Molecular materials are highly concern in materials research today, mainly because the physical properties displayed by pure inorganic materials are mostly found in molecular complexes. Additionally, molecular materials can be structurally designed according to the needs of physical properties through structure-function relationships, which greatly enriches the diversity and performance of materials, and provides great convenience for material research and acquisition.
Molecular magnets are an important direction in the research of many molecular materials. Materials with magnetic properties as the only property include such things as 3D ordering molecular magnets[1], low-dimensional magnets[2-4], and magnetic materials under various external stimuli[5]. When combined or even coupled with other physical properties, it exhibits multi-functionality, such as magnetic refrigeration materials[6], magnetoelectric materials (magnetic capacitors, multi-ferrotes) [7], and magneto-optical materials[8]. Single-molecule/ion magnets (SMMs/SIMs) in low-dimensional magnets have attracted much attention due to their ability to serve as the smallest unit of high-density information storage devices. Especially in recent years, the breakthrough in the operating temperature of SIMs has pushed the research of molecule-based magnetic materials to a new climax[9]. The primary consideration in designing a SIM is the magnetic anisotropy of its spin center. In theory, it is believed that the stronger the uniaxial magnetic anisotropy of the spin center, the higher the energy required for spin reversal, and the higher the blocking temperature. Meanwhile, strong uniaxial magnetic anisotropy can weaken or quench the influence of quantum tunneling magnetization (QTM) on the slow magnetic relaxation of SIMs at low temperatures. Because of this, people prefer to use rare earth ions as spin carriers for designing SIMs. The f electrons of rare earth ions, due to being shielded by outer orbitals, are difficult to be influenced by ligand fields, thus exhibiting approximately gaseous ion characteristics. The f electrons are arranged in a nearly degenerate manner on their occupied orbitals, thus having a large magnetic contribution from first-order orbital angular momentum. At the same time, they can also couple with high-energy orbitals (spin-orbital coupling), resulting in a large magnetic anisotropy parameter D. As early as 2010[10], it was discovered that the single-ion magnetic behavior can be observed even by arbitrarily selecting a Dy-based complex, which is just attributed to the D value. Comparatively, transition metal ions in complexes are easily influenced by ligand fields, which cause d-orbitals to split. Thus, most complexes containing the first transition metal ion exhibit the Jahn-Teller distortion, which further splits the orbitals to form more energy levels. Therefore, the orbital magnetic contribution in transition metal complexes is easily quenched under ligand fields. However, CoⅡ ion is a special exception. It can still provide strong orbital contributions under the influence of ligand fields, as they have very strong spin-orbit coupling and even retain the contribution of first-order orbital angular momentum in some highly symmetric coordination environments[11]. Whether using rare earth ions or CoⅡ ions as spin carriers, to improve uniaxial magnetic anisotropy, people try to increase the axial symmetry of the central ion in the coordination environment when designing complexes. In this way, the axis energy of the highest Kramers double state that causes quantum tunneling coincides with the structural symmetry axis of the spin carrier[12]. Therefore, in complexes with C4[13], C5 [14], C6[15], and C∞[16] symmetry, it is found that magnetic anisotropic and structural symmetric axes can better coincide with each other, and the properties of complexes are also better. Among them, SIMs with particularly C5 symmetry have developed rapidly due to their ease of synthesis.
The ligands used for synthesizing C5 symmetric SIMs are mainly concentrated in ring and semi-open ring ligands with five coordination sites (Scheme 1), such as 15-crown-5 and Schiff base ligands. This can control the formation of a planar configuration between metal ions and ligands through five coordinating atoms, and the axial direction is occupied by other monodentate ligands. At the same time, the axial ligand field can be changed to regulate the properties of SIMs. Due to the large radius of rare earth ions, this design does not seem to be easy to succeed. Instead, the examples with excellent performance were obtained by utilizing the spatial hindrance of monodentate ligands[14a-14b].
