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Dehydration-actuated single-chain magnet through charge transfer in a cyanide-bridged Fe2Co chain
Inorganic Chemistry Communications 112 (2020) 107715
Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Dehydration-actuated single-chain magnet through charge transfer in a cyanide-bridged Fe2Co chain
T
Wen-Jing Jiang, Han-Han Lu, Yin-Shan Meng, Cheng-Qi Jiao, Tao Liu
⁎
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, China
GRAPHICAL ABSTRACT
A cyano-bridged Fe2Co 4,2-ribbon chain, {[Fe(bipy)(CN)4]2[Co(phpy)2]} (1, bipy = 2,2-bipyridine; phpy = 4-phenylpyridine) was obtained by the dehydration of {[Fe(bipy)(CN)4]2[Co(phpy)2]}·2H2O (1·2H2O). The dehydrated 1 exhibits the water-actuated single-chain magnetic (SCM) behaviour through the dehydrationswitchable electron-transfer-coupled spin transition (ETCST).
ARTICLE INFO
ABSTRACT
Keywords: Cyanide-bridged chain Dehydration-switching Electron-transfer-coupled spin transition Single-chain magnet
A cyanide-bridged Fe2Co 4,2-ribbon single-chain magnet, {[Fe(bipy)(CN)4]2[Co(phpy)2]} (1, bipy = 2,2-bipyridine; phpy = 4-phenylpyridine) was obtained by the dehydration of {[Fe(bipy)(CN)4]2[Co(phpy)2]}·2H2O (1·2H2O). The electron-transfer-coupled spin transition (ETCST) was achieved during the dehydration process III III from 1·2H2O to 1, with the electronic configurations transforming from {FeIILS(µ-CN)CoIII LS(µ-NC)FeLS} to {FeLS(µCN)CoIIHS(µ-NC)FeIII } (LS = low spin, HS = high spin). The magnetic characterization indicates that the deLS hydrated 1 exhibits the dehydration-actuated single-chain magnetic (SCM) behaviour.
The study of molecular magnetic materials possessing versatile properties which can be switched by external stimuli is of considerable interest in the view of functionalization and technology [1−3]. Singlechain magnets (SCMs), as a kind of molecular nanomagnets which provide potential applications for molecule-based data storage in electronic devices, have been paid great attention [4−6]. Since Glauber predicted that the one-dimensional systems of ferromagnetically coupled Ising spins could display slow magnetization relaxation at a finite temperature in 1963 [7], it was until 2001 people discovered the first
⁎
single-chain magnet {Co(hfac)2[NIT(C6H4p-OMe)]} [8]. After that, a wide variety of SCMs have been reported based on the CoII, MnIII, NiII, FeII and lanthanide ions owing to their inherent anisotropy [9−14]. During the recent ten years, some approaches have been proposed to design switchable SCMs, but the number of switchable SCMs is still limited. One strategy is to utilize the mechanical forces such as the hydrostatic pressure to tune barrier energy in several MnIII-based complexes [15] or use guest molecules to modify the slow dynamics and magnetic long-range ordering in metal-organic frameworks
Corresponding author. E-mail address: [email protected] (T. Liu).
https://doi.org/10.1016/j.inoche.2019.107715 Received 4 November 2019; Received in revised form 27 November 2019; Accepted 2 December 2019 Available online 09 December 2019 1387-7003/ © 2019 Elsevier B.V. All rights reserved.
Inorganic Chemistry Communications 112 (2020) 107715
W.-J. Jiang, et al.
[16,17]. These switchable magnetic properties mainly result from the interplay and competition of different type of magnetic exchange interactions caused by the structural changes of the paramagnetic metal centers as well as the intra-/intermolecules under external forces. Another strategy is to use metal-to-metal charge transfer (MMCT) and spin crossover (SCO) units to construct the switchable SCMs [18−21]. The MMCT and SCO modules can be conveniently triggered by temperature, light, pressure and guest effect, resulting in the electron rearrangement between two metal centers or within in one metal center [22−25]. Such electron transfer markedly changes the spin states, magnetic anisotropies of the respective metal centers and the magnetic coupling interactions between them. This in turn switches the SCMs property when external stimuli were imbedded. In particular, lattice solvents can also tune the redox potential of metal centers [26−28] and act as supermolecular bridges to tune the intermolecular interactions [29−31], showing an important influence on solvatomagnetic property. However, the subsequent switching-on/off SCM behavior by the action of solvent-dependent electron transfer has not been discovered. To obtain solvent-switching SCMs, a challenge task is the design of predictable 1D assemblies capable of the required functionalization. The cyanide-bridged Fe–Co complexes deriving from the Prussian blue analogues (PBAs) are promising candidates, in which the terminal cyanide nitrogen can act as a hydrogen-bond acceptor from the solvent molecules. Moreover, the electron transfer modules can be rationally manipulated and arranged into desired chain assemblies with versatile structures. Previously, we have synthesized a cyanide-bridged Fe2Co complex, {[Fe(bipy)(CN)4]2[Co(phpy)2]}·2H2O (1·2H2O, bipy = 2,2′dipyridine, phpy = 4-phenylpyridine), based on the use of the tailored tetracyanide-bearing FeIII building blocks Li[Fe(bipy)(CN)4] as