Langmuir 2006, 22, 203-208
203
Immobilization of the [RuII(edta)NO+] Ion on the Surface of Functionalized Silica Gel Patrı´cia G. Zanichelli,† Rosana L. Sernaglia,‡ and Douglas W. Franco*,† Instituto de Quı´mica de Sa˜ o Carlos, UniVersidade de Sa˜ o Paulo, AV. Trabalhador Sa˜ ocarlense 400, CEP: 13566, Sa˜ o Carlos, SP, and Departamento de Quı´mica, UniVersidade Estadual de Maringa´ , AV. Colombo 5790, CEP: 87020-900, Maringa´ , PR, Brazil ReceiVed July 8, 2005. In Final Form: NoVember 7, 2005
The reaction of NO and the immobilized dimer complex (edta)2Ru2(III1/2,III1/2) on silica gel chemically modified with [3-(2-aminoethyl)aminopropyl]trimethoxysilane (AEATS) produces the corresponding immobilized nitrosyl complex AEATS/RuIINO+. This compound, a monomer, was obtained by reducing the immobilized ruthenium dimer either electrochemically or with EuII and reacting this species with NO2- ions. The properties of [Ru(edta)NO]- in solution and anchored (AEATS/RuIINO+) on silica were compared using electrochemical (DPV, CV) and spectroscopic (IR, UV-vis, and ESR) techniques. The results indicate that immobilization does not alter the reactivity of the ruthenium complex and confirm that [Ru(edta)(H2O)]2- may be used, either in solution or immobilized, as a catalyst for the conversion of NO2- to NO+. Both the anchored nitrosyl complex AEATS/RuIINO+ and the [Ru(edta)NO]species in solution, upon one-electron reduction, liberate NO at comparable rates.
1. Introduction Nitric oxide (NO) is a biologically active molecule found in the body. This molecule is known to have several vascular regulatory effects such as vasodilatation, inhibition of platelets, leukocyte adhesion/activation, and inhibition of smooth muscle cell proliferation.1,2 The role of NO in certain physiological functions, either toxic or beneficial, is directly dependent on its source of production as well as its local concentration.3 It has already been shown that NO presents therapeutic effects when released in target areas of living systems in a controlled manner.4 The potential applications of nitrosyl complexes as metallopharmaceutical agents have stimulated investigations of the reaction of ruthenium complexes with NO.5-10 Particular emphasis has been given to the reactivity aspects associated with the association and dissociation of NO as a necessary background to consider whether ruthenium complexes can provide a means to either scavenge or deliver NO under biologically relevant conditions.5,8-12 * To whom correspondence should be addressed. E-mail: douglas@ iqsc.usp.br. Fax: (55)16-33739976. † Universidade de Sa ˜ o Paulo. ‡ Universidade Estadual de Maringa ´. (1) Moncada, S.; Radomski, M. W.; Palmer, R. M. J. Biochem. Pharmacol. 1988, 37, 2495. (2) Jeremy, J. Y.; Rowe, D.; Emsley, A. M.; Newby, A. C. CardioVasc. Res. 1999, 43, 580. (3) Davis K. L.; Marin, E.; Turko I. V.; Murad, F. Annu. ReV. Pharmacol. 2001, 41, 203. (4) Ohwada, T.; Uchiyama, M. J. Synth. Org. Chem. Jpn. 2003, 61, 45. (5) Tfouni, E.; Krieger, M.; McGarvey, B. R.; Franco, D. W. Coord. Chem. ReV. 2003, 236, 57. (6) Fricker, S. P. Platinum Met. ReV. 1995, 39, 150. (7) Shepherd, R. E.; Chen, Y. J. Inorg. Biochem. 1997, 68, 183. (8) Toledo, J. C.; Silva, H. A. S.; Scarpellini, M.; Mori, V.; Camargo, A. J.; Bertotti, M.; Franco, D. W. Eur. J. Inorg. Chem. 2004, 9, 1879. (9) Marmion, C. J.; Cameron, B.; Mulcahy, C.; Fricker, S. P. Curr. Top. Med. Chem. 2004, 4, 1585. (10) Sauaia, M. G.; Oliveira, F. S.; Tedesco, A. C.; Silva, R. S. Inorg. Chim. Acta 2003, 355, 191. (11) Roncaroli, F.; Ruggiero, M. E.; Franco, D. W.; Estiu´, G. L.; Olabe, J. A. Inorg. Chem. 2002, 41, 5760. (12) Wanat, A.; Schneppensieper, T.; Karocki, A.; Stochel, G.; Eldik, R. J. Chem. Soc., Dalton Trans. 2002, 6, 941.