LN5=2, 13-dimethyl-3, 6, 9, 12-tetraaza-l(2, 6)-pyridinacyclotridecaphane-2, 12-diene[14d], H2dapsc=2, 6-diacetylpyridine-bis(semicarbazone)[14d, 17], H2dapb=6-diacetylpryidine-bis(benzoyl hydrazine)[14d].
This design is much easier for transition metal ions. As early as 1978, Palenik and Wester[17] conducted in-depth discussions on the synthesis and structure of such complexes. They found that most first transition metal ions can form pentagonal bipyramidal coordination configurations with H2dapsc. With the deepening of research on SIMs, this type of structure is gradually being paid attention to in the field of molecular magnetism. In 2013, Ruiz et al.[18] predicted through calculations that the metal center (point group D5h) of the pentagonal bipyramidal coordination configuration can exhibit significant magnetic anisotropy. In the second year, Wang et al. [14d] demonstrated the calculation results of Ruiz et al. by studying the magnetic properties of pentagonal bipyramidal CoⅡ complexes, and observed strong field-induced slow magnetic relaxation behavior in the anisotropic CoⅡ complexes. Subsequently, magnetic research on such complexes began to increase significantly, and the number of reports in recent years has shown an upward trend. However, so far, there is still no clear pattern in the relevant research that affects the performance of SIMs.
In the pentagonal bipyramidal CoⅡ SIMs, in addition to the symmetry of coordination configuration playing a decisive role, the electrostatic field effect of coordination atoms also has a significant impact on their magnetic properties. The main manifestation is that a suitable electrostatic field can effectively suppress the QTM, which means that the substituents on the ligand or changing the coordination atoms may regulate the performance of the SIMs[19]. Based on this, unlike previous studies that used the second-period element atoms as axial ligands for pentagonal bipyramidal CoⅡ, we used the third-period Cl atom to adjust its ligand field through axial coordination, to observe its regulatory effect on SIMs. This contrast is stronger than that between coordinating atoms only within the same period. In this work, we selected H 2 dapsc as the main ligand, with a water molecule and Cl- ion at two axial positions, to obtain a pentagonal bipyramidal CoⅡ field-induced SIM [Co(H2dapsc) (H2O)Cl]Cl·2H2O (1). Here, we provide a detailed report on the synthesis, structural characterization, and magnetic properties of this complex.
All chemical reagents used for the synthesis of this work were purchased from Sigma Company and were directly used for experiments without further purification.
The ligand H2dapsc was synthesized according to the reference method[20]. The average yield of 2, 6-diacetylpyridine based on multiple experiments was 73%. Elemental analyses Calcd. for C11H15N7O2(%): C, 47.64; H, 5.45; N, 35.36. Found(%): C, 47.41; H, 5.55; N, 35.40.
The synthesis of complex 1 was slightly modified based on the reference method[17, 20-21]: 1.11 g (4 mmol) of ligand H2 dapsc was added to a mixed solvent of 48 mL ethanol and 12 mL water, and stirred until dissolved. Then, 0.835 g (3.5 mmol) of CoCl2·6H2O was dissolved in a mixed solvent of 60 mL ethanol and 15 mL water. The resulting solution was slowly added to the H2dapsc solution using a dropper funnel and stirred at a constant temperature of 80 ℃ for 24 h. A yellow solid was formed. The solution was filtered and the solid was washed with a small amount of ethanol and water-mixed solvent (4∶1, V/ V) to obtain complex 1. The mother liquor was placed in a dark place and the crystals of complex 1 were obtained after 2-3 months. The precipitate was dissolved in a mixed solvent of ethanol and water (4∶1, V/V), and placed in a dark place. It also took 2-3 months to obtain a crystalline sample that can be used for single crystal measurement. Single crystal X-ray diffraction measurement showed that the crystalline substance in both solutions was the same product 1, with a total of 0.22 g. The yield obtained based on CoⅡ ions was 14%. Due to the good solubility of the product in the solvent used, only a portion of the collected crystals resulted in a lower yield. Elemental analyses Calcd. for C11H21N7Cl2CoO5(%): C, 28.64; H, 4.58; N, 21.26. Found(%): C, 28.59; H, 4.33; N, 21.35.