metalloligands to link the anisotropic CoII ions into a 4,2-ribbon chain III [32]. This complex keeps the low-spin {FeIILS(µ-CN)CoIII LS(µ-NC)FeLS} electronic configuration below room temperature, exhibiting the first example of light switching-on/off SCM properties through the lightinduced MMCT between diamagnetic {FeIILS(µ-CN)CoIII LS} and paraII magnetic {FeIII LS(µ-CN)CoHS} pairs. Herein, the existence of the two lattice water molecules in this complex motivates us to further explore the solvent effect. The structural characterization indicates that the dehydration process accompanies with the MMCT, resulting in the transformation of electronic configuration stabilizing at {FeIII LS(µCN)CoIIHS(µ-NC)FeIII LS} linkages. The dehydrated 1 is characteristic of ferromagnetic coupling interaction within the chain, behaving as the expected single-chain magnet. [Fe(bipy)(CN)4]2[Co(phpy)2] (1) was obtained by the desolvation of {[Fe(bipy)(CN)4]2[Co(phpy)2]}·2H2O (1·2H2O) in N2 atmosphere. Thermogravimetric analysis (TGA) of 1·2H2O showed a weight loss of 3.69% when heating from 25 to 100 °C, corresponding well to the 3.47% loss of two water molecules (Fig. S1). Therefore, the single crystal of 1·2H2O was heating to 390 K, accompanying the loss of lattice water. Single-crystal X-ray diffraction analysis carried out at 390 K revealed that 1 maintained the crystalline integrity and crystallized in monoclinic space group C2/c [33]. The crystal structure is comprised of neutral 2,4-wavelike double zigzag chains (Fig. 1a), similar to that of 1·2H2O. Within the chain, the adjacent Co centers are linked by two [Fe (bipy)(CN)4]− building blocks in a linear manner along the c axis. Each Co center is located in the elongated N6 octahedral environment in which four cyanide nitrogen atoms occupy the equatorial plane (Co–Ncyanide distances: 2.080(5) − 2.050(5) Å). The apical positions are occupied by two nitrogen atoms from monodentate phpy ligands (Co–Nphpy distances: 2.139(5) Å), showing typical high-spin CoII ion character that benefits for the essential SCM behavior. Compared with 1·2H2O at 220 K, the average Co–N bond distances of 1 are lengthened by 0.20 Å. Whereas the Fe–C bond distances (1.892(6)–1.953(8) Å) and the Fe–N bond distances (1.984(5)–1.995(5) Å) keep unchanged, suggesting that the Fe species maintained the FeIII LS state. These structural II results indicate that the formation of {FeIII LS(μ-CN)CoHS} pairs is caused by the dehydration-actuated MMCT. The interlaced planes of triangular
Fe2Co units form a dihedral angle (θ) of 37.0°. From the packing diagram (Fig. S2), 1·2H2O at 220 K shows π⋯π interactions between pyridine rings of bipy (3.816 Å) and benzene rings of phpy (3.828 Å). The distance between uncoordinated water molecules and terminal cyanide nitrogen atoms (O⋯N distance) are found to be ~2.99 Å. For complex 1 stacked in the ab plane (Fig. S3), the π⋯πbipy and π⋯πphpy distances are 3.875 Å and 3.907 Å, respectively (Fig. 1b). The nearest interchain Co⋯Co distance of 1 is 15.573 Å, which is advantage to weaken and/or eliminate the interchain antiferromagnetic interaction. The void space of 1 is estimated to be 20.8% with the PLATON program. Magnetic susceptibilities measured over the temperature range of 2–400 K verify the dehydration-actuated MMCT between 1·2H2O and 1 in the high temperature region (Fig. 2a). Below room temperature, III II 1·2H2O keeps the LS electronic state with {FeIII LSCoLSFeLS} configuration. When slowly increasing the temperature from 300 K to 400 K (2 K min−1), the χT values afford a gradual increase, reaching 4.69 cm3 K mol−1 based on per Fe2Co unit at 400 K, which is in agreement with the typically uncorrelated spins of two LS FeIII ions and one HS CoII ion with significant orbital contributions. Upon cooling, the χT values remain nearly constant. Below 50 K, the χT values increase steeply and reach to a maximum value of 26.45 cm3 mol−1 K at 2.5 K, indicating an intrachain ferromagnetic coupling between the CoIIHS and FeIII LS ions. The magnetic susceptibility data above 50 K follow the Curie-Weiss law and are fitted through the plots of 1/χ versus T, yielding C = 4.54 cm3 mol−1 K and θ = 9.06 K, further confirming the intrachain ferromagnetic interaction (Fig. S4). Below 2.5 K, the little decrease in χT is probably attributed to the presence of weak interchain antiferromagnetic interactions. The field-dependent magnetization at 1.8 K exhibits a rapid increase below 1 kOe, following a linear increase to 4.0 Nβ at 50 kOe (Fig. 2b). The changes of χT values and structural variations from 1·2H2O to dehydrated 1 indicates the key role of lattice water molecules in the charge transfer process from the low-temperaIII II ture phase with {FeIII LSCoLSFeLS} linkages to the high-temperature phase II III with {FeIII Co Fe } linkages. LS HS LS Temperature-dependent infrared (IR) spectra recorded from 150 K to 400 K support the gradually dehydration-actuated electron-transfer process from 1·2H2O to 1 (Fig. S5). For 1·2H2O in the low-temperature range, a broad absorption with peaks at 2192 and 2204 cm−1 is observed, which can be assigned to the bridging-CN stretches of {FeIILS(µIII