Structural and Magnetic Characterization of Two New Coordination Compounds Based on a Fluorene Derivative Ligand①

YANG Qing-Feng YUE Ki WANG Zhi-Hui LAI Xio-Yong WANG Xio-Zhong QIN Ling

a (State Key Laboratory of High-efficiency Utilization of Coal and Green Chemical Engineering,National Demonstration Center for Experimental Chemistry Education, College of Chemistry and Chemical Engineering, Ningxia University, Yinchuan 750021, China)

b (Chongqing Changfeng Chemical Industrial Co., Ltd., Chongqing 401220, China)

ABSTRACT Two new coordination compounds [Co(FDC)2(H2O)2]n (1) and [Cu(FDC)2(2,2΄-bpy)]·DMF·H2O(2) (HFDC = 9-fluorenone-4-carboxylic acid, bpy = 2,2΄-bipyridine, DMF = N,N-dimethylformamide) were prepared by the reactions of corresponding metal salts with a fluorene derivative ligand of HFDC. Both compounds were thoroughly analyzed by X-ray single-crystal diffraction, elemental analysis, IR spectra, PXRD and thermal analysis. Compound 1 features a 1D structure. Via two kinds of O-H···O hydrogen bonds, such 1D chains self-assemble into a 3D supramolecular structure stabilized by the offset face-to-face π···π interactions.Compound 2 features a dinuclear structure which, via O-H···O and C-H···O hydrogen bonds, self-assembles into a 2D supramolecular layer strengthened by strong face-to-face π···π interactions. And variable-temperature magnetic susceptibilities of 1 and 2 indicate weak antiferromagnetic interactions between the Co(II) and Cu(II)metal ions.

Keywords: low-dimensional coordination compounds, fluorene derivative, magnetic properties;

1 INTRODUCTION

Molecular magnetism is an interdisciplinary field of research that has attracted great attention for decades, with studies focusing on revealing the diverse magnetic phenomena in molecular systems, understanding the underlying physics, and constructing new magnetic materials with potential applications[1-3]. The most extensively studied systems are the metal coordination compounds in which paramagnetic metal ions are linked by short bridging groups into finite-sized polynuclear clusters or “infinite” coordination polymers[4-8]. The variety of the structures relies on the presence of suitable metal-ligand interactions and supramolecular contacts, which is directly related to the coordination characteristics of the components, such as the charge and radius of metal ions, the amount of dentate and steric hindrance of the ligands, etc[9,10].

In general, the magnetic CPs are synthesizedviaa bottom-up approach using paramagnetic metal ions and/or metal clusters as building blocks linked by suitable bridging ligands, which can ef fi ciently transmit magnetic couplings between each metal ions[11,12]. The short bridging ligands,such as cyanide, carboxylate and azide, as ef fi cient magnetic transmitting ligands, are dominant in the literature[13-18].Thus, enormous efforts on magnetic CPs have been focused on the design of suitable organic ligands and the coordination tendencies of metal centers for the building of diversi fi ed extended networks with interesting magnetic properties. The N-heterocyclic ligands, such as triazole and tetrazole, are also receiving considerable attention for the preparation of new magnetic CPs[19,20].

Fluorene is a rigid planar structure composed of two directional benzene rings connected through a C-C single bond and a bridged methylene group[21]. The acceptance of methylene group makes the two benzene rings coplanar,which increases their orbital overlap, and also increases the degree of conjugation of the entire system[22,23]. Fluorene has active sites at 2, 4, 5, and 7 positions, which are prone to electrophilic substitution reactions, thus making it easy to obtain a variety of derivatives with a wide range of applications[24-26]. By introducing interactions between heteroatoms andπ-conjugated systems, the electronic structure of plutonium can be changed, and the modifiability of plutonium structure can be increased. However, CPs based on the fluorene derivative have been rarely reported up to now.

Herein, we report two new HFDC-based coordination compounds [Co(FDC)2(H2O)2]n(1) and [Cu(FDC)2(2,2΄-bpy)]·DMF·H2O (2) prepared by solution-diffusion synthesis method, featuring 1D chain and dinuclear structures, respectively. Their preparation, spectroscopic and structural characterization together with their variable-temperature magnetic study are the subject of the present work.

