HUANG Ying-Ying XU Qing-Dou HU Sheng-Min WU Xin-Tao SHENG Tian-Lu②

a (College of Chemistry, Fuzhou University, Fuzhou 350002, China)

b (State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China)

c (University of Chinese Academy of Sciences, Beijing 100049, China)

ABSTRACT Three dinuclear cyanide-bridged complexes [FeII(PY5OMe2)CNMII(PY5OMe2)](OTf)3 (M = Fe 1, M = Co 2, M = Mn 3) (PY5OMe2 = 2,6-bis-((2-pyridyl)methoxymethane)pyridine OTf = CF3SO3-) have been synthesized and characterized. Single-crystal diffraction analyses show these three dinuclear compounds are very similar in structure. The measured ν(CN) results for compounds 1～3 suggest that Mn2+ is electron-poorer than Fe2+ and Co2+. Meanwhile, the temperature dependence of magnetic susceptibilities of complexes 1～3 reveals that in these three complexes, all the cyanide-carbon coordinated Fe(II) is low-spin, and Co(II) for 2 and Mn(II) for 3 are both in a high-spin state through 2～400 K but the cyanide-nitrogen coordinated Fe(II) for complex 1 exhibits spin crossover (SCO) behavior over 300 K and a hysteresis of 36 K in both cooling and heating modes.

Keywords: cyanide-bridged, magnetic property, spin crossover; DOI: 10.14102/j.cnki.0254-5861.2011-3123

1 INTRODUCTION

In the past decades, cyanide-bridged compounds have attracted the attention of many researchers because of their interesting magnetic coupling between paramagnetic metal centers[1,2]and metal-to-metal charge transfer properties[3-5].So far, a large number of cyanide-bridged compounds with novel structures and fascinating magnetic properties such as single-chain magnets (SCM)[6-10], single-molecule magnets(SMM)[11-16]and spin crossover (SCO)[17-22]have been reported. Among them, SCO complexes can switch between low spin (LS) and high spin (HS) states under the stimulation of external conditions such as light[17],pressure[23]and temperature[21]which are usually accompanied by the change of material physical properties like magnetic, optics and dielectric properties. Therefore,spin crossover complex is one of the most attractive advanced switchable materials with the potential to be used as molecular switches, data storage and data displays[24,25].Although this area has been studied for a long time, most of them focused on the effect of ligand field on metal ion spin state. There are few studies on the spin-crossover behavior of different metal ions in the same ligand filed.

In view of many SCO behaviors which are more common on octahedral transition metal complexes ofd4-d7electronic configuration coordinated by six nitrogen-based ligands[26,27],we decide to use 2,6-bis-((2-pyridyl)methoxymethane)pyridine (PY5OMe2) ligand and cyanide-bridge as N-donor ligands to synthesize octahedral coordination compounds and investigate their spin transition behaviors. Herein, we report three dinuclear cyanide-bridged compounds which have been synthesized and characterized by IR spectroscopy,single-crystal X-ray diffraction analysis and magnetic properties analysis. For better comparison, three adjacent transition metal ions Mn(II), Fe(II) and Co(II) are used as the central metal ions to coordinate with 2,6-bis-((2-pyridyl)methoxymethane)pyridine (PY5OMe2) ligand to form the[MII(PY5OMe2)]2+(M = Fe, Co and Mn) fragment and further combine with mononuclear complex [FeII(PY5O Me2)-CN]+through cyanide-bridge. The obtained three dinuclear complexes [FeII(PY5OMe2)CNMII(PY5OMe2)](OTf)3(M =Fe 1, Co 2 and Mn 3) possess similar structures but obviously different magnetic behaviors.

2 EXPERIMENTAL

2. 1 Materials and physical measurements

Unless otherwise description, the experiments are under argon atmosphere and operated with the standard Schlenk techniques. Except that tetrahydrofuran is an ultra-dry solvent (water ≤ 30 ppm), other chemical solvents are purchased at reagent grade without further purification.[MII(PY5OMe2)](OTf)2(M = Fe, Co and Mn) were synthesized through the published papers[28,29]. Infrared (IR)spectra in the range of 4000～400 cm-1were performed on a VERTEX 70 spectrophotometer with KBr pellet. The elemental analyses (C, H and N) were recorded on a Vario MICRO elemental analyzer. Variable-temperature magnetic susceptibility and field dependence of the magnetization measurements were conducted on a Quantum Design Magnetic Property Measurement System MPMS-XL magnetometer. Pascal’s constants were used to correct the diamagnetism of complexes 1～3[30]. And the thermal analyses were performed on a STA449C comprehensive thermal analyzer from 300 to 850 K at a heating rate of 20 K·min-1under nitrogen flow.

