# Investigation on the Synthesis and Photocatalytic Property of Uranyl Complexes of theβ-Diketonates Biscatecholamide Ligand

Authors: Qingchun Zhang; Bo Jin; Rufang Peng; Xiaofang Wang; Zhaotao Shi; Qiangqiang Liu; Shan Lei; Hua Liang
Journal: International Journal of Photoenergy
 (2017)

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## Abstract

A series of uranyl complexes have been synthesized by reacting hexadentate ligands CH2[COO (CH2)nCAM; n=2, 3, 4]2 [CAM = 2,3-Ph(OH)2CONH] containing the catecholamide (CAM) group and β-diketonates framework with uranyl nitrate. They were characterized by FTIR, UV-vis, 1H NMR, XPS, TGA, and elemental analysis. The analysis revealed that oxygen atom of β-diketonate did not bind to uranyl ion in complexes1–3. The photocatalytic degradation properties of the target complexes for degradation of rhodamine B (RhB) were investigated. The result indicated that approximately 74%, 71%, and 67% RhB were degraded in the presence of complexes1–3 after about 210 min, respectively. Consequently, complexes1–3 have excellent photocatalytic degradation property.


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## Body

## 1. Introduction

In recent years, uranyl complexes have attracted much attention for exploring reactivity and coordination behavior of 5f-elements, developing suitable trivalent actinide (An(III)) extractants for nuclear remediation [1], screening highly effective chelators structure for decorporation of trivalent actinide (An(III)) [2], and investigating applications in the fields of catalysis, ion exchange, sensors, and photochemistry [3–5].At present, a large number of uranyl complexes have been synthesized due to their diversified property. Nevertheless, the characteristics of organic ligands play a key role in formation process of the uranyl complexes. Multidentatesβ-diketonates biscatecholamide ligands [6] have six binding sites due to the O-donor of catechol aromatic ring and β-diketonates [7, 8]. As an important chelator for decorporation of uranyl ion, catechol has the favorable stability constant [9, 10]. The β-diketonates compounds were adequate sensitizers to luminescent lanthanide complexes in which either visible or NIR emission is consecutive with photoinduced energy transfer from the sensitizing ligand [11]. The β-diketonates catecholamide ligands (H4L1–3) have been reported as efficient uranyl sequestration agents in neutral pH aqueous media [6], but the detailed functional properties of their uranyl complexes have not been studied.Since early 1800s, the photocatalytic behavior of uranyl complexes has attracted considerable attention. In general, the equatorial plane can be formed between the coordinated atom and the uranyl ion, which appeared to be bulky conjugate system surrounding the rigid rings of ligands centered in uranyl [12]. Then excitation of these species can result in ∗UO22+, which is very active and can trigger a variety of redox reactions to decompose the dye molecules [13, 14]. As compared to TiO2, the excited electrons of dye molecules are injected into TiO2 to decompose the dye molecules [15, 16]. It is generally accepted that, in the photocatalytic reaction, the excitation of uranyl complexes is much more efficient than that of the dye molecules. So, we design the new uranyl-β-diketonates biscatecholamide complexes (UO2L1–3·2H2O), which may have efficient photocatalytic property. In this paper, three complexes UO2L1–3·2H2O have been successfully synthesized in the presence of UO2(NO3)2·6H2O and β-diketonates biscatecholamide ligand. These complexes were characterized by FTIR spectra, UV-vis spectra, 1H NMR, XPS spectra, and elemental analysis. Then photocatalytic degradation properties of the complexes were also investigated by RhB as model compound of organic pollutant.


## 2. Experimental

### 2.1. Materials and Methods

UO2(NO3)2·6H2O (99.9%) was purchased from commercial products from HuBei ChuShengWei Chemistry Co., Ltd. All the organic reagents were pure commercial products from Aladdin. The solvents were purchased from Chengdu Kelong Chemical Reagents Co. The 300–400 mesh silica gel was purchased from Qingdao Hailang. 1H NMR and 13C NMR spectra were recorded on Bruker Avance III 600 spectrometer. The FTIR spectra were obtained from Thermo Scientific Nicolet 380 FTIR spectrophotometer. The UV-vis spectra were obtained from Thermo Scientific Evolution 300 spectrophotometer. The XPS spectra were obtained from Thermo ESCALAB 250 electron spectrometer. Mass spectral analyses were conducted using Varian 1200 LC/MS. Elemental analyses of the complexes were carried out on a Vario EL III CHNS analyzer.


### 2.2. Synthesis of the Ligands

A solution of malonyl dichloride (1.0 mmol) in CH2Cl2 (20 mL) was dropped in the solution of 2,3-bis(benzyloxy)-N-(hydroxyalkyl)benzamide (2.0 mmol) and Et3N (2.0 mL) in CH2Cl2 (20 mL) under ice bath and vigorous stirring conditions. After evaporation of the solvent, the residue was purified by flash column chromatography. Then deprotection of the hydroxyl groups under H2 and Pd/C catalytic conditions in THF resulted in the β-diketonates biscatecholamide ligands H4L1–3 [6].Bis(2,3-bis(hydroxy)-N-(hydroxyethyl)benzamide)malonate (H 4 L 1 ). 1H NMR (600 MHz, DMSO-d6): δ = 8.90 (br s, 2H, CO-NH), 7.25 (dd,J = 7.8, 1.2 Hz, 2H, Ar-H), 6.91 (dd,J = 7.8, 1.2 Hz, 2H, Ar-H), 6.67 (t,J = 7.8 Hz, 2H, Ar-H), 4.20 (t,J = 5.7 Hz, 4H, -CH2-), 3.52 (m, 6H, CO-CH2-CO and -CH2-). 13C NMR (150 MHz, DMSO-d6): δ = 170.4 (C=O), 166.9 (C=O), 150.0 (ArC), 146.7 (ArC), 119.2 (ArCH), 118.4 (ArCH), 117.8 (ArCH), 115.3 (ArC), 63.6 (-CH2-), 41.5 (-CH2-), 38.3 (-CH2-). FTIR (KBr, cm−1): 3398, 2957, 2929, 1750, 1731, 1642, 1590, 1546, 1459. UV-vis (DMSO, nm): 248, 319. APCI-MS (m/z): 463.2 [M+H]+.Bis(2,3-bis(hydroxy)-N-(3-hydroxypropyl)benzamide)malonate ( H 4 L 2 ). 1H NMR (600 MHz, DMSO-d6): δ = 12.74 (br s, 2H, Ar-OH), 9.17 (br s, 2H, Ar-OH), 8.84 (br s, 2H, CO-NH), 7.26 (dd,J = 7.8, 1.2 Hz, 2H, Ar-H), 6.90 (dd,J = 7.8, 1.2 Hz, 2H, Ar-H), 6.67 (t,J = 7.8 Hz, 2H, Ar-H), 4.13 (t,J = 6.4 Hz, 4H, -CH2-), 3.51 (s, 2H, CO-CH2-CO), 3.34 (m, 4H, -CH2-), 1.87 (m, 4H, -CH2-). 13C NMR (150 MHz, DMSO-d6): δ = 169.8 (C=O), 166.5 (C=O), 149.6 (ArC), 146.2 (ArC), 118.8 (ArCH), 117.8 (ArCH), 117.1 (ArCH), 114.9 (ArC), 62.8 (-CH2-), 41.2 (-CH2-), 35.8 (-CH2-), 27.9 (-CH2-). FTIR (KBr, cm−1): 3390, 2956, 1747, 1735, 1641, 1591, 1547, 1459. UV-vis (DMSO, nm): 246, 320. APCI-MS (m/z): 491.1 [M+H]+.Bis(2,3-bis(hydroxy)-N-(4-hydroxybutyl)benzamide)malonate (H 4 L 3 ). 1H NMR (600 MHz, DMSO-d6): δ = 8.80 (br s, 2H, CO-NH), 7.26 (dd,J = 7.8, 1.2 Hz, 2H, Ar-H), 6.99 (dd,J = 7.8, 1.2 Hz, 2H, Ar-H), 6.66 (t,J = 7.8 Hz, 2H, Ar-H), 4.09 (t,J = 6.3 Hz, 4H, -CH2-), 3.50 (s, 2H, CO-CH2-CO), 3.29 (dd,J = 12.5, 6.3 Hz, 4H, -CH2-), 1.59 (m, 8H, -CH2-CH2-). 13C NMR (150 MHz, DMSO-d6): δ = 169.7 (C=O), 166.6 (C=O), 149.6 (ArC), 146.2 (ArC), 118.7 (ArCH), 117.8 (ArCH), 117.0 (ArCH), 114.9 (ArC), 64.5 (-CH2-), 41.1 (-CH2-), 38.3 (-CH2-), 25.5 (-CH2-), 25.2 (-CH2-). FTIR (KBr, cm−1): 3399, 2926, 1745, 1735, 1639, 1594, 1545, 1459. UV-vis (DMSO, nm): 248, 322. APCI-MS (m/z): 519.7 [M+H]+.


