ARTICLE pubs.acs.org/Langmuir
Novel Method for Grafting Alkyl Chains onto Glassy Carbon. Application to the Easy Immobilization of Ferrocene Used as Redox Probe Viatcheslav Jouikov† and Jacques Simonet*,‡ † ‡
UMR 6510, Universite de Rennes 1, Campus de Beaulieu, 35042 Rennes Cedex, France (Equipe MaCSE) Sciences Chimiques de Rennes, CNRS, UMR 6226, Universite de Rennes 1, Campus de Beaulieu, Bat 10C, 35042 Rennes Cedex, France
bS Supporting Information ABSTRACT: Primary alkyl iodides (RI) have been found to react with a cathodically charged glassy carbon surface at potentials more negative than 1.7 V vs Ag/AgCl. In aprotic solvents, this reaction results in grafting of the alkyl chains onto carbon. It is proposed that the process corresponds to the cathodic charge of graphitized and fullerenized zones present in carbon followed by a displacement reaction (analogous to a nucleophilic attack) toward alkyl iodides. This new mode of grafting is applied to the immobilization of ferrocene used as an electrochemical probe. The present work points out the reaction of ω-iodoalkylferrocenes and quantifies the level of grafting of alkyl chains via this promising method for modification of carbon surfaces. Coverage levels were found to be high, reaching the apparent surface concentrations of 8 10 9 mol cm 2. These large values are explained on the basis of swelling of the interface provoked by progressive charging of the carbon surface via insertion of tetraalkylammonium cations concomitantly with the substitution process. Alkylferrocene layers deposited onto carbon were found to be chemically and electrochemically stable.
1. INTRODUCTION Glassy carbon (GC) is generally seen as a convenient electrode material for carrying out analyses and electrolyses over a wide range of cathodic and anodic potentials. For a long time, glassy carbon has been considered as an “inert” material made of nonreactive carbon atoms.1 However, this ideal structure does not seem compatible with the quite recent development of surface modification by addition of aryl radicals, since this reaction obviously implies the presence of unsaturated moieties within the GC matrix. This approach mainly concerns chemical or electrochemical reduction of diazonium salts.2 Therefore, the structure of glassy carbon is somewhat complex. Let us recall that GC is industrially prepared by carbonization of phenolic resins3 at temperatures often higher than 1500 °C, and it contains sheets or ribbons of graphite-like structures in discontinuity with “crystallite” boundaries expected to represent all types of carbons prepared this way. In other words, glassy carbon should be seen as a rather complex patchwork of conducting carbon zones (reticulated structures) with admixture of graphitized areas of different size depending on the pyrolysis conditions and the processing temperatures (up to 2500 °C). It appears quite certain that a tight mixture of sp2 and sp3 carbon zones may coexist in the bulk and at the polished surface. Additionally, graphite particles (or those of parent graphenes and fullerenes) may also coexist in the GC with “polyaromatic materials”4 6 that underscores the concomitant presence of oxidized forms such as complex quinones and ketones found to be progressively formed under aging.8 Therefore, when quite negative potentials are reached, r 2011 American Chemical Society
GC—a very disordered graphitic material with nanographite crystallites and fullerenes4 7—should rather be seen as a highly chemically and electrochemically active material, especially when aprotic solvents are considered (E < 2 V vs Ag/AgCl). Thus, in aprotic organic solvents in the presence of electrolytes M+X (with M+ chosen to be electroinactive within the considered potential range), the polarization of GC at < 1.5 V vs Ag/AgCl yields nucleophilic sites reactive enough to modify the interfacial regions to a large extent. On the other hand, it was already reported that highly oriented pyrolytic graphite (HOPG) forms, under cathodic reduction (or charge) in contact with tetraalkylammonium salts (TAAX),9 well-defined insertion stages [Cn ,TAA+] shown to resemble a kind of “carbon amalgam”, stable in the absence of air and being an efficient reductant (especially when n is small) of organic substrates.10 Additionally, it has been reported that the simultaneous insertion of electrons and cations into HOPG produces a nucleophilic material capable to react with a large panel of electrophilic species. Thus, the contact of this material with CO2 permits the formation of a graphite polycarboxylate.11 This reaction can be perceived as a means of efficient sequestration of this effluent and of a cheap formation of an anionic material via transformation of industrial coke. Alkyl bromides were also reported to react in a similar way (graphite alkylation12). The basic idea is that two concomitant processes Received: June 23, 2011 Revised: November 8, 2011 Published: November 10, 2011 931