2. 2 Synthesis of the complexes

Since complexes 1～3 were prepared in a similar way, only the preparation of complex 1 is described in detail here.

2. 2. 1 [FeII(PY5OMe2)CN](OTf)

KCN (1.95 g, 30 mmol) and [FeII(PY5OMe2)](OTf)2(2.49 g, 3 mmol) were added in methanol (30 mL ). The mixture was refluxed at 80 ℃ for about 0.5 hours. Then the solvent was removed in a vacuum and the remaining solids were extracted with CH2Cl2. The dark red product was obtained by removing CH2Cl2solvent under reduced pressure and recrystallized in methanol. Yield: 1.71 g, 81%. Anal. Calcd.(%) for C31H25F3FeN6O5S·CH3OH: C, 52.04; H, 3.96; N,11.38. Found (%): C, 52.02; H, 3.98; N, 11.33.

2. 2. 2 [FeII(PY5OMe2)CNFeII(PY5OMe2)](OTf)3·2H2O·3CH3CN (1)

[FeII(PY5OMe2)CN](OTf) (0.0706 g, 1 mmol) and[FeII(PY5OMe2)](OTf)2(0.0829 g, 1 mmol) were added in methanol (15 mL). The reaction mixture was refluxed with stirring at 80 ℃ for about 6 hours, after which the mixture solution was concentrated under reduced pressure, and then the product is precipitated by the addition of diethyl ether.The obtained product was redissolved in acetonitrile (8 mL),obtaining brown red crystals by slow diffusion of diethyl ether into the acetonitrile solution. Yield: 0.115 g, 75%. Anal.Calcd. (%) for C62H50F9Fe2N11O13S3·2H2O: C, 47.37; H,3.46; N, 9.80. Found (%): C, 47.27; H, 3.41; N, 9.78.

2. 2. 3 [FeII(PY5OMe2)CNCoII(PY5OMe2)](OTf)3·2H2O·3CH3CN (2)

The synthesis procedure was the same as complex 1 with[FeII(PY5OMe2)CN](OTf) (0.0706 g, 1 mmol) and CoII-(PY5OMe2)[OTf]2(0.0832 g, 1 mmol) in 15 mL methanol.The orange red crystals were obtained. Yield: 0.121 g, 78%.Anal. Calcd. (%) for C62H50CoF9FeN11O13S3·2H2O: C, 47.28;H, 3.46; N, 9.78. Found (%): C, 47.37; H, 3.38; N, 9.87.

2. 2. 4 [FeII(PY5OMe2)CNMnII(PY5OMe2)](OTf)3 (3)

The synthesis procedure was the same as complex 1:[FeII(PY5OMe2)CN](OTf) (0.0706 g, 1 mmol) and MnII-(PY5OMe2)[OTf]2(0.0828 g, 1 mmol) in 15 mL methanol.The dark red crystals were obtained. Yield: 0.109 g, 71%.Anal. Calcd. (%) for C62H50F9FeMnN11O13S3: C, 48.51; H,3.28; N, 10.04. Found (%): C, 48.58; H, 3.77; N, 9.47.

2. 3 X-ray structure determination

The crystallographic data of complex 2 were collected using anω-scan model technique on a Saturn724+ CCD diffractometer with graphite-monochromatic MoKα(λ=0.71073 Å). Complexes 1 and 3 were on a MetalJet D2+diffractometer with graphite-monochromatic GaKα(λ=1.3405 Å). These structures were solved by direct methods using Fourier difference techniques with the SHELXL-2018/3 program package[31]and refined by full-matrix least-squares method onF2with anisotropic thermal parameters for the non-hydrogen atoms. All the hydrogen atoms were calculated and generated in the ideal positions.

3 RESULTS AND DISCUSSION

3. 1 X-ray crystal structure

Complexes 1～3 were synthesized by solution method and their crystals were obtained by slow diffusion of diethyl ether into the acetonitrile solution. Single-crystal X-ray diffraction analyses show that in the cation of compounds 1～3 the two cation fragments [Fe(PY5OMe2)]2+and[M(PY5OMe2)]2+(M = Fe, Co or Mn) with similar coordination structures are linked via a single cyanide bridge.Compounds 1, 2 and 3 crystallize in the monoclinic space groupP21/c,P21/cand triclinic space groupP, respectively(Table 1 and Fig. 1). However, the structure of compound 3 contains two crystallographically independent molecules per asymmetric unit (Table 1 and Fig. S1). The central metal ions of the [M(PY5OMe2)]2+fragment possess the hexa-coordination environment with six nitrogen atoms from one PY5OMe2ligand and one cyanide bridging ligand,forming a distorted octahedral geometry. But for compound 3, the atoms of Fe-C≡N and C≡N-Mn in the backbone structure are disordered in position, as shown in Fig. S1.Thus, herein the structural parameters of compound 3 are not described for comparison.

