Mo6+ Cation Enrichment of the Structure Chemistry of Iodates

Jul 19, 2018 - *H.W.: e-mail, [email protected]; tel, (+86)991-3674558; fax, ... Ba3[(MoO2)2(IO3)4O(OH)4]·2H2O, and Sr[(MoO2)6(IO4)2O4]·H2O were ...
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Mo6+ Cation Enrichment of the Structure Chemistry of Iodates: Syntheses, Structures, and Calculations of Ba(MoO2)2(IO3)4O, Ba3[(MoO2)2(IO3)4O(OH)4]·2H2O, and Sr[(MoO2)6(IO4)2O4]·H2O Yahui Li,†,‡ Hongping Wu,*,† Bingbing Zhang,† Zhihua Yang,† Guopeng Han,†,‡ and Shilie Pan*,† †

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CAS Key Laboratory of Functional Materials and Devices for Special Environments, Xinjiang Technical Institute of Physics & Chemistry, CAS, Xinjiang Key Laboratory of Electronic Information Materials and Devices, 40-1 South Beijing Road, Urumqi 830011, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *

ABSTRACT: The three metal iodates Ba(MoO2)2(IO3)4O (1), Ba3[(MoO2)2(IO3)4O(OH)4]·2H2O (2), and Sr[(MoO2)6(IO4)2O4]·H2O (3) have been successfully synthesized by introducing second-order Jahn−Teller distorted Mo6+ cations by a mild hydrothermal method. Single-crystal X-ray diffraction (XRD) was used to determine the structures of the three title compounds. In compound 1, the [Mo2O11]10− dimers connect with the [IO3]− units by sharing oxygen atoms to form two-dimensional (2D) layers that are separated by the Ba2+ cations. For comparison, the [Mo2O11]10− dimers and the [IO3]− units are isolated in compound 2, and they are connected by the [BaO11]20− polyhedra forming a 3D network. For compound 3, the [MoO6]6− polyhedra link with each other by corner and edge sharing to build 2D corrugated layers with tunnels containing isolated [IO4]3− units. The [SrO9]16‑ polyhedra link the 2D corrugated layers to form a 3D network. The infrared (IR) spectra, the ultraviolet−visible− near-infrared (UV−vis−NIR) diffuse reflectance spectra, and thermal stabilities of compounds 1 and 2 are presented. In addition, the theoretical calculations are also carried out to evaluate their band gaps and density of states.

1. INTRODUCTION Inorganic functional materials are of research interest due to their wide applications in the fields of medicine and health products, food and feed additives, chemical and chemical analysis, nonlinear optical materials, birefringent materials, and so on.1−15 The iodates, as important functional materials applied in visible and near- and mid-infrared (IR) regions, have been extensively explored, and their structure−property relationship has also been widely studied during the past decades. High-performance iodate compounds have been discovered in recent years, including NaI3O8,16 ABi2(IO3)2F5 (A = K, Rb, Cs),17 Bi(IO3)F2,18 Cs2I4O11,19,20 α-AgI3O8,21 βAgI3O8,21 PbPt(IO3)6(H2O),22 BaPd(IO3)4,23 RbAu(IO3)4,24 and Bi3OF3(IO3)4.25 However, notably, if the I−O bond lengths >2.4 Å (the weak I−O interactions) are neglected, the I5+ cation will only contain two types of coordination geometries, i.e. [IO3]− and [IO4]3− units, and they are less prone to condense into polyanions.21 So far, only several types of polyanions are known: for instance, neutral dimeric [I2O5] units and trinuclear [I3O8]− units. These lead to the limited structural diversity of metal iodates. In order to enrich the structural chemistry of iodates and enhance the second-harmonic generation response of metal iodates, d0 transition metal cations (Ti4+, V5+, Cr6+, Mo6+) or cations containing lone pairs (Se4+, Te4+) have been chosen to introduce into metal iodates with different structure types, © XXXX American Chemical Society

