Oxidative Dimerization of Ru ( III )-EDTA Complex on the Surface of Functionalized Silica Gel

Este trabalho descreve um estudo da reação de ancoramento de H[Ru(III)Cl 2 (H 2 EDTA)].4.5 H 2 O na superfície da sílica gel funcionalizada (SF) com o composto [3-(2aminoetil)aminopropil]trimetoxissilano (AEATS), gerando por meio de ligação amida o composto dimérico (EDTA) 2 Ru 2 (IV,IV) na superfície da sílica modificada. A oxidação da água por esse complexo imobilizado na superfície da sílica foi observada pela formação de O 2 e decréscimo do pH, originando a sílica modificada SF-AEATS/(EDTA) 2 Ru 2 (III 1/2 ,III 1/2 ). Este dímero ancorado foi caracterizado por espectroscopia eletrônica e vibracional e também por espectroscopia de EPR. O comportamento eletroquímico de um eletrodo de pasta de carbono modificado com a sílica SFAEATS/(EDTA) 2 Ru 2 (III 1/2 ,III 1/2 ) também foi estudado.


Introduction
The synthesis and characterization of electroactive mono and polymetallic complexes of ruthenium (III)/(II) attached to solid supports have been the subject of extensive studies.5][16][17] The synthetic route to support Ru(III)-EDTA complex on the surface of silica gel functionalized with [3-(2-aminoethyl)aminopropyl]trimethoxysilane (AEATS) is similar to the one described by Anson et al. 18 for the ethylenediaminetetraacetate complexes of ruthenium (II) and (III).The ruthenium complex was attached to graphite electrodes by amide bonds formed by the condensation of an uncoordinated acetate group in Ru(III)-EDTA with amine groups introduced into the graphite surface by a plasma etching procedure. 2n this paper, amine groups are introduced onto the silica surface by silanization reaction with [3-(2aminoethyl)aminopropyl]trimethoxysilane, yielding SF-AEATS.The reaction of this support with 2 This reaction leads to the dimeric complex (EDTA) 2 Ru 2 (IV,IV) attachments on the surface of the SF-AEATS silica.

Experimental
All the chemicals used were Aldrich reagent grade unless otherwise specified, and the solvents were of AR grade and purified when necessary. 19Doubly distilled water was used throughout the work; RuCl 3 .xH 2 O was used as the starting material for the synthesis of the ruthenium complex.
1][22] The product, obtained in the form of yellow crystals, was readily soluble in water, ethanol and N,N'-dimethylformamide. (Found: C, 22.04; H, 4.10; N, 4.99, Anal Calc.for: C, 22.06; H, 4.44; N, 5.14%).Elemental analyses were performed in the Microanalytical Laboratory of the University of São Paulo.The IR spectrum of the product was in agreement with the one reported by Mukaida. 21

Immobilization of Ru(III)-EDTA
2][3] The SF-AEATS silica (0.50g) was added to a solution of dicyclohexylcarbodiimide (0.35 g), in anhydrous DMF (50 mL).Next, a solution of Ru(III)-EDTA complex (0.23 g in 10 mL of DMF) was slowly added and the resulting suspension was refluxed and stirred for 4 h under nitrogen atmosphere.The suspension was cooled to room temperature and the solid was filtered and washed with DMF and anhydrous ethanol and ether.The resulting brown solid was dried under vacuum for 6 h at 393 K.This brown solid was added to water and pH measurements were taken.A rapid evolution of O 2 (Clark electrode was used to confirm the evolution of O 2 ) and a pH decrease were observed.The resulting dark brown solid was separated by filtration, washed with ethanol and ether, dried under vacuum for 4 h at 393 K and stored under vacuum.

