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Mar 10, 2016 - ABSTRACT: The reaction of [γ-SiW10O36]8− with Mn2+ and. Ln3+ in an ... new lanthanide-containing sandwich-type polyoxometalates...
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Probing the Self-Assembly Mechanism of Lanthanide-Containing Sandwich-Type Silicotungstates [{Ln(H2O)n}2{Mn4(B-αSiW9O34)2(H2O)2}]6− Using Time-Resolved Mass Spectrometry and X‑ray Crystallography Lin-Yuan Fan,‡ Zheng-Guo Lin,‡ Jie Cao,* and Chang-Wen Hu* Key Laboratory of Cluster Science, Ministry of Education of China; Beijing Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials, School of Chemistry, Beijing Institute of Technology, Beijing 100081, P.R. China S Supporting Information *

ABSTRACT: The reaction of [γ-SiW10O36]8− with Mn2+ and Ln3+ in an aqueous solution led to the isolation of a series of new lanthanide-containing sandwich-type polyoxometalates (POMs) [{Ln(H2O)n}2{Mn4(B-α-SiW9O34)2(H2O)2}]6− (1− 5a) (Ln = La (1), Nd (2), Gd (3), Dy (4), Er (5); n = 5, 6), which crystallize in the space groups C2/c with a = 33.0900(2)−32.9838(15) Å, b = 12.8044(10)−12.7526(6) Å, c = 22.8273(17)−22.6368(11) Å, V = 9669.2(12)−9519.7(8) Å3, Z = 2 (1, 2); P1̅ with a = 11.9502(4)−11.8447(6) Å, b = 13.2203(4)−13.1164(5) Å, c = 15.8291(5)−15.8524(7) Å, V = 2221.25(13)−2189.95(18) Å3, Z = 1 (3, 4, 5), respectively. Xray diffraction analysis reveals that they consist of twodimensional networks based on a sandwich-type polyanion [Mn4(B-α-SiW9O34)2(H2O)2]12− (6a, {Mn4(SiW9)2}) and lanthanide cations (Ln3+), which are further connected into three-dimensional frameworks by potassium cations for 3, 4, and 5. The unprecedented combination of time-resolved electrospray ionization mass spectrometry (ESI-MS) studies and X-ray crystallography allows us not only to directly observe the in-solution rearrangement of divant anion [γ-SiW10O36]8− into the sandwich-type POM 6a via an intermediate species [Mn3(B-β-SiW8O30(OH))(B-β-SiW9O33(OH))(H2O)]12− (7a, {Mn3(SiW8)(SiW9)}) from ESI-MS results, but also to gain the solid-state structures of intermediate and final product isolated from reaction solutions from X-ray crystallography results, from which the self-assembly mechanism of the lanthanide-containing sandwich-type POMs 1−5a was proposed.

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SiNi 2 W 1 0 O 3 6 (OH) 2 (H 2 O)} 2 ] 1 2 − , 6 [M 4 (H 2 O) 2 (B-αSiW9O34)2]12− [M = Mn2+, Cu2+, Zn2+],7 trimeric [(β2SiW 11 MnO 38 OH) 3 ] 15− , 8 and tetrameric products [{βTi2SiW10O39}4]24−,9 in which the γ-tungsten-oxo framework was not preserved. Obviously, the metastable parent {γ-SiW10} reorganizes into many different fragments (also called building blocks), namely, {γ-SiW9} and {B-β-SiW8} etc., in the presence of transition metal ions in aqueous solutions, and a selective combination of these fragments lead to the formation of a particular cluster type as a result of reaction and crystallization processes (Scheme 16−16). This stimulates us to investigate the mechanism driving such a simple system to generate different Keggin fragments and then self-assemble to form various kinds of sandwich-type POMs. Electrospray ionization mass spectrometry (ESI-MS) has been previously used to investigate the self-assembly mechanism of POMs.17−23 This technique is superior to other

INTRODUCTION Polyoxometalates (POMs) are metal oxide cluster anions containing early transition metals (Mo, W, V, Nb, and Ta) in their highest oxidation states which have been studied extensively due to their interesting structural, catalytic, and redox properties, and their application in areas as diverse as catalysis, analytical chemistry, and medicine.1,2 Sandwich-type POMs represent the largest subclass of transition metal substituted POMs (TMSPs), in which the incorporation of TMs into POM architectures brings about variable structures and tunable physical properties. Synthesis of the sandwich-type POMs is usually accomplished by the reaction of an appropriate lacunary POM precursor (e.g., [PW9O34]9−,3 [SiW10O36]8−,4 and [P2W15O56]12−3,5) with a transition metal ion (e.g., Mn2+, Cu2+, etc.) in a simple one-pot procedure. Usually, the structure of the lacunary POM precursor is preserved in the sandwichtype POM products. However, this is not the case for the divacant decatungstosilicate, [γ-SiW10O36]8−({γ-SiW10}). Kortz et al. demonstrated that the reaction of {γ-SiW10} with a variety of first-row transition metals has led to dimeric [{β© XXXX American Chemical Society

Received: December 2, 2015

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DOI: 10.1021/acs.inorgchem.5b02800 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

The unprecedented combination of time-resolved ESI-MS studies and X-ray crystallography allows us not only to directly observe the in-solution rearrangement of divant anion {γSiW10} into the sandwich-type building block 6a via an intermediate 7a from ESI-MS results, but also to gain the solidstate structures of intermediate and final product isolated from reaction solutions from X-ray crystallography results, from which the self-assembly mechanism of 1−5a was proposed.

Scheme 1. In Situ Transformed Building Blocks (Summarized from refs 6−16) of the Sandwich-Type POMs from [γ-SiW10O36]8− Precursor in the Presence of Transition Metal Ions in Aqueous Solutionsa

