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Reactions Of Urea With Oxides Of Mo(VI), V(V) And Se(IV), At High Temperature

Refat¹, M.S; Sadeek², S.A.; Teleb², S.M.

¹Department of Chemistry, Faculty of Education, Port-Said, Suez Canal University, Egypt.
²Department of Chemistry, Faculty of Science, Zagazig University, Zagazig, Egypt
Fax: +20 66 3341303, Email: msrefat@yahoo.com

Received February 02, 2004. In final form,  May 14, 2004.

Abstract
The molybdenum(VI) nitrate salt, [V₂(O)₄(OH)(NH₂)] complex and selenium metal were obtained by a new reaction of urea with MoO₃, V₂O₅ and SeO₂, respectively, in aqueous media at ~ 90^oC. The solids formed Mo(NO₃)₆, [V₂(O)₄(OH)(NH₂)] and Se metal were characterized through elemental analysis and infrared spectroscopy. Thermo- gravimetric(TG) and differential thermal analysis (DTA) measurements of the Mo(NO₃)₆ salt and the [V₂(O)₄(OH)(NH₂)] complex, along with the infrared spectra of the final decomposition products were also recorded. The data obtained are in agreement with the detected structures and shows that the Mo(NO₃)₆  salt and the [V₂(O)₄(OH)(NH₂)] complex were decomposed to form MoO₃ and V₂O₅.

Resumen
Nitrato de molibdeno (VI), conjuntamente con el complejo [V₂(O)₄(OH)(NH₂)] y selenio metálico fueron sintetizados mediante la reacción entre urea y los reactivos MoO₃, V₂O₅ y SeO₂, respectivamente, en medio acuoso a ~ 90^oC. Los sólidos obtenidos, Mo(NO₃)₆, [V₂(O)₄(OH)(NH₂)] y Se metalico fueron caracterizados mediante análisis elemental y espectroscopía de Infrarrojo. Termogravimetría (TG) y  análisis térmico diferencial (DTA) de la sal Mo(NO₃)₆ y del  complejo [V₂(O)₄(OH)(NH₂)] fueron complementados con espectros  infrarrojo de los correspondientes productos de la descomposición final. Los datos obtenidos concuerdan  con las estructuras propuestas y confirman que la sal Mo(NO₃)₆ y el complejo [V₂(O)₄(OH)(NH₂)] se descomponen para formar MoO₃ y V₂O₅.

Introduction

     Many coordination complexes in which the donor bases are urea or thiourea have been reported. Quagliano and co-workers [1] have established that among the urea complexes, some coordinated through one of the nitrogen atoms and others through the oxygen atom of the urea molecule. The reaction of urea with transition metal ions at room temperature has been extensively studied [2-7].
     The reaction of urea with these metal ions at high temperature is rare in the literature. However, the available publications describe an interesting feature: the reaction products depend not only on the type of metal ions but also on the metal salt used in the reaction [8-12]. Free urea is known to decompose in aqueous media to CO₂ and NH₃ and the decomposition increases with temperature.  The  new idea in this investigation relates to the role of metal ions on the decomposition of urea. 
     The aim of this work was to study the nature of the reaction products of urea with MoO₃, V₂O₅ and SeO₂ in aqueous solutions at high temperature ~ 90^oC, in order to investigate the mechanism of their decomposition. The final reaction products (oxides) were well characterized through elemental analysis, infrared spectra as well as thermogravimetric (TGA) and differential thermal (DTA) analyses. The final decomposition products were identified by means of infrared spectra.

Experimental

     Reagent grade chemicals were used throughout this investigation. The blue solid salt, Mo(NO₃)₆, orange solid complex, [V₂(O)₄(OH)(NH₂)], and dark brown selenium metal were prepared by addition of 0.1M of each solid oxides (MoO₃, V₂O₅ and SeO₂) to  50 mL of an aqueous solution of 1.0M urea. The mixtures were heated with stirring on a water bath to approx. 90^oC for about 18 h. The solid products were precipitated, (in the case of MoO₃ - urea mixture, it gave a blue liquid product which precipitated by adding ethanol), filtered off, washed several times with hot water for V₂O₅ - urea; SeO₂ - urea and with ethanol for MoO₃ - urea, dried at 70^oC in an oven for 3h., and then in vacuo over silica gel. The solid reaction products were characterized by elemental analysis, infrared spectra and thermal analysis (DTA, TGA).
     The elemental analysis data of the solid products are summarized as follows: (calculated values are shown in parenthesis)
i)    MoO₃ - urea system, Mo(NO₃)₆; N, 17.63% (17.95%) ; Mo, 19.89% (20.50%).
ii)    V₂O₅ - urea system, [V₂(O)₄(OH)(NH₂)]; H, 1.48% (1.51%); N, 6.97% (7.04); V, 50.94% (51.21%).
iii)   SeO₂ - urea system, Se metal, the elemental analysis demonstrates the absence of carbon, hydrogen and nitrogen.

