Two Barium Gold Iodates: Syntheses, Structures, and Properties of

Jun 2, 2017 - Two new barium gold iodates, namely, BaAu(IO3)5 and HBa4Au(IO3)12, have been prepared. BaAu(IO3)5 crystallizes in the polar space group ...
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Two Barium Gold Iodates: Syntheses, Structures, and Properties of Polar BaAu(IO3)5 and Nonpolar HBa4Au(IO3)12 Materials Bing-Ping Yang, Chun-Li Hu, Fei-Fei Mao, Xiang Xu, and Jiang-Gao Mao* State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, P. R. China S Supporting Information *

ABSTRACT: Two new barium gold iodates, namely, BaAu(IO3)5 and HBa4Au(IO3)12, have been prepared. BaAu(IO3)5 crystallizes in the polar space group Pca21, whereas HBa4Au(IO3)12 crystallizes in the centrosymmetric space group P21/c. BaAu(IO3)5 consists of unique polar [Au(IO3)4]− anions whose four iodate groups are located at both sides of the AuO4 plane and the polarity points in the [001]̅ direction. BaAu(IO3)5 displays strong second-harmonic-generation (SHG) effects about 0.6KTiOPO4 (KTP) and is phase-matchable. Thermal properties, optical spectra analyses, and theoretical calculations are also reported.

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INTRODUCTION Metal iodates are of current interest because of their valuable second-order nonlinear-optical (NLO) properties, ferroelectric, piezoelectric, and pyroelectric properties.1−3 Other kinds of NLO materials, such as borates with π-conjugated groups,4−8 carbonates,9,10 nitrates,11,12 phosphates with asymmetric POn polyhedra, 13,14 and chalcogenides with polar pyramidal units,15−18 have also been widely studied. The best known examples of iodate crystals are α-LiIO319−21 and KIO3.22,23 Iodates can exhibit large second-harmonic-generation (SHG) effects because of the fact that their special pyramidal IO3− anions, with lone electron pairs of central I5+ cations outward from the base formed by the three O atoms, can lead to large polar distributions of electron density. Several pyroiodates with intense SHG activity have been discovered.24−26 A few iodates that show significant SHG efficiency have been synthesized over the years by introducing other asymmetric structural units such as d0 transition-metal cations with second-order Jahn−Teller distortion27−32 and cations with a lone electron pair into iodates.33−35 Some fluorine-containing iodates with strong SHG effects have also been synthesized recently.36−38 The d8 noble-metal iodates have attracted new attention for their outstanding SHG effects. For instance, α-NaAu(IO3)4, K2Au(IO3)5, β-KAu(IO3)4, RbAu(IO3)4, and α-CsAu(IO3)4 can produce very strong SHG effects, greater than those of KTP.39,40 BaPd(IO3)4 generates a strong SHG response of ∼0.4KTP.41 All of the d8 metal cations © 2017 American Chemical Society

in those compounds are in the square-planar coordination geometry, implying that the square-planar coordination configuration may be favorable for constructing a noncentrosymmetric (NCS) structure. As far as we know, the research of gold iodates is limited to the alkali-metal-cation system.42 In order to investigate the effects of the alkali-earthmetal cations and the square-planar coordination configuration of gold cations on the NCS structure, research of the alkaliearth-metal gold iodate system has been conducted. Our efforts achieved two barium compounds: BaAu(IO3)5 and HBa4Au(IO3)12. BaAu(IO3)5 is NCS and polar, whereas HBa4Au(IO3)12 is centrosymmetric. BaAu(IO3)5 exhibits phasematching behavior with an SHG coefficient of ∼0.6KTP. In this paper, we report their synthesis, structures, spectroscopy, thermal properties, NLO properties, and theoretical calculations. The comparison of the Ba−Au−I−O and AI−Au−I−O systems will also be discussed.

