versión impresa ISSN 0103-5053
J. Braz. Chem. Soc. vol.8 no.4 São Paulo 1997
Structure of cobalt (II) perchlorate adsorbed on silica gel surface chemically modified with benzimidazole molecule
Newton L. Dias Filhoa , Edson Rodriguesb, and Yoshitaka Gushikemc*
aDepartamento de Física e Química, UNESP, Ilha Solteira, CP 31, 15378-000 Ilha Solteira - SP, Brazil
bInstituto de Física de São Carlos, USP, 13560-250 São Carlos - SP, Brazil
cInstituto de Química, Unicamp, CP 6154, 13083-970 Campinas - SP, Brazil
Received: November 4, 1996
Moléculas de benzimidazol covalentemente ligadas à superfície da silica gel, ºSiL (onde L = N-propilbenzimidazol), adsorvem Co(ClO4)2 de solvente não aquoso formando complexos na superfície segundo a reação: m ºSiL + Co(ClO4)2 ® (ºSiL)mCo(ClO4)2. As constantes de equilíbrio e capacidade de adsorção, foram determinadas aplicando-se a equação de Langmuir fornecendo respectivamente, b = 3.0 x 103 L mol-1 e Ns = 0.098 x 10-3 mol g-1. O metal está ligado através do átomo de nitrogênio e o íon perclorato não está coordenado. O estudo de ESR mostrou que o complexo possui essencialmente uma geometria com distorção tetragonal, com os elétrons dos quatro nitrogênios interagindo com o ion metálico central cobalto no plano equatorial. Somente uma espécie complexa foi detectada na superfície.
Covalently attached benzimidazole molecules on silica gel surface, ºSiL (where L = N-propylbenzimidazole), adsorbs Co(ClO4)2 from non-aqueous solvent by forming a surface complex according to the reaction: m ºSiL + Co(ClO4)2 ® (ºSiL)mCo(ClO4)2. The equilibrium constant and the adsorption capacity, determined by applying the Langmuir equation were b = 3.0 x 103 L mol-1 and Ns= 0.098 x 10-3 mol g-1, respectively. The metal is bonded through the nitrogen atom and the perchlorate ion is not coordinated. The ESR study indicated that the complex has essentially an octahedral geometry with tetragonal distortion, with the electrons of the four nitrogen atoms interacting with the cobalt central metal ion in the equatorial plane. Only one complex species was detected on the surface.
Keywords: silica gel, benzimidazole, cobalt-benzimidazole surface complex, cobalt electron spin resonance, silica gel modified with benzimidazole
Organic molecules containing nitrogen atoms have been chemically bonded to a silica gel surface in order to adsorb metal ions from aqueous or non aqueous solvents1-5. As in solution phase, many complex species may be formed at the solid-solution interface by the reaction of the metal ion with the immobilized functional organic molecule6-8. Techniques such as 13C CPMAS NMR9,10 and electron spin resonance (ESR)8,11 have been used to determine the structures of these complex species. The second technique has been particularly useful for copper complexes10,12.
Cobalt (II) is adsorbed from acetone solution by the attached benzimidazole yielding a pale blue complex13. The adsorption of the metal ion occurs according to the reaction: m ºSiL + CoX2 ® (ºSiL)mCoX2, where ºSiL is the attached neutral functional organic group on the silica surface. The stepwise complex formation, which consists of a change in the average number of the ligand, m, coordinating one metal ion is a function of the metal loading in the present case because the surface density of attached ligands is constant. The correlation between the structure of the complexes in the solution phase and that on the solid surface is not straightforward because from a steric point of view the attached ligands have limited mobility while in the solution phase this restriction does not exist.
In this work the structure of cobalt perchlorate adsorbed on the surface of silica modified with benzimidazole is reported. The perchlorate counterion was chosen because it has low coordination ability and thus, the resulting structure of the reaction between Co(ClO4)2 and ºSiL may be exclusively determined by the interaction of the immobilized ligand and the metal ion. On the other hand, this study is of great interest because the silica gel surface is morphologically very heterogeneous, the ligands are not uniformly distributed on the surface and to our knowledge, little information on complex formation in porous materials is available in the literature.
