Assembly of Lanthanide-Containing Tungstotellurates(VI): Syntheses, Structures, and Catalytic Properties

Introduction

Polyoxometalates (POMs) are a unique class of metal-oxo clusters with tunable structures and excellent properties (Hill, 1998; Cronin and Müller, 2012). Due to the existence of abundant surface oxygen atoms, POMs as inorganic ligands can easily coordinate with transition metal or lanthanide (Ln) ions, resulting in the formation of discrete nanoscale clusters or extended structures. Among them, Ln-containing polyoxotungstates have attracted numerous attention owing to their structural diversity (Ma et al., 2015; Zhao et al., 2016b) and attractive applications in luminescence (Granadeiro et al., 2010; Ritchie et al., 2010b), magnetism (Clemente-Juan et al., 2012; Suzuki et al., 2013), and catalysis (Boglio et al., 2006; Suzuki et al., 2014; Li et al., 2018). Up to now, two synthetic strategies have been developed to construct Ln-containing POMs. One is building block method, where different lacunary POM precursors, such as monolacunary [XW₁₁O₃₉]ⁿ⁻ (Zhang et al., 2012; Arab Fashapoyeh et al., 2018; Mougharbel et al., 2020) and trilacunary [XW₉O₃₄]ⁿ⁻ (X = P, Si, Ge) (Zhao et al., 2014, 2017) and [XW₉O₃₃]⁹⁻ (X = As^III, Sb^III) (Chen et al., 2018; Kaushik et al., 2018), were used to coordinate with Ln³⁺ ions. The other is one-pot synthetic strategy, by which intricate POM structures are fabricated through the condensation reaction of simple metal salts with heteroanions (Chen et al., 2013, 2014; Zhao et al., 2016a; Liu J. C. et al., 2018). Although the one-pot method is simple and straightforward, understanding the assembly process is challenging.

Generally, the assembly of Ln-containing POMs can be affected by many synthetic parameters, such as molar ratio of reactants, temperature, pH, solvents, and so on. Among them, the pH value is a key factor. For example, Chen et al. reported five Ce(III)-containing POMs, finding that the structural motif of POM building block can be influenced by the pH value (Chen et al., 2014). Subsequently, they also demonstrated that pH can control the arrangement of lacunary Keggin and Wells–Dawson clusters (Chen et al., 2019). Moreover, the type of Ln³⁺ ions plays a role during the formation of Ln-containing POMs. Ozeki and Yamase reported the effect of lanthanide contraction on the Ln–O bond lengths in [LnW₁₀O₃₆]¹⁰⁻ clusters (Ln = Pr, Nd, Sm, Gd, Tb, Dy) (Ozeki and Yamase, 1994). Previous investigations indicate that the arrangement of POM can also be influenced by the type of Ln³⁺ ions. For example, the assembly of monovacant [α-SiW₁₁O₃₉]⁸− with different Ln³⁺ ions leads to three different structural motifs: 1D linear chain [Yb(α-SiW₁₁O₃₉)(H₂O)₂]⁵⁻, 1D zigzag chain [Eu(α − SiW₁₁O₃₉)(H₂O)₂]⁵⁻, and 2D network [Nd₂(α − SiW₁₁O₃₉)(H₂O)₁₁]²⁻ (Mialane et al., 2003). In 2007, Kortz et al. found that the configuration of POMs can change with the size of Ln³⁺ ions. For example, [Ln(β₂ − GeW₁₁O₃₉)₂]¹³⁻ was isolated when using a larger Ln³⁺ ion (Ln = La, Ce, Pr, Nd, Sm, Gd, or Dy), while [Ln(β₂ − GeW₁₁O₃₉) (α − GeW₁₁O₃₉)]¹³⁻ was prepared when using a smaller Ln³⁺ ion (Ln = Ho, Er, or Tm) (Mougharbel et al., 2020).

