Syntheses, Structures, and Photocatalytic and Sonocatalytic Degradations of Methyl Blue of Cu(II) and Mn(II) Coordination Polymers Based on Tri(triazole) and Dicarboxylate Ligands

Abstract

Cu(II) and Mn(II) coordination polymers [Cu(ttpa)(sub)]_n (Cuttpa or 1) and {[Mn₂(ttpa)₂(nip)₂(H₂O)₂]·3H₂O}_n (Mnttpa or 2) (ttpa = tris(4-(1,2,4-triazol-1-yl)phenyl)amine, H₂sub = suberic acid, nip = 5-nitroisophthalicate) were hydrothermally prepared and the structures were characterized. Cuttpa exhibited a 2D (4,4) network based on [Cu₂(COO)₄] dimers with upper and lower dangled ttpa ligands and a 2D → 3D polythreaded network. Mnttpa showed a 2D (4,4) network with dangled uncoordinated triazole rings from ttpa ligands and nitro groups from nip²⁻ ligands and a 2D → 3D polythreaded network. E_g data of Cuttpa and Mnttpa were 1.88 eV and 2.11 eV. Cuttpa and Mnttpa exhibited good catalytic activity for the decomposition of methyl blue (MB) under visible light and supersound irradiation. The decomposition mechanism using Cuttpa was explored. The holes (h⁺) and ^•OH hydroxyl radicals played the main roles, and the ^•O₂⁻ superoxide radicals played certain auxiliary roles in the decomposition of MB within the Cuttpa catalyst.

1. Introduction

In recent years, more and more inorganic chemistry and materials scientists have become interested in coordination polymers, not only due to their greatly varying and interesting structures [1], but also due to their multifunctional materials for use in applications such as chemical sensors [2], gas storage and adsorption [3], adsorbents for the elimination of toxic chemicals and pollutants [4], catalysis [5], magnetism [6], luminescence [7] and biology [8]. Entanglement is an important topic related to coordination polymers which usually allows for improvement in their packing efficiency when one single network has large free voids [9]. In addition to interpenetration, polycatenation and polyknots, polythreaded systems represent one more intriguing subgroup of entanglement [9,10,11]. Polythreaded frameworks are usually constructed from low-dimensional networks such as 0D, 1D and 2D networks with dangled side arms which are frequency assembled by longer organic ligands [9,10,11]. Even now, polythreaded systems are still less commonly explored. Examples of 2D → 3D polythreaded networks are scarcely documented [12,13,14,15,16].

With the population increasing and social development, the increasing prevalence of organic wastewater is a serious environmental topic. In the past two decades, the effective treatment of organic wastewater has been an important topic. Compared with other methods, such as membrane technology, coagulation and flocculation, to remove dyes from dirty water, photocatalysis has its own unique advantages, such as simple manipulation procedures, low energy consumption, no secondary pollution and high degradation efficiency, and it can use photocatalysts to degrade organic dyes into non-toxic small molecules with solar energy [17]. Nevertheless, the traditional inorganic semiconductor photocatalysts such as TiO₂ and ZnO with high band gaps (E_g) (3.2 eV for TiO₂, 3.4 eV for ZnO) only could utilize a small amount of ultraviolet light (<4%) or visible light or sunlight from solar energy [18,19]. Sunlight contains approximately 45% visible light. Coordination polymer catalysts showing low band gaps could degrade organic dyes under visible light irradiation or sunlight with high efficiency [20,21,22,23,24,25]. This is one good example of utilizing solar energy. Meanwhile, the sonocatalytic degradation of organic dyes under ultrasound also is one good method of removing organic dyes [26].