such as zero-dimensional (0D) clusters, 1D chains, 2D layers, and 3D networks.26−28 For example, the 0D [MoO2(IO3)4]2− clusters are observed in K2MoO2(IO3)4 and [Ti(IO3)6]2− clusters in BaTi(IO3)6,28 1D [VO2(IO3)2]− chains in A[VO2(IO3)2] (A = K+, Rb+)29 and α-KVO2(IO3)2(H2O),30 2D [MoO3(IO3)]− layers in AMoO3(IO3) (A = Li+, K+, Rb+, Cs+),31 and 2D [VO2(IO3)2]− layers in NaVO2(IO3)2(H2O).32 Bi2(IO4)(IO3)3 exhibits a 3D framework through a combination of [IO3]−, [IO4]3−, [BiO8]13−, and [BiO9]15− polyhedra.33 In this research, we are interested in the Mo6+-containing iodates. It is well-known that the d0 Mo6+ cation is prone to second-order Jahn−Teller (SOJT) distortion following the distortion order Mo6+ > V5+ > W6+ > Nb5+ > Ta5+ > Ti4+.35 Clearly, the Mo6+ cation contains the largest distortions among these d0 transition metal cations. In addition, the Mo6+ cation also contains multiple distorted directions: i.e., the Mo6+ cation can move toward a face (C3 direction), an edge (C2 direction), or a corner (C4 direction).35 Therefore, introducing the Mo6+ cations into iodates is an effective method to enrich the structure of iodates.35 In Mo6+-containing iodates, as is known to all, AMoO 3(IO3) (A = Li+, K+, Rb+ , Cs+)31 and K2MoO2(IO3)428 have been reported, which all belong to alkaline Mo6+-containing iodates. In comparison with alkaline Received: May 18, 2018

A

DOI: 10.1021/acs.inorgchem.8b01365 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. Experimental and calculated powder X-ray diffraction patterns of (a) Ba(MoO2)2(IO3)4O and (b) Ba3[(MoO2)2(IO3)4O(OH)4]·2H2O. 2.3. Powder X-ray Diffraction. The powder X-ray diffraction (XRD) data were collected using a Bruker D2 PHASER diffractometer equipped with monochromatic Cu Kα radiation (λ = 1.5418 Å) at room temperature. The 2θ range is from 10 to 70° with a scan step width of 0.02° and a fixed counting time of 1 s per step. The powder XRD patterns of compounds 1 and 2 are shown in Figure 1. Clearly, they are in agreement with the calculated patterns. A pure sample of compound 3 has not been obtained. Many efforts were made to synthesize compound 3 by changing the conditions, but SrMo3O10·4H2O (PDF # 32-1245) was always unavoidable. 2.4. Vibrational Spectroscopy and Optical Properties. The IR spectra were measured on a Shimadzu IRAffinity-1 spectrometer. The samples were mixed thoroughly with dried KBr (6 mg of the sample and 600 mg of KBr), and the spectra were collected in the range of 400−4000 cm−1 with a resolution of 2 cm−1. The UV−vis−NIR diffuse reflectance data for the polycrystalline powders of compounds 1 and 2 were collected at room temperature using a Shimadzu SolidSpec-3700DUV spectrophotometer with the measurement range extending from 190 to 2600 nm. Reflectance spectra were converted to absorbance using the function F(R) = (1 − R)2/2R, where R is the reflectance and F(R) is the Kubelka−Munk remission function.39,40 2.5. Thermal Behavior Analysis. TG analysis and DSC of the sample were investigated using a NETZSCH STA 449C simultaneous thermal analyzer. The samples and reference Al2O3 were placed in a platinum crucible and heated at a rate of 5 °C/min from 25 to 800 °C under a flow of N2. 2.6. Theoretical Calculation Details. The first-principles method was used to calculate the electronic structures of the title compounds using density functional calculations in the CASTEP package.41,42 The Perdew−Burke−Ernzerhof43,44 functional within the generalized gradient approximation was applied for the exchangecorrelation potential. The valence electrons of the three compounds were calculated as Ba 5s25p66s2, Sr 4s24p65s2, Mo 4d55s1, I 5s25p5, O 2s22p4, and H 1s1, respectively. The Brillouin zones were set as 1 × 2 × 1 with a separation of Monkhorst−Pack k-point sampling of 0.07 Å−1. For the aim of energy convergence, the plane-wave cutoff energy was 830 eV. On the basis of the scissor-corrected electron band structures, the imaginary part of the dielectric function was calculated. Consequently, the real part of the dielectric functions was obtained by a Kramers−Kronig transformation, and the refractive indices were determined.45,46

cations, the alkaline-earth cations possess better moistureresisting ability. Here, three new alkaline-earth Mo6+containing iodates, Ba(MoO2)2(IO3)4O (1), Ba 3 [(MoO 2 ) 2 (IO 3 ) 4 O(OH) 4 ]·2H 2 O (2), an d Sr[(MoO2)6(IO4)2O4]·H2O (3), were successfully synthesized through a mild hydrothermal method, in which the interesting [Mo2O11]10− dimers, isolated [IO4]3− units, and edge-sharing Mo−O−I connections have been observed. The IR spectra, the ultraviolet−visible−near-infrared (UV−vis-NIR) diffuse reflectance spectra, and the thermal gravimetric (TG) analysis and differential scanning calorimetry (DSC) of compounds 1 and 2 are presented. In addition, theoretical calculations were also carried out on the three title compounds.

