Spectroscopic and Thermal Studies of [Cr₂(NH₂)₂(H₂O)₂(SO₄)₂]·2H₂O,[Cr(NCO)₃(H₂O)]·3H₂O and [Fe O(OH)]·0.2_H2O Compounds Formed by the Reactions of Urea with Cr₂(SO₄)₃, Cr(CH₃COO)₃ AND Fe₂(SO₄)₃

Abstract

The bridged disulphato complex $[Cr_2(NH_2)_2(H_2O)_2(SO_4)_2]{\cdot}2H_2O$, terminal triisocyanato $[Cr(NCO)_3(H_2O)]{\cdot}3H_2O$ complex and limonite, $[FeO(OH)]{\cdot}0.2H_2O$ compound were prepared by the reaction of $Cr_2(SO_4)_3{\cdot}xH_2O, Cr(CH_3COO)_3$ and $Fe_2(SO_4)_3$, respectively, with urea in aqueous media at $80^{\circ}C$. The infrared spectra of the products indicate that the absence of the bands of urea, but shows the characteristic bands of coordinated amide, water, bridged sulphato and isocyanato groups. Thermogravimetric (TG) and differential thermal analysis (DTA) measurements on the complexes are also recorded. The data obtained agree quite well with the expected structures. A general mechanisms describing the formation and its thermal decomposition of the complexes are suggested.

두개의 sulphato로 다리걸친 착물 $[Cr_2(NH_2)_2(H_2O)_2(SO_4)_2]{\cdot}2H_2O$, 말단 3개의 isocyanato $[Cr(NCO)_3(H_2O)]{\cdot}3H_2O$ 화합물과 갈철광 $[FeO(OH)]{\cdot}0.2H_2O$ 화합물들은 각각의 $Cr_2(SO_4)_3{\cdot}xH_2O, Cr(CH_3COO)_3, Fe_2(SO_4)_3$과 $80^{\circ}C$, 수용액에서 요소와 반응으로 합성되었다. 생성물들의 IR 스펙트럼에서 요소(urea)의 띠가 나타나지 않지만, 결합된 아마이드(amide), 물, 연결된 sulphato와 isocyanato 그룹에 대해서 특징적인 띠를 보인다. 착물들에 대한 열무게분석(TG)과 시차열분석법(DTA) 측정을 기록하였다. 얻어진 데이터들은 예상했던 구조화 잘 일치한다. 형성에 대한 설명과 화합물의 열역학적 분해에 대한 일반적인 메커니즘을 제시하였다.

INTRODUCTION

The reactions between urea and d-block elements at room temperature have been studied extensively.1-6 Most of these studies were indicate that urea coordinates either via the amide nitrogen or the carbonyl oxygen atoms, depending on the type of the metal ion and the nature of such coordination can easily be identified from the shift in both the ν (NH) and ν (C=O) frequencies of the coordinated urea compared with those of free urea. Penland et al.6 studied the infrared spectra of both [Cr(urea)6] X3 and [Fe(urea)6] X3 and they indicate that, oxygento- metal bonds are present in these complexes. However, detailed studies for this class of reaction at high temperature appear to be limited in the literature.7-12 The nature of the reaction products was to be strongly dependent on the type of metal ion and the metal salt used in the reaction.

To continue our investigation in this area,7,10,12 we report in the present article the preparation of the related new compounds [Cr2(NH2)2(H2O)2(SO4)2]·2H2O, [Cr(NCO)3(H2O)]·3H2O and [Fe O (OH)]·0.2H2O. The infrared spectra of the compounds as well as thermogravimetric (TG) and differential thermal (DTA) analysis were carried out.

