Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
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Reversible Phase Transition with Ultralarge Dielectric Relaxation Behaviors in Succinimide Lithium(I) Hybrids Yun-Zhi Tang,* Bin Wang, Hai-Tao Zhou, Shao-Peng Chen, Yu-Hui Tan, Chang-Feng Wang, Chang-Shan Yang, and He-Rui Wen School of Metallurgy and Chemical Engineering, Jiangxi University of Science and Technology, Ganzhou, 341000, P. R. China S Supporting Information *
ABSTRACT: Dielectric relaxations have widely applied on high permittivity capacitors, dielectric switches, ferroelectrics, pyroelectrics, and electrical insulating materials. However, few investigations of large dielectric relaxation behaviors on organic−inorganic hybrid materials have been documented before. Here we present a novel two-dimensional succinimide lithium(I) hybrid compound, [Li(PDD)2ClO4]n, 1, (PDD = 2,5-pyrrolidinedione = succinimide) which shows reversible phase transition behavior in the vicinity of 228 K accompanied by an unusual symmetry breaking from I41/amd to C2/c. X-ray single crystal diffractions analysis indicates the twist motion of pyrrolidine heterocycles, and order−disorder motion of ClO4− anions triggered the reversible phase transition. By means of an intuitive crystallographic model (rattling ion model), we further illustrated the mechanism of the interesting reversible phase transition. Particularly, 1 shows ultralarge dielectric relaxation behavior in the vicinity of the phase transition by its dielectric constant dependence on temperatures and frequencies as well as its Cole−Cole relation.
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INTRODUCTION Research on reversible phase transitions and their dielectric, ferroelectric, and nonlinear optical properties have become one of the hotspots in the past decade, because their special dielectric and polarization properties in the solid−solid state phase transitions have been widely applied on high permittivity capacitors, dielectric switches, ferroelectrics, pyroelectrics, electrical insulating material, etc.1−11 However, most documented dielectric relaxations still have mainly focused on inorganic dielectric materials such as ceramics Pb(Mg1/3Nb2/3)O3) and Pb(Zn1/3Nb2/3)O3;12,13 the investigation of dielectric relaxation behaviors on organic− inorganic hybrid materials have seldom been reported before. In comparison with conventional inorganic dielectric materials, organic−inorganic hybrid materials have shown much advantage, which includes lightweight, friendly environment (not containing lead elements), easily synthesis, highly stability, etc.14−23 In additional, the reported dielectric relaxation research presently primarily exists on the relaxing ferroelectrics such as polyvinylidenefluoride (PVDF),4,24 with very few literature reports documenting the dielectric relaxation behaviors of non-ferroelectrics. What is more, dielectric relaxation phenomenon based on organic−inorganic hybrid molecules with ultralarge dielectric relaxation behaviors, namely, with highly sensitivities dependence on frequency, temperature, etc., are very spare,13−16,25,26 and to design and construct the dielectric relaxation behaviors in organic− inorganic hybrid materials is still a challenging task. Usually, dielectric relaxation molecules should satisfy the following © XXXX American Chemical Society
conditions based on a reversible phase transition: (i) wide width of dielectric peak in the vicinity of Tc (phase transition temperature), (ii) Tc varies with the change of frequency, (iii) Tc exhibits an obvious difference between the heating and cooling cycles, (iv) the dielectric constants should fit exponential law when the temperature is above the Tc. Recently Rempe et al. discovered that a few heterocyclic compounds such as ethylene carbonate (EC) and propylene carbonate (PC) displaying interesting dielectric relaxation,25−27 and then Xiong et al. reported a three-dimensional molecular perovskite ferroelectric: (3-ammoniopyrrolidinium)RbBr3 [(AP)RbBr3], showing high Curie temperature (Tc = 440 K) and dielectric relaxation behaviors.28 This research suggests the twist motion and the order−disorder motion of host guest molecules by the external thermal stimuli can trigger the reversible phase transition and lead to the dielectric relaxation behaviors. Encouraged by their work, we employed the heterocyclic compound succinimide (2,5-pyrrolidinedione) and lithium ions as building blocks and constructed a novel coordination polymer, [Li(PDD)2ClO4]n. Fortunately, 1 exhibits a particular two-dimensional (2D) organic−inorganic hybrid structure and undergoes a fascinating reversible phase transition around 228 K. The most striking is that 1 exhibits an ultralarge dielectric relaxation behavior. With the help of the “rattling ion model” theory model,1 we further reveal the mechanism of dielectric relaxation behavior and structural phase transition. Received: October 12, 2017
