The electrochemistry of ferrocene in non-aqueous solvents

165

J. Electroanal. Chem., 221 (1987) 165-174 Elsevier Sequoia S.A., Latmume - Printed in The Netherlands

STRY OF FJZRROCENE IN NON-AQUEOUS

TImJ!ILEcrRocHEMI SOLVENTS

ALFRED0

J. ZARA, SERGIO S. MACHADO and LUIS OTAVIO S. BULH&.S

Departamento de Qulmica, Universidade Feakral de S& Carlos, Caixa Post4 &io Paul0 (Brazil) ASSIS VICENTE

67613.560~Scio

Carlos,

BENEDETTI

Institute de Quimica de Araraquara,

UNESP,

Caixa Postal, I74-148W-Araraquara,

S& Paul0 (Brazil)

TIBOR RABOCICAI Institute de Qulmica & Universihde SZo Paul0 (Brazil)

de MO Paulo, Caixa Postal, 20780-O.I498&io

Paulo,

(Received 7th June 1985; in revised form 1st October 1986)

ABSTRACT The electrochemical oxidation of ferrocene on Pt in dimethylfo rmamide, ethanol, propylene carbonate and their aqueous solutions was studied at 25O C. The concentration of the supporting electrolyte, NaClO,, was varied from 0.1 to 0.5 M. The results show that the electrode process may be described as a quasi-reversible one-electron charge transfer, followed by slow decomposition of the oxidized species.

INT’RODUCTION

The electrochemical behaviour of ferrocene has been investigated in several media [l-5] and chemically modified platinum-ferrocene electrodes have been employed in acetonitrile [6] and in sulpholane [7]. Little is known, however, about the electrochemical oxidation of ferrocene in propylene carbonate (PC), or even in ethanol (EtOH), while several authors disagree on the interpretation of the overall electrode process when the solvent is dimethylformamide (DMF). There is also disagreement on how the oxidation product behaves: whether or not it decomposes in different media. The influence of adding water, on the oxidation of ferrocene was investigated in methanol only by Duschek and Gutmann [S], who observed a shift of approximately 40 mV in the El,* values, when the water content was increased from 1 to 5%. Recently [9], it was observed that the E” of ferrocene decreases by 100 mV as the water content of an acetone + water mixture is increased from 60 to 95 wt%. 0022-0728/87/$03.50

8 1987 Elsevier Sequoia S.A.

166

The present paper deals with the potentiodynamic behaviour of ferrocene in pure DMF, EtOH and PC and in their aqueous solutions, containing sodium perchlorate as the supporting electrolyte. A platinum electrode was used as the working electrode and the temperature was maintained at 25 o C.

Ferrocene (FeCb) (Fhtka) was purified by sublimation at llO°C and low pressure. Sodium perchlorate (Merck, p.a.), recrystallized in ethanol (p.a.) and dried at 110 o C for 24 h, was used as the supporting electrolyte. Silver nitrate (Carlo Erba, p.a.) was used without any purification. DMF was purified following Brummer’s procedure [lo]. Technical grade EtOH was treated as described elsewhere [II]. Purification of PC (Merck) has been described previously [12]. The water content of the purified solvent (DMF: 0.06% v/v; EtOH: 1.9% v/v and PC: 0.08% v/v) was determined by the Karl Fischer method. The cyclic voltammetric investigations were performed by means of a PAR model 173 ~t~tios~t/~v~~~t, plug-in model 376 and model 175 universal programmer. A PAR model 4102 signal recorder was used to record the cyclic voltammograms for potential scan rates higher than 0.2 V s-t. A three-compartment electrochemical cell was employed with a flat commercial platinum disk (Beckmann; surface area 0.18 cd) working electrode and a platinum wire auxiliary electrode. The working electrode was pretreated by successive triangular potential sweeps in the solvent containing sodium perchlorate. Ag/AgNQ, (10 mM), 0.5 M NaClO,, DMF; Ag/AgNO, (5 mM), 0.1 M NaGlO,, EtOH and Ag/AgClO, (20 mM), 0.1 M NaC10,, PC electrodes were employed as the reference in DMF, EtOH and PC, respectively. They were prepared as described in refs. 11-13. The ohmic drop was compensated following the procedure of Martin et al. 1141. The temperature of the cell was maintained at 25 *C using a MLW-U15 thermostat. Ah the solutions were deaerated by bubb~g with high purity nitrogen, pre-saturated with the blank solution at 25O C. The cyclic voltammograms were recorded in the ranges -0.30 to 0.50 V vs. Ag/Ag” (DMF); -0.70 to 0.09 V vs. Ag/Ag+ (EtOH) and -0.65 to 0.12 V vs. Ag/Ag’ (PC) for DMF, EtOH and PC solutions, respectively. No anodic or cathodic peaks were observed in the absence of ferrocene in these Potential ranges. In calculating the ratio between the cathodic current peak and the anodic current peak a correction was made for the capacitative current. The concentration of the complex was corrected for the addition of water. After each addition of water, the cyclic voltammograms were recorded when the temperature returned to 25 0 C. RESULTS