In prior investigations, [RuIII(edta)(H2O)]- was suggested to be a promising NO scavenger complex.9,12-14 This complex possesses some important characteristics such as solubility in water, fast reaction with NO (kNO ) 2.24 × 107 M-1s-1 at 7.3 °C, pH 7.4)13 to yield [RuII(edta)NO]-, low toxicity (LD50 > 9.0 × 10-5 mol/Kg),14 and inertness toward its being substituted both in biological media and in vitro,11 conditions that satisfy essential requirements to be a metallopharmaceutical compound. Although reduction of [RuII(edta)NO]- to [RuII(edta)NO]2(ENO+/NO ) -0.31 V vs SCE-, k-NO ) 2.1 × 10-3 s-1)12 can be accomplished with biological reductants, this does not preclude the use of [Ru(edta)(H2O)]- as a NO scavenger in vivo since both ruthenium complexes are mostly excreted in urine after 4 h.14 Similar to trans-[RuII(NH3)4(L)NO] (where L ) pyrazine, nicotinamide, or pyridine) complexes,11 [RuII(edta)NO]- is susceptible to nucleophilic attack by OH-, but only at pH g 12, yielding the nitro species:11,14 [RuII(edta)NO]- + 2OH- w [RuII(edta)NO2]3-. Therefore, under physiological conditions [RuII(edta)NO]- is the predominant species present. One major area of application of the present experimental program is in the treatment of the surfaces of surgical implants with NO donors or scavengers. This is very necessary in situations where, for example, a stainless steel stent inserted during angioplasty is rejected by the host, thus leading to restenosis.15 Such a rejection occurs when treated vessels become blocked again as a result of any response to an overstretch injury.15 One important factor is the proliferation of vascular smooth muscle cells, which consist of an abnormal migration of these cells out of the media to produce intimal thickening and sometimes the formation of an obstructive thrombus.16 (13) Cameron, B. R.; Darkes, M. C.; Yee, H.; Olsen, M.; Fricker, S. P.; Skerly, R. T.; Bridger, G. J.; Davies, N. A.; Wilson, M. T.; Rose, D. J. Zubieta, J. Inorg. Chem. 2003, 42, 1868. (14) Zanichelli P. G., Miotto, A. M., Estrela, H. F. G., Soares, F. R., Kassisse, D. M. G.; Bratfisch, R. C. S.; Castellano, E. E.; Roncaroli, F.; Parise, A. R.; Olabe, J. A.; Brito, A. R. M. S.; Franco, D. W. J. Inorg. Biochem. 2004, 98, 1921. (15) Dangas, G.; Kuepper, F. Circulation 2002, 105, 2586. (16) Baek, S. H.; Hrabie, J. A.; Keefer, L. K.; Hou, D. M.; Fineberg, N.; Rhoades, R.; March, K. L. Circulation 2002, 105, 2779.
10.1021/la051852l CCC: $33.50 © 2006 American Chemical Society Published on Web 12/08/2005
204 Langmuir, Vol. 22, No. 1, 2006
Zanichelli et al.
