Fine-tuning of thermally induced SCO behaviors of trinuclear cyanido-bridged complexes by regulating the electron donating ability of C_CN-terminal fragments

Abstract

To achieve fine regulation of Fe^II SCO behavior, a series of trinuclear cyanido-bridged complexes trans-[CpMe_n(dppe)M^II(CN)]₂[Fe1^II(abpt)₂](OTf)₂ (1–4) (1, M = Fe2 and n = 1; 2, M = Fe2 and n = 4; 3, M = Fe2 and n = 5; 4, M = Ru and n = 5; CpMe_n = alkyl cyclopentadienyl with n = 1, 4, 5; dppe = 1,2-bis-(diphenylphosphino)ethane; abpt = 4-amino-3,5-bis-(pyridin-2-yl)-1,2,4-triazole and OTf = CF₃SO₃⁻) were synthesized and fully characterized by using elemental analysis, X-ray crystallography, magnetic measurements, variable-temperature IR spectroscopy and Mössbauer spectroscopy. It is worth mentioning that different from many mononuclear Fe(abpt)₂X₂ (X = NCS, NCSe, N(CN)₂, C(CN)₃, (NC)₂CC(OCH₃)C(CN)₂, (NC)₂CC(OC₂H₅)C(CN)₂, C₁₆SO₃ and Cl) complexes with more than one polymorph, only one polycrystalline form was found in complexes 1–4. Moreover, the thermally induced SCO behaviors of these four complexes are independent of intermolecular π–π interactions. The electron-donating ability of the C_CN-terminal fragment of CpMe_n(dppe)M^IICN can be flexibly regulated by changing the methyl number (n) of the cyclopentadiene ligand or metal ion type (M^II). These investigations indicate that the electron-donating ability of the C_CN-terminal fragment has an influence on the SCO behavior of Fe1^II. The spin transition temperature (T_1/2) of the complexes decreases with the increase of the electron-donating ability of the fragment CpMe_n(dppe)M^II. This study provides a new strategy to predict and precisely regulate the behaviors of SCO complexes.

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Introduction

Spin crossover (SCO) complexes can easily undergo transition between high spin (HS) and low spin (LS) states through external stimuli such as temperature, pressure, light, magnetic fields, and guest molecules,^1–10 and this interesting spin state transition is usually accompanied by changes in the physical properties of the materials, including their color, structural phase transition, dielectric constant and magnetism. Therefore, as controllable functional molecular materials, these novel properties of SCO complexes have been widely studied and applied in high-performance devices such as molecular switches, information storage devices, optical devices, and sensors in the past few decades.^8,11–18 Among them, predicting the SCO behavior of complexes to precisely control their spin transition temperature (T_1/2) is important for practical applications. Due to the direct influence of the ligand field on the SCO properties of complexes, modifying organic ligands directly bound to spin transition metal centers to regulate SCO behaviors is a common research method that has been extensively studied.^19–28 However, the spatial and/or substituent effects caused by the direct modification of ligands often have complicated and unpredictable effects on the properties of SCO.^26,29–34 Thus, although some relevant reports have been published, regulating the SCO behavior of complexes remains a significant challenge.

