Two Coordination Polymers Based on the Flexible N-Substituted 2,2?-Biimidazole Ligand: Solvothermal Synthesis, Crystal Structures and Characterizations

HE Ji-Go ZHANG Ln

a (College of Chemical Engineering, Fuzhou University, Fuzhou 350116, China)

b (College of Chemistry, Fuzhou University, Fuzhou 350116, China)

1 INTRODUCTION

In recent years, synthesis and characterization of coordination polymers have been receiving extensive and enduring attention for their interesting structural topologies and potential applications in gas storage and separation, catalysis, magnetism and optics[1-4].Among them, magnetic coordination polymers have attracted much attention due to the interesting magnetic phenomena and great potential applications in high density information storage,quantum computing and magnetic refrigeration[5-7].Magnetic metal coordination complexes with paramagnetic metal ions as spin carriers and organic groups such as carboxyl, cyano and amine as bridging ligands are one of the important molecular-based magnets[8-10].From the viewpoint of crystal engineering, the most effective and facile approach to build coordination polymer is to utilize an applicable ligand to link metal ions[11-13], or to control the molecular arrangements in crystals by simultaneous use of coordination bonds of a transition metal ion and intermolecular hydrogen bonds, to give infinite architectures[14].An ideal single organic linker should contain several donor atoms bridging metal ions together into extended architectures.Recently, the multifunctional ligand H2Pra2biim (1,1?-di(propionic acid)-2,2?-biimidazole) was selected for the following reason: 1)2,2?-Biimidazole (H2biim) and its derivatives have been widely utilized as biomimetic ligands in bioinorganic chemistry, bridging ligands in oligometallic chemistry for catalysis, and building blocks of supermolecular framework[15-18]; 2) The carboxylic acids have been extensively explored in the design of coordination compounds[19]for various coordination modes to metal ions and the ability to function as nice hydrogen-bond acceptors and donors; 3) The cooperative coordination of imidazole group and propionate arms is expected to exhibit various coordinating modes; 4) The flexibility and chirality induced by the free rotation around the central C–C bond and two propionate arms are expected to display the versatile coordination chemistry.It has yielded a series of Pra2biimbased metal-organic coordination polymers, such as helix-based MOFs of d10transition metals, lanthanide(III)-Pra2biim complexes[20,21], but the complexes of paramagnetic metal ions CuIIand MnIIhave not reported.In the present work, we select the CuIIand MnIIions as the central ion sources, which produced two new Pra2biim coordination polymers,[Cu2(Pra2biim)2(H2O)2]·2H2O (1) and[Mn(Pra2biim)(H2O)2]n(2).

2 EXPERIMENTAL

2.1 Materials and measurements

All commercially available chemicals were of reagent grade and used as supplied without further purification.1,1?-Di(ethylpropionato)-2,2?-biimidazole (Epra2biim) was synthesized in accordance with a published procedure[20].H2Pra2biim was prepared by hydrolyzing Epra2biim.Infrared spectra were recorded on a Bruker Vertex 70 FT-IR spectrometer as KBr pellets in the 4000～400 cm-1range.The C, H and N microanalyses were carried out with a Vario EL III elemental analyzer.TGA was recorded with a Netzsch STA449C apparatus under a nitrogen atmosphere.All of the magnetic measurements were performed using a commercial Quantum Design Physical Property Measurement System (PPMS).

2.2 Synthesis of[Cu2(Pra2biim)2(H2O)2]·2H2O (1)

A mixture of Cu(NO3)2·3H2O (0.05 mmol,0.0121 g), H2Pra2biim (0.05 mmol, 0.0139 g),triethylamine (5 uL), DMF (2 mL) and H2O (1 mL)was sealed in a 20 mL vial, heated at 80 ℃ for 3 days, and then cooled to room temperature.After cooling, blue crystals of 1 were obtained.Yield:27% (based Cu(NO3)2·3H2O).Anal.Calcd.for C24Cu2H32N8O12(Mr= 751.66): C, 38.35; H, 4.29 N,14.91.Found (%): C, 38.46; H, 4.13; N, 14.91%.IR(KBr, cm-1): 3482m, 3134m, 3107w, 2971w, 2249m,2971m, 2940m, 1635w, 1507m, 1466s, 1444s,1417s, 1374w, 1287vs, 1145vs, 1111vs, 1091vs,940w, 917m, 863w, 780s, 721m, 688m, 626s.

