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Synthesis, structure, and magnetic property of a cobalt(Ⅱ) complex based on pyridyl-substituted imino nitroxide radical

Xiaoling WANG ,  Hongwu ZHANG ,  Daofu LIU

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Citation:  Xiaoling WANG, Hongwu ZHANG, Daofu LIU. Synthesis, structure, and magnetic property of a cobalt(Ⅱ) complex based on pyridyl-substituted imino nitroxide radical[J]. Chinese Journal of Inorganic Chemistry, 2025, 41(2): 407-412. doi: 10.11862/CJIC.20240214 shu

基于吡啶取代的单臂氮氧自由基的钴(Ⅱ)配合物的合成、结构和磁性质

  • .

    淮南师范学院化学与材料工程学院, 淮南 232038

    • 通讯作者: 王晓玲, wxlnankai@126.com
摘要: 用对硝基苯甲酸(HPNB)和一种单臂氮氧自由基反应, 合成了一种新的配合物[Co (im4-py)2(PNB)2](im4-py=2-(4'-吡啶基)-4,4,5,5-四甲基咪唑啉-1-氧基自由基), 并对其进行X射线衍射、元素分析、红外和磁性质等表征。晶体结构分析表明, 配合物中每个Co(Ⅱ)处于四配位的变形四面体构型中, 分别与2个im4-py和2个PNB-配位。磁性质研究表明, 配合物中Co(Ⅱ)离子和自由基之间存在着弱的铁磁相互作用。通过磁构交换机理对磁性质进行了解释。

English

  • The design of magnetic compounds arising from the combination of transition metal ions with nitroxide radicals is steadily increasing[1-4]. Nitroxide radicals as stable organic radicals, with a single electron, which produces a strong magnetic exchange effect with paramagnetic metals while coordinating with metal ions, have been widely used as molecular units in constructing magnetic materials[5-7]. Therefore, chemists pay much attention to the design of different kinds of metal-radical complex architectures with appropriate organic nitroxide radicals and coligands in the research. A large number of metal-radical complexes with various structures have been synthesized and magnetically characterized[8-13]. Meanwhile, the cobalt(Ⅱ) ion is a very effective spin carrier and has become a good candidate for the construction of single-molecule magnets (SMMs) and single-chain magnets (SCMs) due to a large spin-orbital coupling[14-15]. Investigation of the fundamental magneto-structural correlation of Co(Ⅱ) imino nitroxides is necessary, especially how the structural factors affect the metal-radical interactions. Such investigation is necessary not only for understanding the magnetic exchange mechanism between the metal and organic radical but also for guiding the design of new organic radical ligands for the development of novel molecular magnetic materials[16-17]. On the other hand, flexible carboxylic acids such as p-nitrobenzoic acid (HPNB) can coordinate with metal ions with great flexibility in the chemical design of molecular assemblies.

    With this in mind, taking advantage of the abilities of imino nitroxide radicals and PNB- to coordinate to transition metals, we have synthesized a new cobalt(Ⅱ)-radical complex: [Co(im4-py)2(PNB)2], where im4-py=2-(4′-pyridyl)-4,4,5,5-tetramethylimidazole-1-oxyl.

    1.   Experimental

    1.1   Material and instrumentation

    All the starting materials were analytical grade. Elemental analyses for C, H, and N were carried out on a Model 240 Perkin-Elmer elemental analyzer. The infrared spectra were taken on a Bruker Tensor 27 IR spectrometer in a 4 000-400 cm-1 range using KBr pellets. Crystal data of the complex were collected on a Rigaku Saturn CCD diffractometer. Variable-temperature magnetic susceptibilities were measured on an MPMS XL-7 SQUID magnetometer at 2-300 K. Diamagnetic corrections were made with Pascal′s constants for all the constituent atoms.

