Single-Molecule-Magnet Behavior and Fluorescence Properties of 8

Aug 25, 2016 - Synopsis. The reaction of RE(dbm)3·2H2O (RE = Y, Eu, Tb, Dy, Lu) and 5-(4-fluorobenzylidene)-8-hydroxylquinoline resulted in five rare...
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Single-Molecule-Magnet Behavior and Fluorescence Properties of 8‑Hydroxyquinolinate Derivative-Based Rare-Earth Complexes Hong-Ling Gao,*,†,‡ Li Jiang,† Wen-Min Wang,† Shi-Yu Wang,† Hong-Xia Zhang,† and Jian-Zhong Cui*,† †

Department of Chemistry, Tianjin University, Tianjin 300354, People’s Republic of China State Key Laboratory of Medicinal Chemical BiologyNankai University, Nankai, Tianjin 300071, People’s Republic of China

‡

S Supporting Information *

ABSTRACT: Five tetranuclear rare-earth complexes, [RE4(dbm)4L6(μ3-OH)2] [HL = 5- (4-fluorobenzylidene)-8hydroxylquinoline; dbm = 1,3-diphenyl-1,3-propanedione; RE = Y (1), Eu (2), Tb (3), Dy (4), Lu (5)], have been synthesized and completely characterized. The X-ray structural analyses show that each [RE4] complex is of typical butterfly or rhombus topology. Each REIII center exists in an eightcoordinated square-antiprism environment. Magnetic studies reveal that complex 4 displays single-molecule-magnet behavior below 10 K under a zero direct-current field, with an effective anisotropy barrier (ΔE/kB = 56 K). The fluorescence properties of complexes 1−5 were also investigated. Complexes 2−4 showed their characteristic peaks for the corresponding REIII center, while complexes 1 and 5 showed the same emission peaks with the ligand when they were excited at the same wavelength.

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INTRODUCTION In recent years, rare-earth (RE) complexes have been a hot topic because of their potential technological applications in various areas such as sensors, information storage, spintronics, and quantum computing.1−4 Quinoline and its derivatives are known as a class of highly luminescent materials as well as promising materials for nonlinear optical properties and electroluminescent applications.5 They exhibit versatile coordination modes including chelating, chelating−bridging in μ- and μ3-phenoxo coordination modes, and bridging in μ-phenol modes.6 Taking advantage of the versatile coordination modes, 8-hydroxyquinoline and its derivatives have been utilized to ligate to transition metals with interesting properties,7 while recent examples have demonstrated the successful incorporation of REIII ions.8 RE complexes based on 8-hydroxyquinoline class ligands have been considered as some of the most promising materials for the design of electroluminescent devices,9 but studies of their magnetic properties are not widely investigated.10 Aside from studying the specific luminescent properties of RE complexes from quinoline derivatives with heteroaromatic highly π-conjugated and βdiketonate systems (Scheme 1), the magnetic properties of RE ions with large unquenched orbital angular momentum and strong spin−orbit coupling have motivated our great interest for the study. Single-molecule magnets (SMMs) featuring slow magnetic relaxation, such as mononuclear, dinuclear, and multinuclear complexes, have been prepared since the first SMM was reported in 1993.11 Up to now, Schiff-based ligands derived from the condensation of different amines and aldehydes have © XXXX American Chemical Society

Scheme 1. Structures of HL and dbm

proven to be particularly suitable for the construction of new SMMs, because of the N- and O-donor chelating groups to accommodate 4f ions.12 Therefore, we have designed and synthesized a Schiff-based ligand by the condensation of 5amino-8-hydroxyquinoline and 4-fluorobenzaldehyde. We have reported a series of RE complexes based on this ligand and acetylacetone (acac).13 In order to study the effects of the second ligand on the structure and properties of the complexes with the same metal center, herein we report five tetranuclear RE complexes, [RE4(dbm) 4L6 (μ3−OH)2 ] [HL = 5-(4fluorobenzylidene)-8-hydroxylquinoline; dbm = 1,3-diphenyl1,3-propanedione; RE = Y (1), Eu (2), Tb (3), Dy (4), Lu (5)], with butterfly or rhombus topology. As a result, slight changes have taken place in the ligand field according to the different perturbations from the second ligand, and the structures, fluorescence properties, and magnetic behaviors have changed. In this research, alternating-current (ac) susceptibility measurements show that complex 4 exhibits Received: June 13, 2016

