Enforcing Ising-like magnetic anisotropy via trigonal distortion in the design of a W(v)–Co(ii) cyanide single-chain magnet

A new octacyanotungstate(v) singe chain magnet with an effective energy barrier of 39.7(3) cm–1 is achieved by enforcing Ising-like magnetic anisotropy via introduction of trigonal distortion with a fac-tridentate capping ligand.


Introduction
Since the rst experimental verication of Glauber dynamics 1 for a cobalt(II)-nitronyl nitroxide radical chain complex in 2001, 2 the phenomenon of slow relaxation in 1-D Ising paramagnets, known as single-chain magnets (SCMs), 3 has received a great deal of attention in the eld of molecular magnetism. 4 The overall energy barrier (D s ) for reversing the direction of magnetization of an SCM is a sum of the correlation (D x ) and anisotropy (D A ) energies. As such, SCMs have the potential to exhibit higher blocking temperatures than their zerodimensional counterpart, namely single-molecule magnets (SMMs) because of the additional correlation energy and the potential to optimize the magnetic coupling (J) and zero eld splitting (D). For example, the introduction of bulky, rigid pyrene into the Co(II)-nitronyl nitroxide radical system led to stronger magnetic coupling, which in turn resulted in a SCM with a record blocking temperature of 14 K. 5 An increasing number of SCMs are being realized from the application of design principles involving the selection of specic bridging units in combination with polynuclear anisotropic metal complexes with ancillary blocking ligands. 6 The use of polynuclear SMMs as building blocks for the construction of SCMs represents a promising strategy, but enforcing the proper alignment of the subunits to afford Ising-like anisotropy is a challenge. 7 In this respect, the use of mononuclear SMMs as building blocks is an excellent approach to SCM design, as a number of these molecules have been found to exhibit high energy barriers due to the presence of large uniaxial anisotropy stemming from spin-orbit coupling and specic geometries and they possess only one easy axis to control. 8 The use of an efficient bridging ligand is of utmost importance in the design of SCMs. In this vein, cyanometallate complexes have received considerable attention because of the efficient exchange coupling through the cyanide bridge and the prospect for preparing homologous series of magnetic materials using a modular approach. 9 Modications of cyanometallate complexes are readily accomplished via installation of suitable capping ligands which allow for effective tuning of the ligand eld, redox potentials, and electronic congurations of the metal centers. Such adjustments had led to the realization of new functional materials including SMMs, 10 SCMs 11 and photomagnets. 12 In the eld of cyanometallate molecular magnets, a common building block that has been widely studied is the octacyanotungstate(V) ion which, as a 5d transition metal complex, 13 exhibits an enhanced ability for magnetic coupling through bridging ligands due to its diffuse orbitals.  4 ]$2H 2 O} n (iPr-Pybox ¼ 2,2 0 -(2,6-pyridinediyl)bis(4-isopropyl-2-oxazoline); tptz ¼ 2,4,6-tris(2-pyridyl)-1,3,5-triazine). 17 Both compounds exhibit the onset of frequency dependent out-of-phase signals in the AC susceptibility data at $2 K but these results do not conform to bona de SCM behavior. Nevertheless the ndings hint at the potential for tuning the magnetic behaviour of such systems by modifying the coordination environment of the Co centers with different capping ligands in order to increase the uniaxial magnetic anisotropy.
Recently, we discovered that axial trigonal distortion of the crystal eld affords a huge uniaxial magnetic anisotropy with a D value on the order of À100 cm À1 for the mononuclear cobalt(II) SMM [Co(Tpm) 2 ](BPh 4 ) 2 (Tpm ¼ 1,1,1-trispyrazol methane). 18 Taking a cue from this work and the preceding discussion, the premise behind the present work was to incorporate these highly anisotropic, trigonally distorted cobalt SMMs into cyanometallate-based chain structures in order to probe if enhanced SCM behavior could be engendered. Herein we report a chain compound with a 3,2-chain structural archetype, viz., {[(Tpm)Co(DMF)W(CN) 8 ] 2 [Co(DMF) 4 ]} n $2nDMF (1). § The structure is a derivative of 1*, where uniaxial anisotropy has been deliberately introduced in the form of trigonal antiprismatic cobalt ions capped by the fac-tridentate ligand Tpm. The magnetic studies indicate that 1 exhibits long range antiferromagnetic ordering below 3.4 K and single chain magnetic behavior with D eff ¼ 39.7(3) cm À1 an astonishing enhancement compared to the properties of 1*.

