Mechanistic Aspects of the Electrochemical Reduction of 7, 7, 8, 8

Physical and Theoretical Chemistry Laboratory, Oxford UniVersity, South Parks ... Department of Materials Chemistry, Graduate School of Engineering, K...
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J. Phys. Chem. B 1998, 102, 6588-6595

Mechanistic Aspects of the Electrochemical Reduction of 7,7,8,8-Tetracyanoquinodimethane in the Presence of Mg2+ or Ba2+ Munetaka Oyama,† Richard D. Webster, Marco Sua´ rez, Frank Marken, and Richard G. Compton* Physical and Theoretical Chemistry Laboratory, Oxford UniVersity, South Parks Road, Oxford OX1 3QZ, U.K.

Satoshi Okazaki Department of Materials Chemistry, Graduate School of Engineering, Kyoto UniVersity, Sakyo-ku, Kyoto 606-01, Japan ReceiVed: April 24, 1998; In Final Form: June 12, 1998

The effect of aggregation of Mg2+ or Ba2+ (M2+) and electrochemically generated reduced forms of 7,7,8,8tetracyanoquinodimethane (TCNQ) in acetonitrile solution was studied by voltammetric and chronoamperometric techniques, the electrochemical quartz crystal microbalance (EQCM), ESR spectroscopy, and atomic force microscopy (AFM). Although the monoreduced form, TCNQ•-, appears not to interact strongly with either metal cation on the voltammetric time scale, the dianion TCNQ2- was found to interact and to form a thin layer deposit of the type M(TCNQ). The potential at which this occurs was less negative than the potential required for the formation of the free dianion in solution, reflecting the binding with the metal cation. Unusually, the initial deposition of M(TCNQ) triggered the deposition of a material possibly M(TCNQ)2 as evidenced by, first, a characteristic voltammetric response, second, a strong ESR signal, and third, the gravimetrically determined deposited mass per charge passed. Both types of deposit gave distinct voltammetric signatures, and an overall scheme for the process is proposed.

Introduction Ion pair formation between electrophilic metal cations and electrochemically reduced species in organic solvent systems can strongly affect the nature and rate of the overall pathway of the associated electrode reaction as detectable in the form of characteristic changes in voltammetric responses.1-3 The key steps in these processes involve ion pair formation or the interaction of ions in aggregates, which, depending on solubility and local concentration, may give rise to deposition processes. In a previous paper4 on the interaction between the 2,3-dichloro5,6-dicyano-1,4-benzoquinone (DDQ) dianion (DDQ2-) and Na+ in acetonitrile, we were able to demonstrate that even in the presence of deposition some interfacial electron-transfer processes may continue and an overall very complex reaction pathway is followed, comparable to the case of a mediatormodified or gated electrode.5 Both the type and reduction state of the anion and the properties of the metal cation are critical in determining the interaction and reactivity. For instance, as observed in our previous work4 the dianion (DDQ2-) was more reactive toward cationic species than the radical anion (DDQ•-). Further, while Na+2DDQ2- is formed as a deposit at the electrode surface, the voltammetric responses for the two-step reduction of 7,7,8,8tetracyanoquinodimethane (TCNQ) are not affected by the presence of Na+,4 indicating little or no interaction between TCNQ2- and Na+. However, in the presence of dications such as Mg2+ or Ba2+, which typically display ion-pairing interactions * To whom correspondence should be addressed. Telephone: +44-1865275413. Fax: +44-1865-275410. E-mail: [email protected]. † On leave from Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Japan.

stronger than those of the alkali metal cations, changes in the voltammetric responses indicating adsorption and deposition reactions associated with the interactions between TCNQ2- and M2+ (Mg2+ and Ba2+) may be expected. In the present paper, we therefore analyze details of the electrode reaction mechanism for the reduction of TCNQ in the presence of the alkaline earth metals M2+. Another aspect of the redox chemistry of TCNQ is the wellknown tendency to form organic charge-transfer complexes; numerous complexes containing the TCNQ anion radical (TCNQ•-) have been reported.6-8 There are only few reports concerning complexes containing TCNQ2-,9,10 although the formation of the dianionic state is known to be a key feature in forming materials with a high electrical conductivity.7,8 Although the electrochemical formation of TCNQ2- in acetonitrile has been known for many years,2,11-13 there has been less work on the interactions of TCNQ2- with metal cations except most notably the studies of Li+ and Na+ interaction by Khoo et al.2,14 A good knowledge of the redox and interfacial chemistry is also of considerable interest for the synthesis of TCNQ salts,8,15 which, when based on electrochemical procedures, could result in the formation of materials inaccessible or not usually obtained by other conventional synthetic techniques.16 Finally, recent developments in the mechanistic understanding of interfacial redox processes involving solid materials17 present in the form of microcrystals at the electrode|solution (electrolyte) interface suggest that ion adsorption and double-layer effects at the surface of the redox-active solid can be important in the context of the overall process. The aim of this paper is to study in detail the reaction of M2+ (Mg2+, Ba2+) with reduced forms of TCNQ by voltammetry, in situ electrochemical ESR, quartz crystal microgravim-

