A series of organic–inorganic hybrid materials consisting of flexible organic amine modified polyoxomolybdates: synthesis, structures and properties

Abstract

A series of organic–inorganic hybrid complexes based on different types of polyoxomolybdates and transition metal complexes, namely, [Zn₂(TPMA)₂(H₂P₂Mo₅O₂₃)]·11H₂O (1), [Zn₂(TPMA)₂(Mo₈O₂₆)] (2), [Co₂(TPMA)₂(Mo₈O₂₆)] (3), [Ni₂(TPMA)₂(Mo₈O₂₆)(H₂O)₂] (4), [Ni₂(TPMA)₂(2-PA)(H₂O)](PMo₁₂O₄₀) (5) [Cu₂(TPMA)₂(Mo₈O₂₆)] (6), 2[Cu(TPMA)(CrMo₆(OH)₆O₁₈)]·H[Cu₂(TPMA)₂(CrMo₆(OH)₆O₁₈)]·4H₂O (7) (TPMA = Tris[(2-pyridyl)methyl]amine, 2-PA = 2-picolinic acid), have been successfully synthesized under hydrothermal conditions. All complexes were characterized by single-crystal X-ray structural analysis, powder X-ray diffraction, IR spectroscopy and TG analysis. All the complexes showed polyoxomolybdate-based zero-dimensional (0D) structures, and could be further extended into three-dimensional (3D) supramolecular frameworks through hydrogen bonding interactions. In addition, the electrochemical properties of complexes 1–7 have been investigated. Interestingly, some complexes have efficient photocatalytic activities to degradate pararosaniline hydrochloride dye molecules.

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Introduction

The development of multifunctional polyoxometalates represents one of the enduring challenges in the field of organic–inorganic hybrid materials.¹ As discrete molybdenum oxide clusters with structural and compositional diversities, polyoxomolybdates include isopolyoxomolybdates and heteropolyoxomolybdates, such as [Mo₆O₁₉]²⁻,² [Mo₇O₂₄]⁶⁻,³ [Mo₈O₂₆]⁴⁻,⁴ [P₂Mo₅O₂₃]⁶⁻,⁵ [PMo₁₂O₄₀]³⁻,⁶ [CrMo₆(OH)₆O₁₈]³⁻ (ref. 7) and so on. These polyoxomolybdates have a number of terminal and bridging O atoms, and can be modified by different metal ions and organic ligands to form organic–inorganic hybrid materials.⁸ As such, the hybrid materials have the dual characteristics of polyoxomolybdates and organic molecules and consequently, they received considerable attention due to their multi-functional properties, such as magnetism,⁹ catalysis,¹⁰ molecular recognition,¹¹ electrochemical activity.¹² However, to date, it still remains a challenge to rationally synthesize organic–inorganic hybrid materials because of a variety of variable factors such as metal ions, organic ligands, solvent, pH and temperature. Therefore, the optimal selection of metal ions and organic ligands plays a key role for the design and synthesis of corresponding functional organic–inorganic hybrid materials.

Even though there are numerous examples in the syntheses of diverse organic–inorganic hybrid materials by controlling the pH value,¹³ reaction temperature,¹⁴ reaction time,¹⁵ synthesis technologies¹⁶ and so on, it is anticipated that more specific crystalline materials could be achieved by systematically studying the experimental parameters and reaction mechanism of these synthetic processes could be elucidated to some extent. To explore intriguing variety of architecture of hybrid materials, chemists have embarked on the combination of different transition metal complexes with POMs cluster, and already got some novel structures.¹⁷ Obviously, this strategy could synthesize a variety of organic–inorganic hybrid materials incorporating different POM systems.

In the previous literature reports, multidentate flexible ligands Tris[(2-pyridyl)methyl]amine (TPMA) and their derivatives chelated with transition metal ions always shows good potential applications,¹⁸ such as magnetism, catalytic activity and drug delivery. However, the combination of these ligands and polyoxometalates haven't been reported. Thus, in this work, Tris[(2-pyridyl)methyl]amine (TPMA) was selected as the multidentate flexible ligand to assemble with different metal ions and various polyoxomolybdates, with the aim for constructing novel organic–inorganic hybrid materials (Scheme 1). Notably, a series of materials consisting of flexible organic amine (TPMA) modified polyoxomolybdates were obtained: [Zn₂(TPMA)₂(H₂P₂Mo₅O₂₃)]·11H₂O (1), [Zn₂(TPMA)₂(Mo₈O₂₆)] (2), [Co₂(TPMA)₂(Mo₈O₂₆)] (3), [Ni₂(TPMA)₂(Mo₈O₂₆)(H₂O)₂] (4), [Ni₂(TPMA)₂(2-PA)(H₂O)](PMo₁₂O₄₀) (5), [Cu₂(TPMA)₂(Mo₈O₂₆)] (6), 2[Cu(TPMA)(CrMo₆(OH)₆O₁₈)]·H[Cu₂(TPMA)₂(CrMo₆(OH)₆O₁₈)]·4H₂O (7). All complexes 1–7 shown polyoxomolybdates-based zero-dimensional (0D) structures, and could be further extended into 3D supramolecular frameworks through hydrogen bonding interactions. In addition, the electrochemical properties of complexes 1–7 have been investigated. Moreover, some complexes have efficient photocatalytic activities to degradate pararosaniline hydrochloride dye molecule.

