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Field-Induced Slow Magnetic Relaxation in an Octacoordinated Fe(II) Complex with Pseudo‑D2d Symmetry: Magnetic, HF-EPR, and Theoretical Investigations Guo-Ling Li,†,‡ Shu-Qi Wu,† Li-Fang Zhang,‡ Zhenxing Wang,§ Zhong-Wen Ouyang,§ Zhong-Hai Ni,*,‡ Sheng-Qun Su,† Zi-Shuo Yao,† Jun-Qiu Li,† and Osamu Sato*,† †

Institute for Materials Chemistry and Engineering & IRCCS, Kyushu University, 744 Motooka Nishi-ku, 819-0395 Fukuoka, Japan School of Chemical Engineering and Technology, China University of Mining and Technology, Xuzhou 221116, People’s Republic of China § Wuhan National High Magnetic Field Center, Huazhong University of Science and Technology, Wuhan 430074, People’s Republic of China ‡

S Supporting Information *

ABSTRACT: An octacoordinated Fe(II) complex, [FeII(dpphen)2](BF4)2· 1.3H2O (1; dpphen = 2,9-bis(pyrazol-1-yl)-1,10-phenanthroline), with a pseudo-D2d-symmetric metal center has been synthesized. Magnetic, highfrequency/-field electron paramagnetic resonance (HF-EPR), and theoretical investigations reveal that 1 is characterized by uniaxial magnetic anisotropy with a negative axial zero-field splitting (ZFS) (D ≈ −6.0 cm−1) and a very small rhombic ZFS (E ≈ 0.04 cm−1). Under applied dc magnetic fields, complex 1 exhibits slow magnetic relaxation at low temperature. Fitting the relaxation time with the Arrhenius mode combining Orbach and tunneling terms affords a good fit to all the data and yields an effective energy barrier (17.0 cm−1) close to the energy gap between the ground state and the first excited state. The origin of the strong uniaxial magnetic anisotropy for 1 has been clearly understood from theoretical calculations. Our study suggests that high-coordinated compounds featuring a D2d-symmetric metal center are promising candidates for mononuclear single-molecule magnets.

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reversal energy barrier of −226 cm−1 in a linear twocoordinated Fe(I) compound.5 After that, many mononuclear complexes based on transition-metal ions, including Fe(I,II,III), Co(II), Mn(III,IV), Ni(I,II), and Cr(I,II), with coordination numbers ranging from 2 to 5 exhibiting SMM behaviors have been reported.6 Another recent method to improve the magnetic anisotropy of a transition-metal ion is using heavymetal ions as “ligands”.7 In general, transition-metal complexes with unsaturated coordination surroundings are far from stable in air. The quest for SMMs in high-coordinated transition-metal complexes not only can afford some air-stable SMMs but also can provide some significant information about the intrinsic nature of the materials. Very recently, SMM behaviors have also been observed in some mononuclear Co(II) compounds with higher-coordinated environments, including in hexa-,8 hepta-,9 and octacoordinated complexes.10 Achieving SMM behaviors in Co(II) compounds are relatively easy since Co(II) is a type of Kramers ion. In contrast, achieving SMM behaviors in highcoordinated mononuclear compounds based on non-Kramers

INTRODUCTION Single-molecule magnets (SMMs), which exhibit slow magnetization dynamics, have enduring appeal to researchers motivated to exploit high-density memory devices, quantum computation, and molecular spintronics.1 Prior efforts in this field were focused on polynuclear SMMs with high ground spin states for the purpose of increasing the magnetization reversal barrier.2 Over the past decade, research interests in SMMs have largely shifted to mononuclear SMMs, since mononuclear compounds can exhibit stronger magnetic anisotropy, which is another important factor determining the reversal energy barrier. It is well-known that strong magnetic anisotropy originating from a large orbital contribution of the lanthanide ions results in numerous excellent mononuclear SMMs.3 In contrast, for 3d transition-metal ions, the orbital angular momentum required for generating magnetic anisotropy is usually quenched by the large ligand field splitting energy. The prevalent strategy in the search for 3d transition-metal-ionbased mononuclear SMMs is decreasing the coordination number to reduce the quenching effect. This strategy started with the discovery of SMM behavior in a trigonal-pyramidal high-spin Fe(II) complex, as reported by Long et al.4 Later the same group achieved the remarkable record of a magnetization © 2017 American Chemical Society

