Controlled Triol-Derivative Bonding and ... - ACS Publications

Apr 13, 2016 - on Cu-Centered Anderson−Evans Polyoxometalates. Yang Wang, Bao Li,* Hujun Qian, and Lixin Wu*. State Key Laboratory of Supramolecular...
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Controlled Triol-Derivative Bonding and Decoration Transformation on Cu-Centered Anderson−Evans Polyoxometalates Yang Wang, Bao Li,* Hujun Qian, and Lixin Wu* State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun 130012, P. R. China S Supporting Information *

ABSTRACT: To create new types of organic ligands covalently grafted onto polyoxometalates and identify the reaction mechanism, we selected CuII as the central heteroatom for the synthesis of a series of disklike Anderson−Evans clusters bearing different triol derivatives on both their faces via one-pot and/or step-by-step routes. By using a [(n-C4H9)4N]4[Mo8O26] precursor cluster and copper acetate as the starting materials, several organically modified χ isomers with CuII heteroatom centers were obtained. Starting from a [(n-C4H9)4N]2[Mo2O7] subcluster, however, a half-malposition coordination fashion of triol ligands with a δ isomer on one face and a χ isomer on the other face of the Anderson−Evans cluster was obtained. By changing the reaction solvent from acetonitrile to methanol, we realized a secondary organic modification of the triol-grafted clusters and obtained a triol ligand/methanol codecoration on the Anderson−Evans polyoxometalate. In addition, by changing the reaction environment, we succeeded in modulating the transformation of triol ligands from one site to another on the polyoxometalate cluster. Importantly, by control of the reaction condition, the methanol molecules were also taken off from the cluster.

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INTRODUCTION Organic moiety grafting through a covalent bond represents one of the effective strategies for synthesizing new structures and functionalizing polyoxometalate (POM) hybrids over recent years.1−6 The obtained organic−inorganic hybrid complexes integrate the advantages of individual segments and can be used for the fabrication of diverse supramolecular assemblies, nanostructured materials, and so forth.7−13 For one of the typical disklike POMs, six hydroxyls surrounding the heteroatom on both faces of B-type Anderson−Evans clusters can be covalently replaced by triol ligands completely and/or partially, forming polyhydroxyl-derivative-decorated architectures.14 When the reported clusters are summarized according to the coordination fashion onto a heteroatom, the known trioldecorated Anderson−Evans POMs can be divided into the first four structure types, as shown in Figure 1. Type 1 represents a single-sided δ isomer, reported by Wei, Wu, and Rompel successively,15−18 with one triol ligand replacing three μ3-O atoms (bridging three metal ions) around a central heteroatom on one face of the disklike clusters with some trivalent heteroatoms. Type 2 bearing a few heteroatoms in trivalence represents a single-sided χ isomer, prepared very recently in strong acidic conditions by Wei’s group.19 The grafting occurs in a malposition coordination fashion in which a triol ligand replaces one μ2-O (bridging two metal ions) and two μ3-O atoms on one face of the POM cluster. Different from these two decorations, Hasenknopf and his co-workers reported type © XXXX American Chemical Society

Figure 1. Schematic drawing of the main decorations (types 1−5) when triol ligands bind to an Anderson−Evans POM cluster, where the blue octahedron represents {MoO6} and the yellow octahedron denotes the O heteroatom {XO6}.

3 (double-sided δ isomer) and type 4 (double-sided χ isomer) in a much earlier stage, in which a double-sided decoration to the Anderson−Evans clusters was built for the first time.20 For type 3, by using MnIII and several other trivalent transitionmetal ions as heteroatoms, two triol ligands replace all μ3-O Received: January 5, 2016

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DOI: 10.1021/acs.inorgchem.6b00008 Inorg. Chem. XXXX, XXX, XXX−XXX