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DOI: 10.1021/acs.inorgchem.7b02625 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry Table 1. Crystal Data and Structure Refinements for 1-RTP and 1-LTP
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compound
1-RTP
1-LTP
empirical formula formula weight crystal system space group a (Å) b (Å) c (Å) β/° volume/Å3 Z ρcalc, g/cm3 μ/mm−1 F(000) radiation 2Θ range for data collection/° index ranges reflections collected independent reflections data/restraints/parameters goodness-of-fit on F2 final R indexes [I ≥ 2σ(I)] final R indexes [all data] largest diff peak/hole/e Å−3
C8H10ClLiN2O8 304.57 tetragonal I41/amd 7.8674(6) 7.8674(6) 20.046(3) 90.00 1240.8(2) 4 1.630 0.389 752.0 MoKα (λ = 0.71076) 8−56.42 −10 ≤ h ≤ 10, −10 ≤ k ≤ 10, −26 ≤ l ≤ 26 5208 444 [Rint = 0.0412, Rsigma = 0.0177] 444/1/36 1.240 R1 = 0.0649, wR2 = 0.1655 R1 = 0.0680, wR2 = 0.1694 0.52/−0.695
C16H20Cl2Li2N4O16 609.14 monoclinic C2/c 15.6084(13) 15.6084(13) 21.306(3) 111.487(4) 4829.9(9) 8 1.670 0.356 2496.0 MoKα (λ = 0.71076) 5.6−61.66 −20 ≤ h ≤ 20, −20 ≤ k ≤ 20, −28 ≤ l ≤ 28 19462 5790 [Rint = 0.0295, Rsigma = 0.0367] 6496/40/392 1.055 R1 = 0.1720, wR2 = 0.3305 R1 = 0.1910, wR2 = 0.3392 1.71/−1.02
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EXPERIMENTAL SECTION
RESULTS AND DISCUSSION Differential scanning calorimetry (DSC) is a powerful method to judge whether there exists a reversible phase transition by thermal stimuli. We measured the thermal behavior for 1 by a PerkinElmer Diamond DSC instrument from 300 to 100 K. As elucidated in Figure 1, one can clearly observe two nearly
Synthesis. All reagents and solvents employed in our work were obtained from commercial sources and used directly without further purification. Compound 1, [Li(PDD)2ClO4]n, was synthesized according to the literature as follows.29 Succinimide (0.1982 g, 2 mmol) and lithium perchlorate (0.2128 g, 2 mmol) were added into 15 mL of methanol solvent or a 15 mL mixture of 1:1 acetonitrile− methanol mixture and then stirred for 10 min. The filtered solution evaporated slowly at room temperature. Colorless block crystals (Figure S1) were obtained from solution before the solvent completely evaporated. Yield: 35.1%. Anal. Calcd. (%) for C8H10ClLiN2O12: C, 26.07; H, 2.73; N, 7.60. Found: C, 26.17; H, 2.75; N, 7.65. IR/cm−1 (Figure S2): 3458(vs), 1775 (w), 1705 (s), 1638 (s), 1387 (w), 1299 (w), 1097 (s), 989 (w), 934 (w), 629 (m), 529(m). According to the IR spectra of Figure S2, the strong absorptions at 1705 and 1638 cm−1 can be assigned to the vibration of carbonyl group of pyrrolidinedione, and then a strong broad peak around 1097 cm−1 suggests there are ClO4 anions in the structure. X-ray Single Crystal Diffraction Analyses of 1-RTP and 1LTP. The crystal structures of 1 were determined at 293 K (1-RTP, CCDC No. 1561115) and 100 K (1-LTP, CCDC No.1561116) respectively by X-ray single crystal diffraction analyses.30,31 The data were corrected for Lp and absorption effects. Their crystal structures were solved by direct methods with the SHELXS-97 program. The crystal data and structures refinements for 1-RTP and 1-LTP are shown in Table 1. Their selected intra-atomic distances and bond angles are given in Table S1. These data are available free of charge from the Cambridge Crystallographic Data Centre (CCDC). Measurement Methods. The frequency-dependent dielectric characteristics for 1 from a powdered sample in form of a pellet were measured using the Novocontrol dielectric spectrometer in the temperature ranges of 150−293 K within the frequency range of 500 Hz to 1 MHz. The heat anomaly behaviors were determined by a TAInstruments (DSC 2920) with a heating rate of 5 K per minute in the 100−300 K domain. Above room temperature, thermogravimetric analysis (TGA) measurements were performed using a TA-Instruments STD 2960 from 293 to 900 K.