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such as charge (in mass) and reduction (at surface) of electron acceptors present in GC should be taken into account. If this is true, the exact nature of interfacial reactions [electron transfer(s) vs SN displacement(s)] is worth being reconsidered. For that, the reduction of organic derivatives featuring electrophilic properties was undertaken. In general, organic halides are known to exhibit electrophilic character, and many studies in the field of electrochemical synthesis and analysis concern these species.13 In particular, 1-iodoalkanes (RIs) are reduced at GC according to a twoelectron bond cleavage process that leads mainly to formation of the corresponding alkanes RH. The free radical R• is generally reduced at a potential less negative than the reduction potential observed—in the absence of catalysis—for the two-electron bond scission.14 In aprotic organic solvents, some homodimers can also be obtained due to the Wurtz reaction that implies transient organic carbanions. Additionally, until now, there is no evidence of the grafting of n-alkyl chains onto carbon via a freeradical reaction.15 Interestingly, cathodic voltammetric experiments with 1-iodohexane have shown a progressive inhibition of the electron-transfer process, however, without evidencing any stable organic deposit at the GC surface.16 In the present work, we have considered anew the reduction of RIs (seen in the widest scope) in aprotic polar solvents with a special attention drawn to seeking for evidence of surface modification of glassy carbon and graphite. The building of compact films from primary RIs is described and discussed as a function of the carbon chain length. Furthermore, quantitation of surface deposits prompted us to consider alkyl chains tagged with a ferrocene group. Thus, ω-iodoalkylferrocenes have been used as redox probes, and for the first time, ferrocene moieties in large amounts have been directly fixed onto carbon. Actually, these probes were chosen as being electrochemically active within the anodic range (reversible formation of ferrocenium cation); they provide valuable information outside of the reduction zone of organic iodides along with a better knowledge of electroactive layers prepared at a GC surface. High-density coverage thus can be obtained proving this method to be very promising for efficient deposition of layers of chemical groups and specific functionalities. Specifically, the present work describes the immobilization of ferrocene moiety at a GC surface using the following precursors:
The typical procedure was as follows: 4.7 mL (6.6 mmol) of a 1.4 M solution of t-BuLi in pentane was added dropwise to a solution of ferrocene (1 g, 5.5 mmol) in THF (18 mL) at 0 °C, and the resulting mixture was stirred for 40 min. The corresponding bromochloroalkane Br(CH2)nCl (7.2 mmol, neat or in 10 mL of THF, if solid) was added, and the solution was stirred for 3 h. The reaction mixture was quenched with a saturated aqueous solution of NaCl and extracted with diethyl ether. The organic layer was dried over Na2SO4, the solvent was stripped on a rotary evaporator, and the crude product was separated on a SiO2 column (hexane). Thus, obtained Fc(CH2)nCl was refluxed overnight with a 5-fold excess of KI in dry acetone. After KCl was filtered off through a Celite filter, the solvent was evaporated to give ω-iodoalkylferrocene (quantitatively). 10-Bromodecylferrocene was prepared from 1,10-dibromodecane according to ref 18; 1,2-di(10-bromodecyl)ferrocene was separated as a byproduct and converted into the corresponding diiodide by the treatment with KI as described above. Physical constants pertaining to the alkyl ferrocenes used in this study are given in the Supporting Information (NMR spectra were obtained with a Bruker 300-MHz spectrometer). Solvents and Electrolytes. In the present study, voltammetric experiments were carried out with solutions of tetra-n-butylammonium tetrafluoroborate (TBABF4) in acetonitrile (ACN) or N,N-dimethylformamide (DMF). All these chemicals were obtained from Aldrich and used as such. Supporting electrolyte concentration was in all cases fixed at 0.1 M. None of the experiments described in this work required any special treatments of the solvent electrolyte solutions. In particular, there was no special need to obtain dry solutions; the average amount of residual water in solutions was found to be about 1000 ppm.