Synthesis of [Mn(Pra2biim)(H2O)2]n (2)

A mixture of MnCl2·4H2O (0.05 mmol, 0.0099 g),H2Pra2biim (0.05 mmol, 0.0139 g), DMF (2 mL)and H2O (1 mL) was sealed in a 20 mL vial, heated at 80 ℃ for 3 days, and then cooled to room temperature.After cooling, colorless crystals of 2 were obtained.Yield: 45% (based MnCl2·4H2O).Anal.Calcd.for C12H16MnN4O6(Mr= 367.22): C, 39.25;H, 4.39; N, 15.26.Found: C, 39.32; H, 4.25; N,15.30%.IR (KBr, cm-1): 3318m, 3147w, 3124w,2997w, 2961w, 1571vs, 1470w, 1410vs, 1352w,1279w, 1259m, 1225w, 1137m, 1029w, 959w, 895w,798w, 780w, 736m, 726m, 666m, 635w, 564w,496w.

2.3 Structure determination

The intensity data were collected on a Saturn724 CCD diffractometer for 1, and Mercury CCD diffractometer for 2 with graphite-monochromatic MoKα radiation (λ = 0.71073 ?).The Crystal Clear software package was used for data reduction and empirical absorption correction[22].The structure was solved by direct methods and refined by full-matrix least squares on F2with the SHELX-97 program[23,24].Crystal data as well as details of data collection and refinement for the complexes are summarized in Table 1.The selected bond distances and bond angles are given in Table 2.The hydrogen bonding parameters are shown in Table 3.

Table 1. Crystallographic Data for Compounds 1 and 2

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

Table 3. Hydrogen Bond Lengths (?) and Bond Angles (°) for Compounds 1 and 2

3 RESULTS AND DISCUSSION

3.1 IR spectra of complexes 1 and 2

In complex 1, the absorption bands at 3482, 3134 and 3107 cm-1could be assigned to the N–H and O–H bond stretching vibrations, suggesting the existence of hydrogen bonding interactions in 1,which is consistent with the result of X-ray analysis.The absorption peaks at 2971 and 2940 cm-1may be attributed to the C–H bond stretching vibrations.The peaks at 1507, 1287 and 1145 cm-1may result from the COO-bond stretching vibrations.The peaks at 1635, 1444, 1342, and 1109 cm-1may belong to the skeletal vibrations of imidazole rings.In complex 2, the absorption band at 3318 and 3124 cm-1may be attributed to the N–H and O–H bond stretching vibrations.The absorption peaks at 2997 and 2961 cm-1may belong to the C–H bond stretching vibrations.The peaks at 1571 and 1410 cm-1could be assigned to the COO-bond stretching vibrations.The peaks at 1470, 1352, 1259 and 1137 cm-1can be assigned to the skeletal vibrations of imidazole rings.All the above IR attribution is in agreement with the structural determination.