SCO-active materials based on octahedral complexes with [Fe^IIN₆] coordination centers have been most frequently reported, and ligands containing 1,2,4-triazoles can usually provide suitable ligand field strength for iron(II) to induce SCO.^35,36 The complex [Fe(abpt)₂(NCS)₂] (abpt = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole) is one particularly interesting example, which has four reported polymorphs (A–D) that show slightly different spin crossover behaviors.^37,38 Polymorph A has been initially reported to have an independent Fe(II) center (Z′ = 0.5) in the asymmetric unit which undergoes a thermal spin transition without hysteresis, with T_1/2 = 180 K. The complex [Fe(abpt)₂(NCSe)₂] has also been reported along with polymorph A of [Fe(abpt)₂(NCS)₂].³⁷ The two derivatives are isostructural, they crystallize in the monoclinic P2₁/n space group, and [Fe(abpt)₂(NCSe)₂] exhibits thermally induced SCO behavior without hysteresis, with T_1/2 = 224 K. Afterwards, many mononuclear SCO complexes with the chemical formula Fe(abpt)₂X₂ (X = NCS, NCSe, N(CN)₂, C(CN)₃, (NC)₂CC(OCH₃)C(CN)₂, (NC)₂CC(OC₂H₅)C(CN)₂, C₁₆SO₃ and Cl) have been reported.^35,39–41 The different magnetic behaviors of these complexes provide us with research ideas for this work. As cyanide (CN⁻) possesses a unique coordination mode, the structure of cyanide-bridged complexes with spin crossover can be more conveniently modified, and this modification is far away from the spin transition metal center, which can partly reduce the complicated effects of steric and substituent effects on SCO behavior. In addition, the stretching frequency of the cyanido group (CN⁻) is characterized by infrared spectroscopy and can be easily observed, particularly because it can sensitively reflect the spin state of iron(II) complexes.

Herein, we report the synthesis and characterization of four trinuclear cyanido-bridged complexes trans-[CpMe_n(dppe)M^II(CN)]₂[Fe1^II(abpt)₂](OTf)₂ (1, M = Fe2 and n = 1; 2, M = Fe2 and n = 4; 3, M = Fe2 and n = 5; 4, M = Ru and n = 5; CpMe_n = alkyl cyclopentadienyl with n = 1, 4, 5; dppe = 1, 2-bis-(diphenylphosphino)ethane; abpt = 4-amino-3,5-bis- (pyridin-2-yl)-1,2,4-triazole and OTf = CF₃SO₃⁻) (Scheme 1). The N-terminal of the CN bridge is the spin transition center (Fe1^II), and ligand field strength is regulated by modifying the CpMe_nM^II(dppe) fragment at the C-terminal of the bridge, namely changing the methyl number (n) of the cyclopentadiene ligand or metal ion type. The magnetic behaviors of these four complexes have been investigated by X-ray crystallography, magnetic measurements, variable-temperature IR, and Mössbauer spectroscopy. The experimental results demonstrate that the electron-donating ability of the CpMe_n(dppe)M^II fragment has a significant influence on the SCO behavior of Fe1^II. The T_1/2 of the complex can be fine-tuned by regulating the electron-donating ability of the metal organic fragment at the C-terminal of CN.

Scheme 1 Synthesis of complexes 1–4.

Results and discussion

Synthesis and X-ray structure determination

Complexes 1–4 were synthesized by reacting CpMe_n(dppe)M^IICN with trans-Fe(abpt)₂(OTf)₂·(CH₃OH)₂ in methanol as shown in Scheme 1 and obtained by slow diffusion of diethyl ether into a mixed solvent of methanol and dichloromethane. Only one polycrystalline form was found for complexes 1–4. Single-crystal X-ray data of 1, 2 and 4 were collected at 293 K. In order to describe the thermal magnetic properties in detail and relate these structures to the crystal structure of 3, three data sets were collected at 100, 293, and 400 K (along the thermal spin transition). Unfortunately, however, the useful crystal data of 3 at 400 K could not be obtained due to the poor crystal quality at high temperatures. 1 and 2 crystallize in the monoclinic P2₁/n space group, whereas 3 and 4 crystallize in the triclinic space group P1̄. Because central Fe1^II is located at an inversion centre, half of the molecules in the four complexes are in the asymmetric unit (Z′ = 0.5). The crystallographic data and structural details are listed in Tables S1 and S2 (see the ESI†), and the structures of the four complexes are shown in Fig. 1 and Fig. S3–S6.† The main structural features of these four complexes are very similar. Each asymmetrical unit of the molecular structures of 1–4 consists of a discrete [M^IILCNFe1^II(abpt)₂NCM^IIL] molecule where two equivalent chelating abpt ligands stand in the equatorial plane and the two equivalent metal–organic fragments (M^IILCN) complete the coordination sphere at trans positions (Fig. 1). Central Fe1^II is in a distorted [FeN₆] octahedral environment and coordinated by six nitrogen atoms: two from the two CN groups and four from the two abpt ligands (two pyridyl nitrogen and two triazole nitrogen).