Polishing Procedures of GC Electrodes and Cleaning the Electrode Surfaces. For polishing the GC electrodes, Waterproof Silicon Carbide Paper (Struers FEPA P #1200, 2400, and 4000) was used with a TegraSystem polishing machine (Struers). Before each deposition procedure, the electrodes were subjected to a deep and careful polishing in order to eliminate the traces of the preceding grafting since the material modification was found to occur within quite thick layers, ca. 100 μm. After the deposits, the electrodes were sonicated for 2 min in the ultrasonic bath (Bransonic 2000), rinsed with alcohol and acetone, and dried under hot air stream. Electrochemical Experiments. All potentials are referred to the aqueous system Ag/AgCl(sat) connected to the analyte by a bridge filled with 0.1 M TBABF4 solution in the corresponding solvent. Electrochemical instrumentation has been previously described.11,15 Electrodes used for voltammetry had an apparent surface area of 0.8 mm2. Coulometric measurements and electrolyses described in this report were carried out in three-electrode cells with a total catholyte volume of 5 7 mL. A fritted glass diaphragm separated the anode and cathode compartments. Experiments were carried out with small amounts of RIs (typically 0.1 mmol). Efficient argon bubbling in the course of voltammetry and coulometry was always completed, allowing good reproducibility of data. Work-up consisted in rinsing the electrodes in the same solvent and ultrasonic cleaning, followed by rinsing with acetone and drying in a hot air stream (60 °C).
The reported method is perceived as a new paradigm of building modified carbon surfaces for catalysis and analytical purposes.
2. EXPERIMENTAL SECTION
3. RESULTS AND DISCUSSION
Chemicals. Carbon samples used for the described experiments
Reduction of Alkyl Iodides. Experimental evidence presented below, principally concerning the reduction of primary alkyl iodides, deals with GC surface changes that occur during recurrent scans over a quite negative potential range (between 1.5 and 3 V vs Ag/AgCl). In CH3CN or in DMF containing a tetraalkylammonium salt such as TBABF4, the progressive modification of the voltammetric response upon repetitive scans is attributable to alterations in the GC surface.
were purchased from Tokai Carbon Co. (code: GC Rod, for building GC stationary electrodes) and from Carbone-Lorraine (large plates, code: VD 1500 and 2500, thickness 3 mm). Aliphatic iodides (RI) (purchased from Aldrich, purity >98%), ferrocene (Aldrich), and α,ω-dihaloalkanes (Acros) were used without further purifications. THF (Acros) was distilled from sodium benzophenone ketyl. Probes used in the present contribution, ω-haloalkylferrocenes, were synthesized according to a method inspired from ref 17. 932
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Figure 2. Voltammetric response of a small piece of GC (left) and, for comparison, of highly oriented pyrolytic graphite HOPG (right) under cathodic polarization within 1 to 2.5 V. Electrolyte: DMF/0.1 M Bu4NBF4. Scan rate v = 50 mV s 1, T = 22 °C.
dimerization or further reduction.22 Thus, radical addition of R• to carbon should be ruled out. The above greatly suggests the occurrence of a “nucleophilic grafting” at the cathode interface, in parallel with the regular heterogeneous electron transfer. Under these conditions, the proximity (if not adsorption) of the RI compound to the carbon surface would appear quite indispensable. Since at the potentials of diffusion-limited current the surface concentration of nonreduced haloalkane is zero ([RX]s = 0), such reactivity must be favored at the onset of the reduction steps when the surface concentration of RX is not zero. Therefore, it seems plausible that the deposition of alkyl groups onto carbon is mainly related to polarization of the C I bond. Similarly, organic deposits on carbon (under the experimental conditions defined above) have only been reported for primary alkyl bromides.23 The immobilization of alkyl groups, described for iodides, is also feasible with parent bromides at more negative potentials. From the standpoint of the presence of graphite and fullerenes in GC, we suggested the structure of this material to play a strategic role in the grafting process. In order to check this, a small fragment of GC was examined (Figure 2) by cathodic voltammetry. A large reversible step (E0 = 1.91 V) was observed, assigned to superficial electrochemical reduction of graphite zones (via an intercalation process of TBA+ cations) with a charge threshold at about 1.75 V. The same experiment with HOPG reveals very similar voltammetric behavior. The SEM features a dramatic change in the morphology of HOPG upon cathodic charge (Figure 3, B upper). In fact, with large amounts of charge injected in the presence of tetraalkylammonium salts, the HOPG cathode was reported to undergo extensive exfoliation10 directly related to the bulkiness of the concerned cations. Quite similarly (Figure 3, B bottom), a single scan in the absence of any alkyl iodide induces many minor modifications of the superficial structure of the GC cathode, most likely due to local swelling of the material caused by insertion of TBA+ cations into graphitized microzones present at the electrode surface. It is expected that quasi-reversible process (with the threshold at about 1.75 V) only pertains to the outermost planes of GC during the few tens of seconds corresponding to the scan time scale in cyclic voltammetry. Thus, total insertion of TBA+ throughout the entire mass of defects contained in ordinary GC cannot take place.
