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Langmuir 1996, 12, 4008-4014
Factors Responsible for the Unusually Strong Adsorption of [Os(bpy)2(Cl)L1]+ (L1 ) 1,2-bis(4-pyridyl)ethane) and Related Complexes on Metal and Graphite Electrode Surfaces John L. E. Campbell and Fred C. Anson* Arthur Amos Noyes Laboratories, Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125 Received March 22, 1996X The extent and dynamics of the adsorption on gold and pyrolytic graphite electrodes of four complexes of Os(II) were compared with the previously reported adsorption of similar complexes on Pt (Acevedo, D.; Abrun˜a, H. D. J. Phys. Chem. 1991, 95, 9590). The complexes investigated were [Os(bpy)2(Cl)L]+ with bpy ) 2,2′-bipyridine and L ) 1,2-bis(4-pyridyl)ethane, 4-phenylpyridine, 4-(1-n-butylpentyl)pyridine, and pyridine. Spontaneous, strong adsorption of all four complexes occurred on graphite electrodes and, except for the complex with L ) pyridine, on gold electrodes. The presence of a pendant pyridine site on ligand L is not required for strong adsorption to occur. Replacement of the [Os(bpy)2(Cl)]+ center by Ru(NH3)52+ also led to a strongly adsorbing complex. The adsorption appears to be driven by hydrophobic interactions of the organic ligands with the electrode surface and with each other as well as specific surface-ligand bond formation when a pendant pyridine group is present. Although the concentration dependence of the quantities of the complexes adsorbed could be fit to a Langmuir isotherm, the dynamics of the adsorption and desorption reactions were not consistent with Langmuirian adsorption. Stabilizing intermolecular electronic interactions among the adsorbed molecules are suggested as a possible explanation for the observed behavior.
Introduction The spontaneous adsorption of [Os(bpy)2(Cl)L1]+ (bpy ) 2,2′-bipyridine; L1 ) 1,2-bis(4-pyridyl)ethane) and closely related complexes at polycrystalline platinum electrodes has been described by Abrun˜a and co-workers.1-4 Monolayers of the adsorbed complex are formed on Pt electrodes exposed to solutions containing sub-micromolar concentrations of the complexes in both aqueous and nonaqueous solvents.1-3 The magnitude of the limiting adsorption obtained, its dependence on pH, and the failure of [Os(bpy)2(Cl)(py)]+ (py ) pyridine) to adsorb led Abrun˜a and co-workers to surmise that the exposed nitrogen atom of the pendant pyridine moiety of ligand L1 was essential for the adsorption to occur. (The structures and abbreviations of the ligands to be discussed in this study are given in Chart 1.) Forster and Faulkner utilized the same complexes in studies of the kinetics of electron transfer and related phenomena involving reactants adsorbed on platinum microelectrodes.5-8 We were attracted to these complexes because of the unusual strength of their adsorption on platinum, even from nonaqueous media. In order to assess the origins of this strong adsorption, we examined the adsorptive behavior of the same and related complexes at gold and graphite electrodes. Included in the set were complexes which lacked the pendant pyridine site. The results show * To whom correspondence should be addressed. E-mail:
[email protected]. Telephone: 818 395 6000. Fax: 818 405 0454. X Abstract published in Advance ACS Abstracts, July 15, 1996. (1) Acevedo, D.; Abrun˜a, H. D. J. Phys. Chem. 1991, 95, 9590. (2) Acevedo, D.; Bretz, R. L.; Tirado, J. D.; Abrun˜a, H. D. Langmuir 1994, 10, 1300. (3) Tirado, J. D.; Acevedo, D.; Bretz, R. L.; Abrun˜a, H. D. Langmuir 1994, 10, 1971. (4) Tirado, J. D.; Abrun˜a, H. D. J. Phys. Chem. 1996, 100, 4556. (5) Forster, R. J.; Faulkner, L. R. J. Am. Chem. Soc. 1994, 116, 5444. (6) Forster, R. J.; Faulkner, L. R. J. Am. Chem. Soc. 1994, 116, 5453. (7) Forster, R. J.; Faulkner, L. R. Langmuir 1995, 11, 1014. (8) Forster, R. J.; Faulkner, L. R. Anal. Chem. 1995, 67, 1232.
