Investigating Electron Transfer in Macromolecular Ruthenium Tris

May 9, 2012 - Further, when these complexes were investigated with collection experiments at a rotating ring–disk electrode, anomalously low values ...
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Investigating Electron Transfer in Macromolecular Ruthenium Tris(bipyridyl) Complexes Using Collection Experiments at a Rotating Ring−Disk Electrode Ryan J. Fealy and Jonas I. Goldsmith* Department of Chemistry, Bryn Mawr College, 101 N. Merion Avenue, Bryn Mawr, Pennsylvania 19010, United States ABSTRACT: Three transition metal complexes, based on the ruthenium tris-bipyridyl unit and containing two, three, and four metal centers, were synthesized and investigated with electrochemical techniques. While the bimetallic complex exhibited very similar properties to the parent ruthenium trisbipyridine, the two larger complexes behaved differently. During cyclic voltammetry, upon the second ligand-based reduction, both the tri- and the tetra-metallic complex precipitated and adsorbed to the electrode surface, as evidenced by the characteristic voltammetric stripping wave observed both upon further reduction and upon reoxidation. Further, when these complexes were investigated with collection experiments at a rotating ring−disk electrode, anomalously low values of the collection efficiency were measured. This effect is attributed to the size of the molecules, which prevents all of the redox centers from being able to simultaneously access the electrode surface, leading to only a portion of the redox active sites on the molecule being oxidized or reduced.

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INTRODUCTION Transition metal complexes have long been utilized for a wide variety of catalytic applications. Owing to their ability to reversibly shuttle between multiple redox states, these species can successfully mediate a variety of processes ranging from bond metathesis1−4 to DNA photocleavage5−7 to the photocatalytic reduction of water.8−19 For various applications, especially those involving light harvesting and energy conversion, the efficiency of the electron transfers mediated by these transition metal-based catalysts is of paramount importance. Much effort has been invested in optimizing such systems, typically via tuning the photo and electrochemical properties of the transition metal complexes by synthetic ligand modifications as well as by varying the nature of the transition metal.20−26 Researchers, including Brewer,17,24,27 Abruña,28,29 and others,30−36 have also investigated the impact of tethering metal centers together, using large multidentate ligands, to create macromolecular assemblies containing multiple transition metal sites. Previous work by Abruña and co-workers has suggested that caution must be taken in assessing electrochemical measurements of such transition metal complex containing macromolecules: the molecules are large enough that electron transfer between the molecule and the surface of a metallic electrode can only occur for a portion of the transition metal complexes present in the macromolecule.37 This results in an artificial (i.e., as an artifact of the molecular structure) decrease in the current observed during electrochemical investigations and the consequent inaccuracies that result from analysis of those experiments. While this issue is of concern during electrochemical experiments, it also has the potential to impact the ability of these macromolecules to serve as effective electron transfer catalysts in light harvesting systems. In a typical quasi-homogeneous system for light © 2012 American Chemical Society

harvesting and water reduction, transition metal complexes serving as photosensitizers (and, as necessary, electron shuttles) are dissolved in a solution that contains colloidal nanoparticles of a noble metal such as platinum or palladium. The reducing equivalents generated when visible light excites the photosensitizers are collected by and accumulated on the colloidal metal particles, which then can reduce water to hydrogen gas. As these systems are solution-based, all components are continually in motion, powered by diffusion and whatever convection happens to be present. As mentioned above, a crucial step in this electron transfer cascade is the electron transfer between a transition metal complex (either the electron shuttle or the photosensitizer depending on the details of the system) and a colloidal metal nanoparticle. As electron transfers are only efficient when partners are very close, the relative time scales of diffusional transport and electron transfer, as described by Abruña et al., are worthy of consideration.37 A possible scenario involves a macromolecule with multiple transition metal complexes encountering a colloidal nanoparticle. If the molecule diffuses away from the colloid prior to adopting appropriate geometric configurations for all of the electron transfer events to occur, much of the light energy captured by that photosensitizer may be wasted (i.e., dissipated through modes other than electron transfer). In order to explore this phenomenon more fully, we have synthesized a series of transition metal complexes containing 1, 2, 3, and 4 ruthenium tris-bipyridine (Ru(bpy)32+) moieties. This well-defined series of molecules was interrogated with a variety of electrochemical techniques, including with collection Received: March 8, 2012 Revised: May 4, 2012 Published: May 9, 2012 13133