Starting materials
The ligand Tpm and [(n-Bu) 3 NH] 3 [W(CN) 8 ] were prepared according to literature procedures. 21 All other chemicals and solvents were of commercially available reagent grade quality and used as received.

Physical measurements
Infrared (IR) spectra were measured as Nujol mulls placed between KBr plates on a Nicolet 740 FT-IR spectrophotometer. Direct current (dc) and alternating current (ac) susceptibility measurements were performed on a Quantum Design SQUID, Model MPMS XL-7 instrument. Single crystal X-ray data for 1 were collected on a Bruker APEX-II diffractometer equipped with CCD detectors at 110 K.

Results and discussion
Synthesis Compound 1 was synthesized by rst reacting Aer layering the reaction mixture with a solution containing the supporting ligand Tpm, crystals of 1 formed over the course of two weeks. An IR spectrum of the compound shows features at 2197 cm À1 , 2176 cm À1 , and 2138 cm À1 , corresponding to the bridging and terminal cyanide stretches, respectively. Thermal gravimetric analysis (TGA) data revealed that the interstitial DMF solvent molecules (calcd 7.4%) are gradually lost before compound 1 begins to decompose at ca. 180 C (Fig. S2 ‡). The phase purity of the product was veried by powder X-ray diffraction (PXRD) (Fig. S3 ‡).

Structural description
Single crystal X-ray diffraction studies revealed that 1 crystallizes in the triclinic P 1 space group as a one-dimensional (  (1) . The coordination geometries of Co1 in both 1 and 1* were compared to an ideal octahedron using the SHAPE program. 21 The coordination geometry of Co1 in 1* is very close to an ideal octahedron (CShM ¼ 0.111) while the coordination geometry of Co1 in 1 exhibits substantially more distortion from an ideal octahedron (CShM ¼ 0.297). This distortion is similar to that observed for [Co(Tpm) 2 ](BPh 4 ) 2 (CShM ¼ 0.450) and is consistent with the assignment of the geometry of Co1 in 1 as trigonal antiprismatic rather than octahedral.