    1.2   Synthesis

    The imino nitroxide radical im4-py was prepared using the literature method[18-19]. The complex [Co(im4-py)2(PNB)2] was synthesized by adding im4-py (0.087 2 g, 0.4 mmol) to a solution of Co(ClO4)2·6H2O (0.073 2 g, 0.2 mmol) in 20 mL methanol. After stirring for 30 min, the resulting red solution was mixed with a solution of NaPNB (0.076 g, 0.4 mmol) in 5 mL water. Further stirring for 2 h gave rise to a red solution which was filtered and kept at room temperature for 4 d, giving red single crystals suitable for X-ray studies. Yield: 0.088 g (53%). Anal. Calcd. For C38H40CoN8O10(%): C, 55.09; H, 4.87; N, 13.53. Found(%): C, 55.10; H, 4.90; N, 13.50. IR data (KBr, cm-1): 1 625 cm-1 [νas(CO2-)], 1 382 cm-1 [νs(CO2-)], 1 525 cm-1 [νas(NO2)], 1 355 cm-1 [νs(NO2)], 1 375 cm-1 [ν(NO)].

    1.3   X-ray crystallography

    A red single crystal of the complex having approximate dimensions of 0.22 mm×0.18 mm×0.16 mm was put on a Rigaku Saturn CCD diffractometer at room temperature with graphite-monochromated Mo Kα radiation (λ=0.071 073 nm). A total of 10 407 reflections were collected in the 3.31° ≤ θ ≤2 5.01° range at 293(2) K with index ranges of -40 ≤ h ≤ 40, -7 ≤ k ≤ 7, and -24 ≤ l ≤ 29 including 3384 independent reflections with Rint=0.064 3. The structure was solved by direct methods using the SHELXS-97 program[20] and refined with SHELXL-97[21] by full-matrix least-squares techniques on F 2. Non-hydrogen atoms were refined anisotropically while the hydrogen atoms were located geometrically and refined isotropically. The corresponding crystal data and collection/refinement parameters are summarized in Table 1. Selected bond lengths and bond angles are listed in Table 2.

    Table 1

    Table 1.  Crystal data and structure refinements for the complex
    下载: 导出CSV
    Parameter [Co(im4-py)2(PNB)2] Parameter [Co(im4-py)2(PNB)2]
    Empirical formula C38H40CoN8O10 μ/mm-1 0.516
    Formula weight 827.72 F(000) 1 724
    Crystal system Monoclinic θ range/(°) 3.31‐25.01
    Space group C2/c Reflection collected 10 407
    a/nm 3.400 1(4) Unique reflection 3 384
    b/nm 0.634 44(7) Data, restraint, number of parameters 3 384, 0, 262
    c/nm 2.461 5(3) GOF 1.062 7
    β/(°) 133.676(2) Final R indices [I > 2σ(I)] R1=0.064 3, wR2=0.156 0
    V/nm3 3.840 4(8) R indices (all data) R1=0.084 6, wR2=0.168 1
    Z 4 Largest diff. peak and hole/(e·nm-3) 609.6 and -679.3
    Dc/(g·cm-3) 1.432

    Table 2

    Table 2.  Selected bond lengths (nm) and bond angles (°) for the complex
    下载: 导出CSV
    Co1—O2 0.196 4(2) Co1—N1 0.203 3(3) N2—O1 0.126 5(4)
    O2A—Co1—O2 94.93(15) O2—Co1—N1 120.60(11) O2—Co1—N1A 106.30(11)
    N1A—Co1—N1 108.54(16)
    Symmetry code: A: -x, y, -z+0.5.

    2.   Results and discussion

    2.1   Description of the crystal structure

    The ORTEP drawing of the molecular structure of [Co(im4-py)2(PNB)2] with the atomic labeling is shown in Fig. 1. X-ray crystallography shows that the Co(Ⅱ) ion is four-coordinated by two pyridyl-N atoms from im4-py radicals (N1, N1A) and two oxygen atoms (O2, O2A) from PNB- anions to form a slightly distorted tetrahedron whose center is occupied by the Co(Ⅱ) ion. The molecular structure of this complex is similar to that of [Co(NIT4-py)2(PNB)2][22] except for the radical ligands. The Co1—N1 (pyridyl) bond distance is 0.203 3(3) nm, while the Co1—O2 (PNB-) bond distance is 0.196 4(2) nm. Two im4-py radicals are symmetrical in space with a bond length of 0.126 5(6) nm for N2—O1, which corresponds to the distance in nitroxide ranges from 0.125 to 0.132 nm[23-24]. In the complex, the radical only behaves as a monodentate ligand through the pyridyl-N atoms while the nitrogen and oxygen atoms of nitroxide moiety remain uncoordinated, which is in agreement with the observation that the N—O stretching vibration at 1 375 cm-1 for free radical in IR spectrum. The fragment O1—N2—C6—N3 is nearly coplanar as expected, indicating the easy delocalization of a single electron by this moiety. The dihedral angles formed by the pyridyl ring and nitroxide groups (O1—N2—C6—N3) of the im4-py ligand is 19.24°.