Scheme 1 The main ligand (TPMA), auxiliary ligand (2-PA) and extra auxiliary reagent (imidazole).

Experimental

Materials and methods

All the chemicals were received as reagent grade and used without any further purification. The ligand TPMA and Na₃[CrMo₆(OH)₆O₁₈]·8H₂O were synthesized according to literatures.¹⁹ The IR spectra were obtained on a Varian 640 FT/IR spectrometer with KBr pellet in 400–4000 cm⁻¹ region. Powder X-ray diffraction (PXRD) was performed on a DX-2600 spectrometer. The thermal gravimetric analyses (TGA) were carried out in flowing N₂ on a SDT 2960 differential thermal analyzer with a rate of 10 °C min⁻¹. Cyclic voltammograms were obtained with a CHI 440A electrochemical workstation at room temperature. Platinum wire was used as a counter electrode and Ag/AgCl electrode was used as a reference electrode. Chemically bulk-modified carbon paste electrode (CPE) was used as the working electrode. The UV experiments were carried out on a Thermo EV 201CP.

Syntheses of TPMA ligand and complexes 1–7

Synthesis of Tris[(2-pyridyl)methyl]amine (TPMA). The synthesis route of TPMA is shown in the Scheme 2. To a suspension of K₂CO₃ (7.87 g, 0.057 mol) in 100 mL of MeCN, 2-(chloromethyl)pyridine hydrochloride (6.28 g, 0.39 mol) was added to produce an orange suspension. Then, 2-picolylamine (2.09 g, 0.19 mol) was added. The mixture was stirred vigorously at 80 °C for 3 days and the product was purified by column chromatography (1 : 1 CH₂Cl₂/MeOH). The final product was brown solid powder (5.51 g, 65.7% yield).

Scheme 2 Synthetic route for the TPMA ligand.

Synthesis of compound [Zn₂(TPMA)₂(H₂P₂Mo₅O₂₃)]·11H₂O (1). A mixture solution of Zn(NO₃)₂ (29.7 mg, 0.1 mmol), TPMA (29 mg, 0.1 mmol), (NH₄)₆Mo₇O₂₄ (123 mg, 0.1 mmol), H₃PO₄ (0.5 mL) and H₂O (10 mL) was stirred at room temperature. The pH was adjusted to 5.0 with 4.0 mol L⁻¹ NaOH, and then the suspension was transferred to a Teflon-lined reactor and kept under autogenous pressure at 160 °C for 3 days. After slow cooling to room temperature, colorless needle-like crystals were filtered and washed with distilled water (yield 47% based on Zn). IR (solid KBr pellet, cm⁻¹): 3500(s), 1687(s), 1513(w), 1498(m), 1024(w), 937(s), 762(w), 733(s).

Synthesis of compound [Zn₂(TPMA)₂(Mo₈O₂₆)] (2). A mixture solution of Zn(NO₃)₂ (29.7 mg, 0.1 mmol), TPMA (29 mg, 0.1 mmol), Na₃[CrMo₆(OH)₆O₁₈]·8H₂O (120 mg, 0.1 mmol) and H₂O (10 mL) was stirred at room temperature. The pH was 5.0, and then the suspension was transferred to a Teflon-lined reactor and kept under autogenous pressure at 160 °C for 3 days. After slow cooling to room temperature, colorless needle-like crystals were filtered and washed with distilled water (yield 31% based on Zn). IR (solid KBr pellet, cm⁻¹): 3588(s), 1692(s), 1501(m), 1428(s), 1395(s), 1312(m), 960(s), 844(s), 712(s).

Synthesis of compound [Co₂(TPMA)₂(Mo₈O₂₆)] (3). A mixture solution of CoCl₂ (23.8 mg, 0.1 mmol), TPMA (29 mg, 0.1 mmol), (NH₄)₆Mo₇O₂₄ (123 mg, 0.1 mmol), imidazole (6 mg, 0.1 mmol) and H₂O (10 mL) was stirred at room temperature. The pH was 5.0, and then the suspension was transferred to a Teflon-lined reactor and kept under autogenous pressure at 160 °C for 3 days. After slow cooling to room temperature, green block crystals were filtered and washed with distilled water (yield 29% based on Co). IR (solid KBr pellet, cm⁻¹): 3429(w), 3117(w), 1648(m), 1481(m), 1277(w), 1039(m), 977(s), 841(s), 732(m).

