Solvothermal Synthesis, Crystal Structure and Characterization of a New Binuclear Nickel(II)Complex with 2-(4-Methylbenzoyl)benzoic Acid①

LI Wei LI Chng-HongLI Yu-Lin KUANG Yun-Fei

a (School of Chemistry and Materials Science, Hengyang Normal University, Hengyang 421008, China)

b (School of Materials and Chemical Engineering, Hunan Institute of Technology, Hengyang 421002, China)

ABSTRACT A new binuclear nickel(II) complex [Ni2(MBBA)2(HPT)2(H2O)2]∙2H2O (1) was synthesized with nickel hydroxide, 2-(4-methylbenzoyl)benzoic acid (HMBBA) and 3-(pyridin-2-yl)-1H-1,2,4-triazole (HPT). It crystallizes in the monoclinic space group P21/n with a = 14.5902(13), b = 8.8191(6), c = 17.6120(14) Å, β =110.830(9)°, V = 2118.0(3) Å3, Mr = 958.26, Dc = 1.503 g/cm3, Z = 2, μ(MoKa) = 0.959, F(000) = 992, the final GOOF = 1.001, R = 0.0353 and wR = 0.09864. Each central Ni(II) ion is coordinated by two oxygen and three nitrogen atoms, forming a distorted square pyramidal geometry. The XRD, TG, spectrum analysis and magnetic properties of 1 were studied.

Keywords: binuclear nickel(II) complex, spectrum analysis, thermal stability, magnetic property;

1 INTRODUCTION

The coordination polymer combines the advantages of inorganic metal ions and organic ligands that can be modified and easily regulated, so it has important applications in the fields of magnetism, catalysis, gas adsorption, superconductor classification, electrochemical sensing, nonlinear optics and ion exchange[1]. As an important and commonly used ligand,carboxylic acid has a relatively rigid structure and can form coordination polymers with different structures[2]. o-(4-Methylbenzoyl)-benzoic acid as a carboxylic acid biomolecular structure containing both carboxyl and formyl groups is a good ligand with multiple coordination sites and a useful intermediate in organic synthesis used chiefly in reducing dyes and fluorescent coatings[3]. 1,2,4-Triazole coordination polymer is not only unique in structure, but also has a wide range of applications in performance, mainly in gas adsorption and storage, which were mainly reported in copper complexes[4], but there are few reports on nickel complexes[5].To extend our previous work and construct new carboxylic acid complexes, we report herein a new binuclear nickel(II)complex [Ni2(MBBA)2(HPT)2(H2O)2]∙2H2O (1) with 3-(pyridin-2-yl)-1H-1,2,4-triazole (HPT) as a main ligand and o-(4-methylbenzoyl)benzoic acid (HMBBA) as an auxiliary ligand. The results show that the complex has antiferromagnetism, good stability at 363 K and strong fluorescence emission at 481 nm.

2 EXPERIMENTAL

2. 1 Materials and instruments

All reagents obtained from commercial sources were analytical grade and used without further purification. HPT was prepared according to the previously reported methods[6].Crystal structure determination was carried out on a Bruker SMART CCD 6000 single-crystal diffractometer. C, H and N analyses were performed on a Perkin-Elmer 2400 elemental analyzer. IR spectra were recorded on a Bruker Vector22 FT-IR spectrophotometer using KBr discs. XRD analyses were performed on a D8 advance with voltage 40 KV, current 40 step 0.02°, test speed 0.1 sec·step-1, copper target and incoming ray wavelength 0.15418 nm. Magnetic measurements in the range of 300～2 K were performed on a MPMS-SQUID magnetometer at a field of 2 kOe on a crystalline sample in the temperature settle mode. Thermogravimetric analyses were performed on a simultaneous SPRT-2 pyris1 thermal analyzer at a heating rate of 10 K·min-1. The fluorescence for the powdered samples was measured on an RF-5301PC spectrofluorometer with a xenon arc lamp as the light source.

2. 2 Synthesis of 1

A mixture of HMBBA (72 mg, 0.3 mmol), HPT (14.6 mg,0.1 mmol) and Ni(OH)2(27.8 mg, 0.3 mmol) was dissolved in 25 mL of mixed solvent (The volume ratio of water and DMF is 5:3). The pH value of the resultant mixture was adjusted to 6.5 by adding sodium hydroxide solution. The reaction was kept at 433 K for 72 h, and cooled to room temperature at the speed of 10 K·h-1. The green crystals of 1 suitable for X-ray diffraction analysis were obtained in 53.2% yield calculated according to HPT. m.p.: 533～535 K. Anal. Calcd. (%) for C44H40N8Ni2O10(Mr= 958.26): C, 55.15; H, 4.21; N, 11.69.Found (%): C, 55.04; H, 4.22; N, 11.66. Main IR (KBr, cm-1):IR (v/cm-1): 3422(w), 3121(w), 1659(vs), 1613(vs), 1481 (m),1362(vs), 1285(m), 1152(m), 934(w), 719(m), 605(w),472(w).