The topologies and properties of coordination polymers are strongly dependent on central metal cations and ligands [27,28,29,30]. N-donor ligands (containing pyridine, triazole, imidazole) and O-donor carboxylate co-ligands are widely employed for constructing coordination polymers [27,28,29,30]. Tris(4-(1,2,4-triazol-1-yl)phenyl)amine (ttpa) contains three triazole rings around the triphenylamine center and can strongly coordinate to metal ions with its three four-position triazole N atoms [31,32,33]. Multicarboxylate ligands such as suberic acid (H₂sub) and 5-nitroisophthalic acid (H₂nip) containing two carboxylate groups can link metal ions with diverse coordination modes and can deprotonate to balance the charge of the coordination network [34,35]. Herein, copper(II) and manganese(II) coordination polymers, [Cu(ttpa)(sub)]_n (Cuttpa or 1) and {[Mn₂(ttpa)₂(nip)₂(H₂O)₂]·3H₂O}_n (Mnttpa or 2), were hydrothermally successfully prepared by employing the N-donor ligand ttpa and O-donor ligands H₂sub and H₂nip. Their structures were characterized. Cuttpa has a 2D (4,4) network with a Cu₂(COO)₄] unit and a 2D → 3D polythreaded framework. Mnttpa has a 2D (4,4)-network and a 2D → 3D polythreaded framework. The catalytic decomposition of methyl blue (MB) under visible light and supersound irradiation were observed. The decomposition mechanism using Cuttpa was studied.

2. Results and Discussion

2.1. Structures

Cuttpa has a 2D (4,4)-network with a Cu₂(COO)₄] unit and a 2D → 3D polythreaded framework. The Cuttpa crystal was monoclinic and P2₁/c. The unsymmetric units were the Cu(II) (Cu(1)), the ttpa, as well as the sub²⁻. Cu(1) was in distorted square pyramidal coordination through four COO⁻ O atoms (O(1), O(2A), O(3B), O(4C)) of four sub²⁻ (Cu(1)-O(1) 1.962(3) Å; Cu(1)-O(2A) 1.984(3) Å; Cu(1)-O(3B) 1.969(2) Å; Cu(1)-O(4C) 1.965(2) Å) in the square plane and one triazole N atom (N(3)) of one ttpa (Cu(1)-N(3) 2.172(2) Å) at the axis (Figure 1a).

Each oxygen atom of two carboxylates from one sub²⁻ coordinates a copper(II) atom. A sub²⁻ connects four copper(II) atomc, forms a [Cu₂(COO)₄] unit, and assembles a [Cu(sub)]_n 2D (4,4) network with a [Cu₂(COO)₄] unit (Figure 1b). The point symbol for this 2D network is 4⁴·6² [36].

The ttpa ligand contains three triazole rings based on the tris(phenyl)amine frame. But only a four-position triazole N (N3) of one ttpa ligand coordinates a copper(II) atom. Two other four-position triazole N atoms (N6, N9) do not coordinate. Therefore, uncoordinated triazole rings serve as dangling arms which are located above and below the [Cu(sub)]_n 2D network (Figure 1c). The dangling arms thread into the windows of adjacent 2D networks and yield the 2D → 3D polythreaded framework (Figure 1d).

Mnttpa has a 2D (4,4)-network and a 2D → 3D polythreaded framework. Crystal Mnttpa is a triclinic crystal system and a $P\overline{1}$ space group. The unsymmetric unit is two Mn(II) atoms (Mn(1), Mn(2)), two ttpa ligands, two nip²⁻ ligands, as well as two coordinated H₂O atoms (O(7), O(14)). The Mn(1) is in the distorted octahedral coordination with three carboxylate O atoms (O(1), O(10A), O(11A)) of two nip²⁻ ligands (Mn(1)-O(1) 2.1357(16) Å; Mn(1)-O(10A) 2.3936(17) Å; Mn(1)-O(11A) 2.2382(17) Å), one oxygen atom, O(7), of coordinated water (Mn(1)-O(7) 2.1469(17) Å), and two triazole N atoms (N(3), N(16B)) of two ttpa ligands (Mn(1)-N(3) 2.238(2) Å; Mn(1)-N(16B) 2.225(2) Å) (Figure 2a). The Mn(2) is in the distorted octahedral coordination with three COO⁻ O atoms (O(3), O(4), O(8)) of two nip²⁻ ligands (Mn(1)-O(3) 2.2362(17) Å; Mn(1)-O(4) 2.2340(17) Å; Mn(1)-O(8) 2.1537(17) Å), one oxygen atom, O(14), of coordinated water (Mn(1)-O(14) 2.1652(18) Å), and two triazole N atoms (N(6C), N(13)) of two ttpa ligands (Mn(1)-N(6C) 2.249(2) Å; Mn(1)-N(13) 2.247(2) Å) (Figure 2a).

