Synthesis, Characterization and Thermal Decomposition of Co(Ⅱ) Complexes Based on Biuret Ligand

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(School of Material Science and Engineering, Southwest University of Science and Technology, Mianyang 621010, China)

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Synthesis, Characterization and Thermal Decomposition of Co(Ⅱ) Complexes Based on Biuret Ligand

WANGMei-ling,ZANGQing,ZHONGGuo-qing

(School of Material Science and Engineering, Southwest University of Science and Technology, Mianyang 621010, China)

AbstractThree kinds of cobalt complexes of biuret were synthesized with cobalt acetate, cobalt chloride or cobalt nitrate and biuret as starting materials in methanol solution. The complexes were characterized by elemental analyses, FTIR spectra, X-ray powder diffraction, and thermogravimetric analysis. The compositions of the complexes were[Co(bi)2(H2O)2](Ac)2·H2O (1),[Co(bi)2Cl2] (2), and[Co(bi)3](NO3)2·2.5H2O (3) (bi = NH2CONHCONH2). The cobalt ion in complex 1 was coordinated by six oxygen atoms from two biuret molecules and two water molecules, the cobalt ion in complex 2 was coordinated by four carbonyl oxygen atoms from two biuret molecules and two chloride atoms, while the cobalt ion in complex 3 was coordinated by six carbonyl oxygen atoms from three biuret molecules. The cobalt ions were all hexa-coordinated. The thermal decomposition processes of complex 1 and 3 under air included dehydration and pyrolysis of the ligands and anions, while the thermal decomposition process of complex 2 was only the pyrolysis of ligands, and the final residues were all cobalt oxide.

Key wordsbiuret; cobalt complex; characterization; thermal decomposition

Biuret is a kind of important chelating ligand, and can act as anO,O′-bidentate neutral ligand orN,N′-bidentate anionic ligand binding to metal ions, such as scandium(Ⅲ)[1], copper(Ⅱ)[2], samarium(Ⅲ)[3], thorium(IV)[4]and so on. In recent years the chemistry of biuret and related compounds attracts increasing attention. A novel bis-cyclometalated complex named [Ir2(m-biuretato-N,N′:O,O′)(ptpy)4] was obtained and studied[5]. What’s more, a research in thermal degradation kinetics of biuret-formaldehyde polymeric ligand was done lately[6]. In animal husbandry, biuret is an excellent feed additive for ruminants, it has the advantages of good palatability, low toxicity and easy to digest, and is safer than other non protein nitrogen feed additives, such as urea. In medicine, biuret can be used for the preparation of hypnotics and sedatives and it is also good at lowering the blood pressure. In chemical industry, biuret can be utilized as raw materials for synthesis of paints, lubricants, and coatings. At home, the complexes of biuret ligand have been rarely reported[7-9], especially in the comparison of different anions in the synthesis of Co(Ⅱ) complexes. The biuret complexes of trace elements as feed additives for ruminants can play a dual role of essential trace elements and non protein nitrogen nutrition supplements. Here we report three Co(Ⅱ) complexes synthesized with biuret and three different cobalt salts.

1Experimental

1.1Materials and physical measurements

All chemicals purchased were of analytical reagent grade and used without further purification. Cobalt acetate tetrahydrate, cobalt chloride hexahydrate, cobalt nitrate hexahydrate and biuret were purchased from Sinopharm Chemical Reagent Co. Ltd. of Shanghai.

Elemental analyses for C, H and N in the complexes were measured on a Vario EL CUBE elemental analyzer, and the content of cobalt was determined by EDTA complexometric titration with murexide as indicator. IR spectra were obtained with KBr pellets on a Nicolet 5700 FT-IR spectrophotometer in the range of 4 000～200 cm-1. The powder X-ray diffraction measurements were recorded on a D/max-Ⅱ X-ray diffractometer in the diffraction angle range of 3～80°. The thermogravimetric analysis data were obtained using a SDT Q600 thermogravimetry analyzer in the air atmosphere in the temperature range of 20～800 ℃ with a heating rate of 10 ℃ min-1.

1.2Synthesis of [Co(bi)2(H2O)2](Ac)2·H2O (1)

Co(CH3COO)2·4H2O (2.49 g, 10 mmol) and biuret (2.06 g, 20 mmol) were weighed and dissolved in 100 mL methanol. The mixed solution was stirred on a magnetic stirrer for about 6 h under reflux reaction. After the solution cooling, the resultant was separated from the reaction mixture by filtration, and washed by methanol and dried in the phosphorus pentoxide desiccator for 1 week. The product was pink powder (3.38 g) and the yield was about 77%.

