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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Organophosphonate-Functionalized Lanthanopolyoxomolybdate: Synthesis, Characterization, Magnetism, Luminescence, and Catalysis of H2O2‑Based Thioether Oxidation Jiawei Wang, Yanjun Niu, Meng Zhang, Pengtao Ma, Chao Zhang, Jingyang Niu,* and Jingping Wang* Henan Key Laboratory of Polyoxometalate, Institute of Molecular and Crystal Engineering, College of Chemistry and Chemical Engineering, Henan University, Kaifeng 475004, Henan, People’s Republic of China S Supporting Information *

ABSTRACT: A novel class of organophosphonate-based polyoxomolybdate derivatives, K4H5[Ln3(H2O)14{(Mo8O24)(O3PCH2COO)3}2]·23H2O (Ln = Gd (1Gd), Tb (2Tb), Dy (3Dy)), have been fully investigated by a few characterization methods such as single-crystal X-ray crystallography, XRPD, elemental analysis, TGA, and IR spectra. The magnetic properties of 1Gd, 2Tb, and 3Dy were investigated, as well as the solid-state luminescence properties of 2Tb and 3Dy. The catalysis properties of 1Gd, 2Tb, and 3Dy for thioether oxidization have been investigated using hydrogen peroxide (H2O2) as an oxidant. The catalysis study demonstrated the efficient and selective conversion of various thioethers to their corresponding sulfones in excellent yields.

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example, {(O2CCH2PO3)2Mo5O15},18 {(HOOCC2H4PO3)2Mo5O15},19 {(HOOCC5H9NCH2PO3)2Mo5O15},20 {(HO 2 CC 6 H 4 PO 3 )Mo 2 O 6 }, {(O 2 CC 6 H 4 PO 3 )MoO 2 F}, {(O 2 CC 6 H 4 PO 3 ) 2 Mo 5 O 15 }, and {(HO 2 CC 6 H 4 PO 3 ) 2 Mo12F4O34(H2O)4}.21 From the details of the examples above, it is easy to figure out that a majority of structures just anchor some common transition metals (Co, Ni, Cu) or even do not anchor any metal ions.22 In contrast, lanthanide (Ln) ions incorporated in the system of Mo/O/RPO32− have been rarely explored. Only a series of Ln species based on 1,1diphosphonic acid (HEDP) functionalized polyoxomolybdates have been reported.23 These compounds form three-dimensional cage structures and have strong dye degradation activity (rhodamine B (RhB)). As a continuation of previous work, we have focused research on Ln-containing organophosphonate-based polyoxomolybdates as they possess 4f electrons, which may lead to novel structures and attractive properties, such as magnetism and photoluminescence properties. Therefore, we chose sodium molybdate and organophosphonate ligands with Ln ions to prepare multidimensional structures by one-pot reactions. Among the vast organophosphonate family, phosphonoacetic acid has been generally used as an organic ligand to obtain inorganic−organic hybrid compounds.19−22 Notably, the polyfunctional bridging ligand phosphonoacetic acid displays the following features: (1) phosphonoacetic acid allows multifunctional coordination modes to coordinate various

INTRODUCTION Polyoxometalates (POMs) have become a family of earlytransition-metal clusters (W, Mo, or V) that possess abundant surface oxygen atoms and controllable structures, which generally show significant structural diversity and intriguing properties such as magnetism, catalysis, electrochemistry, etc.1,2 In particular, organo-functionalized POMs, for instance, carboxylic acid-/alkoxy-/organophosphonate-modified POMs, have become an important and hot research topic in the POM family.3 A large number of organophosphonate-based polyoxomolybdates (Mo/O/RPO32−) have been successfully prepared, and their potential applications have been fully explored4 by Pope,5 Kortz,6 Zubieta,7 Dolbecq,8 et al. A thorough literature survey indicates that monophosphonatefunctionalized polyoxomolybdates, such as the Strandberg type polyanions [(O3PR)2Mo5O15]4− (R = CH3, C2H5, C6H5, C2H4NH3+, CH2C6H4NH3+, CH3CHNH3+, CH3CH(CH3)CHNH3+, etc.),9 have been consistently explored for a long time. In particular, bis-phosphonate-functionalized polyoxomolybdates have achieved a dominant position in the field of Mo/ O/RPO32−: for example, {(O3PCR(OH)PO3)2MoO2} (R = C 3 H 6 NH 2 ), 10 {(Mo V 2 O 4 ) 4 (O 3 PCH 2 PO 3 ) 4 }, 11 {Mo 2 O 5 (O3PCH2CH2PO3)},12 {(O3PCROPO3)2O(Mo3O8)2} (R = CH2S(CH3)2, C4H5N2), {Mo5O15O3P(CH2)nPO3} (n = 2− 4),13 {Mo6O18(O3P(CH2)5PO3)},14 {Mo6O22(O3PCH2PO3)},15 and {Mo7O16(O3PCH2PO3)3}.16 Organotriphosphonates grafted into the Mo/O/RPO32− framework, such as {[N(CH2PO3)3]Mo6O16(OH)(H2O)4},17 have been previously reported. Furthermore, great developments in phosphonocarboxylate derivatives have also been made: for © XXXX American Chemical Society