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starting molybdate clusters, TBA4Mo8O26 and TBA2Mo2O7, were prepared according to literature methods and characterized carefully, consistent with the published results (Figures S1 and S2).38,39 Fourier transform IR spectra were carried out on a Bruker Vertex 80v spectrometer equipped with a DTGS detector (32 scans) with a resolution of 4 cm−1 in KBr pellets. UV−vis spectra were recorded on a Shimadzu 3100 PC spectrometer with a slit width of 2 nm. Elemental analysis for C, H, and N was performed on a Flash EA1112 from Thermo Quest Italia SPA instrument. Elemental analysis for Cu and Mo was performed on a PLASMA-SPEC (I) inductively coupled plasma atomic emission spectrometer. Thermogravimetric analysis (TGA) was carried out on a Q500 Thermal Analyzer (New Castle TA Instruments) in flowing N2 under a heating rate of 10 °C·min−1. Single-crystal X-ray diffraction data were collected on a Rigaku R-AXIS RAPID imaging-plate diffractometer with graphite-monochromated Mo Kα (λ = 0.71073 Å) at 293 K. The empirical absorption correction based on equivalent reflections was applied. All complex crystals were solved by direct methods and refined by full-matrix-least-squares fitting on F2 using the SHELXTL-97 software.40 All non-H atoms, except some lattice solvents, were refined with anisotropic thermal parameters. A summary of the crystallographic data and structural refinements for 1a−6a and 1b (their corresponding polyanion clusters are 1−6) is listed in Table S1. Synthesis of [(n-C4H9)4N]2{H2CuMo6O18[(OCH2)3CCH3]2}·(CH3CN)4 (1a). A mixture of Cu(CH3COO)2·2H2O (0.12 g, 0.55 mmol), TBA4Mo8O26 (0.71 g, 0.33 mmol), and (HOCH2)3CCH3 (0.13 g, 1.10 mmol) in 50 mL of acetonitrile was heated to reflux for 12 h. The blue solution was cooled to room temperature and kept for several days to obtain blue crystals (0.58 g, yield 72.7% based on Mo). Elem anal. Calcd for CuMo6C50N6O24H104: Cu, 3.51; Mo, 31.76; C, 33.13; H, 5.78; N, 4.64. Found: Cu, 3.53; Mo, 31.80; C, 33.15; H, 5.80; N, 4.65. IR (KBr, cm−1): 3622, 2962, 2932, 2873, 1484, 1461, 1117, 1029, 939, 922, 902, 736, 676, 552, 420. Synthesis of [(n-C4H 9)4 N]2 {H2CuMo6O 18[(OCH2)3 CCH2 CH3]2 }· (CH3CN)4 (2a). A mixture of Cu(CH3COO)2·2H2O (0.12 g, 0.55 mmol), TBA4Mo8O26 (0.71 g, 0.33 mmol), and (HOCH2)3CCH2CH3 (0.15 g, 1.10 mmol) in 50 mL of acetonitrile was heated to reflux for 12 h. The reaction solution in blue was cooled to room temperature and kept there for several days to obtain blue crystals (0.50 g, yield 61.8% based on Mo). Elem anal. Calcd for CuMo6C52N6O24H108: Cu, 3.45; Mo, 31.27; C, 33.93; H, 5.91; N, 4.57. Found: Cu, 3.46; Mo, 31.29; C, 33.92; H, 5.90; N, 4.57. IR (KBr, cm−1): 3621, 2962, 2929, 2872, 1483, 1469, 1107, 1046, 936, 922, 909, 736, 677, 552, 413. Synthesis of [(n-C 4 H 9 ) 4 N] 2 {H 2 CuMo 6 O18 [(OCH 2 ) 3 CCH 2 OH] 2 }· (CH3COOH)2 (3a). A mixture of Cu(CH3COO)2·2H2O (0.12 g, 0.55 mmol), TBA4Mo8O26 (0.71 g, 0.33 mmol), and (HOCH2)3CCH2OH (0.15 g, 1.10 mmol) in 50 mL of acetonitrile was heated to reflux for 12 h. The reaction solution in blue was cooled to room temperature and kept there for several days to obtain blue crystals (0.49 g, yield 61.7% based on Mo). Elem anal. Calcd for CuMo6C46N2O30H100: Cu, 3.53; Mo, 31.97; C, 30.69; H, 5.60; N, 1.56. Found: Cu, 3.57; Mo, 32.00; C, 30.67; H, 5.58; N, 1.55. IR (KBr, cm−1): 3429, 2964, 2936, 2874, 1467, 1111, 1018, 939, 913, 896, 731, 673, 554, 416. Synthesis of [(n-C 4 H 9 ) 4 N] 2 {H 2 CuMo 6 O 18 [(OCH 2 ) 3 CNO 2 ] 2 }· (CH3COOH)2 (4a). A mixture of Cu(CH3COO)2·2H2O (0.12 g, 0.55 mmol), TBA4Mo8O26 (0.71 g, 0.33 mmol), and (HOCH2)3CCH2NO2 (0.18 g, 1.10 mmol) in 50 mL of acetonitrile was heated to reflux for 12 h. The reaction solution in blue was cooled to room temperature and kept there for several days to obtain blue crystals (0.60 g, yield 74.4% based on Mo). Elem anal. Calcd for CuMo6C44N4O32H94: Cu, 3.47; Mo, 31.45; C, 28.87; H, 5.18; N, 3.06. Found: Cu, 3.50; Mo, 31.44; C, 28.90; H, 5.20; N, 3.05. IR (KBr, cm−1): 3346, 2963, 2934, 2874, 1472, 1080, 947, 924, 897, 741, 676, 547, 411. Synthesis of [(n-C 4 H 9 ) 4 N] 3 {HCuMo 6 O 18 [(OCH 2 ) 3 CCH 3 ] 2 }· [(HOCH2)3CCH3](CH3CN) (5a). A mixture of Cu(CH3COO)2·2H2O (0.02 g, 0.07 mmol), TBA2Mo2O7 (0.20 g, 0.24 mmol), and (HOCH2)3CCH3 (0.02 g, 0.18 mmol) in 20 mL of acetonitrile was heated to reflux for 12 h. The reaction solution was cooled to room temperature. With solvent evaporation in an ether atmosphere for several days, green crystals were obtained. The crude products were

