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• In recent years, the design and study of multifu-nctional molecule-based magnetic materials has attracted enormous attention owing to their potential applications across a plethora of areas such as in molecular spintronics, and new bifunctional mate-rials^[1-2]. Meanwhile, the exquisite control that can be achieved over the design of coordination compounds via rational structural modifications to the component metal ions and ligands provides unique prospects for designing structures and elucidating fundamental structure-function relationship^[3-5]. Tetrathiafulvalene (TTF) and its derivatives, known as π-electron donors capable of forming stable cation radical and dication species upon oxidation, can be used to construct novel functional materials as versatile building blocks due to its unique structural and electronic diversities^[6-8]. The incorporation of magnetic moment carriers such as Fe(Ⅱ) and Co(Ⅱ) ions into coordination compounds allows for the development of novel multifunctional materials with additional magnetic properties, including ferromagnetism or ferrimagnetism, and spin crossover behavior, which has exceptional potential in technological applications such as data storage and display devices^[9-10]. Recently, several Fe(Ⅱ) coordination compounds based on the terpyridine-tetrathiafulvalene (TTF-terpy) have been studied^[11]. These compounds retain the redox activity of the TTF moiety and moreover some exhibit antiferromagnetic interactions arising from Fe(Ⅱ) ions.

  In this paper, we design a new π-extended ligand consisting of pyrazinyl and pyridine moiety and redox-active TTF core. Based on this novel functional ligand, an iron complex, [Fe(Ⅱ)(L)₂](BF₄)₂·2.75CH₃CN·0.25CH₂Cl₂ (1), is successfully synthesized and the crystal structure is obtained. Its Magnetic, electro-chemical and UV-Vis properties were studied.

  1   Experimental

  1.1   Materials and instruments

  All reagents and solvents were obtained from commercial sources and used without further purification.

  图Scheme 1 Synthesis of ligand L Scheme1. Synthesis of ligand L

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  IR spectra were recorded in range of 400~4 000 cm^-1 on a Vector27 Bruker Spectrophotometer with KBr pellets. C, H, N and S analyses were carried out on a Perkin-Elmer 240C analyzer. Mass spectra were determined on a Varian MAT 311A instrument for ESI-MS. Magnetic susceptibility measurements were performed using a Quantum Design SQUID VSM magnetometer on the crystalline sample for complex 1. Cyclic voltammetry measurements were performed in n-Bu₄NPF₆/CH₃CN electrolyte using a BAS Epsilon electrochemical analyser and three electrode system. All potentials are reported in mV versus Fc/Fc⁺ couple. Absorption spectra were measured on a UV-3100 spectrophotometer.

  1.2   Synthesis of the ligand L

  1.2.1   Synthesis of the formyltetrathiafulvalene (TTF-CHO)

  To a mixture of TTF (696 mg, 3.4 mmol) in anh-ydrous ether (60 mL) was added lithium diisopro-pylamide (LDA, 2.6 mL, 2.0 mol·L^-1 solution in THF) under nitrogen at -78 ℃. After the mixture was stirred for 2 h, Ph(Me)NCHO (1.04 mL) was added dropwise. The mixture was stirred at -78 ℃ for 1 h and then for 12 h at room temperature. Under N₂ protection, hydrochloric acid (15 mL, 1 mol·L^-1) was added and the reactant was stirred for 20 min to quench the reaction. The aqueous phase was extracted with dichloromethane (60 mL×3), and the organic extracts were washed with brine, dried over sodium sulfate, filtered, and concentrated. The residue was purified by a silica column using dichloromethane/petroleum ether as an eluent to afford TTF-CHO as a dark red solid (Yield: 58.8%).

  1.2.2   Synthesis of the ligand L

  A mixture of TTF-CHO (463.8 mg, 2 mmol) in THF (30 mL), 2-acetylpyridine (488.2 mg, 4 mmol) in ethanol (15 mL), and t-BuONa (528.6 mg, 5.5 mmol) in ethanol (15 mL) under nitrogen was stirred at room temperature for 0.5 h. After adding concentrated ammonia (7.2 mL), the reaction mixture was stirred for 20 h. After filtration, the resulting residue was washed several times with methanol and dried by vacuum. The resulting ligand L was obtained as a dark red solid in 20.6% yield. Anal. Calcd. for C₁₉H₁₁N₅S₄(%): C, 52.15; H, 2.53; N, 16.01; S, 29.31. Found(%): C, 52.11; H, 2.55; N, 15.97; S, 29.30. IR (KBr, cm^-1): 3 408m, 3 065w, 1 611m, 1 533m, 1 466w, 1 425w, 1 401 w, 1 328s, 1 261m, 1 219m, 1 192m, 1 034s, 848w, 800m, 732w, 685w, 634w, 525w, 448w. MS (m/z): Calcd. for C₁₉H₁₁N₅S₄ (M⁺) 436.99; Found 437.42.