III CN)CoIII LS} and {FeLS(µ-CN)CoLS} pairs, respectively. The other peaks are attributed to the terminal-CN absorption of [FeIII(bpy)(CN)4]− (2122 cm−1) and [FeII(bpy)(CN)4]2− (2073 and 2085 cm−1). When the temperature was heating to 400 K and kept for 10 min, a new absorption band appears at 2162 cm−1, corresponding to the bridging-CN II stretches of {FeIII LS(µ-CN)CoHS}. Such IR results indicate the occurrence of MMCT process. When further cooling down, this new absorption band does not disappear and maintains the intensity, indicating that 1 II III keeps the {FeIII LSCoHSFeLS} state. These IR results are consistent with the susceptibility experiments. To further investigate the solvomagnetic effect on its magnetic relaxation properties of the dehydrated 1, alternating current (ac) magnetic susceptibility measurements were carried out as a function of both temperature and frequency under zero dc field and 3.5 Oe ac field in the temperature range of 1.8–5.0 K. Both in-phase (χ′) and out-of-phase (χ′′) signals show the frequency dependence, indicating the slow magnetic relaxation (Fig. 2c and d). The Mydosh parameter ϕ = (ΔTp/ Tp)/Δ(logf) (Tp is the peak temperature of χ′,) is calculated to be 0.12, falling in the typical range of SCMs (0.1 ≤ ϕ ≤ 0.3), thus excluding the possibility of spin-glass behaviour. The fitting parameters based on Arrhenius relationship τ = τ0exp(Δ/kBT) give the relaxation energy barrier of Δ/kB = 35.49 K and τ0 = 1.12 × 10−10 s (Fig. S6), which are also in accordance with those reported for SCMs [9]. For an Ising-type chain, a linear region can be found through the fitting of ln(χT) with 1/ T (Fig. S7), following the equation χT = Ceffexp(Δξ/kBT) (Ceff is the effective Curie constant and Δξ is the correlation energy for the finite 2
Inorganic Chemistry Communications 112 (2020) 107715
W.-J. Jiang, et al.
Fig. 1. (a) Crystal structures of 1. The hydrogen atoms are omitted for clarity (Fe, green; Co, pink; C, gray; N, blue). (b) The π⋯π distances between pyridines of bipy in the building block (orange dash) and between benzenes of phpy (red dash) of 1. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 2. (a) Temperature-dependent susceptibilities of 1 (red line) and 1·2H2O (blue line). (b) Isothermal magnetization of 1 after dehydration at 2 K. Temperature dependence of the in-phase (c) and out-of-phase (d) parts of the ac susceptibility of 1. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
regime). The fitting parameters are Δξ/kB = 7.45 K and Ceff = 3.67 cm3 mol−1 K. Below 2.8 K, ln(χT) reaches saturation due to the finite-size effects with [χT]max = 26.45 cm3 mol−1 K. As a result, the correlation unit number (n) is determined to be ~7 based on n = [χT]max/Ceff, which is comparable with reported SCMs [34]. In contrast, the hydrated III II 1·2H2O in the {FeIII LS(µ-CN)CoLS(µ-NC)FeLS} state displays no temperature and frequency dependence behaviour. These results suggest that the uncoordinated water molecules play the key role in determining the magnetic properties of 1. When transforming from 1·2H2O to 1, the hydrogen bonds interaction between water molecules and terminal cyanide nitrogen atoms were destroyed. The hydrogen bonds can tune the redox potential of Fe–Co pairs effectively. For 1·2H2O, the hydrogen bonds (O⋯N distance is ~2.99 Å) show electron-withdrawing effect to terminal CN groups and stabilize the {FeIILS(µ-CN)CoIII LS} pairs in the lowspin state below 325 K. When removing the hydrogen bonds, the redox
potential of FeIILS shifted to a more negative one, leading to the charge III II III II III transfer from {FeIII LSCoLSFeLS} state to {FeLSCoHSFeLS} state. In addition, III the SCM behavior observed in 1 with {FeLS(µ-CN)CoIIHS(µ-NC)FeIII LS} electronic configuration also benefits from the large ratio of intra-/interchain interactions and the symmetry-distortion of anisotropic CoIIHS ions. It is noted that the CoIIHS center of dehydrated 1 affords a more distorted coordination sphere (Σ = 20.76) and stronger magnetic anisotropy in comparison with the reported {Fe2Co} framework showing water-switched charge transfer but with no SCM behaviour in the hydrated phase or dehydrated phase (ΣCo = 6.24) [26]. As a consequence, the anisotropic CoIIHS subunits which are ferromagnetically coupled with [FeIII(bpy)(CN)4]− enhance the overall uniaxial magnetic anisotropy of the chain, resulting the SCM behaviour in 1. In summary, the dehydration-actuated intermetallic charge transfer III III from {FeIILS(µ-CN)CoIII LS(µ-NC)FeLS} state of 1·2H2O to the {FeLS(µ3
Inorganic Chemistry Communications 112 (2020) 107715
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CN)CoIIHS(µ-NC)FeIII LS} state of 1 was observed and characterized in detail. Moreover, the dehydrated 1 shows typical magnetic relaxation behaviour under zero dc field. The discovery of water-switching SCM property provides new access to the read-in and erasure of the bistable states of molecular nanomagnets, which is very important for the potential application of high-density memory or storage devices. Further effort will be put into the reversible control of SCM behaviour by the hydration-dehydration process.