Synthesis of compound [Ni₂(TPMA)₂(Mo₈O₂₆)(H₂O)₂] (4). A mixture solution of NiCl₂ (23.7 mg, 0.1 mmol), TPMA (29 mg, 0.1 mmol), (NH₄)₆Mo₇O₂₄ (123 mg, 0.1 mmol), imidazole (6 mg, 0.1 mmol) and H₂O (10 mL) was stirred at room temperature. The pH was 5.0, and then the suspension was transferred to a Teflon-lined reactor and kept under autogenous pressure at 160 °C for 3 days. After slow cooling to room temperature, light yellow block crystals were filtered and washed with distilled water (yield 26% based on Ni). IR (solid KBr pellet, cm⁻¹): 3362(m), 3127(m), 1766(s), 1714(m), 1501(m), 1484(s), 1122(s), 1006(s), 926(s), 864(s), 689(m).

Synthesis of compound [Ni₂(TPMA)₂(2-PA)(H₂O)](PMo₁₂O₄₀) (5). A mixture solution of NiCl₂ (23.7 mg, 0.1 mmol), TPMA (29 mg, 0.1 mmol), (NH₄)₆Mo₇O₂₄ (12.3 mg, 0.1 mmol), 2-picolinic acid (15.2 mg, 0.1 mmol), H₃PO₄ (0.5 mL) and H₂O (10 mL) was stirred at room temperature. The pH was adjusted to 5.0 with 4 mol L⁻¹ NaOH, and then the suspension was transferred to a Teflon-lined reactor and kept under autogenous pressure at 160 °C for 3 days. After slow cooling to room temperature, black columnar crystals were filtered and washed with distilled water (yield 15% based on Ni).

Synthesis of complex [Cu₂(TPMA)₂(Mo₈O₂₆)] (6). A mixture solution of Cu(NO₃)₂ (24.3 mg, 0.1 mmol), TPMA (29.2 mg, 0.1 mmol), (NH₄)₆Mo₇O₂₄ (123 mg, 0.1 mmol), imidazole (6 mg, 0.1 mmol) and H₂O (10 mL) was stirred at room temperature. The pH was adjusted to 2.5 with 4.0 mol L⁻¹ HCl, and then the suspension was transferred to a Teflon-lined reactor and kept under autogenous pressure at 160 °C for 3 days. After slow cooling to room temperature, green block crystals were filtered, washed with distilled water and separated manually (yield 11% based on Cu). IR (solid KBr pellet, cm⁻¹): 3455(w), 3131(w), 2933(w), 1611(s), 1432(s), 1369(w), 1312(w), 1273(w), 935(s), 824(s), 689(w).

Synthesis of complex 2[Cu(TPMA)(CrMo₆(OH)₆O₁₈)]·H[Cu₂(TPMA)₂(CrMo₆(OH)₆O₁₈)]·4H₂O (7). A mixture solution of Cu(NO₃)₂ (24.3 mg, 0.1 mmol), TPMA (29.2 mg, 0.1 mmol), Na₃[CrMo₆(OH)₆O₁₈]·8H₂O (120 mg, 0.1 mmol) and H₂O (10 mL) was stirred at room temperature. The pH was 5.0, and then the suspension was transferred to a Teflon-lined reactor and kept under autogenous pressure at 160 °C for 3 days. After slow cooling to room temperature, light yellow block crystals were filtered and washed with distilled water (yield 23% based on Cu). IR (solid KBr pellet, cm⁻¹): 3469(w), 3058(w), 1590(s), 1426(s), 1306(m), 1151(m), 1011(s), 944(s), 780(s), 572(w).

Preparation of complexes 1–7 bulk-modified CPEs

The preparation method:²⁰ the bulk-modified Carbon Paste Electrode (CPE) was fabricated by mixing 100 mg graphite powder and 10 mg target compound in an agate mortar for approximately 30 min to achieve a uniform mixture; then two drops methylsilicone oil was added and stirred with a copper rod. The homogenized mixture was packed into a 2 mm inner diameter glass tube and the tube surface was wrapped with weighing paper. The electrical contact was established with the copper wire through the back of the electrode. The bare CPE was prepared by similar process without target complexes.