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off-white solid. 1H NMR (DMSO): δ 9.09 (s, 3H), 8.67 (br m, 3H), 8.39 (d, 3H), 8.30 (t, 6H), 8.28 (m, 3H), 7.93 (t, 3H), 7.46 (m, 3H), 3.37 (m, 6H), 2.51 (m, 6H), 1.80 (m, 6H). 13C NMR (DMSO) δ 164.4, 157.4, 154.9, 149.9, 148.7, 137.9, 136.5, 130.4, 125.1, 121.4, 120.3, 51.4, 38.1, 26.9. ESI-MS in DMF: 757.2 (M + Na)+, 769.2 (M + Cl)−, 733.3 (M − H)−, and 735.5 (M + H)+. G0 PAMAM Dendrimer (Ethylenediamine Core) with Pendant Bipyridines (tetra∼bpy). Bpy-COOH (150 mg, 0.75 mmol) was refluxed in 40 mL of SOCl2 for 18 h. The SOCl2 was removed under vacuum yielding a yellow solid. The solvent was removed from 550 μL of 20% wt/wt PAMAM dendrimer solution in methanol (0.17 mmol) by heating under vacuum until constant mass was achieved. The dendrimer was dissolved in 2 mL of anhydrous DMF, and the acid chloride from above was dissolved in 20 mL of anhydrous dichloromethane. The dendrimer solution and 1 mL of anhydrous triethylamine was added to the acid chloride solution, and the mixture was stirred at room temperature for 18 h. The product precipitated from the reaction mixture and was collected by vacuum filtration, washed with diethyl ether, and dried under vacuum to yield 130 mg (58% yield) of an off-white solid. 1H NMR (DMSO): δ 9.07 (d, 4H), 8.77 (br s, 4H), 8.68 (m, 4H), 8.40 (t, 8H), 8.28 (d, 4H), 8.09 (br s, 4H), 7.94 (t, 4H), 7.47 (m, 4H), 3.33 (m, 8H), 3.23 (m, 8H), 2.85 (m, 4H), 2.67 (m, 8H), 2.21 (m, 8H). 13C NMR (DMSO) δ 171.4, 164.4, 156.8, 154.1, 149.2, 148.0, 137.1, 135.9, 129.5, 124.4, 120.7, 119.6, 50.4, 49.2, 45.4, 37.9, 32.8. ESI-MS in DMF: 1268.9 (M + Na)+, 1244.9 (M − H)−, 1281.0 (M + Cl)−. General Synthesis of Ruthenium Complexes. Ru(bpy)2Cl2·2H2O (1 equiv) and AgPF6 (2 equiv) were stirred under nitrogen for 2 h in 30 mL acetone. The reaction was vacuum-filtered to remove any solid AgCl. A stoichiometric amount of ligand (bis∼bpy, tris∼bpy, or tetra∼bpy) in 20 mL of anhydrous methanol was added to the Ru(bpy)2(acetone)22+ filtrate, and the reaction mixture was refluxed under nitrogen until ESI-MS monitoring showed full complexation of all of the ligand bipyridines. The solvent was reduced to one-third its original volume, water was added to the reaction mixture, and then aqueous NH4PF6 was added to effect precipitation. The mixture was placed in a freezer overnight (at 0 °C) to ensure complete precipitation, and the resulting solid was collected by vacuum filtration, washed with water and ether, and dried under vacuum. Further purification of these complexes is described below. [Ru(bpy)2]2(bis∼bpy)(PF6)4. Ru(bpy)2Cl2·2H2O (152 mg, 0.29 mmol) and AgPF6 (162 mg, 0.64 mmol) were combined as described above to make the activated ruthenium precursor. To this was added bis∼bpy (64 mg, 0.13 mmol), and the reaction was allowed to proceed to completion as described. Then, 209 mg of an orange-red solid (82% crude yield) was purified through chromatography on alumina (CH2Cl2/ CH3OH, 9:1), followed by size exclusion chromatography (SEC) (Sephadex LH20) with a CH3CN eluent. Fractions from SEC were analyzed by TLC on silica (CH3CN/H2O/KNO3 aq. sat., 20:4:1). Recrystallization by ether vapor diffusion yielded 128 mg (51% yield) of ruby-red crystals. 1H NMR (DMSO): δ 8.95−8.81 (m, 14H), 8.51 (d, 2H), 8.21−8.13 (m, 10H), 7.96 (s, 2H), 7.80−7.70 (m, 10H), 7.58−7.48 (m, 10H), 3.15 (m, 4H), 1.41 (m, 4H), 1.14 (m, 4H). 13C NMR (acetone) δ 163.1, 159.7, 158.0, 157.9, 152.8, 152.6, 151.8, 138.9, 136.4, 134.3, 129.1, 128.7, 128.6, 126.0, 125.1, 124.7, 40.1, 26.7, 15.5. ESIMS in CH3CN: 1742.5 (M + 3PF6−)+, 799.3 (M + 2PF6−)+2,

experiments at a rotating ring−disk electrode (RRDE). Deviations from the theoretical collection efficiency, observed in this work, may be the result of incomplete electron transfer processes, whereby only a subset of the redox centers in a macromolecule are able to undergo electrode transfer while in proximity to the electrode, leaving the molecule only partially oxidized or reduced. As complete and efficient electron transfer is a requirement for a functional light harvesting system, the results that follow are significant as they may serve to illuminate some aspects of the complex interplay between the molecular structure of such catalysts and their ability to carry out the electron transfer events necessary for solar energy conversion.

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MATERIALS AND METHODS Synthesis. Unless otherwise noted, all materials were purchased from Aldrich Chemical Co. and used as received. All ligand synthesis reactions were carried out in oven-dried glassware under nitrogen gas. Anhydrous solvents except THF were Aldrich Sureseal; THF was distilled under N2 from Na/ benzophenone. The compounds 2,2′-bipyridine-5-carboxylic acid (bpy-COOH) and cis-Ru(bpy)2Cl2·2H2O were synthesized as described in the literature.38−40 Ru(bpy)3(PF6)2 was synthesized by refluxing 2,2′-bipyridine for 48 h with a stoichiometric amount of ruthenium chloride hydrate in water/ethylene glycol. The reaction mixture was washed with diethyl ether, the desired complex was precipitated by the addition of NH4PF6, and it was recrystallized from acetonitrile via ether vapor diffusion. NMR spectra were acquired on a 400 MHz Bruker NMR spectrometer. ESI-MS data were collected with a direct-injection method into a Waters Micromass ZQ mass spectrometer. UV−vis spectra were acquired on an Agilent 8453 spectrophotometer. N,N′-(Hexane-1,6-diyl)bis(([2,2′-bipyridine]-5-carboxamide)) (bis∼bpy). Following the method of Madhushree and Biradha,41 1,6-hexamethylene diamine (200 mg, 1.72 mmol) was heated under vacuum and dissolved in 10 mL of anhydrous pyridine, and bpy-COOH (1.0 g, 4.98 mmol) was added to 10 mL of pyridine in a second flask under N2. The contents of the 2 flasks were combined, resulting in a pink slurry, and were stirred for 20 min. Then, 2.25 mL of triphenyl phosphite was added, the mixture was heated to reflux, and the yellow homogeneous solution that resulted was refluxed for 18 h. The solvent was removed under vacuum, and the resulting slurry was rinsed with ethanol and methanol and dried under vacuum, yielding 827 mg (81% yield) of a white solid. 1H NMR (DMSO) δ 8.96 (s, 2H), 8.80 (br s, 2H), 8.73 (m, 2H), 8.47 (t, 4H), 8.34 (m, 2H), 8.0 (m, 2H), 7.53 (m, 2H), 3.40 (m, 4H), 1.65 (m, 4H), 1.30 (m, 4H). 13C NMR (DMSO) δ 164.2, 156.8, 154.3, 149.3, 148.1, 136.0, 135.5, 129.9, 124.5, 120.8, 119.7, 42.1, 28.8, 26.1. ESI-MS in DMF: 481.2 (M + H)+, 503.2 (M + Na)+, and 515.2 (M + Cl)−. N,N′,N″-(Nitrilotris(propane-3,1-diyl))tris(([2,2′-bipyridine]-5-carboxamide)) (tris∼bpy). Bpy-COOH (123 mg, 0.615 mmol) was refluxed in 40 mL of SOCl2 for 18 h. The SOCl2 was removed under vacuum yielding a yellow solid. The acid chloride was dissolved in 20 mL of anhydrous dichloromethane and 38 mg (0.20 mmol) of tris(3-aminopropyl)amine, and 1.0 mL of anhydrous triethylamine was added to the flask and stirred for 18 h. Upon completion, the reaction mixture was added to 25 mL of dichloromethane and washed with sat. NaHCO3 (2 × 25 mL) and water (2 × 25 mL). The organic layer was then dried over anhydrous Na2SO4, and the solvent was removed under vacuum yielding 94 mg (62% yield) of an 13134

