Controlled assembly of hybrid architectures based on carboxylic acid ligands and [(O₃PCH₂PO₃)Mo₆O₂₂]¹²⁻

Abstract

The nature of carboxylic acid has a profound effect on constructing POM-based architectures. With different carboxylic acid ligands, three novel diphosphonate-containing compounds have been synthesized and further investigated by elemental analyses, single-crystal X-ray diffraction, IR spectroscopy, UV-vis spectroscopy, X-ray powder diffraction (XRPD), ³¹P NMR spectroscopy, cyclic voltammetric behavior and thermogravimetric (TG) analyses. [(HO₃PCH₂PO₃)Mo₆O₁₈(O₂CC₆H₄OH)₂]⁵⁻ (1a) shows a ring-shaped cluster with a {Mo₆} plane framework. [{(HO₃PCH₂PO₃)Mo₆O₁₈(CH₃CO₂)₂}₂Na₂(H₂O)₂]⁸⁻ (2a) is a reverse symmetrical “crown-type” structure and the subunits are linked by two Na–O(1W)–Na bridges. The polyanion [{(O₃PCH₂PO₃)(Mo₆O₁₈(H₂O)₂)(O₂CCH₂CO₂)}₂(Mo₂O₅)₂(μ₃-O)₂]¹²⁻ (3a) appears as an “S-shaped” structure with a centre of inversion.

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Introduction

The design and synthesis of polyoxometalates (POMs) have gained increasing attention not only for their molecular and electronic structural diversity, but also because of their interesting properties in different fields such as catalysis, magnetism, materials science, optics, biology and medicine.^1–8 Recently, a great deal of research from different groups has been aimed at the functionalization of polyoxomolybdate with organic or organometallic ligands. The prospect of POM-based hybrid materials with novel synergistic effects, exceeding tunable properties and convenience for processability is brilliant.⁹ The covalent binding of organic groups onto POMs may open up a doorway to new classes of POM-based hybrid materials, which combine the intrinsic properties of POMs and organic components. As a matter of fact, interaction of POMs with carboxylic acid ligands has also been synthesized and characterized,¹⁰ which leads to a large number of functionalized species. For instance, {Mo₆O₁₈},¹¹ {Mo₁₂O₄₆},¹² {Mo₂O₄},¹³ and {Mo_2nO_2nS_2n(OH)}_2n.¹⁴ Meanwhile, a part of diphosphates that generally formularized as H₂O₃P–(CH₂)_n–PO₃H₂ are increasingly used in the synthesis of hybrid materials based on POMs, owing to their versatile coordination chemistry and the good stability of P–C bond. {(Mo₂O₄)(O₃PCH₂PO₃)}_n (n = 3, 4, 10),^15a {Mo₇O₁₆(O₃PCH₂PO₃)₃},^15b {(Mo₂O₄)(O₃PCH₂PO₃)},^15c {Mo₂O₅(O₃PCH₂CH₂PO₃)},^15d {Mo₅O₁₅(O₃PCH₂CH₂PO₃)},^15e {Mo₅O₁₅(O₃PCH₂CH₂CH₂PO₃)},^15f,15g [Mo₅O₁₅{O₃P(CH₂)_nPO₃}]⁴⁻ (n = 2–6 (ref. 15h) and 9 (ref. 15i)) and {Mo₆O₁₈{O₃P(CH₂)₅PO₃}¹⁵ⁱ are the examples that have been reported. That is to say, it can not only bind to many different metal ions (M) via P–O–M bonds, but also possess various potential properties which can be utilized in future.¹⁶