Received: October 17, 2017

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DOI: 10.1021/acs.inorgchem.7b02672 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Table 1. Crystallographic Data for 1Gd, 2Tb, and 3Dy formula Mr (g mol−1) space group cryst syst a (Å) b (Å) c (Å) β (deg) V (Å3) Z cryst size (mm3) ρcalcd (g cm−3) μ (mm−1) Rint limiting indices

no. of rflns collected no. of indep rflns F(000) GOF on F2 R1, wR2 (I > 2σ(I)) R1, wR2 (all data)

1Gd

2Tb

3Dy

C12H78K4O111P6Mo16Gd3 4347.71 C2/c monoclinic 29.8359(13) 11.1754(5) 39.2899(17) 94.2560(10) 13064.2(10) 4 0.23 × 0.21 × 0.18 2.210 3.287 0.0224 −35 ≤ h ≤ 35 −13 ≤ k ≤ 8 −46 ≤ l ≤ 46 32630 11548 8272.0 1.068 0.0586, 0.1595 0.0627, 0.1628

C12H78K4O111P6Mo16Tb3 4352.73 C2/c monoclinic 29.7923(12) 11.1666(4) 39.2587(15) 94.2470(10) 13024.7(9) 4 0.21 × 0.19 × 0.15 2.220 3.398 0.0239 −35 ≤ h ≤ 34 −13 ≤ k ≤ 13 −46 ≤ l ≤ 36 32361 11507 8284.0 1.066 0.0573, 0.1546 0.0629, 0.1589

C12H78K4O111P6Mo16Dy3 4363.45 C2/c monoclinic 29.7519(16) 11.1479(6) 39.204(2) 94.2030(10) 12967.9(12) 4 0.28 × 0.16 × 0.15 2.235 3.506 0.0213 −34 ≤ h ≤ 35 −13 ≤ k ≤ 13 −40 ≤ l ≤ 46 32503 11491 8296.0 1.086 0.0598, 0.1642 0.0642, 0.1677

C, 3.20, H, 1.859. Selected IR (KBr, cm−1): 3413 (s), 1711 (m), 1622 (s), 1531 (s), 1434 (m), 1382 (m), 1219 (w), 1142 (s), 1107 (s), 1061 (s), 908 (s), 754 (s), 730 (s), 651 (s), 608 (s). Synthesis of Compound 2Tb. The preparation procedure of 2Tb is similar to that for 1Gd but with TbCl3·6H2O instead of GdCl3·6H2O. Yield: 0.120 g (17.6%) for 2Tb based on TbCl3·6H2O. Anal. Calcd: Mo, 34.64; Tb, 10.76; P, 4.19; K, 3.53; C, 3.25; H, 2.07. Found: Mo, 34.79; Tb, 10.92; P, 4.30; K, 3.62; C, 3.15, H, 1.984. Selected IR (KBr, cm−1): 3411 (s), 1713 (m), 1625 (s), 1532 (s), 1436 (s), 1381 (m), 1216 (w), 1145 (s), 1108 (s), 1063 (s), 905 (s), 750 (s), 733 (s), 652 (s), 610 (s). Synthesis of Compound 3Dy. The preparation procedure of 3Dy is also similar to that for 1Gd but with DyCl3·6H2O instead of GdCl3· 6H2O. Yield: 0.118 g (17.4%) for 3Dy based on DyCl3·6H2O. Anal. Calcd: Mo, 34.56; Dy, 10.96; P, 4.18; K, 3.52; C, 3.25; H, 2.07. Found: Mo, 34.82; Dy, 11.13; P, 4.34; K, 3.72; C, 3.18, H, 1.896. Selected IR (KBr, cm−1): 3408 (s), 1715 (m), 1625 (s), 1532 (s), 1436 (s), 1381 (m), 1219 (w), 1142 (s), 1110 (s), 1063 (s), 906 (s), 756 (s), 728 (s), 653 (s), 607 (s). Catalytic Oxidation of Thioethers. Catalyst (3.6 μmol), substrate (1 mmol), solvent (3 mL), and 30% H2O2 (2 mmol) were loaded into a 50 mL round-bottom tube at a certain temperature with constant stirring. During the reaction, a trace of the reaction solution was taken out from the mixture by using a microsyringe and followed with gas chromatogaphy (GC) analyses. Products were identified from GC-MS spectra with toluene as an internal standard. X-ray Crystallography. Structural measurements for 1Gd, 2Tb, and 3Dy were performed at 296(2) K on a Bruker Apex-II CCD diffractometer using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). This data reduction, including a correction for routine Lorentz and polarization, was solved by an applied multiscan absorption correction SADABS program. 24 Direct methods (SHELXS97) were used to solve their structures, and the heavy atoms were located using the SHELXTL-97 program package.25 In the final refinement cycles, the Mo, P, and Ln atoms were refined anisotropically; the O and C atoms were refined isotropically. The lattice water molecules were determined by the TGA results. Four disordered K+ cations were located by a Fourier map. The hydrogen atoms of organic groups were fixed in calculated positions and then refined using a riding model. All H atoms on water molecules were

