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Synthesis and Spectroscopic Studies of Metal Complexes Formed in the Reaction of Metal Ions with Urea at High Temperature
Synthesis and Spectroscopic Studies of Metal Complexes Formed in the Reaction of Metal Ions with Urea at High Temperature
Journal of the Korean Chemical Society. 2007. Aug, 51(4): 339-345
Copyright © 2007, The Korean Chemical Society
  • Received : March 28, 2007
  • Published : August 20, 2007
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About the Authors
Akmal S. Gaballa
Said M. Teleb
Department of Chemistry, Faculty of Science, Zagazig University, Zagazig, Egypt
El-Metwally Nour
Department of Chemistry, College of Science, Qatar University, Doha, Qatar

Abstract
Urea reacts with PtCl 2 , H 2 [PtCl 6 ]·6H 2 O, H 2 [IrCl 6 ] and Ni(CH 3 CO 2 ) 2 in aqueous solution at high temperature (60-80 ℃) yielding [PtCl 2 (Urea)]·2H 2 O ( 1 ), (NH 4 ) 2 [PtCl 6 ] ( 2 ), (NH 4 ) 2 [IrCl 6 ]·H 2 O ( 3 ) and [Ni 2 (OH) 2 (NCO) 2 (H 2 O) 2 ] ( 4 ) complexes, respectively. In complex 1 , urea coordinates to Pt(II) as a neutral bidentate ligand via amido nitrogen atoms. In complexes 2 , 3 and 4 it seems that the coordinated urea molecules decompose during the reaction at high temperature and a variety of reaction products are obtained. All complexes were isolated in moderate yields as dark green ( 1 ), yellow ( 2 ), pale brown ( 3 ) and faint green ( 4 ) precipitates, respectively. The reaction products were characterized by their microanalysis, IR, 1 H and 13 C NMR spectra as well as thermal analysis. General mechanisms describing the formation of these complexes were suggested.
Keywords
INTRODUCTION
From a coordination chemistry prospective, urea could potentially be a neutral ligand for transition metals and coordinates as a monodentate either via its oxygen 1 or nitrogen atoms 2 or as a bidentate chelating or bridging ligand depending on the type of metal ion. 2 - 5 Biologically, many hydrolytic enzymes employ two metal ions to facilitate the concentrated binding of substrate and addition or elimination of water. 6 - 8 Urease is a nickel-dependent mtalloenzyme that catalyzes the hydrolysis of urea to ammonia and carbon dioxide. 4 Urease employs a nonequivalent pair of nickel ions to facilitate the concerted binding of the substrate at one center and nucleophilic attack of water or hydroxide at the other center. 9 - 11
The reactions between various transition metal ions with urea at room temperature have been studied extensively and many metal–urea complexes have been isolated and characterized. 12 - 16 Recently, we have found that reaction of urea with various metal ions at high temperature proceeds with different mode in which the coordinated urea molecules decompose and a variety of reaction products have been obtained. 17 - 23 The rule played by metal ions in decomposing coordinated urea seems to be depending upon the type of metal ion and the nature of the counter ions involved in the reaction as well as on the reaction temperature.
The present investigation was reported to study the course of the reaction of urea with PtCl 2 , H 2 [PtCl 6 ]·6H 2 O, H 2 [IrCl 6 ] and Ni(CH 3 CO 2 ) 2 ·4H 2 O at ca . 60-80 ℃. The reaction products were characterized by their elemental analyses, infrared, 1 H and 13 C NMR spectra as well as thermal analysis. The obtained complexes are identified as [PtCl 2 (Urea)]·2H 2 O ( 1 ), (NH 4 ) 2 [PtCl 6 ] ( 2 ), (NH 4 ) 2 [IrCl 6 ]·H 2 O ( 3 ) and [Ni 2 (OH) 2 (NCO) 2 (H 2 O) 2 ] ( 4 ), respectively.
EXPERIMENTAL
All of the chemical used throughout this investigation were extra pure grade and used without further purification.