X-ray crystallography

Single crystal X-ray diffraction analysis data for complexes 1–7 were collected with an Oxford Diffraction Gemini R Ultra diffractometer with graphite-monochromated Mo-Kα (λ = 0.71073 Å) or Cu-Kα (λ = 1.54184 Å) at 296 K. All structures were solved by direct methods and refined on F² by full-matrix least-squares methods using the SHELXTL package.²¹ In the structure of compound 1, hydrogen atoms attached to water molecules with partial site occupancy were not located. A summary of the crystal data and structure refinements of complexes 1–7 are provided in Table 1. Selected bond lengths and angles are listed in Table S1.† Crystallographic data for complexes 1–7 have been deposited in the Cambridge Crystallographic Data Center, their crystal structures can be obtained with CCDC reference numbers 1471018–1471020 (1–3), 1471022 (4), 1471021 (5), 1486195 (6), 1486075 (7), respectively.

Table 1 Crystal data and structure refinements for complexes 1–7

Complexes	1	2	3	4
a R1 = ∑||Fo| − |Fc||/∑|Fo|.b wR2 = ∑[w(Fo^2 − Fc^2)^2]/∑[w(Fo^2)^2]^1/2.
Empirical formula	C36H60Mo5N8O34P2Zn2	C36H36Mo8N8O26Zn2	C36H36Mo8N8O26Co2	C36H40Mo8N8O28Ni2
Formula weight	1821.30	1894.99	1882.11	1917.67
Temperature/K	296(2)	296(2)	296(2)	296(2)
Crystal system	Monoclinic	Orthorhombic	Orthorhombic	Monoclinic
Space group	P21/c	Pbca	Pbca	P21/n
a/Å	11.8566(5)	16.6740(4)	16.7082(3)	12.2553(4)
b/Å	19.7359(8)	15.9270(3)	15.9095(3)	15.3666(4)
c/Å	26.2178(10)	19.1977(4)	19.2987(3)	15.4604(5)
α/°	90	90	90	90
β/°	100.164(4)	90	90	110.489(4)
γ/°	90	90	90	90
Volume/Å^3	6038.7(4)	5098.28(19)	5129.96(17)	2727.36(14)
Z	4	4	4	2
ρcalc/g cm^−3	2.003	2.469	2.437	2.3349
μ/mm^−1	1.938	2.915	2.609	2.540
F(000)	3616	3648	3624	1830.6
Radiation	MoKα	MoKα	MoKα	MoKα
Reflections collected	24 542	12 810	15 244	12 597
Independent reflections	10 612	4498	5988	4794
Rint	0.0465	0.0226	0.0215	0.0273
GOOF	1.127	1.055	1.057	1.056
R1,^a wR2^b [I ≥ 2σ(I)]	0.0731, 0.1708	0.0214, 0.0472	0.0237, 0.0508	0.0292, 0.0634
R1, wR2 [all data]	0.0942, 0.1789	0.0291, 0.0495	0.0340, 0.0547	0.0394, 0.0692

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Complexes	5	6	7
Empirical formula	C42H42Mo12N9O43PNi2	C36H36Mo8N8O26Cu2	C72H99Mo18N16O76Cr3Cu4
Formula weight	2660.48	1891.33	4541.75
Temperature/K	296(2)	296(2)	296(2)
Crystal system	Monoclinic	Orthorhombic	Triclinic
Space group	P21/c	Pbca	P1̄
a/Å	11.2559(4)	16.8760(3)	14.4947(6)
b/Å	24.1685(11)	15.9080(3)	15.7549(4)
c/Å	25.7463(9)	18.9788(3)	15.8183(6)
α/°	90	90	99.314(3)
β/°	100.287(4)	90	116.519(4)
γ/°	90	90	91.546(3)
Volume/Å^3	6891.4(5)	5095.10(15)	3169.4(2)
Z	4	4	1
ρcalc/g cm^−3	2.564	2.466	2.380
μ/mm^−1	18.982	2.81	2.718
F(000)	5104	3640	2195
Radiation	CuKα	MoKα	MoKα
Reflections collected	29 399	15 971	23 137
Independent reflections	13 356	5920	11 152
Rint	0.0561	0.0276	0.0227
GOOF	1.059	1.051	1.021
R1, wR2 [I ≥ 2σ(I)]	0.0525, 0.1122	0.0260, 0.0521	0.0280, 0.0610
R1, wR2 [all data]	0.0935, 0.1315	0.0386, 0.0582	0.0392, 0.0651

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Results and discussion

Structural description of complexes 1–7

[Zn₂(TPMA)₂(H₂P₂Mo₅O₂₃)]·11H₂O (1). Compound 1 crystallizes in the monoclinic space group P2₁/c, and consists of one [H₂P₂Mo₅O₂₃]⁴⁻ (abbreviated as P₂Mo₅) polyoxoanion, two TPMA ligands, two Zn(II) ions and eleven interstitial water molecules. Bond valence sum calculations²² show that all molybdenum atoms are in the +6 oxidation state, and zinc atoms are in the +2 oxidation state. Due to the principle of charge conservation, two protons are added in the P₂Mo₅ polyoxoanion.