atoms around the heteroatom on both sides of the disk cluster axisymmetrically. The organic groups grafted on the POM can be further modified symmetrically and asymmetrically21−25 or polymerized,26 and the formed hybrids have become popular hybrid clusters in the self-assembly of POMs.27−35 For the latter type, it is also a double-sided modification fashion, but each triol ligand replaces one μ2-O atom and two μ3-O atoms, forming a nonaxisymmetric structure against the normal of the cluster plane, the so-called full malposition coordination. To date, only two examples bearing ZnII and NiII central heteroatoms involve this malposition fashion,20 and the nature that dominates the modification fashion differing from others is still unknown. Cronin’s group made a detailed analysis for the assembly of a triol-ligand-decorated double-sided δ-isomer Anderson−Evans cluster prepared from a bigger cluster, TBA4Mo8O26 (TBA = tetrabutylammonium cation), and found some smaller cluster intermediates generating in the reaction process for the preparation of [MnMo6O18(OCH2)3CNH2)2]3−.36 In contrast to the productive development of naked Anderson−Evans POMs, their organic decoration motifs have not yet been exploited deeply from the structural point of view. Understanding the influence of the precursors and the reaction environments for the expected modification fashions is still fragmental. As far as we know, few publications concern direct transformations between the structural types, which are significant to building the methodology for creating new modification fashions in the same Anderson−Evans-type cluster.19 Meanwhile, multiple decoration and comprehension of the reactivity of different bridging O atoms is still ambiguous as well. For example, the architecture in type 5 (double-sided χ/δ isomer), in which one triol ligand replaces three μ3-O atoms on one side in regular position while the other replaces one μ2-O and two μ3-O atoms on the other side in malposition, should exist but has never been reported. In addition, both substitution of a central heteroatom and optimization of the reaction route are of interest for the creation of new Anderson−Evans POM hybrids because the change of the heteroatom not only involves control of organic modification but also brings out novel functions.37 By analysis of the grafting possibilities, complementation to the type slot becomes very significant for POM chemistry. Because the divalent transitionmetal ions with larger ionic radii were found to be in close relation to the structural fashion and less cared for,20 we herein selected CuII as the heteroatom to synthesize the triolderivative-decorated Anderson−Evans POMs. To extend the synthetic approach, we used the TBA2Mo2O7 cluster, a small precursor to control the preferred type of triol decoration. With this strategy, a new Anderson−Evans POM (type 5) was obtained successfully. In some cases, the proton solvent methanol was used to modulate the environment of the reaction system. With this strategy, we realized a secondary modification to the triol-ligand-decorated Anderson−Evans POM. Meanwhile, we also successfully controlled the transformation from type 5 to type 4 and dissociation of the methanol molecules attaching to the clusters by changing the acidity of the reaction system.