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etry, and microscopy techniques in order to contribute to the understanding of anion-cation interactions as a stimulus for aggregation, deposition, and potential-controlled solid-phase formation. Experimental Section The experimental details for voltammetric measurements in a channel flow cell have been described previously.18 The channel flow cell employed in this study consisted of a Teflon channel unit and a silica cover plate, sealed together with a lowmelting wax, with the approximate dimensions of 30 mm length × 6.0 mm width × 0.82 mm height. A square platinum electrode (4.0 mm × 4.0 mm) was embedded smoothly in the base. The electrode was polished with a succession of diamond lapping compounds (Kemet International Ltd., Kent, U.K.) from 25 to 1 µm particle size before use. Electrochemical measurements were carried out using an Oxford Electrodes (Oxford, U.K.) potentiostat. A saturated calomel (SCE) reference electrode (Radiometer Copenhagen, Denmark) was situated upstream of the working electrode and a platinum gauze counter electrode downstream. Cyclic voltammograms were recorded with an Autolab PGSTAT20 system (Eco Chemie, Netherlands). The working electrode used was a 1.0 mm diameter platinum disk electrode, and the reference electrode was a saturated calomel electrode. Electrochemical quartz crystal microbalance (EQCM) measurements were carried out by using an Autolab PGSTAT20 system combined with a frequency counter, Fluke PM6680B (Fluke, Netherlands). The 10 MHz oscillator model 230 and the AT-cut gold-coated planoconvex quartz crystals were purchased from the Technical Department, Instytut Chemii Fizycznej Polskiej Akademii Nauk (Poland).19 The working electrode used was a 5 mm diameter gold disk on one side of the quartz crystal exposed to the solution. The resonant frequency of the quartz crystal electrode was monitored simultaneously with the detection of electrochemical responses. The sensitivity of the balance, typically 0.5 ng Hz-1, was calibrated by employing the Ag deposition process from a solution of ca. 0.5 mM AgNO3 and 0.1 M NBu4PF6 in acetonitrile. ESR spectra were measured as reported previously.20,21 First, derivative X-band ESR spectra were recorded on a Brucker ER200D spectrometer operating with a modulation frequency of 100 kHz and using a rectangular TE102 cavity. A flow system suitable for work under an inert atmosphere of nitrogen with a silica channel flow cell using a platinum foil as the working electrode (ca. 4 mm × 4 mm) was used to electrolyze the solution inside the ESR cavity. Atomic force microscopy (AFM) measurements were conducted using a TopoMetrix TMX 2010 Discoverer system in contact mode (typically, 3 Hz scan rate). Pyramidal silicon nitride tips (Topometrix AFM PROBES 1520) were employed. A series of images were recorded at a platinum electrode of 3 mm × 4 mm area before and after 10 s ex situ electrolysis at -0.6 V vs SCE in an acetonitrile solution containing 1.0 mM TCNQ, 0.1 M Bu4NPF6, and 1.0 mM Mg2+ or Ba2+. Before use, the electrode surface was carefully polished with diamond lapping compounds from 1 down to 0.1 µm. After the controlled potential electrolysis, the surface was carefully rinsed with acetonitrile and dried in air before AFM images were recorded. Dried and distilled acetonitrile (Fisons) was used as received. Tetrabutylammonium hexafluorophosphate (Bu4NPF6, electrochemical grade, Fluka) was used as the supporting electrolyte. 7,7,8,8-Tetracyanoquinodimethane (TCNQ, 98%, Aldrich), Mg-

Figure 1. Channel flow cell voltammogram for the reduction of TCNQ in the presence of Mg2+ in acetonitrile, with [TCNQ] ) 1.0 mM, [Bu4NPF6] ) 0.1 M, and [Mg2+] ) (A) 0 mM, (B) 0.25 mM, (C) 0.5 mM, (D) 1.0 mM, or (E) 2.0.mM. Flow rate is 1.9 × 10-2 cm3 s-1. (F) Dependence of E1/2 for the second reduction step on the flow rate in the presence of Mg2+.