The results obtained for the oxidation of ferrocene in pure DMF, EtOH and PC and in their aqueous solutions are assembled in Table 1. The table shows the values

167

of the anodic current peak (i;), the ii/ii ratio, the current function i~u”-“2c-’ (where c is the molar concentration of ferrocene) as a function of the potential sweep rate (u) the anodic peak potential ( J?Z;),the cathodic peak potential (E,C) and the difference AEp between E; and Ei, for different potential sweep rates. DISCUSSION

Table 1 shows that the peak current ratio, ii/i;, is close to unity, never being lower than 0.91, under all the experimental conditions, when the solvent is DMF. A slight increase in the peak current ratio and a slight decrease in the current function i~v-‘/2c-’ with increasing potential scan rate is observed for solutions with a water content lower than 2% (v/v). For higher water concentrations, both the current ratio and the current function remain constant within experimental error. The ii/i; ratio increases slightly with u for ethanolic solutions, mainly with the higher water content, approaching unity. The current function tends to decrease with the potential scan rate. In propylene carbonate and its aqueous solutions, iE/i: is close to unity and tends to decrease with increasing u. The current function remains constant within experimental error. The anodic peak current varies linearly with the ferrocene concentration for all the solvents studied. These results, considering the time scale of the experiments and taking into account the criteria of Nicholson and Shain [15], suggest that very slow decomposition of the oxidized species (FeCpz) occurs in all the ethanolic solutions. In these solutions, a second oxidation peak was observed at a positive potential. After controlled potential electrolysis at -0.065 V vs. Ag/AgNO, (5 mM), NaClO, (0.1 M), EtOH, 2% H,O (v/v), a green product was obtained. The melting point of this product was higher than that of ferrocene. Probably, the second oxidation peak is associated with a ligand oxidation reaction in the FeCp: complex, generating a product that decomposes rapidly. In Fig. 1 the cyclic voltammogram of ferrocene in ethanol containing 22% (v/v) of water is shown. Very slow decomposition of ferrocene also occurs in DMF solutions containing up to 2% of water (v/v), as may be inferred from the current function increase with potential scan rate. In the other DMF solutions and in the PC solutions, no evidence was found of chemical reactions. There are several reports in the literature of a very slow decomposition reaction of the oxidized species in CH,CN, EtOH, DMSO and DMF [l], in water at pH > 4 and in proton acceptor aprotic solvents such as acetone and DMF [16] and in methanol [17]. The idea of the instability of the FeCpl ion in several media is reinforced by the ESR studies of Prins et al. [18]. These authors observed decomposition of the cation, for example, in the presence of Cl- and Br-, producing ferrocene and FeX;, and in the presence of strong donor solvents (S) such as DMSO and DMF following the equation: 2FeCp:+6S+FeCp2+Fe$++2Cp

(VP)

0.06

1.70

Fecp,

2.00

1.97

1.90

1.85

0.500

0.492

0.476

0.462

7.75

4.82

% H,O

c/mM

s-1

0.020 0.050 0.075 0.100 0.200 0.300 0.500 0.750 1.000 2.00 5.00 0.020 0.050 0.075 0.100 0.200 0.300 0.500 0.750 l.OOO 2.000 0.020 0.050 0.075 0.100 0.200 0.300 0.500 0.750 1.00 2.00 5.00 0.050 0.075

u/v

[ E( f 2 mV) vs. Ag/AgNO,(lO

c/M NaClO,

(A) Dimethylformamide

Electrochemistry of ferrwene on platinum at 25 ’ C

TABLE 1

47 72 90 101 143 170 221 270 312 428 680 48 71 80 100 137 166 214 260 301 415 40 63 75 87 125 153 197 238 267 383 615 58 72

lo6 +‘A

mM), 0.5

0.91 0.92 0.91 0.93 0.94 0.97 0.95 0.97 0.96 0.99 0.98 0.93 0.95 0.93 0.96 0.95 0.98 0.98 0.98 1.00 1.00 0.99 0.96 0.98 0.98 0.98 0.99 0.98 0.99 1.02 1.00 0.98 0.96 0.96

i;/ii 0.166 0.161 0.163 0.160 0.160 0.155 0.156 0.156 0.156 0.151 0.152 0.154 0.160 0.148 0.160 0.155 0.154 0.153 0.152 0.153 0.149 0.147 0.148 0.144 0.144 0.147 0.147 0.146 0.144 0.141 0.142 0.145 0.140 0.141

