Two New Sandwich-Type Manganese {Mn5}-Substituted

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Two New Sandwich-Type Manganese {Mn5}-Substituted Polyoxotungstates: Syntheses, Crystal Structures, Electrochemistry, and Magnetic Properties Rakesh Gupta,† Imran Khan,† Firasat Hussain,*,† A. Martin Bossoh,‡,§ Israel̈ M. Mbomekallé,‡ Pedro de Oliveira,*,‡ Masahiro Sadakane,∥ Chisato Kato,⊥ Katsuya Ichihashi,⊥ Katsuya Inoue,⊥ and Sadafumi Nishihara⊥ †

Department of Chemistry, University of Delhi, North Campus, 110007 Delhi, India Laboratoire de Chimie Physique, UMR 8000, CNRS, Université Paris-Sud, Orsay F-91405, France § Université Félix Houphouët-Boigny, 01 BP V34 Abidjan 01, Ivory Coast ∥ Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi, Hiroshima 732-8527, Japan ⊥ Graduate School of Science & Center for Chiral Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi, Hiroshima 732-8526, Japan ‡

S Supporting Information *

ABSTRACT: Herein we report two pentanuclear Mn II -substituted sandwich-type polyoxotungstate complexes, [{Mn(bpy)}2Na(H2O)2(MnCl)2{Mn(H2O)}(AsW9O33)2]9− and [{Mn(bpy)}2Na(H2O)2(MnCl){Mn(H2O)}2(SbW9O33)2]8− (bpy = 2,2′-bipyridine), whose structures have been obtained by single-crystal X-ray diffraction (SCXRD), complemented by results obtained from elemental analysis, electrospray ionization mass spectrometry, Fourier transform infrared spectroscopy, and thermogravimetric analysis. They consist of two [B-α-XW9O33]9− subunits sandwiching a cyclic assembly of the hexagonal [{Mn(bpy)}2Na(H2O)2(MnCl)2{Mn(H2O)}]9+ and [{Mn(bpy)}2Na(H2O)2(MnCl){Mn(H2O)}2]10+ moieties, respectively, and represent the first pentanuclear MnII-substituted sandwich-type polyoxometalates (POMs). Both compounds have been synthesized by reacting MnCl2·4H2O with trilacunary Na9[XW9O33]·27H2O (X = AsIII and SbIII) POM precursors in the presence of bpy in a 1 M aqueous sodium chloride solution under mild reaction conditions. SCXRD showed that the alternate arrangement of three five-coordinated MnII ions and two six-coordinated MnII ions with an internal Na cation formed a coplanar six-membered ring that was sandwiched between two [B-α-XW9O33]9− (X = AsIII and SbIII) subunits. The results of temperaturedependent direct-current (dc) magnetic susceptibility data indicated ferromagnetic interactions between Mn ions in the cluster. Moreover, alternating-current magnetic susceptibility measurements with a dc-biased magnetic field showed the existence of a ferromagnetic order for both samples. Electrochemistry studies revealed the presence of redox processes assigned to the Mn centers. They are associated with the deposition of material on the working electrode surface, possibly MnxOy, as demonstrated by electrochemical quartz crystal microbalance experiments.

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INTRODUCTION Polyoxometalates (POMs) are composed of early transition metal−oxygen clusters with a large structural variety and a wide range of physicochemical features. As a consequence, their inherent properties have given rise to various applications in the fields of catalysis, medicine, photochemistry, and materials science.1−9 Transition-metal-substituted polyoxotungstates © XXXX American Chemical Society

(TMSPs) constitute a very vast class, within which sandwichtype TMSP complexes represent a prominent subclass. In recent years, multinuclear TMSPs have been receiving increasing attention in the fields of catalysis, electrochemistry, and magnetism Received: December 9, 2016

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DOI: 10.1021/acs.inorgchem.6b02994 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. (a) Combined ball-and-stick and polyhedral representation of the polyanions: [{Mn(bpy)}2Na(H2O)2(MnCl)2{Mn(H2O)}(AsW9O33)2]9−. (b) Ball-and-stick representation of the hexagonal [{Mn(bpy)}2Na(H2O)2(MnCl)2{Mn(H2O)}]9+ moiety. Color code: orange, Mn; violet, W polyhedra; bright green, X; red, O; blue, N; yellow, C; rose, H; green, Cl; aqua, H2O; olive green, Na.