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a

EXPERIMENTAL SECTION

Synthesis. K8[γ-SiW10O36]·12H2O was prepared according to the literature.25 All other chemicals were purchased from commercial sources and used without further purification. HPLC grade solvents were generally used. K 5 Mn 0.5 [{La(H 2 O) 6 } 2 Mn 4 (B-α-SiW 9 O 34 ) 2 (H 2 O) 2 ]·26H 2 O (1). Manganese(II) acetate (0.0245 g, 0.1 mmol) was added to a solution of K8[γ-SiW10O36]·12H2O (0.149 g, 0.05 mmol) in water (10 mL). This yellow solution was stirred for 10 min at room temperature. Then lanthanum(III) nitrate (0.0217 g, 0.05 mmol) was added to the above solution and stirred for 40 min at 80 °C. The yellow suspension was filtered into a 20 mL beaker and left to crystallize at room temperature (25 °C). Yellow planar crystals of 1 were obtained after 3 weeks (yield 9% based on W). IR (KBr): ν = 3396.5 (s), 1624.3 (s), 937.5 (s), 905.1 (m), 869.1 (s), 763.9 (s), 719.7 (s), 535.8 (w) cm−1. Elemental analysis: calcd. W 56.14, Si 0.95, La 4.71, K 3.31, Mn 4.19; found W 56.57, Si 1.09, La 4.55, K 3.49, Mn 3.53. K5Mn0.5[{Nd(H2O)6}2Mn4(B-α-SiW9O34)2(H2O)2]·25H2O (2). The preparation method is repeated replacing lanthanum(III) nitrate with neodymium(III) nitride (0.0219 g, 0.05 mmol). Yellow planar crystals of 2 were obtained after 3 weeks (yield 10% based on W). IR (KBr): ν = 3403.3 (s), 1626.4 (m), 938.4 (s), 904.9(m), 869.8 (s), 761.5 (s), 716.2 (s), 536.1 (w) cm−1. Elemental analysis: calcd. W 56.21, Si 0.95, Nd 4.90, K 3.32, Mn 4.20; found W 56.37, Si 1.15, Nd 4.40, K 3.52, Mn 3.81. K4Mn[{Gd(H2O)5}2Mn4(B-α-SiW9O34)2(H2O)2]·27H2O (3). The preparation method is repeated replacing lanthanum(III) nitrate with gadolinium(III) nitride (0.0226 g, 0.05 mmol). Yellow planar crystals of 3 were obtained after10 days (yield 13% based on W). IR (KBr): ν = 3437.8 (m), 1630.0 (m), 1410.8 (m), 942.1 (s), 903.6 (m), 840.0 (s), 775.7 (s), 721.7 (s) cm−1. Elemental analysis: calcd. W 56.07, Si 0.95, Gd 5.33, K 2.65, Mn 4.65; found W 56.31, Si 1.14, Gd 5.39, K 3.02, Mn 4.71. K4Mn[{Dy(H2O)5}2Mn4(B-α-SiW9O34)2(H2O)2]·28H2O (4). The preparation method is repeated replacing lanthanum(III) nitrate with dysprosium(III) nitride (0.0228 g, 0.05 mmol). Yellow planar crystals of 4 were obtained after 1 week (yield 15% based on W). IR (KBr): ν = 3426.6 (m), 1623.4 (m), 944.6 (s), 903.6 (s), 873.2 (m), 772.2 (s), 719.2 (s), 537.3 (m) cm−1. Elemental analysis: calcd. W 55.80, Si 0.95, Dy 5.50, K 2.64, Mn 4.63; found W, 56.14; Si, 1.05; Dy, 5.37; K 2.72; Mn 4.61. K4Mn[{Er(H2O)5}2Mn4(B-α-SiW9O34)2(H2O)2]·28H2O (5). The preparation method is repeated replacing lanthanum(III) nitrate with erbium(III) nitride (0.0222 g, 0.05 mmol). Yellow planar crystals of 5 were obtained after 1 week (yield 18% based on W). IR (KBr): ν = 3403.4 (s), 1624.3 (m), 944.5 (s), 904.1 (s), 872.7 (s), 772.3 (s), 719.8 (s), 536.5 (m) cm−1. Elemental analysis: calcd. W 55.71, Si 0.95, Er 5.63, K 2.63, Mn 4.42; found W 56.32, Si 1.05, Er 5.37, K 2.56, Mn 4.81. K8MnH2[Mn4(B-α-SiW9O34)2(H2O)2]·20H2O (6). Manganese(II) acetate (0.0245 g, 0.1 mmol) was added to a solution of K8[γ-SiW10O36]· 12H2O (0.149 g, 0.05 mmol) in water (10 mL). This yellow solution was stirred for 10 min at room temperature and stirred for 40 min at 80 °C. The yellow suspension was filtered into a 20 mL beaker and left to crystallize at room temperature (25 °C). Yellow planar crystals of 6 were obtained after 1 week (yield 25% based on W). K 1 8 [{Mn(H 2 O) 3 } 2 {Mn(H 2 O) 2 }{Mn 3 (B-β-SiW 8 O 3 0 (OH))(B-βSiW9O33(OH))(H2O)}2]·16H2O (7). The preparation method is repeated at 50 °C instead of 80 °C. 1 M KCl aqueous solution (1 mL) was added to the final filter and left to crystallize at room temperature.

Color scheme: W = teal; O = red; Si = orange.

conventional solution methods such as NMR and UV spectroscopies in terms of sensitivity, speed, and accurate determination of chemical compositions of all species present in the reaction mixture. There are basically two MS-based strategies for the mechanistic study, which are called top-down and bottom-up approaches, respectively, analogous to the methods used in protein mass spectrometry. The top-down approach is essentially to use the fragmentation analysis in tandem mass spectrometry (MS/MS) of an ionized intact molecule to deduce the formation mechanism of POMs, typically combined with theoretical calculations. A bottom-up approach is to analyze the reaction mixture directly from which the mechanistic information was derived, which involves piecing together the building blocks identified in the reaction mixture into a final architecture. Cronin et al. first used both strategies to probe the self-assembly process of POMs including the low-nuclearity molybdates/tungstates ([M6O19]2−, M = Mo, W)17,18 and the polymeric species such as the Keggin clusters [XM12O40]n− (X = P or As, M = W or Mo),19 the silver-linked β-octamolybdate [Ag2Mo8O26]n2n−,20 the MnAnderson anion [MnMo6O18((OCH2)3CNH2)2]3−,21 and the two isomeric Fe-sandwiched silicotungstates [FeIII(H2O)2{γFeIIISiW9O34(H2O)}2]11− and [FeIII(H2O)2{γ-FeIII2SiW8O33(H2O)2}{γ-SiW10O35}]11−,22 etc. Mizuno et al. demonstrated the reversible transformation of Cun-bridged silicodecatungstate dimers (n = 1, 2, or 4) using cold-spray ionization mass spectrometry (CSI-MS) and crystallographic analysis.23 We have studied the reactions of a series of lacunary Keggin silicotungstates with vanadate by ESI-MS, and their distinctive electrospray features were summarized.24 Herein, we report the first direct observation of the self-assembly aggregation and rearrangement processes which govern the formation of a series of new 3d−4f sandwich-type extended POMs 1−5a using timeresolved ESI-MS analysis combined with X-ray crystallography. B

DOI: 10.1021/acs.inorgchem.5b02800 Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