Fig. 1 Molecular structure of 1–4 (a–d). Hydrogen atoms and CF₃SO₃⁻ anions have been removed for clarity. Reddish brown, Fe; Tiffany blue, Ru; pink, P; gray, C; dark blue, N.

The Fe1–N bond lengths of these four complexes are summarized in Table 1. At room temperature, the average Fe1–N bond lengths of 1 and 4 are about 2.071 Å and 2.103 Å, and the Fe–N_py (coordinated with pyridyl nitrogen) bond lengths are 2.103 Å and 2.155 Å, respectively. The average Fe1–N bond lengths are between the typical Fe–N bond lengths of LS and HS Fe(abpt)₂X₂ complexes,^35,37–39 indicating that the Fe1^II centers of 1 and 4 undergo SCO at room temperature. The average length of the Fe1–N bond of 3 is about 1.979 Å (Fe1–N_py = 2.007 Å) at 100 K, which is characteristic of LS Fe^II. When the temperature reached at 293 K, the average Fe1–N and Fe1–N_py bond lengths of 3 are 2.152 Å and 2.201 Å, that are very close to the characteristic bond lengths of HS Fe(abpt)₂X₂. This result may imply that complex 3 undergoes a thermally induced spin state transition from 100 K to 293 K. Similar to 3, the average length of the Fe1–N bond of 2 is about 2.150 Å (Fe1–N_py = 2.221 Å) at 293 K, very close to the typical Fe–N bond lengths of HS Fe(abpt)₂X₂.

Table 1 The space group parameters and spin state parameters of 1–4

Complex	T (K)	Fe1–N (dicyan)	Fe1–N (py)	Fe1–N (trz)	Fe1–N^a	Space group	Z′	Spin state (Fe1^II)	T 1/2 (K)
a The average length of the Fe1–N bond.
1	293	2.037(5)	2.102(4)	2.071(4)	2.070	P21/n	0.5	Mix LS/HS	300
2	293	2.073(2)	2.220(2)	2.155(2)	2.150	P21/n	0.5	Mix LS/HS	232
3	293	2.076(2)	2.201(3)	2.179(3)	2.152	P1̄	0.5	Mix LS/HS	222
3	100	1.937(15)	2.007(14)	1.995(14)	1.980	P1̄	0.5	LS	—
4	293	2.026(3)	2.155(3)	2.131(3)	2.104	P1̄	0.5	Mix LS/HS	275

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The molecular packing of 1 and 2, and 3 and 4 is same (Fig. 2), but that of 1 and 3 is quite different. Both 3 and 4 have a similar 1D molecular chain (Fig. S9 and S10†) often observed in abpt complexes, which are formed by π–π interactions between neighboring parallel abpt planes (Fig. 2c and d). The pyridyl rings on abpt of 3 and 4 are involved in π–π interactions between the intermolecular pyridyl rings with the distances of 3.813 Å for 3 and 3.833 Å for 4. However, in complexes 1 and 2, the abpt planes between the molecules are nearly perpendicular to each other, and no π–π interaction is found (Fig. 2a and b). According to published reports, the SCO behaviors of the mononuclear Fe(abpt)₂X₂ complexes are generally related to the intermolecular π–π interactions. All of the mononuclear Fe(abpt)₂X₂ complexes that undergo at least a partial thermal SCO displayed π–π interactions between pyridyl rings on abpt ligands. Besides, all of the mononuclear Fe(abpt)₂X₂ complexes without intermolecular π–π interactions did not undergo a thermal spin crossover.⁴²

Fig. 2 Diagrams showing the molecular packing of 1–4 (a–d); CF₃SO₃⁻ anions have been removed for clarity. The red dashed lines represent the π⋯π interactions.

