English

• In recent years, remarkable achievements have been made in the design of single-molecule magnets (SMMs) toward high spin-reversal energy barriers and/or blocking temperatures^[1-3]. Typical cases can be observed in Dy^Ⅲ-SMMs with a magnetic blocking temperature of 80 K and an energy barrier up to 1 541 cm^-1[2]. Different from lanthanide compounds, transition metal possesses 3d valence orbitals, as well as the ligand field can quench the orbital angular momentum with ease, which is in favor of the construction of compounds with simpler structures than adjusting structure-function relationship^[4-5]. Especially, the transition metal-Co^Ⅱ ion contains three unpaired electrons, a d⁷ configuration, and unquenched orbital angular momentum during high-temperature regions. Recent investigations have verified that the Co^Ⅱ ion is an excellent candidate for design of the transition metal-based SMMs^[6-10]. In 2003, the first d-block mononuclear SMM [Co(SCN)₂(4-dzbpy)₄] (dzbpy represents diazobenzylpyridine) was reported, which contains a single octahedral Co^Ⅱ ion, coordinating to two NCS and four 4-dzbpy ligands^[11]. An air-stable star-shaped Co^ⅡCo₃^Ⅲ complex was constructed via a chiral Schiff base ligand, exhibiting zero-field SMM behavior^[12]. Furthermore, interest in the Co^Ⅱ-radical (Co^Ⅱ-rad) SMM system also remains flourishing on account of strong 2p-3d magnetic exchange couplings and the unquenched first-order angular momentum giving rise to large uniaxial magnetic anisotropy. In 2016, employing a bis-bidentate nitronyl nitroxide radical NIT-2-Pm, a nitronyl nitroxide-bridged Co^Ⅱ binuclear compound was synthesized, furthermore, field-induced single-molecule magnet behavior was detected firstly in nitronyl nitroxide-bridged 3d compounds^[13]. Long et al. recently introduced the Co^Ⅱ ion into an exchange-coupled binuclear system by a semiquinone radical as the bridging ligand^[14]. Variation of radical ligand donor characteristics offers an avenue to adjust the magnetic anisotropy.

  Along this line, to explore the transition metal-radical chemistry, herein, we synthesized a new nitronyl nitroxide radical NIT-PyzMe-PhCHO, where NIT-PyzMe-PhCHO=2-(3-(1H-imidazol-1-ylmethyl)benzaldehyde)-4, 4, 5, 5-tetramethylimidazoline-1-oxyl-3-oxide, and obtained a 2p-3d hetero-spin complex with the formula[Co(hfac)₂(NIT-PyzMe-PhCHO)] (1), where hfac=hexafluoroacetylacetonate. The crystal structure and the static and dynamic magnetic susceptibilities of the Co^Ⅱ complex were investigated. Furthermore, the Co^Ⅱ-based complex displays field-induced SMM behavior.

  Scheme 1

  

  Scheme 1.  Structure of NIT-PyzMe-PhCHO radical ligand

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  1.   Experimental

  1.1   Materials and instruments

  Starting reagents including heptane and dichloromethane are commercially obtained. The nitronyl nitroxide NIT-PyzMe-PhCHO was synthesized based on the reported literature^[15]. The elemental analysis was performed on a Perkin-Elmer 240 elemental analyzer. Magnetic susceptibility measurements of the powder sample were undertaken on a Quantum Design MPMS magnetometer. Direct-current (dc) magnetic susceptibility was corrected for diamagnetism contribution in the light of Pascal constants^[16].

  1.2   Syntheses of complex 1

  [Co(hfac)₂] hydrate (0.01 mmol) was dissolved in boiling n-heptane (98 ℃) and the mixture was refluxed for 5 h with continuous stirring. After cooling to 85 ℃, a CH₂Cl₂ solution (6 mL) of NIT-PyzMe-PhCHO (0.01 mmol, 0.003 0 g) was slowly added with refluxing for 0.5 h followed, then filtered. The blue filtrate was quiescence at room temperature, generating navy-blue rhombic crystals after three days. FTIR (KBr, cm^-1): 1 785(s), 1 347(m), 1 179(s), 1 158(s), 1 065(s), 952(s), 861(m), 551(s). Elemental Anal. Calcd. for C₂₈H₂₃CoF₁₂ N₄O₇(%): C, 41.29; H, 2.84; N, 6.87. Found(%): C, 41.32; H, 2.83; N, 6.88.

  1.3   Crystal structure determination

  Crystal data of the mononuclear Co^Ⅱ complex was recorded on a Rigaku Saturn CCD diffractometer at 150(2) K using Mo Kα radiation (λ=0.071 073 nm). SADABS^[17] was devoted to empirical absorption correction. The structure of the Co^Ⅱ complex was solved by direct methods and refined by least squares minimization with a suite of SHELX programs^[18]. Anisotropic refinement involving non-H atoms was implemented. In the meantime, H atoms were arranged at ideal positions. Additional single crystal data, selected bond lengths, and bond angles for the Co^Ⅱ complex are displayed in Table 1 and S1 (Supporting information).