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Yellow planar crystals of 7 were obtained after 1 week (yield 20% based on W). Solution Preparation. Reaction mixtures 1−5 (forming compounds 1−5) for ESI-MS, time monitored experiments: Manganese(II) acetate (0.0245 g, 0.1 mmol) was added to a solution of K8[γSiW10O36]·12H2O (0.149 g, 0.05 mmol) in water (10 mL), timing started at this point. This solution was stirred for 10 min at room temperature. Then lanthanide nitride (0.05 mmol) was added to the above solution, and the resultant solution was heated to 80 °C with stirring. An aliquot (20 μL) was removed from the reaction solution every 5 min during the reaction course and immediately diluted with 1 mL of water to produce 1 × 10−4 mol L−1 solution suitable for ESI-MS analyses. The time of removal of each 20 μL aliquot to make up MS dilutions was noted throughout this experiment, with the final dilution made up ca. 50 min after the first dilution. Reaction mixture 6 (forming compound 6) for ESI-MS, time monitored experiments: Manganese(II) acetate (0.0245 g, 0.1 mmol) was added to a solution of K8[γ-SiW10O36]·12H2O (0.149 g, 0.05 mmol) in water (10 mL), timing started at this point. This solution was stirred for 10 min at room temperature. Then it was heated to 80 °C with stirring. An aliquot (20 μL) was removed from the reaction solution every 5 min during the reaction course and immediately diluted with 1 mL water to produce 1 × 10−4 mol L−1 solution suitable for ESI-MS analyses. The time of removal of each 20 μL aliquot to make up MS dilutions was noted throughout this experiment, with the final dilution making up ca. 50 min after the first dilution. Note that for reaction mixture 7 (forming compound 7) for ESI-MS, time monitored experiments were made in exactly the same manner as reaction solution 6 except for the temperature being 50 °C instead of 80 °C. X-ray Crystallography. The crystal data were collected at 120(2) K with a Bruker APEX−II CCD diffractometer with graphite monochromatic Mo−Kα radiation (λ = 0.71073 Å). Crystals were mounted on a glass fiber and fixed with glue. All structures were solved by direct methods and refined by full-matrix least-squares against Fo2 by the SHELXTL program package (Bruker). The active hydrogen atoms were not incorporated in the refinement, and all atoms were refined anisotropically. Mass spectrometry. All MS data were collected using an Agilent 6520 Q-TOF LC/MS mass spectrometer in the negative ion mode. The dual-spray electrospray ionization source condition was as follows: Vcap: 3500 V; skimmer: 65 V; OCT RfV: 750 V; drying and nebulizer gas: N2; nebulizer: 30 psi; drying gas flow: 10 L/min; drying gas temperature: 300 °C; fragmentor: 80 V; scan range acquired 100− 3000 m/z. The sample solutions were made to approximately 10−4 M in water and analyzed by direct injection using an automatic sampler with a flow rate of 0.2 mL/min. All the peak assignments were based on the comparison of experimental and simulated isotopic patterns. Collision induced dissociation (CID) experiments were performed using N2 as the target gas. The desired multiply charged cluster was isolated and subjected to energy-variable CID in which the applied collision energy was raised incrementally. Plots of relative abundance of the parent ion versus applied collision energy were generated with Microcal Origin 8.5 (Microcal Software, Inc., Northampton, MA, USA) to determine the relative dissociation energy, which reflects the gas-phase relative stability of POM clusters. The dissociation curves were measured in triplicate for each cluster. All data were collected and processed using MassHunter (Agilent Technologies (China) Co., Ltd.) workstation software. Other Characterization Methods. IR spectra were recorded on a Nicolet 170SX-FT/IR spectrophotometer with pressed KBr pellets in the range of 400−4000 cm−1. Elemental analysis was performed with a JY-ULTIMA2 ICP-MS. UV/vis spectra were obtained with a TU-1901 spectrophotometer in the range of 200−800 nm. TG analyses were performed under N2 atmosphere from room temperature to 800 °C with a heating rate of 10 °C/min on a DTG-60AH simultaneous DTATG apparatus.

Article

RESULTS AND DISCUSSION The addition of Mn(OAc)2 and Ln(NO3)2 sequentially to an aqueous solution of K8[γ-SiW10O36]·12H2O with molar ratios of {γ-SiW10}/Mn2+/Ln3+ = 1:2:1 upon heating at 80 °C for 40 min led to the isolation of a series of lanthanide-containing sandwich-type extended POMs 1 − 5 with general formulas of KxMny[{Ln(H2O)n}2Mn4(B-α-SiW9O34)2(H2O)2]·zH2O, Ln = La (1), Nd (2) (x = 5, y = 0.5; n = 5; z = 25−26); Gd (3), Dy (4), Er (5) (x = 4, y = 1; n = 6; z = 27−28). Single-crystal X-ray diffraction analysis revealed that compounds 1−5 are inorganic aggregates based on a Weakley-type unit 6a with Ln3+, Mn2+, and K+ cations in the solid states. The 2D network of the inorganic aggregates 1−5a consists of Weakley-type building block 6a13a and Ln3+ linkers with two different linking modes. Compounds 1, 2 and 3, 4, 5 are isomers crystallized in C2/c and in P1̅ space groups, respectively (Figure 1 and Table 1). In

Figure 1. Polyhedral and ball-and-stick representations of compounds 1 and 3: (a) the coordination environment of [Mn4(B-α-SiW9O34)2(H2O)2]12− in 1; (b) the coordination number of La3+ in 1; (c) the coordination environment of La3+ in 1; (d) the 2D network packing arrangement displaying [{La(H2O)6}2Mn4(B-α-SiW9O34)2(H2O)2]6− in 1; (e) the coordination environment of [Mn4(B-α-SiW9O34)2(H2O)2]12− in 3; (f) the coordination number of Gd3+ in 3; (g) the coordination environment of Gd3+ in 3; (h) the 2D network packing arrangement displaying [{Gd(H2O)5}2Mn4(B-α-SiW9O34)2(H2O)2]6− in 3. Color scheme: Ln = green; WO6 octahedron = teal (free), = light blue (bound with Ln3+); Mn = yellow; O = red; Si = orange.