Table 1. Crystallographic Data for Complexes 1～3

As shown in Table 2, the bond angles of C≡N-M (M = Fe and Co) and Fe-C≡N in 1 and 2 are nearly linear in ranges of 176.6(2)～177.1(3)° and 177.8(2)～178.0(3)°. The average Fe-N distances of [Fe(PY5OMe2)CN]+are 1.985(3) Å in 1 and 2.021(2) Å in 2, which are in good agreement of the bond lengths for LS Fe(II) complexes[32], suggesting the cyanide-carbon coordinated Fe(II) is of low-spin. And the average M-N (M = Fe, Co) distances of cyanide-nitrogen coordinated metal ions are 1.997(3) Å in 1 and 2.080(2) Å in 2. These are typical bond lengths for a LS Fe(II) complex and a HS Co(II) complex[34-37], which are consistent with the following magnetic data. We have tried to figure out the changes in bond lengths of 1 at high temperature, at which,however, the crystal was prone to collapse, thus precluding us from obtaining its crystallographic data at 400 K.

Fig. 1a. Structure of complex 1 (All the H atoms, CF3SO3- anions and solvent molecules have been removed)

Fig. 1b. Structure of complex 2 (All the H atoms, CF3SO3- anions and solvent molecules have been removed)

Fig. 1c. Structure of complex 3 (The positional disordered atoms, all the H atoms,F3SO3- anions and solvent molecules have been removed for clarity)

Table 2. Selected Bond Lengths (Å) and Bond Angles (°) for Complexes 1～3

Fe(2)-N(8) 1.996(3) Fe-N(8) 2.013(2) Fe(1)-N(3) 2.091(6) Fe(2)-N(9) 2.097(7)Fe(2)-N(9) 2.032(3) Fe-N(9) 2.055(2) Fe(1)-N(4) 2.000(4) Fe(2)-N(10) 2.005(5)Fe(2)-N(10) 1.991(3) Fe-N(10) 2.008(2) Fe(1)-N(5) 2.120(6) Fe(2)-N(11) 2.093(7)Fe(2)-N(11) 1.997(3) Fe-N(11) 2.013(2) Fe(1)-N(6) 2.108(6) Fe(2)-N(12) 2.106(7)Fe(2)-C(1) 1.910(3) Fe-C(1) 1.904(3) Fe(1)-C(1) 1.948(6) Fe(2)-C(31) 1.957(5)N(1)≡C(1) 1.164(5) N(1)≡C(1) 1.145(3) N(1)≡C(1) 1.153(8) N(7)≡C(31) 1.150(10)C(1)≡N(1)-Fe(1) 177.1(3) C(1)≡N(1)-Co 176.6(2) C(1)≡N(1)-Mn(1) 179.1(15) C(31)≡N(7)-Mn(2) 178.4(9)N(1)-Fe(1)-N(2) 89.63(11) N(1)-Co-N(2) 90.89(8) N(1)-Mn(1)-N(2) 102.3(5) N(8)-Mn(2)-N(9) 80.9(2)N(1)-Fe(1)-N(3) 177.76(2) N(1)-Co-N(3) 177.97(8) N(1)-Mn(1)-N(4) 174.6(6) N(9)-Mn(2)-N(11) 157.8(2)N(1)≡C(1)-Fe(2) 178.0(3) N(1)≡C(1)-Fe 177.8(2) N(1)≡C(1)-Fe(1) 178.3(11) N(7)≡C(31)-Fe(2) 178.5(10)C(1)-Fe(2)-N(7) 92.31(11) C(1)-Fe-N(7) 92.68(9) C(1)-Fe(1)-N(2) 89.5(4) C(31)-Fe(2)-N(8) 89.3(5)C(1)-Fe(2)-N(9) 178.86(3) C(1)-Fe-N(9) 178.98(9) C(1)-Fe(1)-N(4) 77.6(4) C(31)-Fe(2)-N(10) 179.1(6)