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Figure 1. Molecules synthesized in this work. (a) N,N′-(Hexane-1,6-diyl)bis(([2,2′-bipyridine]-5-carboxamide)) (bis∼bpy); (b) N,N′,N″(nitrilotris(propane-3,1-diyl))tris(([2,2′-bipyridine]-5-carboxamide)) (tris∼bpy); (c) G0 PAMAM dendrimer (ethylenediamine core) with pendant bipyridines (tetra∼bpy); (d) {[Ru(bpy)2]2(bis∼bpy)}4+; (e) {[Ru(bpy)2]3(tris∼bpy)}6+; (f) {[Ru(bpy)2]4(tetra∼bpy)}8+. All complexes were synthesized as hexafluorophosphate salts.

484.5 (M + PF6−)+3, 327.1 (M)+4. UV−vis: λmax = 242.0 nm, 280.0 nm, 451.0 nm (ε = 22052 M−1 cm−1). [Ru(bpy)2]3(tris∼bpy)(PF6)6. Ru(bpy)2Cl2·2H2O (256 mg, 0.49 mmol) and AgPF6 (213 mg, 1.08 mmol) were combined

as described above to make the activated ruthenium precursor. To this was added tris∼bpy (103 mg, 0.14 mmol), and the reaction was allowed to proceed to completion as described. Then, 250 mg of a red solid (60% crude yield) was purified 13135

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−0.50 V and +1.20 V until the characteristic cyclic voltammogram of a clean platinum surface was obtained. Electrodes were then rinsed with water, rinsed with acetone, and dried prior to use. The concentration of solutions used for electrochemistry was determined from measuring the absorbance of an aliquot of the solution and was typically in the 0.1−0.3 mM range. Solutions were purged with N2 for 10 min prior to electrochemical experiments, and for all electrochemical experiments where analytical data was extracted, the potential was scanned only over the Ru(II/III) peak.

through chromatography on alumina. Initially, neat acetonitrile was used to elute unreacted precursors and incompletely complexed material, and then, acetonitrile/water (1:1) was used to elute the desired complex, which came off as a tight red band. Size exclusion chromatography (Sephadex LH20) was carried out as described above, and ether vapor diffusion yielded 158 mg (38% yield) of the desired product as red crystals. 1H NMR (DMSO): δ 8.97−8.80 (m, 21H), 8.50 (d, 3H), 8.21−8.12 (m, 15H), 7.82 (s, 3H), 7.80−7.71 (m, 15H), 7.60−7.47 (m, 15H), 3.14 (m, 6H), 2.31 (m, 6H), 1.54 (m, 6H). 13C NMR (acetone): δ 163.3, 159.5, 158.0, 157.4, 153.2, 152.7, 151.9, 138.9, 136.9, 134.4, 129.3, 128.9, 126.3, 125.6, 125.4, 125.0, 55.4, 52.3, 38.8. ESI-MS in CH3CN: 1278.2 (M + 4PF6−)+2, 803.9 (M + 3PF6−1)+3, 566.6 (M + 2PF6−1)+4, 424.3 (M + PF6−1)+5, 329.5 (M)+6. UV−vis: λmax = 242.0 nm, 280.0 nm, 451.0 nm (ε = 35897 M−1 cm−1). [Ru(bpy)2]4(tetra∼bpy)(PF6)8. Ru(bpy)2Cl2·2H2O (209 mg, 0.40 mmol) and AgPF6 (223 mg, 0.88 mmol) were combined as described above to make the activated ruthenium precursor. To this was added tetra∼bpy (100 mg, 0.08 mmol), and the reaction was allowed to proceed to completion as described. Then, 250 mg of a red solid (74% crude yield) was purified through chromatography on alumina as described above for [Ru(bpy)2]3(tris∼bpy)(PF6)6. Size exclusion chromatography (Sephadex LH20) was carried out as described above, and ether vapor diffusion yielded 106 mg (31% yield) of the desired product as red crystals. 1H NMR (DMSO): δ 9.00− 8.85 (m, 28H), 8.55 (m, 4H), 8.18−8.13 (m, 20H), 8.05 (br s, 4H), 7.99 (s, 4H), 7.80−7.65 (m, 20H), 7.60−7.45 (m, 20H), 3.18 (m, 8H), 3.11 (m, 8H), 2.60 (m, 8H), 2.32 (m, 4H), 2.15 (m, 8H). 13C NMR (acetone): δ 173.9, 163.6, 160.0, 158.1, 157.6, 153.0, 152.9, 152.1, 139.1, 136.9, 134.2, 129.3, 128.9, 126.3, 125.6, 125.4, 125.1, 51.3, 50.9, 41.5, 39.4, 34.8. ESI-MS in CH3CN: 1207.9 (M + 5PF6−1)+3, 869.9(M + 4PF6−1)+4, 667.0 (M + 3PF6−1)+5, 531.7(M + 2PF6−1)+6, 435.0 (M + PF6−1)+7, 362.8 (M)+8. UV−vis: λmax = 242.0 nm, 280.0 nm, 451.0 nm (ε = 45030 M−1 cm−1). Electrochemistry. For electrochemical experiments, the reagents employed were acetonitrile dried over molecular sieves, acetone (both 99.9% pesticide residue grade, from Burdick & Jackson), 18 MΩ·cm water, and tetra-n-butylammonium hexafluorophosphate (TBAH) recrystallized from ethanol. Electrochemical measurements were carried out using a Pine Instruments (Grove City, PA) AFCBP1 computer controlled bipotentiostat and AFMSRCE electrode rotator. Measurements were carried out in homemade 1 or 3 compartment cells with 0.1 M TBAH/CH3CN as the supporting electrolyte. A coiled platinum wire was used as a counter electrode, and a silver wire was used as the reference electrode in order to keep the solution nonaqueous. All potentials were subsequently referenced to that of a saturated calomel electrode (SCE) using ferrocene (+0.368 V vs SCE) as an internal standard. The working electrodes (5.0 mm diameter platinum rotating disk electrode and a platinum/platinum rotating ring−disk electrode with a 4.57 mm diameter disk and a ring 0.45 mm thick with a gap of 0.36 mm between the ring and the disk) were purchased from from Pine Instruments. Prior to use, working electrodes were mechanically polished with 1 μm diamond paste. All polishing materials were purchased from Buehler (Lake Bluff, IL). After mechanical polishing, the electrodes were rinsed with water and electropolished by placing the electrode in 0.1 M H2SO4 (Fisher Scientific, TraceMetal grade) and cycling the potential between

