English

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  1   INTRODUCTION

  Researches on the design and synthesis of polyoxometalate- containing hybrids have been increasingly developed in recent years^[1-3]. The reason not only stems from their fascinating structures and topological features, but also from their potential applications in a range of fields^[4-8]. As a kind of important cluster in polyoxometalate family, phosphovanadates (VPOs) have received considerable attention because phosphorus and vanadium can share oxygen atoms with each other to offer the possibility of producing interesting and flexible arrangements^[9-12]. Vanadium centers may present four, five and six coordinated modes in crystals^[13]. The evolution of vanadium oxides is mainly dependent upon the synthesis of new solids possessing unique structures and properties, although the synthesis of these materials still remains a challenge. In this subfield, the (M (II)-ligand)/V_xO_y/PO₄ ^3- system (M (II) = Cu (II) and Zn (II)) with considerable structural diversity has been reported, exemplified by [Zn (phen)(H₂O) V₂O₆] (phen = 1, 10-phenanthroline)^[14], {[Cu (phen)]₂(VO₂)(H₂O)₂(H₂PO₄)₂(PO₄)}^[15], etc. The secondary metal-ligand subunits (SMS) in these compounds are not only as charge compensating and space-falling components but also as intrinsic structural components of bimetallic oxide. To design and construct various kinds of VPOs, several factors must be considered like the ligancy and coordination geometry of metal ions, the coordination capability and spatial extension of organic ligand, and the configuration energy of the structures. We are interested in the synthesis of new polyoxo-metalate-based supramolecular assemblies though weak non-covalent interactions including hydrogenbonding and aromatic ring-stacking. The supramolecular assembly may have unique structures and properties by virtue of the synergistic interactions between inorganic and organic moieties. In our previous work, we have reported a layered vanadate [{Cu (mbpy)}2V₈O₂₁] (mbpy = 4, 4′-dimethyl-2, 2′- bipyridine)^[16], in which the organic ligand is bipyridine decorated with two methyl groups. The -CH₃ pendant is a beneficial factor to form intermolecular C-H…O interaction. Here the ligand was changed to mbpy for constructing new VPO-based polymers. Experimental results indicated that the position of substituent group has an important influence on the crystal linkages. In this paper, the synthesis, crystal structure, and electrochemical properties of Cu (mbpy)(VO₂)(PO₄) (1) are presented.

  2   EXPERIMENTAL

  2.1   Synthesis and characterization

  All the reagents were commercially purchased and used without further purification. Elemental analyses were carried on a Perkin-Elmer 2400CHN elemental analyzer. Fourier transform infrared (FTIR) spectrum was recorded in KBr pellet with a FTIR-8900 IR spectrometer in the range of 4000～ 400 cm^-1. Thermogravimetric (TG) analysis was performed on a Perkin-Elmer Pyris Diamond TG/Differential thermal analysis (DTA) instrument in flowing N₂ at a heating rate of 10 ℃/min^-1. Powder X-ray diffraction (PXRD) was determined by a Bruker AXS D8 Advance diffractometer. Cyclic voltammograms (CV) were recorded on a CHI 660B electrochemical workstation.

  2.2   Synthesis of the compound

  A mixture of Na₂HPO₄·12H₂O (530 mg, 1.48 mmol), NH₄VO₃ (110 mg, 0.94 mmol), CuSO₄·5H₂O (200 mg, 0.80 mmol), mbpy (50 mg, 0.27 mmol) and H₂O (15 mL) was stirred for 30 min at room temperature. The pH value was adjusted to 3.7 with 1 M H₃PO₄, then the solution was transferred to a Teflon-lined reactor and heated at 170 ℃ for 5 days. The reaction was stopped, and the reactor was cooled to room temperature at a rate of 8 ℃/h. The blue crystals were obtained (yield: 70% based on V). The crystals were picked out, washed with distilled water, and dried in air. The elemental analysis found: C, 33.18; H, 2.63; N, 6.48% (Calcd.: C, 33.82; H, 2.81; N, 6.58%).

