Co(II)-substituted heteropolymolybdates

Solid State Sciences 11 (2009) 1433–1438

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Electrochemical and magnetic properties of inorganic polymers constructed from Mn(II)/Co(II)-substituted heteropolymolybdates Xizheng Liu, Guanggang Gao, Lin Xu*, Fengyan Li, Li Liu, Ning Jiang, Yanyan Yang Key Laboratory of Polyoxometalate Science of Ministry of Education, Department of Chemistry, Northeast Normal University, No. 5268 Remin Street, Changchun, Jilin 130024, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 3 January 2009 Received in revised form 19 April 2009 Accepted 24 April 2009 Available online 5 May 2009

Two inorganic polymers constructed from transition metal-substituted heteropolymolybdates, [(CH3)3NH]5n[PMo11MO39]n$xH2O (M ¼ Mn2þ, x ¼ n (1); M ¼ Co2þ, x ¼ 2n (2)), have been synthesized in aqueous solutions and characterized by IR, TGA, and single-crystal X-ray diffraction analysis. Crystal data: 1, monoclinic, C2/c, a ¼ 17.1322(7) Å, b ¼ 17.6062(7) Å, c ¼ 17.6459(7) Å, b ¼ 103.2220(10) , V ¼ 5181.5(4) Å3 and Z ¼ 4; 2, triclinic, P-1, a ¼ 12.1986(7) Å, b ¼ 13.0973(7) Å, c ¼ 16.7736(9) Å, a ¼ 97.1810(10) , b ¼ 98.5040(11) , g ¼ 96.3920(10) , V ¼ 2606.5(2) Å3 and Z ¼ 2. The cyclic voltammograms of 1 and 2 show irreversible redox peaks in DMF solution and there are three reversible couples after addition of 0.1 M H2SO4 aqueous solutions. The cyclic voltammograms of 1/2-modified carbon paste electrode (1-CPE/2-CPE) show two consecutive reversible two-electron redox processes. Especially, 2-CPE shows good electrocatalytic activity toward the reduction of nitrite and hydrogen peroxide. The magnetic properties of the two complexes have also been investigated. Ó 2009 Elsevier Masson SAS. All rights reserved.

Keywords: Inorganic polymer Heteropolymolybdate Magnetic properties Electrochemical properties

1. Introduction Polyoxometalates (POMs), as a well-known class of metal oxygen clusters with a large variety of compositions and structures, have attracted rapidly increasing interest because of their potential applications in catalysis, molecular magnetic materials, photochemical materials, medicine, etc [1–3]. Recently, a growing number of 1D chainlike polyoxometalates which are composed of mono-vacant Keggin heteropolytungstates and transition metals have been extensively studied [4–15]. In contrast, the analogous compounds based on mono-vacant heteropolymolybdates are rarely explored [16–21]. So far, the only reported single-crystal X-ray studies of chainlike heteropolymolybdates refer to the compounds [H2bpy]2[Hbpy][PMo11CuO39]$H2O and [H2bpy]2[Hb[22,23]. However, these two py][PMo11CuO39]$2.75H2O compounds were synthesized under hydrothermal conditions and can hardly be dissolved in common solvents, which limit the further research on their properties in solutions. The other reported mono-substituted Keggin heteropolymolybdates which were synthesized under mild conditions in aqueous solutions show discrete structures [16–21]. Therefore, it is still a challenging task to develop a method (under simple and mild conditions) for the

* Corresponding author. E-mail address: [email protected] (L. Xu). 1293-2558/$ – see front matter Ó 2009 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2009.04.028

self-assembly of multidimensional inorganic polymers containing transition metal-substituted (TMS) heteropolymolybdates. Our group has reported the first example of chainlike TMS heteropolymolybdates synthesized in aqueous solution [24]. On the basis of our previous work, herein, we report two other 1D chainlike complexes constructed by mono-vacant Keggin heteropolymolybdates, [(CH3)3NH]5n[PMo11MO39]n$xH2O (M ¼ Mn2þ, x ¼ n (1); M ¼ Co2þ, x ¼ 2n (2)). Both the compounds 1 and 2 are synthesized in aqueous solutions under mild conditions in high yields. The redox properties and magnetic properties of 1 and 2 have been investigated. 2. Experimental 2.1. Materials and measurements All reagents were purchased commercially and used without further purification. Mo, Mn and Co were determined by Leaman inductively coupled plasma (ICP) spectrometer. IR spectra were obtained on Alpha Centaurt FT/IR spectrometer with KBr pellets in the range 400–4000 cm1. TG analysis was performed on a Perkin– Elmer TGA7 instrument in flowing N2 with a heating rate of 10  C min1. X-ray powder diffraction (XRPD) experiments were performed in the scattering angle range 2q ¼ 4.0–70 with 0.02 steps on a commercial D/max-gA rotating anode X-ray diffractometer using Cu Ka radiation of a wavelength l ¼ 1.54 Å. The