A

DOI: 10.1021/acs.inorgchem.6b01420 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Crystal Data and Structure Refinements for Complexes 1−5 1 formula fw temperature (K) wavelength (Å) cryst syst space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) volume (Å3) Z calcd density (Mg m−3) abs coeff (mm−1) F (000) cryst size (mm3) θ range (deg) limiting indices reflns collected indep refln completeness (%) max and min transmn data/restraints/ param GOF on F2 final R indices [I > 2σ(I)] R indices (all data) largest diff peak and hole (e Å−3) a

2

3

4

5

C156H106F6N12O16Y4 2874.16 113(2) 0.71073 monoclinic P2(1)/c 18.539(4) 18.630(4) 22.426(5) 90 108.72(3) 90 7336(3) 2 1.301

C156H106F6N12O16Eu4 3126.36 113(2) 0.71073 monoclinic P2(1)/c 18.511(4) 18.714(4) 22.515(5) 90 108.94(3) 90 7377(3) 2 1.407

C156H106F6N12O16Tb4 3154.20 113(2) 0.71073 monoclinic P2(1)/c 18.399(4) 18.622(4) 22.385(5) 90 108.83(3) 90 7259(3) 2 1.443

C156H106F6N12O16Dy4 3168.52 113(2) 0.71073 monoclinic P2(1)/c 18.342(4) 18.432(4) 22.232(4) 90 108.94(3) 90 7109(2) 2 1.480

C156H106F6N12O16Lu4 3218.40 113(2) 0.71073 monoclinic P2(1)/c 18.511(4) 18.553(4) 22.350(5) 90 108.85(3) 90 11437(4) 2 1.471

1.639 2928 0.16 × 0.10 × 0.08 1.59−27.96 −16 ≤ h ≤ 24, −24 ≤ k ≤ 24, −29 ≤ l ≤ 29 66846 17518 [R(int) = 0.0733] 99.2 0.8800 and 0.7794

1.749 3120 0.20 × 0.12 × 0.10 1.59−27.92 −22 ≤ h ≤ 24, −23 ≤ k ≤ 24, −29 ≤ l ≤ 29 85145 17649 [R(int) = 0.0501] 99.8 0.8445 and 0.7211

1.998 3136 0.20 × 0.12 × 0.10 1.60−28.13 −23 ≤ h ≤ 24, −24 ≤ k ≤ 24, −29 ≤ l ≤ 29 91780 17580 [R(int) = 0.0634] 98.9 0.8252 and 0.6907

2.153 3144 0.18 × 0.12 × 0.10 1.61−28.32 −24 ≤ h ≤ 21, −24 ≤ k ≤ 24, −29 ≤ l ≤ 29 72642 17637 [R(int) = 0.0944] 99.5 0.8135 and 0.6979

2.768 3184 0.20 × 0.12 × 0.10 1.60−27.98 −24 ≤ h ≤ 17, −24 ≤ k ≤ 22, −29 ≤ l ≤ 29 65294 17381 [R(int) = 0.0407] 99.2 0.7693 and 0.6075