Received: March 24, 2017 Published: July 3, 2017 8018

DOI: 10.1021/acs.inorgchem.7b00765 Inorg. Chem. 2017, 56, 8018−8025

Inorganic Chemistry

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ions, such as Fe(II), remains a challenge. To date, there are still limited examples of high-coordinated mononuclear SMMs based on Fe(II) or other transition-metal non-Kramers ions.11−13 As we know, magnetic anisotropy is deeply influenced by molecular symmetry, which provides a means to achieve SMM behaviors in high-coordinated mononuclear complexes containing non-Kramers ions. We found that the transverse zerofield splitting (ZFS) parameter, E, can be limited to a very small value if the metal complex has a 3- or 4-fold symmetric axis, which may favor the compound to exhibit SMM behavior. This deduction is supported by a six-coordinated Fe(II) complex, [FeII(ptz)6](BF4)2 (ptz = 1-propyltetrazole), which exhibits photoswitchable SMM behaviors.11a [FeII(ptz)6](BF4)2 shows thermal- and light-induced spin transition behaviors and possesses two low-spin-state forms (LSα and LSβ) determined by the rate of lowering the temperature. LSα has Ci symmetry, and LSβ possesses higher symmetry (D3d). SMM behavior can only be achieved for the light-excited state of LSβ. Photoswitchable SMM behavior is also observed for the similar Fe(II) compound [FeII(mtz)6](CF3SO3)2 (mtz = 1-methyltetrazole).11c [FeII(mtz)6](CF3SO3)2 shows a thermal-induced incomplete spin transition, and the high-spin Fe(II) sites at low temperature feature D3d symmetry. Hence, the low-temperature phase of [FeII(mtz)6](CF3SO3)2 exhibits field-induced SMM behavior, and the SMM behavior can be switched off and on via red and green light. Generally, mononuclear complexes characterizing D 2d symmetry are more common in high-coordinated complexes, and these compounds also have a very small E value, which may represent another family of candidates for SMMs. Herein, we present an octacoordinated Fe(II) complex, [FeII(dpphen)2](BF4)2·1.3H2O (1; dpphen = 2,9-bis(pyrazol-1-yl)-1,10-phenanthroline), which possesses an approximately D2d symmetric metal center. Magnetic measurement, high-frequency/-field electron paramagnetic resonance (HF-EPR) studies, and theoretical calculations reveal that this Fe(II) complex features uniaxial magnetic anisotropy and exhibits field-induced slow magnetic relaxation at low temperature. We selected dpphen as the coordination ligand on the basis of the following considerations (Scheme 1): (a) the terminal groups (pyrazole,