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electrochemical experiments were performed on CHI 660 Electrochemical Workstation. A conventional three-electrode cell, glass carbon electrode or 1/2-modified carbon paste electrode (1-CPE/ 2-CPE) as the working electrode, a saturated calomel reference electrode (SCE) and a Pt gauze counter electrode were used. All potentials were measured and reported vs the SCE. Magnetic susceptibility measurements (300–2 K) were performed on polycrystalline samples at field strength of 1000 Oe using a Quantum Design MPMS XL-5 SQUID magnetometer. 2.2. Synthesis of 1 and 2 Synthesis of [(CH3)3NH]5n[PMo11MnO39]n$(H2O)n (1). A mixture of Na2MoO4$2H2O (2.097 g), Na2HPO4 (0.197 g), MnSO4 (0.5105 g) and (CH3)3N$HCl (0.274 g) were dissolved in 40 ml H2O. The pH of the solution was adjusted to 4.52 with 1 M HCl. The obtained brown solution was heated to 70  C for 5 h and then filtrated after cooling to room temperature. The brown filtrate was kept at room temperature for two days, and yellow brown crystals were filtered off, washed with distilled water and ethanol and dried in a desiccator to give a yield of 37% based on Mo. Elemental analysis: calc. (found) (%) Mo, 50.64 (51.13), Mn, 2.64 (2.58). Synthesis of [(CH3)3NH]5n[PMo11CoO39]n$(H2O)2n (2). A mixture of Na2MoO4$2H2O (1.113 g), Na2HPO4 (0.185 g), Co(CH3COO)2 (0.252 g) and (CH3)3N$HCl (0.261 g) were dissolved in 40 mL H2O. The pH of the solution was adjusted to 4.87 with 1 M HCl. The obtained red brown solution was heated to 70  C for 5 h and then filtrated after cooling to room temperature. The brown filtrate was kept at room temperature for two days, and deep brown crystals were filtered off, washed with distilled water and ethanol and dried in a desiccator to give a yield of 46% based on Mo. Elemental analysis: calc. (found) (%) Mo, 50.12 (50.63), Co, 2.80 (2.76). 2.3. Single-crystal X-ray diffraction Single-crystal data of 1 and 2 were collected on a Bruker SMART APEX II CCD single-crystal diffractometer using Mo Ka radiation (l ¼ 0.71073 Å). The structures were solved by direct methods and refined by the full-matrix least-squares methods on F2, which were performed using the SHELXTL-97 software package [25]. All of the non-hydrogen atoms were refined anisotropically. The hydrogen atoms were added according to theoretical models. The crystallographic data and structure determination parameters for 1 and 2 are summarized in Table 1. CCDC reference number: 698739 for 1

and 698738 for 2. The crystallographic data for this paper can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. 2.4. Fabrication of 1-CPE and 2-CPE 1-CPE was fabricated as follows: 0.2 g graphite powder was added to the solution of 2 ml ethanol containing 20 mg of 1 and the mixture was ultrasonically mixed for 20 min, followed by evaporation of ethanol, which produced rather homogenously covered graphite particles. To the graphite particles 0.10 ml Nujol was added and stirred with a glass stick. The homogeneous mixture was used to pack a 3 mm inner diameter glass tube to a length of 0.8 cm from one of its ends and the mixture in the tube was pressed lightly on smooth plastic paper with a copper stick through the back. The electrical contact was established with the copper stick. The surface of the carbon paste electrode was wiped with weighting paper. 2-CPE was fabricated similarly. 3. Results and discussion 3.1. Structure description Because 1 and 2 are isostructural, compound 1 is discussed as an example on the structural features. Single-crystal X-ray analysis reveals that the structure of 1 contains mono-substituted Keggin polyanions [PMo11MnO39]5, [(CH3)3NH]þ cations and water molecules. Fig. 1 shows the structure of polyanions [PMo11MnO39]5 in compound 1. Mo–Ot (terminal oxygen) distances range from 1.639 to 1.671 Å. The Mo–Ob (edge-sharing oxygen atoms), and Mo–Oc (corner-sharing oxygen atoms) distances are in the range of 1.834– 1.907 and 1.914–1.973 Å, respectively. The central four oxygen atoms Oa (bonded to three Mo atoms and one P atom), are observed to be disordered over eight positions and thus each O atom was assigned as half occupancy [26–28]. The average Mo–Oa bond distance is 2.459 Å. The average P–O distance is 1.533 Å. These values are consistent with the reported results [20–23]. The Keggin polyanions are connected through a common oxygen atom to give a onedimensional chain as shown in Fig. 2. In each [PMo11MnO39]5 subunit, the Mn and Mo(6) atoms share the same position and were