1.3Synthesis of [Co(bi)2Cl2] (2)

Complex 2 was synthesized by the same procedure as that for preparation of complex 1 except for using CoCl2·6H2O (2.38 g, 10 mmol) instead of Co(CH3COO)2·4H2O as the start reactant. The product was fuchsia powder (2.36 g) and the yield was about 70%.

1.4Synthesis of [Co(bi)3](NO3)2·2.5H2O (3)

Complex 3 was synthesized by the same procedure as that for preparation of complex 1 except for using Co(NO3)2·6H2O (2.91g, 10 mmol) instead of Co(CH3COO)2·4H2O as the start material. The product was light red powder (3.20 g) and the yield was about 60%.

2Results and Discussion

2.1Composition and property

The results of elemental analyses for the complexes are shown in Tab.1. As shown in Tab.1, the composition formulae of the complexes are CoC8H22O11N6, CoC4H10O4N6Cl2and CoC6H20O14.5N11, respectively. The three complexes are consistent with the target products [Co(bi)2(H2O)2](Ac)2·H2O, [Co(bi)2Cl2], and [Co(bi)3](NO3)2·2.5H2O (bi = NH2CONHCONH2). The water molecules in the complexes come from the crystal water in the raw materials of inorganic salts. In order to make sure whether chlorine atoms were coordinated or ionic, a qualitative test was conducted, namely, a few drops of AgNO3solution was added into aqueous solution containing complex 2 and there was no deposition at first. This indicated that the chlorine atoms were coordinated. However, the transparent solution became turbid with the increasing amount of AgNO3solution, and this showed that the coordination ability of chlorine atom was not very strong. The molar ratio of Co(Ⅱ) to biuret in complex 1 and 2 is 1∶2, while in complex 3 is 1∶3. The three solid complexes are stable in the air, easily dissolved in water, and not easy to absorb moisture.

Tab.1　Elemental analysis results of the complexes (Calculated values are in brackets)

2.2IR spectroscopy analysis

Wavenumber/cm-1　　　　　　　　　　　　　　　　　　　　　　　　　　　2θ/(°)Fig.1　IR spectra of three complexes　　　　　　　　　　　　　　　　　　Fig.2　XRD patterns of three complexes(a)—complex 1, (b)—complex 2, (c)—complex 3　　　　　　　　　　　(a)—complex 1, (b)—complex 2, (c)—complex 3

bi[Co(bi)2(H2O)2](Ac)2·H2O[Co(bi)2Cl2][Co(bi)3](NO3)2·2.5H2Ovibrationtype34143428,338833953422,3401νas(NH2)+ν(H2O)3253,30813201,31123194,30923304,3207νs(NH2)1726,1690168416971697ν(CO═)1620,1580162315821609δ(NH2)1499,1420151214921499ν(C—N)+ν(C—NH2)1330133413341334δ(N—H)1130112911291129ν(C—N)+δ(N—H)1080106511081059ν(C—N)938930937934ν(C—N)+ν(C—NH2)770772765765δ(C—NH2)710634634634δ(CO═)

2.3X-ray powder diffraction analysis

The XRD patterns of the three complexes are shown in Fig.2. As shown in Fig.2, the background of the XRD patterns is small and the diffractive intensity is strong, and the results indicate that the complexes have fine crystalline state. Comparing with the reactants, the strong peak locations of three complexes are changed obviously. The three main strong peaks appear in 2θ=23.81°, 13.52° and 10.29° for complex 1, and in 2θ=21.8°, 28.6° and 23.5° for biuret, while in 2θ=12.84°, 20.08° and 21.03° for cobalt acetate (JCPDS 025-0372). The main strong peaks of complex 1 are different from the main strong peaks of the raw materials. Similarly, the three main strong peaks of complex 2 appear in 2θ=28.49°, 13.23° and 15.12°, while in 2θ=15.24°, 35.74° and 51.44° for cobalt chloride (JCPDS 015-0381). At the same time, the three main strong peaks of complex 3 appear in 2θ=27.91°, 17.19° and 13.37°, and they are distinct from the main strong peaks at 20.69°, 26.83° and 40.42° for cobalt nitrate (JCPDS 019-0356). In short, all these strong peaks of the reactants are disappeared in X-ray powder diffraction patterns of the three complexes. The diffraction angle (2θ), diffractive intensity and spacing (d) of the products are completely different from the reactive materials, which may illuminate that the resultants are new compounds instead of the reactant mixture[15].