2. EXPERIMENTAL SECTION 2.1. Synthesis of Compounds. BaCl2·H2O (99%), MoO3 (99%), HIO3 (99%), Sr(OH)2·8H2O (99%), and I2O5 (99%) were all obtained analytically pure from Alfa Aesar and used without any further purification. Compound 1 was prepared by the pouch hydrothermal method using BaCl2·H2O (0.2 g, 0.82 mmol), MoO3 (0.3 g, 2.08 mmol), HIO3 (0.6 g, 3.41 mmol), and 1 mL of deionized water. Compound 2 was prepared via a reaction mixture of BaCl2·H2O (0.3 g, 1.23 mmol), MoO3 (0.15 g, 1.04 mmol), HIO3 (0.4 g, 2.27 mmol), and 1 mL of deionized water in a pouch. Compound 3 was prepared via reaction of Sr(OH)2·8H2O (0.266 g, 1 mmol), MoO3 (0.33 g, 1 mmol), I2O5 (0.144 g, 0.99 mmol), and 1 mL of deionized water in a pouch. Then the reactions were run in 120 mL autoclaves with PTFE liners (40 mLof deionized water and six pouches in every liner) for 3 days at 220 °C and cooled at a rate of 2.7 °C/h to room temperature. After the reactions, the products were washed with deionized water. 2.2. Structure Determination. Single crystals with approximate dimensions of 0.033 × 0.077 × 0.138 mm3 for compound 1, 0.023 × 0.059 × 0.130 mm3 for compound 2, and 0.035 × 0.071 × 0.101 mm3 for compound 3 were selected and mounted on a glass fiber with epoxy for data collection. Their crystal structures were determined by single-crystal X-ray diffraction (XRD) on an APEX II CCD diffractometer using monochromatic Mo Kα radiation (λ = 0.71073 Å) and integrated with the SAINT program36 at 100(2), 299(2), and 107(2) K, respectively. Their crystal structures were determined by direct methods and refined by full-matrix least-squares fitting on F2 using SHELXL.37 The program PLATON38 was used to verify the possible missing symmetry elements, and no other higher symmetry was found. The details of the crystal parameters, data collection, and structure refinement are given in Table S1. The atomic coordinates, equivalent isotropic displacement parameters, and selected bond lengths and angles are given in Tables S2 and S3.

3. RESULTS AND DISCUSSION 3.1. Crystal Structure. 3.1.1. Crystal Structure of Compound 1. Compound 1 crystallizes in the centrosymmetric space group P21/c (No. 14). In the asymmetric unit (Figure S1a), there are one unique Ba atom, one unique Mo atom, two unique I atoms, and nine unique O atoms. Its B

DOI: 10.1021/acs.inorgchem.8b01365 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 2. Ball and stick structures of Ba(MoO2)2(IO3)4O. The basic building units of Ba(MoO2)2(IO3)4O are (a) [Mo2O11]10− dimers and [IO3]− units, which connect with each other to form (b) [(MoO2)2(IO3)4O]2−∞ layers. (c) The eight-coordinated Ba2+ cations further link these [Mo2O11]10− dimers and the [IO3]− units to form the final (d) 3D network.

