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Synergic on/off Photoswitching Spin State and Magnetic Coupling between Spin Crossover Centers Jun-Li Wang,† Qiang Liu,† Yin-Shan Meng,† Hui Zheng,‡ Hai-Lang Zhu,† Quan Shi,‡ and Tao Liu*,† †

State Key Laboratory of Fine Chemicals, Dalian University of Technology, 2 Linggong Rd., 116024 Dalian, China Thermochemistry Laboratory, Liaoning Province Key Laboratory of Thermochemistry for Energy and Materials, Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China

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ABSTRACT: The existence of a correlation between spin crossover and dielectric properties is a hot topic in the field of multiresponse materials, which has potential applications in the memory devices, switches, and sensors. One formidable challenge is the simultaneous and rapid on/off switching of spin states of the spin carriers and magnetic coupling between them, which is crucial for both reversible photomagnetic behavior and variations in dielectric properties. Here, we report a dinuclear Fe(II) spin crossover complex, wherein each Fe(II) center exhibits an interconversion between FeIIHS (HS = high-spin) and FeIILS (LS = low-spin) achieved upon heating and cooling. Moreover, the spin state of respective Fe(II) ions and the antiferromagnetic interaction between them can be switched bidirectionally under alternating irradiation with 532 and 808 nm light, resulting in interconversion between paramagnetic and diamagnetic properties. Interestingly, the spin crossover can also induce variations in dielectric tensors. This result provides a strategy to simultaneously and bidirectionally switch spin state, magnetic coupling, and dielectric properties using external stimuli.

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INTRODUCTION The design of complexes that exhibit reversible and controllable changes in multifunction upon exposure to an external stimulus is a top topic in the field of molecular materials and attracts lots of attention, because these cooperative properties might result in novel phenomena and provide promising applications in the high-density data storage, sensors, and switches.1−5 Spin crossover (SCO) complexes emerged as an excellent candidate to achieve synergic changes in spin state, magnetic, and dielectric response by temperature changes or light irradiation.6−18 In particular, spin crossover complexes undergoing a LIESST (Light-Induced Excited Spin State Trapping) effect have received significant attention,19−25 because light can be conveniently switched on/off and provide an impetus for interconversion between two electronic states with low power dissipation, superior photostability over successive cycles, and high selectivity. Moreover, the light-induced process is in picosecond scale on the molecular level,26−29 thus providing a fast response to functional signals upon light irradiation. The LIESST effect is unidirectional in most systems, which means that the metastable HS* (HS* = photoinduced high-spin) state from LS could be induced via light irradiation and recovered by thermal treatment. Up to now, complexes showing the bidirectional light-induced SCO are limited and only achieved for one respective metal site.22,30−33 Noteworthily, it is possible to introduce more SCO sites in one molecule and control their spin states and magnetic couplings by the bidirectional light© 2017 American Chemical Society

induced spin transition. Moreover, the increased cooperative interactions between SCO sites will be supposed to increase the lifetime of the metastable HS* state.34,35 On the other hand, the SCO behavior is sometimes accompanied by significance changes in the dielectric tensor which governs the polarization response of the material to applied electric fields.15−18 The existence of a correlation between spin crossover and dielectric properties is also promising. Therefore, the introducing of dielectric response may offer the possibility to improve the functional properties of materials. Some challenges are needed to solve for construction of an SCO system holding both switchable photomagnetic and dielectric properties. One is to realize bidirectional SCO for more metal sites and control the magnetic couplings between them via light irradiation with different visible light sources. The other is to introduce asymmetric SCO centers, because molecular polarization likely occur in a spin transition process. To achieve this goal, the light-responsive asymmetric SCO units are required to connect with each other by short bridging ligands that provide the suitable ligand field for the occurrence of spin transition and transmit the magnetic interaction effectively. Herein, we report an asymmetric dinuclear Fe(II) SCO complex [Fe2L2(μ-L)3(NCSe)4]·2DMF·2H2O (1) (L = 1-naphthylimino-1,2,4-triazole), wherein the electronic state Received: June 28, 2017 Published: August 16, 2017 10674

DOI: 10.1021/acs.inorgchem.7b01633 Inorg. Chem. 2017, 56, 10674−10680

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Inorganic Chemistry changes of the two Fe(II) ions and the magnetic coupling between them can be reversibly achieved by 532 and 808 nm light. The difference of the local electrical dipoles in the HS and LS states leads to a significant change in dielectric constants during spin transition.