2. 3 Determination of the crystal structure

A single crystal with dimensions of 0.22mm × 0.14mm ×0.13mm was put on a Bruker SMART CCD 6000 diffractometer equipped with a graphite-monochromatic MoKα radiation (λ = 0.71073 Å) using an ω scan mode at 100 K. A total of 9525 reflections were collected in the range of 2.25≤θ≤25.01°, of which 3732 were independent (Rint= 0.0472,Rsigma= 0.0633) and 2873 were observed (I > 2σ(I)). All data were corrected by Lp factors and empirical absorption[7]. The crystal structure was solved directly by program SHELXS-2015, and refined by program SHELXL-2015[8]. The hydrogen and non-hydrogen atoms were corrected by isotropic and anisotropic temperature factors respectively through full-matrix least-squares method. The final R =0.0398, wR = 0.0864 (w = 1/[σ2(Fo2) + (0.0419P)2+ 0.0000P],where P = (Fo2+ 2Fc2)/3), (Δ/σ)max= 0.001, S = 1.001, (Δρ)max= 0.383 and (Δρ)min= -0.311 e∙Å-3.

3 RESULTS AND DISCUSSION

3. 1 Crystal structure of 1

The coordination structure of 1 is revealed in Fig.1, square pyramidal coordination geometry of the central Ni(II) ion of 1 is given in Fig.2, and its hydrogen bond linking of the neighboring molecules is shown in Fig.3. Selected bond lengths and bond angles are shown in Table 1, and hydrogen bonds of the complex in Table 2.

Table 1. Selected Bond Lengths (Å) and Bond Angles (°) of Complex 1

Table 2. Hydrogen Bond Lengths (Å) and Bond Angles (°) of Complex 1

Fig.1. Coordination structure of 1 (A: 1-x, 1-y, 1-z)

Fig.2. Square pyramidal coordination geometry of the central Ni(II) ion of 1

Fig.3. Hydrogen bond linking of the neighboring molecule of 1

[Ni2(MBBA)2(HPT)2(H2O)2]∙2H2O (1) is a binuclear complex in space group P21/n. As illustrated in Fig.1, it is inclusive of two Ni(II) ions, two PT-1anions, two MBBA-1anions and two water molecules. The whole molecule displays a symmetric binuclear structure, in which two nickel atoms are linked with two K2:N1:N2-3-(pyridin-2-yl)-1,2,4-triazole molecules, forming a stable six-membered ring structure. In Fig.2, each Ni(1) atom is coordinated with two oxygen atoms(O(1) from MBBA-1anions, O(4) from water molecule) and three nitrogen atoms (N(4) from pyridine ring, N(1) and N(2A)(symmetry code A: 1-x, 1-y, 1-z) from triazole ring), forming a distorted square-pyramidal coordination geometry. The bond lengths and angles around Ni(1) are in the ranges of 1.956(2)～2.264(2) Å and 80.12(10)～176.12(9)°,respectively. The Ni-N bond lengths vary from 1.975 to 2.039 Å, with the average to be 1.998 Å, which is very close to that for the similar complex [Ni2(C7N4H5)3(C9H9O2)]n(Ni-N =1.999 Å)[5d]. The coordination mode of the carboxylate group is mondentate. The Ni-O(carboxyl)distance is 1.956(2) Å, which is shorter than the value for the similar compound[Ni2(C7N4H5)3(C9H9O2)]n(Ni-O = 1.983 Å). More interestingly, two Ni ions, two PT-1and MBBA-1anions form a basic binuclear unit Ni2(MBBA)2(HPT)2. There are six- and five-membered rings in the complex. The six-membered rings consist of N(1), N(2), Ni(1), N(1A), N(2A) and Ni(1A(symmetry code: A: 1-x, 1-y, 1-z) atoms and pyridine ring,and the five-membered ones are composed of Ni(1), N(1),N(4), C(1) and C(3) atoms and triazole ring. In the six-membered ring, the Ni(1)∙Ni(1A) distance is 3.991 Å which is slightly shorter than that in similar Ni(II) complexes([Ni2(C7N4H5)3(C9H9O2)]n, Ni(1)∙Ni(1A) = 4.230 Å and Ni2(HPT)4(H2O)3, Ni(1)∙∙Ni(1A) = 4.125 Å[5e]) and falls in the normal range. In addition, as given in Fig.3, the shortest centroid distance between aromatic cycles of MBBA-1group is 0.3282 Å, smaller than 0.3700 Å, indicating a π-π stacking interaction between the aromatic cycles[9]. It is worthwhile to note that hydrogen bonding interactions are usually important in the synthesis of supramolecular structure. There are O-H∙∙O hydrogen bonding interactions between water molecules and carboxylic acid oxygen atoms: O(4)-H(4A)∙O(5)i, O(4)-H(4B)∙∙O(5)ii, O(5)-H(5B)∙O(3)iii(symmetry codes: i: 1-x, -y, 1-z; ii: -1/2+x, 1/2-y, -1/2+z; iii:1/2+x, 1/2-y, 1/2+z) and O-H∙∙N hydrogen bonding interactions between crystal water molecules and HPT nitrogen atoms: O(5)-H(5A)∙∙N(3). They link the complexes into a three-dimensional structure[10].