Tungstotellurates are an important subfamily of polyoxotungstates with trigonal–pyramidal {Te^IVO₃}²⁻ or octahedral {Te^VIO₆}⁶⁻ as heteroanions. Due to the existence of lone pair electrons in Te(IV) atom, tungstotellurates(IV) tend to form lacunary structures, such as [H₂Te₄W₂₀O₈₀]²²⁻ and [NaTeW₁₅O₅₄]¹³⁻ (Ismail et al., 2009), dimeric [Te₂W₁₇O₆₁]¹²⁻, [Te₂W₁₆O₅₈(OH)₂]¹⁴⁻, and [Te₂W₁₈O₆₂(OH)₂]¹⁰⁻ (Ritchie et al., 2010a), as well as macrocyclic clusters: [W₂₈Te₈O₁₁₂]²⁴⁻, [W₂₈Te₉O₁₁₅]²⁶⁻, and [W₂₈Te₁₀O₁₁₈]²⁸⁻ (Gao et al., 2011). Therefore, Ln³⁺ ions can be readily incorporated into tungstotellurates(IV) (Chen et al., 2013, 2015; Han et al., 2017a,b; Liu J. L. et al., 2018; Liu et al., 2019; Zhang et al., 2020). In comparison, limited attention has been paid on tungstotellurates(VI). The first member of tungstotellurates(VI) is Anderson-type [TeW₆O₂₄]⁶⁻, which was first determined in 1986 (Schmidt et al., 1986). After that, until 2009 two new members, Dawson-like [H₃W₁₈O₅₆(Te^VIO₆)]⁷⁻ ({TeW₁₈O₆₂}) and high nuclearity [H₁₀${\text{Te}}_2^{\text{VI}}$W₅₈O₁₉₈]²⁶⁻, were discovered by Cronin and Long (Yan et al., 2009). As the reported tungstotellurates(VI) are plenary clusters, Ln-containing tungstotellurates(VI) was nearly unexplored. Recently, Ln³⁺ ions (Ln = Eu, Gd, Tb) have been successfully introduced into tungstotellurates(VI) in our group, obtaining dimeric [H₁₀(WO₂){Ln(H₂O)₅(Te^VIW₁₈O₆₅)}₂]¹²⁻ (Ln = Eu, Gd, Tb) and tetrameric [H₁₆{Ln(H₂O)₅(Te^VIW₁₈O₆₄)}₄]²⁸⁻ (Ln = Eu, Gd), dimeric [H₆{Tb(H₂O)₃(Te^VIW₁₇O₆₁)}₂]¹²⁻, mono-substituted [H₂Tb(H₂O)₄(Te^VIW₁₇O₆₁)]⁹⁻, and 3D inorganic framework [HTb(H₂O)₄{Te^VIW₆O₂₄}]²⁻ (Shang et al., 2018). In the synthetic process, we find that the assembly of Tb-containing tungstotellurates(VI) is controlled by pH and that the formation of tetrameric clusters depends on the type of Ln³⁺ ions.

To systematically investigate the effect of Ln³⁺ ions on the assembly of Ln-containing tungstotellurates(VI), herein the Ln source was extended from La³⁺ to Lu³⁺ (except radioactive Pm³⁺). When Ln³⁺ (Tb³⁺, Dy³⁺, Ho³⁺, Er³⁺, Tm³⁺, Yb³⁺, Lu³⁺) with a relatively smaller ionic radius was used in the assembly, dimeric (DMAH)_n[H_22−n{Ln(H₂O)₃[TeW₁₇O₆₁]}₂]^·mH₂O (abbreviated as {Ln₂Te₂W₃₄}, DMAH⁺ = dimethylammonium), mono-substituted (DMAH)₇Na₂{H₂Ln(H₂O)₄[TeW₁₇O₆₁]}^·mH₂O (abbreviated as {LnTeW₁₇}), and 3D inorganic frameworks (DMAH)_n{H_3−nLn(H₂O)₄[TeW₆O₂₄]}^·mH₂O (abbreviated as {LnTeW₆}) were formed at pH 1.7, 1.9, and 2.3, respectively. However, under the otherwise identical conditions, no crystal compounds were generated by using La³⁺-Eu³⁺ with a relatively larger ionic radius (Scheme 1). The three types of Ln-containing tungstotellurates(VI) have been characterized by single-crystal X-ray diffraction, Fourier-transform infrared (FT-IR) spectra, elemental analyses, and TG analyses. Moreover, {LnTeW₆} (Ln = Tb³⁺, Er³⁺, Lu³⁺) were used as Lewis acid catalysts to catalyze the cyanosilylation of aldehydes or ketone with trimethylsilylcyanide (TMSCN) under solvent-free conditions, and the relationship between the radii of Ln³⁺ ions and catalytic activity was also investigated.

SCHEME 1

Scheme 1. The synthetic illustration of the three types of Ln-containing tungstotellurates(VI): {Ln₂Te₂W₃₄}, {LnTeW₁₇}, and {LnTeW₆}.

Experimental

Materials

Most of the chemicals were purchased from the following manufacturers: Aladdin (PrCl₃, 99.9%; TbCl₃·6H₂O, 99.9%; HoCl₃·6H₂O, 99.9%; LuCl₃·6H₂O, 99.9%; citric acid monohydrate, 99.5%; trimethylsilycyanide, 96%), Energy chemical [NdCl₃·6H₂O, 99.99%; SmCl₃·6H₂O, 99.99%; EuCl₃·6H₂O, 99.99%; GdCl₃·6H₂O, 99.99%; DyCl₃·6H₂O, 99.99%; ErCl₃·6H₂O, 99.99%; TmCl₃·6H₂O, 99.99%; dimethylamine hydrochloride, 98%; α-(trimethylsilyloxy)phenylacetonitrile, 97%], Alfa Aesar (YbCl₃·6H₂O, 99.9%), and Sinopharm Chemical Reagent Co., Ltd. (Na₂WO₄·2H₂O, AR; LaCl₃·nH₂O, AR; CeCl₃·6H₂O, 99%; benzaldehyde, AR; acetophenone, AR). These reagents were used without further purification, except benzaldehyde. The Anderson-type tungstotellurates(VI), Na₆TeW₆O₂₄·22H₂O, was prepared according to the literature (Shah et al., 2014).

