Ti2-Containing 18-Tungsto-2-Arsenate(III) Monolacunary Host and the ...

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Ti2‑Containing 18-Tungsto-2-Arsenate(III) Monolacunary Host and the Incorporation of a Phenylantimony(III) Guest Kai-Yao Wang,† Zhengguo Lin,† Bassem. S. Bassil,†,‡ Xiaolin Xing,† Ali Haider,† Bineta Keita,§,▽ Guangjin Zhang,⊥ Cristian Silvestru,∥ and Ulrich Kortz*,† †

Department of Life Sciences and Chemistry, Jacobs University, P.O. Box 750561, 28725 Bremen, Germany Department of Chemistry, Faculty of Sciences, University of Balamand, P.O. Box 100, Tripoli, Lebanon § Laboratoire de Chimie-Physique, Université Paris-Sud, UMR 8000 CNRS, F-91405 Orsay, France ⊥ Key Laboratory of Green Process and Engineering, Chinese Academy of Sciences, 100190 Beijing, China ∥ Department of Chemistry, Supramolecular Organic and Organometallic Chemistry Centre, Faculty of Chemistry and Chemical Engineering, Babes-Bolyai University, RO-400028 Cluj-Napoca, Romania ‡

S Supporting Information *

(AsW9O33)4]20−.5e Now we decided to further investigate the system TiIV and {AsIIIW9} in aqueous solution. Here we report on the synthesis and structural characterization of a novel Ti 2 -containing 18-tungsto-2-arsenate(III), [(TiIVO)2(α-AsIIIW9O33)2]14− (1), comprising a vacant site in the belt position, which can be rationally filled by a phenylantimony(III) guest, resulting in [C6H5SbIII(TiIVO)2(αAsIIIW9O33)2]12− (2); see Figure 1.

ABSTRACT: The novel Ti2-containing, sandwich-type 18-tungsto-2-arsenate(III) [(TiIVO)2(α-AsIIIW9O33)2]14− (1) was successfully synthesized by the reaction of [TiO]2+ species with [α-AsIIIW9O33]9−. The monolacunary polyanion 1 is solution-stable, and a further reaction with 1 equiv of phenylantimony(III) dichloride resulted in [C6H5SbIII(TiIVO)2(α-AsIIIW9O33)2]12− (2). Both polyanions 1 and 2 were structurally characterized in the solid state and solution. Electrochemical studies were also performed on both polyanions.

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olyoxometalates (POMs) represent a unique class of molecular metal−oxygen anions with a remarkable structural variety and manifold properties rendering POMs of interest in catalysis, medicine, magnetism, photochemistry, and materials science.1 The mechanism of formation of POMs is sometimes not fully understood and described as “self-assembly”, but even very large and complex structures can be synthesized in one-step reactions by the simple reaction of water-soluble sources of the composing elements.2 The rational synthesis and controlled derivatization of POMs, perhaps similar to organic chemistry, remain a challenge. The subclass of heteropolytungstates with the lone paircontaining AsIII as the heteroatom has been known for a long time.3 This lone pair of electrons does not allow formation of the plenary Keggin ion, hence enriching the structural variety of this subclass.4 Additionally, the incorporation of TiIV ions into tungstoarsenate(III) POM precursors has led to products exhibiting interesting structural features, combined with unexpected catalytic and biological properties.5 In particular, in the dititanium(IV)-containing 19-tungstodiarsenate(III) [Ti2(OH)2As2W19O67(H2O)]8−,5a the lone pair of electrons on AsIII forces the coordinated TiIV to adopt an unusual squarepyramidal geometry, with the resulting TiIVO5 unit bridging the two trilacunary {AsIIIW9} units. This polyanion exhibited interesting catalytic properties in H2O2-based homogeneous oxidations.5b−d Recently, we reported the Ti7-containing, tetrahedral 36-tungsto-4-arsenate(III) [Ti 6 (TiO 6 )© 2015 American Chemical Society

Figure 1. Rational formation of 1 from trilacunary POM precursor {AsIIIW9} and further reaction of 1 with a phenylantimony(III) electrophile to yield 2. Color code: WO6 octahedra, red; Ti, turquoise; Sb, pink; As, yellow; O, red; C, gray. H atoms are omitted for clarity.