Silica gel 60 (Merck) having a particle size between 0.2 and 0.05 mm was activated at 420 K under vacuum (0.13 Pa). This silica (ca. 50 g) was suspended in dry xylene (200 cm3) and 3-chloropropyltrimethoxysilane (15 cm3) was added; the mixture was stirred for 24 h at 380 K under a nitrogen atmosphere. The resulting modified silica was immersed in pure dimethylformamide (150 cm3) and benzimidazole (17.5 g, 0.15 mol) was added. The mixture was stirred for 24 h at 380 K under a nitrogen atmosphere. The resulting modified silica was filtered off and washed with dimethylformamide and ethanol. The product was heated for 8 h at 353 K under vacuum (0.13 Pa). The amount of benzimidazole attached to the silica surface was No= 0.48 x 10-3 mol g-1.
The reactions which describe the preparation can be represented by the equations in Scheme 1, where for simplification (A) will hereafter be designated as SiL.
Isotherm of Adsorption
The isotherm adsorption of Co(ClO4)2 from an acetone solution by ºSiL was determined at 298 K. About 0.1 g of ºSiL was immersed in 50 mL solutions of Co(ClO4)2 of variable concentrations. The suspension was shaken for 60 min and then allowed to stand for 20 min, time necessary for all the solid material to deposit at the bottom of the flask. About 10 mL of the supernatant solution was evaporated, the residue dissolved in water and the metal was analyzed by conventional EDTA complexometric titration. The quantity of the adsorbed metal was determined by applying the equation:
where Na is the initial metal mole number, Ne the metal mole number in equilibrium with the solid phase and m the mass of the adsorbent.
Infrared spectra of the samples were obtained in the region 2000 to 1000 cm-1, using the technique of self supported disk described elsewhere3. The equipment used was a Nicolet FT-IR spectrophotometer.
The electronic spectra of the samples were obtained by immersing the solid with Co(ClO4)2 adsorbed on the surface, (ºSiL)mCo(ClO4)2, in spectral grade carbon tetrachloride in a quartz cell having 1mm path length. A Cary 2300 spectrophotometer was used. Good quality spectra were obtained since the refractive index of both, silica and the liquid, are nearly the same14.
Spectra of the samples were obtained in a quartz tube at 77 K, at band X (9 Ghz), 40 mW microwave power, modulation frequency of 100 kHz and a magnetic field with sweeping of 1000 Gauss centered at 2850 or 2900 Gauss. The equipment used was a Bruker ECS 106 electron spin resonance spectrometer.
Results and Discussion
Isotherm of adsorption
Figure 1 shows the isotherm of adsorption of Co(ClO4)2 by the solid phase ºSiL, from acetone solution. The adsorption capacity, Ns, and the equilibrium constant, b, were determined by applying the Langmuir equation:
where C is the metal concentration in the solution phase in equilibrium with the solid phase. Plotting C/Nf vs. C gave a straight line with correlation coefficient r = 0.997 was obtained. The calculated constants were: b = 3.0 x 103 L mol-1 and Ns = 0.098 x 10-3 mol g-1. The magnitude of the constant b is the same as that determined for reaction of the pendant benzimidazole ligand and cobalt (II) in non aqueous solvents3. The chemical species formed on the surface is thermodynamically stable and adsorption occurs by nitrogen-metal bond formation. The reaction at the surface can be represented by the equation:
The equation shows that when the metal ion diffuses into the solid-solution interface it is followed by the ClO4- counterion and then, the adsorption occurs as an adsorption of a neutral species. The equation also shows that as the concentration of the metal ion increases at the solid-solution interface, the average number of the attached ligands coordinated to the metal ion decreases because the surface density of the ligands is constant. Therefore, it is very important to study the strucuture of the species taking into account the quantity of the metal adsorbed on the surface8.