Characterizations

FT-IR spectra were recorded on a Thermo IS5 spectrophotometer in the 4,000–400 cm⁻¹ region as KBr-pressed pellets. Thermogravimetric analyses were carried out under flowing N₂ on a Shimadzu DTG-60H instrument at a heating rate of 10°C/min. Elemental analyses (C, H, and N) were performed on ELEMENTAR vario EL cube Elmer CHN elemental analyzer. The Ln, W, Na, and Te elements were measured with ThermoiCAP Q mass spectrometry. The morphologies of {TbTeW₆} were observed on a JEOL model S-4800 field-emission scanning electron microscopy with an accelerating voltage of 5 kV. The catalytic reaction was monitored on a Shimadzu GC-2014C instrument with a flame ionization detector.

X-ray Crystallography

The X-ray single crystal diffraction data were collected on a Bruker APEX-II CCD diffractometer with graphite monochromatic Mo–Kα radiation (λ = 0.71073 Å) at 296 K. All crystals were sealed in capillary glass tubes for testing. All structures were solved using an intrinsic phasing method (SHELXT) (Sheldrick, 2015) and refined by full-matrix least-squares against $F_{\text{o}}^2$ with SHELXL software package (Sheldrick, 2015). Moreover, the residual disordered or crystal solvent molecules and cations were estimated by using the solvent MASK routine of OLEX2 (similar to PLATON/SQUEEZE) (Dolomanov et al., 2009). The hydrogen atoms were not incorporated in the refinements, and all non-hydrogen (Ln, W, Te, C, N, and O) atoms were refined anisotropically. The lattice H₂O molecules and cations can be partly found from the Fourier maps, but not all lattice H₂O molecules and cations can be found from the weak residual electron peaks. Thus, the numbers of the cations and lattice H₂O molecules were determined and added to the molecular formula directly on the basis of elemental analyses, TG analyses, and the charge balance consideration. The crystallographic data for these crystal compounds are summarized in Supplementary Table 1. The crystallographic data have been deposited with the Cambridge Crystallographic Data Center with CCDC 2023272-2023277 for {Ln₂Te₂W₃₄}, 2023278-2023283 for {LnTeW₁₇}, and 2023284-2023289 for {LnTeW₆} (Ln = Dy³⁺, Ho³⁺, Er³⁺, Tm³⁺, Yb³⁺, and Lu³⁺), respectively.

Synthesis of (DMAH)_n[H_22−n{Ln(H₂O)₃[TeW₁₇O₆₁]}₂]^· mH₂O: ({Ln₂Te₂W₃₄}, Ln = Dy³⁺, n = 16, m = 23; Ho³⁺, n = 15, m = 24; Er³⁺, n = 15, m = 25; Tm³⁺ and Yb³⁺, n = 15, m = 28; Lu³⁺, n = 15, m = 30).

A mixture containing Na₂WO₄·2H₂O (4.50 mmol, 1.50 g), Te(OH)₆ (0.44 mmol, 0.10 g), dimethylamine hydrochloride (7.35 mmol, 0.60 g), citric acid monohydrate (0.26 mmol, 0.05 g), and distilled water (20 mL) was charged to a 25 ml glass beaker. The pH was adjusted to 4.5 by dropping 6 M HCl under stirring. After that, LnCl₃·6H₂O (0.33 mmol, 0.12 g) was added to the mixture, forming a uniform suspension and the pH was adjusted to 1.7. The suspension was stirred at room temperature for about 5 min and filtered. The clear filtrate was placed in a refrigerator at 10°C for 5 days and then evaporated at the ambient environment after filtering again. The cubic-shaped crystals were observed after 1 month, and at that time, the pH of the mother liquor is 2.7. Element analysis (%) for {Dy₂Te₂W₃₄}: calcd. C 3.83, N 2.23, W 62.2, Te 2.54, Dy 3.23; found: C 3.39, N 2.06, W 61.2, Te 2.53, Dy 3.12; yield: 34% based on W. Element analysis (%) for {Ho₂Te₂W₃₄}: calcd. C 3.59, N 2.10, W 62.3, Te 2.55, Ho 3.29; found: C 3.41, N 2.10, W 61.3, Te 2.57, Ho 3.15; yield: 30% based on W. Element analysis (%) for {Er₂Te₂W₃₄}: calcd. C 3.59, N 2.09, W 62.2, Te 2.54, Er 3.33; found: C 3.70, N 2.29, W 60.5, Te 2.60, Er 3.10; yield: 32% based on W. Element analysis (%) for {Tm₂Te₂W₃₄}: calcd. C 3.57, N 2.08, W 61.8, Te 2.53, Tm 3.34; found: C 3.61, N 2.24, W 62.0, Te 2.60, Tm 3.30; yield: 35% based on W. Element analysis (%) for {Yb₂Te₂W₃₄}: calcd. C 3.56, N 2.08, W 61.8, Te 2.52, Yb 3.42; found: C 3.57, N 2.11, W 61.2, Te 2.50, Yb 3.23; yield: 36% based on W. Element analysis (%) for {Lu₂Te₂W₃₄}: calcd. C 3.55, N 2.07, W 61.6, Te 2.51, Lu 3.45; found: C 3.61, N 2.14, W 61.3, Te 2.60, Lu 3.40; yield: 37% based on W.