A PerkinElmer 240C elemental analyzer was used to determine the content of C, H, and N elements in ligand H2dapsc and complex 1. The magnetic properties of complex 1 were characterized by the MPMS SQUID-XL7 produced by Quantum Design. The DC variable-temperature magnetic susceptibility was measured in a temperature range from 300 to 1.8 K under the magnetic field of 2 000 Oe. The final data was corrected by a sample tube and Pascal constant[22] for magnetic analysis. The temperature range for AC magnetic susceptibility measurements was 1.8-10 K, the AC magnetic field was 5 Oe, and the frequency range was 1-1 488 Hz.
The diffraction data for complex 1 were collected on a Bruker SMART CCD area-detector diffractometer using graphite monochromatic Mo Kα radiation (λ = 0.071 073 nm) in the ω-scan mode at 293(2) K. The data were treated using SAINT and the absorption corrections were applied using the SADABS program[23] provided by Bruker. The structure was determined by Patterson's method[24] using the SHELXL-97 program[25] and by subsequent Fourier syntheses. Anisotropic treatment was applied to all non-hydrogen atoms refined by full-matrix least-squares on F2. The hydrogen atoms bonded to carbon and oxygen were determined theoretically and refined with isotropic thermal parameters riding on their parents. The details for the structural analyses of complex 1 are given in Table S1 (Supporting information), and selected bond distances and angles are listed in Table S2.
When using the H2dapsc ligand to synthesize CoⅡ-centered complexes, the composition of the resulting complexes varies depending on the inorganic salt used, mainly due to the different auxiliary ligands. In the works of Jubault et al.[14e] and Carcelli et al.[20], the starting materials were perchlorate and nitrate salts, respectively. Due to the electronegativity of the Cl or N atoms in the centers of two acidic ions comparable to the O atom, the Lewis basicity of the acidic ions decreases and the coordination ability weakens, resulting in only water molecules coordinating in the axial direction of complex molecule. This situation also occurs when H2dapb is used as the main ligand[14d, 26]. When salts composed of acidic ions with poor coordinating ability are used as starting materials, the solvent molecules show a strong Lewis basicity and coordinate in the axial direction. In the above references, water, DMF, and MeOH molecules coordinate in the axial direction, while acidic ions are not in the coordination sphere. When other strong Lewis bases are added to the starting materials, such as the auxiliary ligand imidazole reported by Huang et al.[14d], they preferentially coordinate in the axial direction over solvent molecules. Different axial ligands have different ligand field effects on the central ions, which can cause changes in the electronic structure of CoⅡ ions and thus alter their magnetic properties.
The molecular structure of complex 1 has been reported by Palenik et al.[17], but to facilitate the analysis of magnetostructural correlation later, we will briefly describe it from different perspectives.
Complex 1 crystallizes in the space group Cc (namely, Ia in reference[17]), which consists of a [Co(H2dapsc) (H2O)Cl]+ cation, a counterion Cl-, and two lattice water molecules. As shown in Fig. 1, the H2dapsc ligand in a neutral form uses its five coordination atoms to chelate with the CoⅡ ion through the equatorial plane with the bond lengths ranging from 0.216 0(3) to 0.219 8(3) nm. The axial positions are occupied by Cl and O atoms, and the bond lengths are Co—O of 0.214 2(3) nm and Co—Cl of 0.247 03(14) nm, respectively. Therefore, complex 1 shows a distorted pentagonal bipyramidal coordination configuration and the shortest bond length in the axial position. All non-hydrogen atoms on the main ligand and the central Co atom are basically in the same plane, with an average deviation of 0.011 72 nm from the plane. If only considering the plane composed of five coordination atoms and Co, the average deviation is 0.006 96 nm, indicating that five coordination atoms form a good planar pentagon. The axial O3—Co1— Cl1 bond angle is 176.55(8)°, which is very close to a straight line. Therefore, the coordination environment around Co can be considered as a pentagonal bipyramid configuration, which is confirmed by the analysis results of the SHAPE2.1 program (Table S3) [27]. Since the complex molecule contains only one Cl- ion and shows a valence of +1, there is also a counteranion Cl- in the crystal lattice, which forms hydrogen bonds with the axial Cl- ions and the lattice and coordination water molecules to stabilize the solid structure of the complex. The Co···Co distances are 0.673 8 and 0.906 3 nm between the nearest and between the next neighboring coordination ions, respectively. There is no strong intermolecular interaction, so it is believed that the magnetic interaction between molecules is dominated by dipoledipole interaction.
