Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
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Important Role of Intermolecular Interaction in Cobalt(II) Single-Ion Magnet from Single Slow Relaxation to Double Slow Relaxation Zhao-Bo Hu,† Zhao-Yang Jing,† Miao-Miao Li,†,‡ Lei Yin,§ Yan-Dong Gao,† Fei Yu,† Tuo-Ping Hu,*,‡ Zhenxing Wang,*,§ and You Song*,† †
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State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Xianlin Road 163, Nanjing 210023, PR China ‡ Department of Chemistry, College of Science, North University of China, Xueyuan Road 3, Taiyuan 030051, PR China § Wuhan National High Magnetic Field Center & School of Physics, Huazhong University of Science and Technology, Wuhan 430074, PR China S Supporting Information *
ABSTRACT: Two cobalt complexes with similar structures were synthesized using quinoline-2-carboxylic acid (HL) as the ligand. Both complexes are six-coordinated in antitriangular prism coordination geometries. There are one and four molecule units per cell for 1 and 2, respectively, with nearest Co−Co distances of 7.129 and 5.855 Å, respectively, which lead to their intermolecular interactions zj′. Both complexes are field-induced single-ion magnets. Complex 1 shows single slow relaxation under Hdc = 1.5 kOe attributed to the moment reversal, while complex 2 shows double slow relaxation resulting from intermolecular dipolar interaction and moment reversal, respectively.
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INTRODUCTION Single-molecule magnets (SMMs) exhibit slow magnetic relaxation at low temperature, which attract extensive interests in the fields of quantum computing, high-density information storage, and molecule spintronics.1−4 The property is intrinsic in the molecule itself, that is, the long-range ordering or magnetic interactions between neighboring molecules must be inhibited or tuned for achieving SMM behavior and has been proven by magnetic dilution studies.5,6 Hundreds of molecules have been reported with SMMs properties; however, the low blocking temperature (TB) and energy barrier (U) of SMMs have up to now posed some limitations in their use. In order to increase blocking temperature and energy barrier, a variety of methods have been attempted, which can be mainly summed up from two aspects: adjusting the coordination geometry of metal ions and adopting appropriate counterions or solvents.7 Recently, a dysprosium-based SMM has been reported by Goodwin et al. and Guo et al., of which magnetic hysteresis can be observed up to 60 K. This temperature is quite close to 77 K (liquid nitrogen temperature), bringing hope for future application.8 In recent years, the transition metals single-ion magnets (SIMs) have been greatly developed.9−19 For transition metals, the energy barrier can be described as U = |D|S2 (or U = |D|(S2 − 1/4) and thus as a comprehensive effect from the ground state spin S and the negative zero field splitting (ZFS) © XXXX American Chemical Society
parameter D. The cobalt system is one of the most widely studied in SMMs, as it not only has high ground state (S = 3/ 2) but also possesses high magnetic anisotropy.5,20−46 For this reason, a two-coordinated CoII complex reported by Gao’s group in 201728 could display an energy barrier of 413 cm−1. More importantly, according to Kramers theorem, fast quantum tunnelling can be circumvented by utilizing halfinteger spin systems. So, the element Co gives us much opportunity for the study in SIMs. By now, although a large number of cobalt(II) SIMs have been reported with different coordination geometries, such as distorted trigonal bipyramid,42 distorted square antiprism,22 and so on, we found that a complex with low-coordination numbers or uncommon environment is unstable in the film fabrication process, which will largely limit its application. As is well-known, cobalt complex is always stable with six-coordinated geometries. Thus, six-coordinated cobalt SIMs may be a priority for the further study in the process of achieving the device. Trigonal prism and antiprism are the unique configurations in sixcoordinate cobalt complexes showing magnetic relaxation. In 2016, Song et. al and Dunbar et. al reported the trigonal antiprismatic CoII SIMs, [Co(Tp*)2],5 [Co(TPm)2][BPh4]2 and [Co(TPm)2][ClO4]2, which were all field-induced SIMs.39 Received: May 21, 2018
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DOI: 10.1021/acs.inorgchem.8b01389 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
Figure 1. Molecular structure of 1 (left) and 2 (right). For clarity, all hydrogen atoms are removed. Symmetry code: (A) 1 − x, 1 − y, 1 − z.
Figure 2. HF-EPR spectra of 1 (left) and 2 (right) with their simulations at 170 GHz and 4.2 K. The simulation using a negative D value is represented with blue while the simulation using a positive D value with red, showing positive D values in 1 and 2. The parameters of the spin Hamiltonian are obtained from a fit to the 2D field/frequency map as shown in Figure 3.