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EXPERIMENTAL SECTION

General Methods and Materials. All preparations and operations were performed under air conditions. All chemicals used in the reactions, including solvents, were commercially available from Aladdin and were used as received unless otherwise mentioned. The B

DOI: 10.1021/acs.inorgchem.6b00008 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry redissolved in acetonitrile, and the solution was stored at room temperature for several days to obtain green crystals (0.05 g, yield 29.1% based on Mo). Elem anal. Calcd for CuMo6C65N4O27H142: Cu, 3.10; Mo, 28.07; C, 38.06; H, 6.98; N, 2.73. Found: Cu, 3.10; Mo, 28.10; C, 38.03; H, 6.95; N, 2.71. IR (KBr, cm−1): 3411, 2960, 2932, 2873, 1487, 1473, 1457, 1111, 1037, 929, 915, 890, 737, 706, 662, 541, 416. Synthesis of [(n-C4H9)4N]2{CuMo6O16[(OCH2)3CCH3]2(OCH3)2}· (H2O) (6a). The compound was prepared through several approaches. For route 1, a mixture of Cu(CH3COO)2·2H2O (0.12 g, 0.55 mmol), TBA4Mo8O26 (0.71 g, 0.33 mmol), and (HOCH2)3CCH3 (0.13 g, 1.10 mmol) in 5 mL of N,N-dimethylformamide was heated to 85 °C for 2 h, and then 45 mL of methanol was added. The mixture was refluxed for 10 h, cooled to room temperature, and kept there for several days to obtain blue crystals (0.55 g, yield 74% based on Mo). Elem anal. Calcd for CuMo6C44N2O25H98: Cu, 3.75; Mo, 33.97; C, 31.19; H, 5.83; N, 1.65. Found: Cu, 3.80; Mo, 34.02; C, 31.20; H, 5.85; N, 1.65. IR (KBr, cm−1): 3460, 2961, 2931, 2873, 1483, 1471, 1461, 1123, 1030, 939, 922, 904, 735, 674, 552, 416. For route 2, (HOCH2)3CCH3 (0.02 g, 0.18 mmol) and TBA2Mo2O7 (0.20 g, 0.24 mmol) were dissolved in 20 mL of methanol. The mixture was heated to reflux for 2 h, followed by the addition of Cu(CH3COO)2·2H2O (0.02 g, 0.07 mmol). The mixture was then heated to reflux for 10 h, cooled to room temperature, and kept there for several days to obtain blue crystals (0.08 g, yield 47.7% based on Mo). Structural characterization to the product gives results consistent with those from route 1. For route 3, 1a (0.2 g, 0.11 mmol) in 20 mL of CH3OH was heated to reflux for 12 h, and the solution was cooled to room temperature and kept there for several days to obtain blue crystals (0.12 g, 64.2% based on Mo). Structural characterization to the product gives results consistent with those from route 1. For route 4, 5a (0.1 g, 0.05 mmol) in 20 mL of CH3OH was heated to reflux for 12 h, and the solution was cooled to room temperature and kept there for several days to obtain blue crystals (0.04 g, 48.2% based on Mo). Structural characterization to the product gives results consistent with those from route 1. Synthesis of [(n-C 4 H 9 ) 4 N] 2 {H 2 CuMo 6 O 18 [(OCH 2 ) 3 CCH 3 ] 2 }· (CH3COOH)2 (1b). Two routes were adopted for the preparation of 1b. For route 1, acetic acid (0.5 mL) was added to the solution of 5a (0.08 g, 0.046 mmol) in 20 mL of acetonitrile, and the mixture was heated to reflux for 12 h. The reaction solution in blue was cooled to room temperature and stored at room temperature for several days to obtain blue crystals (0.015 g, yield 18.8% based on Mo). Elem anal. Calcd for CuMo6C46N2O28H100: Cu, 3.59; Mo, 32.55; C, 31.24; H, 5.70; N, 1.58. Found: Cu, 3.58; Mo, 32.60; C, 31.25; H, 5.75; N, 1.57. IR (KBr, cm−1): 3619, 2962, 2934, 2872, 1465, 1117, 1033, 936, 925, 900, 730, 667, 550, 419. For route 2, acetic acid (0.5 mL) was added to the solution of 6a (0.04 g, 0.024 mmol) in 20 mL of CH3CN, and the mixture was heated to reflux for 12 h. The reaction solution in blue was cooled to room temperature and kept there for several days to obtain blue crystals (0.035 g, yield 83.8% based on Mo). Structural characterization to the product gives results consistent with those from route 1.

