Single-Chain Magnets Based on Octacyanotungstate with the Highest Energy Barriers for Cyanide Compounds.

By introducing large counter cations as the spacer, two isolated 3, 3-ladder compounds, (Ph4P)[Co(II)(3-Mepy)2.7(H2O)0.3W(V)(CN)8] · 0.6H2O (1) and (Ph4As)[Co(II)(3-Mepy)3W(V)(CN)8] (2, 3-Mepy = 3-methylpyridine), were synthesized and characterized. Static and dynamic magnetic characterizations reveal that compounds 1 and 2 both behave as the single-chain magnets (SCMs) with very high energy barriers: 252(9) K for 1 and 224(7) K for 2, respectively. These two compounds display the highest relaxation barriers for cyano-bridged SCMs and are preceded only by two cobalt(II)-radical compounds among all SCMs. Meanwhile, a large coercive field of 26.2 kOe (1) and 22.6 kOe (2) were observed at 1.8 K.

Since the first observation of the Glauber dynamics1 in a one-dimensional class="Chemical">coordination n class="Chemical">polymer2, the single-chain magnets (SCMs) named in 20023 have attracted considerable attention owing to their potential applications in quantum computing, spintronics, and high-density memory devices345. For the rational design of SCMs of high relaxation barrier and high blocking temperature (TB), strong intrachain magnetic coupling and spin carriers of large magnetic anisotropy are of crucial importance. A variety of SCMs have been synthesized from specific anisotropic metal ion and bridging ligands67891011121314. Among all the bridging ligands, the CN group is a very efficient bridge mediating strong magnetic interaction between metal ions and has played a prominent role in the development of SCMs research5151617181920. Following the first cyano-bridged SCM21 reported in 2003, more and more cyano-bridged SCMs have been synthesized and characterize1516171819202122232425.

class="Chemical">Octacyanometallates [MIV/V(Cn class="Chemical">N)8]4−/3− (M = Nb, Mo, W) are of great importance in the molecule-based magnets2627282930. The stereochemical flexibility of these building blocks facilitates the construction of various topologies and magnetic properties2627282930. Compared with the widely studied hexacyanometallates, octacyanometallates possess a slightly weaker coordination ability to the first row transition metal ions, which facilitates the construction of low dimensional magnetic materials. In addition, the more radially extended valence orbitals of the 4d/5d metal centers might efficiently strengthen the exchange interactions31. However, octacyanometallate-based SCMs are scarcely reported173233, probably due to the difficulty to effectively control the valence of [MIV/V(CN)8]4−/3− for the design and synthesis of 3d-4d/5d heteronuclear 1D chains.

Although the SCM-based magnets showing both the long range magnetic ordering and SCM behavior have added new aspects to the SCM researches3435, a pure SCM with negligible interchain magnetic interaction is still preferred. Synthetically, in order to isolate the 1D Ising magnetic chains, several strategies have been utilized. Among them, large class="Chemical">counter ions are n class="Chemical">considered preferentially to effectively separate the chains and minimize the interchain interaction. For example, in the famous [Mn2Ni] SCM family reported by Clérac and Miyasaka et al., the 1D [Mn2Ni] chains were isolated by various anions such as ClO4−, PF6−, BF4−, ReO4−, and BPh4−, leading to the modification of their magnetic properties3343637. In 2005, we reported the first SMMs based on the 4d/5d metal centers, namely CoII[CoII(CH3OH)3]8[MV(CN)8]6· Solv (M = Mo, W)38. These SMMs suggested the feasibility of using the CoII ion and the octacyanometallates for the construction of low dimensional magnetic materials of strong magnetic anisotropy. Inspired by aforementioned considerations, we devoted to the preparation of the SCMs using the [MV(CN)8]3− units and sterically bulky cations, such as the Ph4P+ and Ph4As+ cations. Here, we report the successful construction of two cyano-bridged 1D ladder chain compounds, or so-called 3, 3-ladder chain39: (Ph4P)[CoII(3-Mepy)2.7(H2O)0.3WV(CN)8]·0.6H2O (1) and (Ph4As)[CoII(3-Mepy)3WV(CN)8] (2). As expected, compounds 1 and 2 exhibit SCM behavior with high energy barriers. Most interestingly, both their energy barriers and the blocking temperatures are the highest for all the cyano-bridged SCMs. In addition, the barriers of 1 and 2 are only next to the cobalt(II)-radical compounds4041 reported by Maria G. F. Vaz et al. among all SCMs.