The carboxylate (O(1)O(2) or O(8)O(9)) of one nip²⁻ ligand exhibits monodentate. The other carboxylate (O(3)O(4) or O(10)O(11)) of one nip²⁻ ligand serves as a chelating coordinated mode (Figure 2a). Each nip²⁻ ligand behaves as a bridge and links two Mn(II) atoms. The ttpa ligand contains three triazole rings based on the tris(phenyl)amine frame. But two four-position triazole N atoms (N(3)N(6) or N(13)N(16)) of one ttpa ligand serve bridging functions and link two Mn(II) atoms. One other four-position triazole N (N(9) or N(19)) was uncoordinated. Therefore, the Mn(II) atoms are linked by nip²⁻ and ttpa bridging and assemble the 2D (4,4) network (Figure 2b). The point symbol of this 2D network is 4⁴·6² [36].

Therefore, uncoordinated triazole rings from the ttpa ligands and nitro groups from the nip²⁻ ligands serve as dangling arms which are located above and below the 2D network (Figure 2c). The dangling arms thread into the windows of adjacent 2D networks and yield a 2D → 3D polythreaded framework (Figure 2c). The hydrogen bond interactions between coordinated H₂O molecules and carboxyl O and triazole N atoms from adjacent 2D networks (O(7)…O(2) (1 − X, 1 − Y, −Z) 2.701(3) Å; O(7)…N(19) (–X, 1 − Y, –Z) 2.788(3) Å; O(14) …O(9) (1 − X, 2 − Y, 1 − Z) 2.675(3) Å; O(14)…N(9) (–X, 1 − Y, –Z) 2.834(3) Å) have important roles in the stability of 3D polythreaded supramolecular architecture (Figure 2c). A polythreaded framework is an interesting topic within crystal engineering [9,10,11,12,13,14,15,16]. The 2D → 3D polythreaded frameworks are uncommon, and few have been documented [12,13,14,15,16]. This research gave two new examples of 2D → 3D polythreaded frameworks.

2.2. Syntheses, PXRDs, FT-IR, and E_g

Cuttpa and Mnttpa were successfully synthesized by using ttpa, H₂suc, or H₂nip and Cu(II) or Mn(II) with the hydrothermal method. The reaction conditions (solvent H₂O/DMF, the reaction temperature, the ratio of reactants, and pH (the reactant NaOH)) were first employed according to our previous synthesis experience. The ratios of ttpa:H₂suc:Cu(II) = 1:2:3 and ttpa:H₂nip:Mn(II) = 1:2:3 were used due to the need to improve yield based on the expensive ttpa ligand. The reaction products from different reaction temperatures and different reaction times were observed. The different reaction temperatures and reaction times for syntheses of Cuttpa and Mnttpa were selected because of the high quality of single crystals suitable for single-crystal X-ray diffraction (91 °C and 3.1 days for the synthesis of Cuttpa; 101 °C and 2.2 days for the synthesis of Mnttpa) that were obtained for these reactions. Meanwhile, the synthesis of coordination polymers was not successful when using ttpa, H₂nip, and Cu(II). No coordination polymer was obtained by using ttpa, H₂suc, and Mn(II).

The powder X-ray diffraction (PXRD) patterns of Cuttpa and Mnttpa were recorded at room temperature (Figures S1 and S2). The peak positions of the simulated and experimental PXRD patterns are in agreement with each other, which confirms their phase purities.

The FT-IR spectra of Cuttpa and Mnttpa were recorded. For Cuttpa, the peaks at 1615, 1592, and 1495 cm⁻¹ are attributed to the asymmetric and symmetric stretching vibrations of carboxylate. There are no absorptions in the 1690~1730 cm⁻¹ region, indicating the complete deprotonation of H₂suc [34]. The peaks at 1515 and 1275 cm⁻¹ are attributed to the stretching vibration region of C=N bonds of the ttpa ligand. For Mnttpa, the broad absorption (3112 cm⁻¹) is due to the O-H group stretching vibration from the water molecules. The peaks at 1604, 1376, and 1349 cm⁻¹ are attributed to the asymmetric and symmetric stretching vibrations of carboxylate [34]. The peak at 1558 cm⁻¹ is attributed to the nitro group from nip²⁻. The peaks at 1517 and 1274 cm⁻¹ are attributed to the stretching vibration region of C=N bonds of the ttpa ligand.

