Investigación

 

Tetraethylortosilicate and Titanium Tetraisopropoxide as Complexing Agents for Chromium(III) and Copper(II)

 

Heilen Arce,¹ Omar Rojas,¹ Gilberto Mondragón,² Alejandro Sáenz,^3,4 Mavis L. Montero^1,3*

 

¹ Escuela de Química, Universidad de Costa Rica, San José, Costa Rica. Fax: (506) 253-5020. E-mail: mmontero@calzada.equi.ucr.ac.cr

² Instituto Nacional de Investigaciones Nucleares. ININ. México.

³ Centro de Investigación en Ciencia e Ingeniería de Materiales. CICIMA, San José, Costa Rica.

⁴ Escuela de Física, Universidad de Costa Rica, San José, Costa Rica.

 

Recibido el 18 de marzo del 2002.
Aceptado el 28 de marzo del 2003.

 

Abstract

Tetraethylorthosilicate and titanium tetraisoproxide, two of the most widely used agents in the sol-gel technique, are employed as ligands for chromium(III) and copper(II). Thermal stability of the obtained compounds is studied through thermogravimetric analysis and the products of their pyrolisis through powder X Ray Spectroscopy, scanning electron microscopy and energy dispersive spectrometry. These products are potential precursors for metallic oxide composites.

Key words: Chromium(III), copper(II), Tetraethylorthosilicate, titanium tetraisoproxide, complexes, metallic oxide composites.

 

Resumen

Tetraetilortosilicato y tetraisopropóxido de titanio, dos de los agentes más comunes en la química Sol-Gel, son usados como ligandos de cromo(III) y cobre(II). Se estudia la estabilidad térmica de los compuestos obtenidos mediante análisis termogravimétricos y los productos de pirólisis se analizan por espectroscopía de rayos X en polvo, microscopía electrónica de barrido y espectroscopía por dispersión de energía. Los compuestos constituyen potenciales precursores de materiales compuestos ("composites") de óxidos metálicos.

Palabras clave: cromo(III), cobre(II), Tetraetilortosilicato, tetraiso-propóxido de titanio, complejos, materiales compuestos de óxidos metálicos.

 

Introduction

Research in Materials Science requires of new synthetic routes to produce materials with complex stoichiometry as those found in superconducting ceramics, or with intricate architectures as those of zeolites. These new strategies should control the solid state structures at atomic level, and represent an important advance in Materials Science. A good approach to materials synthesis is to use the so called molecular precursors, which open the possibility of achieving fine control of the stoichiometry with the extra advantage of the process occurring at low temperatures [1-3].

Particular attention has been given to materials based on metallic oxides due to their importance as catalysts, superconductors, ferroelectrics and semiconductors, among others. The technique known as sol-gel is one of the most widely used to obtain oxide materials, using metallic alkoxides as starting materials, through processes of hydrolysis/condensation. The products can be obtained in different forms including films, monoliths, fibers, etc [4-11].

The present work is a study of the chemistry of transition metals with two of the most commonly used alkoxides in the sol-gel technique, i.e.: tetraethylortosilicate (TEOS) and titanium tetraisopropoxide (TIPO). These alkoxides were used as ligands for chromium(III) and copper(II), obtaining potencial molecular precursors for mixed metal oxides. In addition, the characterization of the pyrolisis products of the synthesized compounds was done.

 

Results and discussion

Anhydrous metal halides such as CrCl₃ are compounds with a strong tendency to form adducts with electron pair donor ligands. TIPO and TEOS are alkoxides with non bonding electron pairs ready to establish coordinated bonds. The reactions must occur in a non coordinating media to preclude that solvent could compete with either TIPO or TEOS. The change in color during the formation reactions of CrCl₃·Ti(OiPr)₄ (1) or CrCl₃·Si(OEt)₄ (2) and the chemical composition of the products are evidence of the coordination of TEOS and TIPO with the chromium(III) ion, without chlorine substitution. Thermogravimetric analysis of compounds 1 and 2 show that thermal decomposition starts at a relatively low temperature: 195 °C and 215 °C respectively (Fig. 1). Compound 1 shows a more pronounce change than 2 at about 200 °C and a second thermal decomposition at 600 °C. These data could be indicative of a weak coordinated bond between TIPO and TEOS and the chromium(III) ion. This conclusion is supported by the fact that when trying to crystallize these compounds in THF, crystals of the well known adduct CrCl₃·3THF are produced. The boiling points of TEOS (168 °C) and TIPO (232 °C) are similar to the thermal decomposition from compounds (1) and (2), fact that also supports the conclusions.

Pyrolisis at 500 °C for both compounds under oxygen produces a mixture of oxides. Elemental microanalysis shows the presence of the expected elements: Cr, O, Si and Ti. Fig. 2 is a typical EDS for compound 2. X Ray diffractograms from the pyrolisis solid residue show eskolaite, Cr₂O₃, as the only crystalline phase (Fig. 3). This fact indicates that both silica and titanium dioxide are amorphous. SEM micrograph from the pyrolisis product from compound 1 shows clusters smaller than 1 µm (Fig. 4), but it is not possible to distinguish crystals from the different oxides, not even at larger magnifications, therefore an interesting composite with very small particles has been prepared.

Electron microscopy (Fig. 5) of a sample corresponding to compound 2 shows crystalline whiskers of Cr₂O₃ up to 6 µm long. These whiskers radiate out of a common origin. The rounded particles were identified with EDS as amorphous silica and have sizes in the order of 4 µm. Also in Fig. 5, one can observe regions in which both Cr₂O₃ and SiO₂ are mixed with particles so small that it is not possible to determine their size even at higher magnifications. X-ray diffractograms of these products show the same as that for compound 1, i.e.: eskolaite, Cr₂O₃, as the only crystalline phase. Therefore, no such XRD is included.