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RESULTS AND DISCUSSION Synthesis and Characterization. In Figure 1, the ligands (1a−1c) and the complexes (1d−1f) synthesized in this work are shown. The general methodology used for ligand synthesis involved coupling a carboxylic acid functionalized bipyridine with an amine-terminated scaffold. While the triphenylphosphite mediated coupling reaction41 gave the desired bifunctional ligand (bis∼bpy) in good yield and good purity, the larger ligands were synthesized more effectively via a traditional acid chloride methodology. Overall, these ligand synthesis reactions proceed relatively smoothly and the large ligands tended, conveniently, to precipitate out of solution. The identity of these ligands was confirmed utilizing both 1H NMR and 13C NMR as well as electrospray ionization mass spectrometry (ESI-MS). Typically column chromatography was not necessary, as the material was of sufficient purity for further reactions to be carried out. To synthesize the ruthenium complexes (1d−1f), the ligands were refluxed with the activated ruthenium bis-bipyridine precursor synthesized by replacing the chlorides of Ru(bpy)2Cl2 with solvent molecules via refluxing with AgPF6 in acetone. Initial attempts utilized a relatively short (12−24 h) high temperature reflux in ethylene glycol but ligand scrambling and polymer formation suggested that a lower temperature reflux (as described in the materials and methods section) was a more advantageous method to synthesize the desired complexes. To ensure that reactions went to completion, the progress of the complexation was monitored by ESI-MS: an aliquot was periodically removed and analyzed to assess whether unreacted ruthenium precursor remained. When the reaction was complete, the complex was purified by column chromatography on alumina, followed by size exclusion chromatography and then by recrystallization. In addition to NMR and ESI-MS analysis, the identity of the ruthenium complexes was confirmed by UV−visible (UV−vis) spectroscopy. In Figure 2, the UV−vis spectra of all four complexes (including Ru(bpy)32+) can be seen. A comparison of the spectra of the three complexes synthesized with that of Ru(bpy) 3 2+ demonstrates that there is no significant perturbation to the electronics of the transition metal complex as a result of tethering multiple metal centers together. This is not particularly surprising as the ligands do not possess any conjugation that would enable long-range electronic communication between ruthenium centers. The molar extinction coefficient of each complex at 451 nm was determined, and the inset in Figure 2 shows the extinction coefficient plotted as a function of the number of redox centers present in the complex. As expected, there is a linear relationship between those parameters, with each subsequent redox center adding approximately 11 000 M−1 cm−1 to the value of the molar extinction coefficient. The determination of the molar extinction coefficient was an important diagnostic 13136

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Figure 2. UV−visible spectra of Ru(bpy) 32+ (−−−), {[Ru(bpy)2]2(bis∼bpy)}4+ ( ), {[Ru(bpy)2]3(tris∼bpy)}6+ (----), and {[Ru(bpy)2]4(tetra∼bpy)}8+ (---) in CH3CN. The spectra are scaled so that the peaks are of similar size and offset from one another for ease of viewing. The inset shows the molar extinction coefficient at 451 nm of each complex as a function of the number of Ru(bpy)32+ centers present in each molecule.

Figure 3. Cyclic voltammograms of Ru(bpy) 3 2 + , {[Ru(bpy) 2 ] 2 (bis∼bpy)} 4+ , {[Ru(bpy) 2 ] 3 (tris∼bpy)} 6+ , and {[Ru(bpy)2]4(tetra∼bpy)}8+ in 0.1 M TBAH/CH3CN at a Pt working electrode and at a scan rate of 100 mV/s. The solutions are of varying concentrations, and for the purpose of presentation, the current has been scaled to make each voltammogram of approximately equal size.

criterion in assessing whether complete complexation had occurred. For example, a tetra∼bpy ligand with only two ruthenium complexes has a molar mass of 2653 g/mol and should have an extinction coefficient at 451 nm of approximately 24 000 M−1 cm−1 (i.e., it is a molecule with 2 Ru(bpy)32+ units). Dissolving 10 mg of this partially reacted complex in enough solvent to make 250 mL of solution will yield a solution with an absorbance of 0.36 at 451 nm. If the extinction coefficient of this material is calculated from such an experiment, with the faulty assumption that it consists of f ully f unctionalized [Ru(bpy)2]4(tetra∼bpy)(PF6)8 material, the concentration used in this calculation will be based on the molar mass of [Ru(bpy)2]4(tetra∼bpy)(PF6)8 (4061 g/mol vs the 2653 g/mol above) and a molar extinction coefficient of 36 500 M−1 cm−1 will be calculated. This value is much smaller than the actual extinction coefficient of [Ru(bpy)2]4(tetra∼bpy)(PF6)8 (45 030 M−1 cm−1) and indicates that the material under investigation is not fully complexed. While ESI-MS was very valuable in the characterization of the ruthenium complexes that were synthesized, the use of UV−vis spectroscopy as described above gave important complementary information to ensure that the materials were correctly synthesized. Electrochemical Analysis. In Figure 3, the cyclic voltammograms (CVs) of the three complexes synthesized in this work, as well as the CV of the parent compound Ru(bpy)32+, can be seen. The electrochemical data garnered from these studies is shown in Table 1. One characteristic that all CVs share is the metal-centered Ru (II/III) redox process that occurs at a potential of approximately 1.26 V vs SCE. That the potential of this redox process does not vary indicates that the inherent nature of the metal center is not perturbed by being incorporated into