Article

RESULTS AND DISSCUSSION

Crystallography. Main structural analysis revealed that compound 1 crystallized in the triclinic space group P1̅ (Table S1 in the Supporting Information). Shape analysis14 indicated that the magnetic unit [FeII(dpphen)2]2+ features an approximately D2d symmetric metal center with a snub diphenoid (J84) FeN8 coordination geometry (Figure 1 and Figure S1 in the Supporting Information). The two N4 planes of dpphen ligands are nearly in vertical form with an interplanar angle of 89.32°. The Fe−Nphen and Fe−Npz bond lengths are almost equal within the range of 2.3088(18)−2.3823(19) Å (Table 1). To the best of our knowledge, there are three other Fe(II) compounds featuring FeN8 coordination surroundings with pseudo-D2d symmetry.15 In comparison with these compounds, 1 has slightly shorter Fe−N bond lengths near the S4 axis but obviously longer bond lengths between the Fe(II) center and the coordination atoms of terminal chemical groups. The very recently reported complex 2 with an FeN4O4 coordination surrounding characterizes a structure similar to that of complex 1.13 The crystal-packing structure of compound 1 shows a layerby-layer stacking of [FeII(dpphen)2]2+ units, as viewed from the a axis direction (Figure S2 in the Supporting Information). In the same layer, the [FeII(dpphen)2]2+ unit interacts with its neighbors through π···π interactions between two pyrazole groups with centroid···centroid distances of 3.710 and 3.727 Å for pyrazole pairs (Figure S3 in the Supporting Information). The adjacent Fe···Fe distances are 8.566 and 8.739 Å, which means that no efficient intermolecular magnetic exchange pathway exists in complex 1. Additionally, the purity of complex 1 was confirmed by powder X-ray diffraction (Figure S4 in the Supporting Information). Static Magnetic Properties. The temperature-dependent direct current (dc) magnetic susceptibility for 1 was obtained under an applied magnetic field of 1000 Oe (Figure 2 and Figure S5 in the Supporting Information). The χmT value is 3.23 cm3 mol−1 K at 300 K, slightly larger than the spin-only value (3.00 cm3 mol−1 K) for an isolated high-spin Fe(II) ion (S = 2, g = 2.0), which is comparable with values for other highspin Fe(II) compounds with pseudo-D2d symmetric metal centers.13,15 When the temperature is lowered, the χmT value remains almost constant until ca. 50 K and then gradually decreases before ca. 15 K. After that, it decreases sharply until finally reaching 2.11 cm3 mol−1 K at 2 K. The diminished χmT value at low temperature may be ascribed to the magnetic anisotropy of compound 1 but can also be caused by intermolecular antiferromagnetic interactions at very low temperature. The isothermal field-dependent magnetization data for compound 1 were collected under an applied field of up to 5.0 T with temperatures of 2−9 K (Figure S6 in the Supporting Information). The magnetization values (2.15−2.71 Nβ) at 5 T are much smaller than the saturation value expected for a magnetic system with S = 2, revealing the existence of magnetic anisotropy in complex 1. Additionally, the obvious separation of the reduced magnetization curves (M vs H/T, inset in Figure 2) in the high-field region further confirms the presence of strong magnetic anisotropy in 1. The reported complex 2 exhibits dc magnetic properties very similar to those for complex 1; however, it has larger χmT (3.58 cm3 mol−1 K at 300 K) and magnetization (3.53 Nβ at 5 T and 2 K) values. As can be seen from the packing structure of compound 1, [FeII(dpphen)2]2+ units are parallel to each other with their

Scheme 1. Synthesis of 2,9-Bis(pyrazol-1-yl)-1,10phenanthroline (dpphen)

pz) located at the 2,9-positions of the rigid phenanthroline (phen) can weaken their ligand field on the metal center and (b) the relatively large steric effect of dpphen can prevent the molecular geometry from seriously deviating from D 2d symmetry. Finally, on the basis of 1 and a very recently reported eight-coordinated Fe(II) complex, [FeII(L)2](ClO4)2 (2; L = 2,9-bis(carbomethoxy)-1,10-phenanthroline),13 the correlation of magnetic properties and structure is carefully discussed; this provides a possible strategy to improve the magnetic properties for these types of Fe(II) SMMs. 8019


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Figure 1. Molecular structure of [FeII(dpphen)2]2+ for complex 1. Fe, C, and N atoms are represented in green, gray, and blue, respectively; H atoms are omitted for clarity. The angle shown is the interplanar angle between the two N4 planes of the two dpphen ligands; the g tensors were determined by calculation.