metal centers to construct multidimensional structures with fascinating properties and (2) phosphonoacetic acid, possessing phosphonate and carboxylate groups, has multiple oxygen atoms which can act in various acidity-dependent coordination modes, allowing the construction of different coordination structures. On the basis of this strategy, our group has successfully synthesized a series of isostructural Ln-containing phosphonoacetic acid based polyoxomolybdates K4H5[Ln3(H2O)14{(Mo8O24)(O3PCH2COO)3}2]·23H2O (Ln = Gd (1Gd), Tb (2Tb), Dy (3Dy)), constituted by two open {Mo8} groups as building blocks, six organic components, and three Ln ions as bridging fragments.

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EXPERIMENTAL SECTION

Materials and Physical Measurements. All chemicals were used as obtained without further purification. Elemental analyses for carbon and hydrogen were performed with a PerkinElmer 2400-II CHNS/O analyzer. XRPD measurements were carried out with a Bruker SMART Apex II CCD-based diffractometer at 293 K. Elemental analyses of Mo, P, K, and Ln were performed with a PerkinElmer Optima 2000 ICPOES spectrometer. Infrared spectra were recorded with a Bruker VERTEX 70 IR spectrometer as KBr pellets. TG analyses were carried out under a N2 atmosphere on a PerkinElmer TGA7 instrument at a range of 25−1000 °C (heating rate: 10 °C min−1). Synthesis of Compound 1Gd. A quantity of phosphonoacetic acid (0.14 g, 1.022 mmol) was dissolved in 25 mL of distilled water followed by the addition of solid Na2MoO4·2H2O (0.606 g, 2.505 mmol) and KCl (0.228 g, 3.058 mmol) with stirring. The pH of the reaction mixture was adjusted to 3.0 by 6 M HCl. To this reaction mixture was added 0.172 g (0.461 mmol) of GdCl3·6H2O, and the pH value was carefully maintained at 3.0 with 6 M NaOH. After it was stirred at 60 °C for 2 h, the mixed solution was gradually cooled to room temperature, filtered, and allowed to evaporate slowly at room temperature. The colorless block-shaped crystals of 1Gd were obtained after about one month. Yield: 0.123 g (18.0%) for 1Gd based on GdCl3.6H2O. Anal. Calcd: Mo, 34.69; Gd, 10.66; P, 4.20; K, 3.53; C, 3.25; H, 2.01. Found: Mo, 34.76; Gd, 10.83; P, 4.29; K, 3.73; B