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The structures were solved in the space group by direct method and refined by the full-matrix least-squares using SHELXTL-2014 fitting on F2. For compound 1, all non-hydrogen atoms were refined anisotropically. The hydrogen atoms of organic ligands were located geometrically and fixed isotropic thermal parameters. Crystal data for 1: Fe2CoC50H34N14, Mr = 1001.54, monoclinic, C2/c, a = 27.776(5) Å, b = 15.573(3) Å, c = 13.589(3) Å, α = 90°, β = 115.591(3)°, γ = 90°, V = 5301.3(17) Å3, Z = 4, F(000) = 2044.0, GOF = 1.165, R1 = 0.0890, wR2 = 0.2790 [I > 2σ (I)]. [34] H. Miyasaka, T. Madanbashi, A. Saitoh, N. Motokawa, R. Ishikawa, M. Yamashita, R. Clérac, Cyano-bridged MnIIIMIII single-chain magnets with MIII = CoIII, FeIII, MnIII, and CrIII, Chem. Eur. J. 18 (2012) 3942–3954.
CRediT authorship contribution statement Wen-Jing Jiang: Conceptualization, Data curation, Formal analysis, Writing - original draft. Han-Han Lu: Investigation. Yin-Shan Meng: Formal analysis, Writing - review & editing. Cheng-Qi Jiao: Formal analysis, Writing - review & editing. Tao Liu: Supervision, Validation. Acknowledgments This work was partly supported by the National Natural Science Foundation of China (Grants 21871039, 21801037, 21421005 and 91422302), Postdoctoral Science Foundation of China (2019M651104) and the Fundamental Research Funds for the Central Universities. Appendix A. Supplementary material Supplementary Information available: Selected bond lengths (Å) and angles (°) for dehydrated complex 1. CCDC-1959389 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/data_request/cif. Supplementary data to this article can be found online at https:// doi.org/10.1016/j.inoche.2019.107715. References [1] O. Sato, Dynamic molecular crystals with switchable physical properties, Nature Chem. 8 (2016) 644–656. [2] M.D. Manrique-Juárez, S. Rat, L. Salmon, G. Molnár, C.M. Quintero, L. Nicu, H.J. Shepherd, A. Bousseksou, Switchable molecule-based materials for micro- and nanoscale actuating applications: achievements and prospects, Coord. Chem. Rev. 308 (2016) 395–408. [3] Y.S. Meng, T. Liu, Manipulating spin transition to achieve switchable multifunctions, Acc. Chem. Res. 52 (2019) 1369–1379. [4] D. Gatteschi, R. Sessoli, J. Villain, Molecular Nanomagnets, Oxford University Press, 2006. [5] C. Coulon, V. Pianet, M. Urdampilleta, R. Clérac, Structure and Bonding vol. 164, (2014) 143–184. [6] O. Kahn, C.J. Martinez, Spin-transition polymers: from molecular materials toward memory devices, Science 279 (1998) 44–48. [7] R. Glauber, Time-dependent statistics of the Ising model, J. Math. Phys. 4 (1963) 294–307. [8] A. Caneschi, D. Gatteschi, N. Lalioti, C. Sangregorio, R. Sessoli, G. Venturi, A. Vindigni, A. Rettori, M.G. Pini, M.A. Novak, Cobalt(II)−nitronyl nitroxide chains as molecular magnetic nanowires, Angew. Chem. Int. Ed. 40 (2001) 1760–1763. [9] R. Lescouëzec, J. Vaissermann, C. Ruiz-Pérez, F. Lloret, R. Carrasco, M. Julve, W. Wernsdorfer, Cyanide-bridged iron (III)–cobalt (II) double zigzag ferromagnetic chains: two new molecular magnetic nanowires, Angew. Chem. Int. Ed. 42 (2003) 1483–1486. [10] M.H. Zeng, Z. Yin, Y.X. Tan, W.X. Zhang, Y.P. He, M. Kurmoo, Nanoporous cobalt (II) MOF exhibiting four magnetic ground states and changes in gas sorption upon post-synthetic modification, J. Am. Chem. Soc. 136 (2014) 4680–4688. [11] R. Clérac, H. Miyasaka, M. Yamashita, C. Coulon, Evidence for single-chain magnet behavior in a MnIII−NiII chain designed with high spin magnetic units: a route to high temperature metastable magnets, J. Am. Chem. Soc. 124 (2002) 12837–12844.