The infrared spectra of (ºSiL)mCo(ClO4)2 as a function of metal loading is presented in Fig. 2. The bands observed between 1570 and 1450 cm-1 are due to the vibrational modes involving the benzimidazole ring. The band at 1500 cm-1 observed in Fig. 2a (sample without adsorbed metal, Nf/No = 0) is slightly shifted to higher frequency, 1504 cm-1 (Fig. 2b, Nf/No = 0.065), and for the saturation condition is shifted to 1518 cm-1 (Fig. 2c, Nf/No = 0.21). This particular band is assigned to the skeleton vibrational mode of the ring which involves coupled vibrations, nCN + nCC15,16. This band normally is shifted toward higher frequency upon metal-nitrogen bond formation3,8. Other bands observed in the spectra are not sensitive to nitrogen-metal bond formation.
If the adsorption occurs exclusively at the benzimidazole basic site i.e., due to the formation of a nitrogen-metal bond, the saturation value should give an idea of the average number of ligands coordinated to the central metal ion. However, it must be remembered that the surface is heterogeneous and the ligands are not distributed uniformly on this surface and thus, many of them may remain uncoordinated even at surface saturation conditions.
The electronic spectra for the complex with Nf/No 0.065, 0.16 and 0.21 were recorded. However, no spectral changes (Fig. 3) were observed as the degree of loading was varied, which may suggest that in every case only one chemical species was formed on the surface. A different behavior was observed for Cu(II) complexes adsorbed on benzimidazole-modified silica, where the spectra changed for different degrees of metal loadings8. In the last case, more than one species was formed on the surface.
Figure 4 shows the esr spectra obtained for (ºSiL)mCo(ClO4)2, under the experimental conditions: Nf/No = 0.065 and 0.21. For Nf/No = 0.21 (Figure 4a) one can distinguish very well nine superhyperfine line structures, characteristic of the interaction of benzimidazole nitrogen atoms with the cobalt unpaired electron, corresponding to the transition Ms = -1/2 to 1/2, and Co nuclear spin I = 7/2. The other expected transitions are barely seen due to the broadening effects but there are residual broadened superhyperfine lines that may be identified by comparing the second harmonic experimental spectrum with the simulated one. The less concentrated species (Fig. 4b) shows broadened superhyperfine lines in all hyperfine transitions including those of M = -1/2 to 1/2. The spectra (Fig. 4b) as a whole resembles that corresponding to the more concentrated perchlorate species (Fig. 4a). This behavior is indicative that the overall symmetry of the cobalt-benzimidazole complex is not changed when cobalt is diluted i.e., the interaction with four benzimidazole molecules persists. The persistent intense line (starred peaks) with g = 2.00 visible in all spectra for ºSiL or (SiL)mCo(ClO4)2, is due to the Fe3+ impurity.
The spectra shown in Fig. 4 may be analyzed considering a square planar structure with the metal ion at the origin and four ligands at the equatorial positions. A powder simulation spectrum was generated using the hamiltonian parameters in Table 1 and the line shape consisting of a ratio of Gaussian to Lorentzian equal to 50 percent and line widths of 35 gauss in the parallel and 30 gauss in the perpendicular positions, respectively. The calculations were carried out using the accepted values for P = 254 cm-1 17 and the spin orbit coupling constant l = 178 cm-1 18. The calculated perpendicular and parallel directions of the superhyperfine coupling constant of the ligand, KN, by simulation up to second order in the perturbation treatment for the superhyperfine spectra19, gave: KN^ = 16 and KN|| = 8 gauss.The resulting simulated spectrum, which agrees fairly well with the experimental spectrum, is shown in Fig. 5.
Only one adsorbed complex species is detected for different surface loadings with the metal ion. The ESR study indicated that we have essentially an octahedral complex with tetragonal distortion in a low spin configuration. Under the D4h crystalline electric field, Co(II) interacts with four nitrogen nuclei of benzimidazole molecules in the equatorial plane. According to the foregoing analysis we have essentially a 2A1 ground state with the electron in a dz2 orbital. About 58 percent of the normalized angular variation of this orbital is in dx2-y2 and this justifies the observation of superhyperfine lines with the four nitrogen nuclei in the equatorial plane of symmetry of the complex.
Y.G. and E.R. are indebted to FINEP and N.L.D.F. to FAPESP for financial support.
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