Table 1. Electrochemical Data E1/2 (V vs SCE) cmpd

Ru2+/ 3+ Ru

L3/ L2L−

L2L−/ LL22−

LL22−/ L33−

Ru(bpy)32+ {[Ru(bpy)2]2(bis∼bpy)}4+ {[Ru(bpy)2]3(tris∼bpy)}6+ {[Ru(bpy)2]4(tetra∼bpy)}8+

+1.26 +1.26 +1.28 +1.28

−1.36 −1.22 −1.17 −1.15

−1.55 −1.55 −1.48a −1.44a

−1.80 −1.78 −1.77b −1.77b

a

The anodic portion of the process has the appearance of a stripping peak, as described below. bThe cathodic portion of this process has the appearance of a stripping peak, as described below, and the anodic wave is considerably diminished in size.

a macromolecular structure. The parent compound, Ru(bpy)32+, displays three ligand localized redox processes at −1.36 V, −1.55 V, and −1.80 V vs SCE, corresponding to the sequential reduction of the three bipyridine ligands. The three complexes synthesized in this work all have their first ligandlocalized redox process at a slightly less negative potential than that of Ru(bpy)32+. This is likely the result of the amide functionality appended to the 5′ position of the bipyridine that is part of each of the ligands shown in Figure 1a,c,e. The presence of this electron withdrawing group will make that particular bipyridine slightly easier to reduce than an unsubstituted bipyridine. The CV of the bimetallic complex {[Ru(bpy)2]2(bis∼bpy)}4+ (Figure 3b) is very similar to that of the parent Ru(bpy)32+ (Figure 3a). The second and third 13137

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ligand-localized redox processes of {[Ru(bpy)2]2(bis∼bpy)}4+ occur at effectively the same potentials as in Ru(bpy)32+ and are attributed to the reduction of the unfunctionalized bipyridine ligands on the outside of each metal center. The electrochemical behavior of {[Ru(bpy)2]3(tris∼bpy)}6+ is considerably different than that of Ru(bpy)32+. In Figure 3c, the complex is subjected to two ligand-localized reductions, and then the potential is reversed prior to the third reduction. The oxidation that occurs after potential reversal has the sharp shape characteristic of a stripping peak that results from the electrochemical desorption of adsorbed material. After the second ligand-localized reduction, the complex is neutrally charged and, as is common for large uncharged molecules, tends to adsorb to the electrode surface. Upon oxidation, the complex reacquires a positive charge and is oxidatively desorbed from the surface, resulting in the observed stripping peak. In Figure 3d, the potential is not reversed after the second ligand-centered reduction process, and the complex is subjected to its third reduction process. As in Figure 3c, the complex is neutrally charged after the second reduction and deposits on the electrode surface. When the complex is reduced for a third time, it becomes negatively charged and is reductively desorbed (in contrast to the oxidative desorption seen in Figure 3c) from the electrode surface, resulting in a sharp, reductive stripping peak at −1.77 V vs SCE. Similar behavior is seen in the CVs of {[Ru(bpy)2]4(tetra∼bpy)}8+ (Figure 3e,f). The first ligandcentered reduction is reversible, but when the complex becomes neutral after the second reduction, it also deposits on the electrode surface and will either be oxidatively or reductively desorbed, depending on the potential program of the CV experiment. While such behavior has been observed for large, dendritic, macromolecules,28,29 the nature of the series of complexes investigated in this work allows for this effect to be investigated in more detail. In this case, a molecule with three or more redox centers tethered together is required for the adsorption and stripping behaviors to begin to arise. Further electrochemical characterization that was carried out on these molecules involved the determination of their diffusion coefficients. These measurements were undertaken as the diffusion coefficient is an important physical parameter to determine for a newly synthesized electroactive molecule; it is via diffusion that electroactive material can come in contact with the surface of an electrode, enabling electron transfer events to occur. A common electrochemical method for determining diffusion coefficients utilizes voltammetry at a rotating disk electrode (RDE) and involves acquiring electrochemical data as various rotation rates of the electrode. The limiting current in such an experiment is described by the Levich equation below:42−44 Il = 0.62nFADo 2/3ω1/2ν−1/6Co*

Figure 4. Voltammograms at a rotating disk electrode of {[Ru(bpy)2]2(bis∼bpy)}4+ in 0.1 M TBAH/CH3CN at a variety of electrode rotation rates. The concentration of redox centers in this solution was 0.36 mM, and the potential sweep rate was 20 mV/s. The inset shows the linear plot of limiting current vs the square root of the electrode rotation rate (see eq 1) used to calculate the diffusion coefficient. The values of the limiting current were measured at 1.50 V vs SCE.

Diffusion coefficient determinations were carried out for all four complexes (including Ru(bpy)32+) via the RDE technique, and the results can be seen in Table 2. The values are an average of the results from several (typically 3−5) separate experiments. Table 2. Diffusion Coefficients, Obtained from RDE Experiments, of All Compounds cmpd Ru(bpy)32+ {[Ru(bpy)2]2(bis∼bpy)}4+ {[Ru(bpy)2]3(tris∼bpy)}6+ {[Ru(bpy)2]4(tetra∼bpy)}8+

diffusion coefficient (cm2/sec) 3.8 3.0 2.5 2.1

× × × ×

10−6 10−6 10−6 10−6

± ± ± ±

0.4 0.3 0.2 0.3

× × × ×

10−6 10−6 10−6 10−6

The diffusion coefficients shown in Table 2 are within a reasonable range of the literature values for similar compounds.37 As expected, the larger molecules have smaller diffusion coefficients although the differences are not particularly dramatic. As the measured values of the diffusion coefficients depend on the extent of solvation, the presence of counterions as well as the size, shape, and configuration of these molecules, it is not clear that significant insight can be drawn from a comparison of the values in Table 2. The purpose for which these molecules were synthesized was to explore a phenomenon, suggested in the literature, whereby large molecules with multiple redox centers may not be adequately sampled by the surface of an electrode during interfacial electron transfer events.37 As electron transfer is very short-range and these molecule are large, not every redox site is simultaneously in close enough proximity to the electrode surface for efficient electron transfer to occur; what may result is the incomplete oxidation or reduction of the molecule. In

