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In Situ Nanostructure Formation of (µ-Hydroxo)bis(µ-carboxylato) Diruthenium Units in Nafion Membrane and Its Utilization for Selective Reduction of Nitrosonium Ion in Aqueous Medium Annamalai Senthil Kumar, Tomoaki Tanase,* and Masayasu Iida Department of Chemistry, Faculty of Science, Nara Women’s UniVersity, Kitauoya-higashi-machi, Nara 630-8285, Japan ReceiVed August 9, 2006. In Final Form: NoVember 16, 2006 Nanostructured molecular film containing the (µ-hydroxo)bis(µ-carboxylato) diruthenium(III) units, [RuIII2(µOH)(µ-CH3COO)2(HBpz3)2]+ ({RuIII2(µ-OH)}), was prepared by an in situ conversion of its µ-oxo precursor, [RuIII2(µO)(µ-CH3COO)2(HBpz3)2] ({RuIII2(µ-O)}), in a Nafion membrane matrix, where HBpz3 is hydrotris(1-pyrazolyl)borate. The conversion procedure results in fine nanoparticle aggregates of the {RuIII2(µ-OH)} units in the Nafion membrane (Nf-{RuIII2(µ-OH)}), where an average particle size (4.1 ( 2.3 nm) is close to the Nafion’s cluster dimension of ∼4 nm. Chemically modified electrodes by using the Nafion molecular membrane films (Nf-{RuIII2(µ-OH)}-MMFEs) were further developed on ITO/glass and glassy carbon electrode (GCE) surfaces, and a selective reduction of nitrosonium ion (NO+), presumably through reaction of a {RuIIRuIII(µ-OH)} mixed-valence state with HNO2, was demonstrated without interference by molecular oxygen in an acidic aqueous solution. The Nf-{RuIII2(µ-OH)}-MMFEs are stable even in a physiological condition (pH 7), where the naked {RuIII2(µ-OH)} complex is readily transformed into its deprotonated {RuIII2(µ-O)} form, demonstrating an unusual stabilizing effects for the {RuIII2(µ-OH)} unit by the Nafion cluster environment.
Introduction A (µ-hydroxo)bis(µ-carboxylato)diiron core is an ubiquitous structure in non-heme diiron metalloproteins; e.g., hemerythrin (Hr) and bacterial respiratory nitric oxide reductase (NOR-FeB) are primarily involved in reversible oxygen storage and irreversible nitric oxide (NO) reduction functionalities, respectively.1-5 Whereas a number of structural models have been developed by using iron-containing coordination compounds, their functional model systems, in particular, electrochemical studies in aqueous medium are quite limited so far.5,6 Complications in preparation of the discrete Fe2(µ-OH)(µ-RCOO)2 core and its instability in aqueous solutions always restrict further biomimicking studies.5 For example, protonation or reduction of [FeIII2(µ-O)(µ-RCOO)2(HBpz3)2] (HBpz3 ) hydrotris(1-pyrazolyl)borate) either by chemical or by electrochemical methods resulted in decomposition of the active core with formation of its monomeric complex, [Fe(HBpz3)2].7 Sterically hindered ligands were later employed to solve the problem,5 but such methodology decreases their solubility in aqueous medium. Recently, a hydrophobically imitated non-heme enzyme model, a dendrimerstabilized complex of [FeIII2(µ-OH)(µ-RCOO)2(Ln3TACN)2]+ (Ln3TACN ) poly(benzyl ether) dendrimer with 1,4,7-triazacyclononane) was reported as a biomimicking system of the non-heme diiron enzyme analogue,6 but the system was reluctant to produce any redox behavior even in a deaerated organic media. * E-mail:
[email protected]. (1) Stenkamp, R. E. In Handbook of Metalloproteins; Messerschmidt, A., Huber, R., Poulos, T., Wieghardt, K. A., Eds.; John Wiley & Sons: New York, 2001; Vol. 2. (2) Lippard, S. J.; Berg, J. M. Principles of Bioinorganic Chemistry; University Science Books: Mill Valley, CA, 1994. (3) Gro¨nberg, K. L. C.; Watmough, N. J.; Thomson, A. J.; Richardson, D. J.; Field, S. J. J. Biol. Chem. 2004, 279, 17120-17125. (4) Wasser, I. M.; de Vries, S.; M.-Loccoz, P.; Schro¨der, K. K. D. Chem. ReV. 2002, 102, 1201-1234. (5) Edit, Y.; Lippard, S. J. Chem. ReV. 2004, 104, 987-1012. (6) Enomoto, M.; Aida, T. J. Am. Chem. Soc. 2002, 124, 6099-6108. (7) Hartman, J. R.; Rardin, R. L.; Chaudhuri, P.; Pohl, K.; Wieghardt, K.; Nuber, B.; Weiss, J.; Papaefthymiou, G. C.; Frankel, R. B.; Lippard, S. J. J. Am. Chem. Soc. 1987, 109, 7387-7396.
