English

• 0.   Introduction

  Spin-crossover (SCO) complexes have attracted much attention due to their promising applications in molecule-based memory devices, sensors and switches^[1-2]. Most of the work has focused on Fe(Ⅱ)^[3-4] and Fe(Ⅲ)^[5-6] complexes, there are still some mononuclear Co(Ⅱ)complexes with abrupt or gradual SCO properties based on derivatives of terpyridine(terpy) ^[7] and Schiff base ligands^[8]. The central Co(Ⅱ)ion is coordinated by N₆^[9-10] and N₄O₂ donor sets^[11]. To the best of our knowledge, there are no reported Co(Ⅱ)SCO complexes with N₂O₄ donor sets. The hydrazone Schiff base ligands have been well studied due to their ease of synthesis, modularity and various coordination modes with keto-enol tautomerism^[12]. Pyrazine-based hydrazone ligand H₂L as shown in Scheme 1 has been used to construct single-molecule magnets (SMMs) with rare earth metals^[13-17]. As a complementary work to this ligand system, herein, we synthesized a mononuclear Co(Ⅱ)complex (NHEt₃)[Co(HL)₂]·3H₂O (1) and characterized it with IR, thermogravimetric analysis (TGA), X-Ray single diffraction and magnetic susceptibility.

  Scheme1

  

  Scheme1.  Structure of H₂L

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  1.   Experimental

  All chemical reagents were commercially available and used without further purification. H₂L was synthesized according to the reported method^[13]. FT-IR spectra were recorded in a range of 4 000~400 cm^-1 on a PerkinElmer FT-IR spectrometer Frontier. Powder X-Ray diffraction (PXRD) patterns were recorded on a Rigaku Smartlab X-Ray diffractometer with Cu Kα radiation (λ =0.154 178 nm, U=40 kV, I=26 mA) in a range of 5°~50° (2θ). Elemental analyses for C, H and N were measured on an Elementar Vario MICRO analyzer. TGA curves were measured in a Al₂O₃ crucible using a PerkinElmer TGA instrument in a temperature range of 25~800 ℃ under a N₂ flow at a heating rate of 20 ℃·min^-1. Magnetic susceptibility measurements for the samples were performed on the Quantum Design MPMS-XL instrument operating under a field of 1 000 Oe in a temperature range of 2~300 K.

  1.1   Synthesis of (NHEt₃)[Co(HL)₂]·3H₂O (1)

  The methanol solution of CoC₂O₄·2H₂O(0.035 5 g, 0.20 mmol) was added dropwise to the mixed solution of H₂L (0.054 5 g, 0.20 mmol) in CH₃OH and CH₂Cl₂ with the addition of NEt₃ (5 drops) as a deprotonation reagent. The color of the mixture changed from yellow to brown slurring. After stirring for 6 h and the filtration, reddish brown filtrate was obtained. After slow evaporation of the filtrate for two weeks, needle like brown crystals were obtained for X-ray diffraction analysis. Yield: 52% (based on CoC₂O₄·2H₂O). Anal. Calcd. for C₃₂H₄₃CoN₉O₉(%): C, 50.79; H, 5.73; N, 16.66. Found(%): C, 51.36; H, 5.28; N, 17.27. IR (KBr, cm^-1): 3 431 (s), 2 934 (w), 2 831(w), 2 681 (w), 1 623(s), 1 600(s), 1 534(s), 1 475(s), 1 437(s), 1 345 (m), 1 243(w), 1 219(s), 1153(s), 1 083(m), 1 019(m), 976(m), 934(m), 858(w), 744(m), 643(w), 581(w), 453(w).

  1.2   Structure determination

  Single-crystal diffraction data were recorded on a Bruker SMART APEX Ⅱ CCD diffractometer with Mo Kα (λ=0.071 073 nm) radiation at 298 K. The crystal structure was solved by direct methods and refined by full-matrix least-squares method on F² using the SHELXTL 2014/7 program and OLEX2^[18-19]. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms on water molecules were generated by the Q peaks. A water solvent molecule was severely disordered without adding hydrogen atoms and was modeled using the squeeze program. The details of singlecrystal diffraction data and selected bond lengths and bond angles are listed in Table 1 and 2, respectively.