Chemical analysis (nitrogen sites)
The amount of nitrogen in the sample of SF-AEATS was quantified taking advantage of its basic character. 24In a typical example, 0.200 mg of the SF-AEATS silica were added to 25.0 mL of standard HCl (0.104 mol L -1 ) solution in a thermostatized cell (25.0 °C), reacting for one hour.The conductivity of the system was measured before and after the reaction.The excess of acid was determined by means of conductivity variation and used to calculate the amount of nitrogen sites.In this case (17.2 ± 0.3)x10 -4 mol g -1 was found.The functionalization efficiency was determined for (silane en) groups as being equal to (8.6 ± 0.3)x10 -4 mol g -1 in the SF-AEATS based on the nitrogen content.
For the ruthenium elemental analysis, the dark brown silica was previously treated with a mixture of HF/HClO 4 / HNO 3 as described in the literature, 25 and ruthenium was analyzed by means of atomic absorption.
The specific area of the silica surface was determined by BET method. 25he surface (silane en) groups density ( ) and their average interatomic distance (l) were calculated, 26 assuming that the (silane en) groups were uniformly distributed on the surface by applying equations 1 and 2.
(1) l = (S BET / N 0 N) 1/2 (2) where N 0 is the amount of (silane en) attached to the surface, N is the Avogadro's number and S BET is the specific area.

Electronic spectra
The electronic spectra for the immobilized species were obtained with the solid suspended in CCl 4 , using a 0.10 cm path length quartz cell and a DU-70 Beckman spectrophotometer.

Cyclic voltammetry
All electrochemical studies were performed using a three-electrode potentiostatic system (Priceton Applied Research assembly including a potentiostat -galvanostat (model 263A) coupled to an IBM -PC computer).The modified carbon paste electrode used as the working electrode was prepared by carefully mixing 20 mg of either SF-AEATS or SF-AEATS/(EDTA) 2 Ru 2 (III 1/2 ,III 1/2 ) with 30 mg of graphite powder and nujol oil, pressing the mixture onto a platinum grid (total area, 0.2 cm 2 ).The studies in the electrochemical cell were performed under argon atmosphere.
The electrochemical behavior of such electrodes was analyzed by cyclic voltammetry and the ionic strength (0.80 mol L -1 ) was controlled with sodium trifluoracetate (NaCF 3 COO), at 25.0 ± 0.5 °C.The potentials were reported to Ag/AgCl (satured KCl) electrode, double junction Orion model 90-02 (E° = 0.197 V vs. SHE, at 25°C).A platinum wire was used as an auxiliary electrode and the modified carbon paste electrode (CPE) was used as a working electrode.

Infrared spectra
The infrared spectrum of the self-supported disk of the material was obtained without any dilution of the immobilized complexes with KBr.The equipment used was an FT-IR Bome Hertmann & Braum, model MB100 spectrophotometer with sample disk containing approximately 10 mg cm -2 .

Electron paramagnetic resonance spectra
Low temperature electron paramagnetic resonance (EPR) spectra were measured using a Bruker ESP-300E spectrometer operating at X-band frequencies at 77 K.The EPR spectra were obtained at the Instituto de Química de São Carlos, USP -São Carlos.

Thermogravimetric Analysis (TGA)
TGA curves were obtained in a Shimadzu (TGA-50) thermogravimetric analyser, at a heating rate of 20 °C min -1 , in the range of 25 to 1000 °C and under N 2 atmosphere flowing at 10 mL min -1 .

