Ligand Substituent Effects on the Spin-Crossover Behaviors of

Jan 8, 2019 - Six analogue dinuclear Fe(II) compounds with Cl- and Me-substituents on various positions show different SCO behaviors corresponding to ...
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Ligand Substituent Effects on the Spin-Crossover Behaviors of Dinuclear Iron(II) Compounds Chun-Feng Wang, Zi-Shuo Yao,* Guo-Yu Yang, and Jun Tao* Key Laboratory of Cluster Science of Ministry of Education, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 100081, PR China

Inorg. Chem. Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/09/19. For personal use only.

S Supporting Information *

ABSTRACT: Six analogue compounds with the general formula [Fe2(xL)5(NCS)4]·yMeOH (x = o-Cl, y = 3 for compound 1; x = m-Cl, y = 5 for 2; x = p-Cl, y = 1 for 3; x = oMe, y = 2 for 4; x = m-Me, y = 2 for 5; x = p-Me, y = 3 for 6; L = N-phenylmethylene-4-amino-1,2,4-triazole) were synthesized. The two Fe(II) ions are triply bridged by the triazole groups of three xL ligands and each Fe(II) is further capped with two NCS− groups and one more xL ligand. These compounds show regular patterns in their magnetic properties that depend on the positions the substituent groups (−Cl or −Me) ride, i.e., ortho-substituted compounds 1 and 4 undergo complete one-step spin crossover (SCO), while meta-substituted compounds 2 and 5 display incomplete one-step SCO with lower transition temperatures, and para-substituted compounds 3 and 6 are in the high-spin states in all temperature ranges. Structural analyses reveal that the molecular geometry and intermolecular interactions of these compounds, which should account for the differences in magnetic properties, are obviously depend on the positions of substituent groups (steric effect), despite them being electron-withdrawing chlorine or electron-donating methyl, whereas theoretical calculations confirm that the electronic effects of substituent groups exert no effect on the magnetic properties.

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INTRODUCTION

behaviors by controlling guest exchanges within a Hofmanntype metal−organic framework. Besides the above-mentioned chemical stimuli, changing the substituent group on ligand can be used to control the SCO behavior more directly, which is usually assigned to the steric and/or electronic effects. When the substituent groups sit on the positions nearest to the coordination atoms, intramolecular steric strain or repulsions are the key point, i.e., larger steric strain stabilizes the high-spin (HS) states of compounds.18,19 If the substituent groups locate on the positions opposite to the pyridyl coordination atoms, as exemplified in [Fe(X-pybox)2](ClO4)2 (pybox = 2,6-bis(oxazolin-2-yl)pyridine, X = H, Cl, Ph, MeO, MeS, Me, 3-thienyl, 2-thienyl, N3, Br, 3-pyridyl, and 4-pyridyl)20,21 and [Fe(4-X-bpp)2]2+ (bpp = 2.6-di(pyrazol-1yl)pyridine) compounds,22 then substituent-induced electronic effect is clearly related to the SCO transition temperatures. Under this circumstance, electron-withdrawing substituents stabilize the low-spin (LS) state of the compound, while electron-donating groups stabilize the HS state. Our motivation to investigate the substituent effects on SCO behavior was inspired by a series of works on dinuclear Fe(II) SCO species, [Fe 2 (L) 5 (NCS) 4 ], 2 3 − 3 2 particularly [Fe2(L1)5(NCS)4]·4MeOH (L1 = N-salicylidene-4-amino1,2,4-triazole)27,28 and [Fe2(L2)5(NCS)4]·4MeOH (L2 =