UV spectra for Cuttpa and Mnttpa showed that the E_g data of Cuttpa and Mnttpa were 1.88 and 2.11 eV, respectively (Figure 3).

2.3. Photocatalytic and Sonocatalytic Decomposition of MB

The photocatalytic and sonocatalytic decomposition of methyl blue (MB) was investigated within Cuttpa and Mnttpa catalysts under visible light and supersound irradiation. The decomposition efficiencies of trade TiO₂ (60 nm) and TiO₂ (20 nm) were also measured.

The decomposition efficiencies of MB were 92.5% within Cuttpa, 68.8% within Mnttpa, 38.2% within TiO₂ (60 nm), 64.3% within TiO₂ (20 nm), and 19.9% within blank H₂O₂ within visible light after 90 min (Figure 4, Figure 5 and Figure 6). After the photocatalytic decomposition experiments, Cuttpa and Mnttpa sustained their structures (Figures S1 and S2). The catalytic efficiency with decomposition MB was Cuttpa > Mnttpa > TiO₂ (20 nm) > TiO₂ (60 nm) within visible light. These results showed that Cuttpa is a good photocatalyst, and Mnttpa is an effective photocatalyst with the decomposition of MB.

The decomposition efficiencies with MB were 91.3% within Cuttpa, 54.9% within Mnttpa, 45.0% within TiO₂ (60 nm), 69.2% within TiO₂ (20 nm), and 22.8% within blank H₂O₂ within ultrasound after 105 min (Figure 7, Figure 8 and Figure 9). After the sonocatalytic decomposition experiments, Cuttpa and Mnttpa sustained their structures (Figures S1 and S2). The catalytic efficiency with the decomposition of MB was Cuttpa > TiO₂ (20 nm) > Mnttpa > TiO₂ (60 nm) within ultrasound. These experimental results showed that Cuttpa is a good sonoocatalyst, and Mnttpa is an effective sonocatalyst for the decomposition of MB.

The stability of the catalyst could be observed in the recycling experiments. After each cycling experiment, the Cuttpa (1) catalyst was recovered by centrifugation, filtration, washing with deionized water and ethanol several times, and then drying under room temperature. Figure 10 is a graph of five cycles of the photocatalytic degradation of MB by Cuttpa (1) under visible light irradiation. It can be seen that the catalytic activity decreased slightly after five cycles, and the final degradation efficiency reached 86.2% after five cycles. The suspension after the last cycle of the degradation experiment was taken out, centrifuged, and filtrated to give a clear solution. The amount of copper species in the solution was measured by ICP. The concentration of the copper species in the solution was low (0.13 mg/L). The removal rate of Cu(II) ions from the Cuttpa (1) catalyst was 0.35%, indicating the stable and reusable nature of the Cuttpa (1) catalyst.

The photocatalytic and sonocatalytic mechanisms are similar [37]. The photocatalytic experiments within Cuttpa, scavenger mannitol, benzoquinone (BQ), and ammonium oxalate (AO) were carried out (Figure 11 and Figure 12). The decomposition efficiency of MB was 92.5% with Cuttpa. However, the decomposition efficiencies declined to 69.0% with BQ, 37.5% with mannitol, and 27.7% with AO. Here, the holes (h⁺) and ^•OH hydroxyl radicals played the main roles, and the ^•O₂⁻ superoxide radicals played certain auxiliary roles in the decomposition of MB within the Cuttpa catalyst.