  Table 1

  

  Table 1.  Crystallographic data and structure refinement for complex 1

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  Parameter	1		Parameter	1
  Empirical formula	C28H23CoF12N4O7		V / nm3	1.608 8
  Formula weight	814.43		Z	2
  Crystal system	Triclinic		Dc / (g·cm-3)	1.681
  Space group	P1		μ / mm-1	0.655
  a / nm	1.095 16(7)		θ range / (°)	3.208-52.788
  b / nm	1.236 27(8)		Reflection collected	35 554
  c / nm	1.342 54(9)		Unique reflection (Rint)	6 569 (0.053 3)
  α / (°)	71.509(3)		GOF	0.957
  β / (°)	79.858(3)		R1, wR2 [I > 2σ(I)]	0.079 8, 0.197 9
  γ / (°)	69.344(2)		R1, wR2 (all data)	0.101 9, 0.224 7

  2.   Results and discussion

  2.1   Description of crystal structure

  Complex 1 crystallizes in the triclinic P1 space group with one independent Co(hfac)₂ moiety and one NIT-PyzMe-PhCHO radical ligand. As shown in Fig. 1, the radical NIT-PyzMe-PhCHO behaves as a bidentate ligand to chelate one Co^Ⅱ ion to form one mononuclear structure. The Co^Ⅱ ion adopts distorted octahedral geometry, encircled by four O atoms from two β-diketonate co-ligands (Co—O_hfac: 0.205 5(4)-0.208 7(4) nm) and one N atom from imidazole ring (Co1—N4: 0.209 0(4) nm) and one O atom from the nitroxide group (Co1—O3: 0.205 5(3) nm). The dihedral angle between the imidazole ring and O—N—C—N—O heterocyclic plane of the PyzMe-PhCHO radical is 30.2(1)°, and the Co—O—N—C torsion angle is 38.5(6)°. The crystal packing of the Co^Ⅱ-rad complex is depicted in Fig. 2. The closest Co^Ⅱ…Co^Ⅱ intermolecular contact is 0.845 nm, and the shortest uncoordinated NO…ON interval is equal to 1.095 nm.

  Figure 1

  

  Figure 1.  Crystal structure of complex 1

  Ellipsoids are set at the 30% probability levels; H and F atoms are not depicted for clarity.

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  Figure 2

  

  Figure 2.  Crystal packing of complex 1

  H and F atoms are removed for clarity.

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  2.2   Magnetic properties

  Static magnetism of the mononuclear Co^Ⅱ complex was recorded under an external field of 1 000 Oe during a temperature range of 2-300 K. As shown in Fig. 3, the χ_MT product at 300 K was 2.33 cm³·mol^-1·K, which was distinctly lower than the theoretical value for one uncoupled Co^Ⅱ ion (χ_MT product of octahedral high-spin Co^Ⅱ: 3.0-3.4 cm³·mol^-1·K)^[19] and one radical (radical: S=1/2, g=2.0, C=0.375 cm³·mol^-1·K). This phenomenon is indicative of extraordinarily strong Co^Ⅱ-radical antiferromagnetic interaction.

  Figure 3

  

  Figure 3.  χ_MT versus T plot of complex 1

  The red line represents the fitting result.

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  For 1, on lowering the temperature, the value of χ_MT continuously decreased to 0.110 cm³·mol^-1·K at 2 K. This phenomenon verifies the prevailing dominant antiferromagnetic coupling. Field dependence of the magnetization (ca. 70 kOe) at 2 K was measured. The M value displayed a gradual increase and reached 0.46Nβ at 70 kOe (Fig.S1), which was significantly lower than the predicted saturation value deriving from the spin-orbit effect of Co^Ⅱ ion and/or strong antiferromagnetic coupling.

  The χ_MT-T data of 1 was fitted by the PHI software^[20] with a spin Hamiltonian of $ \hat{H}=-2 J\left(\hat{S}_{\mathrm{Co}} \hat{S}_{\mathrm{rad}}\right)-$ $D \hat{S}_z^2-E\left(\hat{S}_x^2-\hat{S}_y^2\right) $, where J represents the magnetic coupling between the Co^Ⅱ center and the directly coordinated nitroxide group, zero-field splitting (ZFS) parameters D and E are used to record magnetic anisotropy. The reasonable fitting generates the following magnetic parameters: g_rad=2.00 (fixed), g_Co=2.47, J= -82.90 cm^-1, D=49.30 cm^-1 and E=3.31 cm^-1. The large J value confirms strong antiferromagnetic Co^Ⅱ-rad interaction, which is in accordance with other reported Co^Ⅱ-nitronyl nitroxide compounds^{[6, 13, 21]}. The positive D value declares the easy-plane magnetic anisotropy. In the system, the Co—O—N angle is equal to 120.4°, and the Co—O_rad bond length of 0.205 4 nm is short, which is conducive to the efficient overlap of magnetic orbitals.