Obviously, compared with reported mononuclear Fe(abpt)₂X₂, the influence of intermolecular π–π interactions on the SCO behavior is very weak for the trinuclear complexes 1–4. All 1–4 exhibit thermally induced SCO behaviors (see below), although there is no intermolecular π–π interaction in complexes 1 and 2. In addition, the π–π interactions in 3 and 4 (3.813 Å and 3.833 Å) are significantly weaker than those of previously reported mononuclear Fe(abpt)₂X₂ (3.256–3.684 Å),^{35,37–39,41} which may be related to the modification away from the spin transition metal center.

Solid-state magnetic properties

The temperature dependence of magnetic susceptibilities of 1–4 were investigated on the crystals under an applied field of 1000 Oe in the temperature range of 2–400 K during both heating and cooling processes, and the χ_MT vs. T plots of 1–4 in the range of 2–400 K are shown in Fig. 3. Complexes 2 and 3 exhibit a complete thermal spin crossover in the temperature range of 2–400 K. The χ_MT value is about 3.23 cm³ K mol⁻¹ for 2 and 3 at 400 K, which is in the range of the expected value for one HS Fe^II, indicating one HS Fe1^II in the central fragment and two LS Fe2^II in the terminal CpMe_n(dppe)Fe^II fragments in 2 and 3 at 400 K. As the temperature decreases, the χ_MT values first smoothly decrease down to 3.0 cm³ K mol⁻¹ at 290 K for 2 and 2.94 cm³ K mol⁻¹ at 280 K for 3. Then, the χ_MT values drop more rapidly to reach 0.058 cm³ K mol⁻¹ at 170 K (for 2) and 0.049 cm³ K mol⁻¹ at 150 K (for 3). These are the characteristic SCO behaviors of the transition from a HS state to a LS state. Compared with 2 and 3, complexes 1 and 4 undergo a partial thermal spin crossover in the range of 2–400 K. At 400 K, the χ_MT values of 1 and 4 are about 2.97 and 3.13 cm³ K mol⁻¹, suggesting that neither has reached a complete HS state compared to 2 and 3. During the cooling process, the χ_MT values decrease gradually to 0.03 cm³ K mol⁻¹ for 1 at 200 K and 0.045 cm³ K mol⁻¹ for 4 at 188 K, indicating that 1 and 4 have completely transformed into a LS state. Furthermore, the χ_MT vs. T plots of 1–4 in the cooling mode and the heating mode are basically consistent without thermal hysteresis loops.

Fig. 3 Temperature-dependent susceptibilities under 1000 Oe for the polycrystalline samples of 1–4.

The spin transition temperature T_1/2 of 1–4 are calculated to be 300, 232, 222 and 275 K (see Table 1). Obviously, from 1 to 3, the spin transition temperature decreases with the increase of the number of methyl substituents on the cyclopentadiene ligand, and as for 3 and 4, after replacing C_CN-terminal Fe^II with Ru^II in the fragment (CpMe₅(dppe)M^II), the spin transition temperature increases from 222 K to 274 K. This can be attributed to the stronger electron donating ability of Fe^II than that of Ru^II, which also further explain the similarity in magnetic behaviors between complexes 2 and 3, and complexes 1 and 4. The similarity in magnetic behaviors between 2 and 3 is due to their very similar number (n) of methyl on the CpMe_nM^II(dppe) fragment (4 and 5, respectively), resulting in similar electron donating abilities of the C_CN-terminal fragment. However, for complexes 1 and 4, their C_CN-terminal fragments are CpMeFe^II(dppe) and CpMe₅Ru^II(dppe), respectively. Obviously, the electron donating ability of five methyl groups (for complex 4) is stronger than that of one methyl group (for complex 1), but the electron donating ability of Ru^II (for complex 4) is weaker than that of Fe^II (for complex 1), which results in the similar electron donating abilities of the two fragments, resulting in the similarity in magnetic behaviors of 1 and 4. Therefore, it is evident that the T_1/2 of complexes 1–4 can be regulated by the electron donating ability of the fragment CpMe_n(dppe)M^II. When the electron donating ability of CpMe_n(dppe)M^II weakens, that is, upon reducing the number of methyl substituents on the cyclopentadiene ligand or upon replacing Fe^II with Ru^II in the metal organic fragment, the spin transition temperature of the complex obviously increases.

