

Crystal Structure, Spectroscopic Characterization, and Electrochemical and Thermal Stability Properties of a Dinuclear Nickel(II) Complex①
English
Crystal Structure, Spectroscopic Characterization, and Electrochemical and Thermal Stability Properties of a Dinuclear Nickel(II) Complex①
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1 INTRODUCTION
Recently, increasing attention has been paid to the use of dicarboxylic acid bridging units in the construction of supramolecular architectures[1], and this approach is attractive because the variety and conformational freedoms of such ligands offer the possibility for the construction of unprecedented frameworks[2]. Design and synthesis of metal-organic complexes have attracted much attention not only for their novel structures, but also for their potential applications in catalysis[3], photochemistry[4], magnetism[5], materials science[6], sorption properties[7] and medicine[8]. The aromatic carboxylic acids with a variety of coordinating modes are often used as ligands to construct metal-organic complexes because their complexes can exhibit high thermal, good physical and chemical properties in practical use[9].Methy-bicycle[2.2.1]hept-5-ene-2, 3-dicarboxylic acid (H2L) is an important aromatic carboxylic acid, which is a main raw material of curing agent in the setting system of epoxy resin, applied widely in electroinsulating materials home and overseas, especially those for large electric appliances[10]. In order to obtain knowledge about the structures and functions of metal organic complexes constructed with flexible cyclic carboxylic ligands, we synthesized a new nickel complex Ni2(L)2(2, 2′- bipy)2·5.5H2O (1) using H2L and 2, 2′-bipy as the ligands. Also the preliminary results of the thermal stability and electrochemical properties of this complex were reported.
2 EXPERIMENTAL
2.1 Materials and instruments
All reagents were of analytical grade and used as obtained from commercial sources and used without further purification; H2L was prepared by ourselves[11]. Crystal structure determination was carried out on a Bruker SMART APEX CCD single-crystal diffractometer. Elemental analyses were performed on a Perkin-Elmer 2400 elemental analyzer. The IR spectra were recorded on a Bruker Vector 22 FT-IR spectrophotometer using KBr discs. The ultraviolet absorption spectra were recorded on UV-2501PC UV-visible spectrophotometry (Shimadzu). The electrochemical measurements were performed using an EC 550 (Wuhan Gaoshi Ruilian Company) electrochemical analyzer.
2.2 Synthesis of 1
Complex 1 was prepared by 0.36 mmol of H2L (0.0645 g) and 0.48 mmol of nickel acetate (0.1262 g) added to 20 mL mixed solvents of DMF and water (5:2 in volume). After stirring at 50~55 ℃ for about 3.0 h, 0.46 mmol of 2, 2′-bipy (0.0719 g) was added to the mixture with the pH being adjusted to 6.0~7.0 with dilute sodium hydroxide, then further stirred at 68 ℃ for 12 h. Afterwards, the resultant solution was filtrated, and the filtrate was kept untouched and evaporated slowly at room temperature. Blue block-shaped single crystals suitable for X-ray diffraction analysis were obtained after three weeks. Yield: 46.4%. m.p.: 248~250 ℃. Anal. Calcd. (%) for C40H47N4Ni2O13.50: C, 52.38; H, 5.16; N, 6.11. Found (%): C, 52.22; H, 5.18; N, 6.09. Main IR (KBr, cm-1): 3419(w), 2977(s), 1637(vs), 1559(vs), 1465 (vs), 1350 (vs), 1279(m), 1220(w), 1032(m), 767(vs), 733(m), 651(w), 559.4(w).
2.3 Crystal structure determination
A single crystal with dimensions of 0.18mm × 0.17mm × 0.15mm was put on a Bruker SMART APEX CCD diffractometer equipped with a graphite-monochromatic MoKα radiation (λ = 0.71073 Å) using a φ-ω scan mode at 173(2) K. A total of 11, 494 reflections were collected in the range of 1.60≤θ≤25.01°, of which 6, 969 were independent (Rint = 0.0617) and 4, 491 were observed (I > 2σ(I)). All data were corrected by Lp factors and empirical absorption. The crystal structure was solved directly by program SHELXS-97, and refined by program SHELXL-97[12]. The hydrogen and non-hydrogen atoms were corrected by isotropic and anisotropic temperature factors respectively through full-matrix least-squares method. The final R = 0.0457, wR = 0.0974 (w = 1/[σ2(Fo 2) + (0.1353P)2 + 0.0000P], where P = (Fo 2 + 2Fc 2)/3); (Δ/σ) max = 0.000, S = 1.028, (Δρ) max = 0.0808 and (Δρ) min = -0.2036 e·Å-3.