    Figure 1

    Figure 1.  ORTEP drawing of [Co(im4-py)2(PNB)2] with atom-labeling and 30% thermal ellipsoids

    Symmetry code: A: -x, y, -z+0.5.

    下载: 全尺寸图片 幻灯片

    Fig. 2 shows the packing diagram of the complex. The nearest distance of the uncoordinated NO groups is 0.544 nm. The weak intermolecular hydrogen bonds between the carbon atom of the pyridyl ring of the nitroxide group and the uncoordinated oxygen atom of PNB- (C1…O5B 0.329 9 nm, symmetry code: x-0.5, 2.5-y, z-0.5) exist in the complex. The molecule is linked together via the hydrogen bonds, generating a 3D structure. The 3D net is maintained and stabilized by the hydrogen bonds.

    Figure 2

    Figure 2.  View showing a 3D network formed by hydrogen bonds in [Co(im4-py)2(PNB)2]

    Symmetry code: B: x-0.5, 2.5-y, z-0.5.

    下载: 全尺寸图片 幻灯片

    2.2   Spectroscopic properties

    The infrared spectrum of [Co(im4-py)2(PNB)2] displayed strong bands at 1 382 and 1 625 cm-1, which can be attributed to the νs(CO2-) and νas(CO2-), respectively. The two characteristic strong bands (1 525 and 1 355 cm-1) in the 1 520-1 360 cm-1 region can be assigned to the stretching vibrations of the coordinated CO2- groups. The two bands are separated by 170 cm-1, suggesting a bidentate coordination mode for the CO2- groups. In addition, the N—O stretching vibration at 1 375 cm-1 indicates the existence of an uncoordinated N—O group of the im4-py ligand, which is in agreement with the results of structure analysis.

    2.3   Magnetic property

    The temperature dependence of the magnetic susceptibility of [Co(im4-py)2(PNB)2] was measured in the 2-300 K region under an applied magnetic field of 1 000 Oe and the magnetic behavior is shown in Fig. 3. At room temperature, the value of χMT was 3.06 cm3·mol-1·K, which was higher than spin-only value (2.63 cm3·mol-1·K) expected for non-interacting SCo=3/2 and Srad=1/2 spins, indicating that an important orbital contribution is involved. It can be seen that, upon cooling, the χMT value was almost constant at 3.06 cm3·mol-1·K at 37 K and then it gradually decreased to 2.81 cm3·mol-1·K at 3 K and finally increased up to 3.18 cm3·mol-1·K at 2 K.

    Figure 3

    Figure 3.  Plots of χMT (◇) and 1/χM (○) vs T for [Co(im4-py)2(PNB)2]
    下载: 全尺寸图片 幻灯片