Infrared spectra

The spin states of the cyanido-bridged complexes can be sensitively reflected by the cyanide stretching frequency (ν_CN). Therefore, variable temperature infrared spectra have been performed for 1–4 and the results are shown in Fig. 4. For complex 1–4, clear changes of the cyanide stretching frequency occurred during the spin transition. Only one strong ν_CN peak is observed in 1 (2096 cm⁻¹) and 4 (2115 cm⁻¹) at 170 K, and in 2 (2090 cm⁻¹) and 3 (2084 cm⁻¹) at 150 K, which is attributed to the stretching modes of CN in the LS states. As the temperature increases, the CN stretching peaks for the LS state of 1–4 begin to weaken and a new CN stretching peak at a lower energy appears, attributed to the appearance of the HS state component. Upon further increasing the temperature, the CN stretching peaks of the LS state weaken obviously, while the CN stretching peaks of HS state are significantly enhanced. Furthermore, the integrated peaks of LS and HS states exhibit a regular red shift as the temperature decreases, which may be due to the increase of the high spin state components. Only one strong ν_CN peak remains in 1 (2065 cm⁻¹) and 4 (2073 cm⁻¹) at 450 K, and in 2 (2051 cm⁻¹) and 3 (2044 cm⁻¹) at 400 K, suggesting that complexes 1–4 have completely transformed from the LS to HS state. Evidently, the measurement results of the variable temperature IR of 1–4 are consistent with their magnetic measurements.

Fig. 4 Variable-temperature infrared spectra of 1–4.

Mössbauer spectra

The ⁵⁷Fe Mössbauer spectra can effectively determine the spin states and proportion of Fe^II in the complexes, further explaining its magnetic behaviors. Spectra of complexes 1–4 have been obtained at room temperature, and the results are shown in Fig. 5. The isomer shift (δ), quadrupole splitting (ΔE_Q), peak width (Γ) and the HS/LS area fractions are summarized in Table 2. The spectra of 1 can be deconvoluted into three sets, one with δ = 0.17 mm s⁻¹ and ΔE_Q = 1.84 mm s⁻¹, the second one with δ = 0.36 mm s⁻¹ and ΔE_Q = 0.48 mm s⁻¹ and the last one with δ = 1.02 mm s⁻¹ and ΔE_Q = 2.31 mm s⁻¹. The former is typical of LS Fe2^II of the CpMe_n(dppe)FeCN fragment,^43,44 the second one is attributed to LS Fe1^II of the Fe^IIN₆ motif, and the last one is assigned to HS Fe1^II. The three peak area ratio is 66.7 : 19.3 : 14.0, where the peak area ratio of Fe1^II to Fe2^II is about 1 : 2, which is consistent with the magnetic measurement result and the structural analyses on three Fe^II centers (one Fe1^II and two Fe2^II) of 1. Similar to complex 1, three quadrupole doublets are observed in 2 and 3. The three peak area ratios of the LS Fe2^II/LS Fe1^II/HS Fe1^II species in 2 and 3 are 66.7/3.5/29.8 and 66.7/3.3/30, respectively, which are also in good agreement with their magnetic measurement results. Due to the presence of only one Fe^II center in the structure of 4, only two doublets are observed for LS Fe1^II [δ = 0.39 mm s⁻¹, ΔE_Q = 0.45 mm s⁻¹] and HS Fe1^II [δ = 1.04 mm s⁻¹, ΔE_Q = 2.22 mm s⁻¹], respectively. The two peak area ratio (LS Fe1^II/HS Fe1^II) is 0.35/0.65, which further supports the magnetic susceptibility data of 4.