Figure 1. Voltammetry of alkyl iodides at GC electrodes in DMF + TBABF4. Scan rate: 50 mV s 1. Electrode surface area: 0.8 mm2. Responses of the electrolyte alone given in Aa and Ba. (A) 1-Iodo-octadecane (8 mmol L 1). First 14 scans. (B) 1-Iodooctane (6.5 mmol L 1). First eight scans. Shown below are the responses of rapid test systems at the GC electrodes: A1: (a) response of the solution of 1,2,4,5-tetracyanobenzene at a bare GC electrode; (b) response of the same solution at the modified GC electrode. B1: (a) response of the solution of potassium ferrocyanide in an aqueous saturated KCl at a bare GC electrode; (b) same solution at the modified GC electrode.
Thus, a gradual shift toward more negative potentials is observed. This shift is often quite large (up to 0.4 V) and, concomitantly, decay in the peak current occurs, especially in the case of long alkyl chains. Ultrasonic rinsing shows that the electrode in contact with the original solution does not recover its initial behavior, and the reduction step for the iodoalkane appears definitely shifted. This behavior is exemplified (Figure 1) with 1-iodooctane and 1-iodooctadecane. A surface change that accounts for the progressive potential shift is well supported by voltammetric responses of the resulting carbon surfaces (which is well seen in the case of anodic oxidation of potassium ferrocyanide at a modified and at a freshly polished carbon surface). All primary alkyl iodides CnH2n+1I (with n > 3) exhibit similar behavior. This very profound surface modification puts the question about the mechanism of this process. The hypothesis of free alkyl radical addition to the carbon surface should first be considered, but this mechanism seems very unlikely since the standard potentials of various R•/R couples are precisely located within the potential range (first scan) before the threshold of the two-electron reduction of RI.14 The repetitive scans during the deposition were run at the potentials too negative (E , 1.5 V) to allow reasonably long lifetimes for such transient radicals;19 in addition, rate constants of coupling of free alkyl radicals are close to the diffusion limit (109 1010 L mol 1 s 1 20) which puts heterogeneous interactions with the surface out of competition. Indeed, being of nucleophilic character, Alk• radicals react quite slowly with aromatic compounds nonactivated by electronwithdrawing substituents. Even with benzene, the rate constants amount only to 10 103 L mol 1 s 1 at 25 80 °C.21 At this rate, considering benzene as an elementary homogeneous fragment of graphite, it is clear that even slower heterogeneous radical addition of Alk• to graphite in principle cannot override their 933
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Scheme 2. Schematic Representation of the Grafting of Substituted Alkyl Iodides (RIs) onto a Glassy Carbon Surface (A) via Cathodic Charging of Graphitized Zones Embedded in the Material; (B) Nucleophilic Substitution (SN) Starts at the Charged Graphite Surface (Threshold Potential, 1.7 V) in DMF Containing TBABF4; in the Course of Repetitive Scans Reaching Rather Negative Potentials, the SN Reaction Supposedly Leads to a Quasi-Compact Layer, at Least within the Strongly Graphitized Surface Zones (C)a
Figure 3. Upper row: charge of an HOPG graphite sample in DMF containing 0.1 M TBABF4. (A) Before charge. (B) Morphology change of HOPG after the charge at 2.0 V. Charge injected: 0.1 C cm 2. Bottom row: SEM images of GC electrode surfaces before and after electrochemical reduction. Electrolyte: solution of DMF + 0.1 M TBABF4. (A) Initial glassy carbon surface (Carbone Lorraine). (B) Blank experiment on the same GC sample with a hold of potential at 2.5 V for 5 min.