Results

Crystal Structures Description

Single crystal X-ray diffraction analysis revealed that 1 and 2 are isomorphous and crystallized in the triclinic space group (Supplementary Table S1). Both class="Chemical">compounds are made up of anionic cyano-bridged n class="Chemical">Co-W 3, 3-ladder chains running along the a axis (Fig. 1), Ph4P+ (for 1) or Ph4As+ (for 2) counter cations and lattice water molecules (for 1). Although 1 and 2 are isomorphous, disorder was only found in 1. One of the three 3-Mepy ligands of 1 has a site occupancy fact of only 0.7, while the rest 0.3 is occupied by a water molecule. Thus, each CoII center is in a distorted octahedral N6 environment with six nitrogen atoms from three [WV(CN)8]3− units and three 3-Mepy (for 1 and 2) or in a N5O environment from two 3-Mepy, one bound water molecule and three [WV(CN)8]3− ions (for 1). All three coordinated CN groups from [WV(CN)8]3− are in the meridional position of the octahedron. The Co–N or Co–O bond lengths and the N–Co–N or N–Co–O bond angles are all in the normal range for a slightly distorted octahedron (Supplementary Tables S2 and S3). The continuous shape measures (CShMs) relative to the ideal octahedron calculated using the program SHAPE 2.142 was 0.253 (CoN6)/0.282 (CoN5O) and 0.236 for the CoII centers of 1 and 2, respectively. On the other hand, the polyhedral shape around the W centers does not correspond to ideal symmetry and the [WV(CN)8]3− unit shows a distorted dodecahedral configuration (TDD) for both 1 and 2 with the CShMs to be 0.56 and 0.66 (Supplementary Table S4). The W–C and C–N bond lengths and the W–C–N bond angles are also consistent with the reported values (Supplementary Tables S2 and S3).

Figure 1

Molecular structure of crystal 1.

(a) The asymmetric unit of compound 1. (b) The 1D 3, 3-ladder along the a axis. Hydrogen atoms and Ph4P+ cations were omitted for clarity. Symmetry code: #1 = 2−x, −y, 1−z; #2 = x−1, y, z.

Each [WV(Cclass="Chemical">N)8]3− unit uses only three n class="Chemical">cyanides groups in a relatively T-shaped position to connect to three neighboring CoII centers, with the intrachain Co–W distances being 5.32, 5.38, 5.38 Å for 1 and 5.34, 5.39, 5.41 Å for 2, respectively. Bridged by these cyanide groups, a one-dimensional Co–W chain is formed along the a axis. Within the chain, both the CoII and WV centers connect to three neighbors and this kind of chain can be described as a 3, 3-ladder with regularly alternating WV and CoII along the edges39. However, as both the Co–N–C and W–C–N bond angles and the torsion angles of the W–C–N–Co connection deviate from 180° (Supplementary Tables S2 and S3), the 3, 3-ladders for both 1 and 2 are slightly distorted. Separated by the bulky Ph4P+ or Ph4As+ cations and 3-Mepy ligands (Supplementary Fig. S2), these chains are well isolated to each other, with the shortest interchain Co···Co, Co···W, and W···W distances being 12.8, 11.1, 12.8 Å for 1 and 13.0, 12.9, 13.0 Å for 2, respectively. These large distances will efficiently prevent any interchain magnetic coupling and lead to the SCM behavior of these compounds, as described below.