Dark blue: Co, blue: N, red: O, gray: C, green: Cl; Displacement ellipsoids are drawn at the 50% probability level.
The variable-temperature susceptibility χM of complex 1 was measured on the polycrystalline samples in a range from 300 to 1.8 K under 2 000 Oe of DC external field. Fig. 2a shows the plot of χMT vs T. The shape of the curve and values of χMT are similar to those reported previously[ 14d-14e]. At room temperature, the χMT value was 2.6 cm3·mol-1·K, which was much higher than the spin-only value 1.875 cm3·mol-1·K (S=3/2 and g=2) due to the significant orbital magnetic contribution in CoⅡ ion. However, this value was slightly lower than 3.2-3.4 cm3·mol-1·K for the CoⅡ ion in an octahedral coordination environment. In the octahedral configuration, CoⅡ ion has the magnetic moment from the first-order orbital angular momentum of the t2g state (Fig. 2c). Even if the first-order orbital angular momentum is quenched due to Jahn-Teller distortion in octahedral structure, its spin-orbital coupling is still very strong. On the contrary, in the pentagonal bipyramidal structure, there is only spin-orbital coupling (Fig. 2b) because of no inequivalent electron occupation on the degenerate orbital in the ground state. Upon cooling, χMT slowly decreased and reached 2.5 cm3·mol-1·K at 80 K, and then sharply down to 1.6 cm3·mol-1·K until 1.8 K. This phenomenon is caused by the thermal depopulation of Kramers levels and very common in CoⅡ complexes. It indicates that complex 1 has strong magnetic anisotropy. The evidence can be found in the field dependence of magnetization (Fig. S1). Furthermore, these magnetic properties imply that complex 1 may exhibit the slow relaxation of magnetization. To determine this possibility, the AC magnetic properties were studied in detail.
The red line represents the calculated result as described in the text.
Complex 1 did not show any AC signal without a DC magnetic field. After applying the field, the AC signal could be observed, indicating the existence of QTM in the complex, which can be suppressed by the magnetic field. Subsequently, it was found that 2 000 Oe was the optimal field (Fig.S2), so the AC data were recorded in this field. From the temperature dependence of AC susceptibilities (Fig.S3), we can find that there were no peaks to be observed at low temperatures and low frequencies in both inphase and out-of-phase. With warming, the peaks became apparent, even appearing around 7 K when the frequency was 1.5 kHz. The rich signals in AC measurements and the frequency dependency indicate the presence of slow magnetic relaxation behavior in this complex. The converted variable-frequency AC magnetic properties also illustrate this point (Fig. 3). The preliminary AC results suggest that complex 1 is a field-induced singleion magnet. To prove it, we extracted τ values at different temperatures from the Cole-Cole data (Fig. 4a) and plotted ln τ-T-1, observing a nonlinear curve showing the relationship between τ and temperature (Fig. 4b). A nonlinear fit to all data-based equation 1 afforded A=17.87 K-1·s-1, C=0.65 K-n·s-1, n=3.96, τ0=4.79×10-8 s and Ueff=55.55 K, indicating that the direct and Raman processes dominate the slow relaxation of magnetization. When considering QTM during the fitting process, its coefficient was zero, which fully indicates that QTM has been suppressed by external fields.
|
|
(1) |
where A and C are the coefficients of the direct and Raman processes, respectively; Ueff is the energy barrier for magnetization reversal, kB is the Boltzmann constant and T is temperature.