In the structure of 1, there are two crystallographically independent Zn(II) ions (Fig. 1a). Zn1 ion is five-coordinated in a triangular bipyramid configuration by one O atom from P₂Mo₅ polyoxoanion (Mo–O) and four N atoms from TPMA ligand, in which three N atoms from pyridine generate an approximate triangle plane, and O atom and the fourth N atom serve as vertices. Zn2 ion is also five-coordinated in a distorted triangular bipyramid geometry by four N atoms from another TPMA ligand and one O atom from the same P₂Mo₅ polyoxoanion (P–O). The bond lengths ranges of Zn–O and Zn–N are 1.917(6)–1.977(5) Å and 2.066(8)–2.245(7) Å, respectively (Table S1†). The unit of compound 1 are connected by intermolecular hydrogen bonds [C(26)–H(26)⋯O(16), 3.234(11) Å] into a 1D supramolecular left-handed chain, and the adjacent chains are right-handed chains, therefore compound 1 is achiral complex (Fig. 1b and c). Then adjacent 1D chains are further linked into 2D supramolecular layer through hydrogen bonds [C(36)–H(36)⋯O(22), 3.205(12) Å] (Fig. 1d). 2D layers are extended into a 3D supramolecular framework through H-bonding interaction [C(30)–H(30)⋯O(15), 3.363(12) Å] (Fig. 1e).

Fig. 1 The structure of compound 1: (a) coordination environments for Zn ions and P₂Mo₅ polyoxoanion (the hydrogen atoms are omitted for clarity). (b) and (c) The ball-and-stick and space-filling view of left-handed 1D chain by hydrogen bonding interactions along black arrow direction, respectively. (d) 2D supramolecular layer and (e) 3D supramolecular framework formed by hydrogen bonding interactions.

Zn₂(TPMA)₂(Mo₈O₂₆) (2), Co₂(TPMA)₂(Mo₈O₂₆) (3) and Cu₂(TPMA)₂(Mo₈O₂₆) (6). Complex 2 crystallizes in the orthorhombic space group Pbca, and consists of one [Mo₈O₂₆]⁴⁻ (abbreviated as Mo₈) polyoxoanion, two TPMA ligands and two Zn(II) ions. Bond valence sum calculations²² show that all molybdenum atoms are in the +6 oxidation state, and zinc atoms are in the +2 oxidation state.

In compound 2, the coordination environments of the two Zn(II) ions are similar to each other (Fig. 2a). Zn ion is five-coordinated in a triangular bipyramid geometry by four N atoms from TPMA ligand and one O atom from Mo₈ polyoxoanion (Mo–O). The bond lengths ranges of Zn–O and Zn–N are 1.998(2) Å and 2.046(3)–2.219(3) Å, respectively (Table S1†). Interestingly, it is worth mentioning that the Mo₈ structure is transformed from the starting material CrMo₆ and this kind of transformation between different POM structures have been reported in the literatures.²³

Fig. 2 The structure of compound 2. (a) Coordination environments for Zn ions and Mo₈ polyoxoanion (the hydrogen atoms are omitted for clarity). (b) 2D supramolecular layer by H bonds interactions. (c) 3D supramolecular framework by hydrogen bonding interactions.

TPMA are coordinated with Zn(II) ions and then located on both sides of Mo₈ to form a sandwich structure. The sandwich structures are connected by intermolecular hydrogen bonds [C(16)–H(16)⋯O(3), 3.182(4) Å] into a 2D supramolecular layer (Fig. 2b). 2D layers are extended into a 3D supramolecular framework through H-bonding interaction [C(6)–H(6B)⋯O(8), 3.359(4) Å] (Fig. 2c).

Complexes 2, 3 and 6 are isostructural and possess different metal ions. In compound 3, the bond lengths ranges of Co–O and Co–N are 1.9695(19) Å and 2.045(2) Å–2.208(2) Å, respectively (Table S1†). In compound 6, the bond lengths ranges of Cu–O and Cu–N are 1.930(2) Å and 2.037(3) Å–2.089(3) Å, respectively (Table S1†).

[Ni₂(TPMA)₂(Mo₈O₂₆)(H₂O)₂] (4). Compound 4 crystallizes in the monoclinic space group P2₁/n, and consists of one [Mo₈O₂₆]⁴⁻ (abbreviated as Mo₈) polyoxoanion, two TPMA ligands, two Ni(II) ions and two coordinated water molecules. Bond valence sum calculations²² show that all molybdenum atoms are in the +6 oxidation state, and nickel atoms are in the +2 oxidation state.

