A 3D Silverton-Type Polyoxomolybdate Based on {PrMo12O42}: Synthesis, Structure, Photoluminescence and Magnetic Properties

Introduction

Polyoxomolybdates (POMos) represent a class of metal-oxygen clusters with remarkable structural diversity (Long et al., 2003), which have attracted much attention because of their wide ranging applications (Pope and Müller, 1991; Hill, 1998; Long et al., 2010). The inorganic building units of POMos are generally formed by the flexible Mo−O−Mo and Mo=O bonds, featuring variable coordination numbers and tunable oxidation states between Mo^V and Mo^VI (Kowalewski et al., 2012). Owing to the abundant diversity of molecular structures, examples of POMos with catalysis, magnetic (Maestre et al., 2001; Pati and Rao, 2008; Miras et al., 2011), electrochemical, and luminescent properties (T. Zhang et al., 2005; G. Zhang et al., 2017) have been sufficiently studied in the last few decades. To date, the development of POMos have mainly been focused on the classical structure-type, such as Keggin {XMo₁₂O₄₀} (Dolbecq et al., 2003; Yelamanchili et al., 2008; Leclerc-Laronze et al., 2009; Vasilopoulou et al., 2015) and Dawson {X₂Mo₁₈O₆₂} (López et al., 2002; Sokolov et al., 2008; Hashikawa et al., 2011). For instance, Awaga et al. studied the reversible 24-electron redox during charging/discharging in [PMo₁₂O₄₀]³⁻ and demonstrated the potentials in making rechargeable batteries (Wang et al., 2012). On the basis of the Dawson-type anions [P₂Mo₁₈O₆₂]⁶⁻, Poblet et al. systematically conducted the density functional theory calculations and analyzed their redox properties in detail (López et al., 2002). To the best of our knowledge, only a few of Silverton-type {XMo₁₂O₄₂} compounds functionalized with lanthanide (Ln) ions have been reported and the relevant research is summarized in Supplementary Table S1.

More specifically, Baker et al. reported the first Silverton-type compound in 1953 with the formula of (NH₄)₂H₆[CeMo₁₂O₄₂]·12H₂O and then this structure was further explored by Silverton et al. (Dexter and Silverton, 1968). Subsequently, this configuration was defined as “Silverton” type POMo, in which the [CeMo₁₂O₄₂]⁸⁻ anion is constructed by six corner-sharing {Mo₂O₉} groups and a twelve-coordinated Ce⁴⁺ ion. In 2006, Tsirlina et al. studied the electrochemical and photoluminescence properties of the aforementioned compound (Lu et al., 2006) and Gd³⁺ ion was successfully introduced in the Silverton-type POMos by the one-pot hydrothermal method (Tan et al., 2009). In addition, the transition metal nickel and cobalt ions have also been incorporated in this system, affording three isomorphic 3D frameworks (Tan et al., 2009).

Referring to the relevant systems, Ln ions maintain the inherent photoluminescence properties originated from their 4f → 4f or 5d → 4f transitions. As for the Pr³⁺ ion, the ³P₀ → ³H₄ transitions always emit blue-green light in most oxide lattices while the ¹D₂ → ³H₄ transitions mainly emit red light in some pervoskite lattices (Peng et al., 2012; Liang et al., 2017). Moreover, the forbiddance of d–d transitions of the Mn²⁺ ion limits photoluminescence (Kang et al., 2018), which can be improved by the addition of photosensitizers (Lv et al., 2014) and the suitable crystal field environment (Y. Zhang et al., 2012; Cao et al., 2018).

Herein, the Pr³⁺ ion is employed to construct the Silverton-type POMo and the transition metal Mn²⁺ ion is used to extend the structural dimensionality. As expected, a new 3D Silverton-type POMo, NH₄{Mn₄[PrMo₁₂O₄₂]}·18H₂O (1) has been isolated as single crystals. Under the 375 nm photoexcitation, compound 1 was detected with red emission. Furthermore, the preliminary magnetic property has also been studied by variable temperature magnetic susceptibilities measurement.

Materials and Methods

All chemical reagents were commercially purchased from Sinopharm Chemical Reagent Co., Ltd. and used without further purification. X-ray powder diffraction data were collected on a Bruker D8 Advance diffractometer with Cu Kα (λ = 1.54056 Å) radiation. Infrared spectra were recorded on a Spectrum Two FT-IR spectrometer using KBr pellets in the range of 4,000–400 cm⁻¹. Thermogravimetric analyses were carried out under N₂ atmosphere on a Mettler Toledo TGA/DSC3 synchronous thermal analyzer (heating rate: 10°C·min⁻¹). The vacuum/open 1,200°C tubular electric furnace as well as the ceramic reaction vessel are produced by Tianjin Zhonghuan Co., Ltd. Photoluminescence properties were performed at room temperature using a FLS980 fluorescence spectrophotometer. Magnetic measurements were carried out on a Quantum Design MPMS3 magnetometer in the temperature range of 310–1.8 K.