Table 1 Crystal data and structure refinement for 1 and 2. 1

2

Empirical formula C15H52Mo11N5O40PMn C15H54Mo11N5O41PCo 2083.84 2105.85 Fw Crystal system Monoclinic Triclinic Space group C2/c P1 a (Å) 17.1322(7) 12.1986(7) b (Å) 17.6062(7) 13.0973(7) c (Å) 17.6459(7) 16.7736(9) a (deg) 90 97.1810(10) b (deg) 103.2220(10) 98.5040(11) g (deg) 90 96.3920(10) 5181.5(4) 2606.5(2) V (Å3) Z 4 2 3 Dcalcd (g/cm ) 2.677 2.678 1 2.936 2.997 M (mm ) T (K) 273(2) 273(2) l (Å) 0.71703 0.71073 0.0801, 0.1679 0.0828, 0.1220 Final R1, uR2[I > 2s(I) 0.0869, 0.1714 0.0965, 0.1567 Final R1, uR2 (all data) P P P P Note. R1 ¼ jjFoj  jFcjj/ jFoj, wR2 ¼ { [w(Fo2  Fc2)2]/[ w(Fo2)2]}1/2.

Fig. 1. Representation of the molecular structure unit of 1.

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Fig. 2. A polyhedral view of the 1D chain in 1 or 2. The blue octahedra are MoO6, and yellow octahedra are MnO6 or CoO6.

treated with half occupancy. The occupancy of Mn and Mo(6) were 48.8% and 51.2% after refining. (For compound 2, atoms Mo(4), Co(1), Mo(7) and Co(2) were refined as mixed atoms as Mo(4) (48.2%), Co(1) (51.2%), Mo(7) (48.2%) and Co(2) (51.2%).) This situation is common in other straight-chainlike compounds such as in [ET]8[MnW11PO39]$2H2O [29] and [Cu(en)2(OH2)]2[H2en][{Cu(en)2} P2CuW17O61]$5H2O [4]. In the reported TMS heteropolymolybdates [H2bpy]2[Hbpy][PMo11CuO39]$H2O and [H2bpy]2 [Hbpy] [PMo11ZnO39]$2.75H2O, Cu or Zn with a full occupation and Mo are connected by bridging oxygen atoms to form zigzag chainlike structure [23]. 3.2. Thermal analysis The thermal gravimetric (TG) curves of 1 and 2 are shown in Fig. 3. For 1, the weight loss of 1% at 30–255  C corresponds to the loss of one water molecule per unit (calc. 0.9%). The weight loss of 18.2% at 255–515  C is attributed to the loss of organic molecules and P2O5 (calc. 17.8%). Similarly, compound 2 lost two water molecules per unit (2.1%, calc. 1.7%) from 30 to 250  C, and then lost its organic molecules and P2O5 (16.9%, calc. 17.6%) at 250–530  C.