(ClO4)2 (A.C.S Reagent, Aldrich), and Ba(ClO4)2 (99%, Aldrich) were used as received. All experiments were conducted at 20 ( 2 °C. Results and Discussion Channel Flow Cell Voltammetry of TCNQ in the Presence of Mg2+. As shown in previous papers,4,22 channel flow voltammetry is very effective for the detection of ion-pairing processes between electrochemically reduced species and metal cations in organic solvent systems. We consider first the reduction of a 1 mM solution of TCNQ in acetonitrile/0.1 M NBu4PF6. Figure 1A shows the two electrochemically reversible reduction waves associated with the formation of TCNQ•- and TCNQ2- (eqs 1 and 2). The observed half-wave potentials were

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+0.21 and -0.33 V (vs SCE), respectively, in good agreement with reported values.2,14

TCNQ + e- a TCNQ•-

E1/21 ) +0.21 V vs SCE (1)

TCNQ•- + e- a TCNQ2-

E1/22 ) -0.33 V vs SCE (2)

Under these conditions, the limiting currents observed for the first and the second reduction process are in good agreement with a one-electron and a two-electron reduction and the linear dependence of the limiting currents on (flow rate)1/3 suggests simple diffusion-controlled processes with a diffusion coefficient of DTCNQ ) 1.8 × 10-9 m2 s-1. Parts B-E of Figure 1 show the effect of various quantities of Mg2+ added to the solution on the channel flow cell voltammetric responses for the reduction of TCNQ. The most prominent feature is that in the presence of Mg2+ the second reduction step (eq 2) is suppressed by what appears to be a passiVation process. On the other hand the voltammetric response associated with the first reduction step (eq 1) remains nearly unaffected except for a minute shift in half-wave potential to more negative potentials in the presence of increasing amounts of Mg2+. Further, a new electrochemical reduction response for an acetonitrile solution containing 1.0 mM TCNQ and 0.25 mM Mg2+ (Figure 1B) can now be detected at E1/2 ≈ -0.10 V vs SCE with a rather small, but Mg2+ concentration-dependent, limiting current. The half-wave potential of this response is affected strongly by the flow rate (Figure 1F). It can also be seen (Figure 1) that increasing the concentration of Mg2+ causes the limiting current of the new reduction response to increase. However, the variation of the flow rate has only very little effect on the relative value of the limiting current, and with slightly more than 1 equiv Mg2+ added to the solution containing 1 mM TCNQ the new reduction response appears to be fully developed. With an increase in the Mg2+ concentration, the reduction potential of the second step shifted to more positive potentials (see Figure 1F) and the flow rate dependence of the half-wave potential, E1/2, becomes less pronounced. At a very low flow rate the shift in E1/2 is of the order of 60 mV per decadic change of Mg2+ concentration, but the flow rate dependence (Figure 1F) suggests that a simple interpretation based on the one-to-one reaction of TCNQ2- and Mg2+ is not warranted. A further important feature is the wave shape of the new reduction process. A comparison between the first electrochemically reversible process and the second new voltammetric response (Figure 1E) shows that a much steeper and asymmetric increase in current, possibly associated with nucleation/growth for the latter, occurs. It will be shown below that the reason for the unusual wave shape and the voltammetric features is the formation of a solid deposit at the electrode surface. The results from channel flow voltammetry indicate that the interaction between Mg2+ and TCNQ2-, which may lead to ion pairs or aggregates, further affects the properties of the electrode surface and suppresses the formation of “free” TCNQ2- at the electrode. Because the interaction of TCNQ2- with Mg2+ in a moderately low-permittivity solvent (compared to the permittivity of water) such as acetonitrile is energetically favorable, a stabilization occurs that is reflected by the substantial shift in potential (ca. 0.23 V, Figure 1) for the formation of TCNQ2in the presence and absence of Mg2+. The increase in the cathodic limiting current for the new Mg2+-dependent process also suggests a reaction of Mg2+ with TCNQ2-. To gain more information about the complex pattern of interfering homoge-

Figure 2. Cyclic voltammograms for the reduction of TCNQ in the presence of Mg2+ in acetonitrile at a 1.0 mm diameter Pt disk electrode, with [TCNQ] ) 1.0 mM, [Mg2+] ) 1.0 mM, and [Bu4NPF6] ) 0.1 M. Scan rates are (A) 20 mV/s, (B) 50mV/s, (C) 100 mV/s, and (D) 200 mV/s.

neous and heterogeneous processes, the reactions of TCNQ2with Mg2+ and also with Ba2+ are further analyzed in the following sections using various techniques. Cyclic Voltammetry of TCNQ in the Presence of M2+. To analyze the electrochemical responses, in particular those for the oxidation of any ion pairs or aggregates formed on the electrode surface, cyclic voltammograms were recorded for TCNQ in the presence of Mg2+ at various concentrations and scan rates. Figure 2 shows the cyclic voltammograms observed for an acetonitrile solution containing 1.0 mM TCNQ and 1.0 mM Mg2+ over a range of scan rates from 5 mV s-1 up to 10 V s-1. The first reduction process in the negative scan (denoted by P1red in Figure 2B) exhibits a characteristic diffusion-controlled wave with a peak current proportional to the square of the scan rate and a peak potential of Ep(P1red) ) +0.17 V vs SCE. In contrast, the peak for the second reduction response (P2red), which was detected at Ep(P2red) ) -0.09 V vs SCE, is associated with low peak currents and an unusual shape especially at very low scan rates. The voltammetric response P2red is therefore consistent with the second reduction process observed by channel flow cell voltammetry in the presence of Mg2+. The reduction of TCNQ•- to TCNQ2-, which should be detectable as a diffusion-controlled wave with a formal potential of E1/2 ) -0.33 V vs SCE, was again not observed in the presence of Mg2+. Significant changes in electrochemical responses were observed on the reversed potential scan. Two oxidation processes denoted P2ox (Ep(P2ox) ) 0.12-0.19 V vs SCE) and P1ox (Ep(P1ox) ) 0.25 V vs SCE) are detected with strongly scan-rate-