Figure 3. Resonance field corresponds to the microwave frequency (quantum energy) of EPR transitions for 1 (left) and 2 (right). The curves in blue, red, and black are simulated with the best fitted spin Hamiltonian fitting parameters, where the magnetic field H is parallel to the x, y, and z axes of the ZFS tensor, respectively. Vertical dotted line at 170 GHz represents the frequency where the spectra are recorded or simulated in Figure 2.
Synthesis of [CoL2(H2O)2]·2H2O·2CH3OH (1). Complex 1 was synthesized according to the reported procedure45 with a slight modification. HL (0.2 mmol, 0.034 g) was added to 5 mL water and CoCl2·6H2O (0.1 mmol, 0.0237 g) to 5 mL absolute methanol. After the metal salt was dissolved, two kinds of solutions were slowly mixed. Orange crystals were obtained by slow evaporation after 3 days. Anal. Calculated (%) for C22H28CoN2O10: C, 48.99; H, 5.23; N, 5.19. Found: C, 48.62; H, 5.46; N, 4.91. The diluted sample, i.e., complex 1a, was prepared in the same manner but starting with CoCl2· 6H2O/ZnCl2 = 1:9 molar ratio. Complex 1a has the same crystal structure with complex 1. ICP-AES analyses show that the dilution ration is 10.0(3)% for Co.
To further explore the influence of ligand upon Co-based SIMs with trigonal antiprism, we selected two CoII complexes with similar structure and quinoline-2-carboxylic acid (HL) as the ligand. The slight difference is one of the coordinated O atoms from water for 1 and from ethanol for 2, respectively, which result in only one slow magnetic relaxation process in 1 but two relaxations in 2.
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EXPERIMENTAL SECTION
All solvents and raw material are commercially available and used without further purification. B
DOI: 10.1021/acs.inorgchem.8b01389 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry Synthesis of [CoL2(CH3CH2O)2] (2). The synthetic method of 2 and diluted samples 2a and 2b by Zn are the same as that of 1 and 1a where both water and methanol were replaced by absolute ethanol affording orange crystals as well after 6 days. The diluted sample, i.e., complex 2b, was prepared in the same manner, but starting with CoCl2·6H2O/ZnCl2 = 1:99 molar ratio. Complex 2 is the same as that reported in ref 46. Anal. Calculated (%) for H24C24N2O6Co: H, 4.88; C, 58.19; N, 5.65. Found: H, 4.56; C, 58.32; N, 5.81. The dilution ratios were found by ICP-AES analyses to be 10.0(3)% and 1.0(3)% for Co for 2a and 2b, respectively.
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RESULT AND DISCUSSION Structure Description. The single crystal X-ray diffraction analysis results revealed that the molecules of complexes 1 and 2 have the similar structure, but they have different space groups: triclinic, P1̅ for 1 and monoclinic, P21/n for 2, respectively. The structures of both complexes have been reported in refs 45 and 46. Here, we would briefly describe them for discussion of the following magnetostructural correlation. The metal center CoII ion is six-coordinated and located at the center of inversion symmetry (Figure 1). Two oxygen and two nitrogen atoms provided by ligands and another two oxygen atoms from solvent molecules, forming a distorted trigonal antiprismatic geometry, where O1O3N1 is the base of the prism. All of the Co−O/N bonds are in the normal range for high spin of CoII ion. For 1, the Co−O bond length is 2.037 and 2.117 Å and 2.225 Å for Co−N. These bond lengths correspond to 2.0173, 2.1039, and 2.1979 Å for 2. Packing arrangements are different in complexes 1 and 2. It only has a single molecule in complex 1, while there are four molecules in complex 2 in a cell frame (Figures S1 and S2). The nearest Co···Co distances are 7.129 and 5.855 Å for complexes 1 and 2, respectively. HF-EPR Studies. It is well-known that six-coordinate highspin CoII SIMs having octahedral geometry have been reported to show both negative and positive magnetic anisotropies. To determine the sign of magnetic anisotropy, cracked crystal samples 1 and 2 were studied by high-frequency and high-field electron paramagnetic resonance (HF-EPR) between 60 and 260 GHz (Figures 2 and 3). Since the D values are out of the frequency range in our instrument, no transitions between Kramers doublets Ms = ±1/2 and ±3/2 were observed. Three main signals generated from the intra-Kramers transitions within the Ms = ±1/2 doublets can be observed in the HF-EPR spectra of 1 and 2, which is typical for an S = 3/2 system with a high-spin state and large positive D values. Well-simulated spectra to the HF-EPR data of complexes 1 and 2 are shown in Figure 2. The fitting was done with the D value obtained by SQUID, while adjusting the E and intrinsic g values. The best fit was obtained with the parameters gx = 2.46, gy = 2.46, gz = 2.24, |D| = 62.11 cm−1, |E| = 3.17 cm−1 for 1 and gx = 2.50, gy = 2.50, gz = 2.18, |D| = 72.08 cm−1, and |E| = 7.37 cm−1 for 2. By comparing the line shape and peak positions of the simulated spectra with experimental ones (Figure 3), D values are positive, which demonstrates the easy-plane magnetic anisotropies of 1 and 2.32,47 Direct Current Magnetic Measurements. The direct current (dc) magnetism properties of 1 and 2 were studied on cracked crystals in the range of 1.8−300 K in 1.0 kOe. As shown in Figures 4 and S4, the room-temperature χMT values are 3.12 and 2.88 cm3 K mol−1 for 1 and 2, respectively, larger
Figure 4. Temperature dependence of χMT in 1 kOe between 1.8 and 300 K for complexes 1 and 2 using a MPMS-XL7 SQUID magnetometer. The solid lines represent the best fitted result by PHI (blue) and the best calculated result using CASPT2/RASSI with MOLCAS 8.2 (red). Inset: Experimental M vs H plots at the indicated temperatures for 1.