A s V-’

p”-‘&-‘/

M NaGlO.,, DMF] mole1 1

3 1 0 1 1 2 0 0 0 -4 -10 12 11

-7 -9 -9 -10 -10 -11 -16 -17 -20 -22 -25 -11 -11 -10 -11 -10 -9 -12 -12 -11 -17

- E,P/mV 52 52 52 51 51 51 53 55 55 60 65 52 52 52 53 54 54 58 59 60 64 -64 -64 -64 -62 -66 -66 -66 -70 -72 -75 -76 -72 -73

- Ei/mV 59 61 61 61 61 62 69 72 75 82 90 63 63 62 64 64 63 70 71 73 81 61 63 64 61 65 64 66 70 72 79 86 60 62

AE,/mV

1.79

1.71

1.58

0.448

0.429

0.395

21.1

14.34

10.50

0.100 0.200 0.300 0.500 0.750 1.00 2.00 5.00 0.020 0.050 0.075 0.100 0.200 0.300 0.500 0.75 1.0 2.0 5.0 0.050 0.075 0.100 0.200 0.300 0.500 0.750 1.00 2.00 5.00 0.020 0.050 0.075 0.100 0.200 0.300 0.500 0.750 1.00 2.00 5.00

84 116 137 180 224 256 353 551 32 50 62 75 107 131 169 206 230 338 548 48 58 67 96 117 152 186 211 297 472 26 41 50 59 83 100 132 161 181 262 421

0.95 0.97 0.99 0.98 0.97 0.97 1.00 1.00 0.95 0.97 0.95 0.98 0.99 0.98 0.99 0.98 1.00 0.96 0.92 0.98 0.97 0.98 0.99 0.98 0.98 0.97 1.01 0.98 0.98 0.98 0.98 0.98 0.98 0.98 0.99 0.96 0.98 0.99 0.99 0.94

0.142 0.140 0.135 0.138 0.140 0.138 0.135 0.133 0.128 0.124 0.126 0.133 0.134 0.133 0.133 0.133 0.128 0.133 0.137 0.125 0.124 0.124 0.125 0.125 0.126 0.126 0.123 0.123 0.123 0.114 0.115 0.116 0.118 0.117 0.116 0.118 0.117 0.115 0.117 0.119 13 13 12 12 11 10 6 0 21 19 20 26 25 22 22 22 20 17 9 33 33 31 36 37 34 34 34 27 24 53 51 50 55 56 55 52 51 50 45 39

-75 -77 -78 -80 -80 -81 -85 -94 80 80 81 89 88 87 87 90 92 92 97 92 93 92 100 98 100 101 102 103 112 111 112 111 116 117 117 119 118 121 122 128 (to be continued)

62 64 66 68 69 71 79 94 59 61 61 63 63 65 65 68 72 75 89 59 60 61 64 61 66 67 68 76 88 58 61 61 61 61 62 67 67 71 77 89 s

(v/v)

3.0

4.0

7.0

12.0

Fecp2

3.00

2.91

2.94

2.86

2.73

c/M NaC10,

0.200

0.198

0.1%

0.194

0.192

2.0

I H,O

c/mM

(B) Ethanol [E f 2 mV vs. Ag/AgNO,

TABLE 1 (continued)

0.020 0.050 0.100 0.200 0.500 0.020 0.050 0.100 0.200 0.500 0.020 0.050 0.100 0.200 0.500 0.020 0.050 0.100 0.200 0.500 0.020 0.050 0.100 0.200 0.500

17 112 157 213 337 73 110 154 209 336 72 106 150 206 322 68 98 138 1% 307 58 83 119 168 265

lo6 ii/A 0.94 0.95 0.96 0.97 0.97 0.95 0.97 0.98 0.98 0.98 0.94 0.95 0.95 0.96 0.97 0.90 0.91 0.95 0.96 0.96 0.88 0.90 0.91 0.94 0.96

ii/i;