To the best of our knowledge, only a few analogues of diphosphonate–POMs systems that further functionalized with carboxylic acid ligand have been reported. Herein, the diphosphonate ligand and carboxylic acid ligand synchronously incorporating into POMs grabs our attention. As expected, (C₃N₂H₅)₄Na[{(HO₃PCH₂PO₃)Mo₆O₁₈(O₂CC₆H₄OH)₂}]·(C₃N₂H₄)·9.5H₂O (1) constructed by H₂O₃PCH₂PO₃H₂ and further modified by p-hydroxybenzoic acid has been firstly isolated. (C₃N₂H₅)₈[{(HO₃PCH₂PO₃)Mo₆O₁₈(CH₃CO₂)₂}₂Na₂(H₂O)₂]·8H₂O (2) has been discovered with acetic acid instead of p-hydroxybenzoic acid. To further study the nature effect of carboxylic acid ligands in this area, malonic acid was used to assembly polyoxomolybdate. Under the conventional solution conditions, another novel “S-shaped” structure compound (C₃N₂H₅)₈Na₄[{(O₃PCH₂PO₃)(Mo₆O₁₈(H₂O)₂)(O₂CCH₂CO₂)}₂(Mo₂O₅)₂(μ₃-O)₂]·14H₂O (3) with a centre of inversion has been successfully synthesized.

Experimental

Materials and methods

All chemical reagents were commercially purchased and used without any further purification. Elemental analyses (C, H, N) were performed on a Perkin-Elmer 2400-II CHNS/O analyzer. IR spectra were obtained on a Bruker VERTEX 70 IR spectrometer (using KBr pellets) in the range of 4000–400 cm⁻¹. UV-vis absorption spectra were determined on a U-4100 spectrometer at room temperature. Thermogravimetric analyses (TG) were carried out under the nitrogen gas atmosphere on a Mettler-Toledo TGA/SDTA851^e instrument from 25 to 900 °C with a heating rate of 10 °C min⁻¹. XRPD measurements were performed on a Bruker AXS D8 Advance diffractometer instrument with Cu K_α radiation (λ = 1.54056 Å) in the angular range 2θ = 10–40° at 293 K. ³¹P NMR spectra were detected in 5 mm tubes with ¹H decoupling on a Bruker AV-400 model spectrometer operating at 400 MHz. ³¹P chemical shifts were referenced to the 85% H₃PO₄ as external standard.

Synthesis of 1. Na₂MoO₄·2H₂O (0.73 g, 3.02 mmol) and p-hydroxybenzoic acid (0.14 g, 1.01 mmol) were dissolved in water (10 mL), and H₂O₃PCH₂PO₃H₂ (0.08 g, 0.45 mmol) and imidazole (0.13 g, 1.91 mmol) were added. The pH value was adjusted to 4.6 by 2 mol L⁻¹ HCl. Then the solution was stirred approximately 2 h at 70 °C. After cooling to room temperature, the solution was filtered and the filtrate was left in an open beaker at room temperature. Colourless block crystals of 1 were obtained after about one month. Yield: 0.19 g (22% based on H₂O₃PCH₂PO₃H₂). Elemental analysis (%) calcd for 1: C, 19.06; H, 3.41; N, 7.41. Found: C, 19.54; H, 3.18; N, 7.24. The following abbreviations for selected IR (KBr, cm⁻¹) were used to assign the peak intensities: s, strong; m, medium; w, weak. 3147 (s), 2849 (m), 1595 (s), 1536 (s), 1400 (s), 1275 (w), 1248 (w), 1170 (w), 1051 (s), 929 (s), 894 (s), 788 (m), 665 (s), 626 (s), 525 (m), 452 (w), 428 (w).

Synthesis of 2. The procedure for the formation of 1 was employed but with NaAc (0.25 g, 3.05 mmol) instead of p-hydroxybenzoic acid (0.14 g, 1.01 mmol) and the pH value was adjusted to 4.7 by glacial acetic acid. Yield: 0.33 g (48% based on H₂O₃PCH₂PO₃H₂). Elemental analysis (%) calcd for 2: C, 13.22; H, 2.55; N, 7.26. Found: C, 13.19; H, 2.21; N, 7.43. Selected IR (KBr, cm⁻¹): 3145 (s), 2981 (m), 2845 (m), 2628 (w), 1557 (s), 1426 (s), 1153 (w), 1052 (s), 932 (s), 898 (s), 757 (m), 661 (s), 522 (w), 453 (w), 431 (w).