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EXPERIMENTAL SECTION

Synthesis. BaCO3 (99.8%, Aladdin), BaO (99.5%, Aladdin), Au(OH)3 (Au 79%, Alfa-Aesar), Au2O3 (Au 89%, Fluka), and I2O5 (99%, Aladdin) were used as received. Millipore filtered water with a resistivity of 18.25 MΩ·cm was used in all reactions. All reactions were performed using 1 mL of water in 23 mL Teflon-lined autoclaves. The Received: April 5, 2017 Published: June 2, 2017 7230

DOI: 10.1021/acs.inorgchem.7b00872 Inorg. Chem. 2017, 56, 7230−7236

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Inorganic Chemistry sealed autoclaves were placed in a furnace and heated at 200 °C for 3 days. Then the furnace was cooled to room temperature at a rate of 6 °C h−1. Microprobe elemental analyses were performed with a fieldemission scanning electron microscope (JSM6700F) equipped with an energy-dispersive X-ray spectroscope (Oxford INCA). BaAu(IO3)5 was synthesized from BaCO3 (19.7 mg, 0.1 mmol), Au(OH)3 (24.7 mg, 0.1 mmol), and I2O5 (667 mg, 2 mmol). The pH value of the reaction mother liquid was ∼1. The product consisted of orange prismatic crystals of BaAu(IO3)5 and a small amount of byproducts including colorless Ba(IO3)2 crystals and gold particles, which were removed manually. The yield was 65% (78 mg) based on Au. Energy-dispersive X-ray spectroscopy (EDS) analysis of BaAu(IO3)5 provided an average Ba/Au/I ratio of 0.9:1:4.8. HBa4Au(IO3)12 was synthesized using the same method. The starting materials were BaO (61.3 mg, 0.4 mmol), Au2O3 (22.1 mg, 0.05 mmol), and I2O5 (500.7 mg, 1.5 mmol). The product consisted of yellow rhombic crystals in a yellow mother liquid. The pH value of the mother liquid was ∼1. Yield: 83 mg (58% based on Au). EDS analysis of HBa4Au(IO3)12 revealed an average Ba/Au/I ratio of 3.9:1:11.7. Single-Crystal X-ray Diffraction (XRD). Single-crystal XRD data of the two compounds were collected at 273 K on an Agilent Technologies SuperNova or Rigaku Saturn724+ CCD diffractometer, using Mo Kα radiation (λ = 0.71073 Å). Absorption corrections were performed based on the analytical numeric method43 [for BaAu(IO3)5] or the multiscan method44 [for HBa4Au(IO3)12]. All structures were solved by direct methods and refined by a full-matrix leastsquares fitting on F2 using SHELXTL-97.45 The Flack parameter (absolute structure parameter) of BaAu(IO3)5 was refined to 0.454(18), which indicated that it was a racemic twinning structure. The PLATON program46 was used to check the correctness of the two structures. Crystallographic data are summarized in Table 1. Selected

in Al2O3 crucibles and heated under a N2 atmosphere from 30 to 1000 °C at a heating rate of 10 °C min−1. The IR spectra were collected on a Bruker Optics VERTEX 70 Fourier transform infrared spectrometer from KBr pellets in the 4000−400 cm−1 region. The UV−vis−near-IR (NIR) diffuse-reflectance spectra were recorded on a PerkinElmer Lambda 950 spectrophotometer in the range of 300−2500 nm. Powder SHG measurements were carried out on a modified Kurtz and Perry instrument using a 2.05 μm Q-switched laser.47 Polycrystalline samples of BaAu(IO3)5 were ground, sieved, and divided into six size ranges (25−45, 45−75, 75−105, 105−150, 150−210, and 210− 270 μm) because of the fact that their SHG intensities depend highly on the particle sizes. KTP samples (powder size 210−270 μm) were used as the reference material in order to compare the SHG intensity.