(5 mM), 0.1 M NaClO,, EtOH]3 v/v s-1 0.182 0.167 0.165 0.159 0.159 0.174 0.164 0.164 0.157 0.157 0.173 0.161 0.161 0.157 0.155 0.168 0.153 0.153 0.153 0.152 0.150 0.136 0.138 0.138 0.137

i’v-“2c-‘/ A’s V-’ mol-’ 1 78 79 77 77 75 85 81 87 88 89 89 92 92 94 94 107 109 109 110 110 125 126 128 128 128

- E,^/mV 136 137 136 137 135 144 147 147 148 150 147 149 152 154 154 165 167 168 168 169 184 185 187 188 188

- Ei/mV

58 58 59 60 60 59 60 60 60 61 58 58 60 60 60 58 58 59 58 59 59 59 59 60 60

AEJmV

(v/v)

c/mM

‘+-CP,

1.43

1.42

1.40

1.36

c/M NaCIO,

0.500

0.4%

0.491

0.477

5.0

2.0

1.0

0.2

X H,O 17 21 27 32 37 45 52 64 17 21 27 33 38 46 53 66 17 21 26 33 38 47 54 65 17 21 27 34 39 47 54 66

0.020 0.030 0.050 0.075 0.100 0.150 0.200 0.300

lo6 ii/A 0.96 0.96 0.96 0.95 0.95 0.96 0.95 0.94 0.96 0.98 0.96 0.97 0.95 0.96 0.95 0.95 0.99 0.98 0.97 0.94 1.00 0.96 0.94 0.96 1.00 1.00 0.98 0.96 0.97 0.96 0.96 0.96

i’,/iG

0.088 0.089 0.089 0.091 0.091 0.089 0.089 0.089

0.084 0.085 0.084 0.082 0.082 0.081 0.081 0.082 0.085 0.085 0.085 0.085 0.085 0.084 0.083 0.085 0.086 0.087 0.083 0.086 0.086 0.087 0.086 0.085

APsVmol-’

,a “-l&-l/

(20 mM), 0.1 M NaCIO,, PC]

0.020 0.030 0.050 0.075 0.100 0.150 0.200 0.300 0.020 0.030 0.050 0.075 0.100 0.150 0.200 0.300 0.020 0.030 0.050 0.075 0.100 0.150 0.200 0.300

U/V s-1

(C) Propylene carbonate [E i2 mV vs. Ag/AgClO, 1

0.189 0.189 0.189 0.189 0.189 0.189 0.187 0.187

0.177 0.177 0.177 0.177 0.177 0.177 0.177 0.176 0.183 0.183 0.183 0.183 0.183 0.180 0.180 0.180 0.183 0.183 0.183 0.183 0.183 0.183 0.180 0.180

- E,B/mV 0.243 0.243 0.243 0.243 0.243 0.245 0.245 0.248 0.244 0.245 0.248 0.248 0.248 0.248 0.248 0.249 0.244 0.245 0.245 0.245 0.245 0.248 0.248 0.248 0.251 0.251 0.251 0.251 0.251 0.254 0.254 0.254

- Ec/mV 66 66 66 66 66 68 68 72 61 62 65 65 65 68 68 69 61 62 62 62 62 65 68 68 62 62 62 62 62 65 67 67

AE,/mV

z

172

, -0.70

+ 0.30

-0.20 E/V(vs.

l

0.80

Ag/Ag+)

Fig. 1. Cyclic voltammogram of 3.0 mM fexrocene in ethanol (22% H,O (v/v)).

Scan rate = 0.1 V s-l.

The same authors verified that in weak donor solvents such as acetone, acetonitrile and nitromethane, the FeC$ cation is stable. Table 1 also shows that for potential scan rates up to 0.3 V s-l in DMF solutions, the AEp values are about 60 mV, suggesting a reversible electrode process and a one-electron charge transfer. For higher scan rates, the separation between the anodic and cathodic peaks increases, and the electrode process is described better as a quasi-reversible one. The value of k O, calculated following Nicholson’s procedure [19], is equal to (0.063 f 0.006) cm s-l for potential sweep rates between 0.5 and 5.0 V s-l, in DMF with 0.06% (v/v) of water. This result is somewhat different from the value obtained by Diggle and Parker [l] (k o = 0.033 cm s-l at 1 V s-l). The k o values are approximately constant for all the DMF + H,O mixtures studied (k o = (0.062 f 0.005) cm s-l). In ethanolic solutions, AEr has a value of about 59 f 2 mV, constant within experimental error, under the conditions studied (u values up to 0.5 V s-l). Therefore the charge transfer is reversible. Our results in ethanol disagree with those obtained by Diggle and Parker [l], who suggest that the charge transfer is quasi-reversible (k’ = 0.016 cm s-l). The high values of AEp obtained by those authors are probably associated with ohmic drop. In propylene carbonate solutions containing 0.2% (v/v) of water, AEr is equal to 66 mV for potential scan rates up to 0.1 V s-i and increase for u > 0.1 V s-l. With a 1% (v/v) water content, AEr is approximately 60 mV for u < 0.03 V s-l. Increasing values were observed for higher scan rates. In the case of the other PC + H,O mixtures, the AEr value remains almost 60 mV for potential scan rates up to 0.01 v s-l. Under conditions in which AEr is higher than 60 mV and increases with u, the electrode process is described better as quasi-reversible and the