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because of their highly tunable nature and coupled properties.10−17 The higher-nuclear TMSPs with a close-packed Keggin anion in which the heteroatom has XO4 (X = PV, SiIV, and GeIV) tetrahedral geometry are well-known.18−26 The Mn metal ion also forms multinuclear complexes such as [MnIII13MnIIO12(PO 4 ) 4 (PW 9 O 3 4 ) 4 ] 3 1 − , [Mn 8 (H 2 O) 4 8 P 8 W 4 8 O 1 8 4 ] 2 4 − , [MnIII40P32WVI224O888]144−, and [Mn19(OH)12(SiW10O37)6]34−,27−36 with the close-packed Keggin anion. Notably, not many highernuclear (5 and over) complexes with Keggin anions in which the heteroatom XO3 (X = AsIII, SbIII, and BiIII) moiety bears a lone pair of electrons have been described. The synthesis of higher-nuclearity TMSPs still remains a challenge. However, a few examples, mainly pentanuclear,37 hexanuclear38−45 and nonanuclear46 based on CuII and ZnII-substituted sandwich-type POMs, have been reported. It is worth noting that paramagnetic multinuclear manganesesubstituted complexes are attractive in numerous fields.47−51 Moreover, there is development of manganese complexes exhibiting single-molecule magnet (SMM) behavior.52−62 To date, a few examples of Mn-ion-substituted complexes, such as dinuclear [(α-AsW 9 O 3 3 ) 2 WO(H 2 O)Mn 2 (H 2 O) 2 ] 1 0− , 6 3 [Mn2(H2O)6(WO2)2(SbW9O33)2]10−,64 and [(Mn(H2O)3)2(WO) 2 (β-BiW 9 O 33 ) 2 ] 10− 65 and trinuclear [As2 W 18 {Mn(H2O)}3O66]12−,66 [(MnIII(H2O))3(SbW9O33)2]9−, and [MnII(H2O)Mn2(AsW9O33)2]12−,67 were reported. Yamase et al. reported MnII-substituted sandwich-type polyoxotungstates [(MnCl)6(XW9O33)2]12− (X = AsIII and SbIII) and characterized their magnetic properties.38,44 The synthetic strategy to obtain novel polynuclear TMSPs with new structural varieties and manifold properties continues to be a focus of considerable ongoing research. We are interested in synthesizing polynuclear complexes in the presence of organic moieties, which may lead to isolating TMSP species with new structural functionalities. Our research work mainly concerns the use of simple, one-pot synthesis procedures under ambient reaction conditions in aqueous media. Herein, we report on two pentanuclear MnII-substituted sandwich-type polyoxotungstate complexes, [{Mn(bpy)}2Na(H2O)2(MnCl)2{Mn(H2O)}(AsW9O33)2]9− (1) and [{Mn(bpy)}2Na(H2O)2(MnCl){Mn(H2O)}2(SbW9O33)2]8− (2) (bpy = 2,2′-bipyridine), which consist of two [B-α-XW9O33]9− subunits sandwiching a cyclic assembly of the hexagonal [{Mn(bpy)}2Na(H2O)2(MnCl)2{Mn(H2O)}]9+ and [{Mn(bpy)}2Na(H2O)2(MnCl){Mn(H2O)}2]10+ moieties, respectively, which represent the first examples of the series to date (Figure 1).