In compound 4, the coordination environments of the two Ni(II) ions are same (Fig. 3a). Ni ion is six-coordinated in a distorted octahedral geometry by four N atoms from TPMA ligand, one O atom from Mo₈ polyoxoanion (Mo–O) and one O atom from water molecule. The bond lengths ranges of Cu–O and Cu–N are 1.943(4)–2.782(5) Å and 1.945(5)–1.993(5) Å, respectively (Table S1†). TPMA are coordinated with Ni(II) ions and then located on both sides of Mo₈ to form a sandwich structure. The sandwich structures are connected by intermolecular hydrogen bonds [C(16)–H(16)⋯O(13), 3.102(4) Å] into a 2D supramolecular layer (Fig. 3b). 2D layers are extended into a 3D supramolecular framework through H-bonding interaction [C(2)–H(2)⋯O(12), 3.171(4) Å] (Fig. 3c).

Fig. 3 The structure of complex 4. (a) Coordination environments for Ni ions and Mo₈ polyoxoanion (the hydrogen atoms are omitted for clarity). (b) 2D supramolecular layer by H bonds interactions. (c) 3D supramolecular framework by H bonds interactions.

[Ni₂(TPMA)₂(2-PA)(H₂O)](PMo₁₂O₄₀) (5). Complex 5 crystallizes in the monoclinic space group P2₁/c, and consists of one [PMo₁₂O₄₀]³⁻ (abbreviated as PMo₁₂) polyoxoanion, two TPMA ligands, one 2-PA ligand, two Ni(II) ions and one coordinated water molecule. Bond valence sum calculations²² show that all molybdenum atoms are in the +6 oxidation state, and nickel atoms are in the +2 oxidation state.

In complex 5, the coordination environments of the two Ni(II) ions are different (Fig. 4a). Ni1 ion is six-coordinated in a distorted octahedral geometry by four N atoms from TPMA ligand, one N atom and one O(O–H) atom from 2-PA. Ni2 ion is six-coordinated in a distorted octahedral geometry by four N atoms from another TPMA ligand, one O(C=O) atoms from the same 2-PA and one O atom from water molecule. The bond lengths ranges of Ni–O and Ni–N are 2.002(7)–2.100(7) Å and 2.046(8)–2.112(9) Å, respectively (Table S1†).

Fig. 4 The structure of compound 5. (a) Coordination environments for Ni ions (the hydrogen atoms are omitted for clarity). (b) ‘Z’-type 1D chain by H bonds interactions. (c) 3D supramolecular framework by H bonds interactions.

Two Ni(II) ions are bridged by the carboxyl of 2-PA to produce a dinuclear Ni(II) cluster. The dinuclear Ni(II) cluster are connected by intermolecular hydrogen bonds [C(39)–H(39)⋯O(39), 3.145(11) Å; C(15)–H(15)⋯O(31), 3.128(11) Å] into a 1D supramolecular ‘Z’-type chain (Fig. 4b), and the adjacent chains are extended into a 3D supramolecular framework through H-bonding interaction [C(7)–H(7)⋯O(18), 3.127(12) Å; C(8)–H(8)⋯O(5), 3.125(12) Å] (Fig. 4c).

2[Cu(TPMA)(CrMo₆(OH)₆O₁₈)]·H[Cu₂(TPMA)₂(CrMo₆(OH)₆O₁₈)]·4H₂O (7). Compound 7 crystallizes in the triclinic space group P1̄. Intriguingly, there are two kinds of independent structures and two interstitial water molecules in 7. The first structure consists of one [CrMo₆(OH)₆O₁₈]³⁻ (abbreviated as CrMo₆) polyoxoanion, one TPMA ligands and one copper ion. The second structure consists of one CrMo₆ polyoxoanion, two TPMA ligands and two copper ions. Bond valence sum calculations²² show that all molybdenum atoms are in the +6 oxidation state, chrome atoms are in the +3 oxidation state, and copper atoms are in the +2 oxidation state.

In the first structure of compound 7, Cu1 ion is five-coordinated in a trigonal bipyramid geometry by four N atoms from TPMA ligand and one O atom from CrMo₆ polyoxoanion. In the second structure of 7, the coordination environments of the two Cu ions are same and symmetry related (Fig. 5a), Cu2 ion is also five-coordinated in a trigonal bipyramid geometry by four N atoms from TPMA ligand and one O atom from CrMo₆ polyoxoanion. The bond lengths ranges of Cu–O and Cu–N are 1.914(4) Å–1.994(4) Å and 2.018(6) Å–2.079(7) Å, respectively (Table S1†).

Fig. 5 The structure of compound 7. (a) Coordination environments for Cu ions (the hydrogen atoms are omitted for clarity). (b) The 1D chain by H bonds interactions in the connection mode of ABAB. (c) 2D supramolecular layer and (d) 3D supramolecular framework connected by H bonds interactions.