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RESULTS AND DISCUSSION X-ray Crystal Structure. The yellow crystals of 1 were directly obtained from the reaction mixture by the slow diffusion of diethyl ether. Complex 1 crystallized in the orthorhombic P212121 space group. Two crystallographically independent Fe(II) ions are observed in the dimer unit and bridged by three bidentate ligands (Figure 1). The N6

Figure 2. (a) Raman and (b) IR spectra of 1 at different temperatures.

and were associated with the stretches of NCSe− in the LS state. The coexistence of two modes indicates a mixture of HS and LS states in the sample near the transition temperature. Lowering the temperature further led to a significant increase in stretching intensity at 2106 cm−1 and a clear decrease at 2072 cm−1, thus confirming a thermally induced SCO behavior. Similar to the Raman spectra, the infrared spectra of 1 in the region of NCSe− stretching modes at different temperatures were also recorded to clarify the thermally induced SCO process (Figure 2b). Complex 1 displayed three typical νNCSe stretching bands at 2062, 2072, and 2085 cm−1 at 210 K, which were assigned to the HS state. As compound 1 was slowly cooled to 140 K, the intensity of νNCSe stretching bands corresponding to the HS state decreased, and three new bands were clearly observed at 2095, 2106, and 2116 cm−1. The three new bands were typical for the LS species. The intensity of LS modes increased further upon cooling to 100 K, and that of the HS modes decreased. The observed spectral changes further confirm the occurrence of thermally induced SCO behavior. Mössbauer Spectra Analysis. The phase purity of the bulky samples was confirmed by powder X-ray diffraction (PXRD). The experimental pattern closely matches with the simulated one from single-crystal diffraction data, indicating the identical structure of bulky samples and the measured single crystal (Figure 3). The electronic states of the iron(II) ions in 1 were further characterized by 57Fe Mössbauer spectra at room temperature and 80 K (Figure 4). At room temperature, a single quadruple-split doublet was observed with Mössbauer parameters of δ (isomer shift) = 1.09 mm s−1 and ΔEQ (quadruple splitting) = 2.19 mm s−1, characteristic of the FeIIHS ions. The relative area fraction was 100%, which was consistent with the results of Raman and IR spectra corresponding to the HS phase. As the temperature decreased, the doublet attributed to FeIIHS ions gradually disappeared. An additional doublet with δ = 0.41 mm s−1 and ΔEQ = 0.57 mm s−1 was observed at 80 K, which could be ascribed to the LS species.38 The relative area fraction of FeIILS ions was 100%,

Figure 1. Crystal structure of complex 1 at 280 K. The pink, gray, blue, and orange spheres represent Fe(II), C, N, and Se atoms, respectively. The H atoms of the ligand, H2O, and DMF molecules are omitted for clarity.

coordination environment of each octahedral Fe(II) center is completed by two coordinated NCSe− anions, one monodentate ligand, and three bridging ligands. At 280 K, the Fe1− N and Fe2−N bond lengths are 2.101(11)−2.214(8) Å and 2.134(9)−2.209(8) Å, respectively, which are typical for FeIIHS ions (Table S2). Upon cooling to 92 K, both the average Fe1− N and Fe2−N bond lengths shorten by approximately 0.21 Å (Table S3) and the corresponding Fe1···Fe2 distance decreased from 3.950 to 3.632 Å, indicating that thermally induced spin transition occurs on two Fe(II) sites. When complex 1 was cooled from 280 to 92 K, the distortion parameter, Σ, which is defined as the sum of the absolute deviations of the 12 cis angles at the octahedral Fe(II) center from the ideal value of 90°,36,37 changed from 24.3° to 11.6° for the Fe1 ion and from 20.2° to 13.1° for the Fe2 ion. The relatively small Σ values at low temperature are consistent with the formation of a regular octahedron in the LS state. Variable-Temperature Raman and Infrared (IR) Spectra. Raman and infrared (IR) spectra are useful for characterizing the spin states of Fe(II) ions, since the stretching frequencies of NCSe− ligands coordinating to the metal ions are very sensitive to the electronic states of the metal ions. One single crystal of 1 was excited with a He−Ne laser (632.8 nm) at different temperatures, and the corresponding Raman spectra are depicted between 2000 and 2200 cm−1 in Figure 2a. The two peaks observed at 2058 and 2072 cm−1 were assigned to the stretches of NCSe− coordinating to the FeIIHS ions at 296 K. This result is in agreement with the HS state of 1 shown by the crystal structure at 280 K. As the temperature decreased, two new bands began to appear at 2106 and 2115 cm−1 at 150 K 10675