EXPERIMENTAL SECTION

Syntheses and Structure. The synthesis routes of both complexes are very similar: a dark-brown block-shaped crystalline material has been prepared by the one-pot reaction of MnCl2·4H2O, trilacunary Na9[XW9O33]·27H2O (X = AsIII and SbIII), and 2,2′-bipyridine (bpy) in a molar ratio of 3:1:1 in a 1 M sodium chloride solution under mild reaction conditions. Both molecular clusters were well characterized by single-crystal X-ray diffraction (SCXRD), Fourier transform infrared (FT-IR) spectroscopy, magnetism, electrochemistry, thermogravimetric analysis (TGA), electrospray ionization mass spectromtery (ESI-MS), and elemental analysis. The purities of compounds 1a and 2a , which are the sodium salts of compounds 1 and 2, were confirmed by elemental analysis. SCXRD. SCXRD analysis of the polyanions reveals that both compounds are isostructural and crystallize in the triclinic crystal system of space group P1.̅ The polyanion 1a consists of two [B-α-XW9O33]9− subunits that sandwich the hexagonal [{Mn(bpy)}2Na(H2O)2(MnCl)2{Mn(H2O)}]9+ moiety, resulting in a sandwich-type species with idealized quasi-C2 symmetry. The {XW9O33} units consist of a central XO3 tetrahedron group, which is surrounded by three cornersharing W3O13 trimers. The X−O bond length varies from 1.782(2) to 2.02(2) Å. The central belt of the polyanion is composed of a cyclic assembly consisting of the hexagonal [{Mn(bpy)}2Na(H2O)2(MnCl)2{Mn(H2O)}]9+ moiety in which the MnII ions have two different coordination environments. It is to be noted that, depending on the coordination site, the five MnII atoms can be divided into two groups. One group is [(MnCl)2{Mn(H2O)}]4+ in which three internal MnII atoms (Mn2, Mn4, and Mn5) are five-coordinated and have square-pyramidal geometries. The coordination sites of the latter two MnII atoms (Mn4 and Mn5) are satisfied by four O atoms of two {AsW9O33} units [Mn−OPOM = 2.071(2)−2.183(17) Å] and one terminal chloride ligand [Mn−Cl = 2.375(13) and 2.408(16) Å], whereas the other MnII atom (Mn2) is coordinated with four O atoms of two {AsW9O33} units [Mn−OPOM = 2.085(15)−2.113(15) Å] and one exterior water molecule [Mn−OH2O = 2.139(16) Å]. The other group is [{Mn(bpy)}2]4+, in which two MnII atoms (Mn1 and Mn3) are six-coordinated and the coordination sites are satisfied by four O atoms of two {AsW9O33} units [Mn−OPOM = 2.194(16)−2.251(16) Å] and two N atoms of one bpy ligand [Mn−N = 2.22(2)−2.26(2) Å]. The coordination environment for the polyanion 2a is similar to that of 1a with one exception: a water molecule is coordinated with Mn4 instead of a chloride ion (Figure 2). The alternate arrangement of three five-coordinated MnII ions and two six-coordinated MnII ions with an internal Na cation forms a coplanar sixmembered ring. Furthermore, the Na cation is bound to terminal water molecules and to O atoms coordinated to Mn4 and Mn5. Thus, the Na cation acts as a linker and plays a key role in the construction and stability of the structure because of its size, as shown in Figure 2. In the presence of potassium or cesium salts, the same compounds are formed, but with low yield and poor crystal quality. We have also performed both reactions B