  1.3   Synthesis of 1

  Under nitrogen, to the CH₂Cl₂ solution of L (0.02 mol·L^-1, 10 mL) was added Fe(BF₄)₂·6H₂O (33.8 mg, 0.1 mmol) and a few grains of ascorbic acid in CH₃OH (10 mL). The reaction mixture was stirred under reflux for 20 h. After filtration, the resulting residue was washed several times with methanol, dried by vacuum and recrystallized from 80 mL acetonitrile. The resulting complex 1 was obtained as a dark blue solid in 43.5% yield. Crystals of 1 were grown by slow diffusion of diethyl ether into an acetonitrile solution of 1 at -6 ℃. Anal. Calcd. for C_43.5H_30.75B₂Cl_0.50F₈FeN_12.75S₈(%): C, 42.28; H, 2.51; N, 14.45; S, 20.76. Found(%): C, 42.25; H, 2.48; N, 14.50; S, 20.72. IR (KBr, cm^-1): 3 408m, 3 065w, 1 611m, 1 533w, 1 466w, 1 425 w, 1 401w, 1 328s, 1 261m, 1 219w, 1 192m, 1 034s, 848w, 800m, 732w, 685w, 634w, 522w, 448w.

  1.4   Crystal structure determination

  The crystal data were collected with Mo Kα radiation (λ=0.071 073 nm) on a CCD diffractometer. The cell parameters were retrieved and refined by using computer software (SMART and SAINT, respectively)^[12]. The SADABS^[13] program was applied for absorption corrections. Structure was solved by direct methods using the program package SHELXL-97^[14]. All the non-hydrogen atoms were located in the Fourier maps and refined with anisotropic parameters. Crystallographic data are summarized in Table 1. Selected bond lengths (nm) and bond angles (°) are listed in Table 2.

  Table 1.  Crystallographic data of complex 1

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  Formula	C43.5H30.75B2Cl0.50F8FeN12.75S8
  Formula weight	1 238.74
  Crystal system	Triclinic
  Space group	P1
  a / nm	1.358 8(3)
  b / nm	1.480 0(4)
  c / nm	1.603 0(4)
  α/(°)	110.446(4)
  β/(°)	91.895(4)
  γ/(°)	112.379(4)
  V/nm3	2741.4(12)
  Temperature / K	296(2)
  Z	2
  Dc / (g·cm-3)	1.501
  Size / mm	0.23×0.16×0.09
  F(000)	1 254
  Absorption coefficient / mm-1	0.677
  Reflection collected, used	16 111, 1 064
  Independent reflection (Rint)	10 340(0.078 7)
  Goodness-of-fit on F2	1.027
  θ range for data collection / (°)	1.62~25.68
  R1, wR2 [I > 2σ(I)]	0.091 8, 0.238 6

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

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  Fe(1)-N(2)	0.l98 3(7)	Fe(1)-N(4)	0.l96 7(7)	Fe(1)-N(8)	0.l86 7(7)
  Fe(1)-N(3)	0.l87 8(7)	Fe(1)-N(7)	0.l95 9(7)	Fe(1)-N(9)	0.l94 7(7)
  N(3)-Fe(1)-N(2)	80.8(3)	N(3)-Fe(1)-N(9)	98.l(3)	N(7)-Fe(1)-N(4)	90.7(3)
  N(3)-Fe(1)-N(4)	80.6(3)	N(4)-Fe(1)-N(2)	l6l.3(3)	N(8)-Fe(1)-N(2)	98.4(3)
  N(3)-Fe(1)-N(7)	99.4(3)	N(7)-Fe(1)-N(2)	9l.4(3)	N(8)-Fe(1)-N(3)	l78.6(3)
  N(8)-Fe(1)-N(4)	l00.3(3)	N(8)-Fe(1)-N(7)	8l.7(3)	N(8)-Fe(1)-N(9)	80.8(3)
  N(9)-Fe(1)-N(2)	92.9(3)	N(9)-Fe(1)-N(4)	90.6(3)	N(9)-Fe(1)-N(7)	l62.4(3)

  CCDC: 1565586.