3 RESULTS AND DISCUSSION
3.1 Crystal structure
The molecular structure of 1 is shown in Fig. 1. The selected bond lengths and bond angles are shown in Table 1, and hydrogen bonds in Table 2.
Bond Dist. Bond Dist. Bond Dist. Ni(1)-O(3) 1.947(5) Ni(2)-N(4) 2.002(6) Ni(2)-O(7B) 1.938(5) Ni(1)-N(1) 1.992(6) Ni(1)-O(1) 1.965(5) Ni(2)-N(3) 1.985(6) Ni(1)-O(1A) 2.229(5) Ni(1)-N(2) 1.997(6) Ni(2)-O(5) 2.230(5) Ni(2)-O(5B) 1.965(4) Angle (°) Angle (°) Angle (°) O(3)-Ni(1)-O(1) 90.1(2) O(5B)-Ni(2)-N(4) 93.7(2) N(2)-Ni(1)-O(1A) 96.0(2) O(1)-Ni(1)-N(1) 174.1(2) O(7B)-Ni(2)-O(5) 95.54(19) O(7B)-Ni(2)-N(3) 93.9(2) O(1)-Ni(1)-N(2) 93.3(2) N(3)-Ni(2)-O(5) 101.0(2) O(7B)-Ni(2)-N(4) 166.8(2) O(3)-Ni(1)-O(1A 96.31(19) O(3)-Ni(1)-N(1) 94.7(2) N(3)-Ni(2)-N(4) 81.4(2) N(1)-Ni(1)-O(1A 102.2(2) O(3)-Ni(1)-N(2) 167.6(2) O(5B)-Ni(2)-O(5) 80.9(2) O(7B)-Ni(2)-O(5 90.48(19) N(1)-Ni(1)-N(2) 81.2(2) N(4)-Ni(2)-O(5) 97.5(2) O(5B)-Ni(2)-N(3 175.0(2) O(1)-Ni(1)-O(1A 80.55(19) Symmetry transformations used to generate the equivalent atoms: A: -x + 1, -y + 1, -z + 1; B: -x + 1, -y + 1, -z Table 1. Selected Bond Lengths (Å) and Bond Angles (°) of the ComplexD-H…A D-H H-A D…A ∠DHA O(11)-H(11A)…O(8)i 0.85 1.95 2.793(3) 174 O(11)-H(11B)…O(13)ii 0.85 1.94 2.784(5) 170 O(12)-H(12A)…O(10)ii 0.85 1.95 2.783(4) 167 O(12)-H(12B)…O(9) 0.85 1.86 2.607(6) 146 O(13)-H(13A)…O(2) 0.85 1.92 2.757(3) 169 O(14)-H(14A)…O(12)iii 0.85 1.96 2.806(3) 172 O(14)-H(14B)…O(10) 0.85 1.95 2.794(3) 171 O(11)-H(11B)…O(13) 0.85 1.94 2.779(10) 170 O(12)-H(12A)…O(13) 0.85 1.95 2.796(9) 177 O(12)-H(12B)…O(11) i 0.85 1.96 2.803(10) 171 O(13)-H(13A)…O(4) ii 0.85 1.98 2.822(9) 171 O(13)-H(13B)…O(9) 0.85 2.11 2.753(9) 132 O(14)-H(14A)…O(4)iii 0.85 1.95 2.758(7) 158 O(14)-H(14B)…O(12)iv 0.85 1.98 2.827(9) 175 Symmetry transformations used to generate the equivalent atoms: i: -1 + x, y, z; ii: x, -1+ y, z; iii: 1-x, 1- y, -z ; iv: 1- x, - y, 1 - z Table 2. Hydrogen Bond Lengths (Å) and Bond Angles (°)As shown in Fig. 1, the asymmetric unit in 1 contains two [Ni2(L)2(2, 2′-bipy)2] units and five and half free water molecules. In the independent unit, two nickel (II) ions are bridged by two μ2-η1:η0 3-carboxylate groups of L2- anions, where the end positions are occupied by two 2, 2′-bipy molecules. The coordination geometry of the Ni (1) atom is a distorted square pyramidal geometry with two nitrogen atoms from 2, 2′-bipy and three oxygen atoms from two L2- anions. In NiN2O3, N (1), N (2), O (1) and O (3) are located at the equator plane, and O (1A) is in the axial direction. A parallelogram is made of Ni (1), O (1), Ni (1A) and O (1A). The Ni⋅⋅⋅Ni distance which is bridged by two μ2-η1:η0 3-carboxylate groups of L2- anions is 3.204~3.196 Å. The Ni-N bond lengths range from 1.985 to 2.002 Å. The Ni-O bond lengths fall in the 1.938~2.230 Å range. The bond angle O/N-Ni-N/O is 80.9(2)~ 175.0(2)º. Another noticeable characteristic of the title complex is the strong hydrogen bonding interactions existing between water molecules such as O (12)-H (12A)…O (10) (2.783(3) Å, 167°) and O (14)-H (14B)…O (12) (2.806(3) Å, 172°) as well as between water molecules and H2L ligand like O (13)-H (13A)…O (4) (2.882(3) Å, 171°), O (11)- H (11A)…O (8) (2.793(3) Å, 174°) and O (13)- H (13A)…O (2) (2.757(3) Å, 169°). These hydrogen bonding interactions benefit the stability of the complex structure.