both two types of 2D extended aggregates, the essential sandwich-type building block 6a was constructed by two trivacant B-type Keggin moieties [B-α-SiW9O34]10− sandwiching a central symmetric rhomb-like [Mn4O16(H2O)2] segment through the W−O−Mn and Si−O−Mn connecting modes. All the W and Mn centers exhibit an octahedral coordination environment. The bond lengths of W−O are in the range 1.67(3)−2.44(2) Å, whereas the bond lengths of Mn−O are between 2.02(3) Å and 2.26(3) Å. The detailed crystal structures of compounds 1−5 are described as follows. The structures of compounds 1 and 2 are exemplified as compound 1 (K5Mn0.5[{La(H2O) 6}2Mn4(B-α-SiW9O34) 2(H2O)2]), which is constructed by Weakley-type unit 6a and La3+, Mn2+, K+ cations in solid state (Figure 1a−d)). Six La3+ ions with the same nona-coordination environment are coordinated directly to the surface oxygen atoms (O25, O27, and O28) of three meta-position {WO6} octahedrons in each edge-shared octahedral triad of the two [B-α-SiW9O34]10‑ units. The bond lengths of the three La−O bonds connected to POMs are 2.508(16) Å, 2.547(14) Å, and 2.549(14) Å, respectively. The residual coordination sites of nona-coordinated La3+ ions are fulfilled by six coordinated water molecules. The distances of La−Ow vary from 2.496(16) Å to 2.639(17) Å. C

DOI: 10.1021/acs.inorgchem.5b02800 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Crystallographic Data for Compounds 1−6 compounds

1

2

3

formula Mr crystal system space group temperature (K) a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z Dcalc (g cm−3) F(000) R1 [I > 2σ(I)] wR2 [I > 2σ(I)] R1 (all data) wR2 (all data) GOF CSD number compounds

H160K10La4Mn9Si4W36O216 11788.77 monoclinic C2/c 296 (2) 33.0900 (2) 12.8044 (10) 22.8273 (17) 90 91.346 (1) 90 9669.2(12) 2 4.049 9866 0.0793 0.2034 0.1099 0.2267 0.985 1434212 4

H156K10Nd4Mn9Si4W36O214 11774.08 monoclinic C2/c 296 (2) 32.9838(15) 12.7526(6) 22.6368(11) 90 91.173 (1) 90 9519.7(8) 2 4.108 10082 0.0624 0.1763 0.0745 0.1964 1.054 1434215 5

H78K4Gd2Mn5Si2W18O107 5901.43 triclinic P1̅ 293 (2) 11.9502(4) 13.2203(4) 15.8291(5) 90.364(2) 107.387(3) 110.303(3) 2221.25(13) 1 4.115 2386 0.0716 0.1865 0.0894 0.2063 1.038 1434216 6

formula Mr crystal system space group temperature (K) a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z Dcalc.(g cm−3) F(000) R1 [I > 2σ(I)] wR2 [I > 2σ(I)] R1 (all data) wR2 (all data) GOF CSD number

H80K4Dy2Mn5Si2W18O108 5929.94 triclinic P1̅ 293 (2) K 11.8787(5) 13.1485(6) 15.827(1) 90.656(4) 107.415(4) 110.065(4) 2197.5(2) 1 4.481 2375 0.0713 0.1987 0.0889 0.2174 1.065 1434217

H80K4Er2Mn5Si2W18O108 5939.46 triclinic P1̅ 293 (2) K 11.8447(6) 13.1164(5) 15.8524(7) 90.695(3) 107.355(4) 110.065(4) 2189.95(18) 1 4.503 2395 0.0677 0.1866 0.0862 0.2101 1.045 1434219

Furthermore, the La3+ ions can be viewed as μ3-bridges that link three 6a into a 2D network structure. The crystal structures of clusters 3−5 are different from those of 1 and 2 due to different coordination environment of Ln3+ ions. By taking compound 3 as an example, the solid state structure of 3 (K4Mn[{Gd(H2O)5}2Mn4(B-α-SiW9O34)2(H2O)2]·27H2O) is constructed by the sandwich-type POM 6a and Gd3+, K+, Mn2+ cations (Figure 1e−h). Six Gd3+ ions with the same octa-coordination mode are coordinated to the surface oxygen atoms (O25, O27, O34) of three ortho-position {WO6} octahedrons in each edge-shared octahedral triad of the two [B-α-SiW9O34]10‑ units. The bond lengths of the three Gd−O bonds connected to POMs are 2.354(18) Å, 2.38(2) Å, and 2.400(18) Å, respectively. The residual coordination sites of the octa-coordinated Gd3+ ions are fulfilled by five coordinated water molecules. The distances of Gd−Ow vary

H46K8Mn5Si2W18O90 5439.25 triclinic P1̅ 293 (2) K 12.3688(12) 12.4094(11) 16.7191(17) 89.587(8) 75.471(9) 60.429(10) 2139.7(4) 1 4.221 2356 0.0897 0.2249 0.1261 0.2634 1.021 1434220

from 2.37(3) Å to 2.47(3) Å. All Gd cations can be viewed as μ3-bridges linked with three sandwich-type units 6a to form a 2D planar structure, and the 2D network structure can be further extended to 3D structures by using K+ and Mn2+ cations in solid states. It is interesting to note that in the solid state structures of 1− 5, the lanthanide ions with a different coordination environment due to the different ionic radius (La3+ (103.2 pm) > Nd3+ (98.3 pm) > Gd3+ (93.8 pm) > Dy3+ (91.2 pm) > Er3+ (89 pm)) coordinated directly to the terminal oxygen atoms of the sandwich-type polyanion in different binding modes. For Ln3+ with larger radius, i.e., La3+ and Nd3+, having nine coordination numbers, three of which were bound directly to the meta-oxo ligands connected with the belt-tungsten atoms (La: O25, O27, O28; Nd: O15, O20, O29). In contrast, for Ln3+ with smaller radius, i.e., Gd3+, Dy3+ and Er3+, having eight coordination D