than the expected value of 1.87 cm3 mol−1 K for noninteracting high-spin CoII ion (S = 3/2 and g = 2.0), resulting from the strong orbital contribution.20 Upon cooling, the χMT values first gradually decrease from room temperature to 150 K, then show a more pronounced decrease, finally reaching an ultimate value of 1.54 and 1.72 cm3 mol−1 K at 1.8 K, respectively. This behavior is mainly because of spin−orbit coupling effects. The field-dependence magnetizations of 1 and 2 were measured in the whole field at temperatures 1.8, 2.5, 5, and 10.0 K, respectively (insert in Figure 4 for 1 and Figure S4 for 2). The magnetization at 7 T is considerably lower than the theoretically saturated value. Besides, the nonsuperposition of the M versus H/T plots (Figure S5) indicates the presence of strong magnetic anisotropy. For analyzing the experimental data and considering the first-order orbital angular momentum contained in antitriangular prismatic CoII complexes, it is more complex in this system that simple ZFS models are no longer suitable to describe the electronic structures.48 It is not easy to understand the effect of the first order orbital angular momentum in 4T1g ground terms. Given that there is a correspondence between T and P terms, a T ≡ P isomorphism is employed here to model the system.49 The used spin Hamiltonian is given in eq 1, where λ, α, and B02 represent the spin−orbit coupling constant, orbital reduction parameter and crystal field parameter, respectively. 2 2 Ĥ = −αλLŜ ̂ + α 2B20 [3L̂ z − L̂ ] + βH[−αL̂ + geS ]̂
(1)
II
Generally, for a Co complex of Oh symmetry with a weak ligand field, λ is nearly −170.1 cm−1, and α lies below 1.5. The parameters were obtained as λ = −179.99 cm−1, α = 1.5, B02= 106 cm−1 for 1 and λ = −157.52 cm−1, α = 1.49, B02= 150 cm−1 for 2, in good agreement with the experimental results. Deviation of the fitted λ can be caused by the distortion from an octahedral geometry. The obtained B02 parameters are positive, which are in accordance with the EPR g-values, again indicating the easy-plane magnetic anisotropies of 1 and 2.49 Alternating Current Magnetic Measurements. The alternating current (ac) magnetic susceptibility measurements were performed on complexes 1 and 2 to study the slow C
DOI: 10.1021/acs.inorgchem.8b01389 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
Figure 5. Frequency-dependent χM″ ac susceptibilities in Hdc = 1.5 kOe for 1 (a) and 2 (b) and in Hdc = 0.6 kOe for 2 (c).
Figure 6. Cole−Cole curves of complexes 1 (a) and 2 (c). Solid lines represent the best fit with Debye model. Plot of ln(τ/s) versus T−1 for complexes 1 (b) and 2 (d), where the red solid line represents the fitted results using the Arrhenius formula.
behavior was indicated under high magnetic field (>0.8 kOe) in 2 (Figures S6−S9). To explore the magnetostructural correlation, 1.5 kOe was chosen to test the dynamic magnetization due to the longest relaxation time. Furthermore, the relaxation behavior of 2 was also performed under 0.6 kOe of dc field to deeply understand the single relaxation process.
magnetic relaxation behavior. No signal of out-of-phase (χM″) ac susceptibility without external dc field but obvious frequency dependence of χM″ under external dc fields was observed in both 1 and 2, indicating the efficient quantum tunnelling of magnetization (QTM) in them. On the whole, tested dc field for 1 and under low field for 2 (