products were characterized carefully and identified by singlecrystal X-ray structural analysis (Table S2 and Figures S3− S10). From single-crystal X-ray structural analysis (Table S1), double-sided χ-isomer Anderson−Evans POMs bearing CuII h et er o a t o m c en t e r s w i th t h e s t r u c t u r e f o r m ul a {H2CuMo6O18[(OCH2)3CR]2}2−, where the terminal group R = −CH3, −CH2CH3, −CH2OH, and −NO2, are demonstrated to be obtained successfully. In accordance with triol modification for NiII- and ZnII-centered clusters using the same oxomolybdate starting material,20 as expected, these hybrid POMs display the same decoration fashion on the Anderson− Evans cluster except the central heteroatom having been replaced by CuII, as shown in Figure 2a−d. In contrast to the

RESULTS AND DISCUSSION Synthesis and Characterization of Triol-Grafted CuIICentered Anderson−Evans Clusters. The detailed synthetic procedures for the organically grafted POM clusters are described in the Experimental Section. The reactant TBA4Mo8O26, mixing with copper acetate, was used initially as the starting material to synthesize triol-decorated Anderson− Evans POMs 1a−4a with a CuII central heteroatom under reaction conditions similar to those for the preparation used for other metal-centered Anderson−Evans POMs. However, the synthetic conditions are different from those of the bare Cucentered clusters that were prepared in aqueous solution. Considering the solubility of the reactant precursors used, we conducted the reactions in acetonitrile. All of the prepared

cases in which trivalent heteroatoms act as the central atom, two triol derivative ligands attach on both sides of the cluster in a full malposition coordination fashion in type 4. Although Cu displays properties similar to those of its Ni and Zn neighbors in the periodic table of the elements, the extension for central heteroatoms implies more possibilities regarding organic modification and functionality for Anderson−Evans POMs. The substitution of triol ligands with electron-donating or -accepting terminal groups does not affect the coordination fashion on the CuII-centered POMs. We also tried to synthesize the complex with the triol bearing a −NH2 terminal group, but unfortunately no suitable product for single-crystal X-ray analysis was obtained because of the easy formation of gels, which may be raised from the stronger coordination

Figure 2. Polyanion structures of the obtained triol-ligand-modified Anderson−Evans POM cluster of (a) {H2CuMo6O18[(OCH2)3CCH3]2}2− (1), (b) {H2CuMo6O18[(OCH2)3CCH2CH3]2}2− (2), (c) {H2CuMo6O18[(OCH2)3CCH2OH]2}2− (3), (d) {H2CuMo6O18[(OCH2)3CNO2]2}2− (4), (e) {HCuMo6O18[(OCH2)3CCH3]2}3− (5), and (f) {CuMo6O16[(OCH2)3CCH3]2(OCH3)2}2− (6) with thermal ellipsoids as the atoms at a 30% probability level from the single-crystal structure. All H atoms, except those on μ3-O atoms, are omitted for clarity.

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C

DOI: 10.1021/acs.inorgchem.6b00008 Inorg. Chem. XXXX, XXX, XXX−XXX

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time, forming a multiply modified cluster 6a, as shown in Figure 2f. Interestingly, starting from the TBA 2 Mo 2 O 7 precursor cluster, different from the reaction in acetonitrile for 5a in a half-malposition coordination, a coordination site change from the μ3-O atom to the μ2-O atom occurs when the reaction is conducted in methanol. We obtained the identical product 6a with full malposition coordination. These results indicate that methanol plays the role of both the solvent and reactant. To identify the reactivity of free μ3-O atoms with alcohols, 6a is obtained definitely by refluxing 1a in methanol. In polyanion 6a, the triol ligands remain the same decoration fashion as that of 1a, while two CH3OH molecules neutralize the residue hydroxyls on μ3-O atoms, forming a triol ligand/solventcodecorated Anderson−Evans POM. For all above-mentioned routes for 6a, we obtained single crystals with the same crystallographic parameters. Crystal structure analysis points out that the bond angle between the attached CH3OH and the mean plane of six Mo atoms is 75.2(2)°, and the binding methoxyl group is a bit away from the triol ligand because of steric hindrance from the neighboring triol, indicating less blocking for cografting of the two organic segments on the same side. To the best of our knowledge, the replacement reaction of μ3-O atoms by methanol molecules in trioldecorated Anderson−Evans POMs has never been mentioned before. More interestingly, the methylation process could occur reversibly by simple control of the reaction conditions. With the addition of acetic acid, 6a transforms into 1b by refluxing it in acetonitrile. The only difference between 1b and 1a is the lattice solvent molecules, two CH3COOH in 1b, and four CH3CN in 1a. In contrast to methylation of A-type Anderson− Evans POMs, these reactions demonstrate that without the addition of a strong acid methanol can also be grafted onto Btype Anderson−Evans POMs bearing a Cu central heteroatom. Meanwhile, the release of methanol molecules from the present cluster does not necessarily require breakage of the frame structure of the cluster and the triol-ligand modification fashion. Grafting and Conversion Reactions of Triol Modification. As mentioned above, the triol-ligand-modified Anderson−Evans POM in type 5 is prepared with the subcluster TBA2Mo2O7 as the precursor, which is sourced from hydrolysis of another precursor cluster, TBA4Mo8O26. Depending on the solvents used, both clusters can be directed to the same triolmodified cluster 6 in type 4 or different modified clusters in types 4 and 5. On the other hand, by modulation of the solvent acidity, we also controlled the modified fashion. With the addition of some acetic acid, the triol-modified cluster 1 in type 4 is obtained from acetonitrile through conversion of 5. In comparison to the direct synthesis of 1 from TBA4Mo8O26, the stepped preparation encounters hydrolysis of the precursor cluster, formation of the triol-modified cluster intermediate in type 5, and its final conversion. To identify conversion in multiple paths, we performed the reaction of the intermediate triol-modified cluster in type 5 following a different route. Conversion of the triol-modified cluster 5 to 6 is observed in methanol besides methylation on two residual μ3-O atoms. After the demethylation reaction of 6 by refluxing it in acetonitrile with the addition of acetic acid, we obtain 1 again. Because the reaction also happens without the addition of methanol, apparently, it is not methylation dominating the structural conversion between different modification fashions. Thus, there is an apparent regularity; that is, in the absence of methanol, the addition of acetic acid makes the son cluster