  2.3   Preparation of compound 1 modified carbon paste electrode

  Compound 1 modified carbon paste electrode (1-CPE) was fabricated as follows: 0.5 g graphite powder and 0.05 g crystal were mixed, and ground together by agate mortar and pestle to achieve an even, dry mixture; 0.50 mL paraffin oil was added to the mixture under stirring with a glass rod; then the mixture was used to pack into a 3 mm inner diameter glass tube, and the surface was pressed tightly onto the weighing paper with a copper rod through the back. Electrical contact was established with a copper rod through the back of the electrode.

  2.4   X-ray crystallography

  A suitable single crystal with dimensions of 0.23mm × 0.19mm × 0.17mm for compound 1 was selected for single-crystal X-ray diffraction experiments. Single crystal data were collected on a Smart Apex CCD diffractometer at 296(2) K with Mo-Kα monochromated radiation (λ = 0.71073 Å). The structure of 1 was solved by direct methods and refined by full-matrix least-squares methods on F₂ using the SHELXTL crystallographic software package^[17]. Anisotropic thermal parameters were used to refine all non-hydrogen atoms. The hydrogen atoms were added to their geometrically ideal positions and refined isotropically. The final R = 0.0291 and wR = 0.0980 (R = Σ||F_o| - |F_c||/Σ|F_o|; wR = Σ[w (F_o ² - F_c ²)²]/Σ[w ((F_o ²)²]^1/2).

  3   RESULTS AND DISCUSSION

  3.1   Structural description

  Single-crystal X-ray diffraction analysis revealed that the asymmetric unit of 1 may be grouped as three parts: [Cu-mbpy]²⁺, {VO₂}⁺, and PO₄ ^3- units. The fundamental structural motif is shown in Fig. 1. The selected bond lengths and bond angles are listed in Table 1. The coordination geometry of the metal Cu (II) center is a square-pyramidal [2+2+1] configuration, which is defined by two nitrogen donors of mbpy (Cu-N, 1.991(2) and 2.009(2) Å), two oxygen donors of phosphate (Cu-O, 1.9136(18) and 1.9438(18) Å) and an oxygen atom shared with a vanadium center (Cu-O, 2.342(2) Å). In the {CuN₂O₃} polyhedron, the axial Cu-O bond length is longer than the equatorial values which may be ascribed to Jahn-Teller distortion. One apex of {VO₄} tetrahedron is occupied by O (2) with a shorter V (1)-O (2) distance of 1.608(2) Å, and the other three are shared with two adjacent {PO₄} tetrahedra and one {CuN₂O₃} polyhedron, with their V (1)-O (3), V (1)-O (1), and V (1)-O (5) distances to be 1.617(2), 1.8430(18) and 1.8611(19) Å, respectively. In {PO₄} tetrahedron, the P-O bond lengths are in the 1.5051(19)～1.5745(19) Å range. Each {PO₄} tetrahedron bridges two V and two Cu centers. Resulting from the corner-sharing connection of {PO₄}, {VO₄} and {CuN₂O₃} polyhedra, a neutral chain-like structure of inorganic {Cu (VO₂)(PO₄)}_∞ is constructed (Fig. 2). The organic mbpy ligands are anchored to the Cu (II) centers, which serve further the intermolecular π…π interactions to bridge different chains of {Cu (VO₂)(PO₄)}_∞.

  Figure 1.  Coordination environments of the copper, vanadium, and phosphorus atoms in 1. All hydrogen atoms have been omitted for clarity (ORTEP drawing of 1 with thermal ellipsoids at 30% probability)

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  Figure 2.  Views of the one-dimensional chain of 1 with the alternative arrangement of 4-MRs of {Cu₂P₂} and 3-MRs of {CuVP} subunits

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  Table 1.  Selected Bond Lengths (Å) and Bond Angles (°)

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  BondDist.BondDist.BondDist.Cu(1)-O(4)1.9136(18)P(1)-O(6)1.5141(18)V(1)-O(3)1.617(2)Angle(°)Angle(°)Angle(°)O(4)-Cu(1)-N(2)171.61(9)O(3)-V(1)-O(1)108.15(10)N(2)-Cu(1)-O(3)89.31(9)O(4)-Cu(1)-O(6)#195.64(7)O(4)-Cu(1)-O(3)95.31(8)