17518/0/875

17649/0/875

17580/0/875

17637/0/874

17381/0/875

0.990 R1 = 0.0594, wR2 = 0.1297 R1 = 0.0952, wR2 = 0.1445 0.680 and −0.824

1.104 R1 = 0.0372, wR2 = 0.0799 R1 = 0.0445, wR2 = 0.0838 1.767 and −1.043

1.083 R1 = 0.0478, wR2 = 0.1043 R1 = 0.0623, wR2 = 0.1118 2.097 and −1.536

1.028 R1 = 0.0651, wR2 = 0.1463 R1 = 0.0831, wR2 = 0.1630 5.511 and −2.989

1.093 R1 = 0.0310, wR2 = 0.0666 R1 = 0.0368, wR2 = 0.0690 1.904 and −1.129

R1 = ∑(||Fo| − |Fc||)/∑|Fo|. bwR2 = [∑w(|Fo|2 − |Fc|2)2/∑w(Fo2)2]1/2. the obtained samples for a period of time at ambient conditions, leading to the spontaneous loss of disordered solvent molecules. The crystalline samples used for all measurements were obtained from the same batch. Synthesis of Complexes 1−5. RE(dbm)3·2H2O [0.03 mmol; RE = Y (1), Eu (2), Tb (3), Dy (4), Lu (5)] was dissolved in 15 mL of acetonitrile. Then 5 mL of a CH2Cl2 solution of HL (0.0075 g, 0.03 mmol) was added to the stirred acetonitrile solution, and the mixture was stirred for 5 h at room temperature. Then the solution was filtered. Yellow block crystals suitable for single-crystal X-ray diffraction analysis were isolated by keeping the filtrate at ambient conditions for 1 week. [Y4(dbm)4L6(μ3-OH)2] (1). Yield based on Y: 0.038 g, 43.6%. Elem anal. Calcd for 1 (dried sample): C, 65.10; H, 3.71; N, 5.84. Found: C, 65.26; H, 3.58; N, 5.69. IR (cm−1): 3624(w), 3051(w), 1605(s), 1550(s), 1510(s), 1393(vs), 1315(s), 1220(m), 1087(m), 836(m), 765(m), 718(w), 615(w), 520(w), 467(w). [Eu4(dbm)4L6(μ3-OH)2] (2). Yield based on Eu: 0.036 g, 38.6%. Elem anal. Calcd for 2 (dried sample): C, 59.93; H, 3.42; N, 5.38. Found: C, 59.79; H, 3.56; N, 5.49. IR (cm−1): 3627(w), 3055(w), 1608(s), 1548(s), 1513(s), 1390(vs), 1316(s), 1222(m), 1086(m), 835(m), 762(m), 715(w), 617(w), 523(w), 465(w). [Tb4(dbm)4L6(μ3-OH)2] (3). Yield based on Tb: 0.046 g, 48.3%. Elem anal. Calcd for 3 (dried sample): C, 59.40; H, 3.39; N, 5.33. Found: C, 59.29; H, 3.52; N, 5.48. IR (cm−1): 3626(w), 3056(w), 1610(s), 1547(s), 1515(s), 1392(vs), 1318(s), 1220(m), 1087(m), 836(m), 763(m), 717(w), 615(w), 521(w), 468(w).

frequency-dependent out-of-phase signals, indicating slow relaxation of magnetization with an effective barrier of 56 K.

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EXPERIMENTAL SECTION

All chemicals and solvents used for the syntheses were reagent-grade without further purification. The starting materials RE(dbm)3·2H2O were synthesized according to the method in the literature.14 The Schiff-based ligand was formed from the condensation of 5-amino-8hydroxyquinoline and 4-fluorobenzaldehyde.13 Elemental analysis (EA) for C, H, and N was performed on a PerkinElmer 240 CHN elemental analyzer. IR spectra were recorded in the range of 400−4000 cm−1 with a Bruker TENOR 27 spectrophotometer using a KBr pellet. UV−vis spectra were recorded with a TU-1901 UV−vis spectrometer (Shanghai, China). Powder X-ray diffraction (PXRD) data were examined on a Rigaku Ultima IV instrument with Cu Kα radiation (λ = 1.54056 Å), with a scan speed of 5° min−1 in the range of 2θ = 5− 50°. Thermogravimetric analysis (TGA) was performed on a NETSCH 409 PC instrument with a heating rate of 10 °C min−1. Fluorescence spectra were measured with a Cary Eclipse luminescence spectrophotometer at room temperature. Magnetic measurements were carried out with Quantum Design MPMS-XL7 and PPMS-9 ACMS magnetometers. The diamagnetic corrections for the complexes were estimated using Pascal’s constants, and magnetic data were corrected for diamagnetic contributions of the sample holder.15 All crystalline samples characterized by PXRD, TGA, EA, IR, UV−vis, fluorescent, and magnetic studies were pretreated by keeping B