 

EXPERIMENTAL

All chemicals used through out this work were Analar or extra pure grade. The dark green solid complex [Cr2(NH2)2(H2O)2(SO4)2] ·2H2O was prepared by mixing equal volumes of aqueous solutions of 0.1M of Cr2(SO4)3 · xH2O and 0.6M of urea. The mixture was heated on a water bath to approx. 80 ℃ for about 4-6 h. The complex was precipitated, filtered off, washed several times with hot water and dried in vacuo over P2O5. The two compounds [Cr(NCO)3(H2O)] · 3H2O and [FeO(OH)] · 0.2H2O were prepared in a manner similar to that described above by the reaction of Cr(CH3COO)3 and Fe2(SO4)3 with urea, respectively. Its elemental analysis, infrared spectra and thermal properties DTA and TG characterized the compounds. Analysis of the products obtained:

The infrared spectra of the compounds obtained, [Cr2(NH2)2(H2O)2(SO4)2] · 2H2O,[Cr(NCO)3(H2O)] · 3H2O and [FeO(OH)] · 0.2H2O and of the DTA decomposition products were recorded from KBr discs using a Gensis II FT IR Spectrophotometer. Thermogravimetric (TG) and differential thermal analysis (DTA) of the compounds were carried out using a Shimadzu DT-50H computerized thermal system.

 

RESULTS AND DISCUSSION

Urea reacts with chromium(III) sulphate in aqueous media at ~80 ℃ to form dark green solid complex identified as [Cr2(NH2)2(H2O)2 (SO4)2] · 2H2O. The formation of this complex upon the heating of an aqueous mixture of Cr(III) sulphate and urea may be understood as follows. At room temperature the [Cr(urea)6]+3 complex is formed6 where urea coordinates to Cr(III) ions via its oxygen atom. At high temperature the following reaction may take place:

Fig. 1(a&b) shows the infrared spectra of the free urea along with spectra of the [Cr2(NH2)2(H2O)2-(SO4)2]·2H2O product and its band assignments are given in Table 1. The most probable structure according to the complex formula and the infrared spectrum is shown in formula (I) where the complex contains two bridged sulphato ligands and process C2vsymmetry.

The infrared spectrum of the complex clearly indicates the absence of bands due to coordinated urea and the presence of bands characteristic for coordinated water,13 at 3520, 3500, 3370 and 3350 cm−1. The appearance of these four bands is expected for such a C2v symmetry and can be assigned as follows: the first two bands are assigned to the two antisymmetric vibrations of the type, νas(O-H), B2, while the other two bands are associated with the two symmetric vibrations of the type, νs (O-H), A1 . The bending motion for coordinated water in this Cr(III) complex, δ (H2O), is assigned at 1635 cm−1 while the rocking motion, δr (H2O), is assigned at 885 and 841 cm−1 the assignments for both the bond stretches and the angular deformations of the coordinated water molecules fall in the frequency regions reported for many related aqua complexes.11-17

Fig. 1.Infrared spectra of: (a) urea; (b) [Cr2(NH2)2(H2O)2-(SO4)2] · 2H2O; (c) [Cr(NCO)3(H2O)] · 3H2O and (d) [FeO(OH)] · 0.2H2O.

Table 1.(a): s=strong, w=weak, m=medium, sh=shoulder, v=very, br=broad. (b): ν, stretching; δb, δr, δt and δw correspond to bending , rocking, twist and wagging motions, respectively.

The coordinated -NH2 groups show a set of bands agree quite well with those previously reported for related complexes.11,14,15 The two bands at 3305 and 3290 cm−1 are associated to the N-H stretching modes corresponding to the antisymmetric and symmetric motions, respectively. The angular deformation motions of the coordinated -NH2 groups in this Cr(III) complex can be classified into three types of vibrations: δb (bend), δw (wag) and δt (twist). The assignments of these motions in our complex are as follows. The bending motion, δb(NH2), is assigned at 1490 cm−1. The wagging motion, δw(NH2), is assigned at 985 cm−1. The twisting motion, δt (NH2), is observed at 923 cm−1. The Cr-N stretching frequency is assigned at 516 cm−1.

The bands of bridged coordinated sulphato group are observed at their expected values,18-22 three bands occur in the region above 1000 cm-1 at 1125, 1075 and 1060 cm-1, while three bands of different intensities occupy the region below 1000 cm-1 at 758, 600 and 490 cm-1 (Table 1). The bands at 1125, 1075 and 1060 cm-1 are related to the stretching vibrations of n (SO4)−2 in agreement with the group theoretical analysis for this structure, while the other three bands which observed at 785,600 and 490 cm-1 are associated with the bending vibrations of SO4-2.