### 2.3. Synthesis of Uranyl Complexes

Caution! Even though the UO2(NO3)2·6H2O used in this study contains depleted uranium, standard precautions for handling radioactive substances should be followed.A solution of UO2(NO3)2·6H2O (55.2 mg, 0.11 mmol) in methanol was added to a methanol solution of a β-diketonates biscatecholamide ligands H4L1–3 (0.1 mmol) with stirring. The colorless ligand solution immediately turned orange and shown a strong acidic reaction to pH paper, indicating the formation of a uranyl complex. Following the addition of 1 or 2 drops of pyridine the transparent reaction mixture became turbid. The solution was then heated at reflux overnight, under nitrogen. The uranyl complexes deposited as orange precipitates, which were collected by centrifugation, washed with methanol, and dried in a vacuum oven and obtained orange power.UO 2 L 1·2H2O Complex (1). Yield 72%. The 1H NMR spectra of DMSO-d6 at room temperature show that the complex contains the major and minor species (possibly isomers); the ratio was about 5.2 : 1. 1H NMR (600 MHz, DMSO-d6, major species): δ = 10.03 (d,J = 9.4 Hz, 2H, CO-NH), 7.13–7.03 (m, 2H,ortho cat-H), 6.98–6.88 (m, 4H,meta andpara cat-H), 5.18 (dd,J = 23.8, 12.1 Hz, 2H, -CH2-), 4.78 (d,J = 11.9 Hz, 2H, -CH2-), 4.00 (t,J = 12.0 Hz, 2H, -CH2-), 3.63 (d,J = 14.2 Hz, 2H, -CH2-), 2.83 (d,J = 17.0 Hz, 1H, 0.5× -COCH2CO-), 1.71 (d,J = 17.0 Hz, 1H, 0.5× -COCH2CO-). 1H NMR (600 MHz, DMSO-d6, minor species): δ =10.92 (s, 2H, CO-NH), 7.67 (d,J = 7.5 Hz, 2H,ortho cat-H), 7.44 (d,J = 7.7 Hz, 2H,para cat-H), 6.67 (t,J = 7.8 Hz, 2H,meta cat-H), 4.97 (b, 2H, -CH2-), 4.55 (b, 2H, -CH2-), 4.21 (b, 2H, -CH2-), 3.93 (m, 2H, -CH2-), 3.07 (d,J = 17.3 Hz, 1H, 0.5× -COCH2CO-), 2.83 (d,J = 17.3 Hz, 1H, 0.5× -COCH2CO-). FTIR (KBr, cm−1): 3397, 1746, 1587, 1546, 1461, 1439, 1350, 1309, 1217, 927, 550. UV-vis (DMSO, nm): 258, 328. Anal. Calcd. for UO2C21H20N2O11 (746.40): C, 33.76; H, 2.68; N, 3.75. Found: C, 34.76; H, 2.51; N, 3.69.UO 2 L 2·2H2O Complex (2). Yield 70%. The 1H NMR spectra of in DMSO-d6 at room temperature show that the complex contains the major and minor species (possibly isomers); the minor species signals are too weak to assign. 1H NMR (600 MHz, DMSO-d6, major species): δ = 10.07 (s, 2H, CO-NH), 7.04 (m, 2H,ortho cat-H), 6.93 (m, 4H,meta andpara cat-H), 4.60–4.10 (m, 4H, -CH2-), 4.04–3.78 (m, 4H, -CH2-), 3.18 (d,J = 5.3 Hz, 2H, -COCH2CO-), 2.29–1.97 (m, 4H, -CH2-). FTIR (KBr, cm−1): 3367, 1742, 1588, 1546, 1461, 1444, 1346, 1307, 1217, 929, 550. UV-vis (DMSO, nm): 259, 328. Anal. Calcd. for UO2C23H24N2O11 (774.46): C, 35.64; H, 3.09; N, 3.62. Found: C, 36.71; H, 2.96; N, 3.59.UO 2 L 3·2H2O Complex (3). Yield 74%. The 1H NMR spectra of in DMSO-d6 at room temperature show that the complex contains the major and minor species (possibly isomers); the ratio was about 4.1 : 1. 1H NMR (600 MHz, DMSO-d6, major species): δ = 10.04 (br s, 2H, CO-NH), 7.15–7.04 (m, 2H,ortho cat-H), 6.91 (m, 4H,meta andpara cat-H), 4.10 (m, 4H, -CH2-), 3.94 (m, 4H, -CH2-), 3.19 (d,J = 16.0 Hz, 1H, 0.5× -COCH2CO-), 3.08 (d,J = 16.0 Hz, 1H, 0.5× -COCH2CO-), 1.78–1.56 (m, 8H, -CH2CH2-).1H NMR (600 MHz, DMSO-d6, minor species): δ = 10.54 (br s, 2H, CO-NH), 7.52 (dd,J = 7.6, 1.7 Hz, 2H,ortho cat-H), 7.49 (dd,J = 7.9, 1.7 Hz, 2H,para cat-H), 6.66 (t,J = 7.8 Hz, 2H,meta cat-H), 4.55 (m, 8H, -CH2-), 2.61 (s, 1H, 0.5× -COCH2CO-), 2.38 (s, 1H, 0.5× -COCH2CO-), 1.91–1.79 (m, 8H, -CH2CH2-). FTIR (KBr, cm−1): 3390, 1742, 1588, 1546, 1461, 1444, 1342, 1309, 1218, 927, 550. UV-vis (DMSO, nm): 259, 326. Anal. Calcd. for UO2C25H28N2O11 (802.51): C, 37.38; H, 3.49; N, 3.49. Found: C, 38.45; H, 3.41; N, 3.43.