  Table 1

  

  Table 1.  Crystal data and structure refinement for complex 1

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  Complex	1 (298 K)	1 (100 K)
  Empirical formula	C32H43CoN9O9	C32H43CoN9O9
  Fourmula weight	756.67	756.67
  Crystal system	Monoclinic	Monoclinic
  Space group	P21/c	P21/c
  a / nm	1.582 17(15)	1.576 9(6)
  b / nm	1.015 77(10)	1.011 1(4)
  c / nm	2.194 6(2)	2.171 7(8)
  β/(°)	92.364(2)	92.914(7)
  V / nm3	3.524 0(6)	3.458(2)
  Z	4	4
  Dc / (g·cm-3)	1.426	1.454
  μ/ mm-1	0.549	0.564
  F(000)	1 548	1 580
  θ range / (°)	2.217~25.027	2.223~23.828
  Reflection collected, unique	17 474, 6 225 (Rint=0.051)	17 008, 6 077
  Data, restraint, parameter	6 225, 39, 469	6 077, 12, 474
  Final R indices [I > 2σ(I)]	R1=0.066 3, wR2=0.163 4	R1=0.073 6, wR2=0.176 6
  R indices (all data)	R1=0.106 1, wR2=0.186 7	R1=0.119 8, wR2=0.199 8
  Goodness of fit on F2	1.024	1.043

  Table 2

  

  Table 2.  Selected bond lengths (nm) and angles (°) for complex 1

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  1 (298 K)
  Co(1)—O(1)	0.190 8(3)	Co(1)—O(2)	0.188 4(3)	Co(1)—O(4)	0.189 7(3)
  Co(1)—O(5)	0.188 1(3)	Co(1)—N(2)	0.186 4(3)	Co(1)—N(4)	0.185 7(4)
  O(2)—Co(1)—O(1)	177.32(14)	O(2)—Co(1)—O(4)	89.71(14)	O(4)—Co(1)—O(1)	87.82(14)
  O(5)—Co(1)—O(1)	90.91(13)	O(5)—Co(1)—O(2)	91.58(13)	O(5)—Co(1)—O(4)	178.60(13)
  N(2)—Co(1)—O(1)	83.24(14)	N(2)—Co(1)—O(2)	95.80(14)	N(2)—Co(1)—O(4)	91.20(15)
  N(2)—Co(1)—O(5)	89.22(14)	N(4)—Co(1)—O(1)	92.39(14)	N(4)—Co(1)—O(2)	88.34(14)
  N(4)—Co(1)—O(4)	83.70(16)	N(4)—Co(1)—O(5)	95.78(15)	N(4)—Co(1)—N(2)	173.43(17)
  1 (100 K)
  Co(1)—O(1)	0.188 7(3)	Co(1)—O(2)	0.188 5(3)	Co(1)—O(3)	0.190 7(3)
  Co(1)—O(5)	0.189 9(4)	Co(1)—N(1)	0.186 6(4)	Co(1)—N(2)	0.186 7(4)
  O(1)—Co(1)—O(3)	91.16(15)	O(1)—Co(1)—O(4)	179.14(16)	O(2)—Co(1)—O(1)	91.39(15)
  O(2)—Co(1)—O(3)	177.26(15)	O(2)—Co(1)—O(5)	89.46(15)	O(5)—Co(1)—O(3)	87.99(15)
  N(1)—Co(1)—O(1)	88.80(16)	N(1)—Co(1)—O(2)	95.52(16)	N(1)—Co(1)—O(3)	83.52(16)
  N(1)—Co(1)—O(5)	91.21(17)	N(1)—Co(1)—N(2)	173.35(19)	N(2)—Co(1)—O(1)	96.40(17)
  N(2)—Co(1)—O(2)	88.51(16)	N(2)—Co(1)—O(3)	92.21(17)	N(2)—Co(1)—O(5)	83.53(18)

  CCDC: 1973542, 1(298 K); 20274281, 1(100 K).

  2.   Results and discussion

  2.1   Crystal structure

  Single-crystal X-Ray diffraction analysis reveals that 1 crystallizes in monoclinic system, space group P2₁/c. The coordination environment of Co(Ⅱ)in 1 is shown in Fig. 1. It consists of a Co(Ⅱ)ion, two deprotonated HL^-, one protonated NEt₃ and three free H ₂O molecules. The central Co(Ⅱ)is coordinated by two nitrogen atoms and four oxygen atoms from HL^-. There could be amido-enol tautomerism in one HL^-ligand and the enol was deprotonated in order to balance the charge of the complex. The Co(Ⅱ)ion has a distorted octahedral coordination geometry where four oxygen donors form the basal plane, and the two nitrogen donors occupy the apical position. The distortion parameter, ∑, is defined as the sum of the deviation from the 12 cis-O— Co—O, N—Co—O, N—Co—N angels^[1]. The smaller ∑ value of 35.63° for 1 shows a slightly distorted octahedral geometry. The Co—N bond lengths (Co1—N2 0.186 4(3) nm, Co1—N4 0.185 7(4) nm) are slightly shorter than the reported Co complex with N₂O₄ donor sets. While the Co—O bond lengths (Co1—O1 0.190 8(3) nm, Co1—O2 0.188 4(3) nm, Co1—O4 0.189 7(4) nm, Co1— O5 0.188 1(3) nm) are comparable to the reported Co complex^[20]. We also determined the crystal structure of 1 at 100 K as shown in Fig. 1, it reveals the same structure composition and coordination environment of the central Co ion. The bond lengths of Co—O and Co—N are slightly different with 1 at 298 K. The ∑ value of 37.29° also shows a slightly distorted octahedral geometry. The free water molecules form O—H…O hydrogen bonds with O from phenol and methoxy group as shown in Fig. 2. There are also O —H…N hydrogen bonds with N from pyrazine N and N atom from the protonated triethylamine.