Magnetic properties
The variable temperature magnetic susceptibility data in an applied dc eld of 1 kOe are plotted in Fig. 2. At 300 K, the c m T value of 10.7 cm 3 mol À1 K is much higher than the spin-only value (6.375 cm 3 mol À1 K) for three Co II (S Co ¼ 3/2, g ¼ 2.0) and two W V (S W ¼ 1/2, g ¼ 2.0) centers, in accord with signicant spin-orbital coupling with g Co ¼ 2.66. Upon lowering the temperature, the c m T value remains nearly constant until $50 K and then increases abruptly to a maximum of 68.4 cm 3 mol À1 K at 3.5 K, before decreasing to 51 cm 3 mol À1 K at 2 K. These results indicate overall ferromagnetic coupling between W(V) and Co(II) centers, consistent with other previously reported W V -Co II complexes. 22 No suitable model could be applied for further quantitative analysis of this system because of its complicated topology.
The magnetization at 1.8 K (Fig. 3) rapidly increases above zero eld but does not saturate even at 7 T. The value of 9.82 Nb at 7 T is much lower than the expected saturation value of $14.0 Nb for three Co II (S Co ¼ 3/2, g ¼ 2.66) and two W V (S W ¼ 1/2, g ¼ 2.0) ions, due to signicant magnetic anisotropy as corroborated by the non-superposition of the M vs. H/T plots at higher elds (Fig. S4b ‡). Plots of d(M)/d(H) vs. H indicate a phase transition occurs from antiferromagnetic (AF) ordering to paramagnetic with a critical eld of 300 Oe at 1.8 K (Fig. S6 ‡). A narrow hysteresis loop at 1.8 K was observed with a coercive eld of $40 Oe and a remnant magnetization of 0.6 Nb. zero-eld-cooled (ZFC) and eld-cooled (FC) magnetization data collected under a eld of 20 Oe showed a sharp peak at 3.4 K and irreversibility below ca. 2.4 K; the former feature suggests an AF ordering while the latter behavior represents the blocking temperature (Fig. S6 ‡). Similar behavior has been noted for other cases of magnetic ordering in SCMs. 23 In 1963, Glauber reported his prescient work describing an anisotropic Heisenberg or Ising-like one-dimensional system with the equation: c m T/C eff ¼ exp(D x /k B T), where C eff is the effective Curie constant, and k B is the Boltzmann constant. From this relationship, c m T is expected to increase exponentially with decreasing temperature under a zero applied dc eld. Variable-temperature ac susceptibility data were collected in a zero applied dc eld and an ac eld of 5 Oe oscillating at 1 Hz (Fig. S7 ‡). The resulting plot of ln(c 0 m T) vs. 1/T features a linear region in the temperature range 4.0-12 K, yielding D x ¼ 6.2 cm À1 (inset of Fig. 2). Below 4.0 K, ln(c 0 m T) reaches a maximum and then undergoes a linear decrease with decreasing temperature.
To further probe the dynamics of the magnetization, ac magnetic susceptibility measurements were performed as a function of both temperature and frequency in a 5 Oe ac eld and a zero dc eld. As shown in Fig. 4a, variable-temperature ac susceptibilities for 1 display a strong frequency dependence of both in-phase (c 0 m ) and out-of-phase (c 00 m ) components. The shi of the peak temperature (T p ) of c 0 m , as evaluated by the Mydosh parameter 4 ¼ (DT p /T p )/D(log f) z 0.13, is consistent with normal superparamagnetic behavior (4 ¼ 0.1-0.3). 24  Variable-frequency ac susceptibilities collected in the range 2.5-4.0 K also show highly frequency dependent peaks ( Fig. 4b and S8 ‡). The relaxation time, extracted from the peaks of c 00 m in Fig. 4, follows two thermally activated laws, corresponding to innite-size and nite-size regimes, respectively, marked by the crossover temperature of T* ¼ 2.9 K (Fig. 4c). A tting based on the Arrhenius relationship s ¼ s 0 exp(D/k B T) gave: D 1 ¼ 39.7(3) cm À1 , s 01 ¼ 3.4(5) Â 10 À11 s for the innite regime and D 2 ¼ 31.8(2) cm À1 , s 02 ¼ 2.2(4) Â 10 À9 s for the nite regime. Given the relationship between the energy barriers for a SCM system, D 1 ¼ 2D x + D A and D 2 ¼ D x + D A , the anisotropy (D A ) and correlation energy (D x ) were calculated to be 23.9 and 7.9 cm À1 , respectively. The latter value is consistent with 6.2 cm À1 as estimated from the ln(c 0 m T) vs. 1/T plot. The small correlation energy may be due to the weak magnetic coupling between the W(V) and Co(II) ions across the cyanide bridge, which was found to be $0.7 cm À1 , according to the 2J formalism of the exchange Hamiltonian. 22 Based on |4D A /D x |, the |D/J| value was calculated to be 12.1, suggesting an Ising model for the current case. Cole-Cole plots (Fig. S9 ‡) of c 00 m vs. c 0 m were t to a generalized Debye model, 25 giving a values ranging from 0.24 to 0.50, indicative of a wide distribution of relaxation times, which could be caused by multiple relaxation processes, poly-dispersity of the chain length, magnetic interchain interactions, and/or random defects.