Fig. 5 ⁵⁷Fe Mössbauer spectra of 1–4; the black line represents the sum of contributions of all types of Fe signals in the sample.

Table 2 ⁵⁷Fe Mössbauer fitting parameters of 1–4

Complex	Fe type	δ (mm s^−1)	ΔEQ (mm s^−1)	Area (%)	Γ (mm s^−1)
1	LS Fe2^II	0.17	1.84	66.7	0.271
	LS Fe1^II	0.36	0.48	19.3	0.302
	HS Fe1^II	1.02	2.31	14	0.411
2	LS Fe2^II	0.15	1.84	66.7	0.239
	LS Fe1^II	0.18	0.40	3.5	0.240
	HS Fe1^II	1.03	2.62	29.8	0.334
3	LS Fe2^II	0.19	1.89	66.7	0.244
	LS Fe1^II	0.28	0.30	3.3	0.221
	HS Fe1^II	1.03	2.48	30	0.297
4	LS Fe1^II	0.39	0.45	35	0.451
	HS Fe1^II	1.04	2.22	65	0.468

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Conclusions

In summary, we have synthesized and characterized four trinuclear cyanido-bridged complexes trans-[CpMe_n(dppe)M^II(CN)]₂[Fe1^II(abpt)₂](OTf)₂ (n = 1, 4, 5; M = Fe and Ru). Therein, only one polycrystalline form was found in complexes 1–4 and it was different from the reported mononuclear Fe(abpt)₂X₂ (X = NCS, NCSe, N(CN)₂, C(CN)₃, (NC)₂CC(OCH₃)C(CN)₂, (NC)₂CC(OC₂H₅)C(CN)₂, C₁₆SO₃ and Cl), in which the intermolecular π–π interactions play a key role in the presence of spin crossover. Magnetic measurement results indicate that all four trinuclear cyanido-bridged complexes undergo thermally induced SCO, which is influenced by the electron donating ability of the CpMe_n(dppe)M^II fragment but not by the intermolecular π–π interactions. The spin transition temperature (T_1/2) of the complexes decreases with the increase of the electron-donating ability of the fragment CpMe_n(dppe)M^II, and the variable temperature IR and ⁵⁷Fe Mössbauer spectra have further confirmed this conclusion. This study indicates that this modification away from the spin transition metal center successfully weakens the decisive influence of intermolecular π–π interactions on SCO behaviours, providing a new strategy to predict and precisely regulate the behaviors of SCO complexes.

Experimental

Materials and synthesis

All manipulations were performed under an argon atmosphere with the use of standard Schlenk techniques unless otherwise stated. The ligand abpt⁴⁵ and metal organic fragment CpMe_n(dppe)M^IICN^46–48 were prepared according to published papers. All other chemical solvents were available commercially at reagent grade and used without further purification.

Synthesis of trans-Fe(abpt)₂(OTf)₂·(CH₃OH)₂ (0)

Fe^II(OTf)₂ (0.0354 g, 1 mmol) and abpt (0.0476 g, 2 mmol) were added in 10 mL of methanol. The reaction mixture was refluxed with stirring at 80 °C under a nitrogen atmosphere for 6 h. After that, the mixture solution was concentrated under reduced pressure, and then the product was precipitated by the addition of diethyl ether. The obtained deep red powder was redissolved in methanol (6 mL). The faint yellow crystals of trans-Fe(abpt)₂(OTf)₂·(CH₃OH)₂ were obtained by slow diffusion of diethyl ether into the methanol solution. Yield: 0.052 g, 58%. Anal. Calcd (%) for C₂₈H₂₈F₆FeN₁₂O₈S₂: C, 37.59; H, 3.15; N, 18.79. Found (%): C, 37.16; H, 2.93; N, 18.97.