DOI: 10.1021/acs.inorgchem.5b02800 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry numbers, three of which were bound directly to the orthooxygen atoms connected with the belt-tungsten atoms (Gd: O25, O27, O34; Dy: O4, O11, O17; Er: O2, O4, O16). The bond lengths of the three Ln−O bonds connected to the POMs are 2.354(18) Å, 2.38(2) Å and 2.400(18) Å for La; 2.482(14) Å, 2.560(13) Å and 2.508(13) Å for Nd; 2.354(18) Å, 2.38(2) Å and 2.400(18) Å for Gd; 2.358(18) Å, 2.372(16) Å and 2.330(18) Å for Dy; 2.326(18) Å, 2.349(16) Å, and 2.309(15) Å for Er, respectively. The residual coordination sites of lanthanide cations are fulfilled by coordinated water molecules. The bond lengths of Ln−Ow vary from 2.32(2) Å to 2.639(17) Å. Compound 6 (K 8 H 2 Mn[Mn 4 (B-α-SiW 9 O 34 ) 2 (H 2 O) 2 ]· 20H2O), crystallized in the triclinic space group P1̅, was isolated from the reaction of K8[γ-SiW10O36]·12H2O with Mn(OAc)2 with a ratio of 1:2 ({γ-SiW10}/Mn2+) under the same synthetic conditions as compounds 1−5 except for the absence of Ln(NO3)2. This compound is consisted of the same sandwich-type polyanion 6a as compounds 1−5 and Mn2+, K+, H+ cations (Figure S1a,b in the Supporting Information). The purpose to synthesize compound 6 is to elucidate the role of lanthanide cations in the assembly of the sandwich-type POM 6a from the {γ-SiW10} precursor. ESI-MS was employed to analyze the solution behaviors of POMs 1−6 in aqueous solutions. It was found that the ESI-MS spectra of compounds 1−5 (Figure S2b−f in the Supporting Information) are almost identical with that of compound 6 (Figure S2a in the Supporting Information), all displaying two main ion series, despite a slight shift toward a higher mass range due to the presence of Ln3+ for compounds 1−5 compared to those for compound 6, i.e., the ion series [H8‑xKxMn4(SiW9O34)2]4− (m/z 1165.6−1193.8; x = 0−3) and [H7−xKxMn4(SiW9O34)2]5− (m/z 932.3−943.4; x = 0−1) for compound 6 versus [H8‑x‑3yKxLnyMn4(SiW9O34)2]4− (m/z 1165.6−1261.8; x = 0−3, y = 0−2) and [H7‑x‑3yKxLnyMn4(SiW9O34)2]5− (m/z 932.3−989.7; x = 0−3, y = 0−1) for compounds 1−5. This result indicates that the inorganic aggregates 1−5a are not stable, dissociating into the more stable sandwich-type anion 6a whose structural integrity was maintained in solution. The large and highly negatively charged sandwich-type 6a was stabilized via binding electrostatically with varying numbers of counterions (Ln3+, Mn2+, K+, and H+) as a result of charge reduction reactions occurring during ESI. To investigate how the divacant precursor {γ-SiW10} is rearranged into 6a in the presence of Mn2+, an essential building block for POMs 1−6, and also how this sandwich-type anion is further associated with Ln3+ to build multidimensional structures, the reaction solutions 1−6 (forming POMs 1−6, respectively) are monitored by ESI-MS as a function of reaction time. The reactions were conducted as normal, with 20 μL aliquots removed at specific time intervals during the reaction and crystallization. The aliquots were diluted with 1 mL of HPLC grade water and then analyzed by ESI-MS. By taking compound 3 as an example, the time-resolved ESIMS monitoring data (Figure 2) were interpreted as follows. First, in the mass spectrum of the starting material (K8[γSiW10O36]·12H2O) dissolved in water (pH 8.0), the two major peak envelopes centering at m/z 804.1 and 816.1, correspond to [HSiW10O34]3− and [H5SiW10O36]3−, respectively, which means the {γ-SiW10} unit is stable in water (Figure 2a). After adding 2 equiv of Mn(OAc)2 to the {γ-SiW10} solution with stirring at room temperature for 10 min, remarkable changes

Figure 2. Real-time ESI-MS monitoring on the synthetic process of compound 3: (a) K8SiW10O36 dissolved in water; (b) adding 2 equiv of Mn(OAc)2 to the {γ-SiW10} solution with stirring for 10 min at room temperature (RT); (c−e) adding 1 equiv of Gd(NO3)3 to the solution (b) upon heating at 80 °C for different time intervals; (f) ESIMS spectrum of crystal 3 redissolved in water.

occurred in the mixed solution indicated by a new series of multiply charged clusters in the MS spectrum (Figure 2b), i.e., the main peaks assigned to {MnSiW10} ranging in m/z from 681.0 to 935.7, with m/z 681.0 for [K4MnSiW10O36(OH)2· 2H2O]4−, 911.1 for [Na3K2MnSiW10O36(OH)2·3H2O]3−, 923.0 f or [ N a 2 K 4 M nS i W 1 0 O 3 6 (OH) 3 ·H 2 O ] 3 − , 935.7 for [Na2K5MnSiW10O36(OH)4]3−, serving as the main signals, and the two minor peaks attributed to {Mn2SiW8} and {MnSiW9}, respectively, with m/z 827.4 for [K7Mn2SiW8O31(OH)4·2H2O]3−, and 850.4 for [K6MnSiW9O34(OH)·H2O]3−, respectively, accompanied by a pH drop from 8.0 to 6.4. This result indicates that the parent anion {γ-SiW10} can rapidly react with Mn2+ (in a matter of seconds), yielding the substituted species {MnSiW8}, {MnSiW9}, and {MnSiW10}. Then, after adding Gd(NO3)3 to the mixed reaction solution of {γ-SiW10} + Mn2+ with heating to 80 °C for 10 min, a new species with a formula of [H10−xKxMn3(SiW8O31)(SiW9O34)]4− (m/z 1103.8−1161.3) emerged accompanied by a further pH drop to 5.6 (Figure 2c). Meanwhile the abundant {MnSiW10} species was completely replaced by {LnSiW10} due to higher coordination ability of Ln3+ than that of Mn2+, resulting in an obvious shift toward a higher mass range (i.e., {MnSiW10} (m/z 681.0−935.7) versus {GdSiW10} (m/z 709.8−970.0) (Figure 2c). Moreover, upon continuous heating the solution for 20 min, another new peak envelope assigned to [H8−xKxMn4(SiW9O34)2]4− (m/z 1165.6−1193.8) appeared with a concomitant increase of [H10−xKxMn3(SiW8O31)(SiW9O34)]4− (Figure 2d). Note that both [H10‑xKxMn3(SiW8O31)(SiW9O34)]4− and [H8‑xKxMn4(SiW9O34)2]4− can be viewed as the cation-bound anionic clusters of 7a and 6a, respectively. Finally, after continuous heating the reaction solution for 40 min, the ion of [H 8−x K x Mn 4 (SiW 9 O 34 ) 2 ] 4− is completely in place of [H10−xKxMn3(SiW8O31)(SiW9O34)]4− and reached its maximum at the end of the reaction (Figure 2e). A plot depicting the concentration change of each observed species as a function of reaction time, e.g., {γ-SiW10}([H5SiW10O36]3− at m/z 816.1); {MnSiW10}([Na3K2MnSiW10O36(OH)2·3H2O]3− at m/z 911.1); {Mn2SiW8}([K7Mn2SiW8O31(OH)4·2H2O]3− at m/z 827.4); {MnSiW9}([K6MnSiW9O34(OH)·H2O]3− at m/z 850.4); {Mn 3 (SiW 8 )(SiW 9 )}([H 6 K 4 Mn 3 (SiW 8 O 3 1 )(SiW 9 O 34 )] 4− at m/z 1137.1) and {Mn 4 (SiW 9 O 34 ) 2 }([H7KMn4(SiW9O34)2]4− at m/z 1179.5), by integrating its E