## 2.1. Materials and Methods

UO2(NO3)2·6H2O (99.9%) was purchased from commercial products from HuBei ChuShengWei Chemistry Co., Ltd. All the organic reagents were pure commercial products from Aladdin. The solvents were purchased from Chengdu Kelong Chemical Reagents Co. The 300–400 mesh silica gel was purchased from Qingdao Hailang. 1H NMR and 13C NMR spectra were recorded on Bruker Avance III 600 spectrometer. The FTIR spectra were obtained from Thermo Scientific Nicolet 380 FTIR spectrophotometer. The UV-vis spectra were obtained from Thermo Scientific Evolution 300 spectrophotometer. The XPS spectra were obtained from Thermo ESCALAB 250 electron spectrometer. Mass spectral analyses were conducted using Varian 1200 LC/MS. Elemental analyses of the complexes were carried out on a Vario EL III CHNS analyzer.


## 2.2. Synthesis of the Ligands

A solution of malonyl dichloride (1.0 mmol) in CH2Cl2 (20 mL) was dropped in the solution of 2,3-bis(benzyloxy)-N-(hydroxyalkyl)benzamide (2.0 mmol) and Et3N (2.0 mL) in CH2Cl2 (20 mL) under ice bath and vigorous stirring conditions. After evaporation of the solvent, the residue was purified by flash column chromatography. Then deprotection of the hydroxyl groups under H2 and Pd/C catalytic conditions in THF resulted in the β-diketonates biscatecholamide ligands H4L1–3 [6].Bis(2,3-bis(hydroxy)-N-(hydroxyethyl)benzamide)malonate (H 4 L 1 ). 1H NMR (600 MHz, DMSO-d6): δ = 8.90 (br s, 2H, CO-NH), 7.25 (dd,J = 7.8, 1.2 Hz, 2H, Ar-H), 6.91 (dd,J = 7.8, 1.2 Hz, 2H, Ar-H), 6.67 (t,J = 7.8 Hz, 2H, Ar-H), 4.20 (t,J = 5.7 Hz, 4H, -CH2-), 3.52 (m, 6H, CO-CH2-CO and -CH2-). 13C NMR (150 MHz, DMSO-d6): δ = 170.4 (C=O), 166.9 (C=O), 150.0 (ArC), 146.7 (ArC), 119.2 (ArCH), 118.4 (ArCH), 117.8 (ArCH), 115.3 (ArC), 63.6 (-CH2-), 41.5 (-CH2-), 38.3 (-CH2-). FTIR (KBr, cm−1): 3398, 2957, 2929, 1750, 1731, 1642, 1590, 1546, 1459. UV-vis (DMSO, nm): 248, 319. APCI-MS (m/z): 463.2 [M+H]+.Bis(2,3-bis(hydroxy)-N-(3-hydroxypropyl)benzamide)malonate ( H 4 L 2 ). 1H NMR (600 MHz, DMSO-d6): δ = 12.74 (br s, 2H, Ar-OH), 9.17 (br s, 2H, Ar-OH), 8.84 (br s, 2H, CO-NH), 7.26 (dd,J = 7.8, 1.2 Hz, 2H, Ar-H), 6.90 (dd,J = 7.8, 1.2 Hz, 2H, Ar-H), 6.67 (t,J = 7.8 Hz, 2H, Ar-H), 4.13 (t,J = 6.4 Hz, 4H, -CH2-), 3.51 (s, 2H, CO-CH2-CO), 3.34 (m, 4H, -CH2-), 1.87 (m, 4H, -CH2-). 13C NMR (150 MHz, DMSO-d6): δ = 169.8 (C=O), 166.5 (C=O), 149.6 (ArC), 146.2 (ArC), 118.8 (ArCH), 117.8 (ArCH), 117.1 (ArCH), 114.9 (ArC), 62.8 (-CH2-), 41.2 (-CH2-), 35.8 (-CH2-), 27.9 (-CH2-). FTIR (KBr, cm−1): 3390, 2956, 1747, 1735, 1641, 1591, 1547, 1459. UV-vis (DMSO, nm): 246, 320. APCI-MS (m/z): 491.1 [M+H]+.Bis(2,3-bis(hydroxy)-N-(4-hydroxybutyl)benzamide)malonate (H 4 L 3 ). 1H NMR (600 MHz, DMSO-d6): δ = 8.80 (br s, 2H, CO-NH), 7.26 (dd,J = 7.8, 1.2 Hz, 2H, Ar-H), 6.99 (dd,J = 7.8, 1.2 Hz, 2H, Ar-H), 6.66 (t,J = 7.8 Hz, 2H, Ar-H), 4.09 (t,J = 6.3 Hz, 4H, -CH2-), 3.50 (s, 2H, CO-CH2-CO), 3.29 (dd,J = 12.5, 6.3 Hz, 4H, -CH2-), 1.59 (m, 8H, -CH2-CH2-). 13C NMR (150 MHz, DMSO-d6): δ = 169.7 (C=O), 166.6 (C=O), 149.6 (ArC), 146.2 (ArC), 118.7 (ArCH), 117.8 (ArCH), 117.0 (ArCH), 114.9 (ArC), 64.5 (-CH2-), 41.1 (-CH2-), 38.3 (-CH2-), 25.5 (-CH2-), 25.2 (-CH2-). FTIR (KBr, cm−1): 3399, 2926, 1745, 1735, 1639, 1594, 1545, 1459. UV-vis (DMSO, nm): 248, 322. APCI-MS (m/z): 519.7 [M+H]+.