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Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Dehydration-actuated single-chain magnet through charge transfer in a cyanide-bridged Fe2Co chain
T
Wen-Jing Jiang, Han-Han Lu, Yin-Shan Meng, Cheng-Qi Jiao, Tao Liu
⁎
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, China
GRAPHICAL ABSTRACT
A cyano-bridged Fe2Co 4,2-ribbon chain, {[Fe(bipy)(CN)4]2[Co(phpy)2]} (1, bipy = 2,2-bipyridine; phpy = 4-phenylpyridine) was obtained by the dehydration of {[Fe(bipy)(CN)4]2[Co(phpy)2]}·2H2O (1·2H2O). The dehydrated 1 exhibits the water-actuated single-chain magnetic (SCM) behaviour through the dehydrationswitchable electron-transfer-coupled spin transition (ETCST).
ARTICLE INFO
ABSTRACT
Keywords: Cyanide-bridged chain Dehydration-switching Electron-transfer-coupled spin transition Single-chain magnet
A cyanide-bridged Fe2Co 4,2-ribbon single-chain magnet, {[Fe(bipy)(CN)4]2[Co(phpy)2]} (1, bipy = 2,2-bipyridine; phpy = 4-phenylpyridine) was obtained by the dehydration of {[Fe(bipy)(CN)4]2[Co(phpy)2]}·2H2O (1·2H2O). The electron-transfer-coupled spin transition (ETCST) was achieved during the dehydration process III III from 1·2H2O to 1, with the electronic configurations transforming from {FeIILS(µ-CN)CoIII LS(µ-NC)FeLS} to {FeLS(µCN)CoIIHS(µ-NC)FeIII } (LS = low spin, HS = high spin). The magnetic characterization indicates that the deLS hydrated 1 exhibits the dehydration-actuated single-chain magnetic (SCM) behaviour.
The study of molecular magnetic materials possessing versatile properties which can be switched by external stimuli is of considerable interest in the view of functionalization and technology [1−3]. Singlechain magnets (SCMs), as a kind of molecular nanomagnets which provide potential applications for molecule-based data storage in electronic devices, have been paid great attention [4−6]. Since Glauber predicted that the one-dimensional systems of ferromagnetically coupled Ising spins could display slow magnetization relaxation at a finite temperature in 1963 [7], it was until 2001 people discovered the first
⁎
single-chain magnet {Co(hfac)2[NIT(C6H4p-OMe)]} [8]. After that, a wide variety of SCMs have been reported based on the CoII, MnIII, NiII, FeII and lanthanide ions owing to their inherent anisotropy [9−14]. During the recent ten years, some approaches have been proposed to design switchable SCMs, but the number of switchable SCMs is still limited. One strategy is to utilize the mechanical forces such as the hydrostatic pressure to tune barrier energy in several MnIII-based complexes [15] or use guest molecules to modify the slow dynamics and magnetic long-range ordering in metal-organic frameworks
Corresponding author. E-mail address: [email protected] (T. Liu).
https://doi.org/10.1016/j.inoche.2019.107715 Received 4 November 2019; Received in revised form 27 November 2019; Accepted 2 December 2019 Available online 09 December 2019 1387-7003/ © 2019 Elsevier B.V. All rights reserved.
Inorganic Chemistry Communications 112 (2020) 107715
W.-J. Jiang, et al.