One alternative way of preventing restenosis is to use a stent base containing antiproliferative drugs (drug-eluting stents). Various literature works describe the efficiency of sirolimusand paclitaxel-eluting stents against restenosis.17,18 NO is also known to mediate diverse aspects of blood vessel function, acting as a potent antithrombotic agent19 and inhibitor of vascular smooth muscle cell proliferation.2 Thus, drug-eluting stents containing a NO donor could be a good strategy that can help improve the efficacy of such invasive procedures, thus conferring functionality to the implant.20 Since silica gel and activated stainless steel surfaces both have available hydroxyl groups,21 functionalized silica gel can be useful for the initial investigation of the effects of surface immobilization on the properties of potential metallopharmaceutical compounds. On the basis of these above-mentioned applications, the present study describes the synthesis and reactivity aspects of [RuII(edta)NO]- anchored to a modified silica gel surface. 2. Experimental Section 2.1. Chemical Reagents and Equipment. All reagents and solvents used were of analytical grade (Aldrich) and were used as received. All manipulations with air-sensitive compounds were performed under an atmosphere of argon using standard techniques.22 The specific area of the silica surface was determined by the BET method23 using a CG-2000 instrument, while the group density (δ) and the average interatomic distance (l) were determined by assuming that the [3-(2-aminoethyl)aminopropyl]trimethoxysilane (AEATS) groups are uniformly distributed on the surface and then applying the equations24 δ ) NNA/SBET l ) (1/d)1/2
reference saturated calomel electrode (SCE), and a platinum wire as the auxiliary electrode. The modified carbon paste electrode used as the working electrode was prepared by carefully mixing 20 mg of either AEATS/(edta)2Ru2(III1/2,III1/2) or AEATS/RuNO with 30 mg of graphite powder and one drop of Nujol and then pressing the mixture onto a platinum grid. The reported E1/2 values are conditional ones and are always calculated as the average of (Epa + Epc)/2, where Epa and Epc are the peak potentials of the anodic and cathodic waves. The concentration of NO in aqueous solution was measured using a NO-selective electrode (amino-700) coupled to an inNO-T, NO system from Innovative Instruments. EPR experiments were performed with a Bruker ESR 300E X-band spectrometer at liquid nitrogen temperature. The Ru content was determined using a polarized Zeeman atomic absorption spectrophotometer (Hitachi, model Z-8100) equipped with a Hitachi hollow cathode lamp (12 mA, 349.9 nm wavelength).26 2.2. Functionalization of Silica by AEATS. Silica gel (Aldrich, 70-230 mesh) with a specific surface area of 506 ( 6 m2 g-1 was modified with AEATS following procedures described previously.26,27 The estimated specific surface area (SBET) of functionalized silica (AEATS) was 337 ( 6 m2 g-1. The nitrogen content determined by titrating the NH2 group present in AEATS using conductometric measurements as previously described26,28 was 1.7 ( 0.3 mmol g-1. From eqs 1 and 2, the AEATS group density (δ) was found to be (1.5 ( 0.1) × 1018 M-2, permitting calculation of the average distance between two AEATS groups, which was obtained as 8.1 ( 0.3 Å. 2.3. Immobilization of H2[RuIII(H2edta)Cl2] on Functionalized Silica. The H2[RuIII(H2edta)Cl2] complex was prepared following methodologies based on reaction of RuCl3‚xH2O with Hedta in an acidic medium and detailed in the literature.29,30 In an aqueous solution (pH > 3.0), Cl- ions in this complex are replaced by a water molecule and the coordination of one free carboxylate arm occurs, yielding [RuIII(Hedta)(H2O)]30 according to the equation
(1) (2)
where N is the amount of AEATS attached to the surface (mmol g-1), NA is Avogadro’s number, and SBET is the specific area (m2 g-1). The electronic spectra of the immobilized complexes were obtained by immersing the solid in spectral-grade carbon tetrachloride (CCl4) using a quartz cell with 1 mm path length and a Hewlett-Packard 8451A spectrophotometer. Light scattering was minimized due to the proximity of the refractive indices of CCl4 and the material, resulting in good-quality spectra.25 All IR spectra of the self-supported disk of the material were obtained without any dilution of the immobilized complexes in KBr. The equipment used was an FTIR instrument (Bomem, Hartman & Braun, MB-100 model, 40001000 cm-1). Cyclic voltammetry (CV) and differential pulse voltammetry experiments were performed with a Princeton Applied Research polarographic analyzer (PAR, model 264A). The three-electrode system consisted of a working modified carbon paste electrode, a (17) Sousa, J. E.; Costa, M. A.; Abizaid, A.; Abizaid, A. S.; Feres, F.; Pinto, I. M. F.; Seixas, A. C.; Staico, R.; Mattos, L. A.; Sousa, A. G. M. R.; Falotico, R.; Jaeger, J.; Popma, J. J.; Serruys, P. W. Circulation 2001, 103, 192. (18) Carter, A.; Chronos, N.; Rogers, C.; Robinson, K. A.; Waskman, R.; Weinberger, J.; Wilensky, R. L.; Jensen, D. N.; Zukerman, B. D.; Virmani, R. Circulation 2002, 106, 1867. (19) Cheung, P. Y.; Salas, E.; Schulz, R.; Radomski, M. W. Semin. Perinatol. 1997, 5, 409. (20) Lin, C. E.; Janero, D. R.; Garvey, D. S. Expert Opin. Ther. Pat. 2005, 15, 483. (21) Lin D. Y.; Chang, T. C. Mater. Sci. Eng., A 2003, 359, 396. (22) Shriver, D. F. Manipulation of air-sensitiVe compounds; McGraw-Hill: New York, 1969. (23) (23) Loon, J. C. V. Selected Methods of trace Metal Analysis Biological and EnVironmental Samples; John Wiley & Sons: New York, 1985. (24) Iler, R. K. The Chemistry of Silica; John Wiley & Sons: New York, 1977. (25) Gushikem, Y.; Peixoto, C. R. M.; Rodrigues-Filho, U. P.; Kubota, L. T.; Stadler, E. J. Colloid Interface Sci. 1996, 184, 236.