## 2.3. Synthesis of Uranyl Complexes

Caution! Even though the UO2(NO3)2·6H2O used in this study contains depleted uranium, standard precautions for handling radioactive substances should be followed.A solution of UO2(NO3)2·6H2O (55.2 mg, 0.11 mmol) in methanol was added to a methanol solution of a β-diketonates biscatecholamide ligands H4L1–3 (0.1 mmol) with stirring. The colorless ligand solution immediately turned orange and shown a strong acidic reaction to pH paper, indicating the formation of a uranyl complex. Following the addition of 1 or 2 drops of pyridine the transparent reaction mixture became turbid. The solution was then heated at reflux overnight, under nitrogen. The uranyl complexes deposited as orange precipitates, which were collected by centrifugation, washed with methanol, and dried in a vacuum oven and obtained orange power.UO 2 L 1·2H2O Complex (1). Yield 72%. The 1H NMR spectra of DMSO-d6 at room temperature show that the complex contains the major and minor species (possibly isomers); the ratio was about 5.2 : 1. 1H NMR (600 MHz, DMSO-d6, major species): δ = 10.03 (d,J = 9.4 Hz, 2H, CO-NH), 7.13–7.03 (m, 2H,ortho cat-H), 6.98–6.88 (m, 4H,meta andpara cat-H), 5.18 (dd,J = 23.8, 12.1 Hz, 2H, -CH2-), 4.78 (d,J = 11.9 Hz, 2H, -CH2-), 4.00 (t,J = 12.0 Hz, 2H, -CH2-), 3.63 (d,J = 14.2 Hz, 2H, -CH2-), 2.83 (d,J = 17.0 Hz, 1H, 0.5× -COCH2CO-), 1.71 (d,J = 17.0 Hz, 1H, 0.5× -COCH2CO-). 1H NMR (600 MHz, DMSO-d6, minor species): δ =10.92 (s, 2H, CO-NH), 7.67 (d,J = 7.5 Hz, 2H,ortho cat-H), 7.44 (d,J = 7.7 Hz, 2H,para cat-H), 6.67 (t,J = 7.8 Hz, 2H,meta cat-H), 4.97 (b, 2H, -CH2-), 4.55 (b, 2H, -CH2-), 4.21 (b, 2H, -CH2-), 3.93 (m, 2H, -CH2-), 3.07 (d,J = 17.3 Hz, 1H, 0.5× -COCH2CO-), 2.83 (d,J = 17.3 Hz, 1H, 0.5× -COCH2CO-). FTIR (KBr, cm−1): 3397, 1746, 1587, 1546, 1461, 1439, 1350, 1309, 1217, 927, 550. UV-vis (DMSO, nm): 258, 328. Anal. Calcd. for UO2C21H20N2O11 (746.40): C, 33.76; H, 2.68; N, 3.75. Found: C, 34.76; H, 2.51; N, 3.69.UO 2 L 2·2H2O Complex (2). Yield 70%. The 1H NMR spectra of in DMSO-d6 at room temperature show that the complex contains the major and minor species (possibly isomers); the minor species signals are too weak to assign. 1H NMR (600 MHz, DMSO-d6, major species): δ = 10.07 (s, 2H, CO-NH), 7.04 (m, 2H,ortho cat-H), 6.93 (m, 4H,meta andpara cat-H), 4.60–4.10 (m, 4H, -CH2-), 4.04–3.78 (m, 4H, -CH2-), 3.18 (d,J = 5.3 Hz, 2H, -COCH2CO-), 2.29–1.97 (m, 4H, -CH2-). FTIR (KBr, cm−1): 3367, 1742, 1588, 1546, 1461, 1444, 1346, 1307, 1217, 929, 550. UV-vis (DMSO, nm): 259, 328. Anal. Calcd. for UO2C23H24N2O11 (774.46): C, 35.64; H, 3.09; N, 3.62. Found: C, 36.71; H, 2.96; N, 3.59.UO 2 L 3·2H2O Complex (3). Yield 74%. The 1H NMR spectra of in DMSO-d6 at room temperature show that the complex contains the major and minor species (possibly isomers); the ratio was about 4.1 : 1. 1H NMR (600 MHz, DMSO-d6, major species): δ = 10.04 (br s, 2H, CO-NH), 7.15–7.04 (m, 2H,ortho cat-H), 6.91 (m, 4H,meta andpara cat-H), 4.10 (m, 4H, -CH2-), 3.94 (m, 4H, -CH2-), 3.19 (d,J = 16.0 Hz, 1H, 0.5× -COCH2CO-), 3.08 (d,J = 16.0 Hz, 1H, 0.5× -COCH2CO-), 1.78–1.56 (m, 8H, -CH2CH2-).1H NMR (600 MHz, DMSO-d6, minor species): δ = 10.54 (br s, 2H, CO-NH), 7.52 (dd,J = 7.6, 1.7 Hz, 2H,ortho cat-H), 7.49 (dd,J = 7.9, 1.7 Hz, 2H,para cat-H), 6.66 (t,J = 7.8 Hz, 2H,meta cat-H), 4.55 (m, 8H, -CH2-), 2.61 (s, 1H, 0.5× -COCH2CO-), 2.38 (s, 1H, 0.5× -COCH2CO-), 1.91–1.79 (m, 8H, -CH2CH2-). FTIR (KBr, cm−1): 3390, 1742, 1588, 1546, 1461, 1444, 1342, 1309, 1218, 927, 550. UV-vis (DMSO, nm): 259, 326. Anal. Calcd. for UO2C25H28N2O11 (802.51): C, 37.38; H, 3.49; N, 3.49. Found: C, 38.45; H, 3.41; N, 3.43.


## 3. Results and Discussion

### 3.1. UV-Vis Spectra Analysis

The UV-vis spectra of the ligands H4L1–3 and their corresponding target uranyl complexes1–3 were recorded in DMSO solution (Figure 1), and the data were summarized in Table 1. It was shown from Table 1 that all free ligands π→π∗ transitions absorption bands were in the range of 246–248 nm and n→π∗ transitions absorption bands in the range of 319–322 nm. It was found that the π→π∗ and n→π∗ transitions of complexes were slightly red-shifted as compared to the π→π∗ and n→π∗ transitions of ligands H4L1–3 and had absorptions in the visible regions (380–450 nm), which was ascribed to charge transfer within the U=O double bonds and the ligand to metal charge transfer (LMCT) between the oxygen atoms of the coordinating ligands and the empty orbital of the uranyl ion [17–19]. These results show that the H4L1–3 take part in coordination with uranyl ion.Table 1
UV-vis spectra data of the ligands and of target complexes.
Ligands λ max ⁡ (nm) Uranyl complex λ max ⁡ (nm) H4L1 248, 319 Complex1 258, 328 H4L2 246, 320 Complex2 259, 328 H4L3 248, 322 Complex3 259, 326Figure 1
The UV-vis spectra of (a) the free ligands H4L1–3 and (b) complexes1–3.
(a) (b)


### 3.2. FTIR Spectra Analysis

The FTIR spectra of the ligands H4L1–3 and their corresponding target uranyl complexes were recorded in the region 4000–400 cm−1, which were studied to obtain information on the oxygen-to-uranyl coordination from the shift of the C-O stretching with respect to the nonbonded ligands. Since the FTIR spectra of all ligands have shown similar features, the FTIR spectra of ligands H4L1 as well as their corresponding complex1 were selected for illustration for comparison reasons shown in Figure 2(a) and all complexes1–3 are shown in Figure 2(b). The selected vibrations of the ligands H4L1–3 and their corresponding complexes studied, together with the suggested assignments, were listed in Table 2.Table 2
The FTIR spectral frequencies (cm−1) and tentative assignments for the free ligands and uranyl complexes.
H 4 L 1 Complex1 H 4 L 2 Complex2 H 4 L 3 Complex3 Assignment of vibration 3398 3379 3397 3367 3400 3390 ν(OH) 3104, 2957 3103, 2961 3094, 2956 3101, 2958 3083, 2927 3102, 2954 ν(C-H) 1750 1746 1747 1742 1746 1742 ν(C=O) (β-diketonate) 1643 1588 1641 1587 1640 1587 ν(C=O) 1547 1546 1547 1546 1546 1546 δ(C-N-H) 1488 1461 1487 1461 1487 1461 δ(CH2) s 1460 1439 1460 1444 1460 1444 δ(CH2) s 1364 1367 1361 1384 1361 1384 δ(O-H) 1337 1350 1337 1346 1334 1342 ν(C-N) — 1309 — 1307 — 1309 ν(C-N) and δ(N-H) 1266 1260 1266 1256 1266 1256 ν(C-O) — 1217 — 1217 — 1218 ν(C-O-U) 1178 1174 1177 1173 1171 1171 ν a s(C-O-C) 1156 1157 1158 — 1157 — δ(C-O) 1073 1091 1072 1090 1069 1089 ν s(C-O-C) 1038 1039 1038 1052 1045 1053 δ(C-H) — 927 — 929 — 927 ν(U=O) as 845 856 834 847 842 853 δ(C-H) 767, 743 783, 740 786, 743 793, 742 775, 742 788, 743 δ(C-H) (benzene ring) — 550 — 550 — 550 ν(U=O)sFigure 2
The FTIR spectra of (a) the free ligand H4L1 and complex1 and (b) complexes1–3.
(a) (b)As shown in Table2, the bands at 1746–1750 cm−1 for the free ligands were assigned to the C=O stretching vibration of β-diketonate and the bands at 1742–1746 cm−1 for the corresponding complexes; there is hardly any red shifts to form enol tautomers in complexes [7, 20–22], which confirmed that oxygen atoms of carbonyl group of β-diketonate did not bind to uranyl ion. The bands at 1640–1643 cm−1 were assigned to the C=O stretching vibration of amide groups of free ligands, which shifted to 1588 cm−1 in its corresponding complexes; meanwhile, the peaks at 1266 cm−1 were attributed to the C-O stretching vibration of catechol units shifted to 1256–1260 cm−1 for their complexes, which indicated that oxygen atoms of phenolic hydroxyl take part in coordination with uranyl ion. In addition, the characteristic absorption peaks of C-N groups at 1334–1337 cm−1 shifted to 1342–1350 cm−1 in their corresponding complexes, showing formation of strong hydrogen bond between hydrogen atoms of amide groups and oxygen atoms of phenolic hydroxyls [23, 24]. Frequencies of the stretching and deformation vibrations of the C=C, C-H groups of the benzene rings were also observed. The FTIR spectra of all complexes show strong absorption bands between 927 and 929 cm−1 and weak absorption bands in 550 cm−1. The bands at 927 and 550 cm−1 were assigned to the asymmetric stretching modes of the uranyl moiety [25–27]. Therefore, analytical results of the FTIR spectra indicated uranyl ion complexes were to be obtained.