  2   Results and discussion

  2.1   Crystal structure

  The ORTEP diagram of the complex 1 is depicted in Fig. 1. The crystal structure of 1 adopts the triclinic system (space group P1), and the asymmetric unit contains one [Fe(Ⅱ)(L)₂]²⁺ cation, two BF₄^- anions, a quarter CH₂Cl₂ molecule and 2.75 CH₃CN molecules. In each [Fe(Ⅱ)(L)₂]²⁺ cation, the central iron(Ⅱ) ion lies on the inversion center and adopts a distorted [FeN₆] octahedral coordination environment, which is defined by two nitrogen donors from four pyrazine nitrogen donors and two pyridyl nitrogen donors from two ligands. The Fe-N bonds range from 0.186 7(4) to 0.198 3(7) nm suggesting that the Fe1 ions are in low-spin (LS) states at 296 K^[15]. The TTF moieties of the two L ligands all adopt a U-like configuration with a dihedral angle 13.6°, 6.2°, 8.1° and 25.3° folding at S1 -S2, S3-S4, S5-S6, and S7-S8, respectively. The central C=C bond length of the TTF cores are 0.130 0(13) nm for C3-C4 and 0.127 8(13) nm for C22-C23. The bond lengths and angles are close to those reported for the neutral unit^[16], indicating that the ligand can still be regarded as charge neutral. Complex 1 stacks together to give a two-dimensional (2D) network through noncl-assical hydrogen bonds C(11)-H(11)…F(3), C(21)-H(21)…F(2), C(32)-H(32)…F(6)^ⅰ, C(33)-H(33)…F(4), and C(30)-H(30) …F(5) (d_H…F=0.246, 0.240, 0.240, 0.231 and 0.237 nm, respectively. Symmetry codes: ^ⅰ2-x, 1-y, 2-z) (Fig. 2a). Through nonclassical hydrogen bonds (C(19)-H(19)…F(3), d_H…F=0.246 nm; C(39)-H(39)^ⅱ…F(6), d_H…F=0.242 nm), the solvent CH₃CN molecules are connected to the 2D network. Inter-molecular π…π contacts were found (Fig. 2b). The C…C distances between two neighboring TTF cores are 0.365 2 and 0.376 8 nm and the distances of pyrazine units are 0.372 6 and 0.383 7 nm. In addition, weak S…S interactions ( < 0.370 nm) can be observed in the packing diagram^[17]. The shortest S…S distance between two neighboring TTF cores is 0.368 4 nm.

  图1 Structure of the complex 1 Figure1. Structure of the complex 1

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  图2 Crystal-packing arrangement of 1 showing nonclassical hydrogen bonding interactions (a), π…π (black line) and S…S (red dashed line) interactions (b) Figure2. Crystal-packing arrangement of 1 showing nonclassical hydrogen bonding interactions (a), π…π (black line) and S…S (red dashed line) interactions (b)

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  2.2   Magnetic properties

  Variable-temperature (T) magnetic susceptibility (χ_MT) measurements were performed on the sample of complex 1 in the range of 2~300 K. At room tempera-ture, most of the Fe(Ⅱ) ions are in LS states. As shown in Fig. 3, the χ_MT value is 0.57 cm³·mol^-1·K at 300 K^[18]. As the temperature decreases, the χ_MT value gradually drops close to zero at 2.0 K due to the remain high-spin (HS) Fe(Ⅱ) gradually turned to LS, which indi-cates an gradually SCO transition^[19].

  图3 Temperature dependence of χ_MT for complex 1 at 1 kOe Figure3. Temperature dependence of χ_MT for complex 1 at 1 kOe

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  2.3   Solution-State electrochemistry

  Solution-state electrochemistry was performed on L and complex 1. The electrochemical property for L was investigated by using cyclic voltammetry (CV) in DMF (0.1 mol·L^-1 n-Bu₄NClO₄). For complex 1, cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were used in CH₃CN (0.1 mol·L^-1 n-Bu₄NClO₄). As shown in Fig. 4, two reversible oxidation couples were observed at E_1/2=0.09 and 0.31 V for L, which are associated with the successive oxidation of neutral TTF (TTF⁰) to the radical cation (·TTF⁺) and then to the dication (TTF²⁺). Upon coordination, the CV of 1 shows three distinct redox processes. The two reversible redox peaks at E_1/2=0.18 V and 0.53 V can be assigned to the TTF/·TTF⁺ and ·TTF⁺/TTF²⁺ redox couples, respectively. The third redox peak at E_1/2=1.44 V can be assigned to the Fe(Ⅱ)-centered one-electron oxidation process. Three peaks of DPV correspond to the redox processes of CV, and the ratio of integral area for three peaks is 2:2:1 (the ratio of TTF moieties to Fe(Ⅱ) per complex molecule is 2:1), which suggests that the three-step redox process is independent single-electron transfer process.