Synthesis of trans-[CpMe(dppe)Fe^II(CN)]₂[Fe1^II(abpt)₂](OTf)₂ (1)

CpMe(dppe)FeCN (0.1118 g, 2 mmol) and trans-Fe(abpt)₂(OTf)₂·(CH₃OH)₂ (0.0895 g, 1 mmol) were added in 15 mL of methanol. The reaction mixture was refluxed with stirring at 80 °C under a nitrogen atmosphere for 6 h. After that, the mixture solution was concentrated under reduced pressure, and then the product was precipitated by the addition of diethyl ether. The obtained product was redissolved in a mixed solvent of methanol (6 mL) and dichloromethane (2 ml). The black-red block shaped crystals of 1 were obtained by slow diffusion of diethyl ether into the mixed solvent of methanol and dichloromethane. Yield: 0.150 g, 77%. Anal. Calcd (%) for C₉₂H₈₂F₆Fe₃N₁₄O₆P₄S₂: C, 56.69; H, 4.24; N, 10.06. Found (%): C, 56.22; H, 4.03; N, 10.17.

Synthesis of trans-[CpMe₄(dppe)Fe^II(CN)]₂[Fe1^II(abpt)₂](OTf)₂ (2)

CpMe₄(dppe)FeCN (0.1202 g, 2 mmol) and trans-Fe(abpt)₂(OTf)₂·(CH₃OH)₂ (0.0895 g, 1 mmol) were added in 15 mL of methanol. The reaction mixture was refluxed with stirring at 80 °C under a nitrogen atmosphere for 6 h. After that, the mixture solution was concentrated under reduced pressure, and then the product was precipitated by the addition of diethyl ether. The obtained product was redissolved in a mixed solvent of methanol (6 mL) and dichloromethane (2 ml). The deep-red block shaped crystals of 2 were obtained by slow diffusion of diethyl ether into the mixed solvent of methanol and dichloromethane. Yield: 0.142 g, 70%. Anal. Calcd (%) for C₉₈H₉₄F₆Fe₃N₁₄O₆P₄S₂: C, 57.88; H, 4.66; N, 9.64. Found (%): C, 57.62; H, 4.64; N, 9.82.

Synthesis of trans-[CpMe₅(dppe)Fe^II(CN)]₂[Fe1^II(abpt)₂](OTf)₂ (3)

CpMe₅(dppe)FeCN (0.1230 g, 2 mmol) and trans-Fe(abpt)₂(OTf)₂·(CH₃OH)₂ (0.0895 g, 1 mmol) were added in 15 mL of methanol. The reaction mixture was refluxed with stirring at 80 °C under a nitrogen atmosphere for 6 h. After that, the mixture solution was concentrated under reduced pressure, and then the product was precipitated by the addition of diethyl ether. The obtained product was redissolved in a mixed solvent of methanol (6 mL) and dichloromethane (2 ml). The red block shaped crystals of 3 were obtained by slow diffusion of diethyl ether into the mixed solvent of methanol and dichloromethane. Yield: 0.136 g, 66%. Anal. Calcd (%) for C₁₀₀H₉₈F₆Fe₃N₁₄O₆P₄S₂: C, 58.26; H, 4.79; N, 9.51. Found (%): C, 57.96; H, 4.88; N, 9.72.

