Synthesis,structure and photocatalytic properties of two hybrid compounds prepared by N-methyl-4,4′-bipyridinium chloride

Abstract

Cationic ligand N-methyl-4,4’-bipyridine (MQ) (Scheme 1) reacted respectively with metal halides and pseudohalides to form two organic-inorganic hybrid materials: [MQ][CuI₂] (1), [Cu(MQ)(SCN)₂] (2), which were characterized by single-crystal X-ray diffraction analyses, infrared spectra (IR), elemental analyses, powder X-ray diffraction (PXRD), and thermogravimetric (TG) analyses. The anion structure of compound 1 is binuclear and 2 is two-dimensional. Compounds 1- 2 show the better characteristic of degradating methylene blue (MB) in water under 500 W Xe vapor lamp irradiation.

Keywords

N-methyl-4,4’-bipyridine Organic and inorganic hybrid polymers Photocatalysis

1 Introduction

Over the past decade, the researchers in the field of inorganic chemistry and material chemistry have paid more attention to rational design and construction of coordination polymers (CPs) [1 –3]. At this stage of the study of supramolecular chemistry, scientists are no longer confined to the synthesis of novel structures of supramolecular polymers. Increasing number of researchers is focusing on synthesizing supramolecular polymers with special functions. Ming-Sheng Wang group [4, 5] synthesized a new photochromic and thermochromic bi-functional compound, β-[(MQ)ZnCl₃], with an acentric crystal structure that differs from their previously reported centrical one for α-[(MQ)ZnCl₃](α and β denote different phases). Most of these compounds have adsorption, ion exchange and excellent luminescent properties, in the luminescent materials and nano-size materials and other fields with potential applications [6 –11]. Shuanghong Tian [12] et al. prepared Fe₂(MoO₄)₃ as an effective photo-fenton-like catalyst for the degradation of anionic and cationic dyes in a wide pH range. They found the photocatalytic activity of Fe₂(MoO₄)₃ under visible light (λ>400 nm) was evaluated by the degradation of Acid Orange II (AOII) and methylene blue (MB). This new catalyst is highly effective for the degradation of both dyes in H₂O₂/visible light system at neutral pH. Particularly, a high photocatalytic activity of Fe₂(MoO₄)₃ was obtained in a wide pH range of 3.0–9.0. Feng Wang research group [13] using solvothermal method synthesized a Ni^2 + -based one-dimensional ladder complex, the complex in the visible light can degradate reactive red X-3B.

Wentong Chen group [14] reported the synthesis, structure, and photoluminescence of ZnCl₃(4-methyl-4,4’-bipyridinium) with 4-methyl-4,4’-bipyridinium generated in situ for the first time. To the author’s knowledge, although many chlorometallates with bipyridinium-cations have been documented [15], the MQ⁺-containing chlorometallates are rare, especially the Cu complex of MQ⁺ has not been reported and its photocatalytic activity has not been studied. Therefore, we prepared Cu compounds of MQ⁺ 1 and 2, and then, we also probed their photocatalytic activity by degradation methylene blue (MB).

Scheme 1

The title ligand MQ⁺·Cl^–.

2 Experimental

2.1 Materials and physical measurements

All chemicals and solvents are of A.R. grade and used without further purification. MQ⁺·Cl^– has been synthesized according to the references [16 –18]. IR spectra were recorded on a Shimazu IR435 spectrometer as KBr disk (4000-400 cm^–1). Elemental analyses (C, H, and N) were performed on a Perkin-Elmer 240 elemental analyzer. A model NETZSCHTG209 thermal analyzer was used to record simultaneous TG curves in flowing air atmosphere of 20 mL·min^–1 at a heating rate of 5 °C·min^–1 in the temperature range 45–800 °C using platinum crucibles. The crystallographic data for 1- 2 were collected on a Bruker SMART CCD diffractometer with graphite-monochromated Mo Kα radiation (λ= 0.71073Å). Absorption corrections were applied by using SADABS. All powder patterns were collected on a Philips X-pert X-ray diffractometer at a scanning rate of 4 °·min^–1 in the 2θ range from 5 to 50° with graphite monochromatized Cu-Kα radiation (λ= 0.15418 nm).