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RESULTS AND DISCUSSION Synthesis. BaCO3 can be replaced by BaO and Au(OH)3 can be substituted by Au2O3 as the starting materials. The synthesis conditions in the Experimental Section are optimal. Structure. BaAu(IO3)5 and HBa4Au(IO3)12 represent the first alkali-earth-metal gold iodates. BaAu(IO3)5 is polar, whereas HBa4Au(IO3)12 is centrosymmetric. The ratio of the reactants was supposed to be the principal factor. BaAu(IO3)5 crystallizes in the orthorhombic polar space group Pca21 (No. 29) with axes of a = 7.6880(2) Å, b = 13.0966(4) Å, and c = 13.9866(5) Å (Table 1). The asymmetric unit of BaAu(IO3)5 consists of one Ba2+ cation, one isolated (IO3)− anion, and one polar [Au(IO3)4]− anion unit where a Au3+ cation is coordinated by four iodate groups in a square-planar geometry (Figure 1). The mean deviation from the AuO4 plane is 0.016 Å. The Au−O bond lengths are 1.93(2), 1.96(3), 1.96(3), and 2.01(2) Å. All I−O bonds are also normal, with the bond lengths ranging from 1.72(3) to 1.93(2) Å. The angles around Au are close to those expected for square-planar coordination and are in the ranges of 86.3(10)−92.6(13)° and 176.1(10)−176.7(10)°. The four iodate groups of the [Au(IO3)4]− anion are located at both sides of the AuO4 plane, and the polarity of the [Au(IO3)4]− anion points in the [001̅] direction, which is tilted about 19° away from the AuO4 plane (Figure S1). This kind of [Au(IO3)4]− unit is distinct from that of NCS AAu(IO3)440 or K2Au(IO3)5,39 where all of the iodate groups are located at the same side of the AuO4 plane and the polarity of the [Au(IO3)4]− molecule is perpendicular to the AuO4 plane. Ba2+ cations and isolated (IO3)− anions fill the voids between the [Au(IO3)4]− anions, which stack along the a axis to balance the charge (Figure 2). Each Ba2+ cation is coordinated by eight O atoms, and the Ba−O bond distances range from 2.60(3) to 2.96(2) Å. Bond valence sum (BVS) values have been calculated according to the literature.48,49 The calculated BVS values are 2.205, 2.808, 4.952, 4.792, 5.104, 4.811, and 4.943 for Ba, Au, I1, I2, I3, I4, and I5, respectively. HBa4Au(IO3)12 crystallizes in the centrosymmetric monoclinic space group P21/c (No. 14) with axes of a = 7.663(2) Å, b = 33.001(8) Å, c = 7.729(2) Å, and β = 117.572(3)° (Table 1). The asymmetric unit of HBa4Au(IO3)12 consists of one Au3+ cation, two Ba2+ cations, and six (IO3)− anions. A H ion is required to balance the charge, but it is difficult to determine its exact location. The Au3+ cation is on the special position (1/2, 1 /2, 1/2) of the P21/c space group (Wyckoff position 2b, site symmetry 1̅; Figure 1). One Au3+ cation and four O atoms from four iodate groups are bound in a coplanar arrangement to form a centrosymmetric [Au(IO3)4]− anion. The four iodate groups of the [Au(IO3)4]− anion unit are located at both sides of the AuO4

Table 1. Crystallographic Data for BaAu(IO3)5 and HBa4Au(IO3)12 formula fw cryst syst space group a/Å b/Å c/Å β/deg V/Å3 Z Dcalc/g cm−3 μ(Mo Kα)/mm−1 θmax/deg completeness/% GOF on F2 R1, wR2 [I > 2σ(I)]a R1, wR2 (all data) Flack param

BaAu(IO3)5 1208.81 orthorhombic Pca21 7.6880(2) 13.0966(4) 13.9866(5) 90 1408.26(8) 4 5.701 24.225 31.02 96.9 1.237 0.0728, 0.1681 0.0742, 0.1689 0.454(18)

HBa4Au(IO3)12 2846.13 monoclinic P21/c 7.663(2) 33.001(8) 7.729(2) 117.572(3) 1732.6(8) 2 5.456 19.512 27.68 97.7 1.123 0.0491, 0.0973 0.0558, 0.1013 N/A

R 1 = ∑||F o | − |F c ||/∑|F o |. wR 2 = {∑w[(F o )2 − (F c ) 2 ] 2 / ∑w[(Fo)2]2}1/2.

a

bond lengths and angles are listed in Table S1. CCDC 1550238− 1550239 contain the supplementary crystallographic data for this paper and can also be obtained from the ICSD database (CSD number 432826 and 432827; Web site https://www.fiz-karlsruhe.de/ and email crysdata@fiz-karlsruhe.de). Physical Measurements. The powder XRD data were collected using a Rigaku MiniFlexII diffractometer with Cu Kα radiation. The powder samples were scanned in the 2θ range from 5° to 65° with a detector step width of 0.05°. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were performed on a NETZSCH STA 449F3 instrument. About 5 mg samples were loaded 7231

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Figure 2. View of the 3D structures of BaAu(IO3)5 and HBa4Au(IO3)12.

