii2.661(6) ?; N(2)–H(2N)???O(4)iii2.913(5) ?; sym- metry codes: (i) –,–1/2, –+1/2; (ii) –+1,–1/2, –+1/2; (iii) –, –+2, –+1).

Fig. 2a. Coordination environment of Cu(II) with the thermal ellipsoids at 50% probability in complex 2

Fig. 2b. View of the 2D layer in complex 2

3. 3 IR spectra

In the IR spectra, the absorption bands at 3289 cm-1for 1 and 3189 cm-1for 2 were assigned as the O–H stretching of water molecules. The typical antisymmetric stretching bands of carboxylate groups are at 1612, 1559 cm-1for 1 and 1615, 1586 cm-1for 2, while their symmetric stretching bands are located at 1428(vs) and 1382(vs) for 1 and 1418(m) for 2. The separations (Δ) betweenasym(CO2) andsym(CO2) are 184, 177 cm-1in 1 and 197, 168 cm-1in 2. The IR peaks indicate the presence of bridging modes in paip, which are consistent with their crystal structures.

3. 4 X-ray powder diffraction

In order to check the purity of complexes 1 and 2, the as-synthesized samples were measured by powder X-ray diffraction (PXRD) at room tempera- ture. As shown in Figs. 3a and 3b, the peak positions of the experimental patterns are in agreement with the simulated ones from single-crystal X-ray diffraction, which clearly demonstrates good purity of 1 and 2.

3. 5 Thermogravimetric analysis

Thermal stability of complexes 1 and 2 was stu- died on polycrystalline samples under nitrogen atmosphere. As shown in Fig. 4, thermogravimetric analysis (TGA) curve of complex 1 shows the first weight loss of 5.4% before 180 ℃, which cor- responds to the removal of lattice water molecules (calcd. 5.2%). Complex 1 is stable up to 470 ℃. In complex 2, the weight loss of 4.9% at 150～220℃ is ascribed to the removal of coordinated water molecules (calcd: 5.1%). The framework begins to collapse above 250 ℃. Obviously, complex 1 pos- sesses much higher thermal stability than 2.

Fig. 3a. PXRD patter for complex 1

Fig. 3b. PXRD patter for complex 2

3. 6 Magnetic properties

The magnetic susceptibilities of complex 1 were measured in 2～300 K at an applied field of 1000 Oe. The plots ofmandmversusare shown in Fig. 5. Themvalue at 300 K is 8.43 cm3·K·mol-1. Upon cooling,Tvalue decreases smoothly above 50 K, and then goes down quickly to a minimum value of 1.07 cm3·K·mol-1at 2 K, indicating the antiferromagnetic coupling between Mn(II) ions[16]. The temperature dependence of magnetic suscepti- bilities in the temperature range of 10～300 K follows the Curie-Weiss lawm=m/(?) with a Weiss constant= ?6.49 K and a Curie constantm= 8.69 cm3·mol-1·K. From the viewpoint of crystal structure, complex 1 can be considered as an isolated spin dimer system, and the magnetic susceptibility in the whole temperature range was fitted according to spin Hamiltonian ? = –2?1?2(S1 = S2 = 5/2), whereis the exchange coupling parameter between S1 and S2. The results of the best fitting gave= 1.975(1),= ?0.556(1) cm-1and= 1.39×10-6, which are very close to those from coordination polymers consisting of dinuclear Mn(II)-carboxylate units[15, 16].

Fig. 4. TGA curves of complexes 1 and 2

Fig. 5. Temperature dependence of magnetic suscep tibility in the form ofmandmversusin complex 1. The solid line is the fitting result described in the text

4 CONCLUSION

Two 2D Mn(II) and Cu(II) coordination polymers based on paip were hydrothermally synthesized and characterized. Two,-donor atoms in paip chelate with metal ions to form a stable five-membered ring, but carboxylate groups show different coordination modes. Two carboxylate groups behave in2,2- bridging and chelating modes in 1, while they serve as a bis-monodentate bridge in 2. Thus, paip con- nects four six-coordinated Mn(II) in 1 and three five-coordinated Cu(II) in 2 into 2D structures. Thermal stability of complex 1 is much higher than that of complex 2. In summary, this study has demonstrated that isophthalate derivatives contai- ning chelating,?-donor atoms are promising ligands in the construction of low-dimensional coor- dination polymers.

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22 November 2013;

3 May 2014 (CCDC 951228 and 951229)

① This work was supported by the National Natural Science Foundation of China (21001025), the Natural Science Foundation of Fujian Province (2010J05017) and Provincial Education Department of Fujian (JA12070)

. E-mail: xiaojuli@fjnu.edu.cn