H2[RuIII(H2edta)Cl2] + H2O f [RuIII(Hedta)(H2O)] + 2HCl The anchoring of H2[RuIII(H2edta)Cl2] by means of the amide bonds of AEATS-modified silica gel has recently been described,26,31 but since it is not yet a common practice, a brief outline is presented. Briefly speaking, this is done by adding 500 mg of an SF-AEATS silica to a solution of N,N′-dicyclohexylcarbodiimide (350 mg) in anhydrous DMF (50 mL). A 120 mg solution of H2[RuIII(H2edta)Cl2] (4× excess) dissolved in 10 mL of DMF is then added dropwise and the resulting suspension refluxed for 4 h with stirring under an atmosphere of argon. Upon immobilization of the complex on the matrix, the color of the solid was observed to change from white to brown. The solid was later filtered, washed successively with DMF, ethanol, and anhydrous ether, and then dried under vacuum for 6 h at 393 K. On anchoring the metal complex as a (edta)2Ru2(III1/2,III1/2) dimer as previously reported,26 the surface area was found to decrease to 320 ( 6 m2 g-1. The amount of Ru complex attached to the modified silica surface was 0.20 × 10-3 mol g-1. The IR spectrum of AEATS/Ru exhibited the following relevant frequencies: 1658 cm-1 (ν(CdO) + ν(C-N)), 1573 and 1464 cm-1 (ν(C-N) + δ(N-H) in the plane). These results are found to be in very good agreement with the IR and ESR spectral values previously reported in the literature.26 2.4. Coordination of NO to the Immobilized (edta)2Ru2(III1/2,III1/2) Dimer Complex. AEATS/Ru (200 mg) was added to a round-bottom flask containing 10 mL of 1 × 10-3 mol L-1 HCl. The flask was sealed with a septum and kept under argon for 30 min (26) Codognoto, L.; Zanichelli, P. G.; Sernaglia, R. L. J. Braz. Chem. Soc. 2005, 16, 620. (27) Burggraf, L. W.; Kendall, D. S.; Leyden, D. E.; Pern, F. J. Anal. Chim. Acta 1981, 129, 19. (28) Shopenko, V. V.; Trofimchuck, A. K.; Kaminskii, V. P. SoV. Prog. Chem. 1982, 48, 14. (29) Yoshino, Y.; Uehiro, T.; Saito, M. Bull. Chem. Soc. Jpn. 1979, 52, 160. (30) Zanichelli, P. G.; Codognoto, L.; Sernaglia R. L. Manuscript in preparation. (31) Oyama, N.; Anson, F. C. J. Am. Chem. Soc. 1979, 101, 1634.