The catalytic mechanism using Cuttpa and Mnttpa catalysts was supposed. When the Cuttpa or Mnttpa catalyst was exposed to visible radiation/supersound, the electrons of Cuttpa or Mnttpa could be emitted away from the valence band (VB) to the conduction band (CB), leading to the same amounts of holes (h⁺) in VB. The photoexcited electrons could react with H₂O₂ to result in the ^•OH hydroxyl radicals and react with oxygen and result in ^•O₂⁻ superoxide. The holes (h⁺), ^•OH hydroxyl, and ^•O₂⁻ superoxide radicals perhaps decompose MB to result in CO₂, H₂O, and other inorganic substances.

3. Experimental Section

3.1. Materials and Methods

All other reagents were commercial and used without further purification. The spectra were measured on a Varian 1000 FT-IR spectrometer (Varian, Inc., Palo Alto, CA, USA) in the 4000–400 cm⁻¹ region. Powder X-ray diffractions (PXRDs) were performed using with a D2 Phaser (Bruker, Billerica, MA, USA) X-ray diffractometer with Cu-Kα radiation (λ = 1.5406 Å) was used at room temperature. C, H and N were carried out on a Perkin-Elmer 240C analyser (Perkin-Elmer, Waltham, MA, USA). The UV-vis spectra were collected using a Cary 500 spectrometer (Agilent, Santa Clara, CA, USA). The amount of copper was measured using the inductively coupled plasma (ICP) spectrometer Optima 2100 (Perkin-Elmer, Ningbo, China).

3.2. Preparation of [Cu(ttpa)(sub)]_n (Cuttpa or 1)

The reactants of ttpa (0.10 mmol, 0.045 g), H₂sub (0.20 mmol, 0.035 g), Cu(NO₃)₂·3H₂O (0.30 mmol, 0.075 g), NaOH (0.037 mmol, 0.015 g), H₂O (3.6 mL), and DMF (1.7 mL) were placed into one hard glass tube (8.0 mL). The mixed reactants were heated to 91 °C for 3.1 days, and then chilled to 19 °C. The blue crystals [Cu(ttpa)(sub)]_n were obtained and weighed as 0.038 g with a yield of 56% (based on ttpa). Anal. Calc. for C₃₂H₃₀CuN₁₀O₄ (Cuttpa): C, 56.34%; H, 4.43%; N, 20.54%. Found: C, 56.23%; H, 4.37%; N, 20.45%. IR data (cm⁻¹): 1615 m, 1592 m, 1515 m, 1419 m, 1399 w, 1326 w, 1308 w, 1275 s, 1177 w, 1052 w, 978 m, 954 w, 833 m, 709 w, 654 m, 628 w.

3.3. Preparation of {[Mn₂(ttpa)₂(nip)₂(H₂O)₂]·3H₂O}_n (Mnttpa or 2)

The reactants of ttpa (0.10 mmol, 0.045 g), H₂nip (0.20 mmol, 0.042 g), MnSO₄^.H₂O (0.030 mmol, 0.051 g), NaOH (0.037 mmol, 0.015 g), H₂O (3.6 mL), and DMF (1.7 mL) were placed into the hard glass tube (8.0 mL). The mixed reactants were heated to 101 °C for 2.2 days, and then chilled to 19 °C. The yellow crystals {[Mn₂(ttpa)₂(nip)₂(H₂O)₂]·3H₂O}_n were obtained and weighed as 0.031 g with a yield of 41% (based on ttpa). Anal. Calc. for C₆₄H₅₂Mn₂N₂₂O₁₇ (Mnttpa): C, 50.87%; H, 3.47%; N, 20.40%. Found: C, 50.72%; H, 3.41%; N, 20.29%. IR data (cm⁻¹): 3112 m, 1604 m, 1558 m, 1517 s, 1451 w, 1434 w, 1376 m, 1349 m, 1274 m, 1145 w, 1113 m, 1081 w, 979 m, 832 w, 791 w, 733 m, 724 m, 673 m, 652 w.

3.4. X-Ray Crystallography

Crystal diffractions of [Cu(ttpa)(sub)]_n (Cuttpa or 1) and {[Mn(ttpe)₂(ttpa)₂(nip)₂(H₂O)₂]·3H₂O}_n (Mnttpa or 2) were explored with one Bruker APEX-II CCD. The structures were worked out and improved with the SHELXTL-2018 program [38]. The disordering lattice molecules of Mnttpa were omitted using PLATON (https://www.platonsoft.nl/xraysoft/, accessed on 6 November 2024). The crystallographic data are shown at

Table 1. Crystallographic data for Cuttpa and Mnttpa.