2.2 Synthesis of compounds 1-2

[MQ][CuI₂] (1)The solution of MQ⁺·Cl^–(0.05 mmol) in H₂O/MeCN = 1 : 1 was added to a stirring solution of CuI (0.05 mmol) dissolved in H₂O/MeCN = 1 : 1 in the presence of excess KI. The resulting mixture was stirred for 5 min and filtered. Then the filtrate was slowly evaporated in a vial at room temperature. The yellow crystals of [MQ][CuI₂] suitable for X-ray analysis were obtained after two weeks in about 75% yield. IR (cm^–1, KBr): 3442(w), 2171(m), 1643(m), 1567(m), 1489(s), 1410(s), 1368(m), 1120(w), 1045(m), 910(m), 818(s), 723(w), 559(m); Anal. Calcd for C₂₂H₂₂Cu₂I₄N₄: C, 27.00; H, 2.25; N, 5.72%; Found: C, 27.11; H, 2.31; N, 5.71%.

[Cu(MQ)(SCN)₂](2) Compound 2 was synthesized by a procedure similar to that used for 1, except that CuI/KI was replaced by CuSCN (0.05 mmol)/KSCN. Yield: 60%. IR (cm^–1, KBr): 3438(m),3027(s), 2110(w), 2086(w), 1637(m), 1598(s), 1496(s), 1410(m), 1280(s), 1226(m), 1197(m), 1115(m), 1079(s),993(s), 884(s), 818(w), 760(s), 716(m), 568(s);Anal. Calcd for C₁₃H₁₁CuN₄S₂: C, 44.44; H, 3.13; N, 15.95%; Found: C, 44.47; H, 3.15; N, 15.92%.

2.3 X-ray crystallography study

Crystallographic data for the compounds were collected at 100(2)K on a Bruker APEX-II area-detector diffractometer equipped with graphite-monochromatized Mo-Ka radiation (λ= 0.71073 Å). The non-hydrogen atoms were refined with anisotropic thermal parameters. The hydrogen atoms were assigned with common isotropic displacement factors and included in the final refinement by using geometrical constraint. Data reductions and absorption corrections were performed with the SADABS software packages. All structures were solved by direct methods and refined on F² by full-matrix least-squares using the SHELXL-97 or OLEX-2 program [19]. Anisotropic displacement parameters were refined for all non-hydrogen atoms. The hydrogen atoms were assigned with common isotropic displacement factors and included in the final refinement by using geometrical restraints. The relevant crystallographic data are presented in Table 1. Selected bond lengths and bond angles are listed in Table 2.

Table 1
Crystallographic data

Compound 1 2

Formula C₂₂H₂₂Cu₂I₄N₄ C₁₃H₁₁CuN₄S₂

Formula weight 977.11 350.92

Crystal system orthorhombic monoclinic

Space group Pbcn Cc

a/Å 9.7738(2) 5.9342(3)

b/Å 20.2685(4) 35.9069(16)

c/Å 14.3126(4) 7.1608(4)

α() 90.00 90.00

β() 90 110.834(6)

γ() 90.00 90.00

V/Å³ 2835.32(11) 1426.05(13)

Z 4 4

ρ/Mg cm^–3 2.289 1.634

μ/mm^–1 36.176 1.817

F(000) 1808.0 712.0

Crystal size/mm³ 0.14×0.1×0.05 0.21×0.17×0.15

T/K 291.15 291.15

Reflections collected 6024 5378

data/restrains/parameters 2538/0/146 2787/2/183

GOF on F² 1.036 1.087

Final R indices [I>2σ (I)] R1 = 0.0582, wR2 = 0.1549 R1 = 0.0360, wR2 = 0.0645

R indices (all data) R₁ = 0.0694, wR₂ = 0.1693 R₁ = 0.0422, wR₂ = 0.0683

Largest diff. peak 1.23 0.29

hole(e Å^–3) –1.61 –0.39

Compound	1	2
Formula	C₂₂H₂₂Cu₂I₄N₄	C₁₃H₁₁CuN₄S₂
Formula weight	977.11	350.92
Crystal system	orthorhombic	monoclinic
Space group	Pbcn	Cc
a/Å	9.7738(2)	5.9342(3)
b/Å	20.2685(4)	35.9069(16)
c/Å	14.3126(4)	7.1608(4)
α()	90.00	90.00
β()	90	110.834(6)
γ()	90.00	90.00
V/Å³	2835.32(11)	1426.05(13)
Z	4	4
ρ/Mg cm^–3	2.289	1.634
μ/mm^–1	36.176	1.817
F(000)	1808.0	712.0
Crystal size/mm³	0.14×0.1×0.05	0.21×0.17×0.15
T/K	291.15	291.15
Reflections collected	6024	5378
data/restrains/parameters	2538/0/146	2787/2/183
GOF on F²	1.036	1.087
Final R indices [I>2σ (I)]	R1 = 0.0582, wR2 = 0.1549	R1 = 0.0360, wR2 = 0.0645
R indices (all data)	R₁ = 0.0694, wR₂ = 0.1693	R₁ = 0.0422, wR₂ = 0.0683
Largest diff. peak	1.23	0.29
hole(e Å^–3)	–1.61	–0.39