2 EXPERIMENTAL

2. 1 Materials and physical measurements

All chemicals were of reagent grade, obtained from commercial sources and used without further purification.Elemental analyses for C, H and N were performed with a Perkin-Elmer 2400 LS II element analyzer. The FT-IR spectra were recorded in the range of 400～4000 cm-1on a Perkin-Elmer Spectrum Two FT-IR spectrometer by the dry KBr disks. PXRD data were collected using a D8 Advance A25 diffractometer with Cu-Kαradiation (λ= 1.54060 Å).Thermogravimetric (TG) behaviour was investigated with a Setsys 16 instrument at a heating rate of 10 °C·min-1in air.Fluorescent data were obtained from a Hitachi F-7000 instrument at room temperature. Magnetic measurement was carried out with a SQUID-VSM magnetometer in a field of 1000 Oe.

2. 2 Syntheses of compounds 1 and 2

[Co(FDC)2(H2O)2]n1. Organic ligand HFDC (22 mg, 0.1 mmol) was completely dissolved in N,N-dimethylformamide(DMF) (4 mL) and put on the bottom of a test tube. Then,ethanol solution (v/v = 1:1, 6 mL) was layered on the former.Finally, Co(NO3)2·6H2O (30 mg, 0.1 mmol) was dissolved in ethanol (4 mL), and carefully layered on the top. It was then allowed to stand at room temperature over three weeks. After being washed with distilled water at ambient temperature,orange crystals were obtained in a yield of 47.6% based on Co. Analysis calcd. (%) for C28H18CoO8(Mr= 541.35): C,62.07; H, 3.33. Found (%): C, 61.72; H, 3.17. IR (cm-1):3387 (br., m), 1704 (s), 1610 (s), 1589 (s), 1570 (s), 1468(m), 1455 (w), 1405 (s), 1307 (w), 1250 (w), 1126 (s), 959(m), 871 (w), 802 (w), 770 (w), 731 (s), 618 (m), 455(w).

[Cu(FDC)2(2,2΄-bpy)]·DMF·H2O 2. Compound 2 was synthesized by a method similar to that of 1, except that bpy(16 mg, 0.1 mmol) was also added into DMF. It was then kept at room temperature over three weeks, and green crystals were obtained. Green crystals were obtained in a yield of 48.7% based on Cu after washing with distilled water at ambient temperature. Analysis calcd. (%) for C41H31CuN3O8(Mr= 757.26): C, 64.97; H, 4.09; N, 5.55.Found (%): C, 64.71; H, 3.83; N, 5.31. IR (cm-1): 3426 (br.,m), 1710 (s), 1665 (m), 1602 (m), 1445 (m), 1373 (w), 1352(w), 1303 (w), 1165 (w), 960 (w), 872 (w), 638 (w), 618 (w),488 (w), 482 (w), 474 (w).

2. 3 X-ray crystal structure determination

All data of 1 and 2 were collected at 293 K with a Rigaku R-AXIS RAPID IP diffractometer (MoKα,λ= 0.71073 Å).With the SHELXTL program[27], the structures of 1 and 2 were solved by direct methods and refined by full-matrix least-squares techniques[28]. All the non-hydrogen atoms were refined anisotropically. For 1, all the hydrogen atoms of FDC-and O(2W) were generated geometrically, while those of O(1W) were located from the difference Fourier maps. The hydrogen atoms of 2 were generated geometrically. The relevant crystallographic data of compounds 1 and 2 are listed in Table 1. Selected bond lengths and bond angles are given in Table 2.

Table 1. Crystallographic Data for Compounds 1 and 2

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

3 RESULTS AND DISCUSSION

3. 1 Structural description of compounds 1 and 2

The single-crystal X-ray diffraction analyses reveal that 1 crystallizes in the triclinic system withPspace group. The asymmetric unit consists of two Co2+ions (Co(1) and Co(2);occupancy ratio: 0.5 for each), two FDC-ligands and two coordinating water molecules. As shown in Fig. 1a, both Co(1) and Co(2) are located in a six-coordinated octahedral coordination environment. Co(1) is surrounded by four carboxylate O atoms from two FDC-ligands (O(4), O(4)#1,O(5), O(5)#1, symmetry code: #1: 1-x, 1-y, 1+z) and two water molecules (O(1W), O(1W)#1), while Co(2) is surrounded by two carboxylate O atoms (O(3), O(3)#1) and four water molecules (O(1W), O(1W)#2, O(2W), O(2W)#2,symmetry code: #2: -x, 1-y, 1-z). The Co-O distances change from 1.908(9) to 2.392(1) Å falling in a normal range[29]. As shown in Fig. 1b, the adjacent Co2+ions are linked into an infinite 1D chain along theaaxis through two O atoms (O(4), O(3)) of one FDC-ligand and one coordinating water (O1W). A detailed structural analysis reveals the neighboring 1D chains are further connected into 2D supramolecular layers parallel to theacplane through O(2W)-H(1W)···O(1) hydrogen bonds (O(2W)···O(1)#3 =2.8260(9) Å, O(2W)-H(1W)···O(1)#3 = 166°, symmetry code: #3: -x, 1-y, 2-z) andπ···πstacking interaction with the distance of 3.507 Å (Fig. 1c, Tables 3 and 4). Finally, the adjacent layers are further connected to be a 3D supramolecular structure (Fig. 3)viaO(1W)-H(3W)···O(2) hydrogen bonds (O(1W)···O(2)#2 = 2.7426(7) Å, O(1W)-H(3W)···O(2)#2 = 178°, symmetry code: #4:x, 1+y,z)(Table 3). It is worth noting that there exist two kinds ofπ···πstacking interactions between intermolecular FDCligands with the distances of 3.405 and 3.492 Å,respectively, which strengthen the 3D supramolecular structure of 1 (Table 4).