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Inorganic Chemistry

water molecules; (ii) a second step starting at around 400 °C, attributed to the departure of the bpy ligands (Figure S7). ESI-MS. High-resolution ESI-MS was performed with both compounds after each sample was dissolved in a CH3CN/H2O solution. Parts a and b of Figure 3 show ESI-MS spectra of Na8[{Mn(bpy)}2Na(H2O)2(MnCl){Mn(H2O)}2(SbW9O33)2 ]. Signals assignable to [Na3{Mn2Mn3(SbW9O33)2}]4−, where five Mn atoms are sandwiched by two [SbW9O33] units without any extra ligand to Mn, are observed. This result indicates that the POM structure is stable under the operating ESI-MS conditions. Some fragment signals assignable to [W3O10]2−, [W4O13]2−, [W5O16]2−, and [W6O19]2− are also observed. On the other hand, only some peaks assignable to [AsW9O30]3−, [HAsW9O30]2−, [AsW6 O20] − , [AsW5O17] −, [W3 O10] 2− , [W 4O13 ] 2−, [W5 O16] 2− , [W6O19]2−, and [HW6O19]− are observed in the case of the arsenic derivative (Figure 3c). Signals of [AsW9O30]3− ([H6AsW9O33]3−-3H2O), [HAsW9O30]2− ([H7AsW9O33]2−-3H2O), [AsW6O20]− ([H8AsW9O33]−-4H2O-3WO3), [AsW5O17]− ([H8AsW9O33]−-4H2O4WO3) are assignable to fragments of [AsW9O33]9−. The arsenic derivative is not stable under the ESI-MS conditions. Magnetic Properties. The direct-current (dc) magnetic susceptibility measurements were carried out over a temperature range of 1.8−300 K in a 0.5 T magnetic field on polycrystalline samples. The molar susceptibilities (χM) for 1a and 2a, subtracting diamagnetic susceptibilities of −0.00551 and −0.00197 emu mol−1, respectively, are plotted as χMT versus T in Figure 4a. When the temperature is lowered, the χMT values for both compounds remain quite constant and then increase at around 50 K. At even lower temperatures, they decrease rapidly because of saturation of the magnetization. The χMT curves for 1a and 2a at higher temperatures (>150 K) are consistent with the Curie−Weiss law, with C = 18.5 and 18.1 emu K mol−1 and θ = 9.89 and 15.0 K, respectively. Both of the estimated Curie constants are a little lower than the spin-only value (g = 2) of 21.9 emu K mol−1 for five MnII ions (S = 5/2). The positive Weiss temperatures indicate ferromagnetic interactions between MnII ions. These values correspond to those of the previously reported compounds having a similar spin configuration.38 Although we measured the temperature-dependent alternating-current (ac) magnetic susceptibility in order to investigate the possibilities for ferromagnetic order or SMMs, the typical behaviors for such materials were not observed above 2.0 K under a zero dc magnetic field (Figure S8). Frequency-independent broad peaks for 1a and 2a appeared at ∼4.5 and 3.0 K, respectively, on the χ′ versus T curves with a static bias field of 0.50 T (Figure S9a). The peaks of both samples shifted to higher temperatures as the bias field increased. Peak-top temperatures of the χ′ curve for 1a were observed at ∼6.0 K at 0.75 T and 7.5 K at 1.0 T, and for 2a, the corresponding temperatures were 4.0 K at 0.75 T and 5.5 K at 1.0 T (Figure S9b,c). These results implied that the biased magnetic field improved the ferromagnetic ordering temperature and indicated the existence of a ferromagnetic order below 2 K even under a zero dc magnetic field for both samples. The isothermal magnetizations of 1a and 2a were measured at 2.0 K in the field between −50 and 50 kOe (Figure 4b). The magnetization of both compounds showed a rapid increase for the low fields and then reached 50 kOe at a saturation magnetization of ca. 20 μB, which is slightly smaller than the theoretical value of 25 μB for five MnII ions (S = 5/2 and g = 2). Cyclic Voltammetry. At pH 5.0, the two lacunary fragments AsW9 and SbW9 are stable enough to be characterized by cyclic voltammetry. Their cyclic voltammograms (CVs) were compared with those of compounds 1 and 2, respectively, recorded in the same experimental conditions (Figures S10). It stands out that the presence of the MnII centers in the structure of the two compounds has a very limited influence on the redox processes attributed to the WVI centers. In fact, when the potential is scanned toward the negative side, the redox processes observed between +0.2 and −1.0 V versus saturated calomel electrode (SCE) in the CVs of the four compounds compared in pairs (Figures S10) look alike and are close to being superimposable and reversible. However, when the potential positive side is explored between +0.2 and +1.25 V versus SCE, the CVs of 1 and 2 exhibit new redox processes assigned to the MnII centers. We were particularly interested in these redox waves because they are not present in the CVs of the lacunary fragments AsW9 and SbW9.

