HU Ling-Xia ZHANG Bi-SongWU Chang-Sheng HUANG Dong-Wei QIU Jian-Ping

(College of Pharmaceutics and Material Engineering, Jinhua Polytechnic, Jinhua 321007, China)

Hydrothermal Synthesis, Crystal Structure, Magnetic Properties and Network Topology of a Novel Binuclear Co(Ⅱ) Coordination Polymer(L)4] (bpy = Bipyridine, HL = 2-Fluorbenzoic Acid)①

HU Ling-Xia ZHANG Bi-Song②WU Chang-Sheng HUANG Dong-Wei QIU Jian-Ping

(College of Pharmaceutics and Material Engineering, Jinhua Polytechnic, Jinhua 321007, China)

One novel binuclear Co(Ⅱ) coordination polymer(1, bpy = bipyridine, HL = 2-fluorbenzoic acid) has been hydrothermally synthesized and structurally characterized by elemental analysis, IR and X-ray single-crystal diffraction.Complex 1 crystallizes in the monoclinic system, space group P21/c with a = 11.163(2), b = 21.943(4), c = 9.6381(19) ?, β = 113.92(3)°, V = 2158.2(7) ?3, Z = 4, Dc= 1.518 g/cm3, Mr= 493.31, F(000) =1004, R = 0.0303 and wR = 0.0808 for 3614 observed reflections (I > 2s(I)).Complex 1 exhibits a 3D Schlafli symbol (4.62)(4.66.83) topological framework, and the magnetic characterization shows weak antiferromagnetic coupling exchange between two Co ions.

Co(Ⅱ) complex, coordination polymer, crystal structure, magnetic properties, topological framework;

1 INTRODUCTION

Metal-organic framework (MOF) has similar properties of both composite macromolecule and complexes.Compared with the conventional aluminum phosphate molecular sieves system, MOF has potential applications in nonlinear optics material, magnetism material, superconducting material and catalysis because of its higher yields, micropore size, adjustable shape, structure, function of diverse, etc.The MOF solid has been generated from the assembly with organic bridging ligands and metal ions, in which the ligands or hydrogen bonds, p-pstacking weak interactions, electrostatic interactions and spatial support effect were very important in the regulation of MOF’s topology and performance.The nitrogen aromatic heterocyclic like 4,4?-bipyridyl can be assembled into a one-, two- and threedimensional supramolecules with a variety of metals, and researches about nonlinear optics performance, catalysis, and photoelectric conversion performance have been reported[1-23].Our research group has made some progress in assembling one-dimensional or multidimensional supramolecular structures[24,25]constructed from the central transition metal ions and ligands of 4,4?-bipyridine and halogenated aromatic acid.Recently, a novel binuclear Co(Ⅱ) coordination polymerhas been synthesized by hydrothermal method, its structure was characterized by single-crystal X-raydiffraction, IR, magnetic property, and its topological framework was also analyzed.

2 EXPERIMENTAL

2.1 Reagents and instruments

All chemicals of analytical grade were purchased and used without further purification.Elemental analysis was performed with a Vario III elemental analyzer (Germany Elementar Company).IR spectra were recorded on an EXUS670 FT-IR (American Thermo Nicolet Corporation).Crystal structure determination was carried out on an APEX CCD II single-crystal diffractometer (BIUKER company) and variable temperature magnetic susceptibilities of crystalline of complex 1 were measured on a SQUID Quantum Design Model MPMS-7 magnetometer.

2.2 Synthesis of complex 1

A mixture of CoCO3(0.120 g, 1.00 mmol), 2-fluorobenzoic acid (0.12 g, 0.86 mmol) and 4,4?-bpy (0.080 g, 0.51 mmol) was added to 15 mL CH3OH/H2O (1:2 v/v) mixed solution.After stirring for 2 h, the resulting pink suspension was sealed in a Teflon-lined stainless steel autoclave, then the reactor was placed in an oven at 160 ℃ heated for one week.The autoclave was then cooled to room temperature.The brown insoluble substance in the reaction mixture was removed by filtration and the purple flake crystal was found in the resulting residue.The resulting pale pink filtrate was evaporated naturally at room temperature.The pink crystals were obtained after a month, then washed with distilled water and finally dried in air.Anal.Calcd.(%) for C24H16F2CoN2O4: C, 58.38; H, 3.24; N, 5.68.Found (%): C, 58.52; H, 3.22; N, 5.76.