Synthesis of Compound 1

Solid powder of Mo (0.42 g, 4.38 mmol), Na₂MoO₄·2H₂O (1.06 g, 4.38 mmol) and MoCl₅ (1.20 g, 4.39 mmol) were uniformly mixed in a 25 ml ceramic reaction vessel under the N₂ atmosphere. Then the reaction vessel was carefully placed in the tubular electric furnace. The reactant was successively heated at 150°C (1 h), 200°C (1 h) and 240°C (2 h) and then cooled to room temperature under the protection of argon gas. Black solid powder was collected as the reaction intermediate. Subsequently, the black intermediate product (1.23 g) was dissolved in H₂O (200 ml) and NH₃·H₂O (1 ml, 25–28%), and the turbid solution was stirred for 1 h, then H₂O₂ (24 ml, 30%) was added (pH = 4‐5). After stirred for another 3.5 h the pH of the yellow mixture was automatic adjusted to 1.9, the solution of Mn(CH₃COO)₂ (12 ml, 0.125 mol/L), Pr(NO₃)₃·4H₂O (6 ml, 9.23 mmol/L) and H₂O₂ (6 ml, 30%) were added sequentially to the aforementioned mixture (30 ml). The solution was filtered after stirring for 1 h and the filtrate was left undisturbed at room temperature for crystallization. Golden-yellow block-shape crystals were obtained after about four weeks. Yield: 33.52% based on Na₂MoO₄·2H₂O. Elemental analysis calcd (%) for Mn₄Mo₁₂NO₆₀PrH₄₀: H, 1.60; N, 0.55. Found: H, 1.65; N 0.52. The final formula of 1 should be fixed as NH₄{Mn₄[PrMo₁₂O₄₂]}·18H₂O along with the results of thermogravimetric and elemental analyses. IR (KBr pellet, cm⁻¹): 3,467 (s), 1,624 (m), 957 (m), 926 (w), 887 (w), 643 (s).

Synthetic Discussion

The amount of hydrogen peroxide is key in the isolation of the crystals. Parallel experiments demonstrate that an insufficient amount of hydrogen peroxide leads to an undissolved molybdenum powder at the bottom of the beaker (pH > 5.1). Though the clear solution could also be obtained with excess hydrogen peroxide (pH < 3.9), a mass of flocs is separated in this clear solution in the crystallizable process. However, no single crystals were formed in the parallel experiments as described above.

X-ray Crystallography

Single crystal X-ray structure analyses were performed on a Bruker Apex-II CCD diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) at 296(2) K. The structure was solved by direct methods using Olex2 and the refinements were done by full-matrix least-squares against F². The absorption correction was performed with the SADABS program and all the Mo, Pr, Mn, and O atoms in compound 1 were refined anisotropically. The CCDC reference number is 1904951 for 1. Crystallographic data and the structural refinement results are summarized in Table 1. The selected bond lengths and angles are listed in Supplementary Table S2.

TABLE 1

TABLE 1. Crystallographic data and refinement parameters for 1.

Results and Discussion

Structure Discussion

Crystallographic analyses indicate that compound 1 is crystallized in the cubic space group Fd-3 and consists of one ammonium cation, four manganese cations, a Silverton-type polyanion [PrMo₁₂O₄₂]⁹⁻ and eighteen lattice water molecules. Bond valence sum (BVS) calculations reveal that Mo, Mn and Pr atoms are in +6, +2 and +3 oxidation states, respectively (Supplementary Table S3 in the Supplementary Material). In detail, the dimeric {Mo₂O₉} is formed by two face-sharing {MoO₆} octahedra (Figure 1), in which the Mo−O bond lengths and the O−Mo−O bond angles are in the range of 1.691–2.339 Å and 71–165°, respectively. Six {Mo₂O₉} groups are connected by μ-O atoms forming a unit of {Mo₁₂O₄₂}. The twelve-coordinated Pr ion (Supplementary Figure S7A) is located in the center of the Silverton-type polyanion [PrMo₁₂O₄₂]⁹⁻ with the Pr−O bond length equals 2.559(3) Å and the O−Pr−O bond angles residing in the range of 62–178° (Supplementary Figure S7C). These polyanions are further extensively forming a 3D framework by the distorted {MnO₆} octahedra (Supplementary Figure S7B) with the Mn−O bond length of 2.138(4) Å (Supplementary Figure S7D). Furthermore, the topology configuration of compound 1 could be seen as a 2-connected 3D network (Supplementary Figure S7E), if considering one polyanion [PrMo₁₂O₄₂]⁹⁻ as a node. In this network, each Mn ion connects to two [PrMo₁₂O₄₂]⁹⁻ units via O−Mn−O bridges and these bond angles are in the range of 88–180° (Supplementary Table S2 in the Supplementary Material).