### 3.3. Proton NMR Spectra Analysis

The development of H4L1–3 evolved from the following considerations. The dialkyl malonates spacer not only provides two coordination sites but also enhances the less rigidity and preorientation of the compound and should therefore favor the encapsulation of a uranyl ion unit. Furthermore, model studies show that steric strain in the predisposed ligand reduces the number of possible geometric isomers. For an octahedral liner uranyl complex of an asymmetrically substituted catecholamide ligand there are two conceivable geometrical isomers as shown in Figure 3. So the room-temperature 1H NMR spectra of complexes1–3 compared with free H4L1–3 have shown complex splitting.Figure 3
Possible geometric isomers of uranyl complexes with two asymmetrically substituted catecholate ligands and uranyl oxo atoms omitted for clarity.The1H NMR spectra of all ligands and corresponding uranyl complexes have shown similar features. The complexes in DMSO-d6 at room temperature have shown that the complexes contain the major and minor species (possibly isomers a and b, resp.). The 1H NMR spectra of H4L1 as well as their corresponding complex1 were selected for illustration and shown in Figure 4. The resonance signals of amide protons of the major species isomers shifted from 8.90 ppm to 10.02 ppm, since the downfield chemical shifts of amide protons are generally interpreted in terms of strong hydrogen bonding [23, 24], as shown in Figure 3. The resonance signals of catechol protons of the major species isomers of complex1 showing two peaks at 7.13–7.03 (m, 2H,ortho cat-H) and 6.98–6.88 (m, 4H,meta andpara cat-H), compared with the free ligand H4L1, have appreciable changes and downfield shifts of the aromatic proton resonances of the catechol rings. Similarly, the changes in the level of the signals of the protons of the linker were also shift, turning into complex splitting due to the proximity of the uranyl ion nearby the altered protons. The resonance signals of the methylene protons of the major species isomers showed four peaks at 5.18 (dd,J = 23.8, 12.1 Hz, 2H, -CH2-), 4.78 (d,J = 11.9 Hz, 2H, -CH2-), 4.00 (t,J = 12.0 Hz, 2H, -CH2-), and 3.63 (d,J = 14.2 Hz, 2H, -CH2-). And the active methylene of β-diketonates shows two double peaks at 2.83 (d,J = 17.0 Hz, 1H) and 1.71 (d,J = 17.0 Hz, 1H), but generally, mental complexes present a singlet peak at 5.0–8.0 ppm for the enol tautomers of β-diketonates [20, 28–30]. It confirmed that oxygen atom of β-diketonate did not bind to uranyl ion, also indicated by the FTIR spectra analysis.Figure 4
The1H NMR spectra of ligand H4L1 and of complex1.


### 3.4. XPS Spectra Analysis

The XPS spectra of the samples were conducted at 10 KV and 5 mA under 10−8Pa residual pressure. The peak energies were corrected with C 1s peak at 284.6 eV as a reference. The recorded lines (C 1s, O 1s, N 1s, and U 4f) were fitted using the XPSpeak program after the background subtraction.To study the interaction of uranyl with ligands H4L1–3, XPS spectra were recorded for complexes1–3. The O 1s, C 1s, N 1s, U 4d, and U 4f spectra were shown in Figure 5. Uranyl coordination reaction was observed in complexes evidenced by the appearance of doublet peaks of U 4f5/2 and U 4f7/2 with a splitting value of 10.80–10.90 eV. The values of the binding energies of O 1s, C 1s, N 1s, U 4f5/2, and U 4f7/2 and the splitting values of the U 4f spectra were shown in Table 3. The O 1s and U 4f spectra of complexes1–3 were shown in Figure 6. The peak fitting results of the O 1s and U 4f were shown in Table 4. Because XPS is not a quantitative analysis method, these data could be used for semiquantitative analysis of the distribution of multicomponent [31, 32].Table 3
Binding energy (eV) of complexes1–3 and splitting value of U 4f.
Uranylcomplex O 1s C 1s N 1s U4f5/2 U4f7/2 Splittingvalue Complex1 531.26 284.26 399.66 392.46 381.66 10.80 Complex2 531.71 284.71 399.62 392.43 381.61 10.82 Complex3 531.66 284.66 399.61 392.41 381.51 10.90Table 4
The peak fitting results of the O 1s, U 4f7/2 of complexes 1–3.
Core levels Complex1 Complex2 Complex3 Binding energy (eV) FWHW (eV) Area Binding energy (eV) FWHW (eV) Area Binding energy (eV) FWHW (eV) Area O 1s 533.85 1.31 1468.15 533.98 2.13 7315.93 534.34 1.10 1318.23 533.10 1.42 6015.89 533.50 1.3 5996.96 533.56 0.87 2983.22 532.60 1.08 6718.25 532.70 1.06 6697.41 532.78 0.99 5832.76 532.10 1.14 7356.14 532.30 1.13 7107.48 532.22 1.01 6766.91 531.45 1.21 16239.1 531.60 1.20 15224.1 531.47 1.2 13254.9 530.65 1.52 8646.69 530.69 1.49 8633.66 530.47 1.58 8728.48 U4f7/2 384.49 3.78 12770.3 384.79 4.33 17262.9 384.80 4.12 13500.5 382.10 2.63 36018.1 382.22 2.84 45319.6 382.20 2.74 38513.2Figure 5
XPS survey scans of complexes1–3.Figure 6
Curves fitted high resolution scans of (a) O 1s and (b) U 4f of complexes1–3.
(a) (b)As shown in Figure6(a), it was observed that complexes1–3 presented six main components O 1s spectra for oxygen functionalities. The six peaks position at 530.50–530.69 eV (O=U=O) [33, 34], 531.40–531.55 eV (C-O-U), 532.20–532.33 eV (C=O of β-diketonate), 532.60–532.80 eV (C-O-C), 533.40–533.50 eV (C=O of benzamide), and 533.90–534.00 eV (adsorbed H2O) [35] was observed. And the relative intensities of C-O-U were significantly higher than other oxygen functionalities; meanwhile, the presence of the U=O bonds was confirmed, as expected.The U 4f7/2 spectra were deconvoluted into two components: the peak corresponding to the free uranyl ion that occurred at 381.05 eV and the peak of complexes1–3 that occurred at 382.10–382.22 eV, as shown in Figure 6(b).From the above discussion, it is clear that the ligands H4L1–3 complexation of uranyl ion occurred with the catecholamide moieties.