Article

Inorganic Chemistry

Figure 1. (a) Ball and stick/polyhedral representation of the polyanion 1Gd. (b) Structure of [Gd(H2O)4{(Mo8O24)(O3PCH2COO)3}2]15−. (c) Structure of [(Mo8O24)(O3PCH2COO)3]9−. (d) Structure of {Mo8O24}. Color code: MoO6, aqua octahedra; P, yellow; O, red; C, black; Gd, blue. H atoms are omitted for clarity. directly included in the molecular formula. Crystallographic data for the three compounds reported in this paper have been deposited with the Cambridge Crystallographic Data Center with CCDC numbers 1569825, 1569826, and 1569827 for 1Gd, 2Tb, and 3Dy, respectively. Collected crystal data and structure refinement parameters for 1Gd, 2Tb, and 3Dy are given in Table 1. Analyses of BVS, IR, and XRPD. Bond Valence Sum Calculations. Complete single-crystal X-ray data for the three compounds 1Gd, 2Tb, and 3Dy are available, and selected interatomic distances are presented in Table S1. The bond valence sum (BVS)26 calculations (Tables S2−S4) show that the oxidation states of the Mo and Ln atoms in 1Gd, 2Tb, and 3Dy are +6 and +3, respectively. The BVS values for P atoms are in the range of 2.56−3.145 (average value 2.70), indicating the +3 oxidation state. Furthermore, five protons were added by the charge balance of compounds 1Gd, 2Tb, and 3Dy. IR Spectra. The IR spectra of compounds 1Gd, 2Tb, and 3Dy show very similar characteristic vibrations in the range of 450−4000 cm−1 (Figure S1), suggesting that the polyanions in 1Gd, 2Tb, and 3Dy are isostructural, which is consistent with the results of single-crystal X-ray structural analysis. The peaks at 992, 929, and 904 cm−1 for 1Gd, 993, 928, and 904 cm−1 for 2Tb, and 992, 928, and 903 cm−1 for 3Dy can be regarded as the characteristic Mo−Ot vibrations. The absorption bands at 754−650 cm−1 are attributed to the Mo−O−Mo stretching vibrations. In addition, the characteristic peaks at 1061, 1107, and 1142 cm−1 for 1Gd, 1063, 1108, and 1142 cm−1 for 2Tb, and 1063, 1110, and 1143 cm−1 for 3Dy can be respectively assigned to P−O vibrations. The apparent peaks in the range 1370−1530 cm−1 are characteristic of the carboxyl vibrations from phosphonoacetic acid. The signals appearing around 1630 cm−1 and the peaks around 3400 cm−1 are assigned to the bending and stretching modes of lattice and coordinated water molecules.27 X-ray Powder Diffraction. The experimental XRPD patterns for 1Gd, 2Tb, and 3Dy are consistent with the simulated patterns obtained from single-crystal X-ray diffraction, which indicates that the samples are pure (Figure S2). The differences in intensity between the experimental and simulated patterns might be due to the variation in crystal orientation for the powder samples.

organic ligands incorporated into Ln-containing polyoxomolybdates need to overcome the steric hindrance. It is noteworthy that the polyfunctional bridging ligand phosphonoacetic acid possesses three O−H bonds, three protons of which could be ionizable and coordinate with polyoxomolybdates and Ln ions to form novel structures. We utilized a one-pot synthetic strategy to prepare a series of Ln-containing organophosphonate-substituted polyoxomolybdates by reacting Na2MoO4·2H2O, phosphonoacetic acid, LnCl3·6H2O (Gd, Tb, Dy), and KCl with a molar ratio of 5.43:2.22:1:6.63 in 60 °C aqueous solution with stirring for 2 h. Notably, the pH value of the reaction system plays a key role in the formation of the crystals. Our experimental investigations indicate that the optimal pH range of 2.9−3.1 is helpful for the formation of 1Gd, 2Tb, and 3Dy, whereas their yields will be the highest when the pH value is 3.0 with other conditions kept unchanged. In addition, the amount of phosphonoacetic acid used should be controlled at around 1.022 mmol. Crystals of compounds 1Gd, 2Tb, and 3Dy cannot be obtained if the amount of phosphonoacetic acid used is increased to 1.50 mmol or decreased to 0.50 mmol. During our exploration, we first used LnCl3·6H2O (0.172 g, Ln = Gd3+, Tb3+, Dy3+) to react with other simple materials, giving rise to 1Gd, 2Tb, and 3Dy. Because of the influence of the nature of Ln3+ cations on the structures of the compounds, we replaced Gd3+, Tb3+, and Dy3+ cations by using other Ln3+ cations under the same conditions; however, the desired product were not obtained. In addition, we tried to use Cu2+, Co2+, Ni2+, Fe3+, Zn2+, and Cr3+ cations rather than Ln3+ under the same conditions, while unexpectedly, nothing except for some precipitates were obtained in the end. Now, we will continue to research the influence of Ln-containing organophosphonate-based polyoxomolybdates on the structural variation and relevant properties of the resulting compounds. Description of the Structures. X-ray analyses reveal that compounds 1Gd, 2Tb, and 3Dy are isomorphous and crystallize in the same monoclinic space group C2/c. Therefore, only 1Gd is chosen to describe in detail here as an example. The structure of 1Gd contains 4 K+ ions, 23 water molecules, 1 [Gd3(H2O)14{(Mo8O24)(O3PCH2COO)3}2]9− (1a), and 5 protons for the charge balance. As shown in Figure 1, the