2.4Thermal analysis

Thermogravimetric analysis under air is determined with the samples to investigate the thermal stabilities of three complexes and the TG-DTG curves are shown in Figs. 3～5. In Fig.3, there is a mass loss of 12.41% near 118 ℃ for complex 1, which is ascribed to the loss of three lattice water and coordinated water molecules and the measured value is in agreement with the calculated one (12.36%). There is a large mass loss of 47.20% at 178 ℃, 220 ℃ and 242 ℃ in DTG curve, which can be interpreted as the decomposition of the biuret ligand, and it is close to the calculated result (47.15%). In the last step, it is the process of decomposition of Co(CH3COO)2, and the mass loss of 23.27% is in agreement with the calculated one (23.35%). The residual mass of complex 1 in the TG curve is 17.12%, which agrees with the theoretical value (17.14%), and the final residue is CoO.

Fig.3　TG-DTG curves of the complex 1　　　　　　　　　　　　　Fig.4　TG-DTG curves of the complex 2

In Fig.4, it is not difficult to find that complex 2 is stable up to 223 ℃ comparing with complex 1, whereafter the compound begins to collapse. The complex undergos two mass losses of 34.40% and 26.69% near 267 ℃ and 367 ℃, which represent the oxidation and decomposition of the biuret ligand (calcd. 61.36%). There is a mass loss of 16.25% in the last step, which is considered to be the decomposition of cobalt chloride (calcd. 16.34%), and the final residue is CoO (found 22.66%, calcd. 22.30%).

Fig.5　TG-DTG curves of the complex 3

As shown in Fig.5, the decomposition process of complex 3 is divided into three steps. The mass losses are 7.95%, 56.62% and 20.30% near 133 ℃, 261 ℃ and 452 ℃, respectively, which agree with the loss of 2.5H2O (calcd. 8.38%), 3C2H5O2N3(calcd. 57.57%), and N2O5(calcd. 20.11%). The final residue is CoO (found 14.45%, calcd. 13.95%).

3Conclusions

Three kinds of Co(Ⅱ) complexes, namely [Co(bi)2(H2O)2](Ac)2·H2O (1), [Co(bi)2Cl2] (2), and [Co(bi)3](NO3)2·2.5H2O (3), were synthesized. The Co(Ⅱ) complexes were all hexa-coordinated, and the Co(Ⅱ) was coordinated by all the carbonyl oxygen atoms from the biuret molecules, while the cobalt ion was too coordinated by two oxygen atoms from coordinated water molecules in complex 1 and two chloride atoms in complex 2. This may be the reason that the structures of three complexes were quite different. The thermal analysis results showed that the complexes of 1 and 3 were lost water molecules below 170 ℃, then the biuret ligand in the three complexes was oxidized and decomposed about 200 ℃, and finally inorganic salts of cobalt(Ⅱ) were decomposed into cobalt oxide. The biuret complexes of trace elements can play a dual role of essential trace elements and non protein nitrogen nutrition supplements. With the rapid development of animal industry in our country, the prospects of biuret complexes which are used as feed additives for ruminants are considerable.

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(編輯楊春明)

縮二脲鈷配合物的合成、表征與熱分解

王美玲，臧晴，鐘國清*

(西南科技大學材料科學與工程學院，中國 綿陽621010)

摘要在甲醇溶液中，以乙酸鈷、氯化鈷、硝酸鈷和縮二脲為原料合成了3種結構不同的縮二脲鈷配合物.通過元素分析、紅外光譜、X射線粉末衍射和熱分析對產物進行了表征，其化學組成為[Co(bi)2(H2O)2](Ac)2·H2O (1)，[Co(bi)2Cl2] (2)和[Co(bi)3](NO3)2·2.5H2O (3) (bi = NH2CONHCONH2).配合物1中每個Co2+與2個縮二脲分子中的4個羰基氧原子和2個水分子中的氧原子配位，配合物2中每個Co2+與2個縮二脲分子中的4個羰基氧原子和2個氯原子配位，而配合物3中每個Co2+與6個全部來自縮二脲分子的羰基氧原子配位，均形成了配位數為6的配合物.配合物1和3的熱分解過程包括失水和配體的分解，而配合物2的熱分解過程只是配體的分解過程，最后完全分解形成氧化鈷.

關鍵詞縮二脲；鈷配合物；表征；熱分解

中圖分類號O614.81+2

文獻標識碼A

文章編號1000-2537(2015)05-0053-06

通訊作者*,E-mail：zgq316@163.com

基金項目：四川省教育廳資助重點科研項目(10ZA0160)

收稿日期：2015-04-23

DOI:10.7612/j.issn.1000-2537.2015.05.009