Figure 2. Ball-and-stick representation of the hexagonal moiety for the polyanion: (a) 1a; (b) 2a. Bond lengths are in angstroms. in a 2 M sodium chloride solution and found that, in the case of compound 2a, one water molecule can also be exchanged with a chloride ion at the Mn4 atom in the solid state (Figure S5). This clearly signifies that the exchange phenomenon between chloride and water is relatively fast in solution and the predominant species obtained in the solid state depends on the concentration of chloride. Further, we observed that Mn2 is exclusively coordinated to water in both the polyanions 1a (Mn2−O68) and 2a (Mn2−O141), as well as in the polyanion isolated from a more concentrated sodium chloride solution, which may be due to steric hindrance. Bond-valence-sum (BVS) calculations68,69 show that neither the O atoms in the two {XW9O33} units nor the bridging O atoms in Mn−O−W are protonated. The BVS values of the terminal O atoms (O68 or O141), coordinated with the central Mn2 atom, are found to be 0.39 or 0.47, respectively, which suggests that these O atoms are in water molecules. In the case of 2a, one terminal water molecule (O140) is also coordinated with Mn4, which can be exchanged with a chloride ion in a 2 M NaCl solution. The BVS value for this O atom is found to be 0.26. The BVS values of the O atoms that act as bridges in W−O−W or Mn−O−W are greater than 1.5, which suggests that they are oxo species. In addition, we also calculated BVS values for the Mn, W, and X (X = AsIII, SbIII) atoms and found them in the range of 1.89−2.33 for the Mn atoms, in the range of 6.02−6.37 for the W atoms, and in the range of 2.88−3.10 for the X atoms. So, we conclude that the atoms of the elements Mn, W, and X are present in the 2+, 6+, and 3+ oxidation states, respectively. FT-IR Spectroscopy and TGA. The FT-IR spectra of both compounds show an overall similar stretching frequency pattern with slight shifts in the positions of the vibrational bands, and their comparison clearly indicates that both polyanions are isomorphous (Figure S6). The TGA curves reveal a similar trend shared by 1a and 2a: (i) a progressive weight decrease up to 300 °C, associated with the loss of C


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Inorganic Chemistry

Figure 3. (a) Observed negative ESI-MS spectrum of Na8[{Mn(bpy)}2Na(H2O)2(MnCl){Mn(H2O)}2(SbW9O33)2]. (b) Expanded region and simulated peaks for [Na3Mn2Mn3(SbW9O33)2]4−. (c) Observed negative ESI-MS spectrum of Na9[{Mn(bpy)}2Na(H2O)2(MnCl)2{Mn(H2O)}(AsW9O33)2]. +0.71 and +0.86 V versus SCE and a reduction wave with a single step peaking at +0.59 V versus SCE. Beyond this oxidation wave and when higher potential values are reached (+1.45 V vs SCE), another process is observed that corresponds to the formation of a deposit on the working electrode (Figure 5b), characterized by a progressive increase of the peak current of this irreversible oxidation wave.70 A study of the dependence of the peak current on the square root of the scan rate (Figure S11) seems to confirm that the oxidation of the MnII centers in compound 1 on an edge-plane pyrolytic graphite (EPG) electrode at pH 5.0 remains by and large a diffusion-controlled electrochemical process for potential scans not reaching values beyond +1.2 V versus SCE. The behavior of compound 2 shows marked differences with respect to that of 1 in the same experimental conditions. In fact, even if there are some resemblances between the CVs as far as the waves attributed to the WVI centers are concerned (see above and Figure S12a), the response obtained for the redox processes assigned to the MnII centers in the case of compound 2 is distinct from that observed for compound 1 already described above. Figure 6 reveals that whereas oxidation of the MnII centers of compound 1 comprises two steps (blue CVs), that of compound 2 happens in a single step (pink CVs). There is also a steady increase in the peak current upon consecutive scanning, indicative of the formation of a deposit of manganese oxides on the surface of the working electrode. At pH 6.0, the whole CV shifts to the left, as expected, with the W waves peaking at more negative potentials and the Mn waves peaking at less positive potentials.71 Interestingly, oxidation of the MnII centers for compound 1 occurs now in a single step, related to the formation of a deposit at potential values smaller than +1.0 V versus SCE, whereas at pH 5.0, potential values well beyond this threshold had to be reached in order to observe the formation of a deposit on the working electrode (Figure 7). The fact that the two oxidation steps observed at pH 5.0 merge and become a single wave at pH 6.0 may be explained by the acid−base properties of the POM. Electron transfer in POMs is very often concomitant with proton exchange, resulting in changes in the shapes and positions of the redox waves.