The reaction of [TiO]2+ with the trilacunary [α-AsIIIW9O33]9− heteropolytungstate precursor in a 2 M sodium acetate solution at pH 6 resulted in polyanion 1, which was isolated as a hydrated sodium salt, Na 1 4 [(Ti I V O) 2 (α-As I I I W 9 O 3 3 ) 2 ]·50H 2 O· CH3COONa (Na-1), in 52% yield (see the Supporting Information (SI) for details). Single-crystal X-ray diffraction (XRD) indicated that Na-1 crystallizes in the triclinic space group P1̅ (see SI for details). The novel 1 has a sandwich-type structure, with two [TiO]2+ groups encapsulated by two [αAsIIIW9O33]9− moieties (Figures 1 and 2). Both TiIV ions are pentacoordinated in a square-pyramidal fashion, being bound to two pairs of corner-shared WO6 octahedra via oxo bridges, and a terminal oxo ligand pointing outward, resulting in an overall assembly with idealized C2v symmetry (Figure S1a). The Ti−O Received: August 14, 2015 Published: October 22, 2015 10530

DOI: 10.1021/acs.inorgchem.5b01863 Inorg. Chem. 2015, 54, 10530−10532

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Inorganic Chemistry

On the basis of its structure, polyanion 1 might be considered as an attractive monolacunary host for the incorporation of suitable electrophiles. This hypothesis was confirmed by the reaction of 1 with C6H5SbCl2 in a nearly stoichiometric ratio in a 1 M sodium acetate solution at pH 6 in 62% yield (see the SI for details). We isolated the novel polyanion 2 as a mixed Na/Cs salt, Cs2.5Na9.5[C6H5SbIII(TiIVO)2(α-AsIIIW9O33)2]·40H2O (NaCs2), which crystallizes in the monoclinic space group P21/n (see the SI for details). Polyanion 2 is the expected phenylantimony(III)-embedded derivative of 1, with the SbIII ion occupying the vacant site and the phenyl group pointing outward (Figures 1 and S2), resulting in a structure with idealized point group symmetry C2v (Figure S1b). The SbIII ion is pentacoordinated and exhibits a square-pyramidal coordination geometry, with basal Sb−O bonds ranging from 2.179(8) to 2.234(8) Å and O−Sb−O angles from 84.8(3) to 94.3(3)°. The Sb−C bond length is 2.185(12) Å. We carried out 183W NMR studies on NaCs-2 dissolved in D2O/H2O (pH 6.6) and observed the expected spectrum with signals at −99.5, −108.7, −120.8, −122.6, and −159.3 ppm and relative intensities of 2:2:2:2:1 (Figure 3b). The 13C NMR spectrum of 2 shows four signals at 170.1, 134.1, 128.3, and 127.8 ppm, as expected for the phenyl group on the Sb atom (Figure S6a). This observation is further supported by the 1H NMR spectrum (Figure S6b). The NMR spectra of 2 remained unchanged after 2 weeks, indicating that this polyanion is solution-stable. Replacement of the unique, labile sodium ion in the addenda site of 1 by the phenylantimony(III) electrophile in 2 results in an increased distance between the opposite pairs of oxyges bound to W atoms at the Na/Sb binding site from around 2.99 to 3.20 Å (Figure S3). This is also reflected by an increase in the dihedral angle between the two planes passing through the six “belt” W atoms of each {AsIIIW9} unit in 1 and 2 from about 3.1° to 6.3°. In addition, a slight distortion at the TiO5 units is observed, where, for example, the O−Ti1−O bond angle facing the vacant site increases from 88.2° to 92.8° upon comparison of 1 and 2 (Figure S3). We believe that this structural flexibility at the vacant site of 1 is key for the smooth substitution of Na1 by the phenylantimony(III) electrophile, and potentially by many other guests, rendering 1 a highly interesting monolacunary POM precursor. The electrochemistry properties of 1 and 2 were studied in a 1 M lithium acetate solution at pH 6 and 4, respectively, where the two polyanions are stable, as confirmed by UV−vis spectroscopy (Figures S8 and S9). Figure S10a shows the cyclic voltammogram (CV) of 1 obtained at 100 mV s−1 and restricted to the chemically reversible wave. Whatever the scan rate from 100 to 5 mV s−1, no splitting of the single reduction wave of 1 was observed, but at a scan rate as small as 2 mV s−1, the reduction peak (−0.972 V versus SCE) presents a small prewave at ca. −0.850 V versus SCE (Figure S10b). Upon potential reversal, the reoxidation CV pattern features two waves located at −0.867 and −0.809 V versus SCE, respectively, which confirms the composite nature of the reduction wave. Two closely spaced reduction waves were also reported for Kortz’s polyanion [Ti2(OH)2As2W19O67(H2O)]8− and attributed to reduction of the Ti and W centers, respectively.5a Figure 4a shows the CV of the redox processes of 1 as a function of the potential scan rate. The reduction peak current correlates linearly with the square root of the potential scan rate (inset of Figure 4a), indicating a diffusion-controlled reduction process. The CV pattern of 2 also shows a well-defined quasi-reversible and composite reduction

Figure 2. (a) Representation of polyanion 1 with incorporated, weakly bound Na+ ions. (b) Central belt of 1, with the unique Na1 occupying an addenda site and Na2, Na3, and Na4 occupying the interstitial sites between Ti1, Ti2, and Na1. Color code: Na1, light blue; Na2, Na3, and Na4, dark blue. Otherwise, the color code is the same as that in Figure 1.