The decrease in the magnitude of the maximum intensity of c 00 m upon lowering the temperature and frequencies is attributed to unaccounted for AF interchain interactions. To test this hypothesis, additional ac susceptibilities were measured under a dc eld of 500 Oe, which minimizes the effect of antiferromagnetic interchain interactions (Fig. 5, S10 and S11 ‡). As shown in Fig. S11, ‡ classical SCM behaviour is observed. The effective energy barrier for the innite and nite regimes was estimated to be 38.3(2) and 25.1(7) cm À1 , respectively (Fig. 5b). The difference between the two energy barriers is almost double that of D x , likely originating from the contribution of the applied dc eld. The a values from tting the Cole-Cole plot ( Fig. 5c) based on a generalized Debye model were moderately reduced (in the range of 0.15-0.41), which suggests that the interchain interactions do not entirely account for the wide range of relaxation in 1.
The rapidly increasing number of mononuclear SMMs in the literature indicates the progress that is being made in predicting magneto-structural correlations. Spin-orbit coupling, the main source of magnetic anisotropy, can be tuned with intentional structural modications to target a specic geometry or distortion. 8a Although achieving such correlations still remains a complex issue for octahedral Co(II) systems, recent ndings indicate that a majority of six-coordinate cobalt(II) compounds, such as those in an elongated octahedral geometry, exhibit easyplane anisotropy (D > 0). 26 Very recently, both theory and experiment suggest that uniaxial magnetic anisotropy (D < 0) can be introduced and/or enhanced by increasing the axial trigonal distortion of the crystal eld wherein the easy axis is coincident with the C 3 axis. In 2013, the Gao group corroborated this principle with their report of a mixed-valence [Co 3 III Co II ] cluster with a Co(II) ion in a slightly distorted triangular prismatic geometry that exhibits a large uniaxial magnetic anisotropy (D Co ¼ À115 cm À1 ). 27 In this vein, we recently demonstrated a successful way to realize very large negative D values on the order of À100 cm À1 by preparing trigonal antiprismatic mononuclear cobalt(II) SMMs [Co(Tpm) 2 ][X] 2 , complexes. 18 The SCM behaviour in 1 can be better understood by comparing the properties to the previously reported 1* in terms of their respective structures (Table S1 ‡). In 1*, both Co1 and Co2 centers adopt a slightly elongated octahedral geometry with a dihedral angle of 26.5 between their equatorial planes (easyplane); 16 whereas in 1, while Co2 still adopts an elongated octahedral geometry (easy-plane anisotropy), the Co1 center is now trigonally distorted by virtue of the presence of the Tpm  capping ligand which engenders uniaxial magnetic anisotropy along the C 3 axis (Co1-C9). This very important distinction leads to differences in magnetic behaviour that can be rationalized by considering the easy axes of each compound, indicated by the green lines in Fig. 6. Compound 1* which is not an SCM does not have alignment of the easy plane anisotropy axes and is reported to exhibit glassy magnetic behavior and long range magnetic ordering. In the case of 1, the Co1-C9 bond is nearly parallel to the Co2-O3 bond (one preferred orientation within the easy plane) with a small angle of 2.7 ; therefore, the projection along the Co1-C9 direction of the anisotropy tensors results in Ising-like magnetic anisotropy with g z > g x s g y , consistent with the above estimation (|D/J| > 4/3).

Concluding remarks
This work highlights the advantage of incorporating the principles of mononuclear SMMs into the design of SCMs. The trigonal symmetry that is essential to the SMM properties of [Co(Tpm) 2 ](BPh 4 ) 2 served as a guiding principle in the design of 1. The use of Tpm in the synthesis of 1 engendered coalignment of uniaxial magnetic anisotropies along the C 3 axes of the Co II moieties, resulting in good SCM behaviour, whereas its non-trigonal counterpart, 1*, is not an SCM. Future efforts to incorporate this trigonal distortion into SCMs with other cyanometallates as linkers will be explored.

Conflicts of interest
There are no conicts to declare.