### 3.5. Thermal Properties

To examine the thermal stability of complexes1–3, thermal gravimetric analysis (TGA) was carried out at a heating rate of 10°C min−1 under the conditions of N2 atmosphere with the temperature range from 25 to 1100°C (Figure 7). The TGA curves of complexes1–3 show two steps to weight loss. The first weight loss in the range of 120–220°C should be attributed to the loss of two coordinated water molecules with a percentage weight loss (obs. 5.1%, 4.9%, and 4.8%; calcd. 4.82%, 4.65%, and 4.49%) of complexes1–3, respectively. The second weight loss in the range of 250–1050°C is attributed to the loss of biscatecholamide ligands with a percentage weight loss (obs. 55.1%, 57.6%, and 60.0%; calcd. 56.86%, 58.42%, and 59.87%) of complexes1–3, respectively. The percentage weight of final residue was in good agreement with the calculated value by the stoichiometric chemical formula UO3.Figure 7
The thermal gravimetric analysis curves of (a) complex1, (b) complex2, and (c) complex3.
(a) (b) (c)


### 3.6. Photocatalytic Activity

As we know, methyl blue, methyl orange, and RhB dyes have been widely used in textile, printing, paper, and pharmaceutical industries, causing serious pollution to the environment at the same time [35–38]. In this work, we selected RhB as model compound of organic pollutant to investigate the degradation efficiency using complexes1–3 as catalysts. To rule out the possibility that the photocatalytic activity of the complex arises from molecular or oligomeric species formed through dissolution of the solid samples in the photocatalytic reaction systems, control experiments were conducted. We subjected the catalysts to UV light and continuous stirring in water for 3 h and tested the photocatalytic activity of the solution after filtering off the solid materials. No catalytic activity was observed for the solution. Then filtrates residue was added into fresh RhB for catalysis testing. During the degradation process, a 15 W Hg lamp was used as the UV light source. 10 mg of catalyst was dispersed in 100 mL of 10 mg L−1 RhB solution under ultrasonication for 15 min. A UV-vis spectrophotometer was used to monitor the reaction under the wavelength of 554 nm and continuously at 30 min intervals until the reaction reached steady state.Figures8(a)–8(c) show that the intensity characteristic absorption of RhB had obviously decreased in the presence of complexes1–3 by the UV light irradiation. As it is shown in Figure 9(a), for comparison, the self-degradation of RhB was also assessed under the same experimental conditions. It obviously revealed that, without the solid catalyst in the reaction system, the RhB hardly degraded in 210 min of irradiation under UV light, suggesting that the solution contains no photocatalytically active species. However, in the presence of complexes1–3, the RhB in the solution is degraded approximately 74%, 71%, and 67%, respectively. From the photodegradation constantk in Figure 9(b), we can see that the performance of complexes1–3 (k = 7.1 × 10−3, 6.5 × 10−3, and 6.0 × 10−3min−1, resp.) is higher than commercially available TiO2 (k = 5.9 × 10−3min−1) [39]. The result indicated that the degeneration process was efficient. Meanwhile, complex1 is higher than other complexes. Because complexes1–3 are similar in coordination environment with similar accessibility to active uranyl centers, the higher efficiency is presumably due to the higher content of active uranyl centers in complexes1–3 (36.2%, 34.9%, and 33.6%, resp.)Figure 8
(a) The UV-vis absorption spectra of RhB solution during the photodegradation reaction when complex1 was used as catalyst; (b) the UV-vis absorption spectra of RhB solution during the photodegradation reaction when complex2 was used as catalyst; (c) the UV-vis absorption spectra of RhB solution during the photodegradation reaction when complex3 was used as catalyst; (d) the standard curve of RhB solution.
(a) (b) (c) (d)Figure 9
(a) Photocatalytic degradation; (b) photocatalytic degradation reaction of RhB under UV light irradiation over complexes1–3.
(a) (b)It is well known that the uranyl complexes could degrade the organic pollutants mainly due to the active uranyl centers. At present, two photodegradation mechanisms, hydrogen abstraction and electron transfer, have been generally accepted for the photocatalytic reactions involving uranyl species (Figure10) [12]. Upon photoexcitation, electrons may be promoted from the HOMO to the LUMO to generate excited ∗UO22+ species. And the HOMO and LUMO were ascribed to occupied 2p orbitals of oxygen and the empty uranium orbitals, respectively. The excited electron in the LUMO was not stable; it may return to the HOMO instantly. However, if the electrons from organic molecules (such as RhB) could be abstracted by the ∗UO22+ species, resulting in organic intermediates and protons, the excited electrons in the uranyl unit would remain in the LUMO until they were captured by electronegative substances, such as O2 in the solution. Once O2 captured excited electrons, highly active peroxide anions could be formed. The organic intermediates were decomposed; as a result, peroxide anions further oxidize in the solution.Figure 10
Proposed photocatalytic reaction mechanism of RhB in the presence of a uranyl complex.


## 3.1. UV-Vis Spectra Analysis

The UV-vis spectra of the ligands H4L1–3 and their corresponding target uranyl complexes1–3 were recorded in DMSO solution (Figure 1), and the data were summarized in Table 1. It was shown from Table 1 that all free ligands π→π∗ transitions absorption bands were in the range of 246–248 nm and n→π∗ transitions absorption bands in the range of 319–322 nm. It was found that the π→π∗ and n→π∗ transitions of complexes were slightly red-shifted as compared to the π→π∗ and n→π∗ transitions of ligands H4L1–3 and had absorptions in the visible regions (380–450 nm), which was ascribed to charge transfer within the U=O double bonds and the ligand to metal charge transfer (LMCT) between the oxygen atoms of the coordinating ligands and the empty orbital of the uranyl ion [17–19]. These results show that the H4L1–3 take part in coordination with uranyl ion.Table 1
UV-vis spectra data of the ligands and of target complexes.
Ligands λ max ⁡ (nm) Uranyl complex λ max ⁡ (nm) H4L1 248, 319 Complex1 258, 328 H4L2 246, 320 Complex2 259, 328 H4L3 248, 322 Complex3 259, 326Figure 1
The UV-vis spectra of (a) the free ligands H4L1–3 and (b) complexes1–3.
(a) (b)