S0743-7463(96)00278-8 CCC: $12.00
Chart 1. Structures of Ligands
that the adsorption is not restricted to platinum surfaces nor is the pendant pyridine group required for strong adsorption of the complexes. The experimental data obtained are summarized in this report, and some of the factors that may be responsible for the high degree of adsorbability of this class of complexes are suggested. Experimental Section Materials. K2OsCl6 was used as received from Strem Chemicals Inc. Os(bpy)2Cl2 and [Ru(HEDTA)(OH2)] were prepared using standard procedures from the literature.9,10 All ligands were used as received from Aldrich. Dichloromethane (Merck, Omnisolv), dimethylformamide (Burdick and Jackson), and tetrabutylammonium perchlorate (Southwestern Analytical Chemicals Inc.) were used as received. KClO4 (G. F. Smith) was recrystallized twice from water. The [Os(bpy)2(Cl)L]PF6 complexes (L ) 1,2-bis(4-pyridyl)ethane (L1), 4-phenylpyridine (L2), 4-(1-n-butylpentyl)pyridine (L3), or pyridine (py)) were prepared by refluxing [Os(bpy)2Cl2] with an excess of ligand L in absolute ethanol. In a typical procedure, Os(bpy)2Cl2 (200 mg, 0.349 mmol) was dissolved by refluxing in 125 mL of deaerated absolute ethanol for 20-30 min under argon. A solution of ligand L (4 equiv) in 10 mL of ethanol was added, and the refluxing was continued until thin layer chromatography indicated consumption of the starting material (ca. 20 h, depending on the ligand). After cooling and filtration, NH4PF6 (228 mg in 10 mL of water) was added and the solution was concentrated by rotary evaporation until crystallization commenced. The product was isolated by (9) (a) Buckingham, D. A.; Dwyer, R. P.; Goodwin, H. A.; Sargeson, A. M. Aust. J. Chem. 1964, 17, 325; (b) Inorg. Synth. 1986, 24, 291. (10) Shimizu, K. Bull. Chem. Soc. Jpn. 1977, 50, 2921.
© 1996 American Chemical Society
Adsorption of [Os(bpy)2(Cl)L1]+ filtration, washed with several portions of water (3 × 2 mL) and diethyl ether (5 × 2 mL), and air dried. Yields of crude product were typically ca. 95%. The crude products were purified by chromatography on neutral alumina (Brockman Activity I) using 30:1 CH2Cl2/EtOH as eluent. In each case, the desired complex eluted as a reddish-brown band. After the addition of small amounts of 2-propanol (ca. 3-5 mL) to the eluate, crystallization was induced by rotary evaporation of the solvent. The purified compounds were isolated by filtration, washed with several small portions of ether, and dried overnight in vacuo at room temperature. Elemental analysis. [Os(bpy)2(Cl)L1]PF6‚H2O Calc: C, 43.43; H, 3.42; N, 9.50. Found: C, 43.47; H, 3.24; N, 9.44. [Os(bpy)2(Cl)L2]PF6•H2O. Calc: C, 43.49; H, 3.18; N, 8.18. Found: C, 43.73; H, 3.23; N, 7.77. [Os(bpy)2(Cl)L3]PF6 Calc: C, 45.97; H, 4.43; N, 7.88. Found: C, 45.65; H, 4.40; N, 7.85. [Ru(NH3)5Cl]Cl2 was prepared by the method of Vogt et al.11 and was used to prepare [Ru(NH3)5L1]2+ according to the method of Callahan et al.12 Apparatus and Procedures. Instrumentation for electrochemical measurements included a Princeton Applied Research 173 potentiostat, a 175 Universal Programmer, and a Houston XY recorder. A standard three-electrode cell was employed with a Pt counter electrode. Potentials were measured and are quoted with respect to a sodium chloride saturated calomel electrode (SSCE). The supporting electrolyte was 0.1 M aqueous KClO4. Laboratory deionized water was further purified by passage through a Milli-Q purification system. All solutions were deoxygenated by bubbling with prepurified argon. Edge plane graphite electrodes were prepared by attaching cylinders (6.5 mm diameter, Union Carbide) to glass shafts with heatshrinkable polyolefin tubing (Alpha Wire Products, Inc., FIT300). Au electrodes were prepared by sealing wires (1.0 mm diameter) into Epon 825 epoxy resin (Shell) according to the method of Groat and Creager.13 The electrodes were ground successively with 400 and 600 grit emery to remove epoxy, polished with 0.3 and 0.05 µm Al2O3, and sonicated for 10 min in water prior to use. Au electrodes were etched in 1:1 water/ aqua regia for 