Magnetic Properties

Variable-temperature magnetic susceptibility data for class="Chemical">compounds 1 and 2 were measured in the temperature range of 1.8–300 K in a direct current (n class="Chemical">dc) field of 2 kOe. The χMT vs T plots of 1 and 2 are shown in Fig. 2 and Supplementary Fig. S3. At 300 K, the χMT values of 3.35 cm3 K mol (1) and 3.45 cm3 K mol (2) are much higher than the spin-only value of 2.25 cm3 K mol for one isolated WV center (S = 1/2) and one isolated high-spin CoII center (S = 3/2) with g = 2.00, indicating the significant orbital contribution of high-spin CoII in an octahedral configuration. Upon cooling, χMT increases continuously to a maximum value of 73.17 cm3 mol−1 K (1) and 73.14 cm3 mol−1 K (2) at 14 K, before dropping quickly at lower temperatures. The increase in χMT with decreasing temperature indicates the intrachain ferromagnetic coupling between WV and CoII centers, which has been reported in other CoII-WV systems4344454647. The further sharp decrease below 14 K is attributed to a saturation of the χM value and/or zero-field splitting (ZFS) effect, which urged us to explore the possibility of magnetic blocking. The field-cooled (FC) and zero-field-cooled (ZFC) magnetization was thus measured at Hdc = 10 Oe as shown in Supplementary Figs S4 and S5. The ZFC and FC curves diverge at 10.2 and 8.4 K for compounds 1 and 2, which define the blocking temperatures of these chain compounds.

Figure 2

Variable-temperature dc magnetic susceptibility data in the form of χMT for 1.

Measured in an applied field of 2 kOe. Inset: Plot of ln(χM′T) vs T−1.

To verify the dynamics of the magnetization relaxation, ac magnetic measurements were performed on polycrystalline samples of 1 and 2 under a zero class="Chemical">dc field with Hac = 1 Oe in the frequency range of 1–1500 Hz (Fig. 3a, Supplementary Figs S6a and S7). Below 12 K, the obvious frequency dependence of both in-phase (χM′) and out-of-phase (χM′′) ac susceptibility was observed as typically observed in SCMs. The Mydosh parameter φ = (ΔT/T)/Δ(log fp) (where fp is the frequency at which a maximum appears in the χM′′(f) plot124849, was estimated to be ca 0.13, which falls into the category of superparamagnets (either SMMs or SCMs) and n class="Chemical">considerably greater than that for a spin glass5051. Notably, both the peaks of the χM′ and χM′′ signals are rather broad, indicating the existence of at least two possible relaxation processes. Futhermore, the frequency dependent ac data also show pronounced temperature dependence, from which the semicircular Cole-Cole plots (χM′′ vs. χM′) were obtained (Fig. 3b and Supplementary Fig. S6b). A generalized Debye mode52 was used to extract the values and distribution of the relaxation time τ (Supplementary Tables 5 and 6). The obtained α values are 0.35–0.41 and 0.40–0.42 for 1 and 2, respectively. These values indicated a relatively wide distribution of relaxation times, which might be caused by the variable distributions in chain length and/or interchain magnetic interactions and/or random defects as found in other cases where α ranges from 0 to 0.753.

Figure 3

The ac susceptibility and Cole-Cole diagram for 1.

(a) Temperature dependence of in-phase (top) and out-of-phase (bottom) components of the ac susceptibility for 1 in zero applied static field with a 1Oe oscillating field at a frequency of 1–1500 Hz; (b) Cole-Cole diagram of 1, plotted using χM′ and χM′′ at different temperature. The solid lines represent the fits to a general Debye model.