Synthesis of 3. The procedure for the formation of 1 was employed but with malonic acid (0.21 g, 2.02 mmol) instead of p-hydroxybenzoic acid (0.14 g, 1.01 mmol) and the pH value was adjusted to 4.0 by 2 mol L⁻¹ HCl. Yield: 0.10 g (14% based on H₂O₃PCH₂PO₃H₂). Elemental analysis (%) calcd for 3: C, 10.06; H, 2.22; N, 5.87. Found: C, 10.33; H, 2.50; N, 5.63. Selected IR (KBr, cm⁻¹): 3149 (s), 2991 (m), 2860 (m), 1584 (s), 1445 (w), 1420 (w), 1370 (m), 1279 (w), 1169 (w), 1079 (s), 1049 (m), 974 (m), 929 (s), 900 (s), 671 (s), 624 (m), 522 (m), 444 (w), 420 (w).

X-ray crystallography

A summary of crystal data and structure refinements for 1–3 were listed in Table 1. X-ray structure analysis on single crystal was performed at room temperature on a Bruker Apex-II CCD diffractometer with graphite-monochromated Mo K_α radiation (λ = 0.71073 Å). The crystal data were solved by direct methods and further refined by full-matrix least-squares refinements on F² using the SHELXL-97 software and an absorption correction was performed using the SADABS program.¹⁷

Table 1 Crystal data and structure refinement for compounds 1–3

	1	2	3
Empirical formula	C30H56Mo6N10NaO39.5P2	C34H78Mo12N16Na2O66P4	C32H84Mo16N16Na4O86P4
Formula weight	1849.42	3088.26	3819.96
T (K)	293(2)	296(2)	296(2)
Space group	P1̄	P1̄	P1̄
Crystal system	Triclinic	Triclinic	Triclinic
a/Å	12.725(3)	12.5484(9)	12.5891(9)
b/Å	13.292(3)	15.8024(12)	14.2538(10)
c/Å	20.645(4)	15.9316(11)	14.8335(10)
α/deg	90.526(3)	81.1310(10)	84.5250(10)
β/deg	103.738(3)	71.3690(10)	83.8700(10)
γ/deg	111.016(3)	74.8680(10)	80.4970(10)
V/Å^3	3149.2(12)	2881.1(4)	2602.0(3)
Z	2	1	1
Cryst size (mm^3)	0.50 × 0.35 × 0.11	0.45 × 0.32 × 0.24	0.49 × 0.41 × 0.33
Dc/g cm^−3	1.950	1.780	2.430
μ/mm^−1	1.321	1.415	2.061
F(000)	1830	1508	1840
Limiting indices	−13 ≤ h ≤ 15	−14 ≤ h ≤ 14	−14 ≤ h ≤ 14
	−15 ≤ k ≤ 15	−17 ≤ k ≤ 18	−16 ≤ k ≤ 16
	−24 ≤ l ≤ 24	−18 ≤ l ≤ 18	−17 ≤ l ≤ 15
For data collection/deg	1.80–25.00	2.44–25.00	1.39–25.00
Reflections collected/unique	15 964/10 961	14 731/10 047	13 543/9043
R(int)	0.0264	0.0135	0.0200
Goodness-of-fit on F^2	1.007	1.092	1.054
Final R indices	R1 = 0.0485	R1 = 0.0370	R1 = 0.0360
[I > 2σ(I)]	wR2 = 0.1353	wR2 = 0.1276	wR2 = 0.0876
R indices (all data)	R1 = 0.0645	R1 = 0.0443	R1 = 0.0498
	wR2 = 0.1445	wR2 = 0.1331	wR2 = 0.0929

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Routine Lorentz and polarization corrections were applied at the same time. All non-hydrogen atoms were refined anisotropically. All hydrogen atoms were positioned geometrically and were not included in the refinements.

