English

• 0.   Introduction

  In recent years, research interest in coordination magnetic complexes, especially low-dimensional heteronuclear complexes, is ongoing to deeply understand structural-magnetic correlations in magnetic chemistry^[1-8]. To obtain low‑dimensional heteronuclear magnetic compounds, effective works are focused on skillfully designing and synthesizing prodromic building blocks containing bridging groups. A lot of low- dimensional heteronuclear magnetic compounds have been harvested via the self-assembly of these building blocks with other metal ions and ligands^[9-15]. Among these prodromic building blocks, cyanide ferrates are widely used in the synthesis of low-dimensional magnetic compounds. The various number and arrangements of cyanide groups in cyanide ferrates result in considerable low-dimensional magnetic complexes featuring different topological structures, which provided ideal carriers for the investigation of structural‑ magnetic correlations^[16-20].

  In our previous work, we obtained a series of cyanide-bridged low-dimensional magnetic compounds based on several dicyanideferrates or tricyaniderates^[21-24]. More importantly, the structural‑magnetic relationships of these compounds were carefully discussed. The purpose of our present work is to synthesize more low-dimensional magnetic compounds and to get more information on the relationship between structures and magnetic properties. Besides cyanide ferrates, designing co-assembly ligands is also the key to the synthesis of low-dimensional magnetic complexes. Recently, we designed an almost planar ligand 2, 9‑ di(pyrazo-1-yl)-1, 10-phenanthroline (dpzpen) for the preparation of magnetic complexes^[25]. In this work, a cyanide‑bridged Fe₄^ⅢNi₂^Ⅱ hexa‑nuclear complex [Fe₄Ni₂(Tp)₄(CN)₁₂(dpzpen)₂]·12H₂O·3CH₃OH (1) was obtained by the reaction of tricyaniderate (Bu₄N)[Fe^Ⅲ(Tp)(CN)₃] (Tp=hydrotris(pyrazolyl)borate) with dpzpen and Ni^Ⅱ ion. Herein, we report the crystal structure, magnetic properties as well as structural-magnetic correlation of complex 1.

  1.   Experimental

  1.1   Starting materials and instruments

  All of the reactions were carried out under air. Chemicals were commercially obtained and directly used. The prodromic building block (Bu₄N)[Fe^Ⅲ(Tp)(CN)₃] and ligand dpzpen were synthesized according to the reported literature^[25-26].

  IR spectrum was measured in a range of 4 000-400 cm^-1 on a Thermo Nicolet iS50 FT-IR spectrometer. The powder X-ray diffraction (PXRD) pattern was recorded on a Rigaku Miniflex 600 diffractometer with Cu Kα radiation (40 kV, 15 mA, λ=0.154 056 nm) using a flat plate geometry in a range of 5°-40°. Thermogravimetric (TG) analysis was carried out under argon on a Mettler-Toledo TGA/DSC 1 system with a heating rate of 5 ℃·min^-1. Elemental analysis was performed on an Elementar Vario EL Cube elemental analyzer. Direct current magnetic susceptibilities measurement was carried out on a Quantum Design MPMS SQUID magnetometer using a powdered sample under an external field of 2 000 Oe. Additionally, the experimental susceptibilities were corrected for diamagnetism estimated using Pascal′s constants.

  1.2   Preparation of complex 1

  A solution of ligand dpzpen (15.6 mg, 0.5 mmol) and Ni(ClO₄)₂·H₂O (18.6 mg) in methanol/acetonitrile (5 mL, 1∶1, V/V) was added into a solution of (Bu₄N)[Fe^Ⅲ(Tp)(CN)₃] (59.8 mg, 1.0 mmol) in methanol/water (5 mL, 1∶1, V/V). The reaction mixture was stirred at room temperature for 15 min, after which the insoluble substances were removed via filtration. The resulting clear mixture was located at ambient conditions without any perturbation. After 2 d, red block crystals of complex 1 were obtained with solvent evaporation. Yield: 56.3%. The elemental analysis result and IR spectrum of 1 are listed in the Supporting information.