compound 1 has a magnetic easy axis near the crystalline c axis. Moreover, these magnetic data agree well with the magnetic data along the crystalline c axis obtained by calculation as well as with fitting data based on spin Hamiltonian parameters derived from HF-EPR spectroscopy (Figure S7b). In order to quantify the magnetic anisotropy, the experimental χmT vs T data together with the M vs H data were fitted with the PHI program16 on the basis of the anisotropic Hamiltonian expressed in eq 1

Table 1. Selected Bond Distances (Å) and Angles (deg) for Complex 1 Fe−N(phen) Fe1−N1 Fe1−N6 Fe1−N7 Fe1−N12 Fe···Fe

Fe−N(pz)

2.3454(19) Fe1−N3 2.3265(19) Fe1−N4 2.3130(19) Fe1−N9 2.3823(19) Fe1−N10 8.566/8.739 interplanar angle of dpphen(N4) 89.32

2.3396(19) 2.3218(19)) 2.3088(18) 2.343(2)

2 2 2 H = D(Ŝ z − S(S + 1)/3) + E(Ŝ x − Ŝ y )

+ μB (gx Sx̂ Bx + gySŷ By + gz Sẑ Bz )

(1)

where D, E, Si, Bi, and gi stand for the uniaxial ZFS parameter, transverse ZFS parameter, spin operator, magnetic vector, and g tensor, respectively, and μB is the Bohr magneton. The first two terms in eq 1 represent axial and rhombic crystal-field interactions, and the last term stands for the Zeeman effect. The best-fit parameters are D = −5.85 cm−1, |E| = 0.04 cm−1, gx = gy = 2.04, and gz = 2.16. No admissible fitting can be achieved with a positive D. The negative signal of D and the negligible ratio of |E/D| reveal that compound 1 features strong uniaxial magnetic anisotropy. It is worth noting that the E value approaching zero is in accord with the nature of a magnetic complex with a pseudo-D2d-symmetric metal center. High-Frequency/-Field EPR Spectroscopy. Recently, HF-EPR has become a powerful technique to study the magnetic anisotropy of high-spin Fe(II) compounds.17 In order to directly obtain information about the energy separation between the Ms levels, as well as to further confirm the magnetic anisotropy parameters of compound 1, tunablefrequency HF-EPR at 10 K and variable-temperature HF-EPR at 170 GHz experiments were performed on a ground polycrystalline sample. As expected, complex 1 generated high-quality EPR spectra with strong, well-resolved resonances in the frequency range 120−258 GHz (Figure S8 in the Supporting Information). The field-dependent HF-EPR values at 170 GHz measured at different temperatures together with corresponding simulations based on a negative (left) or a positive (right) D value are shown in Figure 3. When the temperature is lowered, the EPR resonance at approximately 0.3 T becomes weak and the signal at approximately 4.9 T is

Figure 2. Temperature dependence of χmT measured under an applied magnetic field of 1000 Oe for 1. Inset: reduced magnetization data at 2−9 K for 1. The solid lines represent the best fits to the experimental data using the PHI program.

magnetic easy axis close to the crystalline c axis (the angle between the gz axis and the c axis is 15.63°; Figure S7a in the Supporting Information). Hence, to further confirm that compound 1 exhibits a uniaxial magnetic anisotropy, dc magnetic measurements using a single-crystal sample were performed along the c axis in the temperature range 2−100 K (Figure S7b). The χmT value is 3.64 cm3 mol−1 K at 100 K, which is obviously larger than that of a powder sample at the same temperature. With decreasing temperature, the χmT value gradually increases to 5.89 cm3 mol−1 K at 5 K and then decreases to 4.95 cm3 mol−1 K at 2 K, confirming that 8020


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Inorganic Chemistry

Figure 3. Variable-temperature HF-EPR spectrum of 1 with simulated spectra at 170 GHz, which prove that the sign of the zero-field splitting parameter D is negative.