  图4 Solution-state electrochemistry of L (a) and complex 1 (b) Figure4. Solution-state electrochemistry of L (a) and complex 1 (b)

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  2.4   Spectroelectrochemistry

  Given the nature of CV, solution-state spectroele-ctrochemical measurements were conducted to gain insight into the origins of the electrochemical proce-sses for 1 (Fig. 5a). The measurements were also carried out during the electrolysis of the solution of complex 1 in CH₃CN at suitable constant potentials correspon-ding to its redox processes. Upon increasing the applied potential from 0 to 0.45 V, the bands at 466 nm increases due to formation of the ·TTF⁺ radical cation. Upon increasing the potential from 0.5 to 0.7 V, the electronic spectra show a decrease in the characteristic absorptions of the radical cation bands due to formation of TTF²⁺ dication (Fig. 5b).

  图5 Spectroelectrochemistry on 1 over the potential range of 0~0.45 V (a) and 0.5~0.7 V (b) Figure5. Spectroelectrochemistry on 1 over the potential range of 0~0.45 V (a) and 0.5~0.7 V (b)

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  3   Conclusions

  One discrete complex based on the redox-active 4-tetrathiafulvalene-2, 6-di(pyrazin-2-yl)pyridine ligand was synthesized and structurally characterized. At room temperature, most of the Fe(Ⅱ) ions are in LS states. Complex 1 features diverse nonclassical hydrogen bonding interactions, π…π and S…S interactions. The interesting redox activity for complex 1 has been probed via solution-state electrochemistry, revealing multiple processes due to the redox properties of both the TTF and metal ions. The results suggest that the incorporation of redox-active ligands into complexes can be useful for the development of interesting redox-active materials. Further investiga-tions on TTF compounds with new structures and multifunctional properties are ongoing in our group.

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• 

  Scheme 1  Synthesis of ligand L

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  Figure 1  Structure of the complex 1

  Symmetry codes: ^ⅰ2-x, 1-y, 2-z

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  Figure 2  Crystal-packing arrangement of 1 showing nonclassical hydrogen bonding interactions (a), π…π (black line) and S…S (red dashed line) interactions (b)

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  Figure 3  Temperature dependence of χ_MT for complex 1 at 1 kOe

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  Figure 4  Solution-state electrochemistry of L (a) and complex 1 (b)

  Range:-0.1~0.5 V for L, -0.2~1.9 V for complex 1; Arrow indicates the direction of the forward scan

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  Figure 5  Spectroelectrochemistry on 1 over the potential range of 0~0.45 V (a) and 0.5~0.7 V (b)

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  Table 1.  Crystallographic data of complex 1

  Formula	C43.5H30.75B2Cl0.50F8FeN12.75S8
  Formula weight	1 238.74
  Crystal system	Triclinic
  Space group	P1
  a / nm	1.358 8(3)
  b / nm	1.480 0(4)
  c / nm	1.603 0(4)
  α/(°)	110.446(4)
  β/(°)	91.895(4)
  γ/(°)	112.379(4)
  V/nm3	2741.4(12)
  Temperature / K	296(2)
  Z	2
  Dc / (g·cm-3)	1.501
  Size / mm	0.23×0.16×0.09
  F(000)	1 254
  Absorption coefficient / mm-1	0.677
  Reflection collected, used	16 111, 1 064
  Independent reflection (Rint)	10 340(0.078 7)
  Goodness-of-fit on F2	1.027
  θ range for data collection / (°)	1.62~25.68
  R1, wR2 [I > 2σ(I)]	0.091 8, 0.238 6

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

  Fe(1)-N(2)	0.l98 3(7)	Fe(1)-N(4)	0.l96 7(7)	Fe(1)-N(8)	0.l86 7(7)
  Fe(1)-N(3)	0.l87 8(7)	Fe(1)-N(7)	0.l95 9(7)	Fe(1)-N(9)	0.l94 7(7)
  N(3)-Fe(1)-N(2)	80.8(3)	N(3)-Fe(1)-N(9)	98.l(3)	N(7)-Fe(1)-N(4)	90.7(3)
  N(3)-Fe(1)-N(4)	80.6(3)	N(4)-Fe(1)-N(2)	l6l.3(3)	N(8)-Fe(1)-N(2)	98.4(3)
  N(3)-Fe(1)-N(7)	99.4(3)	N(7)-Fe(1)-N(2)	9l.4(3)	N(8)-Fe(1)-N(3)	l78.6(3)
  N(8)-Fe(1)-N(4)	l00.3(3)	N(8)-Fe(1)-N(7)	8l.7(3)	N(8)-Fe(1)-N(9)	80.8(3)
  N(9)-Fe(1)-N(2)	92.9(3)	N(9)-Fe(1)-N(4)	90.6(3)	N(9)-Fe(1)-N(7)	l62.4(3)

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•