	Cuttpa	Mnttpa
Formula	C32H30CuN10O4	C64H46Mn2N22O14
Fw	682.20	1457.11
T/K	188 (2)	189 (2)
Crystal system	Monoclinic	Triclinic
Space group	P21/c	$P\overline{1}$
a/Å	14.7579 (10)	11.5963 (8)
b/Å	18.6867 (13)	17.6243 (11)
c/Å	11.5203 (9)	19.6708 (12)
α (°)	90	113.519 (2)
β (°)	100.047 (2)	94.839 (2)
γ (°)	90	104.434 (2)
V /Å3	3128.3 (4)	3492.6 (4)
F(000)	1412	1492
Z	4	2
ρcalcd (g cm−3)	1.448	1.386
µ(mm−1)	0.753	0.440
Reflections collected	60030	68,021
Unique reflections	7209 [R(int) = 0.0894]	15,953 [R(int) = 0.0951]
Parameter	424	939
Goodness of fit	1.050	1.050
R1 [I > 2σ(I)]	0.0541	0.0591
wR2 (all data)	0.1519	0.1797

. Important bond lengths and angles are presented in Table S1.

3.5. Photocatalytic and Sonocatalytic Decomposition

The photocatalytic experiment was carried out using a PCR-I multipurpose photoreactor (Beijing China Education Au-Light Company Limited, Beijing, China) equipped a CEL-HXF300 Xe lamp with a UV cut-off filter (providing visible light with λ > 400 nm). The amounts 40 mg of catalyst (Cuttpa or Mnttpa, or trade TiO₂ (60 nm), or trade TiO₂ (20 nm)) and 0.50 mL of 30% H₂O₂ were added into 100 mL of methylene blue (MB) solution (10 mg/L). The suspension solutions were stirred in dark conditions for about 30 min to ensure the complete equilibration of the adsorption/desorption of dyes on the photocatalyst surface. Then, the mixture was stirred continuously under visible light irradiation. At a given interval, aliquots of the reaction mixtures were periodically taken, purified by centrifugation, and analyzed with a UV-vis spectrophotometer at an absorption wavelength of 664 nm for MB. In order to evaluate the reusability of the catalyst Cuttpa for MB degradation, recycling experiments were carried out.

The sonocatalytic experiment was carried out in an ultrasonic bath (KQ-200VDE, Kunshan, China) which was operated at a frequency of 100 kHz and with an effective power output of 100 W. The amounts of 40 mg of catalyst (Cuttpa or Mnttpa, or trade TiO₂ (60 nm), or trade TiO₂ (20 nm)) and 0.50 mL of 30% H₂O₂ were added into 100 mL of methylene blue (MB) solution (10 mg/L). The suspension solutions were stirred in dark conditions for about 30 min to ensure the complete equilibration of the adsorption/desorption of dyes on the catalyst surface before ultrasonic irradiation was started. The entire catalytic sonication process took place at 26 ± 2 °C in the dark to prevent the influence of daily or ambient light. Then, the mixture was submitted for ultrasonic irradiation. At a given interval, aliquots of the reaction mixture were periodically taken, purified by centrifugation, and analyzed with a UV-vis spectrophotometer at an absorption wavelength of 664 nm for MB.

4. Conclusions

The copper(II) and manganese(II) coordination polymers were prepared and characterized. Cuttpa exhibited a 2D (4,4)-network with [Cu₂(COO)₄] and a 2D → 3D polythreaded network. Mnttpa showed a 2D (4,4)-network and a 2D → 3D polythreaded network. The catalytic decomposition of methyl blue (MB) by visible light and supersound irradiation were observed. The decomposition mechanism using Cuttpa was studied. The holes (h⁺) and ^•OH hydroxyl radicals played the main role, and the ^•O₂⁻ superoxide radicals played a certain auxiliary role in the decomposition of MB within the Cuttpa catalyst. Cuttpa and Mnttpa are good catalysts in the decomposition of MB within visible light and supersound.