Figure 3. Ball and stick structures of Ba3[(MoO2)2(IO3)4O(OH)4]·2H2O. The basic building units of Ba3[(MoO2)2(IO3)4O(OH)4]·2H2O are (a) [Mo2O11]10− dimers and [IO3]− units. (b) These building units are isolated. (c) The eleven-coordinated Ba2+ cations further connect with each other by sharing common oxygen atoms to form a (d) 3D framework. (e) These isolated anionic units reside in the channels of the 3D framework.

structure is shown in Figure 2. Two [MoO6]6− octahedra are linked with each other by sharing O(1) atoms to form the [Mo2O11]10− dimers (Figure 2a). These [Mo2O11]10− dimers are linked by isolated [IO 3 ] − units to build 2D [(MoO2)2(IO3)4O]2− layers (Figure 2b), which are separated by the Ba2+ cations (Figure 2d). The Ba2+ cations are coordinated to eight O atoms, forming a [BaO8]14− (Figure 2c) bicapped trigonal prism with Ba−O distances in the range of 2.747(3)−2.990(3) Å, which are comparable with those of other reported compounds, such as Ba(H2PO4)2 (2.659(1)− 2.903(2) Å) and Ba2(B5O8(OH)2)(OH) (2.620(6)−2.864(4) Å).47,48 All of the I5+ cations exhibit the typical [IO3]− pyramid with the I−O bond lengths ranging from 1.786(3) to 1.899(3) Å, which are similar to those found in RbAu(IO3)4.24 The Mo6+ cations are coordinated to six O atoms, forming the [MoO6]6− octahedra, which contain three long Mo−O bonds with distances of 2.081(8)−2.210(1) Å and three short Mo−O bonds with bond lengths of 1.702(2)−1.885(1) Å. The

3 + 3 bonding pattern is consistent with that observed in Rb2(MoO3)3(SeO3),49 indicating distortion of the local C3 direction in the [MoO6]6− octahedron. The magnitudes of the out-of-center distortions can be calculated using the method proposed by Halasyamani et al.:35 Δd =

|(M−O2) − (M−O5)| |(M−O1) − (M−O4)| + |cos θ2| |cos θ1| +

|(M−O3) − (M−O6)| |cos θ3|

where the (O1, O4), (O2, O5), and (O3, O6) pairs are the oxygen atoms that constitute the octahedra and are located opposite from each other.35 The calculated result of the [MoO6]6− octahedron is 1.222, which can be quantified as a strong distortion.35 Moreover, bond valence sums (BVS)50,51 result in values of 1.79 for Ba2+, 5.99 for Mo6+, 4.84−4.95 for C

DOI: 10.1021/acs.inorgchem.8b01365 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 4. Ball and stick structures of Sr[(MoO2)6(IO4)2O4]·H2O. The basic building units of Sr[(MoO2)6(IO4)2O4]·H2O are (a) Mo−O polymer units and [IO4]3− units, which connect with each other to form (b) [(MoO2)3(IO4)O2]−∞ layers. (c) The nine-coordinated Sr2+ cations further link the [(MoO2)3(IO4)O2]−∞ layers to form a (d) 3D network structure.

I5+, and 1.74−2.20 for O2− (Table S4), suggesting that the structures of compound 1 are reasonable. 3.1.2. Crystal Structure of Compound 2. Compound 2 crystallizes in the monoclinic space group C2/c (No. 15). In the asymmetric unit, there are two unique Ba atoms, one unique Mo atom, two unique I atoms, twelve unique O atoms, and four unique H atoms (Figure S1b). The structure of compound 2 is shown in Figure 3. Similar to the case for compound 1, compound 2 also contains distorted [Mo2O11]10− dimers and [IO3]− pyramids (Figure 3a). However, different from compound 1, these distorted [Mo2O11]10− dimers and [IO3]− pyramids in compound 2 do not further connect with each other (Figure 3b) but reside in isolation in the channels of a 3D Ba−O framework (Figure 3e). Therefore, in compound 2, there are no Mo−O−I bonds. The Ba2+ cations are coordinated to 11 O atoms, forming a [BaO11]20− (Figure 3c) single capped pentagonal prism with Ba−O distances of 2.722(6)−3.290(9) Å. The bond distances of Ba−O are consistent with those observed in Ba3B6O11F2,52 which shows Ba−O bond lengths from 2.683 to 3.316 Å. In addition, all of the I5+ cations exhibit the typical [IO3]− pyramid with the I−O bond lengths ranging from 1.795(7) to 1.847(6) Å, which are similar to those found in Bi3[IO3]12· Ag4I.53 In compound 2, the [MoO6]6− polyhedra contain three long Mo−O bonds with distances of 1.971(5)−2.339(1) Å and three short Mo−O bonds with distances of 1.695(3)−1.880(8) Å. The 3 + 3 bonding pattern compares well with that observed in AMo2O5(SeO3)2 (A = Sr, Pb, Ba),54 indicating the C3 distortion in bond asymmetry within the [MoO6]6− octahedra. In addition, the value of the out-of-center distortions calculated for [MoO6]6− octahedra is 1.109. The results of BVS50,51 are 1.81−2.33 for Ba2+, 6.36 for Mo6+, 5.01−5.10 for I5+, and 1.81−2.21 for O2−, which match with the expected valences well (Table S4). Meanwhile, the calculated BVS values (H is not calculated) for O(1), O(2), and O(5) are 1.14, 1.07, and 0.51, respectively, which indicate the existence of −OH groups (Table S5). 3.1.3. Crystal Structure of Compound 3. Compound 3 crystallizes in the monoclinic space group C2/c (No. 15). The asymmetric unit consists of one unique Sr atom, three unique