(1)

where n is the number of electrons transferred in a redox event (n = 1), F is the Faraday constant (96 485 C/mol of electrons), A is the surface area of the electrode, Do is the diffusion coefficient, ω is the rotation rate of the electrode, ν is the kinematic viscosity of the solution (taken to be 0.005 cm2/sec for 0.1 M TBAH/CH3CN), and Co* is the bulk concentration of the analyte. In Figure 4, an example of the voltammograms that result from changing the rotation rate of an RDE are shown. The inset shows the plot of the limiting current vs the square root of the rotation rate used to calculate the diffusion coefficient. 13138

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state and cannot be further reduced at the ring electrode. However, when Ru(bpy)32+ begins to be oxidized at the disk electrode, the oxidized Ru(III) that encounters the ring electrode will be reduced back to Ru(II), and reduction current at the ring is observed. If the ratio of the current from the ring electrode to the current from the disk electrode is measured, the collection efficiency that results has a value of 0.22 and does not change as the rotation rate is changed. When the theoretical collection efficiency is calculated, using the dimensions of the electrode stated in the materials and methods section, a value of 0.22 is the result, in good agreement with experiment. One can imagine that the following scenario results in a collection efficiency of 0.22:100 molecules are oxidized at the disk electrode, 22 of those molecules are transported hydrodynamically to the ring electrode, and all 22 of those oxidized molecules get reduced at the ring electrode. In Figure 6 are the collection experiments that were carried out on the other complexes described in this work. As is clear from visual inspection, the relative magnitude of the current at the ring electrode decreases as the size of the complex increases. This clearly indicates that the collection efficiency is different in each experiment; the calculated collection efficiencies can be seen in Table 3. While unlikely, it is conceivable that an irreversible chemical reaction of the oxidized molecule might be the cause of this effect, so to investigate that possibility, cyclic voltammograms at a variety of potential sweep rates were acquired for a solution containing the tetrametallic {[Ru(bpy)2]4(tetra∼bpy)}8+ complex. As can be seen in Figure 7A, the ΔEpeak and the ratio of the peak currents does not change appreciably as the potential sweep rate is modified, indicating that, at least for the time scale investigated, the Ru(II/III) redox process is reversible. In Figure 7B, collection experiments at several rotation rates for the trimetallic {[Ru(bpy)2]3(tris∼bpy)}6+ complex are shown. If there were chemical reactions following the oxidation of the complex, one would expect that the collection efficiency would increase as the electrode rotation rate increased (i.e., as the amount of time for the molecule to get from the disk to the ring decreased). The more quickly the oxidized material traveled to the ring, the less opportunity there would be for degradation to occur. However, the collection efficiency is ca. 0.11 for all three of the rotation rates shown in Figure 7B. As the data shown in Figure 7 are consistent with a reversible redox process, we postulate that it is the incomplete sampling of the macromolecules by the electrode, as described above, that leads to the decrease in the observed collection efficiency. From these results, it appears that the bimetallic {[Ru(bpy)2]2(bis∼bpy)}4+ complex has electrochemical behavior very similar to that of Ru(bpy)32+. The tether linking the two metal centers does not appear to interfere materially with the ability of both redox centers to be in close proximity to the surface of the electrode for sufficient time to allow both electron transfer events to occur. However, this is not the case for the {[Ru(bpy)2]3(tris∼bpy)}6+ species that contains three redox-active metal centers. In order for oxidation to occur, the metal center must be in close proximity to the surface of the electrode, as electron transfer rates are very strongly attenuated by distance. Since molecules are free to move in solution, by diffusive transport, each ruthenium center must remain in the vicinity of the electrode for a long enough time to allow electron transfer to occur. For a single metal center, this is a relatively straightforward process. However, for a complex such as {[Ru(bpy)2]3(tris∼bpy)}6+, when one of the metal centers is

many light-harvesting systems, the reducing equivalents are transferred, in solution, from transition metal complex-based catalysts to colloidal metal nanoparticles.10,12,14,15,20 It is important to understand whether a macromolecule with multiple metal centers can effectively utilize all of its redox capability and transfer electrons from all of its metal centers during an encounter with a metal surface (i.e., the colloid). In order to investigate the possibility of this incomplete electron transfer effect in such systems, collection experiments using a rotating ring−disk electrode (RRDE) were carried out on the four complexes described above. In a RRDE collection experiment, molecules are oxidized at the disk electrode, flung radially outward to the ring electrode, and then reduced at the ring electrode. Because the collection experiment involves sequential electron transfer events, it can serve as a sensitive probe of the efficiency of the electron transfer process. A mathematical description of the behavior of the RRDE was carried out by Levich and a characteristic quantity that results is the collection efficiency, which is defined as the ratio of the current at the ring electrode to the current at the disk electrode.42−44 The collection efficiency is invariant with electrode rotation rate and depends, in the absence of coupled chemical reactions, only on the geometry of the ring and the disk electrodes. If deviations from the theoretical collection efficiency occur, they are often attributed to chemical reactions coupled to redox processes; i.e., the material oxidized at the disk undergoes a chemical reaction and becomes a species irreducible by the ring prior to reaching the ring electrode. However, such chemical reactions are unlikely to be occurring in the systems investigated; the electrochemistry of the Ru(bpy)32+ motiey is well-known to be reversible. Shown below in Figure 5 is an example of a collection experiment using Ru(bpy)32+. When no oxidation is occurring at the disk electrode, the material swept past the ring electrode is already in its reduced

Figure 5. Voltammograms at a rotating ring−disk electrode of Ru(bpy)32+ in 0.1 M TBAH/CH3CN at a variety of electrode rotation rates. The solid symbols indicate the response from the disk electrode, and the open symbols are from the ring electrode. The potential sweep rate was 20 mV/s, and the potential of the ring was held at 0.1 V. 13139

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Figure 6. RRDE collection experiments for (a) {[Ru(bpy)2]2(bis∼bpy)}4+, (b) {[Ru(bpy)2]3(tris∼bpy)}6+, and (c) {[Ru(bpy)2]4(tetra∼bpy)}8+ in 0.1 M TBAH/CH3CN. For all experiments, the electrode rotation rate was 1000 rpm, the potential sweep rate was 20 mV/s, and the ring potential was held at 0.1 V.