the PHI program.16 The EPR resonances at 120 and 258 GHz were representatively identified. The weak EPR resonance at 120 GHz at 2.0 T is the energy transition of |+1⟩ → | 0⟩ states with the field along the z direction, and the resonance at 120 GHz at 3.8 T belongs to the energy transition of |−1⟩ → |+1⟩ states with the field along the x (y) direction. The EPR resonances at 2.0, 3.2, and 6.9 T at 258 GHz all originate from energy transitions between different states with the field along the x (y) direction, which can be unambiguously identified as the energy transitions between the following states: |−1⟩ → |0⟩, |+1⟩ → |0⟩, and |−1⟩ → |+1⟩. There is no resonance observed for z components at higher frequencies, which is due to the fact that D and the g factor are randomly distributed around their mean values, thus significantly broadening the EPR signals for the z components at higher frequencies.18 Dynamic Magnetic Properties. Low-temperature slow magnetic relaxation is a common property for complexes with magnetic anisotropy. To detect the dynamic magnetization behavior of compound 1, its ac magnetic susceptibility was measured under zero and several nonzero external dc magnetic fields (Figure S10 in the Supporting Information). Under zero applied dc field, complex 1 exhibits very fast relaxation behavior with no detectable out-of-phase ac signal (χm″) in the frequency range of 1−1500 Hz. This can be attributed to magnetic tunneling of the ground state, which is not uncommon for mononuclear Fe(II) SMMs.4,6c,11,19 Under a 250 Oe applied dc field, a nonzero χm″ signal with a well-resolved peak at around 165 Hz was observed. When the external dc field was increased, the intensity of the χm″ signal obviously increased, accompanied by the maximum slightly shifting toward lower frequency. This variation becomes small when the external dc field is higher than 1500 Oe. It should be noted that there is a weak ac signal in the low-frequency region (1−5 Hz), indicating the existence of other relaxation processes at low temperature. The magnetic relaxation at low frequency is likely related to the insufficient separation of the magnetic units.20 This weak ac signal merges with the main relaxation signal when the external dc field is higher than 1000 Oe, which prevents some of the data from being fitted well. Hence, the data in the low-frequency region (1−5 Hz) were excluded in the following analyses. The dc field dependent magnetic relaxation time (τ) was determined by fitting the variable-frequency in-phase and outof-phase ac susceptibilities (Cole−Cole plot) using a generalized Debye model (Figure S11 in the Supporting Information). The best-fit parameters are summarized in Table S2 in the Supporting Information, and τ versus H is plotted in Figure S12 in the Supporting Information. The τ value

enhanced. It is clear that the simulations using a negative D value are in accordance with the experimental data. In contrast, the simulations based on a positive D value change in opposite tendency with the experimental data. This result undoubtedly indicates that this compound exhibits uniaxial magnetic anisotropy. To derive the magnetic anisotropy parameters from these HF-EPR spectra, the variable-frequency resonances were combined together to construct a two-dimensional map of resonant field versus frequency and transition energy, which is shown in Figure 4. A least-squares fit was carried out on the

Figure 4. Resonance field vs microwave frequency (quantum energy) of the EPR transition for 1. The black squares are experimental data, while the red and blue solid lines are the best fits to experimental data with magnetic field along the z and x (y) directions, respectively.

entire array of HF-EPR resonances, leading to the best-fit fullsetting spin Hamiltonian parameters of D = −6.00(3) cm−1, |E| = 0.04(1) cm−1, gx = gy = 2.04(2), and gz = 2.10(5). These results are in reasonable agreement with the parameters derived from the dc magnetic data. Additionally, the simulated curves based on the above parameters agree well with the experimental points, which clearly confirms the original assignment of the observed resonances. In order to assign these particular EPR resonances, the simulated field-dependent Zeeman energy splitting plots (Figure S9 in the Supporting Information) for a spin quintet (S = 2) with field along z and x (y) directions were respectively constructed using the above spin Hamiltonian parameters in 8021


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Inorganic Chemistry monotonically increased with increasing applied dc field within the range 500−2000 Oe, which confirms the existence of the tunneling effect at 2 K for compound 1. The α values are larger than 0.2 at high applied dc fields, revealing a relatively broad distribution of relaxation times, which may be related to the fact of a powder sample being used in the measurement or the superposition of several magnetic relaxation processes in the low-frequency range. According to the above analysis, the variable-temperature ac magnetic susceptibility at different frequencies (Figure 5) and

Figure 6. Temperature dependence of relaxation time shown by a ln τ vs T−1 plot for 1. The solid line is the best fit to the data on the basis of eq 2.