Results and Discussion
The immobilization [1][2][3] of H[Ru(III)Cl 2 (H 2 EDTA)] complex on functionalized silica gel SF-AEATS was achieved by the condensation of the uncoordinated acetate groups of Ru(III)-EDTA with amine groups (silane en) of SF-AEATS.Approximately 2.0x10 -4 (mol g -1 ) mol of Ru were immobilized per g of silica, implying that 23% of the nitrogen sites were bonded to the metal centers.A decrease in the surface area of the silica with the functionalization of (silane en) groups was observed.The specific area of the silica decreased from 506 ± 6 m 2 g -1 to 337 ± 6 m 2 g -1 , probably due to the blocking of the pores with the coupling of (silane en) groups on its surface. 12The value of (1.5 ± 0.1)x10 18 M -2 was obtained for the surface bonded (silane en) groups density.Based on equation 2, the estimated value for the average distance between two AEATS groups was 8.1 ± 0.3 Å.This distance could be considered an estimate of the minimum average distance necessary for two metal centers immobilized by amide bonds undergoing oxidative dimerization reactions on the surface of the SF-AEATS or vice-versa.
The specific area of the solid after anchoring the metal complex decreases to 320 ± 6 m 2 g -1 .
Besides the immobilization of the ruthenium complex, metal centers in high oxidation states were observed, probably due to the formation of (EDTA) 2 Ru 2 (IV,IV) dimer.
The infrared spectrum of the SF-AEATS/ (EDTA) 2 Ru 2 (III 1/2 ,III 1/2 ) was an important evidence for amide bond formation in the immobilization of the complex by covalent Ru-N (amide) bonds.Table 1 shows the main infrared frequencies observed in the spectrum of the anchored complex.The spectrum shows the three characteristic bands of amide bonds, i.e., 1658 cm -1 ( (C=O)), 1573 cm -1 and a shoulder at 1464 cm -1 ( (C-N) + (N-H) in the plane). 28he electron paramagnetic resonance spectrum of the SF-AEATS/(EDTA) 2 Ru 2 (III 1/2 ,III 1/2 ) sample was recorded at 77 K, and is shown in Figure 1.
Three g-values were extracted from the EPR spectrum (2.01, 2.25 and 2.39).The spectrum was consistent with the presence of a single unpaired electron confined in an unsymmetrical environment.The signals are in the typical region for Ru(III) compounds and reflect the low symmetry of the dimeric complex.The rhombic anisotropy in Figure 1 indicates that the unpaired electron is substantially delocalized in the bridging ligand. 29he thermoanalytical data obtained by TGA analysis of silicas SPA, SF-AEATS and SF-AEATS/(EDTA) 2 Ru 2 (III 1/2 , III 1/2 ) indicate a similar thermal behavior in an N 2 atmosphere.The mass loss occurs in two steps: up to 400.0 ± 0.5 K, which is attributed to the loss of H 2 O and between 453.0 ± 0.5 and 813.0 ± 0.5 K, attributed to the loss of the organofuntional groups.
The electronic spectrum of the SF-AEATS silica modified with the reduced ruthenium dimeric complex, Ru 2 (III 1/2 ,III 1/2 ), (Figure 2) showed the presence of a band at max = 940 ± 10 nm, with bandwidth value at half intensity of 1/2 = 6152 cm -1 .This band was tentatively attributed as being due to the Ru(III)-Ru(IV) intervalence transition (IT) of the Ru(III)-Ru(IV) dimer anchored on the SF-AEATS silica surface by comparison with the similar compound described in the literature (IT, max at 632 nm). 18Two more absorption bands were observed in the electronic spectrum for the supported complex: max at 370 nm and at 470 nm (insert in Figure 2).
The electrochemical behavior of this electroative anchored species is similar to the one described for the Ru(III 1/2 )-Ru(III 1/2 ) dimer complex characterized by Baar and Anson: 18 From the electrochemical data and assuming the formation of the symmetric binuclear complex, the comproportionation constant (K c ) for reaction: was estimated as K c = 1x10 5 , suggesting a considerable coupling between the metallic centers.
The anchored dimer exhibits pH-dependent E 1/2 values at low pH for the two electrochemical processes.From Pourbaix plots (pH vs. E 1/2 , Figure 4) the pK a values of 3.3 ± 0.2 for the (EDTA) 2 Ru 2 (III 1/2 ,III 1/2 ) anchored dimer and 3.5 ± 0.2 for the (EDTA) 2 Ru 2 (III,III) species were calculated.These pK as values were assigned to the single uncoordinated carboxylate group of the EDTA ligant also present in the dimer complex.
According to the voltammetry data the anchored species in the silica surface do not lixiviate significantly (less than 10 -7 mol L -1 ) even at pH 7.0 for a period of 4 hours.

Conclusions
The results are consistent with the ruthenium dimeric complex (EDTA) 2 Ru 2 (III 1/2 ,III 1/2 ) immobilization with two metal centers immobilized by Ru-N (amide) bonds.This dimeric complex posseses a diamide-bridged and, at least, an uncoordinated carboxylate group in the metal center.
Studies of reactivity and catalytic activity of these supported species are also in development and results will be soon reported.