FIGURE 1

FIGURE 1. The coordination environment of Pr (A) and Mn (B) ions, the structure of the polyanion [PrMo₁₂O₄₂]⁹⁻(C), the polyhedral/ball-stick representation (D) and topology configuration (E) of the 3D framework of compound 1.

Characterizations of IR, TG and XRPD

As shown in Supplementary Figure S1, the IR spectrum of 1 shows the skeletal vibrations in the region of 1,000–600 cm⁻¹. The peaks of 957 and 926 cm⁻¹ are attributed to the stretching vibration of ν(Mo=O) and the peaks of 887 and 643 cm⁻¹ are assigned to ν(Mo−O−Mo). In addition, the peaks appearing at 3,467 and 1,624 cm⁻¹ correspond to the stretching vibration v(O−H) and the bending vibration δ(O−H) of water molecules.

The thermogravimetric (TG) curve of compound 1 exhibits two steps of weight loss (Supplementary Figure S2). The weight loss of the first step (9.79% from 25 to 200°C) is attributed to the loss of one NH₄⁺ and 18 crystallizable water molecules. The remaining weight loss is caused by the decomposition of the skeleton of compound 1.

The peaks in the experimental X-ray powder diffraction (XRPD) pattern are substantially consistent with the simulated peaks, indicating the phase purity of the collected sample (Supplementary Figure S3). The difference in intensity originates from the variation in preferred orientation during the collection of experimental data for the powder sample.

Solid State Photoluminescence Properties

In general, the Ln³⁺ ions possess the atomic properties in Ln-containing structures because of their gradual filling of 4f⁰ to 4f¹⁴ electrons in the 4f orbital and the shielding effect of 5s and 5p electrons (Ban et al., 2017). Therefore, the solid photoluminescence properties of 1 have been studied at room temperature with polycrystalline samples. Excitation of 1 in solid state at 375 nm displays two weak emission bands centered at 466 nm and 488 nm, which are individually attributed to ³P₀ → ³H₅ and ³P₀ → ³H₄ transitions of Pr³⁺ ions (Regulacio et al., 2008). The red emission bands in the range of 600–700 nm can be attributed to many factors. For example, ¹D₂ → ³H₄ (606 nm), ³P₀ → ³H₆ (636 nm) and ³P₀ → ³F₂ (650 nm) transitions of Pr³⁺ ions (Figure 2A) (Q. Zhang et al., 2015). Besides, the strong red emission peak centered at 659 nm is assigned to the transition of Mn²⁺ ions from the higher energy levels of ⁴T₁(⁴G) to the ground state ⁶A₁(⁶S) (L. Wu et al., 2014), which might undergo the intermediate levels of ⁴E(⁴D), ⁴T₂(⁴D), ⁴A₁(⁴G), ⁴E(⁴G), and ⁴T₂(⁴G) (Sharma et al., 2010; Shi et al., 2014). The emission band at 671 nm is attributed to the transitions from ⁵E to ⁴A₂ of Mn²⁺ ions. These strong emission band of Mn²⁺ ions are contributed by the energy transfer from the {PrMo₁₂O₄₂} building block (H. Wu et al., 2018; H. Li et al., 2019). In other words, the Mo−O−Mo linker may play a role of light radiation absorption as well as the energy transfer processes (Pan et al., 2018). In addition, the red emission spectra of Mn²⁺ ions in 1 is in line with the crystal field environment, i.e., orange to deep red emission for octahedral coordination configuration (L. Wu et al., 2014). It should be noted that the intensity of the emission band for Mn²⁺ is greatly stronger than that of Pr³⁺. The photoluminescence intensity is related to the concentration of ions and the concentration of Mn²⁺ is four times that of the Pr³⁺ ion in compound 1 (Mhlongo et al., 2011; Pan et al., 2018; X. Yang et al., 2019).

FIGURE 2

FIGURE 2. (A) Emission spectrum (upon 375 nm excitation) of compound 1, (B) the lifetime decay curve taken by monitoring the emission at 659 nm. Inset: CIE diagram of overall emission of compound 1.