Results and discussion

Structural descriptions

Single crystal X-ray diffraction indicates that compounds 1–3 crystallize in the triclinic P1̄ space group. Bond valence sum calculations (BVS)¹⁸ indicate that all Mo atoms are in +6 oxidation state and P atoms are in +3 oxidation state in 1–3. Considering the charge balance of 1–3, some protons need to be added. To locate the positions of these protons, BVS of all the O atoms on POMs fragments are carried out (Section 6 in ESI†).

The polyanion [(O₃PCH₂PO₃)Mo₆O₁₈(H₂O)]⁴⁻ that has been reported by Kortz and co-workers¹⁹ can be regarded as the motif of compounds 1–3. Both compounds 1 and 2 include the {(HO₃PCH₂PO₃)Mo₆O₂₂} unit (Fig. 1a), which is composed of a central heteroatom P surrounded by a ring of six {MoO₆} octahedra sharing corners and edges alternately. One P from H₂O₃PCH₂PO₃H₂ as the central heteroatom is located slightly above the {Mo₆} plane (Fig. S8†) and coordinates to three μ₃-O atoms and a C atom, which leads to a trigonal-pyramidal coordination geometry. The other P atom is bond to two edge-sharing {MoO₆} octahedra above the {Mo₆} plane. As we known, the reported examples [(CH₃COO)[N(CH₂P-O₃)₃]Mo₆O₁₇(OH)]⁶⁻ and {XMo₆O₂₁} (X = As^III, Sb^III, Bi^III, Se^IV, and Te^IV)^20,21 are modified either with only one carboxylic acid fragment or with the same amino acid species. However, a major feature of 1a is that the six terminal oxygen atoms above the {Mo₆} plane are occupied by two p-hydroxybenzoic acid molecules and one H₂O₃PCH₂PO₃H₂ fragment. More specifically, both the two p-hydroxybenzoic acid molecules are bonded to two edge-sharing {MoO₆} octahedra by their {COO} groups on the same side of the ring (Fig. 1b) and the two {O₂CC₆H₄OH} groups are almost perpendicular to the {Mo₆} plane. Further, the center P atom with the lone pair of electrons is orientated in the same direction with the p-hydroxybenzoic acid ligand.

Fig. 1 Polyhedral/ball-and-stick view of the basic building block {(HO₃PCH₂PO₃)Mo₆O₂₂} (a); 1a (b); 2a (c); 3a (d). {MoO₆}, sea green octahedral; hydrogen atoms are omitted for clarity.

Polyanion 2a is a reverse symmetrical “crown-type” structure composed of two {(HO₃PCH₂PO₃)Mo₆O₁₈(CH₃CO₂)₂} (Fig. 2d) subunits and a linking bridge (Na–O(1W)–Na). The subunit {(HO₃PCH₂PO₃)Mo₆O₁₈(CH₃CO₂)₂} could be considered as the participation in assembly with acetic acid based on the basic building block {(HO₃PCH₂PO₃)Mo₆O₂₂} (Fig. 2b), which is formed from molybdenum-oxide cluster modified by H₂O₃PCH₂PO₃H₂. The six-coordinate center Na1 in 2a (Fig. 3), which exhibits distorted triangular prism coordination geometry are defined by O3, O12, O17 and O27 from the subunit {(HO₃PCH₂PO₃)Mo₆O₁₈(CH₃CO₂)₂} and O1W and O1WA (A: 2 − x, 1 − y, 1 − z) from crystal water molecules [Na–O: 2.357(4)–2.581(4) Å]. Polyanion 3a appears an “S-shaped” structure with a centre of inversion (Fig. 1d), which is constituted by two {(O₃PCH₂PO₃)(Mo₆O₁₈(H₂O)₂)(O₂CCH₂CO₂)(Mo₂O₇)} (Fig. S9†). The {Mo₂O₁₀} linker is coordinated to the basic building block {(O₃PCH₂PO₃)(Mo₆O₁₈(H₂O)₂)} by O28 from H₂O₃PCH₂PO₃H₂ ligand and O31 and O36 from malonic acid molecule. Besides, one {COO} group from malonic acid links two edge-sharing {MoO₆} octahedra from {(O₃PCH₂PO₃)(Mo₆O₁₈(H₂O)₂)} framework and the other {COO} group bonds to two edge-sharing {MoO₆} octahedra from {Mo₄O₁₈} linker (Fig. S10†). BVS values for O15 (O15B, B: 1 − x, 1 − y, 1 − z) and O17 (O17B) above/below the {Mo₆} plane of 3a are 0.242 and 0.266, indicating that they are diprotonated (Fig. S6(c)†).