  1.3   Crystallographic data collection and structure refinement

  Single-crystal X-ray diffraction data of complex 1 was harvested on a Bruker APEX Ⅱ CCD diffractometer using graphite-monochromatized Mo Kα radiation (λ=0.071 073 nm) with ω-scan techniques. The structure was solved using a direct method and refined with the full‑matrix least‑squares technique on the SHELXTL 2014 program. All the non-hydrogen atoms were refined anisotropically. The hydrogen atoms connected to carbon atoms were added theoretically, and the hydrogen atoms connected to other atoms were added by finding the Q peaks from differential Fourier maps. All the B alerts of the crystal data were caused by the unclear solvent molecules in the voids of the unit cell, and the species and the number of solvent molecules were determined by elemental and TG analyses. The conditions and parameters for X-ray diffraction data collection and structure refinement are summarized in Table 1.

  Table 1

  

  Table 1.  Crystallographic data of complex 1

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  Parameter	1
  Formula	C84H62N48B4Fe4Ni2·12H2O·3CH3OH
  Formula weight	2 217.95
  Temperature / K	293(2)
  Crystal system	Monoclinic
  Space group	P21/n
  a / nm	1.441 0(4)
  b / nm	2.803 7(8)
  c / nm	1.461 2(4)
  β / (°)	113.728(5)
  V / nm3	5.404(3)
  Z	2
  Dc / (g·cm-3)	1.407
  μ (Mo Kα) / mm-1	0.940
  F(000)	2 328
  θ range / (°)	1.676-27.585
  Reflection [I > 2σ(I)]	6 806
  R1 [I > 2σ(I)]	0.058 9
  wR2 (all)	0.186 7
  GOF on F 2	1.020

  2.   Results and discussion

  2.1   Synthesis and general characterization

  Crystal sample of complex 1 can be repeatably prepared via the reaction of (Bu₄N)[Fe^Ⅲ(Tp)(CN)₃], Ni(ClO₄)₂·H₂O, and dpzpen in the mixed solvent of methanol and acetonitrile under ambient conditions. The purity of the sample was confirmed by PXRD (Fig.S1, Supporting information). The IR spectrum of complex 1 showed two clear peaks in a range of 2 100- 2 200 cm^-1 (Fig.S2), corresponding to the bridging and the non-bridging cyanide group in this complex. Additionally, the IR absorption wavenumber of the bridging cyanide group (2 170 cm^-1) was higher than that of the non-bridging one (2 120 cm^-1). There were ca. 1.3% (mass percentage) solvent molecules in complex 1, which is determined by TG analysis (Fig.S3). In addition, TG and elemental analyses together with crystal structural analysis indicate that the molecular formula of complex 1 is C₈₄H₆₂N₄₈B₄Fe₄Ni₂·12H₂O·3CH₃OH.

  Figure 1

  

  Figure 1.  Molecular structure of complex 1

  Fe, Ni, C, N, and B atoms are shown in yellow, sky blue, gray, blue, and orange, respectively; H atoms are omitted for clarity; Symmetry code: ⁱ 1-x, -y, -z.

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  2.2   Crystal structure description

  Complex 1 crystallizes in the monoclinic space group P2₁/n (Table 1). The molecular structure of complex 1 contains four [Fe^Ⅲ(Tp)(CN)₃]^- fragments and two [Ni^Ⅱ(dpzpen)]²⁺ fragments, thus resulting in an electroneutral complex (Fig. 1). Two of [Fe^Ⅲ(Tp)(CN)₃]^- units link with two [Ni^Ⅱ(dpzpen)]²⁺ fragments using four cyanide groups, which forms an approximately rhombic skeleton. Additionally, another two [Fe^Ⅲ(Tp)(CN)₃]^- units use each one of the cyanide groups coordinating with the two [Ni^Ⅱ(dpzpen)]²⁺ fragments from another axial position, leading to the final hexa‑nuclear Fe₄^ⅢNi₂^Ⅱ complex.