Immobilization of [RuII(edta)NO+] Ion on Silica Gel to eliminate any oxygen present and then NO bubbled for 6 h. Since mechanical stirring would damage the solid, the mixing process was carried out by allowing gas to flow through the mixture. Excess NO was eliminated by the flow of argon for 30 min. The resulting solid, denominated AEATS/RuIINO+, was then filtered, washed successively with deaerated water, acetone, and anhydrous ether (free of peroxide), and then dried under vacuum. Alternatively, AEATS/ RuIINO+ was obtained from the reaction of NO2- (2.0 × 10-3 mol L-1) with the immobilized Ru-edta complex produced by reducing AEATS/Ru with EuII. In this procedure, 200 mg of AEATS/Ru was added to 20 mL of EuII (2.0 × 10-3 mol L-1) contained in a roundbottom flask under an atmosphere of argon. After 1 h, the solid was washed twice with a deaerated NaCF3COO solution (pH 7.1 ( 0.1). After that, a deaerated aqueous solution of NaNO2 (0.20 mol L-1, pH 7.1 ( 0.1) was added to the flask and the mixture kept under argon for 2 h. The resulting solid was filtered, washed with deaerated water and acetone, and dried under vacuum. 2.5. Dissociation of NO from AEATS/RuIINO+. Spectrophotometric and amperometric measurements carried out indicate that [RuII(edta)NO+]- and AEATS/RuIINO+ do not release NO in the presence of visible light, water, or air. Indeed, no signal indicating the release of NO has yet been observed when a solution of [RuII(edta)NO+]- is irradiated with 355 nm wavelength light. The dissociation of NO from immobilized [RuII(edta)NO+] was investigated by observing the decay in current during the process RuIINO+/RuIINO0 with a carbon paste electrode. This electrode was prepared with AEATS/RuIINO+ and later immersed in a solution containing the reductant EuII (0.02 × 10-2 mol L-1). Further, the dissociation of NO from free and immobilized [RuII(edta)NO+] after the addition of EuII was monitored with a NO-selective electrode. The resulting k-NO values were then determined from current vs time curves. Complementary electrochemical experiments were also conducted in the presence of pyrazine (pz). In these experiments, 100 mg of AEATS/RuIINO+ was added to 20 mL of EuII (0.01 × 10-2 mol L-1) in a round-bottom flask. The mixture was kept under argon for 2 h, after which the solution was decanted and the solid washed twice with a deaerated solution of NaCF3COO (pH 7.1 ( 0.1). A 0.5 mol L-1 pyrazine solution was then added and the mixture kept under air for 1 h. The resulting solid was filtered in a glovebag (under argon), washed with deaerated water and acetone, and then dried under vacuum.
3. Results and Discussion Various methods of immobilizing ruthenium complexes on modified silica gel have already been described.26,32,33 According to the literature, these types of immobilizations can be performed by electrostatic interaction25 or coordination to the ligand present on the modified silica gel surface.32,33 Immobilization of H2[Ru(H2edta)Cl2] on the surface of AEATSfunctionalized silica gel groups has also been recently described.26 In this method of functionalization, the covalent bonding of the H2[Ru(H2edta)Cl2] complex to the amide bonds of AEATS results in strong bonds. Most probably, the initial product is an AEATS/ (edta)2Ru2(IV,IV) dimer complex which, upon washing with water, is reduced by water, yielding the mixed-valence species AEATS/(edta)2Ru2(III1/2,III1/2) (AEATS/Ru). This subject was reported in a related paper.26 The obtained atomic absorption data indicate that (2.0 ( 0.1) × 10-4 mol of Ru is immobilized per gram of AEATS/RuIINO+. This value is similar to that observed for AEATS/Ru ((2.1 ( 0.1) × 10-4 mol g-1), suggesting the ruthenium complex is not leached during the reaction with NO or NaNO2. (32) Carmo, R.; Rodrigues-Filho, U. P.; Gushikem, Y.; Franco, D. W. Polyhedron 2000, 19, 2277. (33) Neiva, S. M. C.; Santos, J. A. V.; Moreira, J. C.; Gushikem, Y.; Vargas, H.; Franco, D. W. Langmuir 1993, 9, 2982.
Langmuir, Vol. 22, No. 1, 2006 205
Figure 1. IR spectra of a self-supported pressed disk of (a) AEATS/ Ru and (b) AEATS/RuIINO+.
Figure 2. EPR spectra (a) before and (b) after reaction of AEATS/ Ru with NO gas (77 K).
3.1. Reaction of AEATS/Ru with NO. The IR spectra before and after reaction of AEATS/Ru with NO are shown in Figure 1. The appearance of a distinct band at 1895 cm-1 following reaction with NO (Figure 1b) coincides with the 1890 cm-1 assumed to be ν(NO+) for solid [RuII(Hedta)NO].34 Upon exposure to NO, the observed ESR g values for AEATS/ Ru disappeared (Figure 2). The observed silent ESR of solid AEATS/RuIINO+ at liquid nitrogen temperature is coherent with the expected diamagnetism of the [RuII-NO+] fragment. The atomic absorption data indicate a ruthenium content of (2.0 ( 0.1) × 10-4 mol g-1 of AEATS/RuIINO+. This value is similar to that observed for AEATS/Ru ((2.1 ( 0.1) × 10-4 mol g-1), suggesting that the ruthenium complex is not leached during the reaction with NO or nitrite. The obtained electronic spectrum of AEATS/Ru showed bands, obtained through Gaussian deconvolution, at 370 nm (not considered in the present study and hence not shown in the accompanying Figure 3) and 470 and 940 nm (shown in the inset of Figure 3). These observations are found to be in accordance with those previously reported.26 After reaction with NO, only two absorption bands are observed (Figure 3) with values of λmax at 344 and 470 nm. The band at 940 nm can be attributed to intervalence transition (IT) due to the Ru(III1/2)-Ru(III1/2) chromophore in AEATS/Ru26 and completely disappears, and the relative absorbance intensity, A470/A350, decreases from 0.98 to 0.40. The observed absorption band of AEATS/RuIINO+ at 344 nm likely corresponds to the band at 370 nm ( ) 180 mol L-1 cm)14 exhibited by the deprotonated form of [Ru(edta)NO]- (the uncoordinated carboxylic acid arm of the edta ligand has pKa ) 2.7) in solution. (34) Rhodes, M. R.; Barley, M. H.; Meyer T. J. Inorg. Chem. 1991, 30, 629.