For an Ising-like or anisotropic Heisenberg one-dimensional system, the χMT value in zero applied field is directly proportional to the class="Chemical">correlation length ξ. Therefore, the χMT increases exponentially with lowering temperature, following the equation: χMT ≈ Ceff × exp (Δξ/kBT)182054, where Ceff is the effective Curie n class="Chemical">constant, and Δξ gives an estimation of the intrachain exchange energy cost to create a domain wall along the chain. Thus, provided the 1D nature of compounds 1 and 2 and the presence of significant anisotropy, plots of ln (χM′T) versus 1/T of both compounds should display a linear region, which is actually observed experimentally. As can be seen in the inset of Fig. 2 and Supplementary Fig. S3, the ln(χM′T) versus T−1 plots measured at Hdc = 0 Oe and Hac = 1 Oe at a frequency of 1 Hz feature a linear region in the temperature range of 11.6–30 K for both 1 and 2, yielding Δξ/kB = 78(0) K and Ceff = 0.83(1) cm3 K mol for 1, and Δξ/kB = 78 (0) K and Ceff = 0.88 (1) cm3 K mol for 2, respectively. Below 11.6 K, ln (χM′T) reaches a maximum and then undergoes a linear decrease with decreasing temperature, which indicates that the correlation length becomes larger than the average distance between two intrinsic defects along the chain, as the interchain interactions are not likely for these well isolated 1D chain1955. From the activation energy Δξ, the exchange interaction J in the Ising limit can be estimated through the expression Δξ = 4|J|Seff2545556. Assuming the system at low temperature to be an Ising chain with the effective spin approach SCo, eff = 1/2 and SW = 1/2, the intrachain interaction is calculated as J/kB = 19.5 K for both 1 and 2. As the Δξ depends not only on the exchange interaction (J) but also the single-ion anisotropy, the J value calculated from the Ising model is a rough estimation.

The Glauber dynamics predicts that the energy barrier for the spin reversal of an ideal Ising chain should be Δτ = 2Δξ for infinite chains and Δτ = Δξ for finite-size chains, where growth of the class="Chemical">correlation length is limited by class="Chemical">naturally occurring defects5758. However, for most real SCMs, an anisotropic Heisenberg chain model is more suitable, which takes into acn class="Chemical">count the magnetic anisotropy energy of each magnetic unit59. As the correlation length ξ increases exponentially with a decrease in temperature, the overall energy barrier Δτ is Δτ = 2Δξ + ΔA for the infinite chain at high temperature and Δτ = Δξ + ΔA for the finite-size chain at low temperature, where ΔA represents the intrinsic anisotropic barrier for the individual spin in the absence of magnetic exchange. This is actually the case for 1 and 2. From the Arrhenius plots of relaxation times τ obtained from the frequency-dependent χM′′ peaks, two thermally activated regions were obviously observed above and below the crossover temperatures of T* = 10.4 and 10.8 K for 1 and 2, respectively. These two regions are corresponding to the infinite-size and finite-size regimes of relaxation, commonly encountered for SCMs. The energy barriers for both of these thermally activated processes were estimated using the Arrhenius law τ = τ0 exp (Δτ/kBT), giving the Δτ1 (τ01) and Δτ2 (τ01) as 252(9) K (1.5(2) × 10s) and 169(2) K (4.6(1) × 10 s) for 1 and 224(7) K (6.3(2) × 10 s) and 154(1) K (4.0(1) × 10 s) for 2, respectively (Fig. 4). To ensure the integrity of data, all the peak values of compound 1 (Fig. 4a) were read from the data of ac susceptibility (Fig. 3a). However, as the peak of 11.4 K is too broad to read accurately, the first spot was excluded in the fitting process. Remarkably, the barriers of 252(9) and 224(7) K observed for 1 and 2 are among the highest for single-chain magnets. The radical bridged chain compounds, [Co(hfac)2PyNN]n40 and [Co(hfac)2NaphNN]n41, were reported to display a relaxation barrier of (396 ± 13) K and (398 ± 14) K, respectively. As such, to the best of our knowledge, the energy barriers of compounds 1 and 2 set the new records for cyano-bridged SCMs and are preceded only by these two CoII-radical compounds among all SCMs. Obviously, for 1 and 2 in both the infinite-size and the finite-size regimes, Δτ1 is larger than 2Δξ and Δτ2 is larger than Δξ. From these values, the intrinsic anisotropic barrier ΔA can be estimated either by ΔA = Δτ1 − 2Δξ or ΔA = Δτ2 − Δξ. The obtained values are 96 or 91 K for 1, and 68 or 76 K for 2, respectively.

Figure 4

Arrhenius plots of relaxation times for 1 (a) and 2 (b). The solid lines show linear fits of the experimental data according to the Arrhenius law.