Fig. 1. a) Coordination environment of the Co2+ in 1; b) 1D chain structure of 1; c) 2D layer of 1 formed via intermolecular O(2W)-H(1W)···O(1) hydrogen bonds and π···π interaction

Single-crystal X-ray diffraction analysis reveals that compound 2 crystallizes in the triclinic system withPspace group. The asymmetric unit is composed of one Cu2+ion, two FDC-ligands and one bpy. As shown in Fig. 2a, a centrosymmetric dinuclear structure appears. The Cu(1)center adopts a distorted square pyramidal geometry con fi guration, completed by three oxygen atoms (O(4), O(6),O(4)#1 and O(6)#1 from two different FDC-ligands,symmetry code: #1, 1-x, 1-y, -z) and two nitrogen atoms(N(2) and N(3) from a bpy ligand). The Cu-O bond lengths are observed in the range of 1.985(18)～2.420(19) Å within the normal range[30], and Cu(1)-N(2) is 2.009(2) Å and Cu(1)-N(3) is 2.019(2) Å. And Cu(1) and Cu(1)#1 are bridged by O(6) and O(6)#1 atoms from different ligands in the dinuclear structure unit. As shown in Fig. 2b, the adjacent dinuclear structure units are linked into a 1D chain along theaaxis through intermolecular hydrogen bonds(O(1W)···O(5) = 2.8700(9) Å, O(1W)-H(1W)···O(5) =174°, O(1W)···O(7) = 3.0072(7) Å, O(1W)-H(1W)···O(7)#1 = 173°, #1: -1+x,y,z). It is worthy of note that there existπ···πstacking interactions between intermolecular bpy with distance of 3.288 Å, which stabilizes the 1D chain. AndviaC(41)-H(41)···O(1W) and C(39W)-H(39)···O(2) (C(41)···O(1W)#2 = 3.5704(12) Å,C(41)-H(41)···O(1W)#2 = 162°, C(39)···O(2) = 3.3192(10)Å, C(39)-H(39)···O(2) = 132°, #2: 2-x, 2-y, 1-z) hydrogen bonds, the neighboring 1D chains are connected into a 2D supramolecular layer.

Fig. 2. a) Molecular structure of compound 2; b) 1D chain of compound 2 along the a axis formed via O(1W)-H(1Wb)···O(7) hydrogen bond;c) 2D supramolecular layer of compound 2 parallel to the ab plane formed via C(41)-H(41)···O(1W) and C(39)-H(39)···O(2) hydrogen bonds

Fig. 3. 3D supramolecular structure of 2 formed via O(1W)-H(3W)···O(2) hydrogen bonds and π···π interactions

Table 3. Hydrogen-bonded Parameters for Compounds 1 and 2

Table 4. Selected π···π Interaction Arrangement for 1 (Plane-to-plane Distance (d), Dihedral Angles (α), Centroid Distance (c))

Table 5. Selected π···π Interactions Arrangement for 2 (Plane-to-plane Distance (d), Dihedral Angles (α), Centroid Distance (c))

4 CHARACTERIZATION

4. 1 Infrared (IR) spectroscopy and powder X-ray diffraction (PXRD)

As shown in Fig. 4, the shapes of the IR spectra of compounds 1 and 2 are roughly similar. The IR spectra of compound 1 and 2 showed strong peaks positioned at 1707 cm-1, which can be attributed to the stretching vibration peak ofv(CO) in the HFDC ligand, indicating that the carboxyl group of the HFDC ligand is coordinated with the metal ion.The peak of the compounds at about 3430 cm-1can be due to the loss of hydrogen ion of the -COOH group of the HFDC ligand, which leads to the weakening of the stretching vibration peak of the O-H bond. The infrared peaks around 1445 and 872 cm-1in compound 2 are attributed to the stretching vibration peak of the C=C double bond and the out-of-plane bending vibration peak of the C-H bond of 2,2΄-bpy, indicating this 2,2΄-bpy is coordinated with Cu2+ions.