DOI: 10.1021/acs.inorgchem.7b01633 Inorg. Chem. 2017, 56, 10674−10680

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= 0 for FeIILS) is 2R ln[(2SHS + 1)/(2SLS + 1)] = R ln 25 (= 26.76 J K−1 mol−1), where R is the gas constant.39−41 The entropy contribution from the change in the orbital degeneracies could be neglectful because the orbital degeneracy has been lifted by the structural distortion to give a nondegenerate orbital, which are also observed in other SCO complexes.39,42 The excess entropy beyond the contribution from the spin multiplicity mainly arises from intramolecular vibrations corresponding to the significant changes in Fe−N stretching and N−Fe−N deformation vibration as described in the crystal structure, Raman and IR spectra. These results suggest that thermally induced spin crossover from LS to HS state is an entropy-driven process.39 Magnetic and Photomagnetic Properties. Magnetic susceptibility measurements were performed in the temperature range of 2−300 K in both cooling and heating modes (Figure 6a). At 300 K, the χT value of 1 was 6.51 cm3 mol−1 K, which is

Figure 3. Experimental and simulated powder X-ray diffraction patterns for 1.

Figure 4. 57Fe Mössbauer spectra of complex 1 at room temperature and 80 K (black dots: observed, red line: HS phase, blue line: LS phase).

indicating that all FeIIHS ions were transformed to FeIILS ions at this temperature. Heat Capacity. To study the driving force of spin crossover, the heat capacity of 1 was measured using a Physical Property Measurement System. A broad heat capacity anomaly centered at 165 K was observed due to the phase transition during heating (Figure 5), indicating the occurrence of spin crossover. Figure 6. (a) Thermal behavior of χT for 1 before and after irradiation in a DC magnetic field of 1000 Oe. (b) Plot of χT vs time under cycles of successive irradiation at 532 nm (green circles) and 808 nm (red circles) at 10 K.

close to the theoretical value of 6.00 cm3 mol−1 K of two isolated FeIIHS ions (S = 2, g = 2.0). The χT value remained nearly constant above 200 K and reached to 6.31 cm3 mol−1 K at 200 K, suggesting that two Fe(II) ions are all in the HS state. Upon further cooling, the χT values decreased steeply in a onestep fashion in the temperature range of 200−70 K, indicating the occurrence of the sharp SCO with a transition temperature T1/2 of 164 K. To probe the possibility of the LIESST effect, samples of 1 were irradiated with green light (532 nm) for 2 h at 10 K and then reheated from 2 K after switching off the laser. The χT value of 1 first increased to a maximum of 2.54 cm3 mol−1 K at 48 K because of the photoinduced spin transition from FeIILS to metastable HS* Fe(II). The superexchange channels were also activated by the LIESST effect. As the temperature decreases, the χT value after irradiation decreases sharply, indicating a dominant antiferromagnetic interaction between two metastable HS* Fe(II) ions transmitted via the rigid triple N1,N2−triazole bridges. However, it is hard to further extract the exact coupling constant because of the

Figure 5. Plot of the molar heat capacity of 1 vs temperature. The solid curve shows the estimated normal heat capacity.

The enthalpy and entropy changes of the phase transition were determined to be ΔH = 13.62 (0.13) kJ mol−1 and ΔS = 85.96 (0.86) J K−1 mol−1, respectively. In general, the entropy change arising from spin crossover consists of contributions from the difference in spin multiplicity, orbital degeneracy, and density of vibration modes. The entropy gain from the difference in spin multiplicity for two FeII centers (SHS = 2 for FeIIHS and SLS 10676