206 Langmuir, Vol. 22, No. 1, 2006
Zanichelli et al. Scheme 1. A Simplified Illustrative Scheme of AEATS/Ru and AEATS/RuNO Anchored on a Silica Surface
Figure 3. Electronic absorption spectrum in CCl4 of (a) AEATS/ RuIINO+ and (b) AEATS/Ru.
Figure 4. Differential pulse voltammograms (NaCF3COO, 0.8 mol L-1, pH 7.0 ( 0.1, scan rate 5 mV s-1, pulse height 25 mV) of (a) AEATS/RuIINO+ and (b) AEATS/Ru electrodes. Inset: cyclic voltammograms of AEATS/RuIINO+ in NaCF3COO (µ ) 0.8 mol L-1, pH 7.0 ( 0.1, scan rate 5 mV s-1).
The disappearance of the band at 940 nm was attributed to the breaking of the dimeric species after NO coordination. Electrochemical experiments carried out before and after the reaction of NO confirm this supposition. Figure 4 shows the cyclic and differential pulse voltammograms obtained for modified carbon paste electrodes (NaCF3COO, 0.8 mol L-1, pH 7.0, scan rate 5 mV s-1) containing AEATS/Ru and AEATS/RuIINO+. From the figure, two processes which can be attributed to the reduction [Ru(IV)-Ru(III)]/[Ru(III)-Ru(III)] and [Ru(III)-Ru(III)]/[Ru(III)-Ru(II)] are quite apparent for AEATS/Ru when E′1/2 ) 0.047 V and -0.260 V vs SCE, respectively26 As can also be observed from the inset in Figure 4, after reaction with NO, only one wave at a value of E1/2 of 0.33 V vs SCE was observed. It is important to emphasize that this potential is comparable to that reported for [RuII(edta)NO+]- (-0.34 V vs SCE)35 in an aqueous solution under similar conditions. Thus, by analogy, this couple can be attributed to the nitrosyl-centered reduction RuIINO+/RuIINO0 in the supported species. For the anchored nitrosyl complex, the observed Ipa/Ipc ratio was 0.92 ( 0.03, while the width of the peak at half-height (w1/2) calculated from differential pulse voltammograms was 0.140 V. This w1/2 value is observed to be slightly higher than that observed for the complex in solution (0.120 V), but lower than the values obtained for the cathodic and anodic processes attributed to [Ru(IV)Ru(III)]/[Ru(III)-Ru(III)] (0.160 V) and [Ru(III)-Ru(III)]/ [Ru(III)-Ru(II)] (0.180 V) couples. The insulator properties of the matrix should therefore account, at least in part, for this observed behavior.36 On the basis of the areas and heights of the (35) Chen, Y, Lin, F.; Shepherd, R. E. Inorg. Chem. 1999, 38, 973. (36) Kubota, L. T., Gushikem, Y. J. Electroanal. Chem. 1993, 362, 219.