173

values of k” can be estimated. For example, the value of (0.023 f 0.005) cm s-l is found for the rate constant when the solution contains 0.2% (v/v) of water. The value of k o increases slightly as the water content in PC increases. For solutions with 5% (v/v) of water, k” = (0.039 f 0.005) cm s-l was obtained. Cabon et al. [2] observed an 88 mV difference between the anodic and cathodic peak potentials for the FeCpJFeCp: couple in ethylene carbonate, with 0.1 M LiClO, as the supporting electrolyte at 40°C. Courtot-Coupez and L’Her [20] report a difference of 100 mV in PC and 0.1 M LiClO,. Andruzzi and Trazza [3], using ac and dc polarography, found that the redox reaction for the FeCp,/FeCpG couple is a reversible process in acetonitrile and DMF + 0.1 M TFXP. The deviation of i, vs. I_?/’ and bEi,/ at high frequencies from the values corresponding to a reversible process was attributed, by those authors, to an uncompensated ohmic drop or, alternatively, to a deviation from reversible behaviour. Chronopotentiometric studies of the oxidation of ferrocene in acetonitrile +0.2 M LiClO, suggest that the electrode process is described better as approximately reversible. In the AlCl, + ethylpyridinum bromide (2 : 1) fused salt system, the oxidation of ferrocene is reversible at sweep rates from 0.1 to 100 V s-l, without any kinetic complications [5]. It may be concluded, therefore, that the addition of water does not give rise to great changes of the electrode process of ferrocene in the three solvents studied. In general, AEr does not vary with the water content if the sweep rate is maintained constant. The addition of water affects mainly the peak potentials. As Table 1 shows, the anodic peak potentials shift to less positive values in all of the solvents, with increasing water content. In DMF, the shift is about 60 mV for a change of = O.l-21% (v/v) in water concentration; in EtOH, the anodic peak potential shifts to 50 mV when water increases from 2 to 12% (v/v). In PC, a 12 mV shift is observed when the concentration of water increases from 0.2 to 5% (v/v). Similar results are reported in the literature for the addition of water to organic solvents [8,9]. It is observed that the oxidation of ferrocene is easier when water is added to the solution. Double-layer effects may also occur when the solvent is changed, and this effect is probably responsible for the potential shift when the water content is increased. The diffusion coefficient of ferrocene (Dn) (estimated considering a reversible charge transfer without kinetic complications) diminishes when water is added to DMF and EtOH and increases when water is added to PC, as expected, since the viscosity of the solvent + water mixture, in the absence of the supporting electrolyte, becomes greater with the water content for DMF and EtOH, and diminishes for PC, in the concentration ranges studied [21-231. Thus the diffusion coefficient of ferrocene in DMF + 0.5 M NaClO, is equal to 1.1 X 10e5 cm2 s-l and with 21% (v/v) of water it decreases to 5.7 X low6 cm2 SK’; in ethanol + 0.2 M NaClO,, D, = 1.1 x low5 cm2 s-l and with 12% Hz0 (v/v), D, = 8.0 X lO-‘j cm2 s-‘; in PC + 0.5 M NaClO, with 0.2% (v/v) of water, DR = 3.0 X 10e6 cm2 s-l and for 5% of H,O, D, increases to 3.4 X 10e6 cm2 s-l.

174 ACKNOWLEDGEMENTS

The authors acknowledge Funda@o de Amparo A Pesquisa do Estado de S5o Paul0 (A.J.Z., proc. 80/1266-9), CNPq (S.S.M., TR proc. 30.1426/79-QU.07, proc. 30.0967/83) and FINEP for financial support. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

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