Furthermore, the slow magnetic relaxation of both 1 and 2 were class="Chemical">coclass="Chemical">nfirmed by the observation of the magnetization hysteresis loops. The loops were firstly measured on the polycrystalline samples at different temperatures and depicted in Fig. 5a and Supplementary Fig. S8a. At 1.8 K, the largest magnetization values at 70 kOe, 2.76 n class="Chemical">NμB (1) and 2.75 NμB (2), are significantly lower than the estimated saturation value of 3.2 NμB for the effective spin approach SCo,eff = 1/2, gCo = 13/36061, and gW = 2, SW = 1/2 indicating the significant magnetic anisotropy344954. Obvious hysteresis loops can be observed below 5 K, with the coercive field of 11.5 kOe for 1 and 9.9 kOe for 2 at 1.8 K. Fortunately, although the single crystals of 1 and 2 are not big enough for the magnetic measurement on one single crystal, these strip-like single crystals grow parallel and form a bundle of about 1.0 × 1.5 × 6.5 mm (Supplementary Fig. S9), which is suitable for the anisotropic measurement. By the face index of the crystal (Fig. 6), we can see that the long edge of the crystal strip (and thus the long side of the crystal bundle) is along the a axis, which is parallel to the direction of the 3, 3-ladder. Thus, the anisotropic magnetization measurements were performed with the dc field applied parallel or perpendicular to the direction of the 3, 3-ladder. As can be seen from the loops at 1.8 K (Fig. 5b and Supplementary Fig. S8b), strong magnetic anisotropy of both compounds is reflected incisively and vividly. When the field is applied along the chain direction, the saturated magnetization value is much lower and exhibits a very large hysteresis loop with a coercive field of Hc = 26.2 kOe for 1 (22.6 kOe for 2) and a remnant magnetization of Mr = 1.28 NμB for 1 (1.27 NμB for 2) at 1.8 K. With the field being perpendicular to the chain direction however, the compounds are quite easily magnetized. At 1.8 K, M reaches 3.4 NμB at 70 kOe (3.9 NμB for 2) with Hc = 12.5 kOe for 1 (7.9 kOe for 2) and Mr = 3.1 NμB for 1 (3.6 NμB for 2). This behavior is consistent with the ferromagnetic coupling between CoII and WV centers43606162. The observation of the loops clearly shows the substantial slow magnetic relaxation, indicative of a “magnetic memory” of both compounds 1 and 2.

Figure 5

The dc variable-field magnetization of 1.

(a) Magnetic hysteresis loops of polycrystalline 1 measured at 1.8, 2, 3, 5 and 10 K with a field sweep rate of 500 Oe/s. Solid line is guide for eyes; (b) Hysteresis loops at 1.8 K on the oriented long crystal bundle of 1 along (black) and perpendicular (red) to the chain direction (a axis).

Figure 6

Face index of single crystal of 1.

In summary, two unique cryano-bridged 3, 3-ladder were synthesized from the anisotropic class="Chemical">CoII center, the octacyanotungsten, and two bulky n class="Chemical">counter cations. Because of the intrachain ferromagnetic interaction, strong magnetic anisotropy, as well as the well isolation of individual chains, these 1D compounds behave as single-chain magnets with record high spin reversal barriers (252(9) and 224(7) K) for the cyano-bridged SCMs. Furthermore, hysteresis loops can be observed for both compounds below 5 K with large coercivities of 26.2 and 22.6 kOe at 1.8 K. Efforts to extend this synthetic strategy towards other SCMs using the 4d/5d cyanometallates and other anisotropic metal centers with higher energy barriers and blocking temperatures are underway.

Methods

Materials

All of the reagents and solvents were purchased from class="Chemical">commercial sources and used as received. The n class="Chemical">complex Cs3[W(CN)8]·2H2O was prepared by reported procedures6364.

Caution! Although no problems were enclass="Chemical">countered in the preparation of the following n class="Chemical">complexes, in the acidic reaction conditions, suitable precautions should be taken when handling potentially poisonous compounds. It is of the utmost importance that all preparations should be performed and stored in well-ventilated areas.