3.2 Spectroscopic characterization
Infrared spectrum of the complex demonstrates an absorption peak at 3, 419 cm-1 ascribed to the characteristic absorption peak of OH in H2O. The anti-symmetric and symmetric stretching vibration absorption peaks of carboxylic group in the ligand appear at 1, 637 and 1, 350 cm-1, which are in contrast with the corresponding peaks of free ligands (at 1, 712 and 1, 224 cm-1, respectively) and shift remarkably. The Δv (vas (coo-) - vs (coo-)) of 287 cm-1 is larger than 200 cm-1, so the carboxyl in the complex is coordinated in a monodentate mode[13]. The IR spectra of strong peaks are at 1, 559, 1, 465 and 767 cm-1 due to the ligand 2, 2′-bipy characteristic absorption peaks.
The ultraviolet absorption spectra of ligand and complexes were determined in methanol solution at the range from 200 to 400 nm. UV spectra showed that the ligand 2, 2′-bipy had strong absorption peak at 284 nm, methy-bicycle[2.2.1]hept-5-ene-2, 3-dicarboxylic acid had strong absorption peak at 226.00 nm, and the strong absorption peak of the complex is located at 232.00 nm. The complex strong absorption peak should belong to π → π* transition absorption of the ligand of the methybicycle[2.2.1]hept-5-ene-2, 3-dicarboxylic acid[14].
3.3 Electrochemical properties
A conventional three-electrode system was employed for the cyclic voltammetric measurement (CV), where a Ag/AgCl electrode, a glassy carbon electrode and a platinum electrode were chosen respectively as the reference electrode, working electrode and counter electrode. The title complex was dissolved in a mixture of methanol and water (volume ratio of 3:1), and the resulting solution had a concentration of 6 × 10-5 mol L-1. A 0.006 M NaClO4 solution was used as the supporting electrolyte. The scan range was -0.20 to 0.60 V. The cyclic voltammogram of the complex at a potential scan rate of 0.10 V s-1 is shown in Fig. 2. There exists an oxidation peak with -0.046 V oxidation potential, which corresponds to the Ni (I)/Ni (II) oxidation process[15].
Under the same conditions, the electrochemical properties of the title complex were also measured by linear sweep stripping voltammetry. In the potential scan rate range of 0.01~0.80 V s-1, the influence of the potential scan rate (v) on the oxidation peak current (Ipa) and the oxidation peak potential (Epa) was studied. ipa is proportional to v, and the linear regression equation is Ipa = 31.5969v1/2 + 0.4341 (Ipa in μA; v in V s-1) with a correlation coefficient of 0.9964 (Fig. 3), which indicates that the electrode reaction process of the complex was controlled by diffusion. In addition, Epa shifts to a more positive value with increasing v, and it is proportional to lnv. The linear regression equation is Epa = 0.0688lnv + 0.4196 (Epa in V, v in V s-1) with a correlation coefficient of 0.9957 (Fig. 3).