interactions between free amino groups and the added Cu ions via the present synthetic path. Because TBA4Mo8O26 always directs the normal structural fashions as those already known, considering that the Anderson−Evans structure could be divided into three {Mo2} oxomolybdate dimers, we selected a smaller cluster, TBA2Mo2O7, as the reactant for the synthesis of triol-liganddecorated Anderson−Evans POMs to examine the possibility of constructing new architectures. Under refluxing conditions in acetonitrile, fortunately, we obtained 5a in type 5. Single-crystal X-ray structure analysis reveals that 5a represents a new binding fashion of the triol ligand on double sides of the disklike POMs, as shown in Figure 2e, which differs from all of the architectures reported. In this coordination fashion, a triol ligand replaces three μ3-O atoms around the heteroatom on one side of the disk like cluster, while the other binds on one μ2-O and two μ3O atoms on the other side, demonstrating an intermediate coordination fashion between types 3 and 4, which are generally prepared from clusters possessing trivalent and divalent metal heteroatoms, respectively, as summarized in Figure 1. For the known triol-decorated Anderson−Evans POMs with divalent heteroatoms on double sides, the charge of the cluster is 2− because two unbound protons attach to the μ3-O atoms on both sides. In the present modified POM, because only one μ3-O atom has been connected by the organic ligand, while the other μ3-O atom is still attached by a proton, the charge of the polyanion increases to 3−. It should be noted that, when TBA2Mo2O7 is used as the starting material to synthesize triol-decorated Anderson−Evans POMs, the obtained yield is lower than that using TBA4Mo8O26 because of loss in the treatment of the crude product. Besides single-crystal X-ray analysis, we analyzed the IR spectra of synthesized complexes in the solid state (Figure S10). All complexes show typical characteristics of the vibrational bands of MoO and Mo−O−Mo of Anderson− Evans POMs. To validate the difference between the χ and χ/δ isomers, we compared the vibrations of 1a and 5a. From the amplified IR spectra, we observe the peak at 736 cm−1 derived from the Mo−O−Mo vibration of the χ isomer according to the literature,20 while for the χ/δ isomer, in addition to the weakened peak at 737 cm−1, another peak emerges at 706 cm−1, which is attributed to the Mo−O−Mo vibration derived from the δ-isomer part.20 We also examined the stabilities of asprepared complexes. All samples in acetonitrile exhibit the characteristic ligand-to-metal charge-transfer band of the Anderson−Evans POM at about 220 nm (Figure S11),19 indicative of their stability in solution even encountering repeated crystallization. Methylation and Demethylation of Triol-Modified Anderson−Evans POMs. The alcohols have already been grafted chemically onto the Keggin− or Wells−Dawson-type POMs.41,42 Yagasaki et al. presented examples of methylation on A-type Anderson−Evans POMs (where six deprotonated μ3O atoms surround the heteroatom), where μ2-O atoms protonated with a strong acid can be replaced by methanol reversibly.43−45 However, this modification has never been found to be effective in B-type Anderson−Evans POMs. Following the same procedure as that used in the synthesis of Anderson−Evans POMs 1a−4a by using TBA4Mo8O26 and copper acetate as the starting reactants but with the solvent changing from acetonitrile to methanol, while the full malposition coordination in type 4 is maintained, two free μ3-O atom positions are grafted with methyl groups at the same D