bond lengths in the square plane of 1 range from 1.957(10) to 1.992(10) Å, and the average terminal TiO bond length is 1.665(12) Å. The O−Ti−O angles range from 85.7(4) to 88.2(4)°. The third addenda site in the central belt is occupied by a hexacoordinated sodium ion (Na1) in the solid state (Figure 2). The interstices in the belt are filled with three additional sodium ions (Na2, Na3, and Na4), in complete analogy to earlier polyanions with {M 2 As III 2 W 19 }, {M 3 (As III W 9 ) 2 }, and {M4(AsIIIW9)2} structural types.6 We also performed 183W NMR on Na-1 dissolved in D2O/ H2O (pH 7.5). The resulting spectrum (Figure 3a) consists of

Figure 3. 183W NMR spectra of Na-1 (a) and NaCs-2 (b) dissolved in H2O/D2O at pH 7.5 and 6.6, respectively, recorded at room temperature. Insets: structural cores of 1 and 2 indicating substitution of Na1 with the phenylantimony(III) group. Color code: W, black. Otherwise, the color code is the same as that in Figures 1 and 2. H atoms are omitted for clarity.

five signals at −104.1, −105.6, −109.6, −110.1, and −153.5 ppm, respectively, with the expected relative intensities of 2:2:2:2:1, which is fully consistent with the C2v point group symmetry of polyanion 1 in the solid state. The 183W NMR spectrum of 1 remained unchanged for 2 weeks, indicating that this polyanion is solution-stable. Polyanion 1 is related to our previously reported [Ti2(OH)2As2W19O67(H2O)]8− and [Ti6(TiO6)(AsW9O33)4]20−.5a,e All three polyanions 1, {Ti2As2W19}, and {Ti7As4W36} can be prepared by the reaction of TiOSO4 with [αAsIIIW9O33]9− but at different reagent ratios (1.25:1 vs 4:1 vs 2.2:1), pH (6.0 vs 4.0 vs 4.6), and solvent (2 M NaCH3COO/ CH3COOH vs H2O vs 1 M NaCH3COO/CH3COOH). 10531


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ACKNOWLEDGMENTS U.K. thanks the German Research Council (Grant DFG KO2288/20-1), Jacobs University, the Chinese Academy of Sciences President’s International Fellowship Initiative (Grant 2015VMA041), and CMST COST Action CM1203 (PoCheMoN) for support. K.-Y.W. acknowledges the China Scholarship Council for a doctoral fellowship. G.Z. thanks the National Natural Science Foundation of China (grant no. 21371173). C.S. acknowledges financial support from the National Research Council of Romania (Grant PN-II-ID-PCE-2011-3-0933). Figures 1 and 2 were generated by Diamond, version 3.2k (copyright Crystal Impact GbR).

Figure 4. CVs as a function of the scan rate for 0.086 mM 1 (a) and 2 (b) in 1 M LiCH3COO/CH3COOH (pH 6 and pH 4 for 1 and 2, respectively). Inset: Variation of the cathodic peak current intensity of the main reduction peak as a function of the square root of the scan rate.

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wave, followed by a wave close to the solvent limit peaking respectively at −0.760 and ca. −0.920 V versus SCE (Figure S11). Figure 4b exhibits the CV of the first redox processes of 2 as a function of the potential scan rate. The peak current varies linearly with the square root of the scan rate, as expected for diffusion-controlled processes (inset of Figure 4b). We also performed preliminary qualitative tests of 1 on the electrocatalytic activity toward nitrate reduction. Several examples of POM-based nitrate reduction electrocatalysts have been reported, but the challenge remains to develop efficient POM electrocatalysts in media at pH > 4.7 Figure S12a shows that 1 is an efficient nitrate reduction electrocatalyst in the pH 6 acetate medium. Moreover, the linear correlation between the nitrate concentration and current (Figure S12b) indicates that 1 is a promising candidate for nitrate quantification. In conclusion, we have synthesized the novel sandwich-type Ti2-containing 18-tungsto-2-arsenate(III) 1 by interaction of the [TiO]2+ species with the trilacunary POM precursor [αAsIIIW9O33]9−. Polyanion 1 was then successfully used as a monolacunary host for the incorporation of a phenylantimony(III) guest, resulting in 2. Both polyanions 1 and 2 were characterized in the solid state by single-crystal XRD, Fourier transform infrared (FTIR), thermogravimetric analysis (TGA), and elemental analysis and in solution by UV−vis and NMR spectroscopy, as well as electrochemistry. Polyanion 1 represents a promising monolacunary POM host platform for functionalization by various main-group and d-block metal-ion guests. This work is currently underway, and the respective results will be reported elsewhere in due course.

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REFERENCES

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b01863. CIF file of Na-1 (CCDC 1413828) (CIF) CIF file of NaCs-2 (CCDC 1413829) (CIF) Experimental section, bond-valence-sum calculations, FTIR, UV−vis, TGA, 1H and 13C NMR, electrochemistry, and elemental analysis (PDF)

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Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. ▽ Retired from Université Paris-Sud, Laboratoire de ChimiePhysique, UMR 8000 CNRS, Orsay, F-91405, France 10532