DOI: 10.1021/acs.inorgchem.6b01420 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. (a) Molecular structure of complex 4, with the C atoms from dbm and the H atoms omitted for clarity. (b) Coordination polyhedra for the adjacent DyIII ions in complex 4. [Dy4(dbm)4L6(μ3-OH)2] (4). Yield based on Dy: 0.041 g, 43.2%. Elem anal. Calcd for 4 (dried sample): C, 59.13; H, 3.37; N, 5.31. Found: C, 59.21; H, 3.22; N, 5.49. IR (cm−1): 3628(w), 3055(w), 1612(s), 1550(s), 1516(s), 1390(vs), 1317(s), 1221(m), 1086(m), 835(m), 765(m), 718(w), 616(w), 520(w), 467(w). [Lu4(dbm)4L6(μ3-OH)2] (5). Yield based on Lu: 0.038 g, 38.9%. Elem anal. Calcd for 5 (dried sample): C, 58.22; H, 3.32; N, 5.22. Found: C, 58.38; H, 3.37; N, 5.28. IR (cm−1): 3625(w), 3055(w), 1610(s), 1552(s), 1518(s), 1391(vs), 1318(s), 1220(m), 1086(m), 835(m), 764(m), 717(w), 615(w), 522(w), 469(w). X-ray Crystallography. Crystallographic studies of 1−5 were carried out on a Bruker SMART-1000 CCD diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) by using a ω−ϕ scan at 113(2) K. The crystals did not degrade during data collection. The structures were solved by direct methods and refined anisotropically by full-matrix least-squares methods based on F2 with the SHELXTL-97 program package for all non-H atoms.16 H atoms were located and included at their calculated positions. The crystals were measured as soon as they were isolated from the solution; thus, there were some disordered solvent molecules. The free solvent molecules were removed via SQUEEZE because of the extreme disorder that could not be solved. Crystallographic data and details of structural determination refinement are summarized in Table 1, and the selected bond lengths and angles are provided in Table S1. CCDC 1428255−1428259 contain the supplementary crystallographic data for complexes 1−5. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/ datarequest/cif.

one N atoms form the coordination sphere around Dy2. The two square bases of the square antiprism for Dy1 consist of O5, O6, N3, and N1 and O3, O4, O8, and O7a, whereas for Dy2, the two square bases are defined by the atoms O1, O2, O5, and O6 and N5, O7, O8, and O8a. A square antiprism can be described by an angle (α) describing its elongation or flatness.17 For a soft-sphere model with a repulsion energy law α ≈ 1/r6, the ideal α value amounts to 57.16°. For complex 4, the α angles vary from 52.7 to 64.9°, indicating that each central DyIII ion displays a rather distorted square-antiprismatic geometry for the large steric hindrance (Figure 1b). The two square antiprisms share three O atoms, O5, O6, and O8, which form a near isosceles triangular face (Figure 1b) with distances of 2.70, 2.71, and 2.61 Å for O8···O5, O8···O6, and O5···O6, respectively. All of the Dy−O distances in 4 are in the range of 2.287(3)−2.391(4) Å, the Dy−N bond lengths are in the range of 2.497(4)−2.521(5) Å, and the O−Dy−O angles are in the range of 67.81(12)−143.99(13)°. The shortest intramolecular Dy ··· Dy distance of 3.5233(6) Å is on the edge of the parallelogram between Dy1 and Dy2 as well as between Dy1a and Dy2a (Figure 2). The packing diagram is provided in Figure S3.

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RESULTS AND DISCUSSION Descriptions of the Complexes. X-ray crystallographic analysis reveals that complexes 1−5 are isomorphous in the monoclinic space group, and the structure of 4 is described as an example. This is typical butterfly or rhombus topology. There are two μ3-hydroxyls at the center of the complex. They bridge the central body ions (Dy2 and Dy2a) to the outer wingtip ions (Dy1 and Dy1a) (Figure 1a) The two hydroxyls lie 0.8716 Å above and below the plane of the metal sites. All eight-coordinated Dy III ions are linked together by a combination of the two O atoms (O8 and O8a) of the μ3OH ligands and phenoxo O atoms (O5, O6, O7, O5a, O6a, and O7a). There are four bidentate anionic dbm− groups above and below the planar core coordinating to four DyIII ions, respectively. In the centrosymmetric complex 4, the Dy1 ion is coordinated to six O and two N atoms, while seven O and