Synthesis of (DMAH)₇Na₂{H₂Ln(H₂O)₄[TeW₁₇O₆₁]}^·mH₂O: ({LnTeW₁₇}, Ln = Dy³⁺, m = 23; Ho³⁺ and Lu³⁺, m = 27; Er³⁺, m = 22; Tm³⁺, m = 21; Yb³⁺, m = 26).

The synthetic procedure of {LnTeW₁₇} was similar to that for {Ln₂Te₂W₃₄}, and the only difference is that the pH of the reaction mixture was adjusted to 1.9. When the rod-shaped crystals were obtained, the pH of the mother liquor is 2.9. Element analysis (%) for {DyTeW₁₇}: calcd. C 3.20, N 1.87, Na 0.88, W 59.6, Te 2.43, Dy 3.10; found: C 3.23, N 1.78, Na 0.96, W 60.6, Te 2.40, Dy 3.06; yield: 17% based on W. Element analysis (%) for {HoTeW₁₇}: calcd. C 3.16, N 1.84, Na 0.86, W 58.7, Te 2.40, Ho 3.10; found: C 3.20, N 1.73, Na 0.86, W 57.2, Te 2.34, Ho 3.09; yield: 17% based on W. Element analysis (%) for {ErTeW₁₇}: calcd. C 3.21, N 1.87, Na 0.88, W 59.7, Te 2.44, Er 3.20; found: C 3.21, N 1.73, Na 0.89, W 60.8, Te 2.50, Er 3.15; yield: 20% based on W. Element analysis (%) for {TmTeW₁₇}: calcd. C 3.22, N 1.88, Na 0.88, W 59.9, Te 2.45, Tm 3.24; found: C 3.23, N 1.75, Na 0.87, W 58.9, Te 2.50, Tm 3.20; yield: 20% based on W. Element analysis (%) for {YbTeW₁₇}: C 3.16, N 1.85, Na 0.87, W 58.8, Te 2.40, Yb 3.26; found: C 3.32, N 1.78, Na 0.83, W 58.2, Te 2.40, Yb 3.26; yield: 19% based on W. Element analysis (%) for {LuTeW₁₇}: C 3.15, N 1.84, Na 0.86, W 58.6, Te 2.39, Lu 3.28; found: C 3.09, N 1.80, Na 0.86, W 59.5, Te 2.46, Lu 3.35; yield: 20% based on W.

Synthesis of (DMAH)_n{H_3−nLn(H₂O)₄[TeW₆O₂₄]}^·mH₂O: ({LnTeW₆}, Ln = Dy³⁺, n = 1, m = 4; Ho³⁺, n = 2, m = 1; Er³⁺, n = 1, m = 10; Tm³⁺, n = 2, m = 8; Yb³⁺, n = 1, m = 7.5; Lu³⁺, n = 1.5, m = 10).

{LnTeW₆} was synthesized by the procedure similar to that for {Ln₂Te₂W₃₄} except that pH = 2.3. The hexagon-shaped crystals were observed after 1 month, and at that time the pH of the mother liquor is 3.2. Element analysis (%) for {DyTeW₆}: calcd. C 1.22, N 0.71, W 56.0, Te 6.48, Dy 8.25; found: C 1.14, N 0.68, W 55.7, Te 6.45, Dy 8.21; yield: 13% based on W. Element analysis (%) for {HoTeW₆}: calcd. C 2.49, N 1.45, W 57.1, Te 6.61, Ho 8.54; found: C 2.56, N 1.43, W 57.5, Te 6.67, Ho 8.59; yield: 16% based on W. Element analysis (%) for {ErTeW₆}: calcd. C 1.15, N 0.67, W 53.0, Te 6.13, Er 8.03; found: C 1.10, N 0.65, W 53.6, Te 6.18, Er 8.10; yield: 16% based on W. Element analysis (%) for {TmTeW₆}: calcd. C 2.30, N 1.34, W 52.7, Te 6.10, Tm 8.07; found: C 2.24, N 1.25, W 52.5, Te 6.06, Tm 8.01; yield: 17% based on W. Element analysis (%) for {YbTeW₆}: calcd. C 1.18, N 0.69, W 54.0, Te 6.25, Yb 8.47; found: C 1.14, N 0.75, W 54.2, Te 6.28, Yb 8.51; yield: 18% based on W. Element analysis (%) for {LuTeW₆}: calcd. C 1.71, N 0.99, W 52.2, Te 6.04, Lu 8.28; found: C 1.74, N 1.03, W 51.8 Te 6.01, Lu 8.23; yield: 20% based on W.