  Figure 1

  

  Figure 1.  Coordination environment of Co(Ⅱ) ion in complex 1 (left: 298 K, right: 100 K) with 50% probability thermal ellipsoids

  Solvent molecules and all hydrogen atoms have been omitted except NH group for clarity

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  Figure 2

  

  Figure 2.  Packing structure of complex 1 (298 K) with hydrogen bonding

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  2.2   PXRD patterns and TGA analysis

  PXRD pattern of the complex was determined at room temperature to check the phase purity as shown in Fig. 3. The peak positions of the experimental PXRD match well with the simulated one, indicating the purity of the complex.

  Figure 3

  

  Figure 3.  PXRD patterns for 1

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  TGA of 1 was carried out in a range of 25~800 ℃ under nitrogen atmosphere. As shown in Fig. 4, the first weight loss of 6.9% from 95 to 185 ℃ is attributed to the loss of free H₂O molecules (Calcd. 6.89%). The second weight loss of 13.1% from 195 to 260 ℃ is attributed to the loss of one protonated NEt₃ (Calcd. 13.7%). After that, the complex began to decompose.

  Figure 4

  

  Figure 4.  TGA curve of 1

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  2.3   Magnetic analysis of complex 1 and dehydrated 1

  The magnetic susceptibilities χ_m of 1 and dehydrated 1 were measured in the 2~300 K temperature range. The χ_mT versus T plots for both in cooling and heating mode are shown in Fig. 5. The χ_mT value of 1.40 cm³·mol^-1·K was lower than the theoretical value of 1.88 cm³·mol^-1·K of the high-spin mononuclear Co(Ⅱ), indicating a mixture of high-spin and low-spin state. The χ_mT decreased gradually as the temperature lowered. It reached 0.848 cm³·mol^-1·K at about 168 K, then it began to increase slowly to 0.935 cm³·mol^-1·K at 147 K, indicating a slightly reverse spin transition due to possible phase changes during this period of temperature range^[21-23]. This was confirmed by the slightly different bond lengths and the ∑ value for complex 1 at 298 K and 100 K. The χ_mT was 0.44 cm³·mol^-1·K at 59 K corresponding to the mononuclear Co(Ⅱ)low -spin state, and it decreased with decreasing temperature until it reached about 0.02 cm³·mol^-1·K at 2 K, there was a small hysteresis loop in the cooling and heating mode as shown in Fig. 5. In order to check the solvent effects on the magnetic properties, the dehydrated complex 1 was obtained by annealing 1 at 115 ℃ for 6 h in vacuo. The χ_mT value of 0.89 cm³· mol^-1·K for dehydrated 1 was significantly lower than the theoretical value of 1.88 cm³·mol^-1·K of the high-spin mononuclear Co(Ⅱ), indicating a majority of lowspin state. The χ_mT decreased gradually as the temperature lowered. It reached 0.43 cm³·mol^-1·K at about 78 K corresponding to the mononuclear Co(Ⅱ)low-spin state. It also kept decreasing to ca. 0.02 cm³·mol^-1·K at 2 K, and no hysteresis loop was observed in the cooling and heating mode. The difference of 1 and the dehydrated 1 can be assigned as the hydrogen bonding effects on the magnetic properties^[24].

  Figure 5

  

  Figure 5.  χ_mT vs T plots for complex 1 (cooling: black, heating: red) and dehydrated 1 (cooling: blue, heating: pink) in cooling and heating mode

  Inset: "reverse"fragment of the curve around 150 K

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  3.   Conclusions

  In summary, we have successfully synthesized and characterized a new mononuclear Co(Ⅱ)complex based on a pyrazine-containing hydrazone Schiff base ligand with rare N₂O₄ donor sets. The complex 1 exhibited a gradual spin transition with an unusual reverse spin state transition due to possible phase changes. The dehydrated 1 also showed a gradual spin transition, and the differences can be assigned as the hydrogen bonding effects on the magnetic properties.