determined by geometry) would still travel from the disk to the ring electrode. However, only 22 of those 44 would be in an oxidized state (i.e., reducible by the ring). At the ring electrode, the 50% incomplete sampling of the 22 oxidized redox centers would occur again: only 11 of those 22 oxidized redox centers would be reduced by the ring electrode. The current at the ring would reflect 11 electron transfers, the current at the disk would reflect 100 electron transfers, and the measured collection efficiency would be 0.11 instead of the expected 0.22. In this way, one may interpret collection efficiencies that vary from the expected value as indicative of complexity in the interaction between the electrode surface and the redox-active molecule and of the existence of the incomplete sampling behavior suggested in the literature. The degree of incomplete sampling that would lead to the measured collection efficiencies can been seen in Table 3. The data from the collection experiments shown in Table 3 suggest that, while this effect is negligible for the bimetallic {[Ru(bpy)2]2(bis∼bpy)}4+ complex, it is significant both for {[Ru(bpy)2]3(tris∼bpy)}6+ and for {[Ru(bpy)2]4(tetra∼bpy)}8+. As would be expected, the collection efficiency becomes smaller for the larger molecules. As a molecule gets larger, when one redox center is near the surface of the electrode, the other redox centers will be farther and farther away, reducing the chance that they will be able to participate in electron transfer reactions. The well-defined

Table 3. Results from RRDE Collection Experiments cmpd

collection efficiency

fraction of redox centers sampled by electrode

Ru(bpy)32+ {[Ru(bpy)2]2(bis∼bpy)}4+ {[Ru(bpy)2]3(tris∼bpy)}6+ {[Ru(bpy)2]4(tetra∼bpy)}8+

0.22 0.21 0.11 0.06

100% 95% 50% 27%

near the electrode, the other ones may be at a considerable distance such that electron transfer events are not feasible. There will be a competition between rotational diffusion, which will bring the distant redox centers closer to the electrode, and translational diffusion, which will move the entire macromolecule away from the electrode.37 If the whole molecule moves away from the electrode before all of the redox centers have had the opportunity to tumble into a position near the electrode surface, the result will be a complex in which only a portion of the redox centers are oxidized. When that incompletely oxidized complex approaches the ring electrode, the same phenomenon will occur, and only a portion of those oxidized sites will be reduced. The diminution in current that results will lead to a collection efficiency smaller than theoretically predicted. For example, if 200 redox centers come to the disk electrode, only 100 might be oxidized (i.e., 50% incomplete sampling), but 44 redox centers (22%,

Figure 7. (A) Cyclic voltammograms at a variety of potential sweep rates for a solution of {[Ru(bpy)2]4(tetra∼bpy)}8+ in 0.1 M TBAH/CH3CN. (B) RRDE collection experiments at various electrode rotation rates for a solution of {[Ru(bpy)2]3(tris∼bpy)}6+ in 0.1 M TBAH/CH3CN; the potential sweep rate was 20 mV/s, and the ring potential was held at 0.1 V. 13140

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The Journal of Physical Chemistry C series of molecules investigated herein clearly shows where the onset of this behavior is and indicates at what juncture the morphology of these macromolecules begins to be a significant factor in their ability to undergo heterogeneous electron transfer processes.

ACKNOWLEDGMENTS

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REFERENCES

This work was supported by the Bryn Mawr College Office of the Provost and the Pittsburgh Conference National Memorial College Grants Program.

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CONCLUSIONS In this work, a series of complexes (bimetallic, trimetallic, and tetrametallic) based on Ru(bpy)32+ was synthesized in order to examine how their electrochemical behavior changed as more redox centers were tethered together. From basic cyclic voltammetry (see Figure 3b) to RRDE collection experiments, the bimetallic complex {[Ru(bpy)2]2(bis∼bpy)}4+ behaved very similar to Ru(bpy)32+. However, once more than two redox centers were a part of a macromolecule, the characteristics of these larger complexes began to differ from those of Ru(bpy)32+. In Figure 3c−f, cyclic voltammetry experiments demonstrate that, upon sufficient reduction to yield a neutral complex, both the trimetallic and tetrametallic complex come out of solution and adsorb to the surface of the electrode; this behavior was not seen for the smaller complexes. When RRDE collection experiments were carried out on {[Ru(bpy)2]3(tris∼bpy)}6+ and {[Ru(bpy)2]4(tetra∼bpy)}8+, the collection efficiencies that resulted were far smaller than expected. The likely explanation for this behavior is that those two complexes are of sufficient size that it is not possible for all of the pendant redox centers to be simultaneously addressed, oxidized or reduced, by the electrode. These results have significant implications in the design of macromolecular systems for catalysis and light harvesting applications, where electron transfers are key processes and where the use of large dendritic macromolecules with multiple redox centers has often been suggested. If, for example, in a classical light harvesting system, a macromolecular electron shuttle is too large to deposit all of its reducing equivalents on a noble metal colloid in a single encounter, it might be more advantageous to use in its place an electron shuttle with fewer redox centers tethered together or one with a more geometrically advantageous configuration of redox centers. As the construction of large, macromolecular catalytic and light harvesting systems is quite complex, a clear assessment of both the benefits and the drawbacks of various geometric architectures employed in these systems is important in guiding such efforts. Further work is currently ongoing to utilize the complexes synthesized herein, as well as related iridium complexes, in light harvesting water reduction systems in order to directly gauge how the presence of multiple redox centers impacts the ability of a molecule to efficiently mediate the electron transfer reactions necessary for using solar energy to convert water to hydrogen. However, the results from the work presented here point toward the conclusion that, for interfacial electron transfer systems such as the ones investigated, the size and the shape of the molecules involved do matter.