  Cu(1)-O(6)#1	1.9438(18)	P(1)-O(5)	1.5615(19)	V(1)-O(1)	1.8430(18)
  Cu(1)-O(3)	2.342(2)	P(1)-O(1)	1.5745(19)	V(1)-O(5)#2	1.8611(19)
  Cu(1)-N(2)	1.991(2)	V(1)-O(2)	1.608(2)	P(1)-O(4)	1.5051(19)
  Cu(1)-N(1)	2.009(2)
  O(6)#1-Cu(1)-N(2)	90.61(9)	O(1)-V(1)-O(5)#2	112.26(9)	N(1)-Cu(1)-O(3)	103.45(9)
  O(4)-Cu(1)-N(1)	91.26(9)	O(4)-P(1)-O(5)	108.62(11)	N(2)-Cu(1)-N(1)	80.84(11)
  O(6)#1-Cu(1)-N(1)	156.56(9)	O(5)-P(1)-O(1)	104.57(10)	O(2)-V(1)-O(3)	109.79(12)
  O(6)#1-Cu(1)-O(3)	98.19(8)	O(4)-P(1)-O(6)	114.27(11)	O(2)-V(1)-O(1)	107.98(10)
  Symmetry transformations used to generate equivalent atoms: #1: -x, -y+1, -z; #2: -x+1, -y+1, -z

  Table 1.  Selected Bond Lengths (Å) and Bond Angles (°)

  It can also be seen in Fig. 2 that two V and two Cu centers form a four-membered ring (4-MR), and a V, a Cu and a P centers form a three-membered ring (3-MR). Then, the simplified V-P-Cu inorganic chain in compound 1 is composed of alternative 4-MRs of {Cu₂P₂} and 3-MRs of {CuVP} subunits. Among different chains, there are extensive intermolecular interactions to strengthen the crystal structure, including hydrogen bonding and aromatic π…π stacking (Fig. 3). The typical C-H…O hydrogen bonding distances are in the range of 2.947(4)～ 3.321(4) Å. The close contact distance between adjacent mbpy ligands is ca. 3.5 Å.

  Figure 3.  Polyhedral packing views of 1 based on the intermolecular interactions

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  The bond valence sum (BVS) calculations^[18] indicate that both V (1) (5.099) and P (1) (4.786) atoms are in the +5 oxidation state. The BVS analysis also shows that Cu (1) (1.883) is in the +2 oxidation state and all oxygen atoms are in the -2 oxidation state.

  3.2   Thermal analysis

  Thermogravimetric (TG) measurement also supports the crystal chemical composition. As shown in Fig. 4, TG curve of 1 reveals a one-step weight loss process. The primary structure of crystal 1 is thermally stable up to ca. 312 ℃. The weight loss of 43.1% in the temperature range of 312～500 ℃ is attributed to the decomposition of mbpy molecules, which is in agreement with the calculated value of calcd. 43.3%.

  Figure 4.  TG curve of compound 1

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  3.3   Infrared spectrum and PXRD measurement

  Infrared spectrum of the crystal sample is recorded in the 4000～400 cm^-1 range at room temperature, using KBr pellet. The principal bands are (cm^-1): 919 and 964 corresponding to the terminal V=O and O-V-O bridge stretching modes; 1057 and 1172 assigned to P-O groups^[19]; 1473 and 1639 assigned to C=C and C=N groups of the organic ligand (mbpy). The broad peak in the 3000～3800 cm^-1 region is associated with the intermolecular hydrogen- bonding interactions in 1.

  Figure 5.  IR spectrum of compound 1

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  Scope of powder diffraction data collection is 5～ 50° (Fig. 6). In the 2θ range of 7～10°, there are strong diffraction peaks. The given position is consistent in the experimental and calculated data of compound 1. Better of matching condition illustrates high purity of crystal phase.

  Figure 6.  Experimental (a) and calculated (b) PXRD patterns based on the results from single-crystal of compound 1

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  3.4   Electrochemical property

  The electrochemical property of 1-modified carbon paste electrode (1-CPE) is investigated in acid aqueous solution^[20]. The cyclic voltammetric (CV) curves for 1-CPE in 1.0 M H₂SO₄ aqueous solution at different scan rates are recorded. There are two pairs of non-ideally reversible redox peaks (I-I′ and II-II′) appearing in the potential range from -0.5 to 1.2 V (Fig. 7). At the scan rate of 20 mV s^-1, the mean peak potentials E_1/2 = (E_pa + E_pc)/2 are 839 mV (I-I′) and 116 mV (II-II′) for 1, respectively, which should be ascribed to the redox of V^[21]. With the scan rates increasing, the anodic and cathodic peak currents are increased. The cathodic peak potentials shift to the negative direction and the corresponding anodic peak potentials to the positive direction. Namely, the peak-to-peak separations between the corresponding cathodic and anodic peaks increase with increasing the scan rates. The plot of peak current (I) vs. scan rate (see insert plot in Fig. 7b) shows that the peak currents are proportional to the square root of the scan rate, indicating that the redox reaction of 1 is controlled by the diffusion.