Figure 4. (a) Plot of the magnetic susceptibility (χMT) versus temperature (T) for 1a and 2a. (b) Field dependence of magnetization for 1a and 2a at 2.0 K. In the case of the redox processes attributed to the MnII centers and, in particular, for compound 1 at pH 5.0, no deposit seems to form on the surface of the working electrode upon consecutive scannings between +0.2 and +1.05 V versus SCE (Figure 5a). However, in many cases reported in the literature, oxidation of the MnII centers present in POMs encompasses the formation of insoluble oxides on the surface of the working electrode.48 In the present case, there is an oxidation wave composed of two distinct consecutive steps peaking at D


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Inorganic Chemistry

Figure 5. CVs of 1 in 1.0 M LiCH3COO + CH3COOH/pH 5.0. (A) Potential scans comprised between +0.2 and +1.05 V. (B) Potential scans comprised between +0.2 and +1.45 V. POM concentration: [1] = 2 × 10−4 mol L−1. Working electrode: EPG. Counter electrode: platinum. Reference electrode: SCE. Scan rate: 10 mV s−1. oxidation of the MnII centers in compound 1 is not concomitant to the formation of a stable manganese oxide film on the surface of the working electrode. Analysis of the curves showing variation of the vibrational frequency (ΔF) of the quartz crystal resonator (which constitutes the support of the carbon electrode in the present EQCM) as a function of the scanned potential reveals that, for both cases (the arsenic- and antimony-containing POMs), a deposit is formed as soon as oxidation of the MnII centers starts, i.e., at +0.80 V versus SCE (Figure 8). When the magnitudes of the current densities (J) and ΔF variations are compared, it is clear that oxidation of compound 2 leads to the formation of a more important deposit than that in the case of its homologous compound 1 (compare parts a and b of Figure 8). The subsequent results obtained upon scanning of the potential revealed different behaviors between the two compounds. The formation of the deposit starts at the same potential for both POMs, at +0.80 V versus SCE, during the anodic scan, and carries on even after the scan direction is reversed at +1.6 V versus SCE. During the cathodic scan, reduction of the deposited manganese oxides and their concomitant detachment from the electrode surface takes place at around +0.50 V versus SCE. In the case of compound 1, this process is close to being fully reversible, resulting in the fact that the vibrational frequency almost recovers the magnitude recorded at the beginning of the experiment. This outcome confirms the previously drawn conclusion that the manganese oxide deposit completely redissolved upon reduction, with the surface of the working electrode remaining unchanged upon consecutive cycling. As far as compound 2 is concerned, redissolution of the manganese oxides upon reduction is not total, and the deposit keeps growing as the potential is cycled. EQCM was helpful in elucidating the existence of different electrochemical behaviors between the two compounds in the medium 1.0 M LiCH3COO at pH 5.0.

Figure 6. CVs of 1 (blue) and 2 (pink) in 1.0 M LiCH3COO + CH3COOH/pH 5.0. Potential scans comprised between 0.2 and +1.05 V. POM concentration: [1] = [2] = 2 × 10−4 mol L−1. Working electrode: glassy carbon. Counter electrode: platinum. Reference electrode: SCE. Scan rate: 10 mV s−1. At pH 6.0, the redox behaviors of the two compounds 1 and 2, with respect to both the reduction processes of the WVI centers and the oxidation processes of the MnII centers, are rather similar, as evidenced by the CVs shown in Figures S13 and S14. EQCM. EQCM provides additional information on the results described above that suggest that, in a LiCH3COO medium at pH 5.0,

Figure 7. (A) CVs of 1 at pH 5.0 (blue) and pH 6.0 (red). (B) CVs of 1 at pH 6.0 for potential scans comprised between 0.2 and +1.05 V. Electrolyte: 1.0 M LiCH3COO + CH3COOH. POM concentration: [1] = 2 × 10−4 mol L−1. Working electrode: EPG. Counter electrode: platinum. Reference electrode: SCE. Scan rate: 10 mV s−1. E


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Figure 8. CVs and ΔF as a function of the scanned potential for (A) 1 and (B) 2. Potential scans comprised between +0.2 and +1.6 V in the forward scan and down to −0.7 V in the backward scan. Scan rate: 10 mV s−1. Electrolyte: 1.0 M LiCH3COO + CH3COOH/pH 5.0. POM concentration: [POM] = 2 × 10−4 mol L−1. Working electrode: carbon (on the quartz crystal resonator). Counter electrode: platinum. Reference electrode: SCE.