DOI: 10.1021/acs.inorgchem.7b01633 Inorg. Chem. 2017, 56, 10674−10680

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Inorganic Chemistry nondeterminacy of the pure HS* state. The χT value was rapidly thermally quenched upon heating to 50 K, indicating that SCO can be induced by light irradiation and recovered by thermal treatment. The HS phase at room temperature has an absorption band around 830 nm (Figure S3), which can be assigned to the d−d transition (5T2 → 5E) of the FeIIHS ion. Thus, an 808 nm diode laser was used as the light source to probe the photosensitivity of the HS* state. A new irradiation at 532 nm was performed and followed by irradiation at 808 nm at 10 K. The χT value decreased rapidly from 1.23 to 0.47 cm3 mol−1 K, which is slightly higher than the value of 0.42 cm3 mol−1 K before irradiation, thus indicating an incomplete and reversible spin transition from the HS* to the LS state via the reverse-LIESST (RLIESST) effect (Figure 6b). The incomplete RLIESST effect may be attributed to the spectral overlap between the 5T2 → 5E band of FeIIHS ions and the spin-forbidden 1A1 → 3T1 and 1A1 → 3T2 bands of FeIILS ions, which has been shown in other bidirectional light-induced SCO complexes.23,30,32 The photoreversibility effects were repeatedly observed by alternating irradiation with 532 and 808 nm lasers at 10 K. Repetitive cycling of the metastable HS* state and the LS ground state shows a reversible light-induced SCO process, further confirming the bidirectional on/off switching of magnetic state. After switching off the 808 nm light source, the χT values increased slowly with increasing temperature, and the curve of χT vs T nearly overlapped with the one before irradiation (Figure 6a), indicating that the antiferromagnetic interaction between two HS* Fe(II) ions was switched off by the RLIESST effect. The relaxation kinetics of the photoinduced metastable HS* state after switching off the irradiation was investigated at different temperatures. The decay of magnetization, normalized to the photoinduced fraction γ (Figure 7a), was fitted by a stretched exponential law [eq 1]:43 γ(t ) = γ(0)exp( −t /τ )β

Figure 7. (a) Relaxation kinetics of the photoinduced fraction in normalized values (t = 0, converted fraction = 1) of 1 at 5 K (red □), 10 K (green □), 15 K (blue □), 20 K (cyan □), 30 K (magenta □), 40 K (dark yellow □), 50 K (navy □), and 60 K (purple □) in the dark. The black solid line is the stretched-exponential fit to eq 1. (b) ln τ versus 1/T curve. The green straight line is obtained with Arrhenius relationship in the high-temperature range.

time increased, the intensity of the three HS modes increased and that of the remaining two LS modes decreased (Figure 8a); this phenomenon is consistent with the photoinduced spin transition from the LS state to the HS* state. When the irradiation was switched to the 808 nm laser, the intensity of the HS vibration band decreased and that of the LS vibration band increased (Figure 8b). This result can be attributed to the reverse photoinduced spin transition from the HS* state to the LS state. Both the disappearance of the HS mode at 2090 cm−1 and the appearance of the LS mode at 2095 cm−1 were caused by irradiation for 85 min. Finally, the photoinduced LS state was irradiated with 532 nm light, leading again to an HS* spectrum (Figure 8c). These results agree well with the photomagnetic measurements and provide spectroscopic evidence for the bidirectional photoinduced SCO behavior. The thermal stability of the photoinduced HS* state was further investigated by following the decay of the IR transmittance as a function of increasing temperature (Figure 8d). Similar to the photomagnetic effect, the infrared spectrum after saturation under 532 nm irradiation can be restored to the thermodynamically favored HS state by thermal treatment. During the heating process, the two LS vibration bands at 2106 and 2116 cm−1 appear at 50 K and again disappear at 190 K. The former are related to the thermal relaxation of HS* → LS and the latter are caused by thermally induced LS → HS spin transition. The HS vibration bands (2090 cm−1) disappeared at 85 K and then reappeared at 140 K, indicating the occurrence of HS* → LS → HS spin transition. Dielectric Property. The dynamic dielectric experiment was also carried out in the 200−1000 kHz frequency range to see the thermal variation of the dielectric constant (ε = ε′ − iε″) upon spin crossover. With decreaseing temperature, the real part (ε′) exhibited an anomaly at approximately 165 K,

(1)