peaks (data not shown), it can be ascertained that the AEATS/ RuIINO+ electrode surface is stable against leaching of the ruthenium complex for immersion periods of up to 6 h in a NaCF3COO/CF3COOH solution. The results of the electrochemical experiments carried out are observed to be in close agreement with the above UV-vis (disappearance of the IT band) and ESR (disappearance of the paramagnetic signal) data and strongly suggest that, in the reaction of AEATS/(edta)2Ru2(III1/2,III1/2) with NO, the anchored mononuclear nitrosyl complex AEATS/RuIINO+ is formed as illustrated in Scheme 1. 3.2. Reduction of the Bound NO in the Immobilized Complex AEATS/RuIINO+. Figure 5 shows the induced changes in the differential pulse voltammograms of a carbon paste electrode containing AEATS/RuIINO+ upon exposure to the reductant EuII. After immersion of the electrode in the supporting electrolyte solution containing the reductant for 20 min, the RuIINO+/RuIINO0 process at -0.330 V is found to decrease while the currents at 0.070 and -0.230 V increase. Since these currents were previously attributed to the [Ru(IV)-Ru(III)]/ [Ru(III)-Ru(III)] couple under similar pH conditions,26,37 these observed changes in the voltammetric spectrum should suggest that, after dissociation of NO, some dimer complex should have been formed again on the silica surface. In prior studies, in a phase solution, of the complex [RuII(Hedta)NO] in the presence of EuII, a similar electrochemical behavior was observed. The current corresponding to the known wave of RuIINO+/RuIINO0 (-0.330 V) decreases simultaneously with an increase in the wave of RuIII/RuII (-0.200 V) due to the formation of the aqua complex.14 Since ruthenium complex dimers are not expected to be formed under these conditions, the dimer formed on the silica surface must be due to the closeness of the metal centers. In solution, pz quickly displaces water14,38 in [RuIIedta(H2O)]and [RuII(Hedta)(H2O)]- complexes following the equation RuII(edta)(H2O)]2- + pz S [RuII(edta)pz]2- + H2O. On the other hand, in aqueous solution (NaCF3COO, 0.8 mol L-1, pH 2.2 ( 0.1), the [RuII(Hedta)pz]- complex shows only one welldefined redox couple at E′1/2 ) 0.05 V vs SCE due to the RuIII/ RuII process. Therefore, pz could be a convenient ancillary ligand to scavenge the aqua complex. The differential pulse voltammogram of the modified carbon paste electrode containing the solid isolated after reaction of AEATS/RuIINO+ with EuII followed by reaction with pz shows a well-defined wave at 0.030 V. This value of E′1/2 is in agreement with the E′1/2 value for the [RuII(Hedta)pz]- complex in solution under comparable experimental conditions, thus indicating that NO is in fact released upon reduction of the immobilized complex. (37) Baar, R. B.; Anson, F. C. J. Electroanal. Chem. 1985, 187, 265. (38) Matsubara, T.; Creutz, C. Inorg. Chem. 1979, 18, 1956.
Immobilization of [RuII(edta)NO+] Ion on Silica Gel
Langmuir, Vol. 22, No. 1, 2006 207
Figure 5. Differential pulse voltammograms of AEATS/RuIINO+ (a) in NaCF3COO (µ ) 0.8 mol L-1, pH 2.2 ( 0.1, scan rate 2 mV s-1) and (b) following 20 min of exposure to EuII.
Figure 7. Cyclic voltammograms of AEATS/Ru (a) in NaCF3COO (µ ) 0.8 mol L-1, pH 7.1 ( 0.1, scan rate 20 mV s-1), (b) after NO2 addition, and (c) from the solid isolated after the NO2- reaction.
Figure 6. Chronoamperogram of NO release after reduction with EuII ions of (a) AEATS/Ru and (b) AEATS/RuIINO+ at pH 2.0 ( 0.1 (NaCF3COO/CF3COOH).