  The alternating current (ac) susceptibility of complex 1 was tested for probing spin dynamics. As shown in Fig.S2, the out-of-phase susceptibility merely displayed frequency dependent at low-temperature region under 0 Oe due to strong quantum tunneling of magnetization (QTM). To weaken QTM, under an external field of 2 000 Oe, the temperature- and frequency-dependent ac behaviors were investigated in detail. For the Co^Ⅱ-rad complex, evident peaks of χ″ signals were discerned (Fig. 4 and S3), a typical SMM characteristic. The relaxation time (τ) and distribution coefficient (α) were simultaneously deduced from χ″=f(ν) curves with a generalized Debye model. The α values lay in a range of 0.18-0.23 (Fig. 5), giving a moderate distribution (α) of relaxation time. Linear fitting of the ln τ vs T^-1 data with the Arrhenius law τ=τ₀exp[Δ/(k_BT)] acquired the energy barrier U_eff=9.0(3) K and pre-exponential factor τ₀=3.0(2) μs (Fig. 4b), which was comparable to observed typical SMMs^[21-26] (10-10^-6 μs).

  Figure 4

  

  Figure 4.  (a) χ″=f(ν) plot of complex 1 under 2 000 Oe; (b) ln τ vs T^-1 plot under 2 000 Oe for 1

  The solid line represents the fitting result.

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  Figure 5

  

  Figure 5.  Cole-Cole plot for complex 1 under 2 000 Oe external field

  The solid line represents the fitting result.

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  Generally, strong magnetic exchange coupling between spin carriers is known to contribute to reducing QTM effects and is desirable for the design of better SMMs. Thus, in complex 1, strong 2p-3d magnetic exchange represents a key step toward the realization of SMM behavior. However, complex 1 still requires an external dc field to display slow relaxation behavior. To improve the magnetic behavior of complex 1, one strategy is to design a more ideal ligand-field environment to generate more strict axial symmetry around the Co^Ⅱ ion, hence weakening transverse anisotropy and restraining QTM^[27]. At this point, modulation of a lower-coordination number at Co^Ⅱ ion may optimize the SMM behavior of complex 1.

  3.   Conclusions

  We successfully isolated a 2p-Co^Ⅱ hetero-spin SMM using nitronyl nitroxide, in which the NIT-PyzMe-PhCHO radical acts as a bidentate ligand to chelate one Co^Ⅱ ion, generating a mononuclear structure. Furthermore, strong antiferromagnetic Co^Ⅱ-nitronyl nitroxide coupling was discerned. The 2p-3d hetero-spin complex showed field-induced SMM behavior with τ₀ of 3.0 μs and U_eff of 9.0 K. To explore the effect of different 3d metal ions on SMM behavior, the construction of analogous complexes with other 3d ions is on the way.

  
  Supporting information is available at http://www.wjhxxb.cn
  
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• 

  Scheme 1  Structure of NIT-PyzMe-PhCHO radical ligand

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  Figure 1  Crystal structure of complex 1

  Ellipsoids are set at the 30% probability levels; H and F atoms are not depicted for clarity.

  下载: 全尺寸图片 幻灯片
  

  Figure 2  Crystal packing of complex 1

  H and F atoms are removed for clarity.

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  Figure 3  χ_MT versus T plot of complex 1

  The red line represents the fitting result.

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  Figure 4  (a) χ″=f(ν) plot of complex 1 under 2 000 Oe; (b) ln τ vs T^-1 plot under 2 000 Oe for 1

  The solid line represents the fitting result.

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  Figure 5  Cole-Cole plot for complex 1 under 2 000 Oe external field

  The solid line represents the fitting result.

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  Table 1.  Crystallographic data and structure refinement for complex 1

  Parameter	1		Parameter	1
  Empirical formula	C28H23CoF12N4O7		V / nm3	1.608 8
  Formula weight	814.43		Z	2
  Crystal system	Triclinic		Dc / (g·cm-3)	1.681
  Space group	P1		μ / mm-1	0.655
  a / nm	1.095 16(7)		θ range / (°)	3.208-52.788
  b / nm	1.236 27(8)		Reflection collected	35 554
  c / nm	1.342 54(9)		Unique reflection (Rint)	6 569 (0.053 3)
  α / (°)	71.509(3)		GOF	0.957
  β / (°)	79.858(3)		R1, wR2 [I > 2σ(I)]	0.079 8, 0.197 9
  γ / (°)	69.344(2)		R1, wR2 (all data)	0.101 9, 0.224 7

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•