    The gradual decrease of the χMT below 37 K may be due to the spin-orbit coupling of cobalt(Ⅱ), while the finally increasing trends of χMT value below 2 K suggest the presence of ferromagnetic interactions between cobalt(Ⅱ) and im4-py radical. The magnetic susceptibility data can be fitted to the Curie-Weiss law with C=3.049 cm3·mol-1·K and θ=0.148 K, implying that a ferromagnetic interaction exists in the complex. The magnetic study showing that the complex is a ferromagnetic interaction between the cobalt(Ⅱ) ion and im4-py radical ligand may be explained by a spin polarization mechanism[25], which is shown in Scheme 1. Based on this model, a spin distribution would induce a contrary spin under the adjacent atom owing to the spin polarization. In the radical ligand, the spin distribution arising from intramolecular spin polarization of the adjacent atoms leads to alternating positive and negative spin density on the carbon backbone of the radical ligands and delocalizes on the pyridyl ring. In this complex, based on the tetrahedral arrangement of the Co(Ⅱ) ion, the magnetic orbits of Co(Ⅱ) are dxy, dxz, and dyz, which are overlapped to the p orbital of the pyridyl nitrogen atom in im4-py radical, so the Co(Ⅱ) spins interact antiferromagnetically with the fractional spin of the p orbital on the coordinating nitrogen atom, namely Co(Ⅱ) spin and the fractional spin of the p orbital on the coordinating nitrogen atom are opposite. Assuming that the positive spin density is located on the Co(Ⅱ) ion, while the negative spin density is located on the coordinating nitrogen atom in im4-py radical, and the alternative delocalization takes place over the pyridine ring because of the spin polarization. This in turn induces a positive spin density on the NO groups of im4-py (Scheme 1). According to the spin polarization mechanism, this leads to a ferromagnetic interaction between the Co(Ⅱ) ion and the im4-py radical.

    Scheme 1

    Scheme 1.  Spin polarization mechanism for intramolecular magnetic coupling in [Co(im4-py)2(PNB)2]
    下载: 全尺寸图片 幻灯片

    3.   Conclusions

    We have successfully obtained a new Co(Ⅱ) complex with imino nitroxide radical ligand. The crystal structural analyses indicate that the radical ligands are coordinated to the Co(Ⅱ) ion via the nitrogen atoms of the pyridine rings to form a spin complex. A ferromagnetic interaction was observed in the complex. The spin polarization mechanism is used to explain the magnetic coupling for the complex.


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  • Figure 1  ORTEP drawing of [Co(im4-py)2(PNB)2] with atom-labeling and 30% thermal ellipsoids

    Symmetry code: A: -x, y, -z+0.5.

    下载: 全尺寸图片 幻灯片

    Figure 2  View showing a 3D network formed by hydrogen bonds in [Co(im4-py)2(PNB)2]

    Symmetry code: B: x-0.5, 2.5-y, z-0.5.

    下载: 全尺寸图片 幻灯片

    Figure 3  Plots of χMT (◇) and 1/χM (○) vs T for [Co(im4-py)2(PNB)2]

    下载: 全尺寸图片 幻灯片

    Scheme 1  Spin polarization mechanism for intramolecular magnetic coupling in [Co(im4-py)2(PNB)2]

    下载: 全尺寸图片 幻灯片

    Table 1.  Crystal data and structure refinements for the complex

    Parameter [Co(im4-py)2(PNB)2] Parameter [Co(im4-py)2(PNB)2]
    Empirical formula C38H40CoN8O10 μ/mm-1 0.516
    Formula weight 827.72 F(000) 1 724
    Crystal system Monoclinic θ range/(°) 3.31‐25.01
    Space group C2/c Reflection collected 10 407
    a/nm 3.400 1(4) Unique reflection 3 384
    b/nm 0.634 44(7) Data, restraint, number of parameters 3 384, 0, 262
    c/nm 2.461 5(3) GOF 1.062 7
    β/(°) 133.676(2) Final R indices [I > 2σ(I)] R1=0.064 3, wR2=0.156 0
    V/nm3 3.840 4(8) R indices (all data) R1=0.084 6, wR2=0.168 1
    Z 4 Largest diff. peak and hole/(e·nm-3) 609.6 and -679.3
    Dc/(g·cm-3) 1.432
    下载: 导出CSV

    Table 2.  Selected bond lengths (nm) and bond angles (°) for the complex

    Co1—O2 0.196 4(2) Co1—N1 0.203 3(3) N2—O1 0.126 5(4)
    O2A—Co1—O2 94.93(15) O2—Co1—N1 120.60(11) O2—Co1—N1A 106.30(11)
    N1A—Co1—N1 108.54(16)
    Symmetry code: A: -x, y, -z+0.5.
    下载: 导出CSV
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  • 发布日期:  2025-02-10
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