A supramolecular 3-D organic-inorganic hybrid structure based on {PMo₁₂} layers arranged in an alternating mode

2.1 Crystal structure of 1

X-ray diffraction analysis has revealed that compound 1 is constructed from six Ag^I cations, two Keggin-type [PMo₁₂O₄₀]^3– polyanions, 12 pz ligands and three coordinating water molecules (Fig. 1). The polyoxoanions of 1 are composed of a central PO₄ tetrahedron surrounded by four vertex-sharing Mo₃O₁₃ units, which emanate from the association of three edge-sharing MoO₆ octahedra. The P–O distances range from 1.533(4) to 1.547(8) Å and the O–P–O bond angles are 109.17(19)° and 109.77(19)°. The Mo–O distances fall into three classes: Mo–Od (terminal), 1.674(5)–1.695(5) Å, Mo–Ob/c (bridge), 1.821(5)–2.006(5) Å, Mo–Oa (central) 2.420(5)–2.452(5) Å.

Fig. 1:

Molecular structure of compound 1 in the crystal.

Each Ag^I cation displays a linear coordination geometry to join two N atoms from two pz ligands via Ag–N bonds to form a short rod [Ag(pz)₂]⁺. The Ag–N distances range from 2.111(8) to 2.169(9) Å. Three such short rods are connected to three water molecules in an alternating mode via hydrogen bonding interactions (N6···O1W 2.975(11) Å and N8···O1W 2.739(10) Å) to form macrocycles (Fig. 2). Two crystallographically independent Keggin clusters, [P(1)Mo₁₂O₄₀]^3– and [P(2)Mo₁₂O₄₀]^3–, are placed at the center of the ring via hydrogen bonding interactions (hydrogen bonds O2···O1W 2.825(8) Å, O17···N8 3.008(10) Å), in two kinds of subunits (Fig. 3). O1W links three short rods [Ag(2)(pz)₂]⁺ to form a supramolecular ring system and further acts to connect Keggin clusters.

Fig. 2:

Supramolecular layer constructed of [P(1)Mo₁₂O₄₀]^3– clusters and hydrogen bonding in a macrocycle (all H atoms omitted for clarity).

Fig. 3:

Hydrogen bonding between crystallographically independent Keggin clusters and the macrocycle.

Each [P(1)Mo₁₂O₄₀]^3– subunit is connected to three adjacent [P(1)Mo₁₂O₄₀]^3– units in AB–AB mode via supramolecular interactions (O11···O11 3.002 Å) to lead to supramolecular layers (Fig. 2) with hexagonal holes. The dimension of the holes is 13.34 × 13.34 Å (O(5)–O(5) distance). Each [P(2)Mo₁₂O₄₀]^3– is further connected to three adjacent [P(2)Mo₁₂O₄₀]^3– units in AB–AB mode via two [Ag(2)(pz)₂]⁺ linkers to form another supramolecular layer along the c axis (Fig. 4). The main interactions are Ag2···O17 2.785 Å and Ag2···O22 3.155 Å. Another kind of hexagonal pores has dimensions of 13.53 × 13.53 Å (O(19)–O(19) distance). Two kinds of layers are further connected in a special alternating type (A-B-A-A-B-A-B-B) by [Ag(pz)₂]⁺ linkers to form a complex 3-D supramolecular network (Fig. 5). The main supramolecular interactions are Ag2···O2 2.918 Å, Ag1···O19 2.791 Å, Ag1···O9 3.237 Å Ag1···O5 3.120 Å. Water molecules play an essential role in forming two kinds of hydrogen bonds (O1W···O2 2.825 Å, O1W···O17 3.076 Å).

Fig. 4:

A supramolecular layer constructed of [P(2)Mo₁₂O₄₀]^3– clusters. Some Ag(pz)₂ units are omitted for clarity.

Fig. 5:

The 3D structure of compound 1 based on {P(1)Mo₁₂} and {P(2)Mo₁₂} layers arranged in an alternating mode (A-B-A-A-B-A-B-B).

According to bond valence sum calculations, all Mo atoms exhibit the +VI oxidation state (average calculated value = 5.99) and the Ag centers are in the +I oxidation state (average calculated value = 0.96), consistent with the overall charge balance.

2.2 Thermal analysis

The thermal stability of compound 1 was determined under nitrogen atmosphere by thermogravimetric analysis (TGA) from 25 to 800 °C. It shows two weight-loss steps: the first weight loss of 1.1 % in the temperature range of 60–93 °C corresponds to the release of the water molecules, which is in accordance with the calculated value of 1.0 % (∼3 H₂O). The second weight loss of 15.3 % in the temperature range of 130–590 °C is attributed to the loss of all pz ligands, which is close to the calculated value of 15.8 % (∼12 pz).

2.3 Fluorescence

When excited at 397 nm, compound 1 exhibits a strong emission at 482 nm in the solid state, which can be assigned to charge transfer between Ag^I and the pz ligand [15]. The result indicates that the title compound is a potential fluorescence-emitting material (Fig. 6).

Fig. 6:

Fluorescence spectra of compound 1 at room temperature.