Reduction of TCNQ

Figure 3. Cyclic voltammograms for the reduction of TCNQ in the presence of Mg2+ in acetonitrile at a 1.0 mm diameter Pt disk electrode, with [TCNQ] ) 1.0 mM, [Mg2+] ) 0.25 mM, and [Bu4NPF6] ) 0.1 M. Scan rates are (A) 20 mV/s, (B) 50 mV/s, (C) 100 mV/s, and (D) 200 mV/s.

dependent relative peak currents. As shown in Figure 2, at scan rates between 20 and 200 mV s-1 the relative current magnitudes for the two oxidation peaks (P2ox and P1ox) can be reversed with P2ox dominating the voltammetric response at high scan rate. At an even faster scan rate (not shown) of 10 V s-1, only one pronounced oxidation wave (P2ox) was observed on the reversed scan possibly superimposed onto a much smaller P1ox. The peak current observed for P2ox scales approximately linearly with scan rate and is therefore likely associated with the removal of a deposit from the electrode surface. But also P1ox, which becomes very large at low scan rates, is associated with a deposit that appears to accumulate at the electrode surface. When the negative-going scan was reversed at a potential positive to P2red, only the reversible redox couple TCNQ0/- as described in eq 1 was observed, without any indication of a deposition process and similar to the case of measurements made in the absence of Mg2+. These results show that (i) two types of electroactive materials (P2ox, P1ox) are formed at the electrode surface by the interaction between Mg2+ and the reduced forms of TCNQ, (ii) a negative potential corresponding to P2red has to be applied for both materials to form, and (iii) the suppression of the reversible reduction for the TCNQ-/2- couple suggests the formation of an absorptive thin layer on the electrode surface. The third point is further supported by the peak shapes, which were similar, for example, to the electrochemical response of TCNQ-modified electrodes.23 In Figure 3 cyclic voltammograms obtained for an acetonitrile solution containing 1.0 mM TCNQ, 0.25 mM Mg2+, and 0.1 M NBu4PF6 are shown also over a range of scan rates from 20

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Figure 4. Cyclic voltammograms for the reduction of TCNQ in the presence of Ba2+ in acetonitrile at a 1.0 mm diameter Pt disk electrode, with [TCNQ] ) 1.0 mM, [Ba2+] ) 1.0 mM, and [Bu4NPF6] ) 0.1 M. Scan rates are (A) 200 mV/s, (B) 500 mV/s, (C) 1.0 V/s, (D) 2.0 V/s, and (E) 5.0 V/s.

to 200 mV s-1. In this case, the magnitude of P2red becomes smaller relative to that of P1red, and the reversible couple TCNQ-/2- described by eq 2 can be observed at faster scan rates in competition with P2. In addition, the same features for the relative peak currents of P2ox and P1ox associated with the deposition processes are observed. The relative results obtained by cyclic voltammetry (see Figures 2 and 3) and by channel flow cell voltammetry (see Figure 1) are governed by the type of mass transport at the electrode surface. In Figure 3, at faster scan rates, it can be seen that the coverage of the electrode surface by the adsorptive thin layer is only partial. In contrast, at a slower scan rate of 20 mV s-1, for the reduction of TCNQ•- a passivated electrode surface appears to be formed so that the shapes of the voltammograms shown in Figures 2A and 3A are almost identical and independent of the concentration of Mg2+. In Figure 4 cyclic voltammograms for an acetonitrile solution containing 0.1 M NBu4PF6, 1.0 mM TCNQ, and 1.0 mM Ba2+ are shown for a range of scan rates from 200 mV s-1 to 5.0 V s-1. Features very similar to those for the voltammetry in the presence of Mg2+ (Figure 2) were found. However, the range of scan rates required is considerably higher. The process denoted P2ox, for example, is virtually absent at scan rates below 200 mV s-1, and the corresponding peak current never dominates the voltammetric current responses. An important observation is a shift of P2ox and P2red toward more negative potentials with Ep(P2ox) ) -0.27 V and Ep(P2red) ) -0.13 V

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vs SCE compared to Ep(P2ox) ) -0.09 V and Ep(P2red) ) +0.12-0.19 V vs SCE for the experiment in the presence of Mg2+. This shift in potential may be attributed to the anticipated difference in interaction between the TCNQ2- dianion and the metal dication. Barium with a considerably bigger ionic radius may interact less strongly compared to the “harder” Mg2+ dication. In the presence of cations of the type M2+, the pathway for the second reduction process appears to change from the simple formation of a soluble product described by eq 2 to more complex adsorption and deposition steps as suggested in eq 3. Because the ion pair formation of the type M2+TCNQ2stabilizes the dianionic state, the second reduction potential of the TCNQ-/2- couple is expected to move to more positive potentials in the presence of M2+ and the overall process may be written as