As shown in Figure 6, a well-fitted curve through all of the data was obtained, which affords an effective energy barrier of 17.0 cm−1, a preexponential factor of τ0 = 1.55 × 10−7 s, and a tunneling relaxation time of τq = 0.017 s for 1. The Ueff value is lower than the values given by U = D|S|2 with the D value determined from the dc magnetic data (23.4 cm−1) or HF-EPR spectroscopy (24.0 cm−1), but close to the energy gap (∼18.0 cm−1) between the pseudodegenerate ground state (|+2⟩, |−2⟩) and the pseudodegenerate first excited state (|+1⟩, |−1⟩) obtained by calculation. This feature indicates that the Orbach process is through the energy barrier of the first excited state, as proposed for some other reported Fe(II) compounds showing field-induced SMM behaviors.11,13,21 The reported octacoordinated Fe(II) complex 2 also exhibits field-induced slow magnetic relaxation.13 The maximum of χm″ for complex 2 appears at a much higher frequency range (>690 Hz) in comparison to that for complex 1. However, the reported results show that compound 2 possesses a higher effective reversal energy barrier (Ueff = 39.1 cm−1) in comparison to that of complex 1; this indicates that a higher reversal energy barrier does not generally lead to slower magnetic relaxation. This may be due to the fact that complex 2 has a large rhombic ZFS parameter (E = 0.08 cm−1) originating from a large deviation of the Fe(II) coordination environment from the ideal D2d symmetry (86.38° for the interplanar angle of two N2O2 planes). Theoretical Calculations. To provide insight into the peculiar electronic structure and thus the origin of the magnetic anisotropy for compound 1, correlated ab initio calculations by the well-established CASSCF/NEVPT2 method have been carried out with the ORCA 4.0.0 package.21 The calculations give a relatively small excitation energy of 6742 cm−1 between the lowest (dxy2dxz1dyz1dz21dx2−y21) and the highest quintet configuration (dxy1dxz1dyz1dz21dx2−y22), confirming the weak overall ligand field from the tetradentate ligands (dpphen). As shown in Figure 7 and Figure S15 in the Supporting Information, the dxy orbital is located on the lowest energy level with relatively large energy gaps to the dxz, dyz, and dz2 orbitals due to the weaker π* interaction in comparison with the σ* interaction. The quasi-degeneracy of the dxz and dyz orbitals results from the slight deviation of the geometry of [FeII(dpphen)2]2+ from D2d symmetry. The stronger σ* repulsion between the Fe(II) center and the pyrazole groups leads to the highest orbital being dx2−y2.

Figure 5. Temperature dependence of ac magnetic susceptibility under 1000 Oe for 1.

isothermal frequency-dependent ac magnetic susceptibility (Figure S13 in the Supporting Information) were obtained under an external dc field of 1000 Oe for 1. These ac data show obvious temperature and frequency dependence. The χm″ vs T curve produced a peak at ∼2.6 K at a frequency of 1488 Hz, and the maximum of χm″ obviously shifted to the lowertemperature region with decreasing frequency. The χm″ vs v curve exhibited a peak at ∼95 Hz at 1.9 K, and the peak quickly shifted toward higher frequencies upon increasing the temperature. The magnetic relaxation time was extracted by fitting the Cole−Cole plot (Figure S14 in the Supporting Information) to a modified Debye model. A plot of ln τ versus T−1 is shown in Figure 6. The best-fit α values are distributed in the range 0.09−0.18 (Table S3 in the Supporting Information), which is an indication of multiple relaxation pathways and the remainder of a tunneling process for 1. Therefore, the dependence of ln τ on T−1 was fitted on the basis of an equation combining Orbach and tunneling terms: τ −1 = τ0−1 exp( −Ueff /kT ) + τq−1