2.3 Crystal structure determination

A suitable single crystal of complex 1 with dimensions of 0.67mm × 0.47mm × 0.14mm was selected for X-ray diffraction data collection performed on an APEX CCD II diffractometer equipped with a graphite-monochromatic MoKa radiation (λ = 0.71073 ?).In the range of 3.01≤q≤25.99°, a total of 18499 reflections were collected by using anw-j scan mode, of which 4230 were independent (Rint≥ 0.0233), and 3614 were observed with I ≥2s(I) for structural correction.Diffraction intensity data were corrected by Lp factors and empirical absorption.All non-hydrogen atoms were located by direct methods, the coordinates and anisotropic displacement parameters of non-hydrogen were refined by full-matrix least-squares techniques, and all hydrogen atoms associated with carbon atoms were calculated by theoretical hydrogenation.All calculations were carried out on PentiumIV 300 by using the SHELX-97 program.The final R = 0.0303 and wR = 0.0808+ (0.0453P)2+ 0.6807P], where P =S = 1.066, (D/s)max= 0.002, (Dr)max= 0.301 and (Dr)min= –0.338 e/?3.

3 RESULTS AND DISCUSSION

3.1 IR spectrum

In the IR spectra of complex 1 (Fig.1), the absorption at 1367 cm-1could be ascribed to the C–N stretching vibration of nitrogen aromatic heterocyclic and 3051 cm-1to the C–H stretching vibration of aromatic ring.The signal at 3419 cm-1was ascribed to the O–H stretching of free water, and the absorptions at 1602, 1564 and 1454 cm-1to the vibration of aromatic ring skeleton.Two peaks were observed at 1625 and 1417 cm-1owing to the presence of symmetrical and asymmetrical stretching vibrations on the bidentate carboxylate anion (-COO–), of which the stronger absorption peak at 1625 cm-1results from the bimodal degeneration of aromatic ring skeleton vibration absorbing peaks and asymmetrical carboxylate ion absorption peaks.

3.2 Crystal structure

Selected bond lengths and bond angles of the title complex [Co(4,4?-bpy)2(L)2] are shown in Table 1 and crystal structure in Figs.2, 3 and 4.Crystal structure of complex 1 showed that it was a binuclear coordination polymer, and in each coordinate unit one Co(Ⅱ) ion is coordinated by four O atoms from three molecules of 2-flurobenzoic acid and twoN atoms from two molecules of 4,4?-bpy, constituting a slightly distorted six-coordinated octahedron.Two of the 2-flurobenzoic acid anions formed a binuclear node by bridging two Co(II) atoms, and the distance of Co(1)–Co(1)I(i = 1–x, 2–y, 1–z) was 4.089(1) ? (Fig.1).By bonding two molecules of 4,4?-bpy on the two Co(II) nodes, a one-dimensional chain was formed along the [101] direction with the bond lengths and bond angles as follows: Co–O 2.010(2)～2.1948(2) ?, Co–N 2.159(2)～2.111(2) ?, N(1)–Co(1)–N(2) = 179.51(1)°, thus those two axial N atoms were almost on the same line.Four 2-flurobenzoic acid anions on the same node could be sorted by coordinate directions into two groups, the bridging and bidentate ligands.The latter were located on the same line of the two Co atoms and the bidentate ligands were perpendicular to the bridging ligands.The ligands were further linked by 4,4?-bpy into a one-dimensional chain.Two-dimensional supramolecular planes were constructed of chains in a staggered manner under the effect of weak hydrogen bonds between C(2) and C(7) of 4,4?-bpy and O(3)ii(ii = 1–x, 2–y, –z) of 2-flurobenzoic acid radical.Moreover, a three-dimensional molecular network was created by weak C–H··O hydrogen bonds formed by C(23) and carbonyl O(4)iii(iii = 1–x, 1/2+y, 1/2–z) of 2-flurobenzoic acid radical between the planes, d(C–H··O) = 3.303(3)～3.390 ?, D(C–H··O) = 159～171° (Table 2).