Mo atoms, one unique I atom, thirteen unique O atoms, and one unique H atom (Figure S1c). Its structure is shown in Figure 4. Two [Mo(1)O6]6− octahedra are connected with each other by edge sharing, i.e. sharing two O(1) atoms, to form the [Mo(1)2O10]8− dimer. Two [Mo(3)O6]6− octahedra form the [Mo(3)2O10]8− dimer in the same way by sharing two O(4) atoms. Further, these two kinds of [Mo2O10]8− dimers are linked by the [Mo(2)O6]6− octahedra to build the 2D Mo−O layers (Figure 4b). These Mo−O layers have parallel channels, which are filled by the [IO4]3− units to build 2D corrugated [(MoO2)3(IO4)O2]− layers, which are separated by the Sr2+ cations (Figure 4d). The Sr2+ cations are coordinated to nine O atoms, forming [SrO9]16− (Figure 4c) tricapped trigonal prisms with Sr−O bond lengths ranging from 2.571(3) to 2.921(0) Å. This is exemplified by Sr2B5O9(OH),55 which has the [SrO9]16− tricapped trigonal prism with Sr−O bond lengths ranging from 2.510(6) to 2.936(9) Å. All of the I5+ cations exhibit the rare [IO4]3− units with I−O bond lengths ranging from 1.827(3) to 2.041(3) Å, which are similar to those found in Bi2(IO4)(IO3)3.33 The corrugated layered structure of compound 3 is very similar to that of Li2Mo3TeO12,34 which has been reported recently. The [MoO6]6− octahedra in Li2Mo3TeO1234 connect with each other by corner and edge sharing to form 2D Mo−O layers with tunnels containing isolated [TeO4]4− units, i.e. [(MoO2)3(TeO4)O2]2− layers with tunnels containing isolated [TeO4]4− units, creating [(MoO2)3(TeO4)O2]2− layers. These layers are separated by the Li+ cations. Similarly, as mentioned above, the [TeO4]4− positions in [(MoO2)3(TeO4)O2]2− layers are replaced by [IO4]3− in [(MoO2)3(IO4)O2]− layers of compound 3. In compound 3, the Mo6+ cations exhibit two types of outof-center distortions for the Mo(1)6+ and Mo(3)6+ cations. The 3 + 3 bonding pattern compares well with that observed in β-BaTeMo2O9;56 the Mo(1)−O and Mo(3)−O bonds show a common 3 + 3 bonding pattern with three long (1.717(5)− 1.862(6) Å) and three short bonds (2.025(6)−2.242(4) Å), exhibiting C3 distortion in bond asymmetries. Meanwhile, the Mo(2)−O bonds show another common 2 + 2 + 2 bonding pattern with two long, two normal, and two short bonds exhibiting C2 distortion in bond asymmetry.35 The two short D

DOI: 10.1021/acs.inorgchem.8b01365 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Investigation of 13 Known Compounds and the Title Compounds in the MoVI−IV−O System compd in MoVI−IV−O system

I−O unit

Mo−O polymer unit

Mo−O−I bridge

anionic group

α-KMoO3(IO3)31 β-KMoO3(IO3)58 Li(MoO3)(IO3)31 Rb(MoO3)(IO3)31 Cs(MoO3)(IO3)31 K2MoO2(IO3)428 Ag2(MoO2)(IO3)459 BaMoO2(IO3)4(H2O)28 Ba((MoO2)6(IO4)2O4)(H2O)60 Ln((MoO2)(IO3)4(OH)) (Ln = La, Eu, Sm, Nd)61,62 Ba(MoO2)2(IO3)4O Ba3[(MoO2)2(IO3)4O(OH)4]·2H2O Sr[(MoO2)6(IO4)2O4]·H2O