DOI: 10.1021/acs.inorgchem.6b00008 Inorg. Chem. XXXX, XXX, XXX−XXX

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are used to depict the synthesis of 1 and 6, both reactions are seen to locate in an acidic environment. When eq 3 is divided into two steps, one can find that 1, acting as a base, neutralizes methanol to yield 6 (eq 3′). For the synthesis of 5 using TBA2Mo2O7 in acetonitrile, as shown in eq 4, the dominated environment becomes less acidic. As an independent evidence, eq 5 points out the conversion from 5 to 1 controlled by acid. Apparently, from eq 6, one can see the role of methanol serving as both the acid and ligand to incorporate onto the cluster.

TBA2Mo2O7 have the same action as that of its mother cluster TBA4Mo8O26, while in the presence of methanol, the reaction of the son cluster also leads to the same modification fashion of the triol ligand as that of the mother cluster. Therefore, methanol plays a role like acid that triggers protonation of the μ2-O atoms and then the following reactions. According to the reaction conditions used in the preparation process and the obtained triol-modified Anderson−Evans clusters with a CuIIcentered heteroatom, we summarized the synthetic routes including new modification types of the triol ligand, conversion between grafting types, and methylation and demethylation in Figure 3.

[Mo8O26 ]4 − + 2H 2O ⇌ 4[Mo2O7 ]2 − + 4H+

(1)

3[Mo8O26 ]4 − + 4Cu 2 + + 8(HOCH 2)3 CCH3 ⇌ 4{H 2CuMo6O18[(OCH 2)3 CCH3]2 }2 − + 6H 2O + 4H+

(2)

3[Mo8O26 ]4 − + 4Cu 2 + + 8(HOCH 2)3 CCH3 + 8CH3OH ⇌ 4{CuMo6O16 [(OCH 2)3 CCH3]2 (OCH3)2 }2 + 14H 2O + 4H+

(3)

{H 2CuMo6O18[(OCH 2)3 CCH3]2 }2 − + 2CH3OH ⇌ {CuMo6O16 [(OCH 2)3 CCH3]2 (OCH3)2 }2 − + 2H 2O (3′) 2−

3[Mo2O7 ]

+ Cu

2+

+ 2(HOCH 2)3 CCH3

⇌ {HCuMo6O18[(OCH 2)3 CCH3]2 }3 − + 2H 2O + OH−

(4)

{HCuMo6O18[(OCH 2)3 CCH3]2 }3 − + H+ ⇌ {H 2CuMo6O18[(OCH 2)3 CCH3]2 }2 − −

−

CH3OH + OH ⇌ CH3O + H 2O Figure 3. Synthesis of organically modified Cu-centered Anderson− Evans clusters by refluxing different precursors with triol and CuAc2 in different solvents, while transformations are conducted by refluxing in different solvents under controlled acidity.

(5) (6)

The published results have demonstrated the necessity of acidity in a stepwise protonation and modification.19 However, it is still important to understand the reactivity difference between the μ3-O and μ2-O atoms for coordination of the alcohol ligands. For A-type Anderson−Evans clusters, it was confirmed that the μ2-O atoms have stronger basicity than the μ3-O atoms because the former is always protonated first when a strong acid is involved and the activated site is then used for methylation,43−45 while for B-type Anderson−Evans clusters, μ3-O atoms have stronger basicity than μ2-O atoms because the μ3-O atoms maintain the protonated state all of the time. Obviously, the basicity reverse between the two kinds of bridging O atoms can be attributed to the change of central heteroatoms, which simultaneously dominate the modification fashions. In the present study, the CuII center makes the Anderson−Evans cluster have largely increased basicity of μ2-O atoms, resulting in its easier neutralization for the formation of malposition coordination. To explain the reason that the triol ligand rather than methanol binds to the μ2-O atom, the coordination selectivity and difference between types 4 and 5 are analyzed. Considering the multidentate chelation for stabilization of the coordination during formation of the cluster,37 the triol ligands bind to the cluster prior to methanol, although the latter displays a bit stronger acidity. As for the coordination selectivity in malposition or axisymmetry, it is subject to the conditions of the reaction environment and