To confirm the phase purity of compounds 1 and 2, PXRD experiments have been carried out at room temperature. As shown in Fig. 5, the experimental PXRD pattern for each compound is in accord with the simulated one generated based on structural data, demonstrating the phase purity of 1 and 2.

Fig. 4. IR spectra of compounds 1 and 2

Fig. 5. Experimental (red) and simulated (blue) powder XRD patterns for compounds 1 and 2

4. 2 Thermal stability analysis

To estimate the thermostability of compounds 1 and 2,thermogravimetric (TG) analyses in purified air were performed and the TG curves are listed in Fig. 6.Compounds 1 and 2 both underwent two steps of weight loss.From the TG curve of compound 1, the first step of weight loss of 6.65% in the range of 30～266 °C is ascribed to the departure of two coordinated water molecules (calcd. 6.66%)per formula unit. The second loss above 266 °C is attributable to the collapse of the whole structure, and the remaining weight of 14.30% corresponds to the percentage (13.84%) of Co and O components, indicating that the final residue may be CoO. The TG curve of compound 2 shows weight loss of 11.49% (calcd. 12.01%) in the range of 30～185 °C due to the loss of one water molecule and one DMF molecule, and then no obvious weight loss is observed until the collapse of the framework at 531 °C, indicating its good thermal stability. The TG curves show that compounds 1 and 2 possess good thermal stability.

Fig. 6. TGA curves of compounds 1 and 2

4. 3 Magnetic properties of compounds 1 and 2

The temperature-dependent magnetic susceptibility was investigated for the crystalline samples of compounds 1 and 2 in the range of 2～300 K with a 1000 Oe applied field. Fig. 7 shows the χMTvs.T and χM-1vs. T curves of compounds 1 and 2. For 1, the χMT value is 0.00316 cm3mol-1K at 300 K,which is smaller than the expected value for two uncoupled high-spin Co(II) ions. As the temperature decreases, the χMT value increases slightly until a maximum value of 0.00302 cm3·mol-1appears at 123.9 K, and then suddenly decreases to 0.00177 cm3·mol-1at 8 K (Fig. 7a). The χMT value decrease with a decreasing temperature indicates the presence of antiferromagnetic interaction between the Co2+ions in compound 1. As shown in Fig. 7b, the observed χMT value of compound 2 at room temperature is 1.0169 cm3·mol-1·K,which is smaller than the expected spin-only value of 1.88 cm3·mol-1·K. Upon cooling, the value of χMT decreases smoothly to a value of 0.8248 cm3·mol-1·K at 60 K. And with further cooling, the sample undergoes a rapid decrease in χMT, reaching 0.4464 cm3·mol-1·K at 2 K. This is a classical magnetic behavior with an antiferromagnetic order,which indicates the antiferromagnetic interactions between the Cu(II) ions. As shown in Fig. 7, The magnetic susceptibilities (χM-1) are well fitted by the Curie-Weiss law in the temperature range of 2～300 K for 1 and 2, thus giving a negative Weiss constantθ= -14.6 K and C = 0.00335 emu·mol-1·K for 1 andθ= -0.099 K and C = 0.101 emu·mol-1·K for 2, confirming the overall intracluster antiferromagnetic interactions again in compounds 1 and 2.

Fig. 7. Temperature dependence of the χMT vs. T and χM-1 vs. T values of compounds 1 and 2 (The red solid lines are the Curie-Weiss law)

5 CONCLUSION

Two coordination compounds based on HFDC ligand with different structures have been synthesized and structurally characterized. Their structures, thermal stabilities and magnetism have been investigated. X-ray single-crystal diffraction analysis reveals that compound 1 features 1D chain structures which further extend into a 3D supramolecular network through hydrogen bonds andπ···πinteractions; compound 2 exhibits dinuclear structures which are connected into a 2D layerviahydrogen bonds andπ···πinteractions. The research on magnetic properties shows that there exist antiferromagnetic interactions between Co(II) and Cu(II) metal ions. Furthermore, the successful syntheses of these two new compounds may provide a useful method to the design and synthesis of novel low-dimensional magnetic CPs.