  In complex 1, the bond lengths of Fe—C and Fe—N are in a range of 0.189 4(5)-0.192 3(6) nm and 0.195 5(3)-0.198 3(4) nm (Table 2), respectively. The Fe—C≡N units are almost linear with corresponding bond angles in a range of 173.6(4)°-179.2(6)°. These features are almost the same as those in the precursor (Bu₄N)[Fe^Ⅲ(Tp)(CN)₃]. Each Ni(Ⅱ) ion coordinates with six nitrogen atoms forming a deformed octahedron, in which three nitrogen atoms from one dpzpen ligand and one nitrogen atom from a cyanide group in the equatorial positions, and two nitrogen atoms from two cyanide groups in the axial positions. Due to the relatively small Ni(Ⅱ) ion size, the Ni—N23 bond length (0.229 3 nm) is longer than other Ni—N bond lengths (0.198 7-0.209 8 nm) and the pyrazolyl group close to N23 is free from the nickel center. The Ni—N≡C segments deviate from lines with angles ranging from 161.6(4)°-169.8(4)°. The nearest intermolecular metal…metal distance is 0.501 0 nm.

  Table 2

  

  Table 2.  Selected bond distances (nm) and angles (°) of complex 1

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  Fe1—C1	0.191 6(5)	Fe1—N7	0.196 9(4)	Ni1—N1	0.206 2(4)
  Fe1—C2	0.191 9(5)	Fe1—N9	0.198 3(4)	Ni1—N4	0.206 5(4)
  Fe1—C3	0.192 3(6)	Fe1—N11	0.198 1(4)	Ni1—N5	0.198 7(4)
  Fe2—C4	0.189 4(4)	Fe2—N13	0.195 5(3)	Ni1—N21	0.229 3(3)
  Fe2—C5	0.190 1(4)	Fe2—N15	0.197 1(4)	Ni1—N22	0.201 6(3)
  Fe2—C6	0.190 8(6)	Fe2—N17	0.195 6(4)	Ni1—N24	0.209 8(4)
  Fe1—C—N1	176.8(4)	Fe2—C4—N4	173.7(4)	Ni1—N1—C1	163.1(4)
  Fe1—C2—N2	177.4(5)	Fe2—C5—N5	175.4(4)	Ni1—N4—C4	161.6(3)
  Fe1—C3—N3	179.2(6)	Fe2—C6—N6	178.1(5)	Ni1—N5—C5	169.8(4)

  2.3   Magnetic properties

  The temperature-dependent direct-current magnetic susceptibility of complex 1 was recorded in a temperature range of 2-300 K under a magnetic field of 2 000 Oe (Fig. 2). At 300 K, the χ_mT value of complex 1 was 3.92 emu·K·mol^-1, a little larger than that (3.50 emu·K·mol^-1) of a system containing isolated four low-spin Fe(Ⅲ) ions and two Ni(Ⅱ) ions. This is due to the spin-orbit interaction in the Fe(Ⅲ) ions. Upon temperature decreasing, the χ_mT value of 1 increased slowly until about 100 K. After that, the χ_mT value rose abruptly to a maximal value of 7.98 emu·K·mol^-1 at 6 K. Following, the χ_mT value decreased rapidly to 6.35 emu·K·mol^-1 at 2 K. The increasing χ_mT value of complex 1 from 300 to 6 K indicates the presence of ferromagnetic interactions between Fe^Ⅲ and Ni^Ⅱ centers. However, the magnetic saturation effect, intermolecular antiferromagnetic interaction, as well as the zero-field splitting of Ni(Ⅱ) ion, leads to the decline of χ_mT value in the low-temperature range. The temperature-dependent magnetic susceptibilities of complex 1 obeyed the Curie-Weiss law in a temperature range of 300-10 K. The plot of χ_m^-1 vsT together with the fitting curve is presented in Fig. 2, from which the Curie parameter of C=3.85 emu·K·mol^-1 and the Weiss parameter θ=9.66 K were obtained. The large positive value of the Weiss parameter also shows that the magnetic coupling is ferromagnetic in complex 1.