Synthesis of trans-[CpMe₅(dppe)Ru^II(CN)]₂[Fe1^II(abpt)₂](OTf)₂ (4)

CpMe₅(dppe)RuCN (0.1320 g, 2 mmol) and trans-Fe(abpt)₂(OTf)₂·(CH₃OH)₂ (0.0895 g, 1 mmol) were added in 15 mL of methanol. The reaction mixture was refluxed with stirring at 80 °C under a nitrogen atmosphere for 6 h. After that, the mixture solution was concentrated under reduced pressure, and then the product was precipitated by the addition of diethyl ether. The obtained product was redissolved in a mixed solvent of methanol (6 mL) and dichloromethane (2 ml). The deep-red block shaped crystals of 4 were obtained by slow diffusion of diethyl ether into the mixed solvent of methanol and dichloromethane. Yield: 0.155 g, 72%. Anal. Calcd (%) for C₁₀₀H₉₈F₆FeN₁₄O₆P₄Ru₂S₂: C, 55.81; H, 4.59; N, 9.11. Found (%): C, 55.50; H, 4.68; N, 9.30.

As reported, many Fe(abpt)₂X₂ complexes have more than one polymorph, and usually, the different polymorphs can be easily distinguished with the color and morphology of the crystals. However, it appears that only one polymorph could be found during the preparation of complexes 1–4.

Physical measurements

X-ray crystallography. The crystals suitable for single-crystal X-ray diffraction were covered in a thin layer of hydrocarbon oil, mounted on a glass fiber attached to a copper pin, and placed under an N₂ cold stream. The crystal data of 1, 2, 3 and 4 at 293 K were obtained on a Synergy Custom (Metal Jet) four-circle diffractometer equipped with Ga-Kα radiation (λ = 1.3405 Å), and the crystal data of 3 at 100 K was obtained on a XtaLAB Synergy R diffractometer equipped with mirror monochromatic Mo-Kα radiation (λ = 0.71073 Å). All the structures were solved using SHELXL-2018 and refined by the full-matrix least-squares method on F² with anisotropic thermal parameters for non-hydrogen atoms.⁴⁹ All the hydrogen atoms were calculated and generated at the ideal position. The crystallographic data have been deposited at the Cambridge Crystallographic Data Centre (CCDC), under deposition numbers CCDC 2313064 for 0, 2313059 for 1, 2313060 for 2, 2313061 for 3 at 100 K, 2313062 for 3 at 293 K and 2313063 for 4.†

Magnetic property measurements. The magnetic susceptibility data were measured with a Quantum Design MPMS-XL SQUID susceptometer under an applied magnetic field of 1 kOe. Measurements of all the samples were performed on microcrystalline powder restrained with a Parafilm film and loaded in a capsule. The experimental susceptibility data were corrected for underlying diamagnetism by using tabulated Pascal's constants and, for the sample holder and Parafilm by using corrected measurement.⁵⁰

Variable-temperature infrared spectra. Variable-temperature IR spectra were recorded on a Nicolet 6700 spectrophotometer with KBr powder. The data were collected using a liquid nitrogen cryostat with a temperature controller between 110 and 450 K.

Mössbauer spectra. The ⁵⁷Fe Mössbauer spectra were obtained using a conventional constant acceleration type spectrometer in transmission geometry. The gamma-ray source was 25 mCi ⁵⁷Co in a Rh matrix and the driver velocity was calibrated using α-Fe foil. All the spectra were fitted with the hyperfine field distribution by a slightly modified version of the Hesse and Rübartsch method using the MossWinn programme.^51,52

Other characterization. The elemental analyses (C, H and N) were performed on a Vario MICRO elemental analyzer, and the thermal analyses (TGA) were performed on an STA449C-QMS403C thermal analyzer from 298 K to 1000 K with a heating rate of 20 K min⁻¹ under a nitrogen flow.

Author contributions

T. L. S. conceived and supervised the project. Y-Y. H. synthesized and characterized the complexes, and performed the data analysis. Y. H., Y. L., J-H. F. and X-L. L. assisted in characterizing the complexes. The paper was written by Y-Y. H. and T-L. S., and all authors discussed the results and commented on the manuscript.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

We are grateful for the financial support from the National Natural Science Foundation of China (21973095 and 92261116), and we acknowledge Lanzhou University for providing the Mössbauer measurements.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2313059–2313064. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3dt04226j

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