above electrolytes, three reversible redox peaks can be observed and the peak potentials E1/2 (E1/2 ¼ (Epa þ Epc)/2) are 0.20 V (I1  I11), 0.065 V (I2  I22) and 0.22 V (I3  I33) for 1 (0.20 V, 0.070 V and 0.22 V for 2) (see Fig. 4b). These three peaks can be ascribed to three consecutive one-electron redox processes of the mono-substituted Keggin heteropolymolybdate framework. Figs. 5a and 6a present the CV behaviors of 1-CPE and 2-CPE in 0.5 M H2SO4 aqueous solution at a scan rate of 0.3 V s1. In the potential range of 0.6–0 V, two reversible redox peaks can be observed and the peak potentials E1/2 are 0.36 V and 0.21 V for 1 (0.37 and 0.22 V for 2). These peaks are ascribed to two consecutive two-electron redox processes of the mono-substituted Keggin heteropolymolybdate framework. At different scan rates from 0.1 to 0.4 V s1 (see Figs. 5b and 6b), the peak potentials do not change notably and the peak currents are almost proportional to the square

3.3. Cyclic voltammetry Different from the reported 1D chainlike TMS heteropolymolybdates which were synthesized under hydrothermal conditions, compounds 1 and 2 are soluble in common organic solvents, which provide us an opportunity to study the cyclic voltammetric (CV) behaviors of 1 and 2 in solutions. Fig. 4a shows the CV curves of 1 and 2 in DMF solution (0.03 mol L1 Bu4NClO4) at a scan rate of 0.2 V s1. In the potential range of 1.0 to 0.5 V, an anodic peak appears at 0.24 V and two cathodic peaks appear at 0.12 and 0.079 V for 1 (0.24, 0.12 and 0.056 V for 2). This result is similar to the reported CV curves of (Bu4N)4H3PMo11O39 in CH3CN [30]. After addition of 6% (v/v) 0.1 M H2SO4 aqueous solution to the

Fig. 3. The TG curve for compounds 1 and 2.

Fig. 4. Cyclic voltammograms of 4.67  104 mol L1 1 and 2 in DMF solution using (a) 0.03 mol L1 Bu4NClO4 and (b) 0.03 mol L1 Bu4NClO4 and addition of 6% (v/v) 0.5 M H2SO4 aqueous solution; (scan rate 0.2 V s1 vs SCE for both (a) and (b)).

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Fig. 5. Cyclic voltammograms of 1-CPE, in 0.5 M H2SO4: (a) scan rate 0.3 V s1 vs SCE; (b) scan rate: 0.10, 0.12, 0.14, 0.16, 0.18, 0.20, 0.22, 0.24, 0.26, 0.28, 0.30, 0.32, 0.34, 0.36, 0.38, 0.40 V s1, potentials vs SCE.

root of the scan rates, indicating that the electrode reactions of 1-CPE and 2-CPE are diffusion-controlled processes (see Figs. S3a and S3b, Supplementary data). The different CV behaviors in the solutions and CPEs indicate that the proton plays an important role in the redox processes of 1 and 2.

Fig. 6. Cyclic voltammograms of 2-CPE, in 0.5 M H2SO4: (a) scan rate 0.3 V s1 vs SCE; (b) scan rate: 0.10, 0.12, 0.14, 0.16, 0.18, 0.20, 0.22, 0.24, 0.26, 0.28, 0.30, 0.32, 0.34, 0.36, 0.38, 0.40 V s1, potentials vs SCE.

behaviors of other POM-modified electrodes in the reduction of chlorate, bromate and hydrogen peroxide [33,34]. Owing to high over potential required at most electrode surfaces for direct electroreduction of nitrite ions, no obvious response is

3.4. Electrocatalytic activity As we know, Keggin POMs can be used as electrocatalysts for the reduction of nitrite and hydrogen peroxide in aqueous solutions [31,32]. 1-CPE and 2-CPE have similar electrochemical properties. Therefore, we only take 2-CPE as an example to explore the electrocatalytic activity in the reduction of nitrite and hydrogen peroxide. Fig. 7 shows cyclic voltammograms for the electrocatalytic reduction of hydrogen peroxide at a bare CPE and 2-CPE in 0.5 M H2SO4 aqueous solution under a scan rate of 0.2 V s1. No obvious voltammetric response is observed at the bare CPE in 0.5 M H2SO4 aqueous solution containing 0.441 M hydrogen peroxide in the potential range from 0.6 to 0 V. It can be seen clearly that with addition of hydrogen peroxide, the first reduction peak current remains nearly unvaried, while the second reduction peak current increases gradually, suggesting that hydrogen peroxide is reduced only by four-electron-reduced species of mono-substituted heteropolymolybdates. This behavior is consistent with catalytic

Fig. 7. Cyclic voltammograms of 2-CPE in 0.5 M H2SO4 solution containing different concentrations of H2O2 (b) 0 mM, (c) 0.088 M, (d) 0.265 M and (d) 0.441 M and (a) a bare CPE in 0.441 M H2O2 and 0.5 M H2SO4 solution (scan rate: 0.2 V s1 vs SCE).