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RESULTS AND DISCUSSION Syntheses. There are rare examples of the organophosphonate ligands and Ln synchronously grafted into the framework of polyoxomolybdates, which have been an enormous challenge to create.28 Ln-containing polyoxomolybdates have generally high surface negative charges, which make the introduction of organic ligands difficult. Simultaneously, C


Article

Inorganic Chemistry polyoxoanion 1a consists of a [Gd(H2O)5]3+ cation and a sandwich polyanion of [Gd(H2O)4{(Mo8O24)(O 3 PCH 2 COO) 3 } 2 ] 1 5 − (1b). The polyanion [Gd(H2O)4{(Mo8O24)(O3PCH2COO)3}2]15− can be viewed as two [(Mo8O24)(O3PCH2COO)3]9− fragments sandwiching a disordered [Gd(H2O)4]3+ ion. The fragment [(Mo8O24)(O3PCH2COO)3]9− (1c) features V-shaped {Mo8O24} building blocks (1d) combining with three organic groups, which could be considered as two corner-sharing {Mo4} units decorated by the H2O3PCH2COOH, one {O3PCH2COO} ligand residing in the center of V-shaped {Mo8} structure unit, and two other {O3PCH2COO} ligands sticking out like two arms. Each {Mo4} subunit is composed of four edge-sharing {MoO6} octahedra, and all of the Mo cations in the {Mo4} unit are basically located in the same plane. The two {Mo4} subunits are placed against each other, forming a “V” type structure, and their dihedral angle is 77.335° (Figure S3). In addition, there are two crystallographically unique types of P atoms in the symmetric unit. One P atom is connected to two corner-sharing {Mo2} units by two μ3-O atoms and a Gd atom by one μ3-O atom; simultaneously, a {COO} group from phosphonoacetic acid links to the remaining two corner-sharing {Mo2} units. The other P atom is bonded to two isolated {MoO6} octahedra by two μ2-O atoms and two edge-sharing {MoO6} octahedra by a μ3-O atom, with one {COO} group from phosphonoacetic acid combining with a Gd atom. As shown in Figure 2, the Gd3+ cation in 1a shows two types of coordination geometries (Gd1 and Gd2). The Gd1 ion

displays a nine-coordinated distorted monocapped quadrangular antiprism and is made up of four μ2-O atoms from the {COO} group (O1, O2, O9, and O10, Gd1−O1 = 2.492(8) Å, Gd1−O2 = 2.475(9) Å, Gd1−O9 = 2.473(8) Å, Gd1−O10 = 2.468(7) Å), and five O atoms from H2O (O1W−O4W and O8W, Gd1−O = 2.411(8)−2.431(9) Å) (Figure 2a). The Gd2 ion adopts an eight-coordinated highly distorted quadrangular antiprism and is achieved by four μ2-O atoms (O12 and O24 from the {Mo8O24} fragment, O26 from the {PO3} group, Gd2−O12 = 2.467(8) Å, Gd2−O24 = 2.540(7) Å, Gd2−O26 = 2.286(7) Å, Gd2−O226 = 2.305(6) Å) and four O atoms from water ligands (O5W and O11W−O13W, Gd2−O(H2O) = 2.182(19)−2.500(18) Å) (Figure 2b). Two Gd ions (Gd2 and Gd2′) in the center of the compound are equivalent with two Gd−O(12W) bonds joining the clusters (average length 2.192(18) Å), while the remaining three coordination sites are occupied by H2O (average length 2.463 Å). As we can observe, two neighboring sandwich [Gd(H 2 O) 4 {(Mo 8 O 2 4 )(O3PCH2COO)3}2]15− (1b) clusters are connected by [Gd(H2O)5]3+ anions, and the structure further extends to a 1D chain in the ac plane (Figure 3a). A more interesting feature of 1a is that a two-dimensional layer (Figure 3b) is constructed by sandwich [Gd(H2O)4{(Mo8O24)(O3PCH2COO)3}2]15− clusters and [Gd(H2O)5]3+ bridges. In this framework, five polyanions 1a are joined together by two [Gd(H2O)5]3+ cations and every 1a links to two [Gd(H2O)5]3+ cations via (C)−O−Gd1−O−(C) bridges. The polyanions of compound 1Gd are further extended into a 3D framework by K+ cations (Figure 3c). Magnetic Properties. Variable-temperature magnetic susceptibilities of 1Gd, 2Tb, and 3Dy have been investigated in the temperature range of 300−2K. The plots of χMT versus T are presented in Figure 4. For 1Gd, the χMT value is 22.56 cm3 K mol−1 at 300 K, slightly lower than the theoretical value of 23.63 cm3 K mol−1 of three isolated GdIII ions (S = 7/2, g = 2) in the 8S7/2 ground state.29 The χMT product slowly drops down and reaches a minimum of 21.88 cm3 K mol−1 at 2 K. The compound 1Gd shows Curie−Weiss behavior in the temperature range of 300−2 K, with a Curie constant of C = 22.56 cm3 K mol−1 and Weiss constant of Θ = −0.018 K (Figure S7a). For 2Tb, the value of χMT is 34.69 cm3 K mol−1 at 300 K, which is quite close to the theoretical value of 35.43 cm3 K mol−1 for three noninteracting TbIII ions (7F6, g = 3/ 2).30 At 2 K, the χMT value decreases to 24.02 cm3 K mol−1 and