TCNQ•- + M2+ + e- f M2+TCNQ2-ads

(3)

The subscript “ads” is used in order to account for the shape of the voltammetric waves that indicate an adsorption or deposition process at the electrode surface. The anodic process P2ox on the reversed anodic scan, at least at the faster scan rates, may be described by the reverse reaction of the process described by eq 3. The voltammograms, in particular in the cases of Figures 3D and 4E, agree well with this adsorption and desorption behavior for M2+TCNQ2- formed at the electrode surface as a thin layer. Electrochemical ESR and AFM Measurements for the Reduction of TCNQ in the Presence of M2+. The voltammetric features such as changes in relative peak current and wave shape for P2ox and P1ox indicate a rather more complex process involving two different types of deposit at the electrode surface and with M2+TCNQ2-ads attributed to P2ox only as a potential-dependent intermediate species. To gain more information about the processes, ESR spectra were recorded in an in situ channel flow electrolysis cell.4,21,22 Data were obtained for an acetonitrile solution containing 0.1 M NBu4PF6, 0.5 mM TCNQ, and 0.5 mM Mg2+. In Figure 5 it is shown that the ESR signal obtained when the potential of the working electrode was held at 0.0 V (vs Ag pseudoreference) consisted of a characteristic multiplet consistent with the solution-phase spectrum of TCNQ•- 24 and therefore consistent with the process described in eq 1. However, at an applied potential of -0.5 V (vs Ag pseudoreference), ion pair interaction occurs according to eq 3, and instead of the solution-phase species TCNQ•- the characteristic single-line ESR spectrum of TCNQ•- in a solid 25,26 was observed (Figure 5). Further, the intensity of the observed resonance, which is proportional to the amount of paramagnetic material, steadily increases with time. Stepping the potential back to a potential that is positive of that for the process P1ox resulted in the loss of the single-line resonance and left only a weak background signal. The formation of the green solid film was confirmed by visual inspection of the electrode surface. The integrated intensity of the single-line ESR signal, which is proportional to the amount of deposit formed, increased linearly over time as shown in Figure 6. Additional information about the nature of the deposit formed at the electrode surface was obtained by atomic force microscopy (AFM). The type of deposit formed when a polished platinum electrode is polarized at -0.6 V vs SCE for a 10 s period in acetonitrile containing 0.1 M NBu4PF6, 1.0 mM TCNQ, and 1.0 mM Mg2+ is shown in Figure 7A. A particulate deposit evenly distributed over the electrode surface with a 200500 nm particle size can be seen. Although the deposit appears

Figure 5. ESR spectra recorded during the reductive electrolysis of TCNQ in the presence of Mg2+ in acetonitrile, with [TCNQ] ) 0.5 mM, [Mg2+] ) 0.5 mM, and [Bu4NPF6] ) 0.1 M. Flow rate is 2.2 × 10-3 cm3 s-1. The modulation amplitude is 0.02 mT. Sweep width is 2 mT. Field sweep time is 100 s. Time constant is 1 s.

Figure 6. Plot of the integrated ESR signal vs time. Other conditions are as in Figure 5.

to be formed with a regular shape, there are no well-developed crystallites. In the background, scratch lines on the polished electrode surface can also be seen. In the case of the reduction of TCNQ in the presence of Ba2+, the features observed in the ESR spectra in the presence of Mg2+ were not observed. In situ ESR spectra for an acetonitrile solution containing 0.1 M NBu4PF6, 0.5 mM TCNQ, and 0.5 mM Ba2+ (not shown) were found to consist only of a weak resonance for the solution-phase TCNQ•- species. However, for an acetonitrile solution containing 5.0 mM Ba2+, the weak and additionally a broad ESR signal, which may be attributed to TCNQ•- in a solid phase, were observed as shown in Figure 8. The solution-phase ESR signals observed in the presence of Mg2+ (Figure 5) as well as in the presence of Ba2+ (Figure 8) do not give any significant indication for a M2+-TCNQ•interaction. Visual inspection of the electrode surface in this case showed a filmlike black precipitate on the electrode surface. A typical image from an AFM study of the deposit formed after a 10 s polarization of a platinum electrode at -0.6 V vs SCE in an acetonitrile solution containing 0.1 M NBu4PF6, 1.0 mM

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Figure 8. ESR spectra obtained during the reductive electrolysis of TCNQ in the presence of Ba2+ in acetonitrile, with [TCNQ] 0.5 mM, [Ba2+] ) 5.0 mM, and [Bu4NPF6] ) 0.1 M. Flow rate is 2.2 × 10-3 cm3 s-1. The modulation amplitude is 0.02 mT. Sweep width is 2 mT. Field sweep time is 100 s. Time constant is 1 s.