  Figure 2

  

  Figure 2.  Temperature dependence of χ_mT (○) and χ_m^-1 (□) under an applied field of 2 000 Oe, and corresponding fitted curves of complex 1

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  To get clear information about magnetic-structural relations for each cyanide-linked ion couple in complex 1, the temperature-dependent magnetic data were fitted using the Mag‑Pack program based on the structural model (Fig. 3) with the spin Hamiltonian Ĥ= -2[J_1d(Ŝ_Fe1Ŝ_Ni1+Ŝ_Fe3Ŝ_Ni2)+J_2d(Ŝ_Fe2Ŝ_Ni1+Ŝ_Fe4Ŝ_Ni2)+J_3d(Ŝ_Fe1Ŝ_Ni2+Ŝ_Fe3Ŝ_Ni1)], and the best-fitting magnetic coupling parameters were derived out with J_1d=3.50(1) cm^-1, J_2d= 3.53(2) cm^-1, J_3d=15.73(2) cm^-1, g=2.07(1), and R= ∑[(χ_mT)_obs-(χ_mT)_cal]²/∑[(χ_mT)_obs]²=2.40×10^-4, which are comparable with those of other cyanide-bridged Fe^Ⅲ-Ni^Ⅱ complexes^[27-32]. It is well known that the ferromagnetic interaction between Fe^Ⅲ and Ni^Ⅱ is produced by the orthogonal overlap of e_g (low-spin Fe^Ⅲ) and t_2g (Ni^Ⅱ) orbitals for unpaired electrons, and the magnetic coupling strength is affected by the structural characteristics. By comparing the structural parameters and magnetic coupling strength of each cyanide-bridged Fe^Ⅲ-Ni^Ⅱ unit (Table 3), we found that a shorter Ni—N bond length and a larger Ni—N≡C angel favor the overlap of e_g (low-spin Fe^Ⅲ) and t_2g (Ni^Ⅱ) orbitals and thus generates stronger ferromagnetic interaction.

  Figure 3

  

  Figure 3.  Magnetic coupling model of complex 1

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  Table 3

  

  Table 3.  Magnetic coupling strength and structural parameters of cyanide-bridged FeⅢ-NiⅡ units in complex 1

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  Parameter	Fe1…Ni1 Fe3…Ni2	Fe2…Ni1 Fe4…Ni2	Fe3…Ni1 Fe1…Ni2
  dFe—C / nm	0.189 4	0.191 6	0.190 1
  dN≡C / nm	0.115 0	0.113 1	0.114 1
  dNi—N / nm	0.206 5	0.206 2	0.198 7
  ∠Fe—C≡N / (°)	173.7	176.8	175.4
  ∠Ni—N≡C / (°)	161.6	163.1	169.8
  Jd / cm-1	3.50	3.53	15.73

  Additionally, the field-dependent magnetization of complex 1 was recorded under a field range of 0-5 T at 2 K, which is presented in Fig. 4. The measured plot of magnetization vs field was good in accordance with the Brillouin curve for a spin system based on S=4 and g=2.00 in the low field range, but was slightly up in the Brillouin curve in the high field range. Moreover, the experimental data strayed away from an uncoupled spin system with S=1/2+1+1/2+1/2+1+1/2 and g=0. These features further confirm that the magnetic exchange of Fe^Ⅲ and Ni^Ⅱ centers through cyanide groups are ferromagnetic and there exists a zero-field splitting effect in complex 1.

  Figure 4

  

  Figure 4.  Field dependence of magnetization of complex 1 at 2  

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  3.   Conclusions

  In summary, a cyanide-bridged Fe₄^ⅢNi₂^Ⅱ hexa‑ nuclear complex was synthesized, whose crystal structure and magnetic properties have been investigated in detail. Importantly, the magnetic interaction strength for each pair of cyanide-bridged Fe^Ⅲ-Ni^Ⅱ units was obtained by fitting the magnetic data using a corresponding structural model. Based on the above results, the magnetic-structural correlation for the synthesized cyanide-bridged Fe^Ⅲ-Ni^Ⅱ complex was discussed, which indicates a larger Ni—N≡C angle and a shorter Ni—N bond length rendering stronger ferromagnetic interaction. This finding would shed light on the development of new high-nuclear clusters with strong ferromagnetic coupling.