Figure 2. Coordination environmenta of the Gd1 (a) and Gd2 (b) atoms.

Figure 3. (a) 1D framework of 1Gd stretching along the ac plane constructed by Gd1 ions. (b) 2D framework of 1Gd stretching along the ab plane constructed by Gd1 ions. (c) Packing arrangement of 1Gd along the b axis. Color code: MoO6, aqua octahedra; P, yellow; O, red; C, black; Gd, blue; K, orange. D


Article

Inorganic Chemistry

spectrum of 2Tb was recorded and included three characteristic bands at 341, 359, and 378 nm, which are assigned to the transitions of 5L6, 5L9, and 5G6 levels, respectively (Figure S6a).33 Similarly, the solid-state sample of 3Dy was excited under 387 nm ultraviolet light (Figure 5b), and the emission spectrum demonstrates three apparent emission peaks at 479, 574, and 662 nm, which can be separately ascribed to the transitions of 4F9/2 to 6HJ (J = 15/2, 13/2, 11/2).34 The yellowgreen emission at 574 nm (Figure S8) from the 4F9/2 → 6H13/2 transition was the most prominent. In addition, the excitation spectrum of 3Dy was obtained, in which four characteristic bands centered at 351, 365, 387, and 427 nm are attributed to the transitions from the 6H15/2 ground state to the different excited states 6P7/2, 6P5/2, 4I13/2, and 4G11/2, in sequence (Figure S6b).35 Decay Analyses. The lifetime measurements of 2Tb and 3Dy were studied under forceful excitation and emission bands. For 2Tb, the decay times under the most intense emission at 544 nm conform to a single-exponential decay behavior by the equation I = A exp(−t/τ), affording a lifetime of 616.31 μs (100%) (Figure 5c), which is evidently longer than most of the reported values for the corresponding TbIII complexes.36 For 3Dy, the lifetime curve (Figure 5d) under emission at 574 nm is fitted successfully by a second-order exponential function by the formula I = A1 exp(t/τ1) + A2 exp(t/τ2), giving τ1 and τ2 values of 5.05 μs (22.58%) and 9.37 μs (77.42%), respectively. Catalysis. In recent years, there has been growing interest in the catalytic oxidation of thioethers owing to their wide potential utilities.37 According to previous reports, POMs as catalysts display excellent catalytic performance in the catalytic oxidation of thioethers.38 Compound 2Tb was selected as a representative example to research the oxidation reactions of thioethers. Then the influences of various reaction parameters on the catalytic activities were investigated by using diphenyl sulfide as a model substrate. Products were identified by GC-

Figure 4. χMT versus T plots of 1Gd, 2Tb, and 3Dy in the 2−300 K range.

the 1/χM versus T plots obey the Curie−Weiss law (C = 34.06 cm3 K mol−1 and Θ = −1.97 K) (Figure S7b). For 3Dy, the experimental χMT value is 37.77 cm3 mol−1 K at room temperature, which is evidently lower than three noninteracting DyIII ions (42.50 cm3 mol−1 K, S = 5/2, g = 4/3).31 The χMT value undergoes a gradual reduction and reaches a minimum of 27.14 cm3 K mol−1 at 2 K. The 1/χM versus T plot obeys the Curie−Weiss law indicating antiferromagnetic coupling with C = 38.15 cm3 K mol−1 and Θ = −1.49 K (Figure S7c). Photoluminescence Properties. Luminescence. When the solid-state sample of 2Tb was excited under visible light at 378 nm, an emission spectrum was observed between 450 and 650 nm (Figure 5a), in which four obvious characteristic emission bands at 489, 544, 584, and 621 nm are respectively attributed to the transitions of 5D4 to 7FJ (J = 6, 5, 4, 3).32 The most intense peak at 544 nm presents a strong green luminescence (Figure S8). Furthermore, the excitation