     Molybdenum(VI), vanadium(V) and selenium contents in the three solid reaction products were determined gravimetrically as metal oxides, and also determined by using atomic absorption. A spectrometer model PYE-UNICAM SP 1900 fitted with a suitable  lamp was used for this purpose.  The  metal percentages obtained by using  atomic absorption  gave values within expected limits.
     The infrared spectra of urea, reactants and solid reaction products were recorded with KBr discs using a Gensis II FT IR spectrophotometer. Thermogravimetry (TG),   and differential thermal analysis were carried out under a N₂-atmosphere using detectors Shimadzu TG-50 H and DTA - 50H, respectively.

Results and discussion

     The interaction between metal oxides and urea has not yet been studied. The solid products molybdenum (VI) nitrate salt, [V₂(O)₄(OH)(NH₂)] complex and selenium metal are produced during the aqueous reaction of urea with MoO₃, V₂O₅ and SeO₂, respectively. The infrared spectra of urea, reactants and the solid products mentioned above are shown in Figure 1. Band assignments of the two products Mo(NO₃)₆ and [V₂(O)₄(OH)(NH₂)], are given in Table 1.
1- Mo(NO₃)₆ Salt:  This compound was obtained through the reaction of MoO₃ with urea at high temperature, approx. 90^oC. Based on elemental analysis data together with a qualitative black-ring test for ionic nitrate using freshly prepared FeSO₄ solution and concentrated sulfuric acid, a black ring of FeSO₄.NO is formed and the infrared spectrum of the obtained compound led to its identification as molybdenum(VI) nitrate, Mo(NO₃)₆.

 

Figure 1. Infrared spectra of a: Urea; b: MoO₃; c: Mo(NO₃)₆; d: V₂O₅ and  e: [V₂(O)₄(OH)(NH₂)]

     The infrared spectrum for this product (Figure 1c) clearly indicates the absence of any bands arising from coordinated urea, instead a group of bands characteristic of ionic nitrate [13] NO₃¯, are observed. The formation of Mo(NO₃)₆ is greatly supported by comparison with the infrared spectrum of the commercially molybdenum(VI) nitrate. The two spectra are very similar indicating the obtained product is Mo(NO₃)₆.
     In order to be sure about the proposed formula and structure for the simple salt, Mo(NO₃)₆, thermogravimetric (TGA) and differential thermal analysis (DTA) were carried out for this salt under a N₂ flow. DTA and TGA thermograms are shown in Figure 2. Table 2 gives the maximum temperature values, Tmax/(°C), together with the corresponding weight loss for each step of the decomposition of the salt. The obtained data strongly support the structure proposed for the salt under investigation as follows.

Table 1.  Infrared frequencies (cm^-1) and tentative assignments for Mo(NO₃)₆ and [V₂(O)₄(OH)(NH₂)] compounds.    

Mo(NO3)6		[V2(O)4(OH)(NH2)]
Frequency	Assignments	Frequency	Assignments
3438 s,br	n(O-H);H2O of KBr	3485 ms,br	n (O-H);OH¯
1405 vs	nas (N-O); NO3	3202 s,br	nas (N-H); NH2¯
949 s	ns (N-O); NO3	3118 w,sh	ns (N-H); NH2¯
902 s	db(NO3)	1630 ms	d (H2O); H2O of KBr
855 sh		1400 vs	db(NH2¯)
750 s	dr(NO3)	1007 ms	dw(NH2¯) + n (V=O)
630 s		970 vs	dt(NH2¯)+n (V=O)
557 s	dw(NO3)	740 vs	d(V-OH-V)
515 sh		672 sh	dr(NH2¯)
		598 sh	n (V-N); NH2¯
		525 s
		467 ms	n (V-O); OH¯
		405 s

Key: s = strong; w = weak; m = medium; sh = shoulder; v = very; br = broad; n = stretching; d = bending; d_w = wagging; d_t = twisting;  d_r = rocking.