[16,17]. These switchable magnetic properties mainly result from the interplay and competition of different type of magnetic exchange interactions caused by the structural changes of the paramagnetic metal centers as well as the intra-/intermolecules under external forces. Another strategy is to use metal-to-metal charge transfer (MMCT) and spin crossover (SCO) units to construct the switchable SCMs [18−21]. The MMCT and SCO modules can be conveniently triggered by temperature, light, pressure and guest effect, resulting in the electron rearrangement between two metal centers or within in one metal center [22−25]. Such electron transfer markedly changes the spin states, magnetic anisotropies of the respective metal centers and the magnetic coupling interactions between them. This in turn switches the SCMs property when external stimuli were imbedded. In particular, lattice solvents can also tune the redox potential of metal centers [26−28] and act as supermolecular bridges to tune the intermolecular interactions [29−31], showing an important influence on solvatomagnetic property. However, the subsequent switching-on/off SCM behavior by the action of solvent-dependent electron transfer has not been discovered. To obtain solvent-switching SCMs, a challenge task is the design of predictable 1D assemblies capable of the required functionalization. The cyanide-bridged Fe–Co complexes deriving from the Prussian blue analogues (PBAs) are promising candidates, in which the terminal cyanide nitrogen can act as a hydrogen-bond acceptor from the solvent molecules. Moreover, the electron transfer modules can be rationally manipulated and arranged into desired chain assemblies with versatile structures. Previously, we have synthesized a cyanide-bridged Fe2Co complex, {[Fe(bipy)(CN)4]2[Co(phpy)2]}·2H2O (1·2H2O, bipy = 2,2′dipyridine, phpy = 4-phenylpyridine), based on the use of the tailored tetracyanide-bearing FeIII building blocks Li[Fe(bipy)(CN)4] as metalloligands to link the anisotropic CoII ions into a 4,2-ribbon chain III [32]. This complex keeps the low-spin {FeIILS(µ-CN)CoIII LS(µ-NC)FeLS} electronic configuration below room temperature, exhibiting the first example of light switching-on/off SCM properties through the lightinduced MMCT between diamagnetic {FeIILS(µ-CN)CoIII LS} and paraII magnetic {FeIII LS(µ-CN)CoHS} pairs. Herein, the existence of the two lattice water molecules in this complex motivates us to further explore the solvent effect. The structural characterization indicates that the dehydration process accompanies with the MMCT, resulting in the transformation of electronic configuration stabilizing at {FeIII LS(µCN)CoIIHS(µ-NC)FeIII LS} linkages. The dehydrated 1 is characteristic of ferromagnetic coupling interaction within the chain, behaving as the expected single-chain magnet. [Fe(bipy)(CN)4]2[Co(phpy)2] (1) was obtained by the desolvation of {[Fe(bipy)(CN)4]2[Co(phpy)2]}·2H2O (1·2H2O) in N2 atmosphere. Thermogravimetric analysis (TGA) of 1·2H2O showed a weight loss of 3.69% when heating from 25 to 100 °C, corresponding well to the 3.47% loss of two water molecules (Fig. S1). Therefore, the single crystal of 1·2H2O was heating to 390 K, accompanying the loss of lattice water. Single-crystal X-ray diffraction analysis carried out at 390 K revealed that 1 maintained the crystalline integrity and crystallized in monoclinic space group C2/c [33]. The crystal structure is comprised of neutral 2,4-wavelike double zigzag chains (Fig. 1a), similar to that of 1·2H2O. Within the chain, the adjacent Co centers are linked by two [Fe (bipy)(CN)4]− building blocks in a linear manner along the c axis. Each Co center is located in the elongated N6 octahedral environment in which four cyanide nitrogen atoms occupy the equatorial plane (Co–Ncyanide distances: 2.080(5) − 2.050(5) Å). The apical positions are occupied by two nitrogen atoms from monodentate phpy ligands (Co–Nphpy distances: 2.139(5) Å), showing typical high-spin CoII ion character that benefits for the essential SCM behavior. Compared with 1·2H2O at 220 K, the average Co–N bond distances of 1 are lengthened by 0.20 Å. Whereas the Fe–C bond distances (1.892(6)–1.953(8) Å) and the Fe–N bond distances (1.984(5)–1.995(5) Å) keep unchanged, suggesting that the Fe species maintained the FeIII LS state. These structural II results indicate that the formation of {FeIII LS(μ-CN)CoHS} pairs is caused by the dehydration-actuated MMCT. The interlaced planes of triangular