In the present communication, by utilizing an in situ formation technique with the naked precursor complex, [RuIII2(µ-O)(µCH3COO)2(HBpz3)2] ({RuIII2(µ-O)}), we have successfully prepared a functional nanostructured inorganic-molecular material of [RuIII2(µ-OH)(µ-CH3COO)2(HBpz3)2]+ ({RuIII2(µ-OH)}) within a polymeric proton/cationic exchanging membrane, Nafion (Nf) (designated as Nf-{RuIII2(µ-OH)}). The {RuIII2(µ-O)} and {RuIII2(µ-OH)} analogues may offer an alternative approach to the redox characteristics of non-heme diiron enzyme models, since they are substitutionally inert and comparatively stable enough for the relevant studies.8-10 Further, chemically modified electrodes by the molecular membrane films of Nf-{RuIII2(µOH)} (designated as Nf-{RuIII2(µ-OH)}-MMFEs) were prepared on a glassy carbon (GCE) and an indium-tin oxide (ITO)/glass electrode surfaces and showed a selective catalytic reduction of nitrosonium ion (NO+), presumably via reaction of a mixed valence species, {RuIIRuIII(µ-OH)}, with HNO2 without molecular oxygen interference. It is noteworthy that redox behaviors of nitric oxide (NO) and its derivates play key roles in many biological systems, and to the best of our knowledge, this is the first redox and electrocatalytic system in aqueous medium utilizing the (µ-hydroxo)bis(µ-carboxylato)dimetal cores chemically supported in Nafion membranes. Experimental Section The discrete diruthenium complexes, [RuIII2(µ-O)(µ-CH3COO)2(HBpz3)2] ({RuIII2(µ-O)}) and [RuIII2(µ-OH)(µ-CH3COO)2(HBpz3)2](PF6) ({RuIII2(µ-OH)}(PF6)) (Scheme 1) were synthesized and characterized as previously reported.9 UV-vis spectra were recorded using an Agilent 8453E UV-vis spectroscopy system (Germany). TEM was performed using a JEOL-2010 instrument for the Nf{RuIII2(µ-OH)}-modified membrane film on a 200-mesh copper grid (8) Tanase, T.; Takeshita, N.; Yano, S.; Kinoshita, I.; Ichimura, A. New J. Chem. 1998, 927-929. (9) Tanase, T.; Takeshita, N.; Inoue, C.; Kato, M.; Yano, S.; Sato, K. J. Chem. Soc., Dalton Trans. 2001, 2293-2302. (10) Valli, M.; Miyata, S.; Wakita, H.; Yamaguchi, T.; Kihiro, A.; Umakoshi, K.; Imamura, T.; Sasaki, Y. Inorg. Chem. 1997, 36, 4622-4626.