10 min prior to sonication followed by cycling between +1.5 and -0.3 V in 1 M H2SO2 for 12 min (20 cycles at 100 mV s-1),14 rinsed with deaerated 0.1 M KClO4, transferred to the cell, and cycled between +0.7 and -0.3 V in 0.1 M KClO4 for 10 min (30 cycles at 100 mV s-1). Graphite electrodes were conditioned by cycling in the region of interest (typically within the range +0.9 to -0.3 V, 100 mV s-1) in 0.1 M KClO4 until a constant background voltammogram was obtained (ca. 5-10 min). Stock solutions of the complexes (0.40 mM) were prepared in 1:1 H2O/DMF. Small aliquots of the stock solutions were injected directly into the electrochemical cell to prepare the test solutions. Unless indicated otherwise, the electrode potential was maintained at 0 V during adsorption of the complexes. In some instances, coatings of the complexes were formed by dipping the electrodes directly in the stock solution (0.40 mM), rinsing with water, and transferring the electrode to a pure supporting electrolyte solution. Experiments with ruthenium complexes were performed under red light. Coverages of adsorbed complexes were estimated by integration of the areas under cyclic voltammograms recorded at 500 mV s-1 to minimize contributions from the dissolved complex (e5 µM). The solutions were not stirred, and at concentrations below ca. 2 µM it can be calculated that significant depletion of the reactant at the electrode surface would have resulted if linear diffusion were the only means by which the adsorbing molecules were transported to the surface. However, the rate of adsorption was so slow that hundreds of minutes were required to reach steady values, and as pointed out by Parsons,15 natural convection will result in transport rates much above the linear diffusion-controlled values at such long times. Accordingly, we followed the procedures employed in the previous studies,2,3 and did not agitate the solutions while the adsorption rates were being measured. The geometric areas of (11) Vogt, L. H., Jr.; Katz, J. L.; Wiberly, S. E. Inorg. Chem. 1965, 4, 1157. (12) Callahan, R. W.; Brown, G. M.; Meyer, T. J. Inorg. Chem. 1975, 14, 1443. (13) Groat, K. A.; Creager, S. E. Langmuir 1993, 9, 3668. (14) Woods, R. In Electroanalytical Chemistry; Bard, A. J., Ed.; Dekker, Inc.: New York, 1976; Vol. 9, Chapter 1. (15) Parsons, R. Adv. Electrochem. Electrochem. Eng. 1961, 1, 1.
Langmuir, Vol. 12, No. 16, 1996 4009
Figure 1. (A) Cyclic voltammograms for [Os(bpy)2(Cl)L1]+ irreversibly adsorbed on a Au electrode from a 1 µM solution of the complex in 0.1 M KClO4. The electrode was placed in the solution for 30 min with its potential maintained at 0 V before the voltammograms were recorded at scan rates of 100, 200, and 500 mV s-1. (B) Quantities of [Os(bpy)2(Cl)L1]+ adsorbed on a Au electrode, Γ, at various times after the electrode was immersed in solutions of the complex. Values of Γ were obtained by measuring the areas defined by cyclic voltammograms like the one in (A) recorded at 500 mV s-1. The concentrations of [Os(bpy)2(Cl)L1]+ were, from the lowest to the highest curve, 0.4, 0.5, 1.0, 2.5, and 5.0 µM. Other conditions as in (A). (C) Quantities adsorbed as a function of concentration for [Os(bpy)2(Cl)L1]+ at Au. The points are the values of Γe obtained by fitting data such as those in (B) to eq 6. The solid line is a fit of the data to a Langmuir isotherm with Γmax ) 1.1 × 10-10 mol cm-2 and K ) 2.9 × 106 M-1. the electrodes were determined from cyclic voltammetric peak currents for the oxidation of Fe(CN)64-. The microscopic, electrochemically active areas of Au electrodes were determined by oxidative iodine desorption according to the method of Rodriguez et al.16 Surface roughness factors for the Au disk electrodes prepared as described above were 1.6 ( 0.2. Au electrodes which had not been etched in 1:1 H2O/aqua regia had higher roughness factors (∼2.5), correspondingly higher background currents, and less well-defined Faradaic responses. The reported quantities of adsorbed reactants are based on the microscopic area of Au but the geometric area of edge plane graphite electrodes.