Spin-crossover (SCO) materials, whose spin states as well as physical properties can be tuned by external physical stimuli (light irradiation, temperature, and pressure), have attracted much attention because of their emerging applications in the areas of information processing, sensing, and/or display.1−3 Some of these SCO materials simultaneously display abrupt transitions and hysteresis loops,4−6 which confer on them a memory effect suitable for molecular switches and data storage.7−10 However, SCO materials can also be tuned by chemical stimuli.11 For example, many SCO materials are extremely sensitive to guest molecules, solvents, or humidity; thus, such sensitivity can be exploited to control their SCO behaviors. This strategy has been widely adopted in the area of SCO materials.12,13 For example, We have systematically studied the solvent effects on a SCO behavior of [Fe(tpa)(NCS)2] (tpa = tris(2-pyridylmethyl)amine) crystals14 and revealed that the exchanges of vaporous solvent molecules can considerably influence the SCO behaviors of parent crystals, while in a porous SCO compound, [Fe(NCS)2(tppm)] (tppm = 4,4′,4″,4‴-tetrakis(4-pyridylethen-2-yl)tetraphenylmethane),15 the different guest solvent molecules in the pores can tune the transition temperatures of materials in a systematic trend. Recently, this strategy was carried forward by Tong and his co-workers,16,17 who realized a reversible modulation of four-, two-, and one-stepped SCO © XXXX American Chemical Society

Received: October 3, 2018

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DOI: 10.1021/acs.inorgchem.8b02789 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

at room temperature for 0.5 h. The white solid product were filtered and dried under vacuum overnight. Ligands m-ClL, p-ClL, o-MeL, m-MeL, and p-MeL were prepared following the same method used for o-ClL. Anal. Calcd (%) for C9H7N4Cl (o-ClL, m-ClL, and p-ClL): C, 52.31; H, 3.41: N, 27.11. Found: C, 51.99; H, 3.47; N, 26.76 for o-ClL. C, 52.72; H, 3.53; N 27.30 for m-ClL. C, 52.78; H, 3.63: N 28.06 for p-ClL. Anal. Calcd (%) for C10H10N4 (o-MeL, m-MeL, and p-MeL): C, 64.50; H, 5.41; N, 30.09. Found: C, 63.72; H, 5.62; N, 30.50 for o-MeL. C, 63.93; H, 5.90; N, 30.02 for m-MeL. C, 63.39; H, 5.98; N, 30.16 for p-MeL. The synthesized ligands were characterized by 1H NMR spectroscopy (Figures S1 and S2). Synthesis of Compound 1. A methanol solution (16 mL) of 1.3 mmol (0.2690 g) o-ClL was added at room temperature into the same volume of methanol solution containing 0.5 mmol iron(II) chloride tetrahydrate (0.1000 g), 1 mmol ammonium thiocyanate (0.0760 g) and 30 mg ascorbic acid. The mixture was stirred for 5 min, with the color changed from limpid to yellow. The resulting solution was then kept for 6 h under nitrogen atmosphere to give large yellow crystals of compound 1. Compounds 2−6 were synthesized following the same method for synthesizing compound 1. Anal. Calcd (%) for C52H47Cl5Fe2N24O3S4 (1): C, 42.39; H, 3.21; N, 22.81. Found: C, 41.83; H, 2.78; N, 22.37. C54H55Cl5Fe2N24O5S4 (2): C, 42.18; H, 3.60; N, 21.87. Found: C, 39.46; H, 2.75; N, 21.88. C50H39Cl5Fe2N24OS4 (3): C, 42.61; H, 2.79; N, 23.85. Found: C, 40.80; H, 2.94; N, 22.37. C56H58Fe2N24O2S4 (4): C, 50.22; H, 4.36; N, 25.10. Found: C, 48.78; H, 4.45; N, 24.03. C56H58Fe2N24O2S4 (5): C, 50.22; H, 4.36; N, 25.10. Found: C, 48.33; H, 4.36; N, 24.10. C57H62Fe2N24O3S4 (6): C, 49.42; H, 4.55; N, 24.51. Found: C, 47.03; H, 4.14; N, 23.22. All samples are unstable in air because the lattice solvent molecules can easily escape or exchange with moisture, so the results of elemental analyses do not match well with the formula obtained by single-crystal X-ray diffraction studies at low temperature. Notably, the calculated results can be substantially improved by adding some amount of interstitial water molecules (Table S1). Crystallographic Data Collection and Structural Determination. Single-crystal X-ray diffraction data of compounds 1−6 were collected on a SuperNova diffractometer. The structures were solved by direct methods and refined on F2 by anisotropic full-matrix leastsquares methods using SHELXL-1997.37 Crystallographic data and structural refinement details are presented in Tables S1 and S2 (CCDC nos. 1870673−1870682).