Scheme 1
a
Concomitant electron transfer toward RI is progressively slowed down as the compactness of the deposit increases.
over that surface. It was established by means of SECM that the deposited organic layer is totally nonconducting. Scheme 1 depicts the different steps that are proposed to occur during the cathodic charging and alkylation of a GC electrode. Equation 1 denotes the partial cathodic charging of GC. Then, as illustrated by eqs 2 and 3, progressive alkylation of the surface takes place via a displacement reaction of superficial sp3 carbons (bearing negative charges) on the C I bond of an organic halide. However, it should be emphasized (see Scheme 2) that these last two processes may occur simultaneously: (a) reduction by electron transfer between the charged mediator(s) and the RI compound on one hand and (b) a purely nucleophilic substitution reaction on the other. The fact that the reduction potential for C I becomes more and more negative during the course of repetitive scans can be understood not only in view of the GC interface blocking but also by the fact that the alkylation progressively modifies the LUMO level of the graphitized mediators. Use of ω-Iodoalkylferrocenes as Probes. At this point, it obviously appears that we do not have reliable information concerning the compactness and the structure of the deposited layer. Processes leading to the deposition could not be perceived on the atomic scale; moreover, the distribution of reacting graphitized islands or nanozones at the GC surface is, by definition, complex and unknown. We decided to study the grafting of RIs
However, when it deals with HOPG, the controlled insertion of electrons leads essentially to almost totally charged substrate which acts as an “electron reservoir” (or a kind of “carbon amalgam”) featuring reducing and nucleophilic properties. The reducing properties of charged graphite toward π-acceptors have been already reported (formation of anion radicals in ESR).10 Moreover, its nucleophilic power is evidenced by a successful and large sequestration of CO2 (an electrophilic species) with electrochemical formation of a graphite polycarboxylate akin to an anion-exchange resin.11 Macroelectrolyses were carried out at a GC surface for obtaining stable deposits that were found to be quite compact, with consequent blocking of the carbon surface as discussed above in the context of voltammetry. In principle, “reductions” by repetitive scans were preferable to those at fixed potential because the latter gave thinner deposits that were much less stable under sonication. In both cases, however, large surface modifications were observed, and the presence of organic deposits could be displayed at different types of carbon such as graphite and GC. Therefore, when a GC electrode is polarized at quite negative potentials, the reactivity of RIs might arise from a similar kind of nucleophilic substitution initiated by reduced zones distributed 934
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Scheme 3
substituted at the terminal (ω) position by a redox probe. For evident reasons, the use of a π-acceptor was discarded; its reduced form should not lead to a homogeneous electron transfer toward the iodide, and its E0 should be more negative than the reduction potential of the RI, both conditions being difficult to satisfy. On the contrary, the use of a π-donor appears suitable because (a) it can be grafted onto GC within the cathodic potential range and (b) its presence can be detected and quantitated by an electroanalysis within the anodic range of potentials. A quite arbitrary choice (presented in Scheme 3) led us to consider ω-iodoalkylferrocenes as suitable target substrates to elucidate the grafting of RIs. Figure 4 displays the electrochemical behavior of Fc C3 I at a glassy carbon electrode. For cathodic scans, the step assigned to the two-electron scission of the C I bond (Ep/2 = 1.77 V) is progressively shifted toward more negative potentials. This behavior is quite similar to that already described above for RIs. On the other hand, the ferrocene moiety of Fc C3 I exhibits a fully reversible process at E0 = 0.4 V (Scheme 3). Ferrocene immobilized onto GC exhibits an almost purely adsorption-like electrochemical response (Figure 4). The difference (ΔE) between the anodic peak and the associated cathodic peak is extremely small (10 mV at a scan rate of 50 mV s 1). Increasing the scan rate reveals that the peak currents are linearly proportional to ν (between 20 and 500 mV s 1). The E0 (+0.40 V) for the soluble substrate Fc(CH2)3I is a little different from that found for immobilized Fc(CH2)3 moieties (+0.58 V). This difference is due to the dative electron-donor effect of pending iodine which stabilizes Fc+ cations; this effect is absent in the grafted Fc. Similarly, n-hexylferrocene has an E0 equal to 0.46 V while the corresponding grafted moiety shows an E0 = 0.51 V. Long chain (n = 10) ferrocenes similarly exhibit E0 = 0.51 V. Immobilization of ferrocene moieties using this process were achieved using several carbon materials (Figure S1). As emphasized above, the apparent amount of deposit depends on the experimental conditions such as the number of cathodic scans and the excursions of potential; however, the amount of deposit cannot be related to the initial surface area of the electrode due to the profound changes that occur during the charging step. Coulometric experiments conducted
Figure 4. 3-Iodopropylferrocene (17 mM). Voltammetry in CH3CN/ 0.1 M TBABF4. Reduction (A1) and oxidation (A2) at a glassy carbon electrode and a scan rate of 50 mV s 1; the two scans are shown separately. (B) Response (v = 100 mV s 1) of the cathodic deposit obtained in the second scan (A1) after a 30 s hold marked with the arrow ( 2.4 V). Ten successive scans between 0 and +0.8 V are superimposed. Surface density of {Fc C3H6} moieties from current integration: 8.2 10 9 mol cm 2. (C) Response of same deposit at scan rates from 20 to 500 mV s 1.