Catalytic Tests

Before the catalytic reaction, ground crystal samples were pretreated in a vacuum oven at 100°C for 3 h. The scanning electron microscopy (SEM) image (Supplementary Figure 9) shows that, after the pretreatment, micron-sized samples with an irregular morphology were obtained. Typically, aldehyde or ketone (1 mmol), trimethylsilycyanide (TMSCN, 2 mmol), and catalysts (2 mol%) were loaded into a 25 ml Shrek tube, which was purged three times with argon gas and heated at 45°C for 12 h. After the reaction, the mixture was diluted with 2 ml acetonitrile and added with naphthalene (0.75 mmol) as internal standard. Finally, the mixture was centrifuged and monitored quantitatively by gas chromatography. The catalysts were collected, washed with acetonitrile, and dried under vacuum for the characterization and the next run.

Results and Discussions

Syntheses and Structures

All crystal compounds were prepared by using Na₂WO₄·2H₂O, Te(OH)₆, dimethylamine hydrochloride, citric acid, and LnCl₃·6H₂O as starting materials in aqueous solution. Although citric acid does not appear in the final structures, the control experiments show that citric acid is important during the synthetic process. When we used another weak organic acid (e.g., acetic acid or amino acid) instead of citric acid, the yields of these crystal compounds dramatically decreased. According to previous investigations, we speculate that citric acid might play the role of a protective agent to coordinate with Ln³⁺ ions in preventing the formation of precipitates (Li et al., 2012; Wang et al., 2015). Moreover, it was found that dimethylamine hydrochloride is indispensable for the synthesis of crystal compounds. In the absence of dimethylamine hydrochloride, under the same reaction conditions, only numerous precipitation was obtained.

Description of {Ln₂Te₂W₃₄} and {LnTeW₁₇} Structures

Single-crystal X-ray crystallographic analyses reveal that both {Ln₂Te₂W₃₄} and {LnTeW₁₇} crystallize in the triclinic space group of P-1. {LnTeW₁₇} is mono-Ln³⁺-substituted Dawson-like POMs, while {Ln₂Te₂W₃₄} has 2:2 dimeric clusters based on {LnTeW₁₇} (Ln = Dy³⁺, Ho³⁺, Er³⁺, Tm³⁺, Yb³⁺, Lu³⁺). Due to the structural similarity, only the structures of {Dy₂Te₂W₃₄} and {DyTeW₁₇} are described here. {DyTeW₁₇} consists of one Dy³⁺ ion, one {TeW₁₇O₆₁} polyanion, seven DMAH⁺ cations, two Na⁺ ions, two protons, and four coordinated and 23 lattice water molecules. Dawson-like {TeW₁₈O₆₂} was first reported by Cronin and Long in 2009 (Yan et al., 2009), where a {Te^VIO₆} octahedron is encapsulated into {W₁₈O₅₄} cage (Figure 1A). The {TeW₁₇O₆₁} can be regarded as a monovacant structure of {TeW₁₈O₆₂} formed by losing one [WO]⁴⁺ from the belt position. Compared with {TeW₁₈O₆₂}, the {Te^VIO₆} octahedron in {DyTeW₁₇} still lies in the center of the cluster but exhibits a slight distortion because one [WO]⁴⁺ unit is replaced by a Dy³⁺ ion. The Te-O bond lengths of {DyTeW₁₇} are in the range of 1.930(5)−2.010(6) Å, deviating from an ideal bond length [1.981(6) Å] in {TeW₁₈O₆₂}. The Dy³⁺ ion is incorporated into the vacant position of {TeW₁₇O₆₁} (Figure 1B) and coordinated with eight oxygen atoms: four from monovacant POM and four from coordinated water molecules. The inserted Dy³⁺ ion exhibits a distorted bicapped trigonal prismatic geometry with Dy-O bond lengths of 2.292(6)−2.464(7) Å (Supplementary Figure 1C). Due to the contraction effect of lanthanide, the Ln-O bond lengths decrease with the decrease of Ln³⁺ ionic radii (shown in Supplementary Table 2).

FIGURE 1

Figure 1. The polyhedral and ball-and-stick view of (A) {TeW₁₈O₆₂}, (B) {DyTeW₁₇}, and (C) {Dy₂Te₂W₃₄}. The cations and the lattice H₂O molecules are omitted for clarity.

{Dy₂Te₂W₃₄} is composed of two Dy³⁺ ions, two monovacant [TeW₁₇O₆₁]¹⁴⁻ polyanions, 16 DMAH⁺ cations, six protons, and six coordinated and 23 lattice H₂O molecules. The dimeric {Dy₂Te₂W₃₄} is composed of two mono-substituted {DyTeW₁₇} subunits, which are connected by a belt-to-belt linking mode through Dy-O bonds (Figure 1C). In this cluster, the two Dy³⁺ ions also display distorted bicapped trigonal prismatic coordination geometries, coordinating by five terminal oxygen atoms (O4, O5, O6, O7, and O8) from two monovacant [TeW₁₇O₆₁]¹⁴⁻ polyanions [Dy-O: 2.305(6)−2.374(7) Å] and three H₂O molecules [O1w, O2w and O3w) (Dy-O: 2.370(8)−2.434(9) Å] (Supplementary Figure 1E).