3. 2 IR spectroscopy

IR spectra of complexes 1～3 are listed in Table 3, and the data of the parent mononuclear compound[FeII(PY5OMe2)CN](OTf) are also listed for the purpose of comparison. Compared with [FeII(PY5OMe2)CN](OTf) (νCN= 2087 cm-1), the CN stretching bands in compounds 1 (νCN= 2095 cm-1) and 2 (νCN= 2099 cm-1) show a clear shift to higher frequencies and a evident shift to lower frequency for compound 3 (νCN= 2077 cm-1). This result can be explained by a combination of two factors: back-bonding from the C-bonded metal into the CN bond and kinematic coupling upon cyanide-bridge formation[38-40]. The former effect is expected to be enhanced by the withdrawal of charge from the cyanide to the second metal and shiftsν(CN) to a lower frequency, while the latter effect is a mechanical constraint on the CN motion and shiftsν(CN) to a higher frequency.For compounds 1 and 2, the raise ofνCNcan be attributed to the stronger effect of the kinematic coupling than the back-bonding effect. For compound 3, however, it can be considered that the back-bonding effect is outweighed,suggesting that Mn2+is electron-poorer than Fe2+and Co2+.

Table 3. CN Stretching Frequencies of Complexes 1～3 and Their Parent Mononuclear Compound

3. 3 Magnetic properties

The temperature dependence of magnetic susceptibilities for complexes 1, 2 and 3 was collected in an applied field of 1000 Oe under the temperature range of 2～400 K (Fig. 2).Complexes 2 and 3 were recorded in a cooling mode from 400 to 2 K, and complex 1 was recorded in both cooling and heating modes from 400 to 2 to 400 K. The TGA results of complexes 1～3 show that compounds 1 and 2 have lost all solvents at 400 K and these three compounds remain thermally stable in the temperature range of 300～400 K(Fig. S3). The thermal variation of the product of the molar magnetic susceptibility times the temperature (χMT) for the Fe(II) complex 1 shows the presence of SCO, as shown in Fig. 2. The value ofχMT at 400 K is 2.38 cm3·K·mol-1slightly below the expected value (3.0 cm3·K·mol-1) of one HS Fe(II) ion (S= 2,g= 2.0), which means the existence of a small amount of LS Fe(II). On further cooling, there is an obvious decrease in theχMT values from 2.38 to 0 cm3·K·mol-1from 400 to 300 K, indicating the completion of spin transition from HS to the LS state. In heating processes, the transition occurs from 400 to 336 K, resulting in a hysteresis loop of 36 K. For the Co(II) complex 2, theχMT value is 2.87 cm3·K·mol-1at 400 K which is consistent with an octahedral HS Co(II) center withS= 3/2, then theχMT product shows a smooth decline and reached 1.74 cm3·K·mol-1at 2 K. This magnetic behavior is typical in Co(II) octahedral complex and attributed to the magnetic anisotropy caused by spin-orbit coupling[33-36]. TheχMT product for the Mn(II) complex 3 basically maintains at around 4.65 cm3·K·mol-1in the temperature range of 2～400 K, which is in agreement with the theoretical spin-only values (4.375 cm3·K·mol-1) for a HS Mn(II) ion (S= 5/2,g=2.0).

Apparently, these three adjacent transition metal ions show different magnetic behaviors, although they are in the same ligand field. The Co(II) ion for 2 and Mn(II) ion for 3 are both in a high-spin state through 2～400 K, but the cyanide-nitrogen coordinated Fe(II) ion for complex 1 possesses a SCO behavior over 300 K. Such differences are related to thed-orbital splitting of these metal ions,indicating that Fe(II) gives a lager orbital splitting in this ligand field[41].

Fig. 2. Thermal variation of the χMT product for complexes 1 (The dashed red and blue lines represent the heating and cooling modes, respectively), 2 (gray) and 3 (green)

4 CONCLUSION

In summary, three dinuclear cyanide-bridged complexes have been designed and synthesized by using the same building unit [FeII(PY5OMe2)CN]+and mononuclear units[MII(PY5OMe2)]2+(M = Fe, Co and Mn) with different metal ions. Single-crystal X-ray diffraction analyses show that the structures of these three dinuclear complexes are very similar.The measuredν(CN) results for compounds 1～3 suggest that Mn2+is electron-poorer than Fe2+and Co2+. And, the temperature dependence of magnetic susceptibilities suggest that these three compounds have different magnetic behavior,namely compound 1 exhibits a SCO behavior and a hysteresis of 36 K, while 2 and 3 are paramagnetic with the high-spin cyanide-nitrogen bound Co(II) and Mn(II).