Figure 7. AFM images for a platinum electrode surface after controlled potential electrolysis for the reduction of TCNQ in the presence of Mg2+ (A, top) and of Ba2+ (B, bottom) in acetonitrile. Applied potential is -0.6 V. Electrolysis time is 10 s. Electrolyzed solution contains the following: [TCNQ] ) 1.0 mM, [Bu4NPF6] ) 0.1 M, (A) [Mg2+] ) 1.0 mM, and (B) [Ba2+] ) 1.0 mM.

TCNQ, and 1.0 mM Ba2+ is shown in Figure 7B. The deposit containing Ba2+ appears more scattered and with a lower density compared to that formed with Mg2+. There is no sign of crystalline material, and especially at high magnification (not shown) only a very poorly resolved structure was visible. Judging from these results, the deposition of the intermediate, M2+TCNQ2-ads, was accompanied or followed by the deposition of a TCNQ•--containing material, which in the case of the presence of Mg2+ steadily accumulates at the electrode surface. EQCM Measurements for the Reduction of TCNQ in the Presence of M2+. The electrochemical quartz crystal microbalance (EQCM) technique19 can be a very powerful tool for the investigation of the adsorption and deposition processes associated with the reduction of TCNQ and for the characterization of the types of deposit formed. Simultaneous gravimetry and voltammetric measurements were performed on a gold electrode deposited on a quartz crystal. To rule out effects of the electrode material, the electrochemical responses obtained with a gold electrode were compared to results obtained on platinum and found to be essentially identical.

Figure 9. Cyclic voltammogram and in situ EQCM data obtained for the reduction of TCNQ in the presence of Mg2+ in acetonitrile, with [TCNQ] ) 1.0 mM, [Mg2+] ) 1.0 mM, and [Bu4NPF6] ) 0.1 M. Scan rate is 20 mV/s.

In Figure 9 the results from a typical EQCM measurement for an acetonitrile solution containing 0.1 M NBu4PF6, 1.0 mM TCNQ, and 1.0 mM Mg2+ are shown. In the negative-going scan the current response P1red is not associated with a change in resonance frequency of the quartz crystal. Hence, no deposition occurs, in agreement with the voltammetric measurements. However, the onset of the second reduction process, P2red, immediately causes a change in mass and therefore deposition. Further, the amount of deposit appears to increase nearly linearly until at P2ox the process stops. At the second oxidation wave, P1ox, all deposit is removed and the electrode surface is converted back into the original state. The total amount of mass increase observed in one potential cycle depended on the potential scan rate chosen. By use of a calibration procedure and assuming that only a rigid solid material attached to the electrode surface is formed, it is possible to calculate the mass increase from the frequency data,19 and the maximum values of mass increase at a given scan rate are

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Figure 10. Plot of the dependence of the maximum mass increase on the reciprocal potential scan rate (V-1) at for several measurements in the presence of Mg2+ (b) and of Ba2+ (2).

Figure 11. Cyclic voltammogram and in situ EQCM data obtained for the reduction of TCNQ in the presence of Ba2+ in acetonitrile, with [TCNQ] ) 1.0 mM, [Ba2+] ) 1.0 mM, and [Bu4NPF6] ) 0.1 M. Scan rate is 50 mV/s.

plotted against the reciprocal of the scan rate in Figure 10. A nearly linear relationship was observed in the presence of Mg2+, consistent with a continuous deposition with time. A typical result for the EQCM measurement in the presence of Ba2+ is shown in Figure 11. The features of the mass change again resemble those for the deposition in the presence of Mg2+ at least qualitatively. The initially strong mass increase can clearly be seen to terminate at a potential corresponding approximately to P2ox on the reversed scan. Compared to the results obtained in the presence of Mg2+, the mass increase initiated in P2red does not appear linear but instead gradually slows down. The dependence of the mass increase on the reciprocal scan rate is also shown in Figure 10, and the amount of the deposit formed can be seen to level off already at moderately slow scan rates. For a more quantitative discussion and the characterization of the deposit formed on the electrode surface, it is necessary to separate the amount of charge passed for the generation of the soluble TCNQ•- from that consumed for the formation of the deposit. Therefore, a potential step directly into the deposition region at a potential negative of P2red, -0.15 V vs

Oyama et al.

Figure 12. Plot of the correlation between mass increase and charge consumed after a potential step has been applied for the reduction of TCNQ in the presence of Mg2+ in acetonitrile. Potential step is from 0.8 to -0.15 V. Measurement period is 100 s. [TCNQ] ) 1.0 mM. [Mg2+] ) 1.0 mM. [Bu4NPF6] ) 0.1 M.