  
  
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      Jiao C Q, Jiang W J, Wen W, Ren Y, Wang J L, Liu T, He C. Single-molecule magnet behavior in a tetranuclear cyano-bridged Fe₂^ⅢNi₂^Ⅱ cluster[J]. Inorg. Chem. Commun., 2016, 74:  12-15.

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  Figure 1  Molecular structure of complex 1

  Fe, Ni, C, N, and B atoms are shown in yellow, sky blue, gray, blue, and orange, respectively; H atoms are omitted for clarity; Symmetry code: ⁱ 1-x, -y, -z.

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  Figure 2  Temperature dependence of χ_mT (○) and χ_m^-1 (□) under an applied field of 2 000 Oe, and corresponding fitted curves of complex 1

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  Figure 3  Magnetic coupling model of complex 1

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  Figure 4  Field dependence of magnetization of complex 1 at 2  

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  Table 1.  Crystallographic data of complex 1

  Parameter	1
  Formula	C84H62N48B4Fe4Ni2·12H2O·3CH3OH
  Formula weight	2 217.95
  Temperature / K	293(2)
  Crystal system	Monoclinic
  Space group	P21/n
  a / nm	1.441 0(4)
  b / nm	2.803 7(8)
  c / nm	1.461 2(4)
  β / (°)	113.728(5)
  V / nm3	5.404(3)
  Z	2
  Dc / (g·cm-3)	1.407
  μ (Mo Kα) / mm-1	0.940
  F(000)	2 328
  θ range / (°)	1.676-27.585
  Reflection [I > 2σ(I)]	6 806
  R1 [I > 2σ(I)]	0.058 9
  wR2 (all)	0.186 7
  GOF on F 2	1.020

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  Table 2.  Selected bond distances (nm) and angles (°) of complex 1

  Fe1—C1	0.191 6(5)	Fe1—N7	0.196 9(4)	Ni1—N1	0.206 2(4)
  Fe1—C2	0.191 9(5)	Fe1—N9	0.198 3(4)	Ni1—N4	0.206 5(4)
  Fe1—C3	0.192 3(6)	Fe1—N11	0.198 1(4)	Ni1—N5	0.198 7(4)
  Fe2—C4	0.189 4(4)	Fe2—N13	0.195 5(3)	Ni1—N21	0.229 3(3)
  Fe2—C5	0.190 1(4)	Fe2—N15	0.197 1(4)	Ni1—N22	0.201 6(3)
  Fe2—C6	0.190 8(6)	Fe2—N17	0.195 6(4)	Ni1—N24	0.209 8(4)
  Fe1—C—N1	176.8(4)	Fe2—C4—N4	173.7(4)	Ni1—N1—C1	163.1(4)
  Fe1—C2—N2	177.4(5)	Fe2—C5—N5	175.4(4)	Ni1—N4—C4	161.6(3)
  Fe1—C3—N3	179.2(6)	Fe2—C6—N6	178.1(5)	Ni1—N5—C5	169.8(4)

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  Table 3.  Magnetic coupling strength and structural parameters of cyanide-bridged FeⅢ-NiⅡ units in complex 1

  Parameter	Fe1…Ni1 Fe3…Ni2	Fe2…Ni1 Fe4…Ni2	Fe3…Ni1 Fe1…Ni2
  dFe—C / nm	0.189 4	0.191 6	0.190 1
  dN≡C / nm	0.115 0	0.113 1	0.114 1
  dNi—N / nm	0.206 5	0.206 2	0.198 7
  ∠Fe—C≡N / (°)	173.7	176.8	175.4
  ∠Ni—N≡C / (°)	161.6	163.1	169.8
  Jd / cm-1	3.50	3.53	15.73

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