(2) 8022


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and Orbach with an effective energy barrier close to the energy gap between the ground state and the first excited state. Our work on this Fe(II) complex and the result of a recently reported octacoordinated Fe(II) complex confirm that D2d symmetry can limit the rhombic ZFS to a small level for high-coordinated magnetic complexes, which is in favor of them exhibiting SMM behavior. More importantly, these results suggest that both molecular symmetry and ligand field should be carefully taken into account for the further design of interesting SMMs in high-coordinated complexes.

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EXPERIMENTAL SECTION

Starting Materials. 2,9-Dichlorophenanthroline (Sigma-Aldrich), pyrazole (TCI), FeII(BF4)2·6H2O (Sigma-Aldrich), K2CO3 (Wako), and all solvents (Wako, reagent grade) were used as received. Synthesis of dpphen (Scheme 1). 2,9-Dichlorophenanthroline (0.50 g, 2.00 mmol), pyrazole (0.32 g, 4.80 mol), and K2CO3 (0.66 g, 4.80 mmol) were mixed in 10 mL of DMF, and the mixture was kept at 120 °C for 3 days. The resulting mixture was slowly cooled to room temperature and then poured into 20 mL of cold water. The target ligand, dpphen, was obtained as a white solid which was collected by filtration, washed with cold water several times, and dried in air. Yield: 0.56 g, 89%. 1H NMR (600 MHz, 25 °C, CDCl3): δ 9.10 (d, J = 2.4 Hz, 2H), 8.40 (dd, J = 20.4 Hz, 8.6 Hz, 4H), 7.84 (d, J = 0.8 Hz, 2H), 7.80 (s, 2H), 6.59−6.62 (m, 4H). Synthesis of [FeII(dpphen)2](BF4)2·1.3H2O (1). FeII(BF4)2·6H2O (16.85 mg, 0.05 mmol) in methanol (10 mL) was added to a suspension of dpphen (31.20 mg, 0.10 mmol) in acetonitrile (5 mL). The resulting orange-red solution was placed under ambient conditions without disturbance. Orange-black crystals can be obtained by evaporation of the solvent within 1 week. Yield: 67%. Anal. Calcd for C36H26.6N12O1.3FeB2F8 (FW 878.01 g mol−1): C, 3.06; H, 49.27; N, 19.15. Found: C, 3.03; H, 49.37; N, 19.19. Physical Measurements. Direct current (dc) and alternating current (ac) magnetic susceptibilities were collected on a Quantum Design MPMS-5S SQUID magnetometer using a crushed crystal sample which was fixed by eicosane. HF-EPR measurement was performed on a locally developed spectrometer at Wuhan National High-magnetic Field Center with a pulsed magnetic field of up to 30 T. Elemental analysis was carried out on a Yanaco CHN CORDER MT-6 elemental analyzer. NMR data were collected on a Bruker AVANCEIII 600 NMR spectrometer. Crystallographic Data Collection and Structure Refinement. Single-crystal X-ray data for compound 1 were collected on an FR-E + CCD diffractometer with Mo Kα radiation at 123 K. The structure of complex 1 was solved using a direct method and refined with the fullmatrix least-squares technique using the SHELXTL 2014 program.22 All non-hydrogen atoms were anisotropically refined, and all hydrogen atoms were located by the HFIX command in the SHELXTL program. The conditions for X-ray data collection and structure refinement are available in Table S1 in the Supporting Information. The coordination bond lengths and some significant geometrical parameters are presented in Table 1. Computational Details. Geometric optimization of complex 1 was performed with the Gaussian09 package23 using the density functional theory (DFT) method. The coordinates of all hydrogen atoms were optimized, and the coordinates of the other atoms were frozen to the X-ray-determined structure. The BP86 functional and TZVP basis was used in the optimization.24 The ground and excited state energies as well as the wave functions for complex 1 were obtained from ab initio calculations. The calculations were performed using the ORCA package (version 4.0.0), in which the completeactive-space self-consistent-field (CASSCF) module for static correlation and the N-electron valence perturbation theory (NEVPT2) for dynamic correlation were used.19 For simplicity, the active space only included the six electrons on the 3d orbitals of the Fe center. Five S = 2 and 30 S = 1 states were taken into consideration.