2.4 Voltammetric behavior of 1-CPE in aqueous electrolyte

Owing to the insolubility of compound 1 in water, its performance in carbon paste electrodes (CPE) can be studied. The cyclic voltammograms for 1-CPE in 1 m H₂SO₄ aqueous solution at different scan rates were recorded. As shown in Fig. 7a, three reversible redox peaks appear in the potential range –1.0 to 1.0 V. The half-wave potentials E_1/2 = (E_pa + E_pc)/2 (scan rate: 20 mV·s^–1) are –0.541 (I–I′), –0.353 (II–II′) and 0.123 (III–III′)V, in accordance with three consecutive two-electron processes of the Mo(VI/V) couples in {PMo₁₂} [23, 24]. When the scan rates varied from 20 to 500 mV·s^–1, the peak potentials changed gradually: the cathodic peak potentials (E_pc) shifted slightly to negative values and the anodic peak potentials (E_pa) shifted slightly to positive values with increasing scan rates. The peak-to-peak separations between the corresponding anodic and cathodic peaks increased, but the average peak potentials did not change. The behavior is consistent with a reversible but nonideal redox process [25, 26].

Fig. 7:

(a) Cyclic voltammograms of the 1-CPE in 1 m H₂SO₄ solution at different scan rates (from inner to outer: 20, 30, 40, 50, 60, 80, 100, 120, 150, 200, 250, 300, 350, 400, 450, and 500 mV·s^–1); (b) Cyclic voltammograms of the 1-CPE in 1 m H₂SO₄ solution containing H₂O₂ at different concentrations (potentials vs. SCE; scan rate: 60 mV s^–1).

2.5 Electrocatalytic activity of 1-CPE for the reduction of H₂O₂

The cyclic voltammograms for the electrocatalytic reduction of hydrogen peroxide in 1 m H₂SO₄ aqueous solution in the potential range from –1.0 to 1.0 V for 1 are shown in Fig. 7b. It can be clearly seen that with increasing H₂O₂ concentration (from 0.0 to 30 mm), all three reduction peak currents increased while the corresponding oxidation peak currents dramatically decreased, suggesting that H₂O₂ was reduced by the PMo₁₂O₄₀^3– polyanions. The results indicate that 1-CPE has good electrocatalytic activity toward the reduction of H₂O₂.

4.1 Materials and measurements

All reagents were purchased commercially and used without further purification. H₃PMo₁₂O₄₀·13H₂O was prepared according to a literature method [27, 28] and verified by the IR spectrum. Elemental analyses (C, H, and N) were performed on a Perkin-Elmer 2400 CHN elemental analyzer. P, Mo, and Ag analyses were performed on a PLASMA-SPEC (I) inductively coupled plasma atomic emission spectrometer. The IR spectra were obtained on an Alpha Centaurt FT/IR spectrometer with KBr pellets in the 400–4000 cm^–1 region. The thermogravimetric analyses (TGA) were carried out in N₂ on a Perkin-Elmer DTA 1700 differential thermal analyzer with a rate of 10 °C min^–1. Fluorescence spectra were performed on a Hitachi F-4500 fluorescence/phosphorescence spectrophotometer with a 450 W xenon lamp as the excitation source. Electrochemical measurements were performed with a CHI660 electrochemical workstation. A conventional three-electrode system was used, with a modified carbon paste electrode (CPE) as a working electrode, a twisted platinum wire as counter electrode, and a commercial Ag/AgCl electrode as reference electrode.

4.2 Synthesis

A mixture of H₃PMo₁₂O₄₀·13H₂O (0.515 g, 0.25 mmol), V₂O₅ (0.045 g, 0.25 mmol), AgNO₃ (0.085 g, 0.5 mmol), pz (0.034 g, 0.5 mmol), and H₂O (10 mL) was stirred for 30 min in air until it was homogeneous. The mixture was then transferred to a Teflon-lined stainless steel autoclave (30 mL) and kept at 180 °C for 2 days. After the autoclave was cooled to room temperature, red block-like crystals were obtained in a yield of 25 % (based on Mo). – Anal. C₃₆H₅₄Ag₆Mo₂₄N₂₄O₈₃P₂ (5162.75): calcd. C 8.37, H 1.05, N 6.51, Ag 12.54, Mo 44.60, P 1.20; found C 8.36, H 1.04, N 6.50, Ag 12.55, Mo 44.62, P 1.23 %. – IR (KBr pellet, cm^–1): 3468 (br), 1613 (s), 1404 (m), 1067 (s), 968 (m), 876 (s), 794 (s).

4.3 X-ray crystallographic studies

The crystal structure of compound 1 was determined from single-crystal X-ray diffraction data. Intensity data were collected on a Bruker SMART CCD diffractometer with graphite-monochromatized MoK_α radiation (λ = 0.71073Å). The structure was solved by Direct Methods and difference Fourier maps and refined by full-matrix least-squares techniques on F² (shelxs/l-97 [29, 30]). Anisotropic displacement parameters were used to refine all nonhydrogen atoms. The positions of hydrogen atoms of the organic molecules were calculated in idealized positions and refined using a riding model. Hydrogen atoms of water molecules were not included. Crystal data and refinement parameters are summarized in Table 1. Selected bond lengths and bond angles of compound 1 are listed in Table 2 and hydrogen bond parameters in Table 3.

CCDC 988814 contains the supplementary crystallographic data for this article. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.