These alkali-earth-metal gold iodates display completely different structures compared with the alkali-metal gold iodates, which is probably caused by the size and coordination mode of the countercations and the flexibility of the [Au(IO3)4]− units.50 IR Spectroscopy. IR spectra of the two compounds show the characteristic (IO3)− vibrational bands at ∼ 647−770 cm−1 (Figure S3). The Au−O vibrational bands locate at 498 cm−1 for BaAu(IO3)5 and at 497 and 555 cm−1 for HBa4Au(IO3)12. These peaks are in good agreement with those of the other gold(III) iodates. UV−Vis−NIR Diffuse-Reflectance Spectroscopy. The diffuse-reflectance spectrum of BaAu(IO3)5 indicates that the polar material is transparent in the 0.53−2.5 μm region (Figure 3). The reflectance data were converted to absorption data (K/ S) using the Kubelka−Munk function:51 F(R) = K/S = (1 − R)2/2R, where R is the reflectance, K is the absorption coefficient, and S is the scattering coefficient. Optical band gaps were extrapolated from the absorption edge to the baseline in K/S versus E diagrams. Approximations of the band gap, Eg, are 2.32 eV for BaAu(IO3)5 and 3.03 eV for HBa4Au(IO3)12. Thermal Studies. The TGA and DSC curves of BaAu(IO3)5 and HBa4Au(IO3)12 are very similar. The DSC curves both show one step of mass loss in the temperature range of 30−500 °C (Figure 4). For BaAu(IO3)5, the mass loss starts at 393 °C and ends at 460 °C and corresponds to the endothermic peaks at 401 and 423 °C of the DSC curve. The total mass loss is 42.8%, which corresponds to the release of 1.5 I2 and 4.5 O2 per formula unit (calcd value 43.4%). As for HBa4Au(IO3)12, the mass loss begins at 410 °C and finishes at 480 °C and corresponds to two endothermic peaks at 418 and 431 °C. The total mass loss is 23.7%, which will be close to the calculated value (24.6%) if the residual is supposed to be a mixture of Ba(IO3)2 and Au.

Figure 1. View of the asymmetric units of BaAu(IO3)5 and HBa4Au(IO3)12. In the BaAu(IO3)5 graph, the green arrows represent the dipole moments of the IO3 groups.

plane, which is similar to that of the centrosymmetric AAu(IO3)4 compound.40 The local dipole moments of all of the IO3 groups are canceled out, corresponding to the centrosymmetric space group P21/c. The Au−O bond lengths range from 1.965(13) to 2.006(9) Å. The bond angles around Au correspond with those expected for coplanar coordination and are in the ranges of 88.7(4)− 91.3(4)° and 179.999(6)−179.999(3)°. Each I5+ cation is in a triangular-pyramidal environment and connects to three O atoms. Least-squares refinements suggest splitting of the site of the three O atoms of the I6O3 group. Therefore, there are six O atoms that are disorderly with a half site occupation around I6. The I−O bond distances range from 1.759(12) to 1.899(10) Å without taking into account the disordered iodate group. The discrete [Au(IO3)4]− molecules stack directly on top of one another along the a axis (Figure 2). The Ba2+ cations and isolated (IO3)− anions lie in the void space between the [Au(IO3)4]− molecules. Ba1 and Ba2 cations are all bound to eight O atoms that are provided by eight (IO3)− anions. The Ba−O bond distances are in the ranges of 2.659(9)−3.074(10) and 2.69(3)−2.891(9) Å for Ba1 and Ba2, respectively. The calculated BVS values of Ba1, Ba2, Au, I1, I2, I3, I4, I5, and I6 are 1.973, 1.951, 2.653, 5.119, 4.963, 4.933, 4.960, 5.019, and 5.252, respectively. 7232

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Figure 4. TGA and DSC curves for BaAu(IO3)5 (a) and HBa4Au(IO3)12 (b). Figure 3. UV−vis−NIR diffuse-reflectance spectra for BaAu(IO3)5 (a) and HBa4Au(IO3)12 (b).