Figure 1. Heat anomalies of 1 in the cooling and heating cycles.
completely opposite sharp peaks at 227.8/228.7 K corresponding to the cooling/heating cycles respectively, indicating a reversible phase transition occuring in the vicinity of 228 K. The large entropy changes for the cooling/heating cycles are 1.94 J·mol−1·K−1 and 1.31 J·mol−1·K−1, further confirming that it belongs to the first order reversible phase transition despite of a small thermal hysteresis (0.9 K) between the cooling/heating cycles. According to the Boltzmann equation, ΔS = nR ln(N), where ΔS is the entropy change extracted from the Cp data, n is the number of guest molecules per mole (n = 1, here), R is the gas constant, and N is the number of possible orientations for B
DOI: 10.1021/acs.inorgchem.7b02625 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry the disordered system, the N values for the heating and cooling cycles can be directly calculated as 1.26 and 1.17. The results agree well with the synergetic motions form microlocal order− disorder of perchlorate anions and twist motion of pyrrolidine groups, which are triggered by the thermal stimuli. In additional, the TGA measurements (Figure S3) show the weight loss of 1 occurring around 500 K (227 °C), far more than that of reversible phase transition temperature (227 K). According to above-mentioned DSC results, we measured the crystal structure for 1 at 300 K (1-RTP) and 100 K (1LTP) through single crystal X-ray diffraction, respectively. As listed in Table 1, some distinct differences can be discovered between 1-RTP and 1-LTP. First of all, the unit cell length of a1‑LTP (15.6084 Å) and b1‑LTP (15.6084 Å) are nearly twice of that in a1‑RTP (7.8674 Å) and b1‑RTP (7.8674 Å); meanwhile, the unit cell length of c1‑LTP = 21.338 Å and c1‑RTP = 20.046 Å, the β angles have changed from 90 to 111.474(3)°, according to the cosine law and define of cosine, and we can obtain a1‑RTP − c1‑RTP = 21.534 Å nearly equal to that of c1‑LTP (21.338 Å) (Figure S4a,b, Supporting Information). Second, significant changes or the reversible phase transitions are that the cell volume of V1‑LTP (4846.0 Å3) is nearly four times of V1‑RTP (1240.8 Å3). Lastly, 1-RTP belongs to the tetragonal crystal system with I41/amd (No. 141) space group, while 1-LTP crystallizes in the monoclinic crystal system with C2/c (No. 15) space group. C2/c possesses a lower centrosymmetric point group C2h with four symmetric elements (E, C2, i, σh), and I41/ amd has a higher centrosymmetric point group D4h with 16 symmetric elements (E, 2C4, C2, 2C′2, 2C″2, i, 2S4, σh, 2σv, 2σd). According to the principle of symmetry breaking, the space group of 1-LTP (C2/c) is the subgroup of 1-ITP (I41/amd), well agreeing with the principle of Aizu notation of 4/mmmF2/ m(s) (Figure 2).32 Notably, a symmetry breaking from 16 to 4 suggests possibly a large molecule motion in the hybrid structure.
Figure 3. (a) Asymmetrical unit for 1-RTP in the room temperature. (b) Asymmetrical unit for1-LTP in the low temperature (100 K).