3.4 Thermal stability properties
The thermogravimetric analysis (Fig. 4) of the title complex demonstrated that the weight loss of the complex in the air from room temperature to 600 ℃ occurred mainly in 3 stages. The first stage takes place from 116 to 175 ℃ with the weight loss of 10.70%, corresponding to the release of five and half free water molecules (calcd. 10.80%). The second stage occurs at 175~285 ℃ with the weight loss of 34.00%, resulting from the loss of two 2, 2′-bipy molecules (calcd. 34.06%). The strong endothermic peak near 249 ℃ can be attributed to melting endothermic of the complexes, which conforms to the melting point of the complex. The third stage is observed from 285 to 430 ℃ with the weight loss of 39.90% due to the departure of two L2- anions (calcd. 38.85%), which is in agreement the crystal structure. In air, the final product is nickel oxide with the residual weight being about 16.30% (calcd. 16.29%). Based on the above judgment, the pyrolytic process of the complex may be divided into the following stages:
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Table 1. Selected Bond Lengths (Å) and Bond Angles (°) of the Complex
Bond Dist. Bond Dist. Bond Dist. Ni(1)-O(3) 1.947(5) Ni(2)-N(4) 2.002(6) Ni(2)-O(7B) 1.938(5) Ni(1)-N(1) 1.992(6) Ni(1)-O(1) 1.965(5) Ni(2)-N(3) 1.985(6) Ni(1)-O(1A) 2.229(5) Ni(1)-N(2) 1.997(6) Ni(2)-O(5) 2.230(5) Ni(2)-O(5B) 1.965(4) Angle (°) Angle (°) Angle (°) O(3)-Ni(1)-O(1) 90.1(2) O(5B)-Ni(2)-N(4) 93.7(2) N(2)-Ni(1)-O(1A) 96.0(2) O(1)-Ni(1)-N(1) 174.1(2) O(7B)-Ni(2)-O(5) 95.54(19) O(7B)-Ni(2)-N(3) 93.9(2) O(1)-Ni(1)-N(2) 93.3(2) N(3)-Ni(2)-O(5) 101.0(2) O(7B)-Ni(2)-N(4) 166.8(2) O(3)-Ni(1)-O(1A 96.31(19) O(3)-Ni(1)-N(1) 94.7(2) N(3)-Ni(2)-N(4) 81.4(2) N(1)-Ni(1)-O(1A 102.2(2) O(3)-Ni(1)-N(2) 167.6(2) O(5B)-Ni(2)-O(5) 80.9(2) O(7B)-Ni(2)-O(5 90.48(19) N(1)-Ni(1)-N(2) 81.2(2) N(4)-Ni(2)-O(5) 97.5(2) O(5B)-Ni(2)-N(3 175.0(2) O(1)-Ni(1)-O(1A 80.55(19) Symmetry transformations used to generate the equivalent atoms: A: -x + 1, -y + 1, -z + 1; B: -x + 1, -y + 1, -z Table 2. Hydrogen Bond Lengths (Å) and Bond Angles (°)
D-H…A D-H H-A D…A ∠DHA O(11)-H(11A)…O(8)i 0.85 1.95 2.793(3) 174 O(11)-H(11B)…O(13)ii 0.85 1.94 2.784(5) 170 O(12)-H(12A)…O(10)ii 0.85 1.95 2.783(4) 167 O(12)-H(12B)…O(9) 0.85 1.86 2.607(6) 146 O(13)-H(13A)…O(2) 0.85 1.92 2.757(3) 169 O(14)-H(14A)…O(12)iii 0.85 1.96 2.806(3) 172 O(14)-H(14B)…O(10) 0.85 1.95 2.794(3) 171 O(11)-H(11B)…O(13) 0.85 1.94 2.779(10) 170 O(12)-H(12A)…O(13) 0.85 1.95 2.796(9) 177 O(12)-H(12B)…O(11) i 0.85 1.96 2.803(10) 171 O(13)-H(13A)…O(4) ii 0.85 1.98 2.822(9) 171 O(13)-H(13B)…O(9) 0.85 2.11 2.753(9) 132 O(14)-H(14A)…O(4)iii 0.85 1.95 2.758(7) 158 O(14)-H(14B)…O(12)iv 0.85 1.98 2.827(9) 175 Symmetry transformations used to generate the equivalent atoms: i: -1 + x, y, z; ii: x, -1+ y, z; iii: 1-x, 1- y, -z ; iv: 1- x, - y, 1 - z -

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