  Figure 7.  (a) Cyclic voltammograms of 1-CPE in 1M H₂SO₄ solution at different scan rates of 20, 50, 80, 110, 140, 170 and 200 mV s^-1; (b) Plot of the anodic and cathodic peak currents against the scan rates

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  4   CONCLUSION

  In summary, a novel phosphovanadate metalorganic polymer Cu (mbpy)(VO₂)(PO₄) has been successfully synthesized under hydrothermal condition. The crystal structure of 1 consists of inorganic {Cu (VO₂)(PO₄)}_∞ neutral chains anchored by organic mbpy ligands. The electrochemical property of compound has also been investigated. Furthermore, we are currently carrying out the other decorated ligands with pendant group, as well different metal centers in similar hydrothermal system and trying to find a clue to the synthesis of some new inorganic-organic materials.

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• 

  Figure 1  Coordination environments of the copper, vanadium, and phosphorus atoms in 1. All hydrogen atoms have been omitted for clarity (ORTEP drawing of 1 with thermal ellipsoids at 30% probability)

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  Figure 2  Views of the one-dimensional chain of 1 with the alternative arrangement of 4-MRs of {Cu₂P₂} and 3-MRs of {CuVP} subunits

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  Figure 3  Polyhedral packing views of 1 based on the intermolecular interactions

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  Figure 4  TG curve of compound 1

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  Figure 5  IR spectrum of compound 1

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  Figure 6  Experimental (a) and calculated (b) PXRD patterns based on the results from single-crystal of compound 1

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  Figure 7  (a) Cyclic voltammograms of 1-CPE in 1M H₂SO₄ solution at different scan rates of 20, 50, 80, 110, 140, 170 and 200 mV s^-1; (b) Plot of the anodic and cathodic peak currents against the scan rates

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  Table 1.  Selected Bond Lengths (Å) and Bond Angles (°)

  BondDist.BondDist.BondDist.Cu(1)-O(4)1.9136(18)P(1)-O(6)1.5141(18)V(1)-O(3)1.617(2)Angle(°)Angle(°)Angle(°)O(4)-Cu(1)-N(2)171.61(9)O(3)-V(1)-O(1)108.15(10)N(2)-Cu(1)-O(3)89.31(9)O(4)-Cu(1)-O(6)#195.64(7)O(4)-Cu(1)-O(3)95.31(8)

  Cu(1)-O(6)#1	1.9438(18)	P(1)-O(5)	1.5615(19)	V(1)-O(1)	1.8430(18)
  Cu(1)-O(3)	2.342(2)	P(1)-O(1)	1.5745(19)	V(1)-O(5)#2	1.8611(19)
  Cu(1)-N(2)	1.991(2)	V(1)-O(2)	1.608(2)	P(1)-O(4)	1.5051(19)
  Cu(1)-N(1)	2.009(2)
  O(6)#1-Cu(1)-N(2)	90.61(9)	O(1)-V(1)-O(5)#2	112.26(9)	N(1)-Cu(1)-O(3)	103.45(9)
  O(4)-Cu(1)-N(1)	91.26(9)	O(4)-P(1)-O(5)	108.62(11)	N(2)-Cu(1)-N(1)	80.84(11)
  O(6)#1-Cu(1)-N(1)	156.56(9)	O(5)-P(1)-O(1)	104.57(10)	O(2)-V(1)-O(3)	109.79(12)
  O(6)#1-Cu(1)-O(3)	98.19(8)	O(4)-P(1)-O(6)	114.27(11)	O(2)-V(1)-O(1)	107.98(10)
  Symmetry transformations used to generate equivalent atoms: #1: -x, -y+1, -z; #2: -x+1, -y+1, -z

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