(ClO4)2] comprises two lithium center atoms, four succinimide ligands, one highly disordered perchlorate anion, and one completely ordered perchlorate anion (Figure 3b). The coordination geometry of Li1 atom can be regarded as a seriously distorted tetrahedron since the bond lengths of Li1− O15 (1.990(13) Å) and Li1−O19 (1.965 (13) Å) are much longer than that of Li1−O13 (1.916(13) Å) and Li1−O18 (1.925(13) Å) (Table S1), while the crystallographically independent Li2 atom possesses a slightly distorted tetrahedral geometry as all the O1−Li1 bond distances are in the normal range of 1.912(18)−1.927 Å with a small changes. The most striking difference between (1-RTP, Figure 4a−c) and (1-LTP, Figure 4d−f) is that they comprise total different building blocks. In 1-RTP, all succinimide ligands act as μ2 bridge mode with an end-to-end modes linking different Li atoms. In this fashion four Li atoms linking four different succinimide ligands construct a regular square grid (A) with a dimensional size 7.867 × 7.867 Å2 (Figure 4a−c). All the atoms of the succinimide ligands lay on the same horizontal plane without any torsion. Such building blocks were linked each other, thus resulting in the formation of a regular 2D sheet, and then these sheets are further linked by the strong hydrogen bond (N(1)−H(1)···O(5)#1, 3.121(7) Å) (Table S2) which assembles them into 3D layer structures (Figure S5). All the perchlorate anions fill in the square grids and appear in a highly disordered state. However, for 1-LTP all the pyrrolidine heterocycles have been seriously twisted in two opposite directions (Figure 4d−f); such a twist motion directly leads to an expanding grid C (Figure 4e,g) and a reducing grid B (Figure 4f,h); namely, all pyrrolidine heterocycles planes in 1LTP deviate away from grid C, and much closer to grid B.
Figure 2. Symmetry breaking for the reversible phase transition in 1.
Crystal Structural Difference between 1-RTP and 1LTP. As we expected, compound 1 exhibits distinct differences between before (1-RTP, Figure 3a) and after (1-LTP, Figure 3b) the reversible phase transition. As illustrated in Figure 3a, the asymmetrical unit for 1-RTP with the empirical formula Li(PDD)2ClO4 contains one lithium center atom, two succinimide ligands, and one completely disordered perchlorate anion. The center Li atom adopts a normal tetrahedral coordination mode which bridges four O atoms from different succinimide ligands. All the O1−Li1 bond distances are identical to each other with 1.943(3) Å, and the bond angles for all O−Li1−O are 104.96(7)°, indicating the Li atom lies in a highly symmetrical center. Totally different from 1-RTP, the asymmetrical unit for 1-LTP formulated as [Li2(PDD)4C
DOI: 10.1021/acs.inorgchem.7b02625 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Figure 4. (a) Two-dimensional sheet of 1-RTP highlighted on the regular square grid A and the disordered perchlorate anion, (b) an extracted grid A from a two-dimensional sheet of 1-RTP, (c) a schematic diagram of grid A, (d) the two-dimensional sheet of 1-LTP highlighted on the two different quadrilateral grids (B and C) and the varied perchlorate anions, (e) the extracted grid B from a two-dimensional sheet of 1-LTP, (f) the extracted grid C from a two-dimensional sheet of 1-LTP, (g) a schematic diagram of grid B which can be regarded as the reducing of grid A, (h) a schematic diagram of grid C which can be regarded as the expanding of grid A.