DOI: 10.1021/acs.inorgchem.5b02800 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

structured species ({Mn3(SiW8)(SiW9)} and {Mn4(SiW9)2}) are two competing processes for the reaction of {γ-SiW10} with Mn2+. After the reaction was completed, yellow crystal of 3 was formed by slow evaporation at room temperature for about 10 days. The ESI-MS of the mother liquor of crystal 3 was shown to have identical speciation with the corresponding reaction solution upon heating at 80 °C for 50 min (not shown). Figure 2f is the ESI-MS spectrum of crystal 3 via dissolution of the prepared crystalline sample in water for comparison (Figure S2d in the Supporting Information). As a control, a pure solution of K8[γ-SiW10O36]·12H2O (0.005 M) was analyzed alongside the manganese reactions to corroborate which fragments were due to the starting material and which were due to the reaction with manganese acetate (see Figure S3 in the Supporting Information). It was surprisingly found that {γSiW10} is stable under the reaction condition (80 °C, 50 min). On the basis of the real-time monitored MS spectra of compound 3, we think the most important procedure dictating the whole synthesis is the formation of an essential sandwichtype building block 6a. Therefore, we carry out the same timeresolved ESI-MS analyses on the binary reaction system consisting of {γ-SiW10} and Mn2+ under the same conditions as the trinary system ({γ-SiW10} + Mn2+ + Ln3+) to investigate the formation mechanism of 6a in detail (Figure S4 in the Supporting Information). It was found from Figure 2 and Figure S4 in the Supporting Information that these two systems follow exactly the same reaction pattern, which is essentially about the in-solution formation of 6a from {γ-SiW10} precursor. Single crystal 6 was obtained in the same manner as compounds 1−5, whose structure was subsequently analyzed by X-ray crystallography and ESI-MS (Figures S1a,b and S2a in the Supporting Information). Throughout the whole experiment, a few points need to be addressed: (1) the conditional stability of {γ-SiW10}: being relatively stable as an isolated form, but becoming extremely unstable in the presence of Mn2+. The directed speciation of {γSiW10} into {SiW9} and {SiW8} fragments undoubtedly occurs in response to the presence of manganese ions; (2) detection of {MnSiW9} and {Mn2SiW8} from reaction solutions supports the hypothesis proposed by Kortz14a that the (B-β-SiW9O34) and (B-β-SiW8O31) fragments are most likely the intermediate transformation products of (γ-SiW10O36) → (B-α-SiW9O34). Note that both fragments are very unstable as free, lacunary ions. The extremely low abundance of {MnSiW9} and {Mn2SiW8} signifies the intrinsic instability of these in situ generated species and the characteristics of intermediate fragments, which will be consumed in the subsequent reaction of forming another notable intermediate species {Mn3(SiW8)(SiW9)} next to the final product {Mn4(SiW9)2}. These results state that metal insertion must be accompanied by isomerization and dimerization as well as loss of tungsten and also manifest the role of transition metal ions (e.g., MnII) in the assembly of large polymeric structures. However, we do not observe any obvious preference of {Mn2SiW8} over {MnSiW9}, although there is a theoretical basis to support the bias.22 The work presented here reinforces earlier results by our group highlighting the flexible nature of the dilacunary precursor {γSiW10} when it encounters 3d transition metal ions such as vanadiumV in aqueous solutions.24 We reasoned that the anion {Mn3(SiW8)(SiW9)} was an intermediate of {Mn4(SiW9)2} according to the following reasons: (1) {Mn3(SiW8)(SiW9)} is only present in the middle stage of reaction of forming {Mn4(SiW9)2}; (2) the intensity

respective peak area in the mass spectrum was shown in Figure 3, from which a semiquantitative relationship among them was

Figure 3. Graphs showing peak intensities of reagent {γ-SiW10}([H5SiW10O36]3− at m/z 816.1), substituted species {MnSiW10}([Na3K2MnSiW10O36(OH)2·3H2O]3− at m/z 911.1), {GdSiW10}([Na 3 K 3 GdSiW 1 0 O 36 (OH) 3 ] 3 − at m/z 937.7), {MnSiW 9 } ([K 6 MnSiW 9 O 34 (OH)·H 2 O] 3 − at m/z 850.4), {MnSiW 8 } ([K7Mn2SiW8O31(OH)4·2H2O]3− at m/z 827.4), intermediate species {Mn3(SiW8 )(SiW9)}([H6 K4 Mn3(SiW8 O31)(SiW9O34)] 4− at m/z 1137.1) and product {Mn4(SiW9)2}([H7KMn4(SiW9O34)2]4− at m/z 1179.5) plotted against the time of ESI-MS data acquisition during the reaction of {γ-SiW10} + Mn(OAc)2 + Gd(NO3)3 at 80 °C. The peak identities and m/z values of the peaks studied are shown along with representations of the determined structures of these species. Color scheme: Ln = green; W = teal; Mn = yellow; O = red; Si = orange.

established. It can be seen from Figure 3 that the intensity of the reagent {γ-SiW10} decreases as that of {MnSiW10}([Na3K2MnSiW10O36(OH)2·3H2O]3− at m/z 911.1) increase. Interestingly, the three intermediate species {Mn2SiW8}([K7Mn2SiW8O31(OH)4·2H2O]3− at m/z 827.4), {MnSiW9}([K6MnSiW9O34(OH)·H2O]3− at m/z 850.4), {Mn3(SiW8)(SiW9)} ([H6K4Mn3(SiW8O31)(SiW9O34)]4− at m/z 1137.1) as well as the final product {Mn 4 (SiW 9 ) 2 }([H 7 KMn 4 (SiW9O34)2]4− at m/z 1179.5) remain at low concentrations throughout the reaction course. However, by closer inspection of these species (see the inset of Figure 3), it can be still found that (1) the growth in intensity of intermediate fragments {Mn2SiW8} and {MnSiW9} remain at the same pace with that of {MnSiW10}, reaching their maxima at t ≈ 10 min; (2) the ion intensities of {MnSiW9} and {Mn2SiW8} start to decrease as the substantial increase of {Mn3(SiW8) (SiW9)} and moreover the decrease of {MnSiW9} and {Mn2SiW8} and the sharp increase of {Mn3(SiW8) (SiW9)} occur simultaneously; (3) the ion intensities of {Mn3(SiW8)(SiW9)} and {Mn4(SiW8)(SiW9)} increase concomitantly at the initial reaction stage of forming dimeric silicotungstates (10 min ≤ t ≤ 26 min) with heating (80 °C) and reach their maxima progressively. These observations suggest that (1) certain relationships may exist between the pair of {MnSiW9}, {Mn2SiW8}, and {Mn3(SiW8)(SiW 9 )} and the pair of {Mn 3 (SiW 8 )(SiW 9 )} and {Mn4(SiW9)2}; (2) the reactions of forming the substituted fragments ({MnSiW9} and {Mn2SiW8}) and the sandwichF