Fe2Co units form a dihedral angle (θ) of 37.0°. From the packing diagram (Fig. S2), 1·2H2O at 220 K shows π⋯π interactions between pyridine rings of bipy (3.816 Å) and benzene rings of phpy (3.828 Å). The distance between uncoordinated water molecules and terminal cyanide nitrogen atoms (O⋯N distance) are found to be ~2.99 Å. For complex 1 stacked in the ab plane (Fig. S3), the π⋯πbipy and π⋯πphpy distances are 3.875 Å and 3.907 Å, respectively (Fig. 1b). The nearest interchain Co⋯Co distance of 1 is 15.573 Å, which is advantage to weaken and/or eliminate the interchain antiferromagnetic interaction. The void space of 1 is estimated to be 20.8% with the PLATON program. Magnetic susceptibilities measured over the temperature range of 2–400 K verify the dehydration-actuated MMCT between 1·2H2O and 1 in the high temperature region (Fig. 2a). Below room temperature, III II 1·2H2O keeps the LS electronic state with {FeIII LSCoLSFeLS} configuration. When slowly increasing the temperature from 300 K to 400 K (2 K min−1), the χT values afford a gradual increase, reaching 4.69 cm3 K mol−1 based on per Fe2Co unit at 400 K, which is in agreement with the typically uncorrelated spins of two LS FeIII ions and one HS CoII ion with significant orbital contributions. Upon cooling, the χT values remain nearly constant. Below 50 K, the χT values increase steeply and reach to a maximum value of 26.45 cm3 mol−1 K at 2.5 K, indicating an intrachain ferromagnetic coupling between the CoIIHS and FeIII LS ions. The magnetic susceptibility data above 50 K follow the Curie-Weiss law and are fitted through the plots of 1/χ versus T, yielding C = 4.54 cm3 mol−1 K and θ = 9.06 K, further confirming the intrachain ferromagnetic interaction (Fig. S4). Below 2.5 K, the little decrease in χT is probably attributed to the presence of weak interchain antiferromagnetic interactions. The field-dependent magnetization at 1.8 K exhibits a rapid increase below 1 kOe, following a linear increase to 4.0 Nβ at 50 kOe (Fig. 2b). The changes of χT values and structural variations from 1·2H2O to dehydrated 1 indicates the key role of lattice water molecules in the charge transfer process from the low-temperaIII II ture phase with {FeIII LSCoLSFeLS} linkages to the high-temperature phase II III with {FeIII Co Fe } linkages. LS HS LS Temperature-dependent infrared (IR) spectra recorded from 150 K to 400 K support the gradually dehydration-actuated electron-transfer process from 1·2H2O to 1 (Fig. S5). For 1·2H2O in the low-temperature range, a broad absorption with peaks at 2192 and 2204 cm−1 is observed, which can be assigned to the bridging-CN stretches of {FeIILS(µIII III CN)CoIII LS} and {FeLS(µ-CN)CoLS} pairs, respectively. The other peaks are attributed to the terminal-CN absorption of [FeIII(bpy)(CN)4]− (2122 cm−1) and [FeII(bpy)(CN)4]2− (2073 and 2085 cm−1). When the temperature was heating to 400 K and kept for 10 min, a new absorption band appears at 2162 cm−1, corresponding to the bridging-CN II stretches of {FeIII LS(µ-CN)CoHS}. Such IR results indicate the occurrence of MMCT process. When further cooling down, this new absorption band does not disappear and maintains the intensity, indicating that 1 II III keeps the {FeIII LSCoHSFeLS} state. These IR results are consistent with the susceptibility experiments. To further investigate the solvomagnetic effect on its magnetic relaxation properties of the dehydrated 1, alternating current (ac) magnetic susceptibility measurements were carried out as a function of both temperature and frequency under zero dc field and 3.5 Oe ac field in the temperature range of 1.8–5.0 K. Both in-phase (χ′) and out-of-phase (χ′′) signals show the frequency dependence, indicating the slow magnetic relaxation (Fig. 2c and d). The Mydosh parameter ϕ = (ΔTp/ Tp)/Δ(logf) (Tp is the peak temperature of χ′,) is calculated to be 0.12, falling in the typical range of SCMs (0.1 ≤ ϕ ≤ 0.3), thus excluding the possibility of spin-glass behaviour. The fitting parameters based on Arrhenius relationship τ = τ0exp(Δ/kBT) give the relaxation energy barrier of Δ/kB = 35.49 K and τ0 = 1.12 × 10−10 s (Fig. S6), which are also in accordance with those reported for SCMs [9]. For an Ising-type chain, a linear region can be found through the fitting of ln(χT) with 1/ T (Fig. S7), following the equation χT = Ceffexp(Δξ/kBT) (Ceff is the effective Curie constant and Δξ is the correlation energy for the finite 2
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Fig. 1. (a) Crystal structures of 1. The hydrogen atoms are omitted for clarity (Fe, green; Co, pink; C, gray; N, blue). (b) The π⋯π distances between pyridines of bipy in the building block (orange dash) and between benzenes of phpy (red dash) of 1. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 2. (a) Temperature-dependent susceptibilities of 1 (red line) and 1·2H2O (blue line). (b) Isothermal magnetization of 1 after dehydration at 2 K. Temperature dependence of the in-phase (c) and out-of-phase (d) parts of the ac susceptibility of 1. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
regime). The fitting parameters are Δξ/kB = 7.45 K and Ceff = 3.67 cm3 mol−1 K. Below 2.8 K, ln(χT) reaches saturation due to the finite-size effects with [χT]max = 26.45 cm3 mol−1 K. As a result, the correlation unit number (n) is determined to be ~7 based on n = [χT]max/Ceff, which is comparable with reported SCMs [34]. In contrast, the hydrated III II 1·2H2O in the {FeIII LS(µ-CN)CoLS(µ-NC)FeLS} state displays no temperature and frequency dependence behaviour. These results suggest that the uncoordinated water molecules play the key role in determining the magnetic properties of 1. When transforming from 1·2H2O to 1, the hydrogen bonds interaction between water molecules and terminal cyanide nitrogen atoms were destroyed. The hydrogen bonds can tune the redox potential of Fe–Co pairs effectively. For 1·2H2O, the hydrogen bonds (O⋯N distance is ~2.99 Å) show electron-withdrawing effect to terminal CN groups and stabilize the {FeIILS(µ-CN)CoIII LS} pairs in the lowspin state below 325 K. When removing the hydrogen bonds, the redox