Figure 5. (a) Solid-state emission spectrum of 2Tb under excitation at 378 nm. (b) Solid-state emission spectrum of 3Dy under excitation at 387 nm. (c) Lifetime decay curve of 2Tb monitored under excitation at 378 nm. (d) Lifetime decay curve of 3Dy monitored under excitation at 387 nm. E


Article

Inorganic Chemistry MS spectra. The selectivity to sulfoxide and sulfone products and the conversion of initial sulfides were taken for the qualitative analysis with GC. The influences of reaction time, reaction temperature, and amounts of catalyst and H2O2 are shown in Table 2. Blank reactions were performed for diphenyl

Table 3. Selective Oxidation of Various Thioethers to Sulfones Using 2Tb by H2O2

Table 2. Oxidation of Diphenyl Sulfide with H2O2a entry

catalyst (μmol)

H2O2 (mmol)

temp (°C)

time (min)

conversnb (%)

selc (%)

seld (%)

1 2 3 4 5 6

3.6 2 3.6 3.6 3.6 3.6

2.0 2.0 1.0 1.5 2.0 2.0

30 50 50 50 50 50

120 120 120 120 60 120

82.3 99 85 97 98 100

51.4 77 24 45 70 99

48.6 23 76 55 30 1

a

Reaction conditions: diphenyl sulfide (1 mmol), 30% H2O2 (1.5 mmol, 2 mmol), and catalyst (2 μmol, 3.6 μmol) in CH3CN (3 mL). b Conversion was determined by GC-FID using an internal standard technique based on diphenyl sulfide. cSelectivity to diphenyl sulfone. d Selectivity to diphenyl sulfoxide.

sulfide, giving a 37% conversion and 52% selectivity for sulfone in the absence of the catalyst 2Tb after 2 h; the conversion of diphenyl sulfide and selectivity to diphenyl sulfone of the reaction are low (Figure S9). The conversion was significantly increased to 99% (Table 2, entry 2) by the addition of 2 μmol of 2Tb. If addition of 2Tb was incrementally increased from 2 to 3.6 μmol, the conversion increased from 99 to 100% (Table 2, entries 2 and 6) and the selectivity to sulfone increased from 77% to 99%. With these conditions, conversions were observed to increase gradually and selectivity to sulfone increased dramatically with a rise in the dosage of oxidant and catalysts (Table 2, entries 2−4 and 6). Furthermore, the results of parallel experiments indicated that the conversion and selectivity increased as the reaction time was prolonged (Table 2, entries 5 and 6) and remarkably declined with a decrease in reaction temperature (Table 2, entries 1 and 6), which demonstrated that both reaction time and temperature were crucial factors for the transformation from diphenyl sulfide to diphenyl sulfone. A series of experimental results showed that 1 mmol of diphenyl sulfide was completely converted to oxidation products and excellent selectivity (99%) was obtained under the optimal conditions, 3 mL of MeCN with 2 mmol of 30% H2O2 and 3.6 μmol of 2Tb at 50 °C for 2 h (Table 2, entry 6), which were superior to the results for the catalyst [Bmim]4Mo8O26 (Bmim = 1-butyl-3-methylimidazolium) with 3 mmol of 30% H2O2.39 Encouraged by the catalysis of 2Tb for the oxidation of diphenyl sulfide, catalytic oxidations of various thioethers were carried out under the optimal conditions (Table 3). The results of experiments demonstrate that alkyl thioethers (Table 3, entries 1−3) and aryl alkyl thioethers (Table 3, entries 4−7) give excellent conversion (100%) and selectivity to sulfone (≥99%). Diphenyl sulfide shows a relatively slow reaction, which might be due to the low electron density on the sulfur atom of the substrate (Table 3, entry 8). It is noted that refractory sulfur-containing compounds benzothiophene (BT), dibenzothiophene (DBT), and 4,6-dimethyldibenzothiophene (4,6-DMDBT) are also oxidized to sulfones with excellent conversion and selectivity despite their expressed super conjugation and large steric hindrance for the oxidative process at mild temperatures (up to 60 °C; Table 3, entries 9−11). Compound 2Tb, as a

a

Reaction conditions for entries 1−8: substrate (1 mmol), 30% H2O2 (2 mmol), and catalyst (3.6 μmol) in CH3CN (3 mL) at 50 °C. b Reaction conditions for entries 9−11: sulfide (1 mmol), 30% H2O2 (3 mmol), and catalyst (3.6 μmol) in CH3CN (3 mL) at 60 °C. c2 MPaO2. dConversion value determined by GC-FID. eSelectivity to sulfones.