IO3 IO3 IO3 IO3 IO3 IO3 IO3 IO3 IO4 IO3 and IO4 IO3 IO3 IO4

1D chain 1D chain 1D chain 1D chain 1D chain isolated isolated isolated 2D layer isolated [Mo2O11] dimer [Mo2O11] dimer 2D layer

corner sharing corner sharing corner sharing corner sharing corner sharing corner sharing corner sharing corner sharing corner and edge sharing corner sharing corner sharing no Mo−O−I bonds corner and edge sharing

[MoO3(IO3)]− layer [MoO3(IO3)]− layer [MoO3(IO3)]− layer [MoO3(IO3)]− layer [MoO3(IO3)]− layer [MoO2(IO3)4]2− cluster [MoO2(IO3)4]2− cluster [MoO2(IO3)4]2− cluster [(MoO2)3(IO4)O2]− layer [(MoO2)(IO3)4]3− cluster [(MoO2)2(IO3)4O]2− layer [Mo2O11]10− dimer and IO3− pyramid [(MoO2)3(IO4)O2]− layer

Figure 5. (a) IR and (b) UV−vis−NIR diffuse reflectance spectra of Ba(MoO2)2(IO3)4O and Ba3[(MoO2)2(IO3)4O(OH)4]·2H2O.

structure. Therefore, compound 2 should be a molybdate iodate. As we all know, this is the first example of s double salt in the MoVI−IV−O system. The I−O and Mo−O units in compounds 1 and 3 are linked by sharing O atoms. Therefore, the two compounds can be called molybdenum(VI) iodates. On the basis of Table 1, it is also clear that the I/Mo molar ratio affects the polymerization of the anionic groups. Further, the following rule can be summarized on the basis of the compounds given in Table S6: when the molar ratio of I/Mo is not less than 4, the anionic groups exist in the form of 0D clusters. Otherwise, the anionic groups usually exist in the form of 2D layers. That is, with an increasing proportion of I5+ cations, the degree of polymerization of anionic groups is decreased. However, the anionic group in compound 2 is isolated. The rule mentioned above seems to be also applicable to metal iodates in the VV−IV−O system (Table S7). At the same time, the anionic groups are isolated in Zn2(VO4)(IO3)64 and Ln(VO3)2(IO3) (Ln = La, Ce, Pr, Nd, Sm, Eu).65,66 A detailed structure comparison was carried out among compound 2, Zn2(VO4)(IO3),64 and Ln(VO3)2(IO3) (Ln = La, Ce, Pr, Nd, Sm, Eu).65,66 The A/(I + Mo) or A/(I + V) molar ratios are 0.5, 1, and 0.33, respectively. If the difference in the valence of the cations is also taken into account, the charge number of cations of these three compounds is not less than the number of anions. That is, with an increasing proportion of the valence of cations, the degree of polymerization of anionic groups will further decrease.

bonds, two normal bonds, and two long bonds range from 1.704(4) to 1.706(4) Å, 1.931(2) to 1.980(5) Å, and 2.263(5) to 2.318(3) Å, respectively, which are comparable to those found in Rb4Mo5P2O22.57 These features are also observed in β-BaTeW2O9;56 the two unique W6+ cations distort either along the local C3 [111] direction or the C2 [110] direction as well. In addition, the calculated results of [Mo(1)O6]6−, [Mo(2)O6]6−, and [Mo(3)O6]6− octahedron distortions are 1.144, 1.294, and 1.172, respectively. The BVS50,51 values for the Sr2+, Mo6+, I5+, and O2− cations are 2.19, 5.92−6.15, 5.04, and 1.79−2.18, respectively (Table S4). In addition, the calculated BVS (H is not calculated) for O(8) is 0.24, which indicates the existence of −OH groups (Table S5). 3.1.4. Structural Analysis Based on the Connection between I−O and Mo−O Polyhedra. To the best of our knowledge, there have been 13 compounds reported for Mo6+containing iodate systems. They are given in Table 1. In comparison with the 13 known structures, the following structural features exhibited by the title compounds seem to be interesting for the structural chemistry of Mo6+-containing iodates. (i) Compounds 1 and 2 contain [Mo2O11]10− dimers, which are the first cases found in Mo6+-containing iodates. (ii) Compound 3 contains four-coordinated I5+ cations, which have only been observed in Ln((MoO2)(IO3)4(OH)) (Ln = La, Eu, Sm, Nd), 61,62 Ba((MoO 2 ) 6 (IO 4 ) 2 O 4 )(H 2 O), 60 Ag4(UO2)4(IO3)2(IO4)2O2,63 and Bi2(IO4)(IO3)333 among all iodates. (iii) The [IO3]− units and the [Mo2O11]10− dimers in compound 2 are isolated: i.e., no Mo−O−I bridge in the E