## 3.2. FTIR Spectra Analysis

The FTIR spectra of the ligands H4L1–3 and their corresponding target uranyl complexes were recorded in the region 4000–400 cm−1, which were studied to obtain information on the oxygen-to-uranyl coordination from the shift of the C-O stretching with respect to the nonbonded ligands. Since the FTIR spectra of all ligands have shown similar features, the FTIR spectra of ligands H4L1 as well as their corresponding complex1 were selected for illustration for comparison reasons shown in Figure 2(a) and all complexes1–3 are shown in Figure 2(b). The selected vibrations of the ligands H4L1–3 and their corresponding complexes studied, together with the suggested assignments, were listed in Table 2.Table 2
The FTIR spectral frequencies (cm−1) and tentative assignments for the free ligands and uranyl complexes.
H 4 L 1 Complex1 H 4 L 2 Complex2 H 4 L 3 Complex3 Assignment of vibration 3398 3379 3397 3367 3400 3390 ν(OH) 3104, 2957 3103, 2961 3094, 2956 3101, 2958 3083, 2927 3102, 2954 ν(C-H) 1750 1746 1747 1742 1746 1742 ν(C=O) (β-diketonate) 1643 1588 1641 1587 1640 1587 ν(C=O) 1547 1546 1547 1546 1546 1546 δ(C-N-H) 1488 1461 1487 1461 1487 1461 δ(CH2) s 1460 1439 1460 1444 1460 1444 δ(CH2) s 1364 1367 1361 1384 1361 1384 δ(O-H) 1337 1350 1337 1346 1334 1342 ν(C-N) — 1309 — 1307 — 1309 ν(C-N) and δ(N-H) 1266 1260 1266 1256 1266 1256 ν(C-O) — 1217 — 1217 — 1218 ν(C-O-U) 1178 1174 1177 1173 1171 1171 ν a s(C-O-C) 1156 1157 1158 — 1157 — δ(C-O) 1073 1091 1072 1090 1069 1089 ν s(C-O-C) 1038 1039 1038 1052 1045 1053 δ(C-H) — 927 — 929 — 927 ν(U=O) as 845 856 834 847 842 853 δ(C-H) 767, 743 783, 740 786, 743 793, 742 775, 742 788, 743 δ(C-H) (benzene ring) — 550 — 550 — 550 ν(U=O)sFigure 2
The FTIR spectra of (a) the free ligand H4L1 and complex1 and (b) complexes1–3.
(a) (b)As shown in Table2, the bands at 1746–1750 cm−1 for the free ligands were assigned to the C=O stretching vibration of β-diketonate and the bands at 1742–1746 cm−1 for the corresponding complexes; there is hardly any red shifts to form enol tautomers in complexes [7, 20–22], which confirmed that oxygen atoms of carbonyl group of β-diketonate did not bind to uranyl ion. The bands at 1640–1643 cm−1 were assigned to the C=O stretching vibration of amide groups of free ligands, which shifted to 1588 cm−1 in its corresponding complexes; meanwhile, the peaks at 1266 cm−1 were attributed to the C-O stretching vibration of catechol units shifted to 1256–1260 cm−1 for their complexes, which indicated that oxygen atoms of phenolic hydroxyl take part in coordination with uranyl ion. In addition, the characteristic absorption peaks of C-N groups at 1334–1337 cm−1 shifted to 1342–1350 cm−1 in their corresponding complexes, showing formation of strong hydrogen bond between hydrogen atoms of amide groups and oxygen atoms of phenolic hydroxyls [23, 24]. Frequencies of the stretching and deformation vibrations of the C=C, C-H groups of the benzene rings were also observed. The FTIR spectra of all complexes show strong absorption bands between 927 and 929 cm−1 and weak absorption bands in 550 cm−1. The bands at 927 and 550 cm−1 were assigned to the asymmetric stretching modes of the uranyl moiety [25–27]. Therefore, analytical results of the FTIR spectra indicated uranyl ion complexes were to be obtained.


## 3.3. Proton NMR Spectra Analysis

The development of H4L1–3 evolved from the following considerations. The dialkyl malonates spacer not only provides two coordination sites but also enhances the less rigidity and preorientation of the compound and should therefore favor the encapsulation of a uranyl ion unit. Furthermore, model studies show that steric strain in the predisposed ligand reduces the number of possible geometric isomers. For an octahedral liner uranyl complex of an asymmetrically substituted catecholamide ligand there are two conceivable geometrical isomers as shown in Figure 3. So the room-temperature 1H NMR spectra of complexes1–3 compared with free H4L1–3 have shown complex splitting.Figure 3
Possible geometric isomers of uranyl complexes with two asymmetrically substituted catecholate ligands and uranyl oxo atoms omitted for clarity.The1H NMR spectra of all ligands and corresponding uranyl complexes have shown similar features. The complexes in DMSO-d6 at room temperature have shown that the complexes contain the major and minor species (possibly isomers a and b, resp.). The 1H NMR spectra of H4L1 as well as their corresponding complex1 were selected for illustration and shown in Figure 4. The resonance signals of amide protons of the major species isomers shifted from 8.90 ppm to 10.02 ppm, since the downfield chemical shifts of amide protons are generally interpreted in terms of strong hydrogen bonding [23, 24], as shown in Figure 3. The resonance signals of catechol protons of the major species isomers of complex1 showing two peaks at 7.13–7.03 (m, 2H,ortho cat-H) and 6.98–6.88 (m, 4H,meta andpara cat-H), compared with the free ligand H4L1, have appreciable changes and downfield shifts of the aromatic proton resonances of the catechol rings. Similarly, the changes in the level of the signals of the protons of the linker were also shift, turning into complex splitting due to the proximity of the uranyl ion nearby the altered protons. The resonance signals of the methylene protons of the major species isomers showed four peaks at 5.18 (dd,J = 23.8, 12.1 Hz, 2H, -CH2-), 4.78 (d,J = 11.9 Hz, 2H, -CH2-), 4.00 (t,J = 12.0 Hz, 2H, -CH2-), and 3.63 (d,J = 14.2 Hz, 2H, -CH2-). And the active methylene of β-diketonates shows two double peaks at 2.83 (d,J = 17.0 Hz, 1H) and 1.71 (d,J = 17.0 Hz, 1H), but generally, mental complexes present a singlet peak at 5.0–8.0 ppm for the enol tautomers of β-diketonates [20, 28–30]. It confirmed that oxygen atom of β-diketonate did not bind to uranyl ion, also indicated by the FTIR spectra analysis.Figure 4
The1H NMR spectra of ligand H4L1 and of complex1.