The two independent structures are linked by intermolecular hydrogen bonds [C(21)–H(21)⋯O(21), 3.024(11) Å] into a 1D supramolecular chain with the connection mode of ABAB (Fig. 5b), and the adjacent chains are connected by intermolecular hydrogen bonds [C(3)–H(3)⋯O(23), 3.074(11) Å] into a 2D supramolecular layer (Fig. 5c). Further, 2D layers are extended into a 3D supramolecular framework through H-bonding interaction [C(20)–H(20)⋯O(7), 3.156(12) Å] (Fig. 5d).

FT-IR spectra and powder X-ray diffraction

The IR spectra of complexes 1–7 are shown in Fig. S1.† For compound 1, the bands at 856, 793, 627 and 963 cm⁻¹ could be ascribed to the characteristic peaks of ν_as(Mo–O_t), ν_as(Mo–O–Mo) and ν_as(P–O). For compound 2, the bands at 1125, 826 and 750 cm⁻¹ could be ascribed to the characteristic peaks of ν_as(Mo–O_t) and ν_as(Mo–O–Mo). For compound 3, the bands at 985, 779 and 742 cm⁻¹ could be ascribed to the characteristic peaks of ν_as(Mo–O_t) and ν_as(Mo–O–Mo). For compound 4, the bands at 994, 829 and 728 cm⁻¹ could be ascribed to the characteristic peaks of ν_as(Mo–O_t) and ν_as(Mo–O–Mo). For compound 5, the bands at 936, 803, 728, 1090 cm⁻¹ could be ascribed to the characteristic peaks of ν_as(Mo–O_t), ν_as(Mo–O–Mo) and ν_as(P–O), the bands at 1619 cm⁻¹ could be ascribed to the characteristic peaks of ν_as(C=O) from the ancillary ligand of 2-PA. For compound 6, the bands at 963, 832 and 709 cm⁻¹ could be ascribed to the characteristic peaks of ν_as(Mo–O_t) and ν_as(Mo–O–Mo). For compound 7, the bands at 1016, 954 and 793 cm⁻¹ could be ascribed to the characteristic peaks of ν_as(Mo–O_t) and ν_as(Mo–O–Mo). For 1–7, the bands in the region of 1700 to 1200 cm⁻¹ could be ascribed to the characteristic peaks of the TPMA ligand. The bands in the region of 3750 to 3000 cm⁻¹ could be ascribed to the characteristic peaks of the ν_as(–OH) from the water molecules.

To indicate the phase purities of complexes 1–7, PXRD experiments were carried out. As shown in Fig. S2,† diffraction peaks of both simulated and experimental patterns match well in the key positions, which indicate the phase purity of bulky samples. The differences in intensity may be aroused from the preferred orientation of the powder samples.²⁴

Thermal stability analyse. As shown in Fig. S3,† the thermal stabilities of complexes 1–7 were investigated under N₂ atmosphere from room temperature to 800 °C. For 1, two steps of weight loss are observed. The first weight loss of 2.77% (calc. 10.87%) occurs before 296 °C, which is attributed to the loss of eleven free water molecules. The second weight loss is ascribed to the decomposition of the remaining structure. As for 2, its structure starts to collapse when the temperature reaches to 363 °C. Similarly, the collapsing temperature for 3 is 358 °C. For 4, there are two steps of weight loss. The first weight loss of 2.00% (calc. 1.88%) occurs before 247 °C, which is ascribed to the loss of two coordinated water molecules. The second weight loss is attributed to the decomposition of the remaining structure. For 5, its structure starts to collapse when the temperature reaches to 134 °C. For 6, its structure starts to collapse when the temperature reaches to 307 °C without the process of the loss of water molecules. For 7, two steps of weight loss are observed. The first weight loss of 0.38% (calc. 0.58%) occurs before 294 °C, which is attributed to the loss of two free water molecules. The second weight loss is ascribed to the decomposition of the remaining structure. According to the results of the above analysis, the basic skeletons of all the complexes display good thermal stability.