Under the most intense emission at 659 nm, the characteristic excitation peaks of Pr³⁺ ion are centered at 475, 479, and 496 nm for the transition of ³H₄ → ³P₂, ³H₄ → ³P₁ and ³H₄ → ³P₀, respectively (Regulacio et al., 2008). The transition of ⁶A_1g(⁶S) → ⁴T_2g(⁴D) (Shang et al., 2015), ⁶A₁ → ⁴E(⁴D), ⁶A₁ → ⁴T₂(D) and ⁶A₁ → [⁴A₁(G), ⁴E(⁴G)] (K. Li et al., 2016) for Mn²⁺ ion are obtained at 375, 390, 425, and 457 nm (Supplementary Figure S9).

Decay Analyses

The decay lifetime of 1 in solid state was measured under the strongest emission band at 659 nm (Ban et al., 2017). As shown in Figure 2B, the decay curve is fitted well using the second order exponential function with lifetimes of τ₁ = 1.15 μs(50.02%) and τ₂ = 8.94 μs (49.98%) (H.-L. Li et al., 2017). The average decay time of 5.04 μs (τ*) can be obtained using the following equation: τ* = (A₁τ₁²+A₂τ₂²)/(A₁τ₁+A₂τ₂) (A₁ = 2008.28, A₂ = 257.76) (Tian et al., 2016). In addition, the chromaticity coordinates (x, y) are always used to determine the emission color of samples and the standard chromaticity coordinates of white light emission is (0.33, 0.33). For compound 1, the chromaticity coordinates are found to be (0.641, 0.359). Therefore, the overall emission of 1 can be simulated to be red emission, which is close to the reported analogues (Geng et al., 2013).

Magnetic Property

The variable-temperature magnetic susceptibility of compound 1 in the polycrystalline state has been investigated at 1,000 Oe at temperature ranges of 310–1.8 K. As shown in Figure 3, the experimental χ_MT product of 19.9 cm·K·mol⁻¹ at room temperature is close to the expected value of 19.1 cm·K·mol⁻¹ for one Pr³⁺ ion and six uncoupled high-spin Mn²⁺ ions (L. Yang et al., 2015; Martínez-Coronado et al., 2012; Jia et al., 2018). Upon cooling, the χ_MT value gradually decreases to 14.3 cm·K·mol⁻¹ at 50 K, below which the χ_MT value sharply drops to 3.5 cm·K·mol⁻¹ at 1.8 K. The reciprocal susceptibility 1/χ_Mvs. T plots align well with the Curie-Weiss law from 300 to 20 K with a negative Weiss constant of −24.96 K (Supplementary Figure S10), which further proves that the magnetic properties of compound 1 is dominated by antiferromagnetic interactions (Nunzi et al., 2007; Zhao et al., 2019). These antiferromagnetic coupling might be transferred by O−Mo−O bonds (Castellano et al., 2011).

FIGURE 3

FIGURE 3. Plots of _χM and _χMT vs. T from 310 K to 1.8 K of compound 1.

Conclusion

In summary, a new 3D POMo framework imbedding Ln and transition metal ions has been synthesized and characterized. To the best of our knowledge, our work, employing the Silverton-type building block {XMo₁₂O₄₂}, is able to enrich the Silverton-type POMo family. The twelve-coordinated Pr³⁺ ion in compound 1 is encapsulated at the center of the Silverton-type {PrMo₁₂O₄₂} and the bridged Mn²⁺ ions further extend the structure to 3D frameworks. The overall red emission spectra of 1 originates from the nature of Pr³⁺ and Mn²⁺ ions and the lifetime decay behavior has also been systematically probed [τ₁ = 1.15 μs (50.02%) and τ₂ = 8.94 μs (49.98%)]. Magnetic property studies indicate that 1 displays antiferromagnetic interactions. The successful isolation of 1 is not only beneficial for the development of Ln-containing POMs, but also opens a way to explore multifunctional material with photoluminescence and magnetic properties.

Data Availability Statement

The datasets presented in this study can be found in online repositories. The names of the repository/repositories and accession number(s) can be found in the article/Supplementary Material.

Author Contributions

YZ and YJ performed the experiments. XS assisted in results analyses. HK, SC, and PM conducted the structural and magnetic measurements. JN and JW designed the study and discussed the results. All authors contributed to the article and approved the submitted version.

Funding

This work was financially supported by the Natural Science Foundation of China (20771034 and 21401042).

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Supplementary Material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fchem.2021.615595/full#supplementary-material.

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