between +0.4 and +0.8 V allowed the quantity of surface-grafted ferrocene moieties to be measured with respect to the initial area of the supposedly perfectly planar electrode. A theoretical surface coverage value for a compact layer of ferrocene, based on the assumption that this species can be represented as a sphere of 6 Å in diameter, is available (1.2 10 10 mol cm 2); this value has been more or less confirmed by the experiment (5 10 10 mol cm 2 for a planar Au electrode24). Other reports concerning ferrocene-terminated self-assembled monolayers and postformation modifications of SAMs based on the use of click chemistry to functionalize organic surfaces were recently published.25 27 Data concerning polymers decorated with ferrocene groups are also available.28 It is noteworthy that all surface coverages given in these reports are significantly smaller than those found in the present work, obtained by integration of ferrocene oxidation peaks shown in Figure 5, curve B. For instance, coverage levels for layers with ferrocene connected to the surface by C4 and C6 linkers are both similar and significantly higher than the above values, being 8.7 10 9 and 8.1 10 9 mol cm 2, respectively. These levels of coverage correspond to the largest values obtained with ferrocenes under the conditions of the present study. Figure 5 shows the SEM images revealing two kinds of deposit. As seen in image A, the singly branched ferrocene causes strong corrosion of the carbon surface. On the other hand, the doubly branched ferrocene Fc(C6 I)2 (image B) leads to the appearance of grooves in quite thick layers that could be due to an intermolecular Wurtz-type polymerization process that takes place along with some localized graftings that permits the polymeric form to resist against intensive ultrasonic rinsing. Such anionic 935
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Figure 5. SEM images of GC modified with Fc C4 I (A) and Fc(C6 I)2 (B). RI concentrations: 10 2 M. Electrolyses in CH3CN. Applied potential: 2.2 V. Charge: 2 C cm 2. Figure 7. Fc C4 I. Concentration: 15 mmol L 1. Electrolyte: DMF/ 0.1 M TBABF4. (A) One-step deposit of Fc at 2.1 V. GC electrode surface area: 7 mm2. Deposit level found: 8.3 10 9 mol cm 2. (B) The same (bare) GC electrode submitted to a charge at 2.8 V in the absence of Fc C4 I and, after charging with a known amount of electricity Q, dipped into the solution of RI (0.02 mol L 1 in CH3CN). Contact time: 2 min. The responses of the resulting deposits are exhibited in B1 (Q = 6 10 3 C; deposit: 7 10 9 mol cm 2) and B2 (Q = 4 10 3 C; deposit: 3 10 9 mol cm 2). (C) For comparison, a smaller GC electrode (area: 0.8 mm2) treated as in (B) and giving, after a charge of 0.8 10 3 C, a deposit of 12 10 9 mol cm 2. All experiments (charge and the following reaction with RI) were performed in a glovebox under an argon atmosphere.