NLO Properties. Because BaAu(IO3)5 is polar and the UV−vis−NIR diffuse-reflectance spectrum of BaAu(IO3)5 indicates that the material is transparent in the 0.53−2.5 μm region, the SHG properties of BaAu(IO3)5 have been measured using the 2.05 μm Q-switched laser. The diagrams of the SHG intensity versus various particle sizes indicate that BaAu(IO3)5 is phase-matchable based on the Kurtz and Perry definition. Moreover, using a 2.05 μm Q-switched laser, the SHG signals that were produced by BaAu(IO3)5 and KTP samples of the same size (210−270 μm) have been compared, which reveals that BaAu(IO3)5 exhibits a strong SHG effect of approximately 0.6 times that of KTP (Figure 5). The SHG effect of BaAu(IO3)5 is slightly smaller than that of the reported K 2 Au(IO 3 ) 5 (1.0KTP) and β-KAu(IO 3 ) 4 (1.3KTP). In order to analyze the polarity of BaAu(IO3)5 and the causes for the differences of the SHG effects between BaAu(IO3)5 and A−Au−I−O compounds, the local dipole moments of the (IO3)− groups and the net dipole moment in the unit cell have been calculated (Table S2). The calculated magnitudes of the local dipole moments of the (IO3)− groups fall within the range of 14.92−17.36 D. The x and y components of the dipole moments of the (IO3)− groups over a unit cell are canceled out completely; hence, the net dipole moment over a cell is 186.19 D and points in the [001̅] direction. The dipole moment per unit volume of BaAu(IO3)5 (0.132 D Å−3) is slightly higher than those of K2Au(IO3)5 (0.114 D Å−3) and β-KAu(IO3)4 (0.099 D Å−3). The possible reason for the inverse SHG tendency and the relatively low

Figure 5. Diagram of the particle size versus SHG intensity of BaAu(IO3)5 under laser radiation at λ = 2.05 μm. Oscilloscope traces of the SHG signals for BaAu(IO3)5 (212−270 μm) and KTP samples of the same size are plotted in the inset.

SHG response of BaAu(IO3)5 may be its racemic twinning structure. Theoretical Studies. To investigate the relationship between the structure and the SHG properties of BaAu(IO3)5, a detailed theoretical study was performed based on density functional theory (DFT) methods (S7 in the Supporting Information). The band structure of BaAu(IO3)5 is displayed in Figure 6. Obviously, BaAu(IO3)5 has a direct band gap with both the valence band (VB) maximum and the conduction band (CB) minimum locating at the X point (Table S3). The calculated band gap (2.10 eV) is slightly lower than the experimental one (2.32 eV). The underestimation of the band gap is due to the common limitation of the generalized gradient approximation Perdew−Burke−Ernzerhof function. To accurately study the 7233

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symmetry made it have three independent SHG tensors. The SHG tensors d31, d32, and d33 were calculated to be 2.16 × 10−8, 9.11 × 10−9, and 2.42 × 10−8 esu, respectively. Despite the fact that the calculated coefficients based on a periodic crystal are greater than the experimental value measured on powder (0.6 times that of KTP), it still confirms that BaAu(IO3)5 possesses a high SHG effect. Meanwhile, we also calculated the frequency-dependent refractive indices of the compound (Figure 8). The birefringence of the compound is calculated to be 0.224, which is large enough to achieve phase matching in the SHG process. To gain overall knowledge of the SHG-contributed electronic states, spectral decomposition of the largest tensor of d33 for BaAu(IO3)5 was performed, as shown in the bottommost panel of Figure 7. Obviously, the most contributing region is just between −6.0 and +5.0 eV, in which the top VB (−1.1 to 0 eV) corresponding to the O 2p nonbonding states mixing with some Au d states and the bottommost CB (0 to 5 eV) corresponding to the unoccupied I 5p, O 2p, and Au dx2−y2 states give positive contributions, while the states of −6.0 to −1.1 eV play a negative role for the SHG response of the material. To exhibit intuitive SHG-contributed orbitals, SHG density analyses have been made for BaAu(IO3)5, as shown in Figure 9.