In the low-temperature region (5−20 K), the HS* → LS relaxation is very slow and probably attributable to a nearly temperature-independent tunneling process of the system from the metastable HS* state to the normal LS state. These results confirm the stability of the metastable HS* state and that the conversion of HS* → LS at 10 K was activated by the 808 nm laser irradiation rather than the thermal relaxation. In the hightemperature range, thermal activation of the relaxation process becomes more important. The relaxation time τ obeys an Arrhenius relationship (τ(T) = τ0 exp(ΔE/(kBT)), where τ0 and ΔE are the pre-exponential factor and energy barrier, respectively. The plot of ln τ vs 1/T (Figure 7b) shows the linearity in the high-temperature range (30−60 K), with an activation energy ΔE = 282.7 (±20.0) cm−1 and a preexponential factor τ0 = 5.79 (8) s. The activation energy of the thermally induced relaxation is comparable with those of previously reported binuclear Fe(II) SCO complexes.44 These results indicate that the relaxation process of HS* → LS becomes thermally activated in the high-temperature region. Irradiation-Time Dependence of IR Spectra. The irradiation-time dependence of the IR spectra was assessed in the dark to further verify the occurrence of the bidirectional SCO behavior by alternating irradiation with 532 and 808 nm lasers at 10 K. Both the appearance of the HS mode at 2090 cm−1 and the disappearance of the LS mode at 2095 cm−1 were induced by irradiation at 532 nm for 10 min. As the irradiation 10677

DOI: 10.1021/acs.inorgchem.7b01633 Inorg. Chem. 2017, 56, 10674−10680

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Figure 8. (a) Irradiation-time dependence of the IR spectra of 1 irradiated at 532 nm at 10 K. (b) Photoreversibility upon irradiation at 808 nm at 10 K. (c) Irradiation-time dependence of the IR spectra irradiated at 532 nm for the light-induced LS phase generated from an 808 nm laser. (d) Variable-temperature solid-state infrared spectra in the warming mode for 1 after irradiation using a 532 nm laser.

both dielectric loss (tan δ) and the imaginary part (ε″) during the spin transition process (Figure S5). The peak temperature (T) varied from 230 K at 200 kHz to 256 K at 1000 kHz (Figure 9b). Complex 1 does not exhibit the change of crystal symmetry, and the bulk sample can be seen as an assembly of randomly orientated dipole moments in the spin transition process. Therefore, this anomaly of dielectric constants is not so large. The anomaly of dielectric properties is related to the intramolecular structural change but not the magnetoelectric effect, which is confirmed by Raman and IR studies. The structural change causes changes in the local polarizability that contributes to the dielectric constant. The large entropy gain extracted from heat capacity further confirms the conclusion.

indicating that a phase transition occurred (Figure 9a). This is in agreement with the magnetic susceptibility and heat capacity

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CONCLUSIONS In summary, we have successfully realized the photoswitching in the bidirectional light-induced spin transition compound [Fe2L2(μ-L)3(NCSe)4]·2DMF·2H2O. The electronic structure, reversible light-induced spin transition, magnetic couplings in the metastable HS* state, and the existence with the anomaly of dielectric properties are fully characterized and discussed. Each Fe(II) ion exhibits bidirectional light-induced spin transition. Moreover, the antiferromagnetic interaction between two SCO sites can be switched on/off by the selected wavelength of the laser, representing the first example with on/off switchable magnetic interaction between SCO centers via the bidirectional LIESST effect. Meanwhile, the dielectric constant also makes a response to the spin crossover process, although there is no direct correlation between spin state changes and dielectric anomaly. Further work on this topic is ongoing.

Figure 9. Temperature dependence of (a) dielectric constant (ε′) and (b) dielectric loss (tan δ) for 1 at different frequencies.

measurements. On heating, the ε′ increased and returned to the initial value without a thermal hysteresis loop. During the HSto-LS transition, the electronic polarizability and the relative permittivity decrease as the volume and distortion of the HS molecule are larger than that of the LS one. Moreover, the ionic polarizability would also decrease to some extent with the HS → LS spin state change since the HS state possesses a relatively stronger ionic nature. Indeed, a decrease of the macroscopically measurable ε′ was observed for most of the investigated SCO compounds.6,16 There was observable frequency dependence in

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MATERIALS AND METHODS

General Procedures. A methanol solution of Fe(NCSe)2 (0.50 mmol) in the presence of a small quantity of ascorbic acid was added into a DMF solution of L (1.50 mmol). The resulting limpid mixture was stirred for an hour and then placed in diethyl ether vapor. Yellow crystals were obtained after several weeks. Elemental analysis for C75H68Fe2N26O4Se4: calcd. H, 3.76; C, 49.36; N, 19.95. Found H, 10678