Table 2

Selected bond distances(Å) and angles(°) for compounds 1-2

Compound 1
Cu(1)-I(1)	2.6077(17)	Cu(1)-I(2)	2.6992(19)	Cu(1)-N(1)	2.116(8)
N(1)-Cu(1)-I(1)	104.5(3)	N(1)-Cu(1)-I(2)	103.7(3)	C(1)-N(1)-Cu(1)	121.6(6)
C(5)-N(1)-Cu(1)	120.6(7)	I(1)-Cu(1)-I(2)	113.24(7)	I(1)-Cu(1)-I(2)#1	118.97(7)
I(2)-Cu(1)-Cu(1)#1	56.53(6)	I(2)#1-Cu(1)-Cu(1)#1	57.51(6)	I(2)#1-Cu(1)-I(2)	108.74(6)
N(1)-Cu(1)-Cu(1)#1	137.1(2)	N(1)-Cu(1)-I(2)#1	106.2(3)	Cu(1)#1-I(2)-Cu(1)	65.96(6)
I(1)-Cu(1)-Cu(1)#1	118.12(4)
Compound 2
Cu(1)-S(1)	2.4910(14)	Cu(1)-S(2)	2.2913(15)	Cu(1)-N(1)#1	1.951(4)
N(1)-Cu(1)#3	1.951(4)	Cu(1)-S(1)#2	2.3639(14)	S(1)-Cu(1)#4	2.3639(14)
N(1)-Cu(1)-S(1)#2	113.28(15)	N(1)#1-Cu(1)-S(1)	98.39(13)	N(1)#1-Cu(1)-S(2)	125.29(16)
S(1)#2-Cu(1)-S(1)	102.71(4)	S(2)-Cu(1)-S(1)#2	103.55(5)	S(2)-Cu(1)-S(1)	111.62(6)
C(1)-N(1)-Cu(1)#3	166.6(5)	C(1)-S(1)-Cu(1)#4	105.07(18)	C(1)-S(1)-Cu(1)	105.12(17)
Cu(1)#4-S(1)-Cu(1)	131.29(7)	C(2)-S(2)-Cu(1)	108.9(2)

3 Results and discussion

3.1 Description of crystal structure

3.1.1 Crystal Structure of 1

Structure of [MQ][CuI₂] (1). Single-crystal X-ray diffraction analysis revealed that 1 crystallized in the monoclinic system with the space group Pbcn. As shown in Fig. 1(a), the structure of 1 consists of a pair [MQ] CuI₂ molecules, The copper atom is four-coordinated in a distorted tetrahedral environment with a nitrogen atom from the MQ⁺ ligand and three iodine atoms.(Cu-I:2.6077(17)–2.6692(18)Å, Cu-N:2. 1167(88)Å, N1-Cu1-I1 : 104.5(3)°, I1-Cu1-I2 : 118.97(7)° and 113.24(7)°, N1-Cu1-I2 : 103.7(3)° and 106.2(3)°, I2-Cu1-I2 : 108.74(6)°). The dihedral angle between the two pyridyl rings is 4.3°, indicating the torsion angle between the two pyridyl rings smaller than range between 11.05(6)° and 16.48(1)°, which are comparable to those previously documented [20].

Fig.1

(a) The structure unit diagram of 1. (b) Packing structure of 1. The H atoms were omitted for clarity.