phenyl-N-(4H-1,2,4-triazol-4-yl)methanimine).29,30 These two compounds show quite different SCO-fluorescent properties (one-step complete SCO versus three-step incomplete SCO), only because the o-H on terminal phenyl group was replaced with o-OH. To systematically study the substituent effects, we have designed two kinds of ligands with electronic-withdrawing and -donating characters, respectively (Scheme 1), and report Scheme 1. Schematic Representation of Ligands Designed for Preparing Compounds 1−6

here the syntheses, crystal structures, and magnetic properties of six analogue compounds, namely, [Fe2(o-ClL)5(NCS)4]· 3MeOH (1), [Fe2(m-ClL)5(NCS)4]·5MeOH (2), [Fe2(pClL)5(NCS)4]·1MeOH (3), [Fe2(o-MeL)5(NCS)4]·2MeOH (4), [Fe 2 (m-MeL) 5 (NCS) 4 ]·2MeOH (5), and [Fe 2 (pMeL)5(NCS)4]·3MeOH (6). Compounds 1 and 4 display complete one-step SCO, 2 and 5 show incomplete one-step SCO, and 3 and 6 are in HS states in all temperature ranges. The results indicate that the differences in magnetic properties arise mainly from steric effect rather than the electronic effect.

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EXPERIMENTAL SECTION

Materials and Physical Measurements. Ammonium thiocyanate and 4-NH2-1,2,4-triazole were purchased from Aladdin and Alfa Aesar, respectively. Iron(II) chloride tetrahydrate was obtained from Tianjin Chemical Reagent Company (China). 2-Chlorobenzaldehyde (o-Cl), 3-chlorobenzaldehyde (m-Cl), 4-chlorobenzaldehyde (p-Cl), 2-methylbenzaldehyde (o-Me), 3-methylbenzaldehyde (m-Me), and 4-methylbenzaldehyde (p-Me) were purchased from Adamas Reagent Co. Ltd. Ascorbic acid and all solvents were purchased from Sinopharm Chemical Reagent Co., Ltd. All starting materials were used without further purification. Elemental analyses for C, H, and N were performed on a Vario EL III elemental analyzer. Thermogravimetric analyses (TGA) were performed under nitrogen on SDT-Q600 equipped with an alumina pan and heated at a rate of 10 K min−1. Absorption spectra of ligands were recorded on a Shimadzu UV2550 spectrophotometer with a scan rate of 600 nm min−1. Infrared spectra (KBr pellets) were recorded in the range of 400−4000 cm−1 on a Nicolet 5DX spectrometer. 1H NMR spectra of ligands in CDCl3 were recorded on a Bruker AV-500 spectrometer. Chemical shifts are reported in ppm with the internal TMS signal at 0.0 ppm as a standard. Diffuse reflectance spectra (DRS) were obtained with a Varian Cary 5000 UV−Vis spectrometer with scan rate of 600 nm min−1, using BaSO4 as reference. Magnetic measurements were carried out on a Quantum Design MPMS XL7 magnetometer working in the 10−300 K temperature range with 1 K min−1 sweeping rate under a magnetic field of 5000 Oe; the freshly prepared crystals with a drop of mother liquor were sealed in an NMR tube. Density functional theory (DFT) calculations were performed with Gaussian 09. Geometry optimization of ligands were done with the functional of Perde−Burke−Ernzerhof (PBE). The 6-31G(d) basis set is used for all of the atoms of the ligands. Geometry optimization, single-point energy calculation of the LS and HS geometries of the complex, and subsequent frequency analyses were done with the functional of B3LYP and Perde-Burke-Ernzerhof (PBE).33−35 The TZVP basis set is used for all atoms of the complex.36 Syntheses of o-ClL. 4-NH2-1,2,4-triazole (24 mmol) and o-Cl (24 mmol) were dissolved in 20 mL ethanol, and the mixture was refluxed for 5 h. After that, the resulting solution was left undisturbed