DOI: 10.1021/acs.inorgchem.7b01633 Inorg. Chem. 2017, 56, 10674−10680

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4.11; C, 48.98; N, 19.63. Elemental analyses were performed on an Elementar Vario EL III analyzer. Raman spectra were obtained by using a Raman microscope (LabRAM HR Evolution); the excitation source was a He−Ne laser (632.8 nm and 15 mW). Variabletemperature Raman data were collected by using a Linkam THMS600 liquid nitrogen cryostat with a temperature controller between −196 and 600 °C, with the cooling rate of 10 K·min−1. Heat-capacity measurements were performed in zero magnetic field and in the temperature range 2−300 K, using the heat-capacity option of a Quantum Design Physical Property Measurement System (PPMS).45 The temperature intervals adopt logarithmic spacing below 100 K, 3 K temperature intervals were used in the temperature region of 100− 231, and 10 K intervals were used above 231 K. The mass of the sample used in this measurement was 7.92 mg. The ultraviolet−visible absorption spectrum was obtained using a Hitachi U-4100 spectrometer. Magnetic and Photomagnetic Measurements. Magnetic measurements of the samples were performed on a Quantum Design SQUID (MPMS XL-7) magnetometer in both cooling and heating modes in the temperature range from 2 to 300 K at ±1 K min−1. Data were corrected for the diamagnetic contribution calculated from the Pascal constants. The photomagnetic experiments were performed at 10 K with a green laser (λ = 532 nm and 10 mW/cm2) and red laser (λ = 808 nm and 35 mW/cm2).46 The temperature-dependent magnetization was measured after irradiation in the temperature range from 2 to 100 K at a rate of 0.5 K·min−1. Infrared Spectra. The temperature dependence of infrared spectra was measured on KBr pellet samples using a Nicolet iS10 FT-IR spectrometer equipped with a liquid helium type cryostat (OptistatCF2). The cooling and heating rates are 3 K·min−1. For infrared spectra after irradiation, the sample was irradiated via a flexible optical fiber guided a laser diode pumped laser at 10 K. Mö ssbauer Spectra. Zero-field 57Fe Mössbauer spectra were recorded on a Topologic 500A spectrometer. 57Co(Rh) moving in a constant acceleration mode was used as the radioactive source. The temperature of the sample was controlled by a Model 9700 digital temperature controller from Scientific Instruments company. The sample was encapsulated in a sample holder to avoid loss of solvents when pulling a vacuum on the cryostat. The Doppler velocity of the spectrometer was calibrated with respect to α-Fe. Dielectric Constant Measurement. The temperature-dependent ac (alternate current) dielectric permittivity measurements were carried out on a TH2828 Precision LCR meter, at a temperaturesweeping rate of 2 K min−1 and in dried N2 flow. Samples were ground, and pressed into tablets under the pressure of 1 GPa. The capacitor was made by painting the two faces of tablet pieces with silver conducting paste and copper wires as the electrodes. The capacitor was kept dried over silica gel for 2 days and finally coated with AB glue before dielectric measurements, in order to avoid the influence of moisture. The area (7.01 mm2) and thickness (0.26 mm) of the capacitor were measured under a microscope with a Phenix CCD eye and the software. Single Crystal Crystallography. X-ray data were collected at different temperatures using the SMART and SAINT programs on a Bruker Smart APEX II X-ray diffractometer equipped with a graphitemonochromated Mo−Kα radiation source (λ = 0.71073 Å). The structures were solved by a direct method and refined by full-matrix least-squares on F2 using the SHELX program with anisotropic thermal parameters for all non-hydrogen atoms. The hydrogen atoms of organic ligands were located geometrically and fixed isotropic thermal parameters. Attempts to add the hydrogen atoms for the solvent water molecules at 280 and 92 K, and one DMF molecule at 280 K, in the crystal structure through Fourier electron density failed, respectively. The supplementary crystallographic data used in this study are listed in Table S1.

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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.7b01633. Further information on crystal data, molar heat capacity data, magnetic properties, UV−visible absorption spectroscopy, IR spectra, and dielectric constant (PDF) Accession Codes

CCDC 1506799 and 1506800 contain 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 [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 Author

*E-mail: [email protected]. ORCID

Tao Liu: 0000-0003-2891-603X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partly supported by the NSFC (Grants 91422302 21421005, and 21322103), the Open Fund of National Laboratory of Molecular Science (20140116), and the Fundamental Research Funds for the Central Universities, China.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.7b01633 Inorg. Chem. 2017, 56, 10674−10680

Article

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DOI: 10.1021/acs.inorgchem.7b01633 Inorg. Chem. 2017, 56, 10674−10680