According to the edge lengths between two carbon atoms of each opposite pyrrolidines, one can further discover that the four edge lengths in grid C are 8.0806 Å, 8.0673 Å, 8.0736 Å, 8.0642 Å (Figure 4f), while those in grid B are 7.5416 Å, 7.5549 Å, 7.5487 Å, 7.5580 Å (Figure 4e). Obviously grid C has much larger “cavity” than grid B. All the perchlorate anions in grid C are still highly disordered, while all the perchlorate anions in grid B are completely ordered. Similar to that of 1-RTP, all grids B and C in 1-LTP are cross-linked to each other and produced the 3D sheets; then many strong hydrogen bonds exist such as N(2)−H(2)···O(15) (2.999(16) Å), N(4)− H(4)···O(20), (2.955(9) Å), and N(3)−H(3)···O(16), (3.016(17) Å) (Table S2) which assemble them into a 3D layer structure (Figure S6). To further explain the unusual structural reversible phase transition, we proposed an intuitive crystallographic model (rattling ion model).1,33 Here the ClO4− anions were regarded as the guest ions which fill in the 2D grids constructed by Li cations and succinimide ligands. In the high temperature of 1RTP, a large “rattling” space with normal square grid (7.867 × 7.867 Å2) is expected for the fast rotating ClO4− anions in the disordered structure, because the disordered ClO4− anions with fast rotating motion prop occupy a larger cavity from the framework (Figure 4a). When it is below the phase transition temperature (1-LTP), much less rattling space is expected in the ordered arrangement (Figure 4d). Thanks to the twist motion of pyrrolidine heterocycles, the lengths of the part quadrilateral grids (B) have changed to 7.5416 Å, 7.5549 Å, 7.5487 Å, 7.5580 Å (Figure 4g), reducing 0.33, 0.31, 0.32, and 0.31 Å respectively, in comparison with grid A (Figure 4c); moreover, the dihedral angle between the neighbor pyrrolidine
planes in grid B have been changed from 90° to 78° (Figure S7), indicating only a compressed and cramped space for the perchlorate anion. In such a situation, the rotating ClO4− anions freeze to a still and ordered state. However, another expanding type of quadrilateral grids (C) (8.0806 Å, 8.0673 Å, 8.0736 Å, 8.0642 Å, Figure 4f) became even much larger than that of grid A (Figure 4c); there is still much space for the rotating motion of perchlorate anion, and therefore, perchlorate anions in grid C still lie in a highly disordered state. Since 1-RTP has D4h point group with 16 symmetric elements (E, 2C4, C2, 2C′2, 2C″2, i, 2S4, σh, 2σv, 2σd) and 1-LTP possesses C2h point group with four symmetric elements (E, C2, i, σh), it compels us to make clear how the symmetry breaking happened in such a reversible structural phase transition. As shown in Figure 5, the symmetric elements have been labeled in 1-RTP (Figure 5a, left) and 1-LTP (Figure 5b, right) respectively when viewed in the same b-axis direction. First, we
Figure 5. Symmetric elements in 1-RTP (a, left) and 1-LTP (b, right) respectively when viewed in the b-axis direction. D
DOI: 10.1021/acs.inorgchem.7b02625 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry can easily discover that the two C4 symmetrical operations in 1RTP pass through the regular lithium center and parallel to the a- and b-axis respectively (Figure 5a), and the rotatory reflection axis S4 also exists in the structure, while in 1-LTP Li1 adopts a seriously distorted tetrahedral geometry, the C4 rotating axis and the rotatory reflection axis S4 vanishes in the structure. Second, since there are two different Li1 centers as well as the torsion of the pyrrolidine heterocycles in 1-LTP, the β angle has changed from 90 to 111.474(3)°, 2σv and 2σd mirrors which are parallel to the ab and bc planes as well as the secondary axis (2C′2, 2C″2) also disappear in 1-LTP, while the σh mirror (ac plane) and C2 rotating axis (along the b-axis) still exist in both 1-RTP and 1-LTP. Lastly, one can discover the inversion center (i) has always not changed during the phase transition. The most remarkable features are that compound 1 displays ultralarge dielectric relaxation behaviors in the vicinity of 228 K (Tc). Figure 6 provides the real part (ε′) of the dielectric
also discovered in the low dielectric states (below the phase transition). Agreeing well with the structural changes of the reversible phase transition, the ClO4− anions with a large rattling space can shift easily without distorting the 2D framework when an electric field is applied to a disordered structure. A larger dielectric constant and larger Curie−Weiss constants should be typical in this case. On the other hand, in partly ordered structure with a very small rattling space, the ClO4− anions cannot shift easily without distorting the structure (cavity).1 A smaller dielectric constant and CurieWeiss constants are expected.1 Interestingly, Xiong et al. also reported a h ybrid multif unctional material, (3ammoniopyrrolidinium)RbBr3, [(AP)RbBr3], which shows interesting dielectric relaxation behaviors. Different from compound 1, dielectric relaxation behaviors in [(AP)RbBr3] occur in a para-ferroelectric phase transition. Besides, compound 1 shows ultralarge dielectric relaxation behaviors, namely, with very large dielectric constant differences dependence on temperatures and frequencies than that in [(AP)RbBr3]. It should be noted that whether in low frequency or high frequency, the real part of dielectric constants on the high dielectric states is near 2.5 times of that on low dielectric states. What is more, the imaginary part (ε″) of dielectric constants also exhibits similar changes as that of the real part (Figure S8). Such a sharp contrast suggests 1 is an ideal dielectric switch in a widely frequency application range. To further intensively investigate the dielectric relaxation process of 1, we simulate the Cole−Cole diagram of 230 K
Figure 6. The real part (ε′) of the dielectric constant dependence on temperature at different frequencies highlighted on the differencevalue between cooling and heating runs.