3.2 Structure of[Cu2(Pra2biim)2(H2O)2]·2H2O (1)

Compound 1 crystallizes in the triclinic group P1 and the asymmeric unit consists of one CuIIion, one Pra2biim2-ligand, one coordination water molecule and one lattice water molecule.As shown in Fig.1,the Cu ions are centrosymmetrically doubly-bridged by oxygen atoms and nitrogen atoms of two Pra2biim2-ligands to form the binuclear[Cu2(Pra2biim)2(H2O)2] with the Cu···Cu separation of 4.217 ?, which is longer than that in[Cu2(H2O)2(Me2biim)4]4+(Me2biim = N,N?-dimethyl-2,2?-biimidazole) (3.213 ?)[25].The Cu(II) ion is coordinated by two carboxyl oxygen atoms and two nitrogen atoms of imidazole rings from two distinct Pra2biim2-ligands, and one coordination water molecule with a distorted [CuO3N2] square pyramidal geometry.Two imidazole nitrogen atoms and two carboxyl oxygen atoms form the basal plane of the pyramid, and the metal is displaced by 0.290 ? from this plane towards the apical ligand,while the apical position is occupied by one water molecule.The imidazole rings of the ligand are not coplanar with the dihedral angle of the imidazole rings of 80.4° in 1, which is significantly larger than that in [Cu2(H2O)2(Me2biim)4]4+(55.9 and 57.5°)[25].The coordinating made of the Pra2biim2-ligand is indicated in Scheme 1a.There are one intramolecular hydrogen bond (O(5)···O(2) 2.772(3) ?)between the uncoordinated carboxyl oxygen atom(O(2)) and the coordinating water molecule (O(5)).The lattice water molecules link the binuclear[Cu2(Pra2biim)2(H2O)2] into a one-dimensional structure running along the 101 direction through the intermolecular hydrogen bond (O(1W)···O(2)2.862(4) ?, O(1W)···O(4) 2.701(4) ? and O(5)···O(1W) 2.780(4) ?).The chains were further stacked into a 3D supramolecular framework via the C–H···O association (C(2)···O(1W) 3.357 ?,C(5)···O(1) 3.516 ?, C(8)···O(1W) 3.439 ? and C(9)···O(2) 3.377 ?).

Scheme 1. Coordination modes of the Pra2biim2- ligand observed in 1 and 2

Fig.1. View of the coordination environment of Cu(II) ions in 1.Symmetry code: A: 1–x, 1–y, 1–z

Fig.2. View of the 1D structure interconnected by O–H···OOC hydrogen bonds

Fig.3. View of the 3D supramolecular framework of 1

3.3 Structure of [Mn(Pra2biim)(H2O)2]n (2)

Complex 2 belongs to the monoclinic Pc space group.The asymmetry unit contains one MnIIion,one Pra2biim2-ligand and two coordinating water molecules.As shown in Fig.4, the MnIIion has a distorted octahedral geometry with the equatorial sites occupied by two nitrogen atoms of the imidazole moiety and two carboxylic oxygen atoms from four different Pra2biim2-ligands, and the apical positions are occupied by two coordinating water molecules.The MnIIions were interconnected by Pra2biim2-ligands through the N,N?-bridged biimidazole moiety and two deprotonated propionate arms into a 2D-layered structure in the ac plane(Fig.5).The Pra2biim2-ligand has a coordinating mode (Scheme 1b) similar to that found in[Cd(Pra2biim)(H2O)2]n·2H2O[20], but the dihedral angle in 2 (93.1°) is significantly larger than that in[Cd(Pra2biim)(H2O)2]n·2H2O (62.7°).The sheet structure of 2 has a 63topology with both the MnIIion and the ligand acting as four-connected nodes.The layer is further stabilized by the hydrogen bonds between the coordinated water molecules and the carboxylic oxygen atoms (O(5)···O(1) 2.835(10)?, O(5)···O(2) 2.689(10) ?, O(6)···O(3) 2.860(9)?, O(6)···O(1) 3.001(11) ? and O(6)···O(4)2.686(11) ?) (Fig.5 and Table 2).The adjacent layers interact with each other through weak hydrogen bonding interactions between the carbon atoms of imidazole moiety and the coordinated water molecules or uncoordinated carboxylic oxygen (C(2)···O(4) 3.251 ?, C(3)···O(6) 3.675 ?,C(8)···O(2) 3.393 ? and C(9)···O(5) 3.819 ?),yielding a 3D supramolecular framework (Fig.6).