The reaction of CuCl₂(H₂O)₂ with TEOS and TIPO allows to explore the possibility of bonding these ligands with the water coordinated to the copper atom and its effect on the strength of the bond produced.

Thermogravimetric analysis of the copper compounds CuCl₂(OH)₂ · Ti₂(OiPr)₆ (3) and CuCl2(OH)2·Si2(OEt)₆ 4, shows that the thermal decomposition starts at 590 °C and 560 °C respectively (Fig. 6). There is a clear difference with the thermal decomposition temperatures for the chromium compounds, this is indicative of the greater strength of the copper-ligands bonds. This fact suggests that the presence of water in the coordination sphere of the metal is determinant in the type of product of the reaction, producing partial hydrolysis of the ligand and formation of a bond type Cu-O-Ti,(Si).

The copper compounds were also burnt in oxygen at 500 °C. EDS analysis from the products so obtained (not included) show the expected presence of Cu, O, Si, and Ti. The pyrolisis products from compound 3 show the tenorite phase for CuO and the anatase phase for TiO₂ (Fig. 7). SEM electrograph for this solid show CuO crystals between 3 and 10 µm embedded in the TiO₂ matrix (Fig. 8). The X-ray diffractogram (not included) from the pyrolisis product of compound 4 shows only the tenorite phase, thus SiO₂ is amorphous. SEM observation is very similar to the one already described, with crystals up to 10 mm in the SiO₂ matrix.

 

Experimental

General method

All manipulations were carried out under nitrogen atmosphere, using Schlenk or drybox techniques. Toluene was distilled over sodium under nitrogen. CrCl₃ was prepared according to the literature [12]. The synthesized samples were analyzed with an X Ray powders diffractometer, Siemens D5000 and observed with a scanning electron microscope (SEM), Philips XL-30, equipped with an EDAX energy dispersive spectroscopy probe (EDS). Thermogravimetric analysis were performed in a V2.5H TA Instruments.

General synthetic procedure:

Compounds were prepared by refluxing for 5 h 5 mmol of CrCl₃ or CuCl₂(H₂O)₂ with 25 mmol of TEOS or TIPO in 40 mL of toluene.

CrCl₃·Ti(OiPr)₄ (1): Dark red microcrystalline powder, unstable to humidity. I.R. (KBr, cm⁻¹):1380, 1136, 1102, 614, 548. Mag. Suscep.: 1.26 × 10⁻⁵ c.g.s. Anal. C. (%): C: 32.6, H: 6.34, Cl: 24.1, Cr: 11.8, Ti: 10.7. Anal. Exp (%): C: 31.5, H: 5.84, Cl: 23.6, Cr: 10.9, Ti: 9.84.

CrCl₃·Si(OEt)₄ (2): Fuchsia microcrystalline powder, unstable to humidity. I.R (KBr, cm⁻¹):1447, 1405, 1085, 1024, 872, 536. Mag. Suscep.: 1.94 × 10⁻⁵ c.g.s. Anal. C. (%): C: 26.2, H: 5.46, Cl: 29.1, Cr: 14.2, Si: 7.64. Anal. Exp (%): C: 25.5, H: 5.25, Cl: 28.6, Cr: 13.2, Si: 7.50.

CuCl₂(OH)₂ · Ti₂(OiPr)₆ (3): Dark green microcrystalline powder, unstable to humidity. I.R (KBr, cm⁻¹): 3270, 1365, 1120, 1017, 932, 860, 738, 667. Mag. Suscep.: 5.01 × 10⁻⁶ c.g.s. Anal. C. (%): C: 35.0, H: 7.10, Cl: 11.5, Cu: 10.3, Ti: 15.5. Anal. Exp (%): C: 32.3, H: 6.83, Cl: 10.5, Cu: 9.65, Ti: 13.9.

CuCl₂(OH)₂ · Si₂(OEt)₆ (4): Dark yellow microcrystalline powder, unstable to humidity. I.R (KBr, cm⁻¹): 3288, 1620, 1586, 1459, 1076, 722. Mag. Suscep.: 6.55 × 10⁻⁶ c.g.s. Anal. C. (%): C: 29.1, H: 6.47, Cl: 14.4, Cu: 12.8, Si: 11.3. Anal. Exp (%): C: 27.5, H: 5.92, Cl: 13.6, Cu: 11.9, Si: 10.8.

Concluding Remarks

It has been shown that regular ligands such as TEOS and TIPO are able to bond to CrCl₃ by oxygen free electron pairs. This influences the decomposition temperatures of the obtained compounds, which are relatively low and close to the boiling points of the ligands. Pyrolisis under oxygen at 500 °C for these compounds yields composites of crystalline eskolaite and amorphous SiO₂ or TiO₂.

TEOS and TIPO are also able to bond to CuCl₂(H₂O)₂ through partial hydrolysis of these alkoxides induced by the water molecules bound to the copper(II) ion.

Compounds produced with CuCl₂ · 2H₂O have decomposition temperatures three times higher than those observed for the case of CrCl₃. This is indicative of a covalent bond metal-ligand established during the hydrolysis. The products obtained when these two compounds were burnt under oxygen at 500 °C form crystalline composites of tenorite and anatase (with TIPO) and a composite of crystalline tenorite with amorphous SiO₂ (with TEOS).

Thus, it is possible with simple reactions, to coordinate the TEOS and TIPO molecules to the chromium(III) and copper (II) ions, obtaining potential precursors for composites based on metallic oxides.

 

Acknowledgements

To Consejo Nacional de Investigaciones Científicas (CONICIT), Costa Rica for financing the research. To Instituto Nacional de Investigaciones Nucleares (ININ), México for the samples characterization with XRD, SEM and EDS.

 

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