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RESULTS Magnetic Properties. The thermal dependence of χMT, in which χM is the molar magnetic susceptibility and T is temperature, for crystalline samples of 1−3 was shown in the temperature range of 10−250 K (Figure 1a). At a first glance, compound 1 displays complete one-step SCO. The χMT value changes from 7.21 cm3 K mol−1 at 180 K (HS state) to 0.30 cm3 K mol−1 at 10 K (LS state), sharply in a narrow temperature range. The transition temperatures T1/2, which are defined as the temperatures where the fractions of the HS and LS species are equal, were calculated to be T1/2↓ = 134 K in the cooling mode and T1/2↑ = 140 K in the warming mode, respectively, giving a thermal hysteresis loop of 6 K width. Compound 2 undergoes an incomplete one-step SCO. The χMT value at 180 K is 7.25 cm3 K mol−1, which is almost the same as that of compound 1, indicating all Fe(II) ions are in the HS states. On cooling, the χMT value decreases gradually to 0.89 cm3 K mol−1 at 10 K, suggesting a remnant of 12% HSFe(II) population or the existence of paramagnetic impurities. Compared to those of compound 1, the critical transition temperatures of compound 2 shift to lower temperature with a small thermal hysteresis loop of ca. 3 K (T1/2↓ = 104 K and T1/2↑ = 107 K) (Figure S3). Compound 3 is in the HS state in B

DOI: 10.1021/acs.inorgchem.8b02789 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 2. Molecular structure of compound 1 (100 K). Lattice methanol molecules and hydrogen atoms are omitted for clarity. Brown, Fe; blue, N; gray, C; yellow, S; green, Cl. Symmetric code: (a) −x + 1, y, −z + 1/2. All ligands are in fact highly disordered, in which each o-Cl phenyl group occupies mirror-symmetric positions.

N atoms, one η1-triazole N atom, and two cis-isothiocyanato N atoms, which affords an average Fe−N bond length of 1.965(4) Å that falls in the range typical for LS Fe(II)−N bonds.38,39 At 293(2) K, the corresponding average Fe−N bond length and intramolecular Fe···Fe distance are 2.171(5) Å and 3.996(2) Å, respectively, showing an increase of 0.20 Å in the average Fe−N bond length that is a characteristic signal of LS-to-HS transition. Compound 4, [Fe2(o-MeL)5(NCS)4], shows SCO behavior very similar to that of compound 1; thus, the variation value of its average Fe−N bond length also shows similar temperature-dependent trend to that of 1 (Table 1). Along with the spin-state changes form LS to HS, expansions of 0.20 and 0.33 Å are observed for the average Fe−N bond length and the Fe···Fe distance, respectively, in compound 4. For compounds 2 and 5, the average Fe−N bond lengths and Fe···Fe distances at 85 K are obviously longer than those of compounds 1 and 4 at the LS states, corresponding to their incomplete SCO behaviors. As to the HS compounds 3 and 6, their average Fe−N bond lengths and Fe···Fe distances are the longest among these compounds, even at 100 K.

Figure 1. Magnetic properties of compounds 1−3 (a) and 4−6 (b). Temperature scanning rate: 1 K min−1; applied magnetic field: 5000 Oe.

all temperature range, and the smooth decrease in the χMT value in the low temperature range may reflect weak intramolecular antiferromagnetic interactions. As shown in Figure 1b, compound 4 shows a SCO behavior similar to that of compound 1. The χMT value for compound 4 is 7.16 cm3 K mol−1 at 180 K, referring to the existence of two HS Fe(II) ions. The χMT value at 10 K is 0.34 cm3 K mol−1, indicating the occurrence of a complete one-step SCO with a small residual HS population. The transition temperatures of T1/2↓ = 138 K and T1/2↑ = 144 K define a thermal hysteresis loop of 6 K width. The χMT value of compound 5 demonstrates a slight decrease from 7.14 cm3 K mol−1 at 180 K to 6.99 cm3 K mol−1 at 105 K, and then a remarkable change was detected around 97 K. The χMT value of 6.56 cm3 K mol−1 at 80 K implies that only ∼8.2% HS Fe(II) ions switch to the LS states. Compound 6 closely resembles compound 3 in the magnetic properties, i.e., HS-only in the whole temperature range. By comparing the magnetic properties of compounds 1−3 and 4−6, we can find that the ortho-substituted compounds 1 and 4 display complete onestep SCO, meta-substituted compounds 2 and 5 undergo incomplete one-step SCO with lower transition temperature, and para-substituted compounds 3 and 6 are HS-only in the measured temperature range. These results indicate that the SCO behaviors of these compounds mainly depend on the positions where the substituent groups locate. Crystal Structures. The molecular structures of [Fe2(xL)5(NCS)4] are only different from each other in the substituent groups, so compound 1 is taken as an example for the detailed description of crystal structures of these analogue compounds. Figure 2 shows the molecular structure of [Fe2(oClL)5(NCS)4] at 100(2) K. The two Fe(II) ions are bridged by three o-ClL ligands, resulting in a separation of 3.644(1) Å between Fe1 and Fe1a (Table 1). The octahedral {FeN6} coordination symmetry of Fe1 is completed by three μ-triazole