Fig.4. View of the coordination environment of the Mn(II) ions in 2.Symmetry codes: A: x, 1+y, z; B: 1+x, 1–y, 0.5+z; C: 1+x, 2–y, 0.5+z

Fig.5. View of a 63 2D sheet structure of 2

Fig.6. View of the 3D supramolecular framework of 2 viewed along the b axis

Fig.7. Plots of temperature dependence of the magnetic susceptibility of compound 1 in the form of χM and χMT vs T

3.4 Thermal stability

Thermogravimetric analysis (TGA) has been performed to examine the thermal stability of compounds 1 and 2.For 1, the combined TGADTG experiments revealed that the loss of lattice and coordination water molecules (9.8%, expected 9.6%) occurred in the temperature range of 70～136oC.The ligand began to decompose at ca.230oC.For 2, the first weight loss of 10.2% from 130～210oC corresponds to the departure of two coordination water molecules (calculated 9.8%).Further weight loss has not been observed until 250oC.

3.5 Magnetic properties

The dc magnetic susceptibility study of 1 has been carried out in an applied magnetic field of 1000 Oe in the temperature range of 300～2 K.The temperature dependence of the molar magnetic susceptibility of 1 is presented in Fig.7 in the form of χMand χMT vs.T.χMT is 0.888 cm3·K·mol-1at 300 K, which is significantly higher than that calculated for two high spin CuIIions value of 0.750 cm3·K·mol-1(g = 2, S = 1/2)[26]because of the high-spin CuII(S = 3/2) centers that are expected to afford orbital contributions to the magnetic moment and afford g values that deviate significantly from 2.0.As the temperature is lowered, the χMT product decreased monotonously and slowly within the entire temperature range and reaches 0.66 cm3·K·mol-1at 2 K.A best fit of the experimental data to the Curie-Weiss law in the temperature range of 2～300 K leads to the Curie and Weiss constants of 0.89 cm3·K·mol-1and –0.60 K,respectively, indicating antiferromagnetic interactions between the two CuIIions.The best fit to the data has been achieved by using the equation (Eq.1)for the exchange-coupled copper(II) dimers, which results from a consideration of the eigenvalue of H= –2JS1S2, where the symbols have the usual meanings.An excellent fit was obtained when J/k =–0.83, g = 2.158 and R = 2.85 × 10-7.The magnetic coupling of the two Cu(II) atoms in 1 (J/k = –0.83)is much weaker than that in[Cu2(H2O)2(Me2biim)4]4+(J/k = –7.88)[25].The dihedral angle of the two imidazole rings of the bridging ligand of 80.4° in 1 is significantly larger than that in [Cu2(H2O)2(Me2biim)4]4+(55.9 and 57.5°) and the Cu-Cu separation of 4.217 ? is clearly longer than 3.215 ? in[Cu2(H2O)2(Me2biim)4]4+, which lead to the poor magnetic communication between the two unpaired electrons of the Cu(II) atoms.

4 CONCLUSION

In summary, two new coordination compounds 1 and 2 constructed from Cu2+or Mn2+ions and the Pra2biim2-ligand with two different coordinating modes have been obtained characterized by X-ray crystallography under the same reaction conditions.In 1, two Pra2biim2-ligands serve as bridging ligand and link two Cu2+ions through coordination bond,forming a binuclear complex.In 2, each Pra2biim2-ligand serves as the 4-connected bridging ligand and links the Mn2+ions, forming a 2D polymer.Their three-dimensional supramolecular architectures are stabilized by hydrogen bonds.It indicated that the geometry of the coordination complex is determined not only by the coordination environment but also by the metal entity itself.The Cu-Cu antiferromagnetic coupling with interaction has been observed and the greater twist angle of the two imidazole rings and Cu···Cu separation are responsible for much weaker magnetic exchange coupling between the Cu2+ions in 1.

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