Figure 2. Parallelogram view of the crystal structure of 4, with the H atoms and partial HL omitted for clarity. C

DOI: 10.1021/acs.inorgchem.6b01420 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry PXRD and TGA. The crystalline products of 1−5 have been characterized by PXRD at room temperature (Figure S1). The observed PXRD patterns are largely consistent with the results simulated from the single-crystal data. Several missing or extra minor peaks could be attributed to the loss of the solvent molecules in the unit cell during the sample preparation, which is unavoidable. TGA was performed on complexes 1−5 to investigate their thermal stabilities with a heating rate of 10 °C min−1 in the temperature range of 30−800 °C under an air atmosphere. The TGA curves of 1−5 are provided in Figure S2. For 1−5, the TGA curves have similar profiles, exhibiting one main weight loss step between 300 and 600 °C. UV−Vis Spectra. The UV−vis absorption spectra of complexes 1−5 and HL were measured in a methanol solution (10−4 mol L−1) at ambient temperature (Figure 3). HL consists

respectively. The observed main peaks at ca. 262 nm might be attributed to the intraligand transitions of L− and dbm− ligands. Also, the distinct absorption bands at 360 nm of complexes 1− 5 can be ascribed most likely to the extended n → p* transitions in Schiff-based ligands bound to the REIII ions. In addition, the broad low-energy band is slightly blue-shifted compared to that of the free Schiff-based ligand, which might be assigned to the coordination effect between the Schiff-based ligand and REIII cations. Luminescence Properties. The photoluminescence of complexes 1−5 in a methanol solution was investigated at room temperature. For 2, the characteristic peak of the EuIII ion at 617 nm was observed when excited at 308 nm (Figure 4a). However, the emission spectrum of complex 3 exhibits four major TbIII emission peaks at 492, 545, 589, and 621 nm. The first emission band at 491 nm can be assigned to the transition of 5D4 → 7F6, while the other bands at 545, 585, and 626 nm can be attributed to the 5D4 → 7F5, 5D4 → 7F4, and 5D4→ 7F3 transitions, respectively 18 (Figure 4b). For 4, typical luminescence peaks at 574 and 627 nm can be assigned to the transitions of 4F9/2 → 6H13/2 and 4F9/2 → 6H11/2, respectively19 (Figure 4c). For complexes 1 and 5, because of the full-filled orbital electron configuration of Y and Lu ions, they would not show their typical emission peaks. The main emission bands shown in Figure 4d can be attributed to π → π* transitions of the ligand. When complexes 1 and 5 were excited at 250 nm, their emission spectra exhibit several major emission peaks consistent with the ligand in a methanol solution. From the emission spectra, the luminescence intensity of the two complexes is not higher than that of the free ligand, indicating that Y and Lu ions cannot induce the luminescence enhancement for the ligand. Magnetic Properties. The magnetic properties of complexes 3 and 4 were investigated by solid-state magnetic

Figure 3. UV−vis absorption spectra of complexes 1−5 and HL.

of three main absorption bands centered at ca. 210, 249, and 368 nm, respectively. In contrast, complexes 1−5 display three analogous sets of absorption bands at ca. 210, 263, and 360 nm,

Figure 4. Luminescence spectra of complexes 1−5 and HL in a methanol solution (a) for 2, (b) for 3, (c) for 4, (d) and for 1, 5, and HL. D

DOI: 10.1021/acs.inorgchem.6b01420 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

signals of 4 show a temperature dependence, which also reveals the presence of slow relaxation of magnetization in complex 4. The relaxation time τ data derived from the χ″ peaks follows the Arrhenius law [τ = τ0 exp(ΔE/kBT)] at a temperature range of 7−10 K, with the preexponential factor τ0 = 2.64 × 10−7 s and the effective anisotropy barrier ΔE/kB = 56 K (Figure 7),

susceptibility measurements in the range of 2−300 K on a Quantum Design PPMS XL-7 SQUID magnetometer in an applied magnetic field of 1000 Oe. The crystalline samples for all magnetic measurements were vacuum-dried to avoid the influence of the interstitial solvent molecules on the magnetic measurements. The plots of χMT versus T for complexes 3 and 4 are shown in Figure 5. At room temperature, the χMT values are 46.66 (3)

Figure 7. Plot of ln(τ) versus T−1 fitting to the Arrhenius law for complex 4 under a zero dc field.