## 3.4. XPS Spectra Analysis

The XPS spectra of the samples were conducted at 10 KV and 5 mA under 10−8Pa residual pressure. The peak energies were corrected with C 1s peak at 284.6 eV as a reference. The recorded lines (C 1s, O 1s, N 1s, and U 4f) were fitted using the XPSpeak program after the background subtraction.To study the interaction of uranyl with ligands H4L1–3, XPS spectra were recorded for complexes1–3. The O 1s, C 1s, N 1s, U 4d, and U 4f spectra were shown in Figure 5. Uranyl coordination reaction was observed in complexes evidenced by the appearance of doublet peaks of U 4f5/2 and U 4f7/2 with a splitting value of 10.80–10.90 eV. The values of the binding energies of O 1s, C 1s, N 1s, U 4f5/2, and U 4f7/2 and the splitting values of the U 4f spectra were shown in Table 3. The O 1s and U 4f spectra of complexes1–3 were shown in Figure 6. The peak fitting results of the O 1s and U 4f were shown in Table 4. Because XPS is not a quantitative analysis method, these data could be used for semiquantitative analysis of the distribution of multicomponent [31, 32].Table 3
Binding energy (eV) of complexes1–3 and splitting value of U 4f.
Uranylcomplex O 1s C 1s N 1s U4f5/2 U4f7/2 Splittingvalue Complex1 531.26 284.26 399.66 392.46 381.66 10.80 Complex2 531.71 284.71 399.62 392.43 381.61 10.82 Complex3 531.66 284.66 399.61 392.41 381.51 10.90Table 4
The peak fitting results of the O 1s, U 4f7/2 of complexes 1–3.
Core levels Complex1 Complex2 Complex3 Binding energy (eV) FWHW (eV) Area Binding energy (eV) FWHW (eV) Area Binding energy (eV) FWHW (eV) Area O 1s 533.85 1.31 1468.15 533.98 2.13 7315.93 534.34 1.10 1318.23 533.10 1.42 6015.89 533.50 1.3 5996.96 533.56 0.87 2983.22 532.60 1.08 6718.25 532.70 1.06 6697.41 532.78 0.99 5832.76 532.10 1.14 7356.14 532.30 1.13 7107.48 532.22 1.01 6766.91 531.45 1.21 16239.1 531.60 1.20 15224.1 531.47 1.2 13254.9 530.65 1.52 8646.69 530.69 1.49 8633.66 530.47 1.58 8728.48 U4f7/2 384.49 3.78 12770.3 384.79 4.33 17262.9 384.80 4.12 13500.5 382.10 2.63 36018.1 382.22 2.84 45319.6 382.20 2.74 38513.2Figure 5
XPS survey scans of complexes1–3.Figure 6
Curves fitted high resolution scans of (a) O 1s and (b) U 4f of complexes1–3.
(a) (b)As shown in Figure6(a), it was observed that complexes1–3 presented six main components O 1s spectra for oxygen functionalities. The six peaks position at 530.50–530.69 eV (O=U=O) [33, 34], 531.40–531.55 eV (C-O-U), 532.20–532.33 eV (C=O of β-diketonate), 532.60–532.80 eV (C-O-C), 533.40–533.50 eV (C=O of benzamide), and 533.90–534.00 eV (adsorbed H2O) [35] was observed. And the relative intensities of C-O-U were significantly higher than other oxygen functionalities; meanwhile, the presence of the U=O bonds was confirmed, as expected.The U 4f7/2 spectra were deconvoluted into two components: the peak corresponding to the free uranyl ion that occurred at 381.05 eV and the peak of complexes1–3 that occurred at 382.10–382.22 eV, as shown in Figure 6(b).From the above discussion, it is clear that the ligands H4L1–3 complexation of uranyl ion occurred with the catecholamide moieties.


## 3.5. Thermal Properties

To examine the thermal stability of complexes1–3, thermal gravimetric analysis (TGA) was carried out at a heating rate of 10°C min−1 under the conditions of N2 atmosphere with the temperature range from 25 to 1100°C (Figure 7). The TGA curves of complexes1–3 show two steps to weight loss. The first weight loss in the range of 120–220°C should be attributed to the loss of two coordinated water molecules with a percentage weight loss (obs. 5.1%, 4.9%, and 4.8%; calcd. 4.82%, 4.65%, and 4.49%) of complexes1–3, respectively. The second weight loss in the range of 250–1050°C is attributed to the loss of biscatecholamide ligands with a percentage weight loss (obs. 55.1%, 57.6%, and 60.0%; calcd. 56.86%, 58.42%, and 59.87%) of complexes1–3, respectively. The percentage weight of final residue was in good agreement with the calculated value by the stoichiometric chemical formula UO3.Figure 7
The thermal gravimetric analysis curves of (a) complex1, (b) complex2, and (c) complex3.
(a) (b) (c)


## 3.6. Photocatalytic Activity

As we know, methyl blue, methyl orange, and RhB dyes have been widely used in textile, printing, paper, and pharmaceutical industries, causing serious pollution to the environment at the same time [35–38]. In this work, we selected RhB as model compound of organic pollutant to investigate the degradation efficiency using complexes1–3 as catalysts. To rule out the possibility that the photocatalytic activity of the complex arises from molecular or oligomeric species formed through dissolution of the solid samples in the photocatalytic reaction systems, control experiments were conducted. We subjected the catalysts to UV light and continuous stirring in water for 3 h and tested the photocatalytic activity of the solution after filtering off the solid materials. No catalytic activity was observed for the solution. Then filtrates residue was added into fresh RhB for catalysis testing. During the degradation process, a 15 W Hg lamp was used as the UV light source. 10 mg of catalyst was dispersed in 100 mL of 10 mg L−1 RhB solution under ultrasonication for 15 min. A UV-vis spectrophotometer was used to monitor the reaction under the wavelength of 554 nm and continuously at 30 min intervals until the reaction reached steady state.Figures8(a)–8(c) show that the intensity characteristic absorption of RhB had obviously decreased in the presence of complexes1–3 by the UV light irradiation. As it is shown in Figure 9(a), for comparison, the self-degradation of RhB was also assessed under the same experimental conditions. It obviously revealed that, without the solid catalyst in the reaction system, the RhB hardly degraded in 210 min of irradiation under UV light, suggesting that the solution contains no photocatalytically active species. However, in the presence of complexes1–3, the RhB in the solution is degraded approximately 74%, 71%, and 67%, respectively. From the photodegradation constantk in Figure 9(b), we can see that the performance of complexes1–3 (k = 7.1 × 10−3, 6.5 × 10−3, and 6.0 × 10−3min−1, resp.) is higher than commercially available TiO2 (k = 5.9 × 10−3min−1) [39]. The result indicated that the degeneration process was efficient. Meanwhile, complex1 is higher than other complexes. Because complexes1–3 are similar in coordination environment with similar accessibility to active uranyl centers, the higher efficiency is presumably due to the higher content of active uranyl centers in complexes1–3 (36.2%, 34.9%, and 33.6%, resp.)Figure 8
(a) The UV-vis absorption spectra of RhB solution during the photodegradation reaction when complex1 was used as catalyst; (b) the UV-vis absorption spectra of RhB solution during the photodegradation reaction when complex2 was used as catalyst; (c) the UV-vis absorption spectra of RhB solution during the photodegradation reaction when complex3 was used as catalyst; (d) the standard curve of RhB solution.
(a) (b) (c) (d)Figure 9
(a) Photocatalytic degradation; (b) photocatalytic degradation reaction of RhB under UV light irradiation over complexes1–3.
(a) (b)It is well known that the uranyl complexes could degrade the organic pollutants mainly due to the active uranyl centers. At present, two photodegradation mechanisms, hydrogen abstraction and electron transfer, have been generally accepted for the photocatalytic reactions involving uranyl species (Figure10) [12]. Upon photoexcitation, electrons may be promoted from the HOMO to the LUMO to generate excited ∗UO22+ species. And the HOMO and LUMO were ascribed to occupied 2p orbitals of oxygen and the empty uranium orbitals, respectively. The excited electron in the LUMO was not stable; it may return to the HOMO instantly. However, if the electrons from organic molecules (such as RhB) could be abstracted by the ∗UO22+ species, resulting in organic intermediates and protons, the excited electrons in the uranyl unit would remain in the LUMO until they were captured by electronegative substances, such as O2 in the solution. Once O2 captured excited electrons, highly active peroxide anions could be formed. The organic intermediates were decomposed; as a result, peroxide anions further oxidize in the solution.Figure 10
Proposed photocatalytic reaction mechanism of RhB in the presence of a uranyl complex.


## 4. Conclusions

Theβ-diketonates biscatecholamide ligands H4L1–3 were obtained via the reported procedure; their complexes with UO22+ were also prepared successfully and determined by the means of UV-vis spectra, FTIR spectra, 1H NMR, and elemental analyses. The complex 1H NMR spectra show that the uranyl complexes have two conceivable geometrical isomers. The photodegradation property of the target complexes was investigated by the RhB which is a model compound of organic pollutant. The results show that complexes1–3 exhibit good photocatalytic property, which will be used as the promising candidate photocatalytic materials for nuclear organic waste treatment in nuclear industry.



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*Source: 8041647-2017-01-29.xml*