DOI: 10.1021/acs.inorgchem.8b01365 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 6. Band gaps, density of states and partial density of states of (a) Ba(MoO2)2(IO3)4O, (b) Ba3[(MoO2)2(IO3)4O(OH)4]·2H2O, and (c) Sr[(MoO2)6(IO4)2O4]·H2O.

In addition, the marked structural differences of the three title compounds may stem from not only the I/Mo ratio but also several other factors, such as the following. (i) Different Mo−O bond lengths cause different values of the out-of-center distortions ranging from 1.702(2) to 2.339(1) Å and different directions of the out-of-center distortions along either the C2 [110] direction or the local C3 [111] direction. (ii) Different coordination numbers of the I atoms affect the degree of polymerization of the compound. The [IO4]3− units in compound 3 are further connected with the [MoO6]6− octahedra by corner and edge sharing. However, the [IO3]− units in compounds 1 and 2 are further connected with the [MoO6]6− octahedra only by corner sharing. (iii) The coordination numbers of the Ba2+ cations in compounds 1 and 2 and the coordination numbers of the Sr2+ cations in compound 3 are 8, 11, and 9, respectively. The largest coordination number is that of Ba2+ cations in compound 2. This situation leads to a 3D Ba−O framework with tunnels occupied by isolated [Mo 2 O 11 ] 10− dimers and [IO 3 ] − pyramids. 3.2. Vibrational Spectroscopy and Optical Properties. To confirm the existence of the I−O and Mo−O units in compounds 1 and 2, IR spectral measurements were carried out. As shown in Figure 5a, their IR spectra are similar except for the absorption bands at 3432−2949 cm−1 in compound 2, which are caused by the stretching and bending modes of [OH]− units.61,62 The IR absorption bands at 1017−857 cm−1 in the two compounds are caused by the [MoO6]6− units stretching vibrations, the absorption band in the range 857− 525 cm −1 is attributed to the symmetric (ν1) and antisymmetric stretching vibrations (ν3) of the [IO3]− units, and the bands in the range 525−400 cm−1 are due to symmetric bending vibrations (ν2) of the [IO3]− units.61,62

The band splitting and position shift may be related to I(V) and Mo(VI) in markedly different environments. Therefore, the existence of the [MoO6]6− and [IO3]− units was verified by IR spectra, which is consistent with the experimental results of the single-crystal XRD analyses. Figure S2 shows the UV−vis−NIR diffuse reflectance spectra of compounds 1 and 2 in the region of 190−2600 nm. Evidently, the cutoff edges of compounds 1 and 2 are 309 and 253 nm, respectively. The corresponding energy gaps of compounds 1 and 2 are 3.06 and 3.32 eV, respectively (Figure 5b). 3.3. Thermal Behavior Analysis. It can be seen from the TG-DSC curves in Figure S3a that compound 1 is thermally stable up to 400 °C. It can be seen from the DSC curve that there are three remarkable endothermic peaks appearing at 448, 495, and 654 °C, respectively. The TG curve in the range of 315−743 °C shows a total mass loss of 60.4%, which corresponds to a complicated decomposition process of I2O5.67 The total mass loss is 60.4%, which is consistent with the loss of four molecules of I2O5 with a calculated value of 60.2%. Although endothermic peaks and mass loss cannot be exactly assigned due to the complexity of decomposition reactions, the possible reaction results are a guess based on the powder XRD of the sample after melting. It is obvious that the experimental value matches well with the calculated value. It can be seen that compound 2 is thermally stable up to 350 °C from the TG-DSC curves in Figure S3b. It is clear that there are four distinct endothermic peaks at 362, 516, 546, and 583 °C in DSC curves. The mass loss of compound 2 can be divided into three steps. Step 1 in the range of 308−369 °C has a mass loss of 4.3%, which is consistent with the loss of four molecules of H2O with a calculated value of 4.8%.68 Steps 2 and 3 in the range of 370−733 °C have a mass loss of 44.9%, F