DOI: 10.1021/acs.inorgchem.5b02800 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

availability of Mn and high temperature definitely facilitate the formation of sandwich-type POMs. What is more, we also run variable-temperature experiments in which the reaction solution of {γ-SiW10} and Mn(OAc)2 was first kept at room temperature for 40 min, then raised to 50 and 80 °C for 40 min, respectively (Figure 5). It is clear to see the

decrease of {Mn3(SiW8)(SiW9)} and the intensity growth of {Mn4(SiW9)2} are closely correlated (Figure 3). Therefore, we think it might be feasible to “frozen” the reaction at an intermediate level by lowering the temperature below 80 °C in order to maximize the yield of the intermediate from which solids, better single crystals, can be obtained. In order to test our hypothesis, we repeat the same reaction at lower temperatures (50 °C and RT, respectively) with an expectation that the low temperature may favor the formation of the intermediate {Mn3(SiW8)(SiW9)} and hence facilitate to capture such anion in the crystalline form. Fortunately, we succeeded in “frozening” the reaction at 50 °C from which only 7a-based crystals (compound 7: K18[{Mn(H2O)3}2{Mn(H 2 O) 2 }{Mn 3 (B-β-SiW 8 O 30 (OH))(B-β-SiW 9 O 33 (OH))(H2O)}2]·16H2O) was isolated (Figure 4). The molecular

Figure 5. Real-time ESI-MS monitoring on the reaction mixture 6 upon heating at variable temperatures from RT → 50 °C → 80 °C continuously: (a) K8SiW10O36 dissolved in water; (b) adding 2 equiv of Mn(OAc)2 to the {γ-SiW10} solution with stirring for 10 min at RT; (c−e) heating the solution (b) at 50 °C for different time intervals; (f−h) heating the solution (e) at 80 °C for different time intervals. Figure 4. Real-time ESI-MS monitoring on the synthetic process of compound 7: (a) K8SiW10O36 dissolved in water; (b) adding 2 equiv of Mn(OAc)2 to the {γ-SiW10} solution with stirring for 10 min at RT; (c−e) heating the solution (b) at 50 °C for different time intervals; (f) ESI-MS spectrum of crystal 7 redissolved in water.

stepwise changes of the reaction: at room temperature, only substituted species, i.e., {MnSiW 10 }, {MnSiW 9 } and {Mn2SiW8}, were formed (Figure 5b); upon heating to 50 °C for 40 min, the intermediate species {Mn3(SiW8)(SiW9)} was optimally generated as a final “product” at this stage (Figure 5c−e) which will be gradually and completely transformed into {Mn4(SiW9)2} with heating to 80 °C for 40 min (Figure 5f −h). This temperature game suggests that the stable sandwich-type POM 6a can only be formed under specific conditions which normally requires heating and an excess amount of sandwiched transition metals. Therefore, the overall reaction can be written as follows: {γ-SiW10} + Mn2+ → {MnSiW 9 } + {Mn 2 SiW 8 } → {Mn 3 (SiW 8 )(SiW 9 )} → {Mn4(SiW9)2}. The processes of aggregation of the intermediate fragments ({MnSiW9} and {Mn2SiW8}) into an intermediate {Mn3(SiW8)(SiW9)} which is finally converted into {Mn4(SiW9)2} are evidently seen in the time-resolved ESIMS experiments and the variable-temperature experiment. Finally, the relationship between {Mn3(SiW8)(SiW9)} and {Mn4(SiW9)2} was further tested via analyzing an aqueous solution of crystal 7 by ESI-MS with heating at 80 °C as a function of standing time (Figure 6). It was found that {Mn3(SiW8)(SiW9)} was gradually and spontaneously transformed into {Mn4(SiW9)2} in solution in the absence of any external source of Mn2+. Note that {Mn3(SiW8)(SiW9)} and {Mn4(SiW9)2} are identified by two peak envelopes (ranging from −4 to −5), respectively, i.e., [H8−xKxMn4(SiW9O34)2]4− and [H 7−x K x Mn 4 (SiW 9 O 34 ) 2 ] 5− for {Mn 4 (SiW 9 ) 2 } and [H10−xKxMn3(SiW8O31)(SiW9O34)]4− and [H9−xKx(SiW8O31)(SiW9O34)]5− for {Mn3(SiW8)(SiW9)}. The temperaturedependent diagram depicting these two mechanistically related

structure of 7 was determined to be composed of a rare centric tetrameric [{Mn(H2O)3}2{Mn(H2O)2}{Mn3(B-βSiW8O30(OH))(B-β-SiW9O33(OH))(H2O)}2]18‑ (7a′) subunit, 18 K+ counter cations, and 16 lattice water molecules (Figure S1c−e in the Supporting Information). The structure of 7a′ is constructed from two structurally equivalent asymmetric sandwich-type moieties 7a held together by two structurally equivalent [Mn(H2O)3]2+ cations and a unique [Mn(H2O)2]2+ cation. The asymmetric sandwich-type moiety 7a in 7a′ is built up of three manganese ions encapsulated by two unequivalent lacunary Keggin fragments [B-β-SiW9O34]10− and [B-βSiW8O31]10−. It is of our special interest to see if the dimeric nature of 7a′ is maintained in solution. The ESI-MS spectrum of 7 obtained via dissolution of the corresponding crystalline sample in water presents two main series of ions, [H9‑xKxMn3(SiW8O31)(SiW9O34)]5− (m/z 878.9−901.9; x = 0−3) [H10‑xKxMn3(SiW8O31)(SiW9O34)]4− (m/z 1103.8− 1161.3; x = 1−7), suggesting that the dimeric cluster 7a′ was dissociated into monomeric moiety 7a in aqueous solutions whereas the sandwich structure of the latter was maintained (Figure S2(g) in the Supporting Information). It is also worthwhile to mention that at room temperature, the reaction of {γ-SiW10} with Mn2+ led to the substituted products ({MnSiW10}, {MnSiW9}, and {Mn2SiW8}) only without any identifiable peaks related to the sandwich-type POM (Figure S5 in the Supporting Information). This indicates that the excess G

DOI: 10.1021/acs.inorgchem.5b02800 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 6. Real-time ESI-MS monitoring on the solution of pure crystal 7 upon heating at 80 °C as a function of standing time: (a) t = 0; (b) 10 min; (c) 20 min; (d) 30 min; (e) 40 min. The identified clusters are [H 9 ‑ x K x Mn 3 (SiW 8 O 3 1 )(SiW 9 O 3 4 )] 5 − (m/z 878.9−901.9), [H10‑xKxMn3(SiW8O31)(SiW9O34)]4− (m/z 1103.8−1161.3) for {Mn3(SiW8)(SiW9)} and [H7‑xKxMn4(SiW9O34)2·nH2O]5− (m/z 932.3−943.4), [H8‑x KxMn4 (SiW9O34)2 ·nH2 O] 4− (m/z 1165.6− 1193.8) for {Mn4(SiW9)2}, respectively.