Results The previous studies of the adsorption of [Os(bpy)2(Cl)L1]+ were confined to Pt electrodes.1-8 However, adsorption also occurs on other electrode materials. Adsorption on Gold. In Figure 1A is shown a representative set of cyclic voltammograms obtained with a Au electrode in a solution containing 1 µM [Os(bpy)2(Cl)L1]+. The currents, corresponding to the Os(III)/Os(II) couple of the adsorbed complex, gradually increased (16) Rodriguez, J. F.; Mebrahtu, T.; Soriaga, M. P. J. Electroanal. Chem. 1987, 233, 283.
4010 Langmuir, Vol. 12, No. 16, 1996
Campbell and Anson Table 1. Variation with L in the Adsorption and Retention of [Os(bpy)2(Cl)L]+ Complexes at Au and EPG Electrodes ligand La
1010Γ,b mol cm-2
L1 L2 L3 py
0.9 0.2 0.6 0
L1 L2 L3 py
1.0 2.0 2.4 1.0
1010Γ,c mol cm-2
% retentiond
Au Electrode
Figure 2. (A) Repeat of Figure 1A using an EPG electrode. (B) Examples of the time dependence of the adsorption of [Os(bpy)2(Cl)L1]+ on EPG. The concentrations of the complex were (9) 0.61 and (b) 1.08 mM. The data are representative but variations as great as twofold in the quantity adsorbed were obtained in replicate experiments.
after the electrode was placed in the solution until they became steady after about 30 min. The contribution to the currents from the dissolved complex was negligible because of its low concentration, and the peak currents were linearly dependent on scan rate, as expected for an adsorbed reactant. The difference between the anodic and cathodic peak potentials (20-30 mV) and the peak width at half height (120-140 mV) were somewhat greater than expected for an ideally Nernstian couple. The time dependence of the adsorption of the [Os(bpy)2(Cl)L1]+ complex at Au is shown in Figure 1B, and the quantities adsorbed after there were no further changes are shown in Figure 1C as a function of the concentration of the complex in solution. The behavior shown in Figure 1 for Au electrodes closely resembles that reported by Abrun˜a and co-workers at Pt electrodes,1-3 so that similar factors appear to be involved in the strong adsorption at both metals. When the Au electrode was pretreated by exposure to a 10 mM solution of NaI to form an adlayer of iodine atoms on its surface,16 the subsequent adsorption of [Os(bpy)2(Cl)L1]+ was not significantly different in rate or magnitude from that observed at the iodine-free Au surface. Adsorption on Graphite. Strong adsorption of the [Os(bpy)2(Cl)L1]+ complex also occurs at edge plane and basal plane pyrolytic graphite electrodes. We confined our measurements to edge plane graphite (EPG) because the voltammetric wave shapes obtained were more nearly ideal. Shown in Figure 2A are cyclic voltammograms recorded with an EPG electrode in a 1 µM solution of [Os(bpy)2(Cl)L1]+. The voltammograms resemble those in Figure 1A for the Au electrode. The rates of adsorption at EPG were also similar to those on Au, but the total quantities adsorbed were not as reproducible and tended to increase slowly even after hundreds of minutes (Figure 2B). The extent of adsorption was strongly dependent on the pretreatment of the EPG, and variations as large as 100% in the quantities adsorbed were observed in replicate experiments. Electrodes polished to mirror finishes adsorbed less of the complex than more roughly polished electrodes. The primary conclusion to be drawn from Figures 1 and 2 is that unusually strong adsorption of the [Os(bpy)2(Cl)L1]+ complex occurs on Au and EPG as well as on Pt1-7 electrodes.
1.2 0.8 1.0