  
  
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• 

  Scheme1  Structure of H₂L

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  Figure 1  Coordination environment of Co(Ⅱ) ion in complex 1 (left: 298 K, right: 100 K) with 50% probability thermal ellipsoids

  Solvent molecules and all hydrogen atoms have been omitted except NH group for clarity

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  Figure 2  Packing structure of complex 1 (298 K) with hydrogen bonding

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  Figure 3  PXRD patterns for 1

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  Figure 4  TGA curve of 1

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  Figure 5  χ_mT vs T plots for complex 1 (cooling: black, heating: red) and dehydrated 1 (cooling: blue, heating: pink) in cooling and heating mode

  Inset: "reverse"fragment of the curve around 150 K

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  Table 1.  Crystal data and structure refinement for complex 1

  Complex	1 (298 K)	1 (100 K)
  Empirical formula	C32H43CoN9O9	C32H43CoN9O9
  Fourmula weight	756.67	756.67
  Crystal system	Monoclinic	Monoclinic
  Space group	P21/c	P21/c
  a / nm	1.582 17(15)	1.576 9(6)
  b / nm	1.015 77(10)	1.011 1(4)
  c / nm	2.194 6(2)	2.171 7(8)
  β/(°)	92.364(2)	92.914(7)
  V / nm3	3.524 0(6)	3.458(2)
  Z	4	4
  Dc / (g·cm-3)	1.426	1.454
  μ/ mm-1	0.549	0.564
  F(000)	1 548	1 580
  θ range / (°)	2.217~25.027	2.223~23.828
  Reflection collected, unique	17 474, 6 225 (Rint=0.051)	17 008, 6 077
  Data, restraint, parameter	6 225, 39, 469	6 077, 12, 474
  Final R indices [I > 2σ(I)]	R1=0.066 3, wR2=0.163 4	R1=0.073 6, wR2=0.176 6
  R indices (all data)	R1=0.106 1, wR2=0.186 7	R1=0.119 8, wR2=0.199 8
  Goodness of fit on F2	1.024	1.043

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  Table 2.  Selected bond lengths (nm) and angles (°) for complex 1

  1 (298 K)
  Co(1)—O(1)	0.190 8(3)	Co(1)—O(2)	0.188 4(3)	Co(1)—O(4)	0.189 7(3)
  Co(1)—O(5)	0.188 1(3)	Co(1)—N(2)	0.186 4(3)	Co(1)—N(4)	0.185 7(4)
  O(2)—Co(1)—O(1)	177.32(14)	O(2)—Co(1)—O(4)	89.71(14)	O(4)—Co(1)—O(1)	87.82(14)
  O(5)—Co(1)—O(1)	90.91(13)	O(5)—Co(1)—O(2)	91.58(13)	O(5)—Co(1)—O(4)	178.60(13)
  N(2)—Co(1)—O(1)	83.24(14)	N(2)—Co(1)—O(2)	95.80(14)	N(2)—Co(1)—O(4)	91.20(15)
  N(2)—Co(1)—O(5)	89.22(14)	N(4)—Co(1)—O(1)	92.39(14)	N(4)—Co(1)—O(2)	88.34(14)
  N(4)—Co(1)—O(4)	83.70(16)	N(4)—Co(1)—O(5)	95.78(15)	N(4)—Co(1)—N(2)	173.43(17)
  1 (100 K)
  Co(1)—O(1)	0.188 7(3)	Co(1)—O(2)	0.188 5(3)	Co(1)—O(3)	0.190 7(3)
  Co(1)—O(5)	0.189 9(4)	Co(1)—N(1)	0.186 6(4)	Co(1)—N(2)	0.186 7(4)
  O(1)—Co(1)—O(3)	91.16(15)	O(1)—Co(1)—O(4)	179.14(16)	O(2)—Co(1)—O(1)	91.39(15)
  O(2)—Co(1)—O(3)	177.26(15)	O(2)—Co(1)—O(5)	89.46(15)	O(5)—Co(1)—O(3)	87.99(15)
  N(1)—Co(1)—O(1)	88.80(16)	N(1)—Co(1)—O(2)	95.52(16)	N(1)—Co(1)—O(3)	83.52(16)
  N(1)—Co(1)—O(5)	91.21(17)	N(1)—Co(1)—N(2)	173.35(19)	N(2)—Co(1)—O(1)	96.40(17)
  N(2)—Co(1)—O(2)	88.51(16)	N(2)—Co(1)—O(3)	92.21(17)	N(2)—Co(1)—O(5)	83.53(18)

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•