3. 2 Spectrum analysis

Infrared spectrum of the complex demonstrates an absorption peak at 3422 cm-1ascribed to the characteristic absorption peak of water molecules. The antisymmetrical and symmetrical stretching vibration absorption peaks of carboxylic group in the ligand appear at 1659, 1612 and 1362 cm-1, which are in contrast with the corresponding peaks of free ligands (at 1691, 1673 and 1449 cm-1, respectively) and shift remarkably. The Δvcoo-(vas(coo-)- vs(coo-)) of the complex is 297 cm-1, which is larger than 200 cm-1, so the carboxyl is coordinated with monodentate modes[9]. The strong peaks at 1481, 934 and 719 cm-1are ascribed to the characteristic absorption peaks of the HPT ligand.

Fig.3 shows the emission spectra of the title complex and free ligands in solid state at ambient temperature recorded in the range of 360～580 nm. As seen in Fig.4. HPT, HMMBA and 1 display the characteristic emission peaks at 450, 408 and 481 nm, respectively. The emission spectra of 1 have the same characteristic emission peak and similar figure with the free ligand HPT, but its luminescence is stronger. The emission bands of 1 are red-shifted by 31 nm as compared to the HPT ligand, which is attributed to the coordinative interactions between the metal atom and the ligand. Such emission bands may be tentatively assigned to ligand-to-metal charge transfer (LMCT)[11].

Fig.4. Luminescence property curves of 1 and the ligands(a: complex of 1; b: HPT; c: HMMBA)

3. 3 Powder XRD and magnetic properties

Powder X-ray diffraction (PXRD) experiment at room temperature was carried out to investigate the purity of compound 1 (Fig.5). The main peaks observed match well with the simulated ones, indicating the phase purity of the as-synthesized product[12]. The temperature dependence of the molar magnetic susceptibility of the complex, investigated in the range of 300～2 K with an applied magnetic field of 2 kOe, in the forms of XmT and 1/Xmvs. T is revealed in Fig.6.The product of XmT is 1.2887 cm3· K·mol-1at 300 K, which is a little larger than the same complex Ni2(HPT)6(H2O)3(XmT =1.843 cm3·K·mol-1)[5e], [Ni4(L)2(m2-OAc)2(m3-OAc)2]∙CH3OH∙H2O (XmT = 3.42 cm3·K·mol-1)[13]and[Ni(5-Br-ip)(bib)]n(XmT = 2.51 cm3·K·mol-1)[14]. Upon cooling, the XmT gradually decreases until 22 K to reach a product of 0.00836 cm3· K·mol-1. 1/Xm decreases slowly with the decrease of temperature and rises suddenly when reaching 100 K. When the temperature drops to 18 K, 1/Xmshows the maximum value, then drops sharply.

In addition, as shown in Fig.6, the data are in linear relationship in the form of 1/Xmvs. T. The linear regression equation is 1/Xm= 0.0045T + 0.0014 with a correlation coefficient of 0.9969. According to the Curie-Weiss law, from Xm= C/(T − θ), the Weiss constant θ can be obtained, C =5.308 and θ = -0.00743 K. These magnetic behaviors show antiferromagnetism for the title complex. In this work, the main magnetic interactions may be considered to occur between adjacent nickel(II) ions bridged by triazole rings(N(1), N(2), N(1A), N(2A)) (symmetry code: A: 1-x, 1 -y,1-z), with the same type of bridge in the Ni(II) complexes[15].

Fig.5. XRD patterns of 1 simulated from X-ray single-crystal diffraction and experimental data

Fig.6. Temperature dependence of the magnetic susceptibility of 1 in the form of XmT vs. T and 1/Xm vs. T

3. 4 Thermal stability

The thermogravimetric analysis (Fig.7) of 1 demonstrated that the weight loss of the complex in the air from room temperature to 873 K occurred mainly in 3 stages. The first stage takes place from 363 to 408 K with the weight loss of 7.50%, corresponding to the release of free and coordinated water molecules (calcd.: 7.52%). At the same time, one thermal absorption peak of DTG appears at about 390 K,indicating the decomposition of bound water. The second one occurs from 523 to 728 K with the weight loss of 30.0%,resulting from the loss of two HPT molecules (calcd.:30.30%). The second DTG thermal absorption peak occurs at about 533 K, which belongs to a strong thermal decomposition of 1. From stages 2 to 3, the weight loss of 47.0% (theoretical value: 46.59%) is found at 728～833 K due to the departure of 30 carbon, 22 hydrogen and 5 oxygen atoms, which are from two MBBA anions. The final product is nickel oxide, with the final residual rate to be 15.50%(calcd.: 15.59%).

4 CONCLUSION

This paper reports the synthesis and structure of a new binuclear nickel compound[Ni2(MBBA)2(HPT)2(H2O)2]∙2H2O based on HPT and HMBBA. Its XRD, TG analysis and magnetic properties were studied. Our research shows that 1 is antiferromagnetic, and gives off an intense fluorescence at around 481 nm.