potential of FeIILS shifted to a more negative one, leading to the charge III II III II III transfer from {FeIII LSCoLSFeLS} state to {FeLSCoHSFeLS} state. In addition, III the SCM behavior observed in 1 with {FeLS(µ-CN)CoIIHS(µ-NC)FeIII LS} electronic configuration also benefits from the large ratio of intra-/interchain interactions and the symmetry-distortion of anisotropic CoIIHS ions. It is noted that the CoIIHS center of dehydrated 1 affords a more distorted coordination sphere (Σ = 20.76) and stronger magnetic anisotropy in comparison with the reported {Fe2Co} framework showing water-switched charge transfer but with no SCM behaviour in the hydrated phase or dehydrated phase (ΣCo = 6.24) [26]. As a consequence, the anisotropic CoIIHS subunits which are ferromagnetically coupled with [FeIII(bpy)(CN)4]− enhance the overall uniaxial magnetic anisotropy of the chain, resulting the SCM behaviour in 1. In summary, the dehydration-actuated intermetallic charge transfer III III from {FeIILS(µ-CN)CoIII LS(µ-NC)FeLS} state of 1·2H2O to the {FeLS(µ3
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CN)CoIIHS(µ-NC)FeIII LS} state of 1 was observed and characterized in detail. Moreover, the dehydrated 1 shows typical magnetic relaxation behaviour under zero dc field. The discovery of water-switching SCM property provides new access to the read-in and erasure of the bistable states of molecular nanomagnets, which is very important for the potential application of high-density memory or storage devices. Further effort will be put into the reversible control of SCM behaviour by the hydration-dehydration process.
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CRediT authorship contribution statement Wen-Jing Jiang: Conceptualization, Data curation, Formal analysis, Writing - original draft. Han-Han Lu: Investigation. Yin-Shan Meng: Formal analysis, Writing - review & editing. Cheng-Qi Jiao: Formal analysis, Writing - review & editing. Tao Liu: Supervision, Validation. Acknowledgments This work was partly supported by the National Natural Science Foundation of China (Grants 21871039, 21801037, 21421005 and 91422302), Postdoctoral Science Foundation of China (2019M651104) and the Fundamental Research Funds for the Central Universities. Appendix A. Supplementary material Supplementary Information available: Selected bond lengths (Å) and angles (°) for dehydrated complex 1. CCDC-1959389 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/data_request/cif. Supplementary data to this article can be found online at https:// doi.org/10.1016/j.inoche.2019.107715. References [1] O. Sato, Dynamic molecular crystals with switchable physical properties, Nature Chem. 8 (2016) 644–656. [2] M.D. Manrique-Juárez, S. Rat, L. Salmon, G. Molnár, C.M. Quintero, L. Nicu, H.J. Shepherd, A. Bousseksou, Switchable molecule-based materials for micro- and nanoscale actuating applications: achievements and prospects, Coord. Chem. Rev. 308 (2016) 395–408. [3] Y.S. Meng, T. Liu, Manipulating spin transition to achieve switchable multifunctions, Acc. Chem. Res. 52 (2019) 1369–1379. [4] D. Gatteschi, R. Sessoli, J. Villain, Molecular Nanomagnets, Oxford University Press, 2006. [5] C. Coulon, V. Pianet, M. Urdampilleta, R. Clérac, Structure and Bonding vol. 164, (2014) 143–184. [6] O. Kahn, C.J. Martinez, Spin-transition polymers: from molecular materials toward memory devices, Science 279 (1998) 44–48. [7] R. Glauber, Time-dependent statistics of the Ising model, J. Math. Phys. 4 (1963) 294–307. [8] A. Caneschi, D. Gatteschi, N. Lalioti, C. Sangregorio, R. Sessoli, G. Venturi, A. Vindigni, A. Rettori, M.G. Pini, M.A. Novak, Cobalt(II)−nitronyl nitroxide chains as molecular magnetic nanowires, Angew. Chem. Int. Ed. 40 (2001) 1760–1763. [9] R. Lescouëzec, J. Vaissermann, C. Ruiz-Pérez, F. Lloret, R. Carrasco, M. Julve, W. Wernsdorfer, Cyanide-bridged iron (III)–cobalt (II) double zigzag ferromagnetic chains: two new molecular magnetic nanowires, Angew. Chem. Int. Ed. 42 (2003) 1483–1486. [10] M.H. Zeng, Z. Yin, Y.X. Tan, W.X. Zhang, Y.P. He, M. Kurmoo, Nanoporous cobalt (II) MOF exhibiting four magnetic ground states and changes in gas sorption upon post-synthetic modification, J. Am. Chem. Soc. 136 (2014) 4680–4688. [11] R. Clérac, H. Miyasaka, M. Yamashita, C. Coulon, Evidence for single-chain magnet behavior in a MnIII−NiII chain designed with high spin magnetic units: a route to high temperature metastable magnets, J. Am. Chem. Soc. 124 (2002) 12837–12844.
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