Description of {LnTeW₆} Structures

Single-crystal X-ray crystallographic analyses reveal that the six {LnTeW₆} compounds are isostructural and crystallize in the orthorhombic space group of Cccm. As a result, only the structure of {DyTeW₆} is described in detail. {DyTeW₆} contains one Dy³⁺ ion, one [TeW₆O₂₄]⁶⁻ polyanion, one DMAH⁺ cation, two protons, and four coordinated and four lattice H₂O molecules. The [TeW₆O₂₄]⁶⁻ cluster shows an A-type Anderson structure where the central Te(VI) is surrounded by six edge-sharing WO₆ octahedra (Figure 2A). The octahedral {TeO₆} has a slight distortion with O-Te-O bond angles in the range of 85.0(2)° to 95.0(2)°.

FIGURE 2

Figure 2. (A) View of the coordination of an Anderson-type [TeW₆O₂₄]⁶⁻ polyanion to one hydrated Dy³⁺ cation. (B) View of 3D inorganic structure of {DyTeW₆} compound. Simplified view of (C) [TeW₆O₂₄]⁶⁻ polyanion and (D) Dy³⁺ cation as the four-connecting sites. (E) 3D schematic illustration of {DyTeW₆} (yellow: [TeW₆O₂₄]⁶⁻ polyanion; purple: Dy³⁺ cation). The DMAH⁺ cations and the lattice H₂O molecules are omitted for clarity.

As shown in Figure 2C, each [TeW₆O₂₄]⁶⁻ acts as a four-connecting node and connects with four Dy³⁺ ions through the terminal O2 atoms. The Dy³⁺ center exhibits a square antiprismatic geometry completed by four H₂O molecules (O1w) and four terminal oxygen atoms (O2) from four adjacent [TeW₆O₂₄]⁶⁻ polyanions, and the Dy-O bond lengths are from 2.410(4) to 2.342(4) Å (Supplementary Figure 2A). As shown in Figure 2B and Supplementary Figure 2B, the alternating connection of [TeW₆O₂₄]⁶⁻ polyanions and Dy³⁺ cations by sharing O2 atoms leads to a 3D inorganic framework. From the topological views, both [TeW₆O₂₄]⁶⁻ and Dy³⁺ can be regarded as four-connecting sites (Figures 2C,D) and a (4,4)-connected PtS topology can be abstracted (Figure 2E).

It was found that the assembly of three types of Ln-containing tungstotellurates(VI) is pH dependent. Using {TeW₁₇O₆₁} as a building block, dimeric {Ln₂Te₂W₃₄} and mono-substituted {LnTeW₁₇} were isolated at pH 1.7 and 1.9, respectively. At pH 1.8, {Ln₂Te₂W₃₄} and {LnTeW₁₇} crystallized together. When the solution pH was adjusted to 2.3, 3D inorganic frameworks {LnTeW₆} based on Anderson-type {TeW₆O₂₄} were obtained. The results indicate that the size of tungstotellurates clusters decreases with the increase of pH. A similar trend is reported in Tb-containing tungstotellurates(VI) (Shang et al., 2018). In addition, we found that the assembly of Ln-containing POMs is influenced by the type of Ln³⁺ ions. The use of Ln³⁺ ions with a relatively smaller ionic radius [e.g., Tb³⁺ (Shang et al., 2018), Dy³⁺, Ho³⁺, Er³⁺, Tm³⁺, Yb³⁺, or Lu³⁺] leads to the formation of {Ln₂Te₂W₃₄}, {LnTeW₁₇}, and {LnTeW₆} in the pH range of 1.7–2.3. However, when starting from Ln³⁺ ions with a relatively larger ionic radius (e.g., La³⁺, Ce³⁺, Pr³⁺, Nd³⁺, Sm³⁺, or Eu³⁺), under the otherwise identical conditions only precipitate or {TeW₁₈O₆₂} clusters were observed. Notably, dimeric clusters, [H₁₀(WO₂){Ln(H₂O)₅(TeW₁₈O₆₅)}₂]¹²⁻, were prepared at pH 1.5 by using La³⁺, Ce³⁺, Pr³⁺, Nd³⁺, Sm³⁺, Eu³⁺, or Tb³⁺, where two {TeW₁₈O₆₅} units are bridged by one {WO₂} and two Ln³⁺ (Shang et al., 2018; Yang et al., 2018). Nevertheless, such structural motif cannot be obtained when using Dy³⁺, Ho³⁺, Er³⁺, Tm³⁺, Yb³⁺, or Lu³⁺ as starting materials. Therefore, it is obvious that both pH and the type of Ln³⁺ ions have a significant impact on the assembly of these Ln-containing tungstotellurates(VI) (Scheme 1).