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reduction peak currents increased gradually with the addition of nitrite. These results indicate that 2-CPE possesses good electrocatalytic activity toward the reduction of nitrite. 3.5. Magnetic properties

Fig. 8. Cyclic voltammograms of 2-CPE in 0.5 M H2SO4 solution containing different concentrations of NaNO2 (b) 0 M, (c) 0.01 M, (d) 0.02 M, (e) 0.03 M, (f) 0.04 M and (g) 0.05 M and (a) a bare CPE in 0.05 M NaNO2 and 0.5 M H2SO4 solution (scan rate: 0.2 V s1 vs SCE).

observed for nitrite at a bare CPE in the range of 0.6–0 V in 0.5 M H2SO4 aqueous solution. Fig. 8 shows cyclic voltammograms for the electrocatalytic reduction of nitrite at a bare CPE and 2-CPE in 0.5 M H2SO4 aqueous solution under a scan rate of 0.2 V s1. Both the

In complex 1 or 2, Mn2þ (3d5; S ¼ 5/2) or Co2þ (3d7; S ¼ 3/2) is located in the mono-vacant Keggin heteropolymolybdates, coordinated by six oxygen atoms from two adjacent polyanions (Fig. S4, Supplementary data). The Mo6þ (3d0, S ¼ 0) ions do not possess an effective magnetic moment and do not contribute to the bulk properties [35,36]. As shown in Fig. 9, the magnetic moments (meff) of 1, determined from the equation meff ¼ 2.828(cMT)1/2, is 5.96 mB at 300 K, close to the expected value for an uncoupled manganese ion (g ¼ 2.0, S ¼ 5/2, 5.91 mB) [37–39]. As the temperature decreases, the meff value decreases slowly until 30 K and then sharply falls to a minimum value of 5.31 mB at 2 K. Such behaviors may result from the zero field splitting of Mn2þ ions, which is similar to that reported for [PW11Mn] chain [36]. The magnetic moment of 2 is 5.39 mB at 300 K, which is much higher than that expected for an isolated S ¼ 3/2 ion (meff ¼ 3.87 mB) due to the contribution of orbital angular momentum at high temperature [36,40,41]. The meff value decreases with lowering temperature (3.64 mB at 2 K); this is typically the magnetic behavior for single Co2þ ion with spin–orbit coupling occurring at lower temperature [40,41]. 4. Conclusions In summary, we have reported two 1D chainlike inorganic polymers constructed by TMS Keggin heteropolymolybdate subunits. Both these compounds present 1D linear chainlike structure and are synthesized in aqueous solutions under mild conditions. The cyclic voltammograms of 1 and 2 show irreversible redox peaks in DMF solution and there are three reversible couples after addition of 0.1 M H2SO4 aqueous solutions. The cyclic voltammograms of 1-CPE and 2-CPE show two consecutive reversible two-electron redox processes. Furthermore, 2-CPE exhibits electrocatalytic activity toward the reduction of nitrite and hydrogen peroxide. The magnetic studies of the two compounds indicate typical single ion magnetic behaviors of Mn2þ and Co2þ. The successful isolation of 1 and 2 provides us an opportunity to design and synthesize new heteropolymolybdates under a mild condition. Acknowledgements The authors are thankful for the financial supports from the National Natural Science Foundation of China (Grant No. 20671017; 20731002), This work was also supported by the Program for Changjiang Scholars and Innovative Research Team in University and the Analysis and Testing Foundation of Northeast Normal University. Appendix. Supplementary data

Fig. 9. Thermal evolution for cmT curve for (a) 1; (b) 2.

Crystallographic data for the structures reported in this paper in the form of CIF file have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication number CCDC-698739 for 1 and number CCDC-698738 for 2. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (Fax: (þ44)1223-336-033; E-mail: [email protected]). XRPD patterns and IR spectrum for 1 and 2 are available in supporting information. Supplementary data associated with this article can be found in the online version, at doi:10.1016/j.solidstatesciences.2009.04.028.

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