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At pH 6.0, 1 exhibits the same behavior as 2: they both give rise to a deposit that will not totally redissolve upon reduction (Figure S15).

ASSOCIATED CONTENT

* Supporting Information

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02994. Additional experimental details, crystallographic data for 1a and 2a, additional structural figures, FT-IR spectra, TGA curves, and additional figures of ESI-MS, magnetic properties, and CV and EQCM (PDF)

CONCLUSIONS In conclusion, we have synthesized two novel pentanuclear MnII-substituted sandwich-type polyoxotungstate complexes, [{Mn(bpy)}2Na(H2O)2(MnCl)2{Mn(H2O)}(AsW9O33)2]9− and [{Mn(bpy)}2Na(H2O)2(MnCl){Mn(H2O)}2(SbW9O33)2]8−, following a one-pot reaction procedure under mild reaction conditions. Solid crystalline materials of sodium salts were well characterized with different analytical techniques including SCXRD, elemental analysis, ESI-MS, FT-IR spectroscopy, and TGA; these isomorphous POMs consist of two [B-α-XW9O33]9− subunits sandwiching a cyclic assembly [{Mn(bpy)} 2 Na(H 2 O) 2 (MnCl) 2 {Mn(H 2 O)}] 9+ and [{Mn(bpy)}2Na(H2O)2(MnCl){Mn(H2O)}2]10+ moieties, respectively. The temperature dependence of the dc magnetic susceptibilities showed a ferromagnetic interaction between Mn ions. Moreover, the ac susceptibility data with a dc-biased magnetic field implied the existence of a ferromagnetic order for both samples. As far as the electrochemical behavior is concerned, the response due to the W centers in the lacunary ligands, on the negative side of the potential axis, is hardly affected by the presence of the Mn centers when they are both present in the POMs 1 and 2. Redox processes assigned to the Mn centers are also observed on the positive side of the potential axis. They are associated with the deposition of material, possibly MnxOy, on the working electrode surface, as demonstrated by EQCM experiments. These manganese-containing POMs may turn out to be promising catalysts for the electrooxidation of water. To the best of our knowledge, pentanuclear MnII-substituted sandwich-type POMs are reported for the first time. We are currently exploring this work with other transitionmetal ions.

Accession Codes

CCDC 1520936−1520937 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: +91 11 27666646. Fax: +91 11 2766 6605. *E-mail: [email protected]. Tel: +33 1 69 15 47 34. Fax: +33 1 69 15 61 88. ORCID

Firasat Hussain: 0000-0003-0639-7638 Israel̈ M. Mbomekallé: 0000-0003-3440-8066 Masahiro Sadakane: 0000-0001-7308-563X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS F.H. is grateful to the Department of Chemistry, University Scientific Instrumentation Centre, University of Delhi, New Delhi, F


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Inorganic Chemistry

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India, for providing instrumental facilities and IIT Bombay for ICP-AES. F.H. acknowledges University of Delhi for DU-DST Purse grant phase (II) for financial assistance. R.G. thanks the CSIR, New Delhi, India, for financial support. We thank T. Amimoto at the Natural Science Center for Basic Research and Development, Hiroshima University, for ESI-MS measurements. M.S. and S.N. are thankful for Grants-in-Aid for Scientific Research (C) 26420787 and (B) 16H04223, respectively, from the Ministry of Education, Culture, Sports, and Science of Japan. A.M.B. thanks the French Embassy in Ivory Coast for his scholarship and Dr. Bernadette Avo Bile from the University Félix Houphouët-Boigny for cosupervision. I.M.M. and P.d.O. thank the Université Paris-Sud and CNRS for financial support.

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