Fig.1.Infrared spectrum of complex 1

Fig.2.Molecular structure of the title compound 1 (i = 1–x, 2–y, 1–z)

Fig.3.View along the [101] direction of crystal structure of complex 1 (50% probability)

Fig.4.Structure of complex 1 along the [101] direction with hydrogen bonds (dashed lines)

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

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

This structure had its own unique features.Firstly, the ligands including 2-flurobenzoic acid anion and 4,4?-bpy formed their linkage with two Co atoms from [101] and [010] directions to construct the chains, and two- and three-dimensional supramolecular networks were retained by C–H··O hydrogen bonds between the chains and planes, all of which were built into three-dimensional 4.40? ′ 18.5? and 4.40? ′ 4.6? channels.Secondly, the structure was physically and thermally stable, which contained no crystal or coordinated water and would not decompose below 315 ℃.These properties would meet the basic standard of MOF materials.

To discuss the topic coordination from a topological viewpoint, bridged 2-fluro benzoic acid anion was linked to three Co ions by coordinate linkages and hydrogen bonds.They formed three nodes, among which the top one was assigned (4.62).Each Co(II) ion was linked with three 2-fluro benzoic acid anions and two adjacent Co(II) ions in five nodes, with the top one to be (4.66.83).Topological symbol of the whole skeleton was (3,5)-c and the total Schlafli symbol was (4.62)(4.66.83) (Fig.5).This is a new topological network structure[26].

Fig.5.Topological representation of the 3D supramolecular architecture

3.3 Magnetic properties

Variable temperature magnetic susceptibility in 2～300 K range was recorded on a SQUID Quantum Design Model MPMS-7 magnetometer.Theeffmfor each Co2+ion at room temperature is 4.83 μB, which is significantly larger than the spin-only value of 3.87 μBexpected for isolated high-spin Co(II) (S = 3/2 and g = 2.0).The value falls into a reasonable range for complexes containing Co(II) centers in the high-spin state.The magnetic behavior should be due to a larger orbital contribution arising from the4T1gground state of Co(Ⅱ).Upon cooling, the cmT value gradually decreases from 2.92 to 1.94 cm3·K·mol–1at 10 K, and dramatically increases to 2.11 cm3·K·mol–1at 8 K, then drops to 1.57 cm3·K·mol–1at 2 K, indicating an overall antiferromagnetic coupling between the Co(Ⅱ) ions.The abnormal behavior at 8～10 K may be attributed to little ferromagnetic impurity.The cmT value was complied with the Curie-Weiss law as cm= C/(T – q), thus the Curie and Weiss constants were worked out by cm-1vs.T curve fitting, where C = 3.02(1) cm3·K·mol–1and q = –9.00(2) K.The onedimensional chain Co(Ⅱ) coordination polymer was simplified into a binuclear Co model in order to explore magnetic exchange of the topic coordination according to Heisenberg’s Hamiltonian of spin, while magnetic exchange between the binuclear Co units was taken into consideration.

Fitting was conducted according to formula (1)[27].

Where

Best fit constants: g = 1.63(1), J = –0.54(1) cm–1, zJ′= –0.09(2), and R = 1.0×10-5(R = ∑(cm·Tcalc–cm·Tobs)2/∑(cm·Tobs)2) (Fig.6).A negtive J indicated weak antiferromagnetic interaction in the Co units and a negative zJ′ suggested weak antiferromagnetic coupling between the binuclear Co, which was consistent with the trend of the curve.

Fig.6.Temperature dependece of the magnetic susceptibility of cm–T and cmT–T

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16 January 2015; accepted 29 April 2015 (CCDC 1059934)

① This project was supported by the National Natural Science Foundation of China (51343003) and the Scientific Research Projects of Zhejiang Provincial Education Department (Y201224707)

② Corresponding author.Professor, majoring in coordination chemistry.E-mail: zbs_jy@163.com

10.14102/j.cnki.0254-5861.2011-0640