representative example of POMs, possess a strong redox nature, which is responsible for the splendid catalytic activity of the thioether−POM system. As shown in Figure S9, the catalytic activity of 2Tb is superior to that of its raw material, which causes us to speculate that the catalytic reaction may be concentrated on the isopolymolybdate units. On addition of 30% H2O2 to a suspension of 2Tb in CH3CN at 50 °C, the solid is completely dissolved and finally changes to a yellow solution, which indicate the formation of an active species. The IR spectra of the catalyst 2Tb (a), the sample after the reaction with excess H2O2 (b), and the recycled catalyst 2Tb (c) are provided in Figure 6. It is observed that the IR spectrum of 2Tb has Mo−O−Mo bridge vibrations in the range between 650 and 754 cm−1. However, as can be seen from the spectrum after the reaction with excess H2O2, the characteristic Mo−O−Mo peaks disappear after the reaction with excess H2O2. However, a strong peak at 861 cm−1 can be observed, which is assigned to the peroxo bond ν(O−O) stretching vibration (Figure 6b).40 Therefore, the species MoO(O2)2 is the possible active species in catalysis.40 On the basis of previous reports41 and the experimental results in the current work, a possible mechanism of the reaction has been provided. Peroxomolybdic active species can be formed by the nucleophilic attack of H2O2 on F


Article

Inorganic Chemistry

for the synthesis and design of a novel benign catalytic system for sulfoxidation reactions. In subsequent work, more intriguing structures and properties of organophosphonate-functionalized Ln-containing POMs will be introduced.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02672. Details on equipment used for complexes 1Gd, 2Tb, and 3Dy and Figures S1−S11 as described in the text (PDF) Accession Codes

Figure 6. Infrared spectra of compound 2Tb before the catalytic reaction (a), after the reaction with H2O2 (b), and for recycled compound 2Tb (c).

CCDC 1569825−1569827 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.

the Mo atom of the polyoxometalates. Electrons are withdrawn from the peroxyl moiety, thereby increasing the electrophilic character of the peroxidic oxygens, oxidizing the sulfur compounds into sulfones.42 To confirm that the 2Tb is a truly heterogeneous catalyst, a hot filtration experiment was performed under the optimal conditions. The catalyst was separated after a reaction time of 45 min, and the reaction was allowed to proceed with the filtrate. There was no further conversion observed in the filtrate, which suggested that this reaction process is a heterogeneous oxidation process (Figure S12). The catalyst was subsequently removed by filtration after completion of the oxidation reaction, and the leaching of Mo species was determined by ICP-AES analysis. These species are essentially undetectable in the filtrates (below the detection limit). These results show that 2Tb is truly heterogeneous and catalyst leaching is negligible under these conditions. The recycling of 2Tb in the oxidation of diphenyl sulfide was also investigated. After each run, the catalyst was separated by simple filtration, followed by washing with CH3CN, and then placed into a fresh reagent mixture. The recycled catalyst 2Tb can be used at least three times (Figure S10) with a general decline in selectivity that may result from the loss of catalyst during the catalyst separation process, indicating that 2Tb is a potentially heterogeneous and stable catalyst. Moreover, IR spectra confirmed that 2Tb could maintain its structural integrity after catalytic reactions (Figure S11). Finally, oxidation reactions of thioethers with 1Gd and 3Dy as catalysts were performed, which shows that they also have superior catalytic performance (Figure S9).

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for J.N.: [email protected]. *E-mail for J.W.: [email protected]. ORCID

Chao Zhang: 0000-0002-7400-5803 Jingyang Niu: 0000-0001-6526-7767 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge support from the NSFC (2117205) and the Natural Science Foundation of Henan Province (162300410015).

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REFERENCES

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CONCLUSIONS In summary, a novel class of phosphonoacetic acid based polyoxomolybdate derivatives, 1Gd, 2Tb, and 3Dy, was successfully prepared by a one-pot reaction. In the research system of Mo/O/RPO32−, the compounds 1Gd, 2Tb, and 3Dy represent the first examples of Ln-containing phosphonocarboxylate-based polyoxomolybdate derivatives, in which the synchronous incorporation of the organic ligand and Ln atoms into a polyoxomolybdate is an important breakthrough. Moreover, the magnetic properties of 1Gd, 2Tb, and 3Dy and the photoluminescence properties of 2Tb and 3Dy have been systematically probed. The results of catalysis by 1Gd, 2Tb, and 3Dy with thioethers indicate that it can efficiently oxidize various kinds of sulfides to the corresponding sulfones with H2O2 under mild conditions, which provides a direction G


Article

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