SCE, was chosen in order to minimize the formation of soluble products. The mass increase was monitored by combining the potential step chronoamperometry measurement with in situ gravimetry. A plot of the mass increase versus the charge consumed for the case of the reduction of TCNQ in the presence of Mg2+ is shown in Figure 12. Most importantly, after an initial period the mass-charge dependence becomes nearly linear (line A) with a slope of 2.07 × 10-3 g C-1 and an agreement with the theoretically expected slope for the deposition of Mg(TCNQ)2 per two electrons (line B, slope of 2.24 × 10-3 g C-1) rather than that for Mg(TCNQ) per two electrons (line C, slope of 1.18 × 10-3 g C-1) can be seen. The initially smaller slope of the experimental line may be attributed to the deposition of a less heavy material, e.g., Mg2+TCNQ2- (see line C), and/or the initial loss of some material into the solution phase. However, we emphasize that these results cannot rule out the inclusion of solvent in the deposit or particularly the formation of mixed salts such as Mg2(TCNQ2-)(TCNQ•-)2 (theoretical slope of 1.71 × 10-3 g C-1). The correlation between the mass and charge permits a more quantitative characterization of the deposited species. In the case of Mg2+, on the basis of the results from ESR measurements, a paramagnetic material such as Mg2+(TCNQ•-)2 is expected. If the additional incorporation of solvent and foreign ions into the deposit is assumed to be negligible, then the EQCM results appear to support the formation of Mg2+(TCNQ•-)2 at least after an initial period of deposition of a less heavy deposit. A similar analysis for the deposition process in the presence of Ba2+ is shown in Figure 13. The expected gradients, assuming the deposition of Ba(TCNQ) (line C, slope of 1.77 × 10-3 g C-1) and Ba(TCNQ)2 (line B, slope 2.83 × 10-3 g C1-) per two electrons, are indicated. The mass increase per charge in this case can be seen to result from the initial deposition period, and only for a short period of time the experimental curve is parallel to the Ba(TCNQ)2 slope (line B). Then the slope indicates the formation of a less heavy material (slope of 2.37 × 10-3 g C-1). That is, at first the formation of soluble material and Ba(TCNQ) dominates, then the formation of a material such as Ba(TCNQ)2 appears to occur, and finally the deposited species appears to change again. Mechanistic Speculation on the Deposition Process. In the presence of Mg2+, the results from ESR spectroscopy and in situ EQCM monitoring suggest the formation of a paramagnetic

Reduction of TCNQ

J. Phys. Chem. B, Vol. 102, No. 34, 1998 6595 was studied by voltammetry, ESR, AFM, and EQCM. Although TCNQ•- does not strongly interact with M2+, TCNQ2- was found to interact and to form a thin-layer deposit probably of the type M(TCNQ). The potential at which this occurs was less negative than the potential required for the formation of “free” TCNQ2- in solution and reflected binding by M2+. Surprisingly, the initial deposition of M(TCNQ) triggered the deposition of a different material, probably M(TCNQ)2, as evidenced by a characteristic ESR signal and the gravimetrically determined deposited mass per charge passed. Both types of deposit gave distinct voltammetric signatures. However, the mechanistic interpretation based on the data reported in this study will need confirmation by independent spectroscopic techniques or structural data necessary to rule out the formation of materials containing both TCNQ•- and TCNQ2-, or other ionic species. Such work is presently under active investigation.

Figure 13. Plot of the correlation between mass increase and charge consumed after a potential step has been applied for the reduction of TCNQ in the presence of Ba2+ in acetonitrile. Potential step is from 0.8 to -0.3 V. Measurement period is 100 s. [TCNQ] ) 1.0 mM. [Mg2+] ) 1.0 mM. [Bu4NPF6] ) 0.1 M.

material of the possible composition Mg(TCNQ)2. Taking into account these results and the results obtained by cyclic voltammetry that an intermediate species of the type Mg2+TCNQ2-ads is present, a conceivable pathway for the formation of a Mg2+(TCNQ•-)2 solid appears to be the successive growth by deposition of Mg2+(TCNQ•-)2 unit by unit after an initial formation of Mg2+TCNQ2-ads at the electrode surface. The following oversimplified mechanistic scheme illustrated in eqs 4-7 may be proposed on the basis of the above results and the known tendency of stacking of the radical anion TCNQ•- in conducting salts.6-8,23,27

Pt|Mg2+TCNQ2-ads + TCNQ•-

(4)

Pt|Mg2+TCNQ2-adsTCNQ•- + Mg2+

(5)

Pt|Mg2+TCNQ•-TCNQ2-Mg2+ + TCNQ•-

(6)

Pt|Mg2+TCNQ•-TCNQ•-Mg2+TCNQ2-

(7)