Figure 5. Temperature dependence of the χMT product for complexes 3 and 4 at 2−300 K with a dc applied field of 1000 Oe.

which are comparable to those of reported Dy-based SMMs.22 The Cole−Cole plots of χ″ vs χ′ were obtained and fitted to the generalized Debye model to obtain α values (Figure 8). It

and 55.03 (4) cm3 K mol−1, respectively, which are in good agreement with the expected values of 47.28 and 56.68 for four uncoupled LnIII ions (7F6, g = 3/2 for TbIII; 6H15/2, g = 4/3 for DyIII). As the temperature is lowered, the χMT values of complexes 3 and 4 gradually decrease from 300 to 50 K and then further decrease rapidly to reach a minimum of 36.73 cm3 K mol−1 for 3 and 30.61 cm3 K mol−1 for 4 at 2 K. The drop of χMT for both complexes is due to depopulation of the MJ sublevel and antiferromagnetic interactions between Ln ions.20 In order to investigate the dynamics of magnetization of complex 4, both the temperature and frequency dependencies of the ac susceptibility were measured under a zero directcurrent (dc) field. From the temperature dependence of the ac susceptibility (Figure 6), complex 4 shows strong frequencydependent in-phase (χ′) and out-of-phase (χ″) signals, indicating the presence of slow magnetic relaxation at lower temperature, typical of SMM behavior. The parameter ϕ = (ΔTp/Tp)/Δ(log ν) = 0.32, calculated by the data extracted from the temperature dependence of in-phase ac susceptibility, excludes the possibility of spin-glass behavior (0.01 < ϕ < 0.08).21 From the frequency dependence of ac susceptibility (Figure S4), both the in-phase (χ′) and out-of-phase (χ″)

Figure 8. Cole−Cole plots for complex 4 measured in a zero dc field. The solid lines are the best fit to the experimental data, obtained with the generalized Debye model with α = 0.39−0.48.

shows that nearly a semicircle shape and α = 0.39−0.48 were found over the temperature range of 4−10 K. The relatively large distribution coefficient α is due to two symmetrically

Figure 6. Temperature dependence of the in-phase and out-of-phase components of the ac magnetic susceptibility for complex 4 under a zero dc field with an oscillation of 3.0 Oe. E

DOI: 10.1021/acs.inorgchem.6b01420 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry independent magnetic centers in complex 4.13 For complex 3, the imaginary component χM″ does not show frequency dependence (Figure S5). This indicates that complex 3 does not express SMM behavior.

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CONCLUSION In summary, a series of rhombus-shaped tetranuclear complexes containing the [RE4(dbm)4L6(μ3-OH)2] unit have been successfully synthesized. Magnetic investigation reveals that a Dy4 complex displays slow magnetic relaxation behavior. Under a zero dc field, the energy barrier ΔE/kB = 56 K with the preexponential factor τ0 = 2.64 × 10−7 s can be obtained via the ac susceptibility study. So, it seems that the magnetic properties would be affected by the steric hindrance of the second ligand. Also, the coordination environment around the metal center of high steric hindrance is not conducive to obtaining higheffective-barrier SMMs.

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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.6b01420. PXRD patterns, TGA curves, packing diagram of 4, selected bond lengths and angles, and kinetic data (PDF) X-ray crystallographic data in CIF format (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.-L.G.). *E-mail: [email protected] (J.-Z.C.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported financially by the State Key Laboratory of Medicinal Chemical Biology (Nankai University; Opening Foundation No. 201603002) and the National Natural Science Foundation of China (Grants 21473121, 21271137, and 21571138).

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REFERENCES

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DOI: 10.1021/acs.inorgchem.6b01420 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acs.inorgchem.6b01420 Inorg. Chem. XXXX, XXX, XXX−XXX