Fig. 2 The schematic stepwise assembly of 2a.

Fig. 3 (a) The sticks representation of polyanion 2a; (b) ball-and-stick representation of Na–O(1W)–Na bridges; (c) the distorted triangular prism coordination geometry of Na1 in 2a.

Making a general survey of 1a–3a, there are two points that worth mentioning. Firstly, both 1a and 2a possess two crystallographically unique types of P atoms: P1 exhibits PO₃C tetrahedral geometry [P1–O: 1.517(3)–1.535(3) Å; P1–C: 1.800(5)–1.804(5) Å] residing the apical site in the center of the ring where the basal plane is defined by three μ₃-O atoms and a C atom from H₂O₃PCH₂PO₃H₂ ligand; P2 also shows a PO₃C tetrahedral configuration [P2–O: 1.491(4)–1.534(4) Å; P2–C: 1.799(5)–1.812(5) Å] with one C atom from H₂O₃PCH₂PO₃H₂ ligand, two μ₂-O atoms and a terminal oxygen atom (O_t). While a little different is that P2 in 3a is defined by C1, two μ₂-O atoms and a μ₃-O atom [P2–O: 1.519(4)–1.527(4) Å; P2–C1: 1.797(5) Å] (Fig. S7†). Secondly, only two terminal oxygen atoms above/below the {Mo₆} plane in {(HO₃PCH₂PO₃)Mo₆O₂₂} unit are not shared by p-hydroxybenzoic acid for 1a and acetic acid for 2a. On the contrary, there are only two terminal oxygen atoms from {(O₃PCH₂PO₃) (Mo₆O₁₈(H₂O)₂)} unit in 3a are decorated by malonic acid ligand. The differences may be result from the stereohindrance effect. Compared with 2a, 3a is a sandwich-type structure and the sandwich layer {Mo₄O₁₈} group is decorated by malonic acid molecule, which possess an “S-shaped” structure. It is proven that using carboxylate ligands to functionalize heteropolymolybdates is feasible. At the same time, this fully shows the profound effect of carboxylic acid ligand in the self-assembly process on constructing POM-based architectures.

Synthesis discussion

To date, the noncovalent POM-based hybrids are well-developed. However, there still remains much research on the hybrids in which the POMs and the organic groups are linked through covalent interactions. A large number of diphosphonate-containing compounds in the literature are insoluble or unstable,²² while the Mo^VI complexes reported here are soluble and stable in aqueous solution as evidenced by ³¹P NMR spectroscopy. In this paper, carboxylic acid and diphosphate were chosen as ligands, and three compounds were synthesized in the aqueous solution by the one-pot reaction from sodium molybdate. The synthetic routes of compounds 1–3 are shown in Scheme 1. Further, in order to explore the best conditions, we adjusted the solution in terms of pH, counterion, molar ratio and temperature. Table S5† shows a summary of the reaction conditions and the factors affecting are analysed as follows: (a) the counterion is especially important. During the preparation of 1–3, when using K⁺ instead of [C₃N₂H₅]⁺ as the counterion, no crystals could be obtained. To our knowledge, the reason may be that the larger organic counterion [C₃N₂H₅]⁺ undergo extensive C–H⋯O hydrogen bonding interactions with the cluster, which is preferable for the formation of the framework.²³ (b) The molar ratio of Mo/P plays an important role in the composition and yield of the product and the best ratio of Mo/P is 10/3. (c) As it turns out, the reaction temperature between 60 °C and 80 °C has almost no significant effect on the quality of crystals. However, when the reaction temperature keeps below 60 °C, we failed to obtain the pure crystalline phase of compounds 1–3. (d) The pH value should be controlled in the range of 3.5 to 5.5. We could not obtained good quality crystals when the pH value is lower than 3.5. While the pH value is higher than 5.5, some amorphous precipitates in the course of crystallization are discovered and we filtrated it right now but it did not work to the formation of crystals.