Figure 7. (left) Calculated d orbital splitting and electronic configuration of the ground state for compound 1. (right) Calculated splitting of the free ion quintet state under the effect of a D2dsymmetric ligand field.

Spin−orbit coupling was taken into account by quasidegenerate perturbation theory, which leads to the splitting of the ground state (Figure S16 in the Supporting Information). Projecting the lowest five energy states of the ground state obtained from CASSCF/NEVPT2 calculations onto a spin quintet S = 2 produced a full set of spin Hamiltonian parameters of D = −6.32 cm−1, |E| = 0.04 cm−1, gx = gy = 2.05, and gz = 2.11, which are in good agreement with the results derived from the HF-EPR spectra. The very small E value was generated by the nonrigorous D2d symmetry of the actual molecular structure, leading to quantum tunneling relaxation between pseudodoublets. Regardless of the small deviation from ideal symmetry, the interaction between the 5B2 and 5B1 states contributes negatively (−6.36 cm−1) to the overall D value, leading to the highly axial nature of the Fe(II) center. The contribution from the interaction between the 5E states (+2.42 cm−1) is approximately canceled by the contributions from interactions between different spin multiplicities (−2.50 cm−1). On the basis of the above theoretical analysis, one possible way to improve the axial anisotropy for this species of compounds is to enhance the donating ability of the coordination atoms near the S4 axis to increase the antibonding interactions or to reduce the donating effect of terminal groups to weaken the antibonding interactions. This strategy is supported by compound 2, for which two ester groups exert a weaker ligand field on the Fe center, and thus the compound shows stronger uniaxial anisotropy with D = −11.7 cm−1.13 However, compound 2 exhibits faster relaxation behavior in comparison with complex 1, which may be due to the fact that complex 2 features a large deviation of Fe(II) coordination environment from the ideal D2d symmetry or distinct crystal lattice effects. This suggests that both molecular symmetry and ligand field should be carefully considered for further development of SMMs in high-coordinated complexes.

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CONCLUSIONS The work here presents an octacoordinated Fe(II) complex with a pseudo-D2d-symmetric metal center showing uniaxial magnetic anisotropy with a negative axial ZFS and a small rhombic ZFS. The magnetic anisotropy of this compound has been well determined by magnetic measurements, HF-EPR spectroscopy, and ab initio calculations. Similar to some other mononuclear Fe(II) SMMs, the relaxation behavior of this Fe(II) complex is combined relaxation processes of tunneling 8023


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Inorganic Chemistry Basis sets of TZVP quality and the auxiliary sets (TZVP/C) were used through all ab initio calculations.24,25

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00765. Structural and magnetic parameters, packing diagrams, and additional magnetic, HF-EPR, and calculation data (PDF) Accession Codes

CCDC 1528305 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for Z.-H.N.: [email protected]. *E-mail for O.S.: [email protected] ORCID

Osamu Sato: 0000-0003-3663-5991 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS G.-L.L. and S.-Q.W. thank the China Scholarship Council for support. This work was supported by the Natural Science Foundation of Jiangsu Province (BK20140181) and by JSPS KAKENHI Grant Numbers 17H01197, 16H00843, and 15H01018.

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