10.1021/la0623578 CCC: $37.00 © 2007 American Chemical Society Published on Web 12/10/2006
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Scheme 1. Conventional Preparation of the Naked {RuIII2(µ-O)} and {RuIII2(µ-OH)} Complexes in Organic Media
(Nisshin EM Co., Ltd., Tokyo, Japan). Electrochemical experiments were carried out with a Hokuto Denko HZ-3000 (Japan) electrochemical workstation at room temperature (25 ( 2 °C). A threeelectrode assembly consists of platinum wire counter electrode, Ag/ AgCl (in a saturated KCl aqueous solution) or Ag/AgPF6 (in CH3CN containing 0.1 M [nBu4N][PF6]) reference electrode, and Pt, GCE (surface area ) 0.07 cm2, BAS), and ITO plate (approximately 1 cm × 4 cm, Kinoene optical industry, Tokyo, Japan) or its chemically modified working electrodes. A Nafion-stabilized {RuIII2(µ-OH)} (Nf-{RuIII2(µ-OH)}) membrane solution was prepared first by mixing the precursor {RuIII2(µ-O)} complex (7.3 mg) with a 2.5% Nafion ethanolic solution (0.4 mL), followed by gentle ultrasonification. It served as a stock solution to prepare the molecular membrane modified film electrodes (MMFEs) of Nf-{RuIII2(µ-OH)} on the GCE and ITO surfaces. The GCE/Nf-{RuIII2(µ-OH)}-MMFE preparation procedure is as follows; a 2.5 µL of the Nf-{RuIII2(µ-OH)} stock solution is dropped on a clean GCE surface, which was then allowed to dry for 10 min and resulted in uniform formation of the MMFE. A similar procedure except 10 µL of the coating volume was used for the case of ITO/ Nf-{RuIII2(µ-OH)}-MMFE. After the physical modification, the electrodes were electrochemically cycled in a potential window of -0.4 to 1.1 V vs Ag/AgCl until peak currents for the redox process at E1/2 ) 0 V (vs Ag/AgCl) became constant in a KCl/HCl solution at pH 2 with a scan rate (V) of 50 mV/s. During the process, the in situ formed {RuIII2(µ-OH)} cations were properly doped into the anion sites of the Nafion membrane. Solid-state UV-vis spectra of ITO/Nf-{RuIII2(µ-OH)}-MMFEs were measured in air after immersion into aqueous solutions with different pH and reaction conditions.
solid-state Nf-{RuIII2(µ-OH)} film on the surface of the ITO plate was closely similar to that of the solution spectrum for the discrete {RuIII2(µ-OH)} complex (Figure 1A(a),B(a)), which directly demonstrated an efficient in situ conversion of {RuIII2(µ-O)} to {RuIII2(µ-OH)} in the Nafion matrix. The TEM images of the Nf-{RuIII2(µ-OH)} films showed fine nanoparticle dot structure with an average particle size of 4.1 ( 2.3 nm (Figure 2), which was close to the cluster-core size of the Nafion (ca. 4.0 nm) and that of the recently reported cadmium sulfide nanoparticle (4.1 nm) in Nafion membrane systems.11 A plausible mechanism is thus sketched in Scheme 2. Since the polymeric Nafion ionic cluster contains highly concentrated H+ ions, the naked precursor complex {RuIII2(µ-O)} initially underwent a proton-transfer reaction to result in the hydroxobridged cationic form, {RuIII2(µ-OH)}, which was subsequently
Results and Discussion Scheme 1 indicates the conventional preparation of the naked complex, {RuIII2(µ-OH)} (λmax ) 530 nm in CH3CN, dark pink) from its oxo-bridged derivative, {RuIII2(µ-O)} (λmax ) 570 nm in CH3CN, blue) by treatment with a strong organic acid.8,9 In the present work, a homogeneous Nafion solution containing the hydroxo-bridged diruthenium(III) cation, {RuIII2(µ-OH)}, (Nf{RuIII2(µ-OH)}) was successfully prepared by simple mixing of the naked precursor {RuIII2(µ-O)} with a 2.5% diluted Nafion membrane ethanolic solution, where the blue color of {RuIII2(µ-O)} immediately changed to clear dark pink similar to the discrete {RuIII2(µ-OH)} complex. Control experiments on direct mixing of the {RuIII2(µ-OH)} with the Nafion solution (Nf+{RuIII2(µ-OH)}) yielded an insoluble mixture with a solid sedimentation in the bottom of vial, unlike the homogeneous Nf-{RuIII2(µ-OH)} case. The UV-vis spectral pattern of the
Figure 1. (A) UV-vis spectra of the discrete complexes {RuIII2(µ-O)} (solid line) and {RuIII2(µ-OH)} (broken line) dissolved in CH3CN (a) and those in pH 7 PBS (few drops of CH3CN is added to prepare the naked complex dissolved aqueous solutions) (b). (B) Solid-state UV-vis spectrum of the molecular membrane film of Nf-{RuIII2(µ-OH)} on an ITO electrode surface (ITO/Nf-{RuIII2(µ-OH)}-MMFE) without (a) and with (b) immersion into aqueous solutions at pH 7 and 11.