     The thermal decomposition of the molybdenum (VI) nitrate salt proceeds approximately with one main degradation step. The only one stage occurs at a maximum temperature of 245°C. The weight loss associated with this stage is 69.05% which is closer to the theoretical value of 69.32% corresponding to the loss of 6NO₂ + 3/2 O₂ as will be described by the mechanism of the decomposition. The final thermal product obtained at 600°C is MoO₃ judging from its measured infrared spectrum shown in Figure 1b.

[Mo(NO3)6] MoO3 + 6NO2 + 3/2O2

Figure 2. Thermal analysis TGA, DTG and DTA for Mo(NO₃)₆ salt.

Table 2. Maximum temperatures, Tmax/^oC, and weight loss values of the decomposition stages for the Mo(NO₃)₆ and  [V₂(O)₄(OH)(NH₂ )] compounds.

Complexes	Decomposition	Tmax/oC	Lost species	%weight losses
				Found	Calc.
Mo(NO3)6	First stage	≥245oC	6NO2 + 3/2O2	69.05%	69.32%
	Total loss			69.05%	69.32%
	Residue			30.95%	30.68%
[V2(O)4(OH)(NH2)]	First stage	≥230oC	NH3	8.48%	8.55%
	Total loss			8.48%	8.55%
	Residue			91.52%	91.45%

2- V₂(O)₄(NH)(OH)] Complex:

     The infrared spectrum of the new complex, [V₂(O)₄(OH)(NH₂)], in a KBr disc, is shown in Figure 1e, its band assignments are given in Table 1. The spectrum shows no bands due to coordinated urea, whereas characteristic signals arising from coordinated O₂^-, NH₂¯ and OH¯ groups are observed. Accordingly, the most probable geometrical structure for this complex is shown in structure (I) where both NH₂¯ and OH¯ exist as bridging ligands.

(I)

     The coordination -NH₂ group shows a set of bands that agree with those previously reported [1,13-15] for related complexes. The two bands at 3202 and 3118 cm^-1 are assigned to the N-H stretching modes corresponding to the antisymmetric and symmetric modes, respectively. The four bands at 1400, 1007, 970 and 672 cm^-1 are due to the bending, wagging, twisting and rocking vibration of the coordinated -NH₂ group, respectively. The V-N stretching frequency is of particular interest, because it provides direct information about the coordination bond. According to Nakamoto [13], the bands at 525 and 598 cm^-1 are attributed to the n(V-N) stretching mode. The n(O-H) vibration of the bridged hydroxide, OH¯ ligand was assigned  to the weak intensity band at 3485 cm^-1. Like many other compounds containing the M=O groups, that exhibit the M=O vibration bands in 1050 - 800 cm^-1 region [16, 17], the stretching frequency of the type n(V=O) in our complex is assigned to the doublet bands that observed at 1007 and 970 cm^-1. The bending OH (V-OH-V) appears at ca. 740 cm^-1 [18], while V-O (oxygen of OH¯) is observed at 467 and 405 cm^-1 [19].
     The thermal degradation of [V₂(O)₄(OH)(NH₂)] complex take place in one degradation stage, Figure 3. This stage of decomposition occurs at the maximum temperature of 230°C, accompanied by a weight loss 8.48% corresponding to the loss of NH₃ in agreement with the theoretical weight loss value of 8.55%. This mechanism of decomposition is supported by the infrared spectrum of the final thermal decomposition product, V₂O₅, Figure 1 d.

[V2(O)4(OH)(NH2)]   V2O5 + NH3

     The formation of selenium metal upon the reaction of urea with selenium dioxide at high temperature indicates that the reduction process of Se(IV) oxide occurs during the decomposition of urea into NH₃ + CO₂. Finally, the formation of these three solid reaction products, Mo(NO₃)₆, [V₂(O)₄(OH)(NH₂)] and Se metal upon the reaction of urea with MoO₃, V₂O₅ and SeO₂ respectively, in aqueous media  at high temperature is proposed to occur as follows:

i) MoO3+ 3(NH2CONH2) + 12O2 Mo(NO3)6 + 3CO2 +6H2O

ii)V2O5+NH2CONH2+H2O  [V2(O)4(OH)(NH2)]+ CO2 + NH3

iii) SeO2+ 2(NH2CONH2) + 2O2 Se +2CO2 + 2N2 + 4H2O

Figure 3. Thermal analysis TGA, DTG and DTA for the  [V₂(O)₄(OH)(NH₂)] complex.

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