Infrared spectra of the reactants and the obtained complexes were recorded from KBr discs (4000-400) using a Buck scientific 500-IR spectrophotometer. Thermal gravimetric analysis (TG) and (DTG) were carried out using a Perkin-Elmer TGA-7 computerized thermal analysis system. The rate of heating of samples was kept at 10 ℃ min -1 . Sample mass of 0.397 mg was analysed under N 2 flow of 30 ml min -1 . Microanalyses were performed by the Microanalysis Unit of Cairo University, Egypt using CHNS-932 (LECO) and vario-EL (elmentar Analysensysteme) elemental analyzers.
1 H and 13 C NMR spectra were recorded on Varian spectrometers Gemini 200, VXR 400 and Unity 500 operating at 200, 400 and 500 MHz for 1 H, respectively. Solvent signals ( 1 H, 13 C) were used as internal references.
- [PtCl2(Urea)]·2H2O (1)
To a suspension of platinum(II) chloride (0.531 g, 0.01 mol) in 10 ml of distilled water, a solution (10 ml) of urea (0.720 g, 0.06 mol) was added. The mixture was stirred at room temperature for about 24 h. The obtained clear solution was then stirred for about 8 h at ca . 60 ℃ (in a water bath). The resulting precipitate of [PtCl 2 (Urea)]·2H 2 O ( 1 ) (dark green) was then filtered out, washed several times with hot distilled water and dried at 50 ℃ in an oven for 3 h and then in a desiccator in vacuo . Yield: 0.465 g (64.3%).
Anal. found: C, 3.29; H, 2.35; Cl, 18.98; N, 7.84.
Anal. calc. for CH 8 Cl 2 N 2 O 3 Pt (362.07): C, 3.32; H, 2.23; Cl, 19.58; N, 7.74.
- (NH4)2[PtCl6] (2)
To a solution, (5 ml) of H 2 [PtCl 6 ]·6H 2 O (0.518 g, 0.01 mol) in water, a solution (10 ml) of urea (0.360 g, 0.06 mol) was added. The resulting clear yellow solution was stirred for about 8 h at ca . 60 ℃ (in a water bath). The resulting yellow precipitate of compound 2 was filtered off, washed several times with hot distilled water and MeOH and then dried in a desiccator in vacuo . Yield: 0.190 g (42.8%).
Anal. found: Cl, 45.94; H, 1.96; N, 6.05.
Anal. calc. for Cl 6 H 8 N 2 Pt (443.87): Cl, 47.92; H, 1.82; N, 6.31.
- (NH4)2[IrCl6]·H2O (3)
To a water solution (10 ml) of H 2 [IrCl 6 ] (0.814 g, 0.01 mol), a solution (10 ml) of urea (0.720 g, 0.06 mol) was added. The resulting clear solution was stirred for about 8 h at ca . 65 ℃ (in a water bath), there was changing in colour of the reaction mixture from dark brown to red. The precipitate of compound ( 3 ) (pale brown) was then obtained by the addition of acetone dropwisly with stirring that was filtered off, washed several times with acetone and then dried in a desiccator in vacuo . Yield: 0.103 g (11.2%).
Anal. found: Cl, 45.58; H, 2.50; N, 6.55.
Anal. calc. for Cl 6 H 10 IrN 2 O (459.03): Cl, 46.34; H, 2.20; N, 6.10.
- [Ni2(OH)2(NCO)2(H2O)2] (4)
To a solution (20 ml) of nickel(II) acetate tetrahydrate, Ni(CH 3 CO 2 ) 2 ·4H 2 O (0.498 g, 0.01 mol) in water, a solution (20 ml) of urea (0.720 g, 0.06 mol) was added. The clear solution was stirred for about 4-6 h at ca . 80 ℃ (in a water bath) resulting in precipitation of a faint green compound that was filtered off, washed several times with hot distilled water, dried at 50 ℃ in an oven for 4 h and then in a desiccator in vacuo . Yield: 0.200 g (36.8%).
Anal. found: C, 9.01; H, 2.41; N, 9.84.