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Langmuir, Vol. 23, No. 2, 2007 393
Figure 2. A typical TEM image for the Nafion membrane molecular film containing {RuIII2(µ-OH)} units (abbreviated as Nf-{RuIII2(µOH)}). Scheme 2. Cartoon for in Situ Nf-{RuIII2(µ-OH)} Formationa
a (A) A proton transfer occurred from Nafion to {RuIII (µ-O)} 2 within the matrix. (B) Modified Nafion cage can stabilize the {RuIII2(µ-OH)} with hydrophilic and -phobic, electrostatic, and H-bonding interactions.
stabilized by the anionic sites together with hydrophobic, hydrophilic, and hydrogen-bonding interactions inside the Nafion cage. Such a Nf-{RuIII2(µ-OH)} core is unusually stable in an aqueous system of the physiological condition (pH 7). The solution-phase UV-vis spectrum of the naked complex {RuIII2(µ-OH)} in a phosphate buffer solution (PBS) at pH 7 showed unstable behavior of the hydroxo-bridged dimetal structure, due to its rapid deprotonation into the {RuIII2(µ-O} complex (Figure 1A(b)). On the other hand, the film of Nf-{RuIII2(µ-OH)} on ITO showed a stable UV-vis spectral pattern in a pH 7 PBS, characteristic of the {RuIII2(µ-OH)} species, and it was deprotonated only at higher pH of around 11 (Figure 1B(b)). Namely, the {RuIII2(µ-OH)} species are stabilized only in the cave of a Nafion film in the aqueous solution. This observed phenomenon is quite similar to the protein-stabilized active sites in non-heme metalloproteins. Electrochemical behavior of the Nf-{RuIII2(µ-OH)} film was probed initially with cyclic voltammetry (CV) using the chemically modified glassy carbon electrodes (GCE/Nf-{RuIII2(µ-OH)}-MMFEs) in a N2-saturated KCl-HCl solution at pH 2. As can be seen in Figure 3A, a well-defined reversible electrontransfer behavior (E1/2 ≈ 0 V vs Ag/AgCl sat KCl) was observed with a mixed adsorption and diffusion-controlled fashion (slope of a plot log(ipc) vs log(V) ) 0.73; ipc ) cathodic peak current), and the peak was stable upon multiple cyclings. Surface-active Ru concentration, ΓRu, is calculated as 1.2 nmol cm-2 by using the equation Q ) nFAΓRu (Q ) area under a cathodic peak (V ) 10 mV/s), and other symbols are as their usual significance). (11) Wang, S.; Liu, P.; Wang, X.; Fu, X. Langmuir 2005, 21, 11969-11973.
Figure 3. CVs of GCE/Nf-{RuIII2(µ-OH)}-MMFEs (A) with N2 (dotted lines) and O2 (solid lines) saturated KCl/HCl solutions at pH 2, and (B) those with and without addition of 25 mM of NO+ in deaerated pH 2 KCl/HCl solutions at V ) 50 mV/s. Dotted line in (B) corresponds to the GCE/Nf response with NO+ under identical working conditions. (C) CVs of GCE/Nf-{RuIII2(µ-OH)}-MMFE before and after NO+ (15 mM) reduction; other conditions are same as in (B). (D) GCE response of a homogeneous mixture containing 0.63 mM of the naked {RuIII2(µ-OH)} complex, without (dotted line) and with (solid line) 26 mM of NO+ in a deaerated CH3CN solution containing 0.1 M [nBu4N][PF6] and 1.2 mM of p-toluene sulfonic acid at V ) 100 mV/s.