FT-IR Spectra

Since both {Ln₂Te₂W₃₄} and {LnTeW₁₇} are based on monovacant {TeW₁₇O₆₁}, their FT-IR spectra exhibit similar characteristic absorption peaks. As shown in Supplementary Figures 3, 4, the characteristic peaks at 1,020 cm⁻¹ correspond to the antisymmetric stretching vibrations of Te-O bonds and the peaks at 942–948, 812–818, and 743–757 cm⁻¹ are attributed to the vibrations of terminal W=O bonds, bridging W-O_b-W (b: edge-shared O atoms) and W-O_c-W (c: corner-shared O atoms), respectively. Compared with the plenary Dawson-like [TeW₁₈O₆₂]¹⁰⁻ (1,019, 949, and 799 cm⁻¹) (Yan et al., 2009), some characteristic absorption peaks of POMs are slightly shifted, which might be caused by inserting Ln³⁺ ions in the vacant site of monovacant {TeW₁₇O₆₁}. The FT-IR spectra of {LnTeW₆} are illustrated in Supplementary Figure 5. The characteristic peaks in the range of 1,000–400 cm⁻¹ are very similar. The peaks between 970 and 950 cm⁻¹ are assigned to the terminal W=O bonds, and those in the range of 920–630 cm⁻¹ are the characteristic stretching vibrations of the W-O-W bridges.

Moreover, the characteristic peaks of DMAH⁺ countercations can be observed in these compounds. The peaks at 3,110–3,132 and 2,778–2,790 cm⁻¹ are assigned to the stretching vibrations of N-H and C-H bonds, respectively, while the peaks at 1,558–1,640 and 1,456–1,466 cm⁻¹ are attributed to the bending vibrations of N-H and C-H bonds, respectively. The broad peaks at 3,400–3,432 cm⁻¹ are the stretching vibrations of H₂O molecules. In addition, we observed that the crystals of Ln-containing tungstotellurates(VI) can easily turn to powder when leaving the mother liquor due to the loss of lattice water molecules. However, the POM skeleton is still maintained, which has been demonstrated by the FT-IR spectra.

Catalytic Activities of {LnTeW₆}

Our previous electrospray ionization–mass spectrometry investigations show that the {Tb₂Te₂W₃₄} and {TbTeW₁₇} clusters are unstable in solution, dissociating into stable {TeW₁₈O₆₂} and other fragments (Shang et al., 2018). In comparison, the 3D inorganic framework {LnTeW₆} is stable, and the Lewis acidic centers (Ln³⁺) are accessible after removing the lattice and the coordinated water molecules. The cyanosilylation reaction is an important method to prepare cyanohydrins, which can be further converted into value-added chemicals (e.g., α-hydroxy ketones, α-hydroxy acids, and β-amino alcohols) and drug molecules (Brunel and Holmes, 2004; Jia et al., 2014). Therefore, the cyanosilylation of benzaldehyde with TMSCN under solvent-free conditions was used as a reaction model to evaluate the Lewis acid catalytic activity of {LnTeW₆} (Figure 3A). As no byproduct was obtained in the reaction, the yield of cyanohydrin trimethylsilyl ethers was calculated based on the conversion of benzaldehyde.

FIGURE 3

Figure 3. (A) The cyanosilylation reaction of benzaldehyde with trimethylsilylcyanide (TMSCN). (B) The Ln–O₂ bond lengths and the yield of cyanosilylation catalyzed by {TbTeW₆}, {ErTeW₆}, and {LuTeW₆}, respectively. (C) Effect of temperature on cyanosilylation using {TbTeW₆}. (D) Time profile for cyanosilylation catalyzed by {TbTeW₆}. (E) Effect of the amounts of {TbTeW₆} on cyanosilylation. Reaction conditions: benzaldehyde (1 mmol), TMSCN (2 mmol), {TbTeW₆} (2 mol%, relative to the benzaldehyde), naphthalene (0.75 mmol, internal standard) under Ar atmosphere, 45°C, 12 h.

In our work, seven isostructural {LnTeW₆} frameworks (Ln = Tb³⁺, Dy³⁺, Ho³⁺, Er³⁺, Tm³⁺, Yb³⁺, and Lu³⁺) were synthesized, which provides a good platform to investigate the effect of Ln ionic radius on the Lewis acid catalytic activity. As the decrease of ionic radius from Tb³⁺ (0.0923 Å) to Lu³⁺ (0.0848 Å) is unobvious, only three representative catalysts, {TbTeW₆}, {ErTeW₆}, and {LuTeW₆}, were used. As shown in Figure 3B, all of {TbTeW₆}, {ErTeW₆}, and {LuTeW₆} can promote the cyanosilylation reaction with a yield of 69–97%. In contrast, only 20% yield was obtained in the absence of a catalyst. Interestingly, it was found that the catalytic activity of {LnTeW₆} decreased with the decrease of Ln ionic radius, giving the order of {TbTeW₆} (97%) > {ErTeW₆} (80%) > {LuTeW₆} (69%). On the one hand, the Ln-O2 bond lengths (O2: terminal oxygen atom of {TeW₆O₂₄}) decrease from Tb³⁺ to Lu³⁺ in the order of Tb-O2 (2.360 Å) > Er-O2 (2.312 Å) > Lu-O2 (2.290 Å). The decrease of Ln-O2 bond length leads to the increase of steric hindrance around Ln³⁺ ions, and as a result, it becomes difficult for the substrates to access the Lewis acidic centers. On the other hand, the Ln-O1w bond lengths (O1w: the coordinated H₂O molecules) also decreases from Tb³⁺ to Lu³⁺ (Tb-O1w: 2.410 Å, Er-O1w: 2.382 Å, and Lu-O1w: 2.369 Å), making the removal of coordinated H₂O molecules difficult and resulting in the catalytic activity of Ln³⁺ ions being reduced. With the good performance of {TbTeW₆}, it is used in the following experiments.