Thermogravimetric (TG) and differential thermal analysis (DTA) were carried out for the [Cr2(NH2)2-(H2O)2(SO4)2] · 2H2O complex under a N2 flow. Fig. 2(A&B) represents the DTA and TG curves and Table 2 gives the maximum temperature values for decomposition along with the corresponding weight loss values. These data support the proposed complex structure and also indicate that the decomposition of the complex occurs in four degradation steps. The first stage of decomposition occurs at maximum temperature of 120 ℃ and is accompanied by a weight loss of 8.68% corresponding to the loss of the two uncoordinated water molecules. The relative low value of temperature of this step may indicate that these water molecules undergoes less H-bonding. The second step of degradation occurs at 286 ℃ with a weight loss of 8.79%, this associated with the loss of the two coordinated water molecules. The third decomposition stage occurs at a maximum temperature 352 ℃ and is accompanied by a weight loss of 7.89%, this is associated with the loss of the two amide groups. The final step occurs at 416 ℃ without loss weight, this stage should be attributed to the rupture of the Cr-O bonds (O of sulphato). No weight loss in this step is related to the fact that SO4−2 is still associated with Cr(III) but in ionic form. The infrared spectra of the different decomposition steps (Fig. 3), were supported these conclusions which shows the absence of any bands associated to the amido and bridged sulphato groups, but shows a group of bands characteristic of ionic sulphate, Cr2(SO4)3 at 1085, 940 and 885 cm-1.

Accordingly to these conclusions, the decomposition mechanism for this complex is as follows:

Fig. 2.Thermal analysis diagrams of [Cr2(NH2)2(H2O)2(SO4)2] · 2H2O. (a) TG and DTG, (b) DTA.

Table 2.The maximum temperature, Tmax / ℃, and weight loss values of the decomposition stages for the [Cr2(NH2)2(H2O)2-(SO4)2] · 2H2O, [Cr(NCO)3(H2O)] · 3H2O and [FeO(OH)] · 0.2H2O compounds

The green solid complex, [Cr(NCO)3(H2O)] · 3H2O, formed by the reaction of urea with chromium(III) acetate in aqueous media at ~80 ℃. The infrared spectrum of this complex is given in Fig. 1c and its vibrational assignments are listed in Table 1. The i.r. spectrum also clearly indicates the absence of bands due to coordinated urea and the presence of bands characteristic for isocyanate ions21 at 2190 and 2044 cm−1 and for coordinated water13 in the 3360-3220 cm−1 region. The complex is well characterized through the elemental analysis, infrared spectra and thermal analysis TG and DTA. The νas (N≡C) and νs (N≡C) are observed as expected at 2190 and 2044 cm−1, respectively, while the ν (C-O) and δ (NCO) are assigned at 1458, 1416 and 588, 530 cm−1, respectively. These results for the NCO− ions agree quite well with those known for isocyanato complexes.21,23 The various ν(O-H) vibrations characteristic of coordinated water and uncoordinated water in the 3435-3220 cm−1 region. These bands can be assigned as follows, the first band at 3360 cm−1 is assigned to the antisymmetric vibration of the type νas (O-H), B2 while the other two bands at 3270 and 3220 cm−1 are associated with two symmetric vibration of the type ν (OH); A1, and exhibit intensities expected from their symmetry characteristic (A1&B2) under C2v symmetry. The bending vibration, δ(H2O) is assigned at about 1645 cm−1. The stretching and angular deformations (δr δt and δw) of coordinated water molecules fall in their expected frequency values, Table 1 like many other compounds.11-17

Fig. 3.Infrared spectra of the different decomposition steps for [Cr2(NH2)2(H2O)2(SO4)2] · 2H2O.