In the studies of SIMs, the theoretical calculation is the effective and common means. It can help to determine the magnetic anisotropy and the magento-structural correlation of complexes. In this work, complete-active-space self-consistent field (CASSCF) calculations of individual CoⅡ (S=3/2) fragments of complex 1 have been carried out with the OpenMolcas program package[28]. The computational model was based on the X-ray single crystal structure of 1, but only one H2O and one Cl- on the axial sites of Co remained (Fig. S4). The basis sets for all atoms are atomic natural orbitals from the MOLCAS ANORCC library[29]: ANO-RCC-VTZP for CoⅡ ions; VTZ for the coordinated O, N, and Cl atoms; VDZ for other O, N, C, and H atoms. The calculations employed the second-order Douglas-Kroll-Hess Hamiltonian, where scalar relativistic contractions were taken into account in the basis set and the spin-orbit couplings were handled separately in the restricted active space state interaction (RASSI-SO) procedure. For CoⅡ, the active space was defined as seven electrons in five d orbitals, CAS (7, 5). We considered 10 quadruplets and 40 doublets for the restricted-active-space self-consistent-field (RASSCF) calculations and employed all of the states for RASSI-SO calculations. Based on the above CASS-CF/RASSI-SO calculations, the SINGLE_ ANISO program was then employed for obtaining the energy levels, g tensors, mJ values, magnetic axes, etc[30-32]. The calculation results were given the zero-field splitting parameters D=45.68 cm-1, E=-0.32 cm-1, and g tensors of the lowest spin-orbit states of CoⅡ for complex 1: gx= gy=2.44, g z =2.00 with the anisotropic axis as shown in Fig. 5. The simulations of χMT-T and M-H are shown in Fig. 2a and S1a.
This result indicates that complex 1 is a field-induced SIM with rhombic anisotropy because of gx=gy > gz, which is similar to that reported previously by Wang et al[14d]. The higher D value than their result may be because of the higher electrostatic field of axial Cl- ion than neutral water or pyridine molecules. However, this electrostatic field is not strong enough to make complex 1 an axially anisotropic SIM, as the shortest bond length is in the axis, but the D value is still positive.
In this work, we used 2, 6-diacetylpyridine-bis (semicarbazone) (H2dapsc) as the main ligand to coordinate to CoⅡ ion form a mononuclear complex, [Co(H2dapsc) (H2O)Cl]Cl·2H2O (1), with a pentagonal bipyramidal configuration. The magnetic study and the theoretical calculation indicate that complex 1 is a field-induced SIM with rhombic anisotropy and the D value was 45.68 cm-1. This D value was higher than those reported analogs previously because of the strong electrostatic field of axial Cl- ion. Our work provides a reference for understanding the influence of ligand electrostatic field effects on SIMs.
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Scheme 1 Some commonly used ligands for synthesizing SIMs with pentagonal bipyramidal configuration
LN5=2, 13-dimethyl-3, 6, 9, 12-tetraaza-l(2, 6)-pyridinacyclotridecaphane-2, 12-diene[14d], H2dapsc=2, 6-diacetylpyridine-bis(semicarbazone)[14d, 17], H2dapb=6-diacetylpryidine-bis(benzoyl hydrazine)[14d].
Figure 1 Molecular structure diagram and stacking diagram of complex 1
Dark blue: Co, blue: N, red: O, gray: C, green: Cl; Displacement ellipsoids are drawn at the 50% probability level.
Figure 2 Variable-temperature magnetic properties of complex 1 (a); Electronic structures of CoⅡ ion in pentagonal bipyramidal (b) and octahedral coordination environments (c), respectively
The red line represents the calculated result as described in the text.
Figure 4 Cole-Cole plots of complex 1 (a) and the relationship between relaxation time and temperature (b)
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