Electrochemical properties

The electrochemical properties of 1–7 have been investigated in detail in 0.5 M Na₂SO₄ + 0.1 M H₂SO₄ aqueous solution (Fig. 6 and S4†). In the potential range from 0–500 mV, there are two pairs of reversible redox peaks (I–I′ and II–II′) for compound 1. The mean peak potentials E_1/2 = (E_cp + E_ap)/2 (scan rates: 200 mV s⁻¹) are 273 and 83 mV, respectively. In the potential range from 0–500 mV, there are three pairs of reversible redox peaks (I–I′, II–II′, III–III′) for compound 2. The mean peak potentials E_1/2 = (E_cp + E_ap)/2 (scan rates: 200 mV s⁻¹) are 394, 248 and 101 mV, respectively. While compound 3 shows two pairs of weak redox peaks. For compound 4, there are three pairs of reversible redox peaks (I–I′, II–II′, III–III′) in which the mean peak potentials E_1/2 = (E_cp + E_ap)/2 (scan rates: 200 mV s⁻¹) are 372, 239 and 112 mV, respectively. In the potential range from −200 to 500 mV, there are three pairs of obvious reversible redox peaks (I–I′, II–II′, III–III′) for compound 5. The mean peak potentials E_1/2 = (E_cp + E_ap)/2 (scan rates: 200 mV s⁻¹) are 344, 125 and −168 mV, respectively. In the potential range from 0–400 mV, there are two pairs of reversible redox peaks (I–I′ and II–II′) for 6, and their mean peak potentials E_1/2 = (E_cp + E_ap)/2 (scan rates: 200 mV s⁻¹) are 222 and 110 mV, respectively. In the potential range from 100–600 mV, there is only one pair of weak redox peak (I–I′) for complex 7. The mean peak potentials E_1/2 = (E_cp + E_ap)/2 (scan rates: 200 mV s⁻¹) is 221 mV.

Fig. 6 Cyclic voltammograms of 1–6 CPEs in 0.5 M Na₂SO₄ + 0.1 M H₂SO₄ aqueous solution at distinct scanning rates (from inside to out: 50, 100, 150, 200, 250, 300, 350, 400, 450, 500 mV s⁻¹) respectively.

As for 1–7, the redox peaks may arise from the redox process of Mo-based polyoxoanion clusters, but the pairs number of redox peaks are different which may be caused by the diverse coordination modes of those complexes.²⁵ The cathodal peak potentials of complexes 1–7 move to the minus orientation, and the corresponding anodal peak potentials move toward the plus orientation with increasing scanning rates.

Photocatalytic activities

It is well known that a wide range of polyoxomolybdates possess photocatalytic activities in the degradations of organic dyes under UV irradiation.²⁶ Thus, we explored the photocatalytic performances of complexes 1–7 for the photodegradation of pararosaniline hydrochloride with UV irradiation through a typical process: complexes 1–7 (20 mg) were added to the 50 mL aqueous solution of pararosaniline hydrochloride (6 mg L⁻¹), respectively, and stirred for about 15 minutes to ensure the equilibrium of the resulting solution. Then the solution was exposed to UV irradiation from a 100 W Hg lamp (λ = 365 nm). The solution was kept for stirring during the irradiation. At every 30 min intervals, 5 mL of sample was taken out for UV analysis.

As observed in Fig. 7, complexes 1, 4, 5, 6 and 7 display excellent degradation effect for pararosaniline hydrochloride. It is obvious that the absorption peaks of pararosaniline hydrochloride decreased significantly with illumination time. The calculation results show that the conversion of pararosaniline hydrochloride after irradiation in 3 hours is 93.25% for 1, 87.87% for 4, 76.48% for 5, 81.31% for 6, 90.16% for 7, respectively. Though complexes 2 and 3 have similar structures with complex 6, but their photocatalytic activities are far less than that of complex 6. As shown in Fig. S5† and 7f, their degradation efficiency for pararosaniline hydrochloride are only 31.71% and 32.17%, respectively, this result maybe caused by the difference of their center metal ions. By contrast, the order of photocatalytic activity is 1 > 7 > 4 > 6 > 5 (Fig. 7f). The photocatalytic behaviors of those five complexes indicate that they may be good photocatalysts for the photodegradation of organic dyes.

Fig. 7 Photocatalytic activities of complexes 1 (a), 4 (b), 5 (c), 6 (d), 7 (e) and the comparison for their degradation rates of complexes 1–7 (f), inside photo: before (left) and after (right) illumination.

Conclusions

In this contribution, a series of organic–inorganic hybrid complexes based on different types of polyoxomolybdates and transition complexes have been successfully synthesized under hydrothermal conditions. Complexes 1–7 show polyoxomolybdates-based zero-dimensional (0D) structures, and can be further extended into three-dimensional (3D) supramolecular frameworks through hydrogen bonding interactions. Intriguingly, four kinds of polyoxometalates (P₂Mo₅, Mo₈, PMo₁₂, CrMo₆) present in complexes 1–7 and this indicates the diversity of polyoxomolybdates. Additionally, the electrochemical properties of complexes 1–7 have been investigated. Moreover, some complexes have efficient photocatalytic activities to degradate pararosaniline hydrochloride dye molecule. It is believed that this work could provide useful information for the construction of multifunctional polyoxomolybdates-based organic–inorganic hybrid materials and various architectures of those materials could be observed consistently.

Acknowledgements

Financial support from the National Natural Science Foundation of China (21371078, 21401077, 21472029), the National Key Basic Research Program of China (2013CB934200) is gratefully acknowledged.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. CCDC 1471018–1471020 (1–3), 1471022 (4), 1471021 (5), 1486195 (6) and 1486075 (7). For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra21603j

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