Scheme 1 The synthetic pathways of 1–3 (all of 1–3 were stirred approximately 2 h at 70 °C).

XRPD and FT-IR patterns

The experimental XRPD patterns for 1–3 are in good agreement with the simulated XRPD patterns from the single-crystal X-ray diffraction, demonstrating the good phase purity for 1–3 (Fig. S2†). The differences in intensity between the experimental and simulated XRPD patterns might be due to the variation in preferred orientation of the powder sample during collection of the experimental XRPD.

The peak shapes of the IR spectra of 1–3 (Fig. S1(a)†) show similar features for the skeletal vibrations from 400 to 1700 cm⁻¹, indicating that 1–3 almost have the same basic framework. Meanwhile they are in good agreement with the results of single-crystal X-ray structural analyses. The peaks at 929 and 894 cm⁻¹, 932 and 898 cm⁻¹, 929 and 900 cm⁻¹ are attributed to the Mo–O_t characteristic vibration for 1, 2 and 3, respectively; and the peaks below 815 cm⁻¹ assigned to the bridging Mo–O–Mo vibration. The P–O vibrations are seen at 1170 and 1051 cm⁻¹ for 1, 1153 and 1052 cm⁻¹ for 2, and 1169 and 1079 cm⁻¹ for 3, which are respectively assigned to two environment phosphorus. Compared with the IR spectra of the uncoordinated carboxylic acid (1620–1740 cm⁻¹) (Fig. S1(b)†), the vibration bands of 1–3 show obvious red shifts, as a result of the coordination of carboxylic acid ligand with metal ion.²⁴ In addition, the peaks at 3360–3500 cm⁻¹ could be attributed to the characteristic vibrations of hydroxyl from lattice water molecules.

³¹P NMR spectroscopic characterization of compounds 1–3

NMR spectroscopy is a powerful tool to confirm the structure of polyanions and investigate their stability in solution. To verify the coordination environment of P atoms, the ³¹P NMR spectra of compounds 1–3 have been explored after they are dissolved in D₂O at room temperature. X-ray analysis reveals that there are two types of P atoms in the three structures. P1 should give one signal and P2 should give another signal. As shown in Fig. S4(d)† and Fig. 4, ³¹P NMR spectrum of H₂O₃PCH₂PO₃H₂ ligand possesses one resonance at 19.05 ppm and the ³¹P NMR spectra of compounds 1–3 all exhibit two resonances. Indeed, The singles at 17.98 ppm, 17.93 ppm and 18.18 ppm may be attributed to the presence of the central heteroatom P surrounded by a ring of six {MoO₆} octahedra for 1, 2 and 3, respectively. The resonance peak found at 25.45 ppm for 1, 25.45 ppm for 2 and 25.30 ppm for 3 can reveal the presence of the other species of P sitting above/below the {Mo₆} plane. This result agrees with the aforementioned conclusion by X-ray analysis.

Fig. 4 ³¹P NMR spectra of 1–3 dissolved in D₂O at room temperature.