To explore the optimal reaction conditions, the influences of reaction temperature, time, and amount of catalyst on the cyanosilylation reaction were systematically investigated. As shown in Figure 3C, the reaction was conducted in the temperature range of 30–75°C, and a satisfactory yield (97%) was achieved at 45°C. The yield of 2-phenyl-2-[(trimethylsilyl)oxy]acetonitrile increases with the reaction time, and the maximum yield was reached after 12 h at 45°C (Figure 3D). As shown in Figure 3E, the catalyst amount of 2 mol% (relative to benzaldehyde) is an optimized dosage for this reaction.

To verify the heterogeneity of {TbTeW₆}, the catalyst was incubated in solvent at 45°C for 12 h, and the inductively coupled plasma result reveals that a negligible amount of Te, Tb, and W was detected in the filtrate. Moreover, the reusability and the stability of {TbTeW₆} was tested under the optimized conditions. As shown in Figure 4A, {TbTeW₆} could be reused for five times without a significant loss of its catalytic activity. The FT-IR spectra and powder X-ray diffraction (PXRD) patterns of the {TbTeW₆} used were basically identical to those of the fresh ones (Figures 4B,C), suggesting that the structure of {TbTeW₆} was maintained after five cycles and that Lewis acidic center Tb³⁺ is successfully stabilized by {TeW₆O₂₄} clusters. The SEM images show that the ground {TbTeW₆} has an irregular morphology in micrometers before the reaction and after five recycles (Supplementary Figure 9). In addition, Na₆TeW₆O₂₄ and TbCl₃·6H₂O were used in the reaction and gave a yield of 100%. However, Na₆TeW₆O₂₄ is unstable under the turnover conditions, as confirmed by FT-IR and PXRD characterization (Supplementary Figure 10), and TbCl₃·6H₂O cannot be reused.

FIGURE 4

Figure 4. (A) Reusability of {TbTeW₆} catalyst. (B) Fourier-transform infrared spectra and (C) powder X-ray diffraction patterns of the activated {TbTeW₆} catalyst before reaction and after each cycle. Reaction conditions: benzaldehyde (1 mmol), trimethylsilylcyanide (2 mmol), {TbTeW₆} (2 mol%, relative to the benzaldehyde), naphthalene (0.75 mmol, internal standard) under Ar atmosphere, 45°C, 12 h.

The effect of substituents on aromatic aldehydes was investigated, and the results are shown in Table 1. Aromatic aldehyde with electron-withdrawing group (chloro) is beneficial to the cyanosilylation reaction, giving a yield of up to 100% (2 in Table 1). However, a significant decrease of yield was observed by using aromatic aldehydes with electron-donating groups (methoxy) (3–5 in Table 1). Among them, the yield of 5 (29%) with two methoxy substituents is much lower than those of 3 (49%) and 4 (66%) with one methoxy substituent. Generally, ketones are less reactive than aldehydes in cyanosilylation reaction, and thus only 8% yield was obtained using acetophenone as substrate (6 in Table 1).

TABLE 1

Table 1. The cyanosilylation reaction catalyzed by {TbTeW₆}^a.

Conclusions

In summary, a series of Ln-containing tungstotellurates(VI) have been isolated and structurally characterized. The assembly of dimeric {Ln₂Te₂W₃₄}, mono-substituted {LnTeW₁₇}, and 3D inorganic framework {LnTeW₆} is pH dependent, which were formed at pH 1.7, 1.9, and 2.3, respectively. Importantly, the type of Ln³⁺ ions plays an important role in the assembly process. The three types of Ln-containing POMs can be synthesized by using Tb³⁺-Lu³⁺ ions with a relatively smaller radius, while when starting from La³⁺-Eu³⁺ ions only precipitation or {TeW₁₈O₆₂} clusters were observed. Moreover, three {LnTeW₆} (Ln = Tb³⁺, Er³⁺, Lu³⁺) are selected as heterogeneous Lewis acid catalysts for the cyanosilylation reaction. It was found that the catalytic activity of {LnTeW₆} decreases with the decrease of Ln³⁺ ionic radius. The {TbTeW₆} catalyst exhibits excellent stability and can be reused for five times without a significant loss of activity. The successful isolation of Ln-containing POMs not only contributes to the understanding of Ln-containing POM assembly but also provides a good platform to investigate the influence of Ln ionic radius on their Lewis acid catalytic activity.

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

YC and CH supervised the project. JL, SS, and NZ prepared the catalysts. ZLin and ZY analyzed the crystallographic data. JL performed the catalytic experiments and wrote the manuscript. ZLi participated in the analysis of the results. All authors contributed to the article and approved the submitted version.

Funding

This work was financially supported by the National Natural Science Foundation of China (21671019, 21871026, 21771020, and 21971010).

Conflict of Interest

SS was employed by company China Resources Double-Crane Pharmaceutical Co., Ltd.

The remaining 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.2020.598961/full#supplementary-material

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