3.1.2 Structure of [Cu(MQ)(SCN)₂](2)

Considering the flexibility and complexity of MQ in the self-assembly process of supramolecule, we studied the anion effect on the same condition and got 2 with a more beautiful architecture. The single-crystal X-ray diffraction indicates that 2 crystallized in the orthorhombic system, space group Cc. The coordination environment around the Cu in 2 is presented in Fig. 2(a) with systematic numberings. The copper of 2 is four coordination positions with three sulfur atoms and one nitrogen atom. Compared to 1, the torsion angle between the two pyridyl rings in structure of 2 was smaller, in that the dihedral angle between the two pyridylrings was 2.75°. The polyanionic structure of 2 exists as a two-dimensional network which can be regarded as sheets of [Cu(SCN)₂]_n. Within the sheets each copper(I) center is coordinated by three (two μ-S and one μ-N) μ₃-S, S, N thiocyanato ligands, which leads to the formation of ten-membered (Cu-S-Cu-NCS-Cu-SCN-) rings. Each of the rings is fused to six other rings to give a distorted honeycomb pattern [11]. The fourth ligand points out of the plane of sheet as a terminal mode. The packing structure of compound 2 is 3D structure connected by hydrogen bond, and the hydrogen bond range is 2.3∼2.9Å as shown in Fig. 2(b).

Fig.2

(a) The structure unit diagram of 2. (b) Packing structure of 2. The H atoms were omitted and the others were shown as ball and stick models for clarity.

3.2 Power X-ray diffraction (PXRD) and thermogravimetric (TG) analyses

Powder X-ray diffraction (XRD) has been used to confirm the purity of the samples in the solid state. For compounds 1- 2 the as-synthesized PXRD patterns closely match the simulated patterns generated from the results of the single crystal diffraction data, indicative of pure products which were shown in Fig. 3. In order to investigate the thermostability of compounds 1-2, the TGA experiments were performed up to 800°C in flowingN₂atmosphere, as shown in Fig. 4. The curves exhibited similar thermal behavior and the samples gradually lost weight in two main steps. The first step is the main weight-loss process corresponding to the decomposition of organic ligand. The second weight-loss step is corresponds to the collapse of inorganic bulks. Thermogravimetric analyses reveal that there is no weight loss in the temperature range of 50–250 °C for two complexes, indicating that they are potential heat-resistant hybrid materials [21, 22].

Fig.3

Power X-ray Diffraction (PXRD) of complex 1-2, 1(a), 2(b).

Fig.4

The TG curves of compounds 1-2.

3.3 Photocatalytic properties

Recently, a lot of attention has been focused on semiconductor photocatalysts because of their potential utilization in purifying waste water by thoroughly decomposing organic compounds. methylene blue (MB) is commonly used as a representative of widespread organic dyes that easily contaminate textile effluents and are very difficult to decompose in waste streams [23, 24]. Herein, the photodegradation of MB was selected for evaluating the photocatalytic performances of compounds 1- 2 under a 1000 W high-pressure mercury vapor lamp irradiation.

The title compounds (100 mg) were added to an MB (100 mL, 1.0×10^–5 mol L^–1) aqueous solution and magnetically stirred in the dark for 30 min to ensure the equilibrium of adsorption/desorption. Then the solution was exposed to visible irradiation, meanwhile, kept stirring. The visible light was from high-pressure mercury vapor lamp which the UV light was removed via optical filter 3.0 mL sample was sucked out for analysis at set intervals. As shown in Fig. 5, the absorption spectra of MB decreased obviously along with the prolonging of the reaction time. Besides, changes in the concentration of the aqueous MB solution versus irradiation time were plotted in Fig. 4 (C₀: the initial concentration of the MB; C: the concentration of the dye at any given time). It can be seen that the photodegradation activities increase from basically no change (without any catalyst) to 99% for 1 after 10 min of irradiation, but they are 99% for 2 after 100 min of irradiation. These phenomenon show that 1 has better properties of catalysis due to the different structure.

Fig.5

Photocatalytic degradation of MB solution under UV light irradiation with the use of compounds 1- 2 and the control experiment without any catalyst.

4 Conclusions

In this paper, two polymers 1- 2 were prepared by self-assemble method with the metal salts and the cation ligand, and researched their photocatalytic activity in the visible light. After exploration found that 1- 2 can degraded MB in visible light in 10 or 100 min. Organic-inorganic hybrid supramolecular polymers may have different properties due to their different functional groups. In addition to photocatalytic performance, they may have potential application value in other photochemical and electrochemistry, and we deserve further development and application.

Supplementary data

Supporting information: important crystallographic data for 1 and 2, experimental and simulated powder XRD patterns for 1 and 2. CCDC reference numbers: 1582805 (1), 1582834 (2). These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: t44 1223 336 033 or Email: deposit@ccdc.cam.ac.uk.

Footnotes

Acknowledgments

Research efforts in the Niu group are supported by the National Science Foundation of China (No. 21671177).

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