Furthermore, to determine whether the structures characterized in the crystal state are stable in solution, the ³¹P NMR spectra with time from 0 to 72 h have been researched (Fig. S4†). The two resonances attributed to 1 and 2 have almost no changing suggest the coordination environment of P atoms unchanged with time. For 3, it shows very slightly shift of the resonance at 18.18 ppm after four hours. Twelve hours later, the two resonances at 25.30 ppm and 18.18 ppm are all slightly shifts that show the slow decomposition of 3. The ³¹P NMR spectroscopic studies show that the skeleton of 1 and 2 are stable in aqueous solution at least 72 h.

Cyclic voltammetric behavior of 1–, 2– and 3–GCEs in aqueous electrolyte

To explore the electrochemical property of compounds 1–3, cyclic voltammetric (CV) behavior in the range of 0–0.6 V were measured at different scan rates in Na₂SO₄/H₂SO₄ aqueous solution at pH = 1.0 (Fig. 5). The electrochemical behaviors of the 1–, 2– and 3–glassy carbon electrodes (GCEs) are similar except for some slight potential shift and the behavior of 1–GCE has been taken as an example. As shown in Fig. 5a, two pairs of redox peaks (I–I′, II–II′) are observed of compound 1. At the scan rate of 250 mV s⁻¹, the CVs of 1–GCE show the mean peak potentials E_1/2 = (E_pa + E_pc)/2 at +121 mV (I–I′) and +248 mV (II–II′). The half-wave potentials are at +131 mV (I–I′) and +265 mV (II–II′) for 2–GCE (Fig. 5b), +135 mV (I–I′) and +272 mV (II–II′) for 3–GCE (Fig. 5c), respectively. The redox peaks I–I′ and II–II′ can be ascribed to reversible Mo^VI/Mo^V and Mo^V/Mo^IV redox reactions. In other words, it is the result from two reversible one-electron processes of Mo atoms.²⁵ The cathodic peak potentials shift slightly towards negative potential values and the anodic peak potentials shift towards positive potential values with the increasing scan rates from 50 to 250 mV s⁻¹.

Fig. 5 Cyclic voltammograms of 1–3 (1 × 10⁻³ mol L⁻¹) under scan rates from inner to outer: 50, 100, 150, 200 and 250 mV s⁻¹; insert figures: the plots of the anodic and the cathodic peak I–I′ currents against scan rates for 1–3. Working electrode, glassy carbon; reference electrode, SCE.

In addition, the peak currents are proportional to the scan rate (insert figures in Fig. 5), which indicates that the redox processes are surface-controlled, and the exchanging rate of electrons is fast.²⁶ These results exhibit almost no structural effect of 1a–3a on the redox properties, except for some slight potential shifts of the redox peaks.

Conclusions

In this paper, three novel diphosphonate-containing POMs functionalized by carboxylic acid ligand have been isolated. The different structural characterizations of 1a–3a reveal that the coordinated carboxylate pendants play an important role in the formation of polyanions and show the strong chelating ability of this class as ligand. These results may open a way to the synthesis of a large family of hybrid POMs, which are functionalized with variable diphosphonate and carboxylic acid ligands, synchronously. The following work we will focus on synthesizing and further investigating more multidimensional hybrids diphosphonate–POMs by means of introducing different POMs framework or other organic ligands.

Acknowledgements

We gratefully acknowledge financial support from the Natural Science Foundation of China, the Natural Science Foundation of Henan Province for financial support.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: IR spectra of 1–3 and carboxylic acid ligands (Fig. S1). XPRD patterns of 1–3 (Fig. S2). The UV-vis spectra of 1–3 (Fig. S3). ³¹P NMR spectroscopic characterization of 1–3 with time and H₂O₃PCH₂PO₃H₂ (Fig. S4). Thermogravimetric analyses of 1–3 (Fig. S5). The bond valence sum calculations of Mo, P and O atoms on POM fragments in 1–3 (Fig. S6, Table S1, Table S2, Table S3 and Table S4). The coordination environment of P1 and P2 atoms in 1–3 (Fig. S7). Some other supporting figures (Fig. S8–S10) and Table S5. CCDC 1030525, 1030526 and 1030527. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra01955a

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