It is well known that urea coordinates to Cr(III) ions at room temperature via its oxygen atom.6 Forming the [Cr(urea)6]+3 ion complex. At high temperature the following reaction may take place:

To make sure about the proposed formula and structure of the formed complex, [Cr(NCO)3(H2O)] · 3H2O, thermogravimetric (TG) and differential thermal analysis (DTA) were carried out under N2 flow. DTA and TG thermograms are shown in Figs. 4 (A&B). Tables 2 gives the maximum temperature values, Tmax/℃, together with the corresponding weight loss for each step of the decomposition reaction. The data obtained support the proposed structure and indicate that, the thermal decomposition of these complex proceeds with two main degradation steps. The first stage of decomposition occurs at a temperature maximum of 195℃. The found weight loss associated with step is 22.07% and may be attributed to the loss of the three uncoordinated water molecules which is in good agreement with the calculated values of 21.60%. The second stage of decomposition occurs at a temperature maximum of ≥350 ℃. The weight loss found at this stage equals to 43.13% corresponds to loss of 2CO+3/2N2+1/2H2O+1/2H2. To supporting our conclusion of the absence of NCO and H2O, we make infrared spectra for the residue of ignition at different temperatures (200, 350, 500, 650 and 800 ℃), Fig. 5, which clearly indicate that the NCO− group disappeared at 350℃ and the characteristic stretching vibration of Cr2O3 which containing the Cr=O groups that exhibit the Cr=O vibration bands in the 1050-800 cm-1 region24,25 the stretching frequency of the type ν (Cr=O) in our complex is assigned at 1133, 892 cm-1 and doublet bands observed at 619 and 572 cm-1. The agrees with the elemental analysis of the existence of no nitrogen or carbon elements.

Fig. 4.Thermal analysis diagrams of [Cr(NCO)3(H2O)] · 3H2O. (a) TG and DTG, (b) DTA.

Fig. 5.Infrared spectra of the different decomposition steps for [Cr(NCO)3(H2O)] · 3H2O.

The weight found for the residue after decomposition is 34.80% giving an actual total weight loss of 65.20%. However the calculated total weight loss of is 64.80%.

Finally, The thermal decomposition reactions of the complex can be summarized as follows:

Urea reacts with iron(III) sulphate in aqueous media at ~85 ℃ to form ferric hydroxide oxide (Limonite), [FeO(OH)] · 0.2H2O. The infrared spectrum obtained for this compound, Fig. 1d clearly indicates the absence of any bands due to coordinated urea and the presence of bands characteristic for hydroxide ion26 at 3170 and 1285, 1027 cm-1. The bands associated for the stretching of Fe-O [27] is observed at 900 and 800 cm-1 (Table 1). Based on these facts, and with the elemental analysis, the compound [FeO(OH)] · 0.2H2O is formed. The formation of ferric hydroxide oxide is greatly supported by measuring the infrared spectrum of the commercially obtained ferric hydroxide oxide, [FeO(OH)] · nH2O.28 We noted that the two spectra are typically (fingerprint) indicating that, the obtained product is [FeO(OH)] · 0.2H2O.

The formation of [FeO(OH)] · 0.2H2O compound upon the heating of an aqueous mixture of the ferric(III) sulphate and urea may be understood as follows at room temperature, ferric(III) ions react with urea to give the parent complex6 of the type [Fe(urea)6] X3 where (X=SO4−-). At higher temperature, the following reaction may take place:

Fig. 6.Thermal analysis diagrams of [FeO(OH)] · 0.2H2O. (a) TG and DTG, (b) DTA.

The thermal decomposition DTA and TGA Fig. 6 (A&B) of the compound [FeO(OH)] · 0.2H2O exhibits two main degradation steps. The first step of decomposition occurs from 30 to 163 ℃ is accompanied by a weight loss of 3.91% in agreement with the theoretical values 3.88% for the loss of 0.2H2O. The second step of decomposition occurs at a maximum temperature 230 ℃ with a weight loss of 9.37%. This step is associated with the loss of 1/2H2O giving the oxide product, FeO1.5. The infrared spectrum of the final product (Fig. 7) supports the above discussion. It only shows the bands associated with the ferric oxide, Fe2O3.

Accordingly, the following mechanism is proposed for the thermal decomposition of the [FeO(OH)] · 0.2H2O compound as follows:

Fig. 7.Infrared spectra of the different decomposition steps for [FeO(OH)] · 0.2H2O.