Anal. calc. for C 2 H 6 N 2 Ni 2 O 6 (271.47): C, 8.85; H, 2.23; N, 10.32.
RESULTS AND DISCUSSION
Urea reacts with platinum(II) chloride in aqueous solution at 60 ℃ to form a dark green complex identified as [PtCl 2 (Urea)]·2H 2 O ( 1 ). In this complex, urea coordinates to Pt(II) as a bidentate chelating ligand via its nitrogen atoms. The infrared spectrum of the complex shows the characteristic bands of N-coordinated urea. Assignments of the infrared bands in the complex spectra in comparison with those of free urea were given in 1 . The formation of N→Pt coordination bond causes significant changes in many of characteristic urea vibrations. The N-H stretching vibrations are shifted to lower values 24 . On the other hand, the observed stretching vibration of the carbonyl group, n(C=O) in the complex spectrum is shifted to a higher value indicating that oxygen atom is not the donor site, see 1 . Furthermore the stretching vibration, ν(N-C) was observed at a lower frequency (1390 cm -1 ) in comparison with its corresponding value in free urea (1463 cm -1 ). This is in consistent with the previously known 3 for N-coordinated urea. The deformation (bending and in-pane rocking) motions of NH 2 group are observed at 1615 and 1184 cm -1 , respectively. The appearance of only one band in the region characteristic for the bending vibration of NH 2 at 1615 cm -1 indicate that both NH 2 groups are coordinated to Pt(II) ion and urea acts as chelating ligand. 25 The geometry associated with complex is likely square planar with two chloro and two nitrogens constituting the coordination sphere of platinum(II) ion. This conclusion was also supported by the data obtained from 1 H and 13 C NMR measurements. 4 . Only one signal is observed in 1 H NMR spectrum of [PtCl 2 (Urea)]·2H 2 O complex and strongly down field shifted (7.36 ppm) in comparison with its value in free urea (5.64 ppm). Further more the 13 C NMR spectra of the complex and free urea show only one signal at approximately the same value. These two observations are consistent with a chelating bidentate nature for urea through the two nitrogen atoms.
Characteristic infrared frequencies (cm-1) and tentative assignments for urea and complex1, [PtCl2(Urea)]·2H2O
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*s, strong; m, medium; v, very; w, weak; sh, shoulder. **υ, stretching; as, asymmetric; s, symmetric; δb, bending; δr, rocking; δw, wagging.
Urea seems to interact with hexachloroplatinic or hexachloroiridic acid in a different mode giving complexes 2 and 3 , respectively. The formation of such complexes upon heating of an aqueous mixture of urea with H 2 [PtCl 6 ] and H 2 [IrCl 6 ], (see 1 ), were clearly identified from their infrared and 1 H NMR spectra . 1 , 2 , 4 . The asymmetric and symmetric stretching motions for N-H were observed at lower frequencies in both complexes, 2 , in comparison with their values in free urea and in consistent with those known for ammonium ion. 3 , 6 - 29 Such way of interaction was further supported by observing the stretching motion for M-Cl bond in [PtCl 6 ] 2− and [IrCl 6 ] 2− moieties at 337 and 306 cm −1 , respectively [30]. 1 H NMR spectra of the complexes (NH 4 ) 2 [PtCl 6 ] and (NH 4 ) 2 [IrCl 6 ]·H 2 O strongly support this conclusion, there is no signals correspond to urea observed but instead a new broad singlet signal around 7.3 ppm assigned to N H 4 + is observed.
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IR spectra of (A) urea, (B) Complex 1, (C) Complex 2 and (D) Complex 3.
Characteristic infrared frequencies (cm-1) and tentative assignments for complexs (NH4)2[PtCl6] (2) and (NH4)2[IrCl6]·H2O (3)
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*s, strong; m, medium; v, very; w, weak; sh, shoulder. **υ, stretching; as, asymmetric; s, symmetric; δb, bending; δt, twisting.