Synthesis of 1

Single crystals of 1 were achieved by the slow diffusion of the reactants in the presence of the bulky class="Chemical">Ph4P+ and n class="Chemical">Ph4As+ cations in H-shaped tube. 3-methylpyridine (0.5 mmol, 46.5 mg) was added dropwise into a stirred aqueous solution (2.5 mL) of CoCl2·6H2O (0.25 mmol, 59 mg), and Ph4PCl (0.25 mmol, 94 mg) was added after 30 minutes. Then the solution was carefully added to one side of an H-shaped tube. The other arm contained an aqueous solution (2.5 mL) of Cs3[W(CN)8]·2H2O (0.125 mmol, 103.5 mg). A methanol-water (v/v, 1:3) mixture was used as a buffer between the two arms. The dark red strip single crystals suitable for X-ray diffraction of 1 were obtained. Elemental analysis (%): Calcd. for C48.2H40.7N10.7O0.9CoWP: C 54.72, H 3.88, N 14.17; found: C 54.45, H 4.10, N 14.33%. IR: ν(RO–H) 3407.7(w), ν(C–Hpy) 3026.8(w), 1107.5(vs), ν(C–HMe) 2923.1(w), ν(C≡N) 2146.0(w), ν(C=C, C=N) 1583.4(m), 1483.5(s), 1439.5(s).

Synthesis of 2

The synthesis of class="Chemical">compound 2 is analogous to that of 1 but using n class="Chemical">Ph4AsCl (0.25 mmol, 105 mg) instead of Ph4PCl. The dark red strip block crystals of 2 suitable for single-crystal X-ray analysis were obtained after 4 weeks. Elemental analysis (%): Calc. for C50H41N11CoWAs (2): C 53.93, H 3.71, N 13.84. Found: C, 54.09; H, 3.82; N, 13.43. IR: ν(RO–H) 3411.3(w), ν(C–Hpy) 3081.1(w), 1109.2(vs), ν(C–HMe) 2922.6(w), ν(C≡N) 2145.6(w), ν(C=C, C=N) 1579.4(s), 1482.9(s), 1439.7(vs).

X-ray Crystallography

Crystallographic data of class="Chemical">compounds 1 and 2 were n class="Chemical">collected on Photon100 CMOS detector and Bruker Smart CCD area-detector diffractometer with Mo-Kα radiation (λ = 0.71073 Å) by using an ω scan mode at 123 K and 296 K, respectively. The diffraction data were treated using SAINT65, and all absorption corrections were applied by using SADABS66. All non-hydrogen atoms were located by Patterson method67 using the SHELXS programs of the SHELXTL package and subsequent difference Fourier syntheses. Hydrogen bonded to carbon were determined theoretically and refined with isotropic thermal parameters riding on their parents. H-atoms of water and methanol were first located by difference Fourier E-maps and then treated isotropically as riding. All non-hydrogen atoms were refined by full-matrix least-squares on F2. All calculations were performed by SHELXTL-976869.

Measurements

The IR spectra were carried out with a class="Chemical">Nexus 870 FT-IR spectrometer using KBr pellets in the range of 400–4000 cm. Elemental analyses of C, H, n class="Chemical">N were recorded on a PerkinElmer 240 C elemental analyzer. The temperature-dependent magnetic susceptibility, the zero-field cooled magnetization and field-cooled magnetization, the anisotropy measurement at 1.8 K and AC magnetic susceptibility were measured using a Quantum Design MPMS-XL7 superconducting quantum interference device (SQUID) magnetometer. Magnetization hysteresis loop at 1.8–10 K from 7 to −7 T and back were measured using an eicosane-constrained sample to prevent sample torquing on a Quantum Design VSM SQUID magnetometer. Experimental susceptibilities were corrected for the diamagnetism estimated Pascal′s tables and for sample holder by previous calibration.

Additional Information

Accession class="Chemical">codes: Crystallographic data for the structural analysis of the n class="Chemical">compounds have been deposited with the Cambridge Crystallographic Data Centre, under CCDC no. 1419479 (1) and 1419480 (2). These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www. ccdc.cam.ac.uk/data_request/cif.

How to cite this article: Wei, R.-M. et al. Single-Chain Magnets Based on class="Chemical">Octacyanotungstate with the Highest Energy Barriers for n class="Chemical">Cyanide Compounds. Sci. Rep. 6, 24372; doi: 10.1038/srep24372 (2016).