The dissociation of NO from AEATS/RuIINO+ in the presence of EuII was also monitored by FT-IR, UV-vis, and a NO-selective electrode. After exposure to EuII, the characteristic ν(NO+) at 1895 cm-1 (Figure 1) was no longer observable in the IR spectrum (data not shown). Additionally, the solid isolated after the reduction of AEATS/RuIINO+ by EuII then followed by the addition of pz exhibited a UV-vis spectrum with a band at 550 nm (obtained by spectral deconvolution, data not shown). By analogy with the pz derivative in solution (545 nm, ) 1.4 × 104 mol-1 L cm-1),14 this spectrum corresponds to anchored [RuII(Hedta)pz]. The concentration of NO released from AEATS/ RuIINO+ in the presence of EuII was monitored with a NOspecific electrode. From plots of current vs time (Figure 6), a value of k-NO ) (4 ( 1) × 10-3 s-1 (25 °C) was determined. This rate constant is found to be in very good agreement with that similarly estimated (data not shown) for [RuII(Hedta)NO0] in solution ((3 ( 1) × 10-3 s-1). The method was validated by comparison with a previously reported k-NO value of 2.1 × 10-3 s-1 for [RuII(Hedta)NO0].14 These results strongly suggest that the immobilization of [RuII(edta)NO]- on silica does not significantly alter its chemical properties compared to those when it is in solution. 3.3. Reaction of AEATS/Ru with NO2-. The reaction between [RuII(edta)(H2O)]2- and NaNO2 (phosphate buffer, pH 7.4) was monitored in solution spectrophotometrically. The electronic spectrum of [RuII(edta)(H2O)]2- in solution (1.5 × 10-3 mol L-1) exhibited two bands, one at 427 nm ( ) 2.6 × 102 mol-1 L cm-1) and the other at 280 nm ( ) 2.9 × 103 mol-1 L cm-1).38 After addition of NaNO2 (2.2 × 10-3 mol L-1), only absorbance at 370 nm ( ) 1.8 × 102 mol-1 L cm-1) became apparent. These
values are in agreement to those extracted by us for the [RuII(edta)NO]- complex spectrum under similar experimental conditions. The cyclic voltammograms of the AEATS/Ru electrode in the absence and presence of NO2- are shown in Figure 7. Following the addition of nitrite to the electrolyte solution, the two electrochemical processes initially present (curve a of Figure 6) are observed to fade and a prominent new process is apparent at E1/2 ) -0.330 V (Figure 7b). This value coincides with that obtained for AEATS/RuIINO+ (Figure 4), thus indicating that NO2- can be converted to NO in the presence of the immobilized ruthenium complex. The presence of two poorly defined processes, one at Epa1 ≈ 0.13 V and Epc1 ≈ 0.02 V and the other at Epa2 ≈ -0.12 V (Figure 7b), which represent the same region as that of the immobilized Ru dimer complex (Figure 7a), indicates that a small amount of this species probably remains on the silica surface inside the electrode. Additional electrochemical experiments were performed with an electrode made of solid isolated from the reaction of AEATS/ Ru with EuII followed by reaction with NO2-. In this case, the cyclic voltammograms displayed only one coupled redox process with an E1/2 value of -0.330 V (Figure 7c), indicating that all of the immobilized dimer complex was converted to a nitrosyl complex. The Ipa/Ipc ratio was 0.96 ( 0.03, and W1/2 calculated from differential pulse voltammetry data was 0.186 V. These values are shown to be in very good agreement with those observed for the RuIINO+/RuIINO0 process in AEATS/RuIINO+. In all, the spectrophotometric and voltammetric data strongly suggest that [Ru(edta)NO]- is formed at pH 7.0 through the reaction between NO2- and [RuII(edta)(H2O)]2- in solution or the immobilized (edta)2Ru2(III1/2,III1/2) species after reduction. On the basis of the data presented here, the catalytic conversion of NO2- into NO using Ru-edta species either in the homogeneous phase or supported on AEATS-functionalized silica can be envisioned to occur according to the cycle shown in Scheme 2. The rate-determining step of this cycle, which is shown to be
208 Langmuir, Vol. 22, No. 1, 2006 Scheme 2. Potential Catalytic Cycle for Conversion of NO2- into NO+ by RuII(edta) or AEATS/Ru
Zanichelli et al.
for H2O in [Ru(edta)(H2O)]- at a rate of kNO ) 2.24 × 107 M-1 s-1 at 7.3 °C, pH 7.4.13 Therefore, since anchoring does not drastically change the properties of [Ru(edta)NO]-, it is reasonable to assume that the rates of the reaction of NO2- with RuII species in solution and anchored would not differ very much. Since nitrite is present in the blood stream (ca 0.5 µmol)39 and the reaction between NO2- and the immobilized ruthenium species is fast, this system may represent an interesting method to increase the useful lifetime of a stent, for example, on medical applications where a local low constant NO concentration is required. Experiments aiming at elucidating the kinetic details of this catalytic cycle as well as the activation of stainless steel as a support are now occurring in our laboratory and will soon be reported. Acknowledgment. We are indebted to FAPESP and CNPq for the financial support given to this research.
slow (k-NO ) 2.1 × 10-3 s-1), is expected to be the aquation of the coordinated NO. Kinetic data for the substitution of NO in anchored AEATS/ Ru are not yet available. However, in solution, NO2- substitutes
LA051852L (39) Himeno, M.; Ishibashi, T.; Nakano, S.; Furuya, K.; Yoshida, J.; Kigashi, T.; Uchida, K.; Nishio, M. Clin. Exp. Pharmacol. Physiol. 2004, 31, 591.