Discussion. The nature of the ferrocene grafting process was worth being discussed quite deeply in order to strongly support the importance of the preliminary charge of the material. For that, we run the experiments given below. We attempted to relate the quantity of charge injected into the carbon cathode to the quantity of electricity subsequently required to oxidize the grafted ferrocene moieties, both being measured in coulombs as illustrated in Figure S2. Surprisingly, when the amount of electricity used to charge the carbon cathode (at the onset of the reduction step) is made smaller and smaller, the amount of grafted ferrocene moieties does not vary much. Thus, the amounts of electricity necessary for the grafting become quite close to that of the electrochemical response of immobilized ferrocene, which could possibly be explained by preadsorption of RI onto carbon, followed by a concerted twoelectron nucleophilic transfer which could be quite similar to an SN process.30 Of course, this pathway omits the intermediacy of radicals in the grafting process. Furtherermore, charge experiments in the absence of Fc Cn I fully support Scheme 1. First, graphite or a glassy carbon electrode was cathodically charged according to reaction 1 in the absence of Fc Cn I under an inert atmosphere in the glovebox. Then, after the cell was disconnected, the electrode was dipped into the solution of Fc C4 I and allowed to react for a few tens of seconds. After the electrode was ultrasonically rinsed and washed with acetone, the grafting of the ferrocene moiety to the electrode was confirmed by voltammetry. These experiments are summarized in Figure 7, showing that the levels of charge in the conventional one-step method and in the separate reaction of the iodide with the preliminary charged GC are quite similar. However, one notes somewhat diffusional character of the immobilized Fc oxidation after the two-step grafting. This phenomenon is supposedly provoked by a strong modification of the surface structure: in two-step experiments the morphology of the
Figure 6. Nyquist plots for the oxidation of ferrocene (5 mM) at a GC electrode bearing immobilized Fc(CH2)6 groups: (a) freshly grafted, fully covered electrode (upper and right scales); (b) same electrode after first “gentle” polishing; (c, d, e) consecutive “harsh” polishing according to the procedure given in the Experimental Section; (f) bare GC electrode after removal of more than 100 μm of the surface layer. Frequency range from 1 MHz to 0.01 Hz. Electrolyte: 0.1 M Bu4NPF6 in CH3CN. Eapp = 0.3 V; ΔE = 10 mV.
polymerization is well-known for α,ω-dihaloalkanes at strongly reducing potentials.29 We verified that the extent of ferrocene grafting is about 10 times greater for image B than for image A. Finally, EDX analyses revealed that these polymeric deposits have a high iron content. Electrochemical impedance measurements have shown that the charge-transfer resistance for ferrocene oxidation at a GC electrode, freshly modified with {Fc(CH2)6} groups, is quite high (RCT = 13 MΩ). This observation attests to the fact that the grafted layer acts as an insulator entirely blocking the solventexposed electrode surface (Figure 6, curve a). Upon progressive polishing of this electrode, RCT drops down to sub-kΩ values (curves b f). Since grafting is expected to occur at a substantial depth into the substrate, these experiments emphasize that several quite harsh buffing’s are necessary to eliminate the original modified layer, whose thickness greatly exceeds (roughly ≈100 μm) the roughness of the polished GC surface, until the impedance of a shiny bare GC electrode is progressively regained. Finally, surface reflectance FTIR spectra have also confirmed the presence of Fc(CH2)n moieties at the carbon surface (Figure S3). 936
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Figure 8. Voltammetry of Fc C6 I (15 mmol L 1) in DMF/0.1 M TBABF4. GC electrode, surface area: 0.8 mm2. Scan rate: 50 mV s 1. Influence of styrene as radical scavenger (concentration: 22 mmol L 1) on the ferrocene deposition at GC electrode (area: 7 mm2) at a fixed potential of 2.2 V. (A) and (A1): No styrene present. Amount of applied charge: 0.1 10 3 C. Estimated grafting level: 10 10 9 mol cm 2. (B) and (B1): Styrene added. Amount of applied charge: 0.15 10 3 C. Grafting level found: 9.2 10 9 mol cm 2.
charged interface evolves in time before the contact with Fc C4 I; this idle time is absent in one-step process when grafting occurs immediately, once the charged site is formed. Second, several reductions of Fc Cn I were achieved in the presence of radical scavengers (like styrene at different excess ratios, as depicted in Figure 8) and spin traps (like N-tert-butyl-αphenylnitrone in large excess). In all cases, the level of grafting was found to remain similar, which provides an additional support to the occurrence of the process according to Scheme 1. Lastly, the grafting of a radical clock such as 6-bromo-1-hexene (whose reductive cleavage would lead to a free hexenyl radical reported to undergo fast first-order exocyclization with ks = 106 s 1 31) was tested under identical conditions. The coverage by 1-hexenyl groups, proved by bromination or iodination with ensuing cathodic dehalogenation of 1,2-dihalo derivatives monitored by coulometry, was found to be high and equivalent to that with alkylferrocenes. This practically excludes the reaction of the cyclized saturated radical that could be quite certainly occurring owing to the low level of free hexenyl radicals at so negative potentials (actually, 0.9 V more negative than the estimated standard potential proposed for the reduction of free parent radicals14).