Figure 7. CID mass spectra of (a) {Mn3(SiW8)(SiW9)} ([H7K3Mn3(SiW 8 O 3 1 )(SiW 9 O 3 4 )−H 2 O] 4 − at m/z 1123.1) and (b) {Mn4(SiW9)2}([H5K3Mn4(SiW9O34)2−H2O]4− at m/z 1193.8) precursors at appropriate collision energies and (c) the relative stability diagram of {Mn3(SiW8)(SiW9)} (black line) and {Mn4(SiW9)2} (red line) clusters. The precursor ion in each case is shown in an empty square box.

Scheme 2. Proposed Self-Assembly Mechanism of [{Ln(H2O)n}2{Mn4(B-α-SiW9O34)2(H2O)2}]6−a

species is shown in Figure S6 in Supporting Information, in which the anions of [H6K4Mn3(SiW8O31)(SiW9O34)]4− at m/z 1137.1 and [H7KMn4(SiW9O34)2]4− at m/z 1179.5 are favorably formed at 50 and 80 °C (both at t = 40 min), respectively (Figure S6a in the Supporting Information). Interestingly, the gas-phase relative stability study on the two sandwich-type clusters shows that {Mn3(SiW8)(SiW9)} is substantially unstable than {Mn4(SiW9)2} based on the energy-variable CID of the two corresponding species with the same charge, [H7K3Mn3(SiW8O31)(SiW9O34)−H2O]4− at m/z 1123.1 and [H5K3Mn4(SiW9O34)2−H2O]4− at m/z 1193.8 (Figure 7c), although their CID mass spectra display similar dissociation patterns (Figure 7a−b). The above two different experiments illustrate the causal relationship of {Mn3(SiW8)(SiW9)} and {Mn4(SiW9)2} and the characteristic role of {Mn3(SiW8)(SiW9)} as an intermediate. In summary, we suggest that the overall transformation of (γSiW10O36) to Mn4(B-α-SiW9O34)2 in the presence of Mn2+ proceeds stepwise and involves the following intermediate fragments: (γ-SiW10O36) + Mn2+ → (B-β-MnSiW9O34)/(B-βMn2SiW8O31) → Mn3(B-β-Mn2SiW8O31)(B-β-MnSiW9O34) → Mn4(B-α-SiW9O34)2. The main driving force is the conversion from γ → β → α-type rotational isomers although the number of lacunary sites increases during this transformation. Of particular importance is the detection of two intermediate fragments: one is unstable Mn-substituted species of {Mn2SiW8} and {MnSiW9}, the other is metastable manganese-containing sandwich dimer 7a′, which was first identified in solution by time-resolved ESI-MS and then determined by X-ray crystallography for the solid-state structure. The lanthanide ions, Ln3+, are coordinated with 6a during the process of crystallization and formed two types of network structures, respectively (Scheme 2).

a Color scheme: Ln = green; WO6 octahedron = teal (free), = light blue (bound with Ln3+); Mn = yellow; O = red; Si = orange.

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CONCLUSION We synthesized and characterized a series of lanthanidecontaining sandwich-type POMs [{Ln(H2O)n}2Mn4(B-αSiW9O34)2(H2O)2]6− (Ln = La (1), Nd (2), Gd (3), Dy (4), Er (5); n = 5, 6) with two types of extended network structures using [γ-SiW10O36]8− as a starting material. By combing timeresolved ESI-MS with single-crystal X-ray diffraction analysis, we can not only identify the dynamic changes of the solution state species involved in the self-assembly process as a function of reaction time, establish the semiquantitative relationship among these observed species by plotting its concentration change against reaction time, but also analyze the structures of the solids isolated from the reaction solutions. On the basis of the time-resolved ESI-MS data and single-crystal X-ray diffraction analyses, we proposed the overall transformation of (γ-SiW10O36) to Mn4(B-α-SiW9O34)2 in the presence of H

DOI: 10.1021/acs.inorgchem.5b02800 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Mn2+ proceeds stepwise and involves the following intermediate fragments: (γ-SiW10O36) + Mn2+ → (B-β-MnSiW9O34)/(B-β-Mn2SiW8O31) → Mn3(B-β-Mn2SiW8O31)(B-βMnSiW9O34) → Mn4(B-α-SiW9O34)2. The lanthanide ions, Ln3+, are coordinated with the as-synthesized sandwich-type building block [Mn4(B-α-SiW9O34)2(H2O)2]12‑ during the crystallization process and formed multidimensional structures. Future work will be focused on the effect of other first-row transition metal ions in the assembly process of the Weakley dimer.

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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.5b02800. Polyhedral and ball-and-stick representations of compounds 6 and 7, ESI-MS spectra of crystals 1−7 redissolved in water upon heating at 80 °C for different time intervals, real-time ESI-MS monitoring on an aqueous solution of pure K8SiW10O36, real-time ESIMS monitoring on the synthetic process of compound 6, real-time ESI-MS monitoring on the reaction of K8SiW10O36 with Mn(OAc)2 at RT, graphs showing peak intensities of the intermediate species {Mn3(SiW8)(SiW9)} and product species {Mn4(SiW9)2} plotted against reaction temperature and reaction time (PDF) Crystallographic information files (CIF1, CIF2, CIF3, CIF4, CIF5, CIF6)

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AUTHOR INFORMATION

Corresponding Authors

*(J.C.) E-mail: [email protected]. *(C.-W.H.) E-mail: [email protected]. Author Contributions ‡

L.-Y.F. and Z.-G.L. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the National Natural Science Foundation of China (21371025, 21173021, 21231002, 21276026), 973 Program (2014CB932103), the 111 Project (B07012), and the Fundamental Research Grant (20121942006) by Beijing Institute of Technology.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.5b02800 Inorg. Chem. XXXX, XXX, XXX−XXX