The catalytic decomposition of urea in presence of hexchloroplatinic or hexachloroiridic acids could be explained as follows:
NH 2 CONH 2 + H 2 O → NH 2 CO 2 + NH 4 +
NH 2 CO 2 + 2H + → NH 4 + + CO 2
An aqueous solution of nickel(II) acetate was found to interact with urea at ca . 80 ℃ and a faint green powder precipitate of complex (4) is obtained. The complex was isolated in a good yield and identified by microanalysis, IR, 13 C NMR spectra and thermal analysis. The IR spectrum of the complex 4 is shown in . 2 A and its band assignments are given in 3 . The infrared spectrum shows no bands either due to coordinated urea or acetate ions but instead a group of bands characteristic for coordinated isocyanato, coordinated water molecules and bridged hydroxo groups. The complex spectrum show the presence of bands characteristic for coordinated terminal isocyanato group at 2240 and 2080 cm -1 at higher wavenumbers as expected in comparison with the corresponding bridged isocyanato ions. 3 , 20 , 35 The stretching vibration of the hydroxo group was observed as a strong band at 3633 cm -1 and for coordinated water in the 3444-3381 cm -1 region. 36 - 38 Tentative assignments of bands associated with bending motions of NCO , OH and H 2 O ligands were listed in 3 . Another group of bands lying at lower wavenumbers 478-393 cm −1 associated with the vibrational motions of Ni-O and Ni-N bonds were also observed [39]. 1 H and 13 C NMR spectra of the complex [Ni 2 (OH) 2 (NCO) 2 (H 2 O) 2 ] shows no signals due to free or coordinated urea, but instead a new signal at 124.0 in the 13 C NMR spectrum is observed corresponds to NCO ligand. Based on this data together with the elemental and thermal analysis we identified the obtained product as binuclear nickel(II) complex, [Ni 2 (OH) 2 (NCO) 2 (H 2 O) 2 ] and the most probable structure associated with this formula is shown in Formula 1 .
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IR spectra of : (A) complex 4 (B) The residue left after the first stage (200 ℃), [Ni2(OH)2(NCO)2], (C) The final thermal decomposition product, NiO.
Characteristic infrared frequencies (cm−1) and tentative assignments for complex4, [Ni2(OH)2(NCO)2(H2O)2]
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*s, strong; m, medium; v, very; w, weak; sh, shoulder. **υ, stretching; as, asymmetric; s, symmetric; δb, bending; δt, twisting, δr, rocking.
Selected1H and13C NMR chemical shifts (δ in ppm) of urea and its compounds in dmso-d6
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Selected 1H and 13C NMR chemical shifts (δ in ppm) of urea and its compounds in dmso-d6
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It is well known that urea coordinates to Ni(II) ions at room temperature forming the [Ni(urea) 4 ] 2+ complex ion. 2 , 30 - 32 At high temperature the following reaction may take place:
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The catalytic decomposition of urea and the formation of NCO group in presence of nickel acetate salt could be explained as follows: 33 , 34
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Thermogravimetric (TG) and derivative of thermogravimetric analysis (DTG) were carried out for complex 4 under N 2 flow, . 3 . The obtained ther-mal data strongly support the proposed complex formula and indicate that the mode of thermal decomposition occurs in two stages. The first stage lies at a maximum temperature of 175.6 ℃ and is accompanied by a weight loss of 13.4% corresponding to the loss of two water molecules in consistent with the calculated value of 13.26%. The infra red spectrum obtained for the residue at this stage (at 200 ℃) clearly show the disappearance of bands due to water molecules and the presence of the characteristic bands due to OH- and NCOgroups, see . 2 B. The second stage was observed at 414.0 ℃ and is accompanied by a weight loss of 31% associated with the loss of two HNCO units in good agreement with the calculated values of 31.17% according to the following reactions:
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Thermogravimeteric (TG) and derivative (DTG) of complex 4.
The infrared spectrum for the final residue left after this temperature strongly supports this conclusion see . 2 C. The spectrum shows only the characteristic bands associated with NiO.
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