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oxidation of the grafted ferrocene permitted estimating the apparent surface concentration (ΓFc) of the latter to be ∼8 10 9 mol cm 2. This value is quite high compared to that expected for a compact film at an inert flat surface. Such high concentrations of surface-bound ferrocene are attributable to the fact that cathodic charging of GC and graphite (at E < 1.7 V) occurs concomitantly with a nucleophilic attack on alkyl iodides (RIs) that are present at the electrode solution interface. The occurrence of this process has been demonstrated by experiments in which a carbon electrode is first cathodically charged and then put in the contact with Fc Cn I under an inert atmosphere. Since carbon material has a high porosity, both natural and that induced by cathodic charging, ferrocene moieties are immobilized in a kind of three-dimensional layer. The method presented herein has been found to be successful with a number of electrophilic organic species and should be highly valuable for immobilizing, via polymethylene links of various lengths, a variety of chemical functionalities to afford many kinds of modified carbon electrodes.
’ ASSOCIATED CONTENT
bS
Supporting Information. Voltammetry of iodopropylferrocene at different carbon materials, coulometry of its deposition at glassy carbon, FTIR spectra of the grafted alkylferrocenyl layers, and physical constants of the synthesized haloalkyl ferrocenes. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Tel: +33-2323-6292; Fax: + 33-2323-6732; e-mail: jacques.simonet@ univ-rennes1.fr.
’ ACKNOWLEDGMENT The authors gratefully thank Prof D. Peters (Indiana University, Bloomington) for valuable discussions and comments on the present work. ’ REFERENCES (1) For a review, see: Organic Electrochemistry, 4th ed.; Lund, H., Hammerich, O., Eds.; Marcel Dekker: New York, 2001, and references cited therein. (2) Pinson, J.; Podvorica, F. Chem. Soc. Rev. 2005, 34, 429–439. (3) Jenkins, G. M.; Kawamura, K. Nature 1971, 231, 175–176. (4) Gogotsi, Y.; Libera, J. A.; Kalashnikov, N.; Yoshimura, M. Science 2000, 290, 317–320. (5) Harris, P. J. F. Philos. Mag. 2004, 84, 3159–3167. (6) Bernier, P.; Lefrant, S. Le carbone dans tous ses etats; Gordon and Breach Science Publishers: Amsterdam, 1997, and references cited. (7) McCreery, R. L. Chem. Rev. 2008, 108/7, 2646–2687. (8) Gennaro, A.; Isse, A.; Bianchi, C. L.; Mussini, P. R.; Rossi, R. Electrochem. Commun. 2009, 11, 1932–1935. (9) Besenhard, J. O. Carbon 1976, 14, 111–115. (10) Simonet, J.; Lund, H. J. Electroanal. Chem. 1977, 75, 719–730. (11) Dano, C.; Simonet, J. J. Electroanal. Chem. 2004, 564, 115–121. (12) Bernard, G.; Simonet, J. J. Electroanal. Chem. 1980, 112, 117–125. (13) Peters, D. G. In Organic Electrochemistry, Lund, H., Hammerich, O., Eds.; Marcel Dekker: New York, 2001; p 341 and references cited therein. (14) The reduction of RIs corresponds to a two-electron cathodic scission with a free alkyl radical as transient. Generally, the alkyl radical is
4. CONCLUSION This contribution has revealed a new and efficient protocol for the immobilization of redox-active groups onto carbon through the use of iodoalkyl vectors. For the first time, an electroactive probe such as ferrocene has been covalently grafted onto carbon or graphite to a large extent. Maximum ferrocene coverage has been assessed to be large whatever the length of the alkyl group. Experiments performed under the same conditions with gold or platinum do not allow one to obtain similar deposits, which strongly supports the conclusion that the immobilization of ferrocene is specific for the complex structure of glassy carbon. Measurements of the amount of electricity corresponding to the 937
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