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DISCUSSION As shown above, these compounds show different magnetic properties, i.e., complete SCO (1 and 4) versus incomplete SCO (2 and 5) and HS-only (3 and 6), accompanied by changes in their Fe−N and Fe···Fe distances. Because the molecular structures of these compounds are only distinguished in the terminal phenyl groups, such a phenomenon should arise from the substituent effects (given that solvent effects were excluded). Taking into account the electronegative nature of chlorine atom and methyl group and their substituent positions, the substituent effects including electronic effect and steric effect are considered. Electronic Effect. Charge densities (σ) on the coordination atoms (N1 and N2) of all ligands are calculated by replacing phenyl hydrogen atoms (H1−H5) with chlorine and methyl group, respectively (Figure 3 and Table S4). First, all methods (Mulliken/Hirshfeld/NBO) produce similar results showing that σ(N1) is larger than σ(N2) despite the kind and position of the substituent, and the values of Me-substitution are only slightly higher than those of Cl-substitution ( τm‑ > τp‑ is established for either Cl- or Me-substituted species. Therefore, a correlation between the substituent position, τ value and SCO behavior can be concluded for these compounds, i.e., the farther the substituent, the smaller τ value achieved and the greater the possibility that SCO cannot occur. One of the reasons that the τ value correlates with the substituent position may be assigned to the different hydrogen bonds and crystal lattice that NCS− involved in. As shown in Figure 5, complexes [Fe2(xL)5(NCS)4] show different packing modes in the case of different substituent positions, in which more hydrogen bonds formed between NCS− groups and ligand H atoms result in smaller τ values, as evidenced in compounds 3 and 6. L−Fe···Fe−L Bending Angles (α and β). From the structural point of view, the monocoordinated terminal xL ligands diverge from the plane formed by the two Fe(II) ions and the two terminal ligand triazole groups (Figure 6). Here, we adopt a bending angle that describes the relationship between this divergence and SCO behaviors, which is defined as the angle between the line connecting the centers of phenyl D

DOI: 10.1021/acs.inorgchem.8b02789 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 5. Perspective view of the hydrogen bonds between NCS− and ligand H atoms in compounds 1−6.

Figure 6. Molecular structures of compounds 1−6 showing the bending angles (named as α and β).

beneficial for the spin transition in compounds 1 and 4. One possible reason for this relationship is that ligand with higher bending angles may exert weaker ligand-field strength on Fe(II) ion; thus, compounds 3 and 6 favor HS-only behavior in the measured temperature range. Besides the variation of molecular geometry arising from substituents, molecular packing effects may also play an important role in affecting their magnetic properties. These analogue compounds can be divided into three types, o-1/4, m2/5, p-3/6, based on their SCO behaviors and molecular structures. In fact, their molecular packing structures show similar types (Figures 7 and S4 and S5); here we only try to compare intermolecular interactions of the Me-substituted compounds 4−6. In these compounds, each molecule of [Fe2(MeL)5(NCS)4] contacts with neighboring ones through interligand C NSchiff base···πphenyl interactions. Among these, compound 4 shows the strongest interactions (3.389 Å) between the monocoordinated terminal MeL ligands and the neighboring ones, while its interactions between bridging MeL ligands and neighboring ones are the weakest (3.552 Å in average). Taking

and triazole groups and the plane comprising of Fe(II) ions and terminal ligand triazole coordinate-N atoms. As the o-substituted groups are disordered on 2-fold axis, the whole molecules are therefore symmetric, and the α and β values of compounds 1 (or 4) are identical. As shown in Table 2, the average α and β values of either Cl- or Me-substituted Table 2. L−Fe···Fe−L Bending Angles (α and β, °) of Compounds 1−6 α β average