Catalytic activity of the electrode was first examined with bare molecular O2-saturated KCl-HCl solution at pH 2 (Figure 3A), and unexpectedly, the GCE/Nf-{RuIII2(µ-OH)}-MMFE did not show any marked alteration in either redox potential and current values with the dissolved O2 (Figure 3A). On the other hand, the GCE/Nf-{RuIII2(µ-OH)}-MMFE system with NO+ resulted in a selective catalytic reduction response with a peak current sensitivity of 0.16 µA/mM (i.e., ipccat/[NO+], where ipccat ) net catalytic reduction peak current, 4 µA) (Figure 3B). The catalytic GCE/Nf-{RuIII2(µ-OH)}-MMFE electrode has appreciably identical CV response before and after the NO+ reduction (Figure 3C). The same behavior was also confirmed by solid-state UV-vis spectra of ITO/Nf-{RuIII2(µ-OH)}-MMFE, indicating the stable existence of the regenerated {RuIII2(µ-OH)} units in the Nafion film (Supporting Information Figure S1). No catalytic redox response was observed with NO2 and NO3- in N2-saturated KCl-HCl solutions at pH 2. It was noted from the literature that all of the most exciting mediators had a serious interference problem with O2 during electrochemical reductions of NO and its derivatives, with examples being Nf/methyl viologen,12,13 Nf/toluindine blue,14 metallo-porphyrins,15,16 and Nf/electropolymerized [Cr(v-tpy)2]3+ (v-tpy ) 4-vinyl-2,2′,6′,2′′(12) Litong, J.; Ping, J.; Jiannong, Y.; Yuzhi, F. Talanta 1992, 39, 146-147. (13) Ferreyra, N. F.; Dassie, S. A.; Solis, V. M. J. Electroanal. Chem. 2000, 486, 126-132. (14) Pin, K.-C.; Chuang, C.-S.; Cheng, S.-H.; Su, Y. O. J. Electroanal. Chem. 2001, 501, 160-165. (15) Chi, Y.; Chen, J.; Miyake, M. Electrochem. Commun. 2005, 7, 12051208. (16) Fuerte, A.; Corma, A.; Iglesias, M.; Morales, E.; Sa´nchez, F. J. Mol. Catal., A 2005, 246, 109-117.
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terpyridyl).17 Control experiments with unmodified electrodes (i.e., GCE and GCE/Nf) failed to show any sign of the NO+ reduction under the present working conditions. It should be noted that NO has complicated chemistry in biological systems, and several other forms like NO+ are also implicated in various biological processes, especially in acidic environments.18,19 Since NO+ is known to undergo rapid reaction with water, NO+ + 2H2O f HNO2 + H3O+,20 the true active species in the present electrochemical reduction was assumed to be nitrous acid, HNO2. In order to confirm this, a salt of nitrous acid, NaNO2, was further subjected to the electrochemical reaction, since the pKa value of HNO2 is 3.3.21 Expectedly, the GCE/Nf-{RuIII2(µ-OH)} electrode in the presence of NO2- also yielded appreciable electrocatalytic reduction with a peak current sensitivity (0.11 µA/mM) close to that with NO+ (Supporting information Figure S2), suggesting that nitrous acid is a possible true active species in the present electrocatalytic system. The hydroxo form of the RuIII2 complex is necessary for the reduction process in this work, because the oxo complex is not a good precursor for the RuIIRuIII mixed-valence state (vide infra). In addition, the hydroxo RuIII2 species exclusively existed in the Nafion film under the present electrochemical conditions (pH 2), since the pKa value of the hydroxo complex is ∼4. In order to pursue the mechanism, parallel electrochemical experiments were carried out with the naked {RuIII2(µ-OH)} complex in a homogeneous nonaqueous condition in the absence of both water and O2 as in Figure 3D. On the basis of the established redox behavior, the reversible and irreversible redox peaks centered at -0.41 V (E1) and -1.16 V (E2) vs Ag/AgPF6 were assigned as one-electron redox processes of {RuIII2(µ-OH)}/ {RuIIRuIII(µ-OH)} and {RuIIRuIII(µ-OH)}/{RuII2(µ-OH)}, respectively.9 The CV of {RuIII2(µ-OH)} measured in the presence of NO+ ions markedly showed a sharp catalytic reduction response at the E1 process, where the mixed-valence {RuIIRuIII(µ-OH)} species selectively existed. These observations imply that the redox process observed with GCE/Nf-{RuIII2(µ-OH)}-MMFE corresponds to generation of a RuIIRuIII mixed-valence species in a Nafion matrix, which may further be responsible for the catalytic reduction with NO+. While appreciable retention of the (17) Maskus, M.; Pariente, F.; Wu, Q.; Toffanin, A.; Shapleigh, J. P.; Abruna, H. D. Anal. Chem. 1996, 68, 3128-3134. (18) Broillet, M.-C. Cell. Mol. Life Sci. 1999, 55, 1036-1042. (19) Peri, L.; Pietraforte, D.; Scorza, G.; Napolitano, A.; Fogliano, V.; Minetti, M. Free Radical Biol. Med. 2005, 39, 668-681. (20) Zhao, Y.-L.; McCarren, P. R.; Houk, K. N.; Choi, B. Y.; Toone, E. J. J. Am. Chem. Soc. 2005, 127, 10917-10924. (21) Burner, U.; Furtmu¨ller, P. G.; Kettle, A. J.; Koppenol, W. H.; Obinger, C. J. Biol. Chem. 2000, 275, 20597-20601.
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{RuIII2(µ-OH)} species within the Nafion matrix before and after NO+ reduction (Figures 3C and S2) demonstrated an absence of direct coordination of NO+ (or NO2-) to the RuIII2 centers, a preliminary electrochemical kinetic study indicated a presence of a weak intermediate adduct like [Nf-RuIIRuIII(µ-OH)-NO+ (or HNO2)] with inner-sphere mechanistic reduction pathways like the chemically modified electrode with Nafion/leadruthenium oxide pyrochlore.22 The new approach established in the present study is of potential importance in relation to biomimicking electron-transfer systems in aqueous medium for non-heme diiron proteins, especially bacterial nitric oxide reductase. Further detailed studies relating to physicochemical characterization, biomimicking properties, and chemical sensors are in progress.
Conclusions We have successfully prepared a new (µ-hydroxo)bis(µcarboxylato) diruthenium(III) complex within a polymeric Nafion membrane (Nf-{RuIII2(µ-OH)}) using its (µ-oxo)bis(µ-carboxylato) diruthenium(III) precursor, in which the RuIII2(µ-hydroxo) species was unusually stabilized in aqueous medium. This in situ conversion was found to form fine nanoparticle aggregates of the {RuIII2(µ-OH)} units with a particle size closer to the Nafion ionic cluster dimension. Chemically modified Nf-{RuIII2(µ-OH)} molecular film electrodes can selectively catalyze reduction of NO+, presumably through a key reaction of the {RuIIRuIII(µ-OH)} mixed-valence state with in situ formed HNO2. The material design along this study will provide a new platform for better understanding, probing, and mimicking the redox and catalytic functions of the non-heme metalloproteins in aqueous medium. Acknowledgment. This work is partially supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science, and Technology, Japan. A. S. Kumar gratefully acknowledge the Japan Society for the Promotion of Science (JSPS) for the postdoctoral fellowship and research grant. Supporting Information Available: Solid-state UV-vis spectra of ITO/Nf-Ru2(µ-OH)-MMFE before and after the NO+ reduction and CV responses of GCE/Nf-Ru2(µ-OH)-MMFE for the electrocatalytic reduction of NaNO2 in a KCl-HCl solution at pH 2. This material is available free of charge via the Internet at http://pubs.acs.org. LA0623578 (22) Zen, J.-M.; Kumar, A. S. Acc. Chem. Res. 2001, 34, 772-780.