Mechanism Analysis of the Grafting and Conversion Reactions of Triol Modification. Combining the present study and the published results, three main factorsthe acidity in the reaction solution, the activation of bridging O atoms, and the reaction competition between the triol ligand and methanol moleculecould be confirmed to dominate the modified structural motifs of B-type Anderson−Evans POMs. To elucidate the dependence of the reactions on these factors, we first compare the impact of the starting reactants. Because TBA4Mo8O26 is larger in the number of Mo atoms than the target Anderson−Evans POM, proper hydrolysis into smaller intermediate fragments and/or rearrangement of the subclusters are unavoidable.36 When eq 1 is used to describe hydrolysis of [Mo8O26]4− into [Mo2O7]2−, protons can be found to generate in the reaction. Thus, when TBA2Mo2O7 acts as the starting cluster, its reaction in the nonproton solvent acetonitrile suffers a weaker acidic environment than the case using TBA4Mo8O26 directly, resulting in an incomplete protonation on μ2-O atoms and an axisymmetric coordination of the triol ligands. In agreement with TBA4Mo8O26, as illustrated in Figure 3, methanol plays a similar role and provides protons in all involved reactions. When eqs 2 and 3 E

DOI: 10.1021/acs.inorgchem.6b00008 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

triol ligands to the polyanions in the final products. Methanol, which serves a gentle acidic solvent, not only governs the final coordination but also joins the coordination on the O atom that connects to the central heteroatom, which has not been reported as far as we know. Following the present study, we believe that other diverse organically modified Anderson− Evans-type POMs bearing different divalent metal-ion centers can be prepared and modulated by precisely controlling the acidity of the solution and ligands.

coordination geometry. When the acidity in solution is not high enough to protonate μ2-O atoms, the axisymmetric coordination becomes the only choice for triol ligands. In the case of protonation of μ2-O atoms, the structure stability of the final cluster and reactivity of the μ2-O atoms become the key factors. As shown in Figure 4, after coordination of the triol ligands, the

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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.6b00008. Detailed information on the structure characterization, crystal structures, and parameters, TGA curves, IR, and UV−vis absorption spectra, proposed reaction equations, and theoretical calculation (PDF) X-ray crystallographic data for 1a−6a and 1b in CIF format (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Basic Research Program of China (Grant 2013CB834503), the National Natural Science Foundation of China (Grants 51203059, 91227110, and 21574057), the Ministry of Education of China (Grant 20120061110047), and the Changbaishan Scholars of Jilin Province.

Figure 4. Distance (Å) analysis between μ3-O atoms and between the μ3-O and μ2-O atoms on one side of Anderson−Evans polyanions: (a) 1, (b) 6, (c) and 5 with the triol ligand at the malposition position; (d) 5 with the triol ligand at the central position. The distances in purple are bound, while the distances in black are unbound.

distances between coordinated O atoms become shorter, which implies that the chelation fashion is favorable for increasing the stability of the cluster structures. On the other hand, the coordination triangle consisting of one μ2-O and two μ3-O atoms, calculated from the bond lengths in the crystal structure, is obviously smaller than the triangle comprised of three μ3-O atoms, indicating that the former strains the cluster framework, affording higher stability than the latter during chelate modification. The result also indicates that, in the case where both the μ2-O and μ3-O atoms are in protonated state, malposition coordination is preferential to axisymmetric coordination. The energy calculation summarized in Table S3 supports analysis that the reactivity on protonated μ2-O and μ3O atoms dominates the reaction sites.

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REFERENCES

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CONCLUSIONS In conclusion, by introducing CuII as the central heteroatom, a series of triol-derivative-decorated Anderson−Evans POMs were synthesized. In addition to the commonly used TBA4Mo8O26 affording a double-sided χ isomer of the Anderson−Evans cluster, small subcluster TBA2Mo2O7 directs a new coordination fashion of the organic ligand, a halfmalposition coordination to the CuII-centered Anderson− Evans structure. The introduction of a CuII ion changes the reactivity property of μ2-O and μ3-O atoms during reaction with the triol ligand and alcohol. The acidity change in solution is deduced to play a key role for the attachment fashion of the F

DOI: 10.1021/acs.inorgchem.6b00008 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.6b00008 Inorg. Chem. XXXX, XXX, XXX−XXX