The deposition process is initiated by the formation of a Mg2+TCNQ2-ads deposit (eq 3). After the formation of Mg2+TCNQ2- the successive interaction with TCNQ•- formed at the electrode surface (eq 5) and Mg2+ (eq 6) allows the formation and growth of Mg2+(TCNQ•-)2 (eq 7). A closely related mechanism would also account for the formation of a mixed salt of the type Mg2+2(TCNQ2-)(TCNQ•-)2. The repetition of steps 4-7 is responsible for the continuous growth. The formation of the TCNQ dimer unit is well-known from several examples of metal salts,6,23,27 and it may be inferred that the presence of Mg2+ encourages the formation of the dimer unit and the growth of the structure. In the case of the reduction of TCNQ in the presence of Ba2+, the paramagnetic deposit was formed only in small quantity as shown by voltammetry, ESR, and EQCM measurements. The mechanistic differences between Mg2+ and Ba2+ are thought to be due to the difference in interaction with the TCNQ anion or dianion, and on the basis of this for Ba2+, an earlier termination of successive growth (steps 4 to 7) may be proposed. Conclusions The effect of aggregation and electrocrystallization of M2+ and electrochemically generated TCNQ2- in acetonitrile solution

Acknowledgment. We thank the Royal Society and Japan Society for the Promotion of Science for our cooperative research work awarded as a U.K.-Japan Joint Science Program and for support for M.O. under their Bilateral Exchange Program. R.D.W. thanks the Ramsay Memorial Fellowships Trust for a Postdoctoral Fellowship and Wadham College, Oxford, for a Lectureship. F.M. thanks New College, Oxford, for the award of a Stipendiary Lectureship and the Royal Society for a University Research Fellowship. M.S. thanks COLCIENCIAS for a scholarship. References and Notes (1) Nagaoka, T.; Okazaki, S. J. Electroanal. Chem. 1983, 158, 139. (2) Khoo, S. B.; Foley, J. K.; Pons, S. J. Electroanal. Chem. 1986, 215, 273. (3) Oyama, M.; Takei, A.; Okazaki, S. J. Chem. Soc., Chem. Commun. 1995, 1909. (4) Oyama, M.; Marken, F.; Webster, R. D.; Cooper, J. A.; Compton, R. G.; Okazaki, S. J. Electroanal. Chem. 1998, 451, 193. (5) Fujihira, M. Topics in Organic Electrochemistry; Fry, A. J., Britton, W. E., Eds.; Plenum: New York, 1986; Chapter 6. (6) Kaim, W.; Moscherosch, M. Coord. Chem. ReV. 1994, 129, 91. (7) Garrito, A. F.; Heeger, A. J. Acc. Chem. Res., 1973, 7, 232. (8) Ward, M. D. Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1989; Vol. 16, pp 181. (9) Siedle, A. R. J. Am. Chem. Soc. 1975, 97, 5931. (10) Siedle, A. R.; Candela, G. A.; Finnegan, T. F. Inorg. Chim. Acta 1979, 35, 125. (11) Acker, D. S.; Hertler, W. R. J. Am. Chem. Soc. 1962, 84, 3370. (12) Peover, M. E. Trans. Faraday Soc. 1964, 60, 417. (13) Suchanski, M. R.; Van Duyne, R. J. Am. Chem. Soc. 1976, 98, 250. (14) Khoo, S. B.; Foley, J. F.; Korzeniewski, C.; Pons, S.; Marcott, C. J. Electroanal. Chem. 1987, 233, 223. (15) Kathirgamanathan, P.; Rosseinsky, D. R. J. Chem. Soc., Chem. Commun. 1980, 839. (16) Bond, A. M.; Colton, R.; Daniels, F.; Fernando, D. R.; Marken, F.; Nagaosa, Y.; Van Steveninck, R. F. M.; Walter, J. N. J. Am. Chem. Soc. 1993, 115, 9556. (17) Bond, A. M.; Colton, R.; Marken, F.; Walter, J. N. Organometallics 1994, 13, 5112. (18) Compton, R. G.; Dryfe, R. A. W. Prog. React. Kinet. 1995, 20, 245. (19) Koh, W.; Kutner, W.; Jones, M. T.; Kadish, K. M. Electroanalysis 1993, 5, 209. (20) Compton, R. G.; Waller, A. M. In Spectroelectrochemistry: Theory and Practice; Gale, R. J., Ed.; Plenum: New York, 1988; Chapter 7. (21) Webster, R. D.; Bond, A. M.; Coles, B. A.; Compton, R. G. J. Electroanal. Chem. 1996, 404, 303. (22) Cooper, J. A.; Alden, J. A.; Oyama, M.; Compton, R. G.; Okazaki, S. J. Electroanal. Chem. 1998, 442, 203. (23) Inzelt, G.; Day, R. W.; Kinstle, J. F.; Chambers, J. Q. J. Phys. Chem. 1983, 87, 4592. (24) Fischer, P. H. H.; McDowell, C. A. J. Am. Chem. Soc. 1963, 85, 2694. (25) Chesnut, D. B.; Phillips, W. D. J. Chem. Phys. 1961, 35, 1002. (26) Bond, A. M.; Fiedler, D. A. J. Electrochem. Soc. 1997, 144, 1566. (27) Michaud, M.; Carlone, C.; Hota, N. K.; Zauhar, J. Chem. Phys. 1979, 36, 79.