constant dependence on temperature at different frequencies. First, the reversible phase transition temperature (Tc) exhibits great differences for cooling and heating runs at a given frequency. For example under frequency 500 Hz, the phase transition temperature for the cooling cycle is 214.4 K, while that for the heating cycles is 228.3 K, enhancing 13.9 K. With the same manner, we can discover a similar increase of 14.6, 15.0, 13.7, 16.6, and 14.3 K can be obtained for the cooling and heating runs under 1 kHz, 5 kHz, 10 kHz, 100 kHz, and 1 mHz, respectively. Second, Tc of compound 1 displays ultralarge dielectric relaxation dependence at different frequencies. Such as for the cooling cycle, when the frequency varies from 1 kHz, 5 kHz, 10 kHz, 100 kHz to 1 mHz, the Tc correspondingly changes from 214.4, 216.3, 219.3, 222.0, 227.1 to 236.8 K, increasing 1.9, 3.0, 2.7, 5.1, and 9.7 K respectively. The total increase reaches 22.4 K, a similar increase can be clearly disclosed for the heating cycle, and the total increase run up to 32.8 K, indicating a high sensitivity dependence on frequency. Lastly, dielectric constants show great changes with the increase of frequency. When the frequency varies from 1 kHz, 5 kHz, 10 kHz, 100 kHz, to 1 mHz, the real part of dielectric constants corresponding to high dielectric states (above the phase transition) is 21.5, 19.2, 16.8, 15.2, 12.6 and 11.3, decreasing 2.3, 2.4, 1.6, 2.6, and 1.3 respectively. A similar situation can be
Figure 7. Cole−Cole diagram of 230 K for compound 1.
deviating from semicircles. As illustrated in Figure 7, the curves are well fitted by the Cole−Cole equation: ε0 − ε∞ ε = ε∞ + 1 + (iωτ )1 − α (1) where ε0 and ε∞ are the static and high frequency permittivity, respectively, ω is the angular frequency, τ is the macroscopic relaxation time, and α represents the distribution of the relaxation time parameter. The points (ε′, ε″) lie on the semicircle centered at ((ε0 + ε∞)/2, 0). As expected, the experimental Cole−Cole plots at 230 K are fitted to eq 1, and the fitting parameters ε0, ε∞, and τ were determined. The E
DOI: 10.1021/acs.inorgchem.7b02625 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry values of parameter α indicate that the relaxation is nonpure Debye characteristics.
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CONCLUSION In summary, we have successfully constructed a novel 2D organic−inorganic hybrid [Li(PDD)2ClO4]n, 1, which uncommonly display ultralarge dielectric relaxation behavior around 228 K. The investigation suggests that the twist motion of pyrrolidine heterocycles and the order−disorder motion of ClO4− anions caused the reversible phase transition. This research opens a new avenue for exploring switchable dielectric materials and phase transition materials.
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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.7b02625. Experimental procedures, thermogravimetric curves, and additional analysis data (PDF) Accession Codes
CCDC 1561115−1561116 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
Yun-Zhi Tang: 0000-0003-1791-5234 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Y.-Z.T. is grateful for the support from the National Natural Science Foundation of China (Grant No. 21671086, No. 21261009, No. 21461010, No. 21471070) and Jiangxi Province Science and Technology Support Program (20133BBE50020).
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REFERENCES
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DOI: 10.1021/acs.inorgchem.7b02625 Inorg. Chem. XXXX, XXX, XXX−XXX