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(1) Trnka, T. M.; Grubbs, R. H. Acc. Chem. Res. 2001, 34 (1), 18−29. (2) Suzuki, A. J. Organomet. Chem. 1999, 576 (1−2), 147−168. (3) Miyuara, N.; Suzuki, A. Chem. Rev. 1995, 95 (7), 2457−2483. (4) Negishi, E. Acc. Chem. Res. 1982, 15 (11), 340−348. (5) Armitage, B. Chem. Rev. 1998, 98 (3), 1171−1200. (6) Erkkila, K. E.; Odom, D. T.; Barton, J. K. Chem. Rev. 1999, 99 (9), 2777−2796. (7) Holder, A. A.; Swavey, S.; Brewer, K. J. Inorg. Chem. 2004, 43 (1), 303−308. (8) Balzani, V.; Credi, A.; Venturi, M. ChemSusChem 2008, 1 (1−2), 26−58. (9) McDaniel, N. D.; Bernhard, S. Dalton Trans. 2010, 39 (42), 10021−10030. (10) Moradpour, A.; Amouyal, E.; Keller, P.; Kagan, H. Nouv. J. Chim. 1978, 2, 547−549. (11) McCormick, T. M.; Calitree, B. D.; Orchard, A.; Kraut, N. D.; Bright, F. V.; Detty, M. R.; Eisenberg, R. J. Am. Chem. Soc. 2010, 132 (44), 15480−15483. (12) Kalyanasundaram, K.; Kiwi, J.; Gratzel, M. Helv. Chim. Acta 1978, 61 (7), 2720−2730. (13) Esswein, A. J.; Nocera, D. G. Chem. Rev. 2007, 107 (10), 4022− 4047. (14) Ziessel, R.; Hawecker, J.; Lehn, J. M. Helv. Chim. Acta 1986, 69 (5), 1065−1084. (15) Brown, G. M.; Brunschwig, B. S.; Creutz, C.; Endicott, J. F.; Sutin, N. J. Am. Chem. Soc. 1979, 101 (5), 1298−1300. (16) Goldsmith, J. I.; Hudson, W. R.; Lowry, M. S.; Anderson, T. H.; Bernhard, S. J. Am. Chem. Soc. 2005, 127 (20), 7502−7510. (17) Elvington, M.; Brown, J.; Arachchige, S. M.; Brewer, K. J. J. Am. Chem. Soc. 2007, 129 (35), 10644−10645. (18) Du, P.; Schneider, J.; Luo, G.; Brennessel, W. W.; Eisenberg, R. Inorg. Chem. 2009, 48 (11), 4952−4962. (19) Du, P.; Knowles, K.; Eisenberg, R. J. Am. Chem. Soc. 2008, 130 (38), 12576−12577. (20) Curtin, P. N.; Tinker, L. L.; Burgess, C. M.; Cline, E. D.; Bernhard, S. Inorg. Chem. 2009, 48 (22), 10498−10506. (21) Wang, X.; Goeb, S.; Ji, Z.; Pogulaichenko, N. A.; Castellano, F. N. Inorg. Chem. 2011, 50 (3), 705−707. (22) Cline, E. D.; Adamson, S. E.; Bernhard, S. Inorg. Chem. 2008, 47 (22), 10378−10388. (23) Lowry, M. S.; Bernhard, S. Chem.Eur. J. 2006, 12 (31), 7970− 7977. (24) Arachchige, S. M.; Brown, J. R.; Chang, E.; Jain, A.; Zigler, D. F.; Rangan, K.; Brewer, K. J. Inorg. Chem. 2009, 48 (5), 1989−2000. (25) Probst, B.; Guttentag, M.; Rodenberg, A.; Hamm, P.; Alberto, R. Inorg. Chem. 2011, 50 (8), 3404−3412. (26) Du, P.; Schneider, J.; Jarosz, P.; Zhamg, J.; Brennessel, W. W.; Eisneberg, R. J. Phys. Chem. B 2007, 111 (24), 6887−6894. (27) Knoll, J. D.; Arachchige, S. M.; Wang, G.; Rangan, K.; Miao, R.; Higgins, S. L. H.; Okyere, B.; Zhao, M.; Croasdale, P.; Magruder, K.; et al. J. Inorg. Chem. 2011, 50 (18), 8850−8860. (28) Takada, K.; Díaz, D. J.; Abruña, H. D.; Cuadrado, I.; Casado, C.; Alonso, B.; Morán, M.; Losada, J. J. Am. Chem. Soc. 1997, 119 (44), 10763−10773. (29) Storrier, G. D.; Takada, K.; Abruña, H. D. Langmuir 1999, 15 (3), 872−884. (30) Alonso, E.; Valerio, C.; Ruiz, J.; Astruc, D. New J. Chem. 1997, 21 (11), 1139−1141. (31) Moulines, F.; Djakovitch, L.; Gloaguen, B.; Thiel, W.; Fillaut, J.L.; Delville, M.-H.; Astruc, D.; Boese, R. Angew. Chem., Int. Ed. 1993, 32 (7), 1075−1077.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 13141

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(32) Denti, G.; Serroni, S.; Campagna, S.; Ricevuto, V.; Balzani, V. Inorg. Chim. Acta 1991, 182 (2), 127−129. (33) Constable, E. C. Chem. Soc. Rev. 2007, 36, 246−253. (34) Astruc, D. Acc. Chem. Res. 2000, 33 (5), 287−298. (35) Balzani, V.; Campagna, S.; Denti, G.; Juris, A.; Serroni, S.; Venturi, M. Acc. Chem. Res. 1998, 31 (1), 26−34. (36) Newkome, G. R.; Moorefield, F.; Vögtle, F. Dendritic Molecules: Concepts, Synthesis, Properties; VCH: Weinheim, Germany, 1996. (37) Goldsmith, J. I.; Takada, K.; Abruña, H. D. J. Phys. Chem. B 2002, 106 (34), 8504−8513. (38) Kiehne, U.; Bunzen, J.; Staats, H.; Lutzen, A. Synthesis 2007, 7, 1061−1069. (39) Panetta, C. A.; Kumpaty, H. J.; Heimer, N. E.; Leavy, M. C.; Hussey, C. L. J. Org. Chem. 1999, 64 (3), 1015−1021. (40) Sullivan, B. P.; Salmon, D. J.; Meyer, T. J. Inorg. Chem. 1978, 17 (12), 3334−3341. (41) Madhushree, S.; Biradha, K. Cryst. Growth Des. 2006, 6 (1), 202−208. (42) Bard, A. J.; Faulkner, L. R. Electrochemical Methods, 2nd ed.; J. Wiley & Sons: New York, 2001; Chapter 9. (43) Albery, W. J.; Hitchman, M. L. Ring Disc Electrodes; Clarendon Press: Oxford, U.K., 1971. (44) Levich, V. G. Physicochemical Hydrodynamics; Prentice Hall: Englewood Cliffs, NJ, 1962.

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