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Inorganic Chemistry which is consistent with a complex decomposition process of I2O5.67 The mass loss for the two steps is 44.9%, which is consistent with the loss of four molecules of I2O5 with a calculated value of 45.9%. The powder XRD analysis shows that the residues after TG-DSC are BaMoO4 and some unknown phases (Figure 1). 3.4. Electronic Structure. The electronic structures of compounds 1−3 are shown in Figure 6. It is found that compound 2 possesses a direct band gap of 1.70 eV. In contrast, compounds 1 and 3 possess indirect band gaps of 1.45 and 1.93 eV, respectively. To illuminate the band structures of the three compounds, the density of states graph is divided into three parts: (i) under −15 eV, (ii) from −15 eV to the valence band maximum, and (iii) the conduction band minimum. For compound 1, part i is occupied by Ba-5s6s, Mo-4d, I-5s5p, and O-2s orbitals. Part ii is made up of Ba-5p, Mo-4d, I-5s5p, and O-2p orbitals. Part iii is composed of Mo-4d, I-5s5p, and O-2p orbitals. For compound 2, the band structure is slightly different from that of compound 1. The partial density of states of compound 2 contains the addition of H-1s in the bands. For compound 3, part i is made up of Sr-4p, Mo-4d, I-5s5p, O-2s, and H-1s orbitals. Part ii is occupied by Sr-4p, Mo-4d, I-5s5p, O-2p, and H-1s orbitals. Part iii is composed of Mo-4d, I-5p, and O-2p orbitals. For compounds 1−3, the bottom conduction bands are mainly Mo-4d states. The top of the valence bands mainly comes from the O-2p states. In other words, the O-2p states and the Mo-d states determine the Fermi level position.

1−3, asymmetric units of compounds 1−3, UV−vis− NIR diffuse reflectance spectra, and TG-DSC curves of compounds 1 and 2 (PDF) Accession Codes

CCDC 1845840−1845842 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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*H.W.: e-mail, [email protected]; tel, (+86)991-3674558; fax, (+86)991-3838957. *S.P.: e-mail: [email protected]; tel, (+86)991-3674558; fax, (+86)991-3838957. ORCID

Hongping Wu: 0000-0003-0975-1700 Bingbing Zhang: 0000-0002-1334-5812 Zhihua Yang: 0000-0001-9214-3612 Shilie Pan: 0000-0003-4521-4507 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Key Research Project of Frontier Science of CAS (Grant No. QYZDB-SSW-JSC049), the Western Light Foundation of CAS (Grant 2016-QNXZ-A2), Xinjiang Scientific and Technological Innovation Talents Project (Grant No. QN2016YX0339), Xinjiang International Science & Technology Cooperation Program (Grant No. 2017E01014), and the Youth Innovation Promotion Association CAS (Grant No. 2015353).

4. CONCLUSION Three new alkaline-earth Mo6+-containing iodates, compound 1 (P21/c), compound 2 (C2/c), and compound 3 (C2/c), have been successfully obtained by a mild hydrothermal method. In all of the reported materials in the MoVI−IV−O system, the Mo−O units mostly exist in the form of isolated [MoO6]6− octahedra or connect with each other by corner sharing to form 1D chains. However, the interesting [Mo2O11]10− dimers are observed for the first time in compounds 1 and 2. In addition, in all of the materials reported, the d0 transition metal is bonded to six oxygen atoms to form an octahedral coordination environment. The ligands of these six oxygen atoms either are terminal or are bonded to an I5+ cation. However, we never observe the Mo−O−I bonds in compound 2. Compound 2 is the first case without Mo−O−I bridges in reported Mo6+-containing iodates, as we all know. Compound 3 contains isolated [IO4]3− and edge-sharing Mo−O−I connections. In summary, the introduction of molybdenum greatly enriches the structure chemistry of iodates. Furthermore, continuous efforts are underway in the MoVI−IV−O system to explore new compounds with characteristic structures or interesting properties.

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AUTHOR INFORMATION

Corresponding Authors

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Table of the Crystallographic data for compound 1 (CIF) Crystallographic data for compound 2 (CIF) Crystallographic data for compound 3 (CIF) The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01365. Crystal data and structure refinement details, atomic coordinates and equivalent isotropic displacement parameters, bond lengths, and angles for compounds G

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DOI: 10.1021/acs.inorgchem.8b01365 Inorg. Chem. XXXX, XXX, XXX−XXX