Figure 6. Calculated band structure of BaAu(IO3)5.

optical properties of BaAu(IO3)5, a scissor value of 0.22 eV (the difference between the measured and calculated gaps) was added in subsequent analyses. The scissor-added partial density of states is plotted in Figure 7. The electronic states of I and Au overlap fully with those of

Figure 8. Calculated frequency-dependent refractive indices of BaAu(IO3)5. Figure 7. Scissor-added partial density of states (the top four panels) and the spectral decomposition of d33 (the bottommost panel) for BaAu(IO3)5.

In the VB, the O 2p nonbonding states, especially those of the isolated IO3 groups, have the main SHG contribution, and the Au dxy states also make a small contribution, but some orbitals contribute negatively. In the CB, the empty orbitals of Au dx2−y2

O in almost the whole energy range, indicating the strong bonding interactions of the I−O and Au−O bonds. The peaks in the lowest VB range (−25.0 to −16.4 eV) mainly originated from the O 2s, I 5s5p, and Ba 5s states, mixing with a mite of O 2p. The bands in the higher range (−11.2 to −8.5 eV) are comprised of the Ba 5p, I 5p, and O 2s2p states. The outmost p electronic states of O and I, as well as Au 5d, take charge of the region near the Fermi level (−7.8 to 0 eV). The CB regions are contributed by the empty s, p, and d orbitals of almost all of the atoms in the structural unit. The VB top and CB bottom are dominated by the nonbonding O 2p states and the unoccupied dx2−y2 states of Au, respectively, indicating that the band gap of BaAu(IO3)5 is determined by Au and O atoms. The static SHG coefficients of BaAu(IO3)5 have been calculated by employing the independent particle approximation method. Restriction of the space group and Kleinman’s

Figure 9. SHG density of d33 in the VB (a) and CB (b) for BaAu(IO3)5. 7234

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ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (Grants 21231006, 91622112, and 21003127), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant XDB20000000), and the Natural Science Foundation of Fujian Province (Grant 2015J05045) for their financial support.

and the O 2p bonded to Au are the major SHG contributors, while I 5p and other O 2p states contribute a little to SHG. The contribution values of all kinds of ions or groups have been further calculated by summing the SHG density in the VB and CB, and the SHG-contributed ratios of the Ba2+, AuO4, and IO3 groups are 0.82%, 17.38%, and 80.81%, respectively. The results indicate that AuO4 groups are also important in the SHG process of BaAu(IO3)5 in addition to the IO3 groups; in comparison, the alkali-earth-metal Ba2+ cations are negligible. The facts are very similar to our previous study on K2Au(IO3)5.

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CONCLUSIONS In summary, two new alkali-earth-metal gold iodates, namely, BaAu(IO3)5 and HBa4Au(IO3)12, were prepared and studied. With the same kinds of starting materials but different reaction conditions, BaAu(IO3)5 is polar, while HBa4Au(IO3)12 is centrosymmetric. Their structures are distinct from those of the alkali-metal gold iodate system, although they are composed of [Au(IO3)4]− anion units, isolated iodate anions, and countercations. A new type of [Au(IO3)4]− polar unit, where four iodate groups are located at both sides of the AuO4 plane, has been discovered in the polar material BaAu(IO3)5. The calculated net dipole moment per unit volume of BaAu(IO3)5 is slightly larger than that of AAu(IO3)5. Moreover, theoretical calculations demonstrate that BaAu(IO3)5 has a large SHG coefficient and the birefringence is big enough to achieve phase matching in the SHG process. Powder SHG measurements using 2.05 μm laser radiation reveal that BaAu(IO3)5 exhibits a strong SHG effect about 0.6KTP. The SHG effect of BaAu(IO3)5 is relatively lower than that of AAu(IO3)4, which may be caused by the intrinsic racemic twinning quality. Furthermore, BaAu(IO3)5 has a high thermal stability up to 393 °C and has a high transmittance in the spectral range of 530−2500 nm. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00872. Additional tables and figures and computational methods (PDF) Accession Codes

CCDC 1550238−1550239 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

Xiang Xu: 0000-0003-4132-5322 Jiang-Gao Mao: 0000-0002-5101-8898 Notes

The authors declare no competing financial interest. 7235

DOI: 10.1021/acs.inorgchem.7b00872 Inorg. Chem. 2017, 56, 7230−7236

Article

Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.7b00872 Inorg. Chem. 2017, 56, 7230−7236