1

2

3

4

5

6

8.176 8.176 8.176

10.44 42.522 26.481

57.343 31.778 44.561

7.176 7.176 7.176

21.552 7.236 14.394

62.035 30.195 46.115

compounds follows the same trend, i.e., p- > m- > o-. To correlate this trend with the magnetic properties, we can find that p-substituted compounds (3 and 6) are HS-only, whereas o-substituted compounds (1 and 4) undergo complete SCO, which suggest the smaller α and β values induced by intermolecular interactions and rigid crystal lattice are E

DOI: 10.1021/acs.inorgchem.8b02789 Inorg. Chem. XXXX, XXX, XXX−XXX

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a monotonic decrease. This trend is consistent with the SCO behaviors of compounds 4−6, as it is for compounds 1−3. Notably, the intermolecular interactions and rigid crystal lattice play important roles in determining the molecular geometry in the solid state. As shown in Figure S8, the optimization of the molecular structure in the gas phase induces a remarkable change in the molecular geometry. The energy difference between the HS and LS state of complex 2, ΔEHS‑LS, which is 3862 cm−1 based on the structures obtained directly from single crystal X-ray diffractions (Table S6), changes to an unreasonable negative value after structural optimization (Table S7). This result further verifies the deformation of molecular geometry induced by the intermolecular interactions and rigid crystal lattice is important for the spin transition behaviors found in these compounds. Moreover, the deformation of the molecular geometry engenders a distortion in the first coordination sphere of the metal centers. The octahedral distortion parameters, Σ for the HS Fe(II) of different complexes, are Σ(3) > Σ(2) ≈ Σ(1) and Σ(6) > Σ(5) > Σ(4) (Table S8), agreeing well with τ, α, and β found in the respective compounds. The stronger deviation of the ideal octahedral geometry usually leads to further splitting of the t2g and eg orbitals of metal centers, which potentially destabilize the LS state of the compounds. Correspondingly, HS-only magnetic behaviors were detected in the complexes of 3 and 6.

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CONCLUSIONS In summary, we have reported the crystal structures and magnetic properties of six dinuclear Fe(II) compounds with Cl- and Me-substituents on various positions. Through the detailed analysis on some structural factors and magnetic properties, we found that the geometric distortion in molecular structure accompanied by the intermolecular interactions play important roles in the spin transition of these compounds. These structural factors have been unveiled to be mainly determined by positions the substituents reside (steric effect) rather than their electronic nature (electronic effect). These results may suggest an effective way to control the SCO behaviors of similar dinuclear compounds.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b02789. Information on molecular and crystalline structures, magnetic properties, diffuse reflectance spectra, IR and TGA of complexes 1−6. 1H NMR spectra of o-ClL, mClL, p-ClL, o-MeL, m-MeL, and p-MeL (PDF) Accession Codes

Figure 7. Intermolecular interactions (Å) observed in compounds 4− 6.

CCDC 1870673−1870682 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.

account of its complete SCO behavior, it is possible to conclude that stronger interactions involving terminal ligands and weaker interactions involving bridging ligands favor SCOactive species. As the bridging ligands take part in the formation of more intermolecular interactions (six CN···π interactions in each compound), which ought to be one key point to distinguish the SCO properties. In fact, we can find that the average distances of CN···π interactions are 3.552, 3.496, and 3.476 Å for compounds 4−6, respectively, showing

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. F

DOI: 10.1021/acs.inorgchem.8b02789 Inorg. Chem. XXXX, XXX, XXX−XXX

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Guo-Yu Yang: 0000-0002-0911-2805 Jun Tao: 0000-0003-0610-4305 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 21325103, 21671161, 21701013, and 21701014) and the National Postdoctoral Program for Innovative Talents of China (BX201600015).

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DOI: 10.